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Jet Hopper Simple Modifications And Upgrades

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Hi, in addition to this shelf queen Jet Hopper with perfect rear tyres that I bought a few months ago ;-

http://www.tamiyaclub.com/forum/index.php?showtopic=48441

...I just bought another one recently for 15 GBP from the Bay, see pictures below from the Ebay page, with the intention of it being thrashed mercilessly around my back garden mini Off Road track. I also have a spare black body (with a few scuffs and with 1 of the spot lights that has broken off but I have it in a box) so I might be tempted to repaint one in yellow since I like the idea. I would like to use reproduction decals if there are any available for the Jet Hopper?

This JH was in used condition and can be seen here along with it's box which is different to the box off the shelf queen one ;-

http://www.tamiyaclub.com/forum/index.php?...20&start=20

I first tried this used one out in my kitchen on linoleum floor using Alkaline cells, and found that it would barely move in non-turbo mode. So I then put NiMH 2700mAh Vapex cells into it (AA) and then I tried it out in the short grass in my back garden mini Off Road track and found as regards power that the stock Jet Hopper couldn't pull the skin off a rice pudding :lol: . It was rather hopeless, it kept being unable to decide whether to go forwards or backwards (in non-turbo mode), it kept reversing and then going forwards several times a second as though the inernal control board PCB was being overloaded. Maybe mine is faulty?! :lol:. Also it would not steer properly due to the poor torque from the built in servo, the lack of proportional controls really sucks, but what is worst is that when I tried to steer the voltage to the motor was drastically cut since it goes into non-turbo mode when steering, to the point where the car won't hardly even move anymore at all on short grass. On the high gear ratio on turbo mode it's OK but could do with more poke, so at least the Turbo function still works on the TX (but with no steering) but the performance wasn't that startling!. But if it can't even turn corners it's useless to me. Finally the radio control range was very poor indeed (12 feet approx.), and for the record this deep red car is on 27MHz.

This was completely impractical for me to continue with, so I decided that my Jet Hopper MUST be modified in order to give satisfaction.

Elsewhere on the forum TA-Mark is doing an excellent and exciting build thread showing brushless power going into his JH, but I will here offer my comments on much more modest upgrades (e.g. a 5x more powerful very cheap brushed motor and a very much more powerful servo, and proper proportional radio control with excellent range).

So time to get to the hard work part. Here we go.

Cheers,

Alistair G.

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The tyres on this used one are just around the amount of wear where I won't be terrified to use them hard, and they haven't dried out. I am trying to reproduce the Jet Hopper tyres (I have 4 of them in NIP condition), see ;- http://www.tamiyaclub.com/forum/index.php?showtopic=49589 . Wheels are in nice condition. The front bumper is missing it's reinforcement but the bumper is in fair condition. The underside of the chassis is in good condition.

The body is particularly good condition (better than I had hoped) and you're right Mark it is a deep red! My other one is a uniform red-orange http://www.tamiyaclub.com/forum/index.php?showtopic=48441 . There are a few decals missing (including the rear wing one) so I'm going to need a repro. decal set. Anyone?!

I just noticed that they put "K C DAXLIGHTER" on the lights instead of Daylighter, maybe to side step copyright issues! However a little touch of yellow paint would put things right there!

Someone painted my roll bar and nerf bars!

Blimey there is quite a difference in colour between the as new condition one and my deep red one (see picture)!

NOTE that the red-orange body says "27km/h" on the front decal, whereas the deep red one says "25km/h", which leads me to believe that the red-orange one is a newer version of Jet Hopper? As manufacturers got into a war of whose car went fastest! Maybe I have one of the very last production runs of the JH for Europe ? Because as you can see the decals on the new orange-red condition car are perfect and completely unfaded it would seem. The body is a completely uniform color inside and outside which leads me to beleive it has not been faded by UV light? One of the SAYS decals on the right hand side was skewed so I removed and repositioned it and it was the same body color underneath the sticker.

It was a slightly used black body with 49MHz chassis, supplied with a new red-orange body, new front tyres, new rear tyres, new front bumper, and new rear wing, suggesting that the red-orange body was spare replacement parts sourced, which may indicate that it's from a production run near the end of the JH 1's life just before it became JH2 ?

This may be true as I have seen someone's JH2 and it was quite an orange color!

Cheers,

Alistair G.

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OK here we go with the modifications ;-

First thing needed was to remove the top half of the chassis, this is done by unscrewing the 4 screws on the top of this section. Also each nerf (side) bar has 3 screws in it and they have to be unscrewed as the nerf bars are holding the top half onto the bottom half of the chassis. Then you can pull the top half off and it has the LED cover / surround and LED itself lightly pressed into it so remove those latter 2 items, but it will still have wires connecting the battery box to the PCB underneath. I pulled / cut all the wires off as I would be discarding the PCB. There are also wires connecting the PCB to the charge socket (on the later Jet Hopper) (BTW you can't remove this charge socket) and the PCB itself is held down by a screw. Then we can remove the internal servo and it's very weak servo saver, the servo is held on by 2 screws in the top of it on the outsides of it. I discarded the main PCB. I removed the on/off switch.

Then I undid the 2 screws holding in the motor endcap and took the motor out to finally find out what size motor the Jet Hopper has, as this fact had fascinated me for years. I was expecting to see a 380 motor but it turned out to be quite a bit smaller. In fact it's a 280 motor!

Cheers,

Alistair G.

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I took the motor out of my stock Jet Hopper and it is a Mabuchi motor , model number RE280 SA , and appears to be variant - 2295 with metal brushes, not carbon, nominal 6V, 9600 RPM, 0.14A no load, 4.4A current when stalled, peak power 6W. ;- http://www.mabuchi-motor.co.jp/en_US/cat_f.../re_280rasa.pdf . Motor simulation is here ;- http://www.mabuchi-motor.co.jp/en_US/produ.../0.0019/neutral . Since as Mark mentioned elsewhere the board doesn't seem to supply the motor with 12V (8 cells alkaline), that seems to be for the radio section instead, and appears to supply the motor with less (4 cells, 6V) for the Turbo mode. If powered on NiCd / NiMH the motor would receive 4.8V on Turbo but would have much more punch since alkalines have a very high (comparatively) internal resistance.

When I examined the output stage to the motor there were about 6 fairly meaty power transistors with quite large metal heatsinks. This motor draws 4.4 A when stalled at 6V, which is consistent with the gauge of wire that you can see attached to the motor (on alkalines, stalled at 6V, current would be lower as there is a large volt drop when you start drawing "heavy" currents from alkalines due to high internal resistance of alkalines), and on 6V no load (NiCd / NiMH) the motor is at 9600 RPM. Indeed I just put 6V through the motor and I heard a very average RPM, not that slow but nowhere near a stock RS540SH which does 17,550 RPM at 7.2V (model 6527 , that is to say 0.65mm wire, single, 27 turns). Maximum output power is just under 6W at 6V with this variant of this 280 motor.

There are 2 other motors in the 280 series, model 2865 http://www.mabuchi-motor.co.jp/en_US/cat_files/rc_280sa.pdf and 2485, but 2865 draws 9A when stalled at 6V so would flatten the alkaline batteries quite quickly, and 2485 draws 1A more than 2295 when stalled even at 6V, and the peak power is higher, and the designers would want to operate a motor at it's highest rated voltage for highest torque and efficiency, since the JH supplies only 6V to the motor (on alkalines).

I have included a couple of photo's below of the 280 motor out of my JH compared to a stock Mabuchi RS540S .

Taking the motor out of the JH and finding out what it was felt like a major milestone for me, as I always wondered what was in it!

Cheers,

Alistair G.

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A little while ago as I was pulling apart my deep red JH I noticed that it has a charging jack so it seems like it's a later model of JH, possibly just before the change to the 9.6V Jet Hopper (the 9.6V Jet Hopper just came with a rechargeable battery pack instead of working off 12 V from 8 cells of alkaline). However at the time I was so frustrated by the rubbish control that I pulled the main PCB out of it and pulled all the wires out so I didn't get much chance to see how it was wired in detail.

Today though I just pulled my red-orange early model JH apart (no charging jack, early revision TX board, plain silver painted logo on TX) and took some pictures and now I understand how the stock Jet Hopper is wired (I think). See the pictures. It has 12V available between purple and yellow going to the main board. Also has 6V available between purple and red. 9V connection does not appear to exist. I am of course assuming the use of alkalines here. All input wires are very thin. The 2 motor output wires (red, white) are reasonable gauge (a lot thicker) and could carry 5A or so, which is consistent with the 4.4A stalled current at 6V of the Mabuchi RE280 SA 2295 motor. However the way it's wired, it seems that there isn't enough thickness from the input wires to allow more than about 1A to come out of the alkalines without a horrid voltage drop!

So from what I can see, the radio part is powered off 12V (presumably to a voltage regulator at 12V) and the motor in turbo mode is powered from 6V and when on non-turbo mode must have only maybe 3V across it.

The trouble is that the motor out of my Jet Hopper doesn't have anything written on it apart from Mabuchi Motor and Made in Taiwan and Patented.

Indeed, Mark told me that if you run the standard motor on 9.6v for a few minutes under load it will burn out. I can believe it now that I have examined the wiring and that found the motor appears to use only 6V on Turbo.

However the main board is using 12V (look at the purple to red wire loop in the pictures and the way it connects the purple to orange then to yellow through the switch and then onto the main board) as far as I can best make out, for the radio section, but even so it has very poor range, the range was little more than 12 feet outdoors when using an 8.4V NiMH block in the TX and NiMH cells in the car. Maybe the poor range is also because the main board requires 12V for 'normal' reception and NiMH only gives the board 9.6V.

I would be reasonably happy with the stock motor I think, as long as it was permanently in Turbo mode, which means at 4.8V on NiMH, but since it's rated to 6V I'd rather use it on 6V so I get the full speed and torque along with the highest efficiency. The problem comes when you try and use an Electronic speed controller with it such as my Futaba MC330 CR which only accepts 7.2 to 8.4V input! Which would burn out the motor or make it run hot!

Cheers,

Alistair G.

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I was thinking that I could use a pack of 6V of sub-C cells for my JH since I just happened to have such a pack. However I weighed it and the pack is 281g. Also it is large, see picture below.

In comparison I just weighed 5 Vapex 2700 AA cells (6V) and they came to 148g! Half the weight! Looks like I'll be using a custom made pack of AA cells then! I am off to research where to buy separate AA cells that I can solder to, with safety vents, preferably with about 2500mAh capacity minimum.

Ahh then again thinking about it I can rewire the existing internal battery box to have thicker wires on it and just use 5 cells. Maybe I can fit a receiver in the space that is left by not using the other 3 cells!

Cheers,

Alistair G.

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Regarding the much needed stock JH servo upgrade ;- Here is the size of the aperture at the front when you remove the existing stock servo from the original Jet Hopper. The space available is 40.5mm wide x 40mm deep x 25mm height approx. See pictures below.

Also I just noticed that my later version chassis has much larger springs both front and rear and the rear shocks are a lot larger diameter (compared to my early JH chassis).

Cheers,

Alistair G.

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Interestingly I just compared the Mabuchi website motor simlations for an RS540SH variant 6527 (which is a stock kit motor for most 1/10 cars, .65mm wire, 27 T single) and the only (non-torque band) RS380SH motor available , variant 4535, which presumably means .45mm wire, 35 T single, and the RC280SA variant 2485 (Jet Hopper uses a 280 size motor), and the range seems to have been rather cleverly designed, since we have ;-

RS-540 SH #6527 , Peak output at 7.2V is 74W , torque at this output is 817gcm (stock kit motor in most cars)

RS-380 SH #4535 , Peak output at it's 6V rated maximum is 35.6W, torque at this output is 393gcm (Tamiya Grasshopper motor)

RC-280 SA #2485 , Peak output at it's rated 9.6V maximum is 18W, torque at this output is 213gcm

...notice the pattern developing! So as we can see each one in the series (if the right variant is chosen) has roughly half or double the performance when you go up one or down one.

Just thought that was an interesting curiosity :lol:

However the Jet Hopper unfortunately uses the RE-280 SA #2295 , which is the same size but is wound differently, which has a peak output of 5.42W at it's maximum 6V rating , torque at this output is 113gcm, which is 1/7th the power of the Grasshopper and 1/4 of the torque ! What a weed :lol:

Cheers,

Alistair G.

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So then, to replace the stock Jet Hopper with a motor that fits exactly but has much higher performance it looks like it would be ideal to use the RC-280 SA #2865 , which has identical dimensions to the stock Jet Hopper motor but has a maximum output of 28.4W at 8.4V which means we could use 7 cells of AA NiMH, which would mean the car gets 1 cell lighter, and the motor power is then 5.24x higher (!) than the stock JH, and torque at this output is 221gcm which is twice that of the stock motor, efficiency being 37.8 per cent at this max. power, and no load RPM is 19,600 RPM which is twice that of the stock JH.

This motor should cost peanuts also, which suits me just fine! You can buy an awful lot of these motors for the price of a brushless set up, and with the modern AA cells motor efficiency is almost irrelevant I would think, plus any reasonable reversible double tap type (back for brakes, return to neutral and pull back for reverse) ESC should work very well as the motor can be stopped quickly and doesn't need a very powerful braking circuit in the ESC, and you're not going to burn out a reasonable ESC with a 28W motor! :lol:

I just checked and there are several of these types of motors available on Ebay at the moment, but not that exact one. Initially I thought I saw the exact one but it was the RE not the RC, with different spec's, you have to be careful! However the price is usually around 3.50 GBP it seems, and about the same for postage. Sounds good to me LOL. I'll be able to operate it with a 'double tap' type ESC, my Futaba MC330 CR on 7 cells.

I just learned that JP distribution have these motors (so available at UK model shops to order) ;-

http://www.jperkinsdistribution.co.uk/deta...bcatname=Motors.

At least the picture shows the exact needed motor to nicely upgrade the stock Jet Hopper by a factor of 5x more power. However their description says "4.8 to 12 volt DC", which is incorrect for this motor. According to the official Mabuchi data sheet the rated voltage is 4.5 to 9.0V DC ;-

http://www.mabuchi-motor.co.jp/en_US/cat_files/rc_280sa.pdf

...this means that either the voltage listed on the JP web page, or the picture on JP's web page showing RC280SA 2865, is wrong.

So I have e-mailed them to ask them please can they take a picture of one of these motors from their stock so that I can see the label on the actual motor that they are selling and thus confirm that it is indeed the RC280SA 2865. Also because there are also variants 20120 and 2485 of this RC280SA as you can see from the data sheet, which have different specifications as these latter 2 motors are wound differently, and I would hate to order this 5510343 motor from them and find that it wasn't what I was expecting from the picture on JP's web page :lol: Also you have to be aware that there is a dimensionally identical Mabuchi motor called the RE280SA which has a few variants and different spec's also. They all perform with vary different output powers so it's important to get the correct one!

Cheers,

Alistair G.

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Yesterday I tried to take the pinion off the JH motor shaft and get the rear wheels off and the little car put up one heck of a fight :o

I tried to find a hardened steel pin to hammer into the end of the pinion's central motor shaft hole but realised that I first had to get the motor shaft free as otherwise I couldn't do that operation. Seeing as how I didn't actually need the stock JH motor anymore I gripped it in a bench vice and the ended up putting the standard long nose pliers into the gap between the pinion and the motor and trying to lever the pinion off the shaft but it pulled the shaft out of the armature :lol: (which is only a press fit anyway)! Then I was free to put the pinion plus shaft into a bench vice where I could put an undersize hardened steel pin in one end and hammer the shaft out. However I couldn't find a suitable sized hardened pin. I tried hammering it with the short end of Allen keys but bent them, even when I heated the pinion in boiling water to try and expand it. It's one mother of a press / interference fit ! It just wouldn't come off... so I got frustrated and called Mark for help.

The great guy helped out marvellously and suggested a couple of very useful things. He said "You need to make a few tools to remove the pinion. First take a 1mm thick sheet of steel and cut a slice into it just bigger than the motor shaft. You will need to lay this sheet over a sturdy vice. Heat the pinion with a butane torch, then while it's hot use a cut off hex key (a rounded useless one is a good candidate to cut) to drift the motor shaft from the pinion."

In the end today I managed to finally get the pinion off the motor shaft, I did it by as Mark suggested cutting an allen key (part of the short bit) with a Junior Hacksaw to make a short hardened steel pin, and then I put the pinion plus motor shaft in the vice, loosley held, and then I played a disposable lighter flame on the pinion for 1 minute 10 seconds on fullest flame setting, as I have no torch, and then I hit the section of Allen key into the hole with a large hammer and it made it almost come out (luckily the JH motor shaft is not pressed quite all the way into the pinion, there's a 3mm or so gap at the top where we can insert a 'hammering pin' as it were), but I went to the depth of the allen key section and there was still a little bit of motor shaft still in the pinion, so I then gripped the motor shaft in the vice, with pinion sticking out of the left of the vice, with pliers inbetween pinion and vice jaw left ends, and I levered the pinion off with reasonable ease, without damaging the pinion on the middle section of the pinion where the the spur runs on it. It seemed that heating the pinion to expand it was the key to getting it off, I don't think it would have come off otherwise as I was hitting it hard the other night and got no effect! Thank goodness that's now done!

I included some pictures below. Levering the motor shaft out of the motor with pliers, motor and motor shaft & pinion separated, Allen key chopped up to make the hardened steel 'knocking out' pin, a picture showing the way I heated the pinion with a disposable Ronson lighter, and the free'd pinion by itself (finally!).

Progress! :lol:

Cheers,

Alistair G. (Live_Steam_Mad)

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Getting the gearbox off the chassis was quite easy, you just use a 3 x 12mm self tap screw or similar and hold the assembly down on a non-markable surface (like a cutting mat) and you hold the screw end against the steel shaft that goes all the way through the gearbox and chassis mouldings, with pliers and hit the screw head with a small hammer and the shaft comes out with reasonable ease. The large sized Tamiya Allen key can then be used to drive the shaft fully out once the screw has gone as far as it can. Pictures to illustrate are below.

Now for the rear wheels. I twisted them as hard as I could as I thought that they might be held on with the nuts on each end with slow setting superglue on their threads, but the wheels just wouldn't come off. I couldn't get a nut driver on the nuts either as they don't stick out far enough. So I called on Mark again and he told me "Almost the same method to remove the press fit hexes from the axle. The slot you cut in the sheet (7mm MDF works for this and won't damage the inside of the rim) needs to be big enough to go around the axle housing and backs to the wheel. Drift the axle from the hex with a suitable drift (larger cut hex key again). You cannot of course use heat on the axle or it will melt the axle housing and rim. Be careful you don't drop the gearbox as it will pop off suddenly. Do not attempt to twist the hexes from the axles as this will just destroy the hex seat in the plastic rim. Use many small taps with the hammer rather than one large thud. I did say they were a PITA to remove, hence why it's a good idea to use a motor that will last a long time with large amounts of power pumping through it. Not a job you want to do every other week. If you have access to a set of pin punches they are much easier to hold onto than cut off hex keys. A trick to hold the hex key and not hurt your fingers is to grab them in a pair of pointy nose vice grips. Putting the pinion and axle hexes back on is just as much fun as removing them without doing any damage to the sourounding parts."

Now I can see why Mark is thinking that brushless is such a good idea. They don't have any brushes to wear or commutator to damage, and swapping the JH pinion between motors can be a real chore.

So it looks like I'll be stumping up the money at some point and going the brushless route also. I'll show details of what motor I end up putting into it here or on Mark's brushless JH topic.

I'll still need to give my stripped out Jet Hopper a lot more powerful servo and proportional radio gear, so I'll show that here.

However the thought occurs that if I can ream the pinion out and cross drill the pinion and use a grub screw then I can use any of the cheap brushed motors and swap the pinion between motors quite easily.

I was looking for any evidence of a shaft flat on the JH motor shaft and there isn't one. None of the 280 motors from Mabuchi have a shaft flat. Also none of the brushless replacement motors of 280 size have shaft flats either it seems?! For example the AE RC18T uses a motor with no shaft flat (so it appears)

but does have a grubscrew ;-

http://aedownloads.com/downloads/manuals/18t_manual.pdf

(page 11, bag G, step 1, magnify the motor)

Also of importance, some of the brushless motors of this size like the LRP one ;-

http://www.lrp.cc/en/products/electric-mot...d-6900/details/

...have shaft diameters of 2.3mm whereas the original Mabuchi motor shaft is 2.0mm (+ / - zero as it's Superfinished). Thus the JH pinion would have to be reamed on a lathe to a press fit I assume. Hence the attaction of putting a grub screw in the pinion. Doesn't have to be super accurate then, as you have the grub screw biting into the motor shaft, but not by much admittedly as any decent motor shaft tends to be hardened and Superfinished.

I just found this 1/18 brushless motor by Losi, it's got a shaft flat so would be suitable for use with a JH pinion with grub screw ;-

http://losi.com/Products/Default.aspx?ProdID=LOSB9463

...I saw the shaft flat in an advert with picture of it in a magazine. However it's a sensoreless motor (Losi don't do sensored ones in 1/18) so it will 'cog' at lower RPM's so driving will apparently not be smooth which may be an issue.

The specification and manual for the motor is here ;-

http://losi.com/ProdInfo/Files/1-18SensorlessMotorManual.pdf

They start at 135W power and go upwards! Which is 25x the power of the original motor!

Finally there is also the matter of motor attachment. The original JH motor has two 2.3mm holes on a 14mm diameter. The holes in the motor are NOT tapped they are just plain holes. Only the holes on the 380 motor are tapped. The motor is held into the gearbox by a plastic moulded pip. Thankfully the above LRP motor has two 2.5mm holes at 14mm diameter so it should drop straight in as it's dimentionally identical to the original RE280 motor it seems.

Cheers,

Alistair G.

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Finally I managed to get one of the wheels off today. Wow it was more of a hassle than getting the pinion off and that was bad enough!

To do this, I took Mark's advice and grabbed some MDF, in this case off an old drawer rotting away outside at the back of the house, it was all I could lay my hands on and was 16mm thick with thin plastic facing front and back. I used a Coping Saw and bench vice to cut a piece about 50mm square. Then I cut a slot in it 18mm wide by 27mm deep. This completes the wooden 'tool' needed. Pictures below show this. Then I put the wheel (minus tyre) and axle / gearcase in the tool and tried to hold it in the vice but could not close the jaws enough to provide a base for the tool i.e. too much wood was unsupported in my small bench vice and would probably split apart (as it was partially rotten anyway). I supposed that plywood would have been a lot stronger but it's harder and might not cusion the shock of the hammer so well when we come to knock the axle out resulting in a cracked wheel (it's only plastic, looks like ABS). So what the heck to do? I grabbed 2 pieces of 3/4" (19mm) equal sides mild steel angle (L shape), both about 14" long, and put them into the vice on either side against the jaws, and I put an off cut of solid wood which was about 1 inch thick (25mm) (anything as long as it's quite a bit bigger than 18mm wide otherwise it will crush the gearbox when you close the vice jaws) and of at least the full length of the vice jaws, into the gap between the 2 steel angles and tightened up the vice very tight, and put the wooden tool plus wheel / gearcase up as far as I could get against the side of the vice with the wooden tool resting on top of the steel angles, which made a very effective support. Pictures below show this. I was all ready to hit the axle end inside the wheel nut (as the axle is a press fit in the wheel nut, there are NO threads in either).

Now then, what to hit it with? I initially started hitting it with a small hammer and the short end of a 3mm Allen key but that didn't seem to do anything at all, too little force being transmitted. So I tried cutting the short end of the Allen key to 12mm length with a Junior Hacksaw and used it as a hardened steel pin to hammer out the axle. However I had to hold the pin with pliers and at a strange angle to stop me hitting the top and sides of the wheel as the nut is it's deeply recessed (wheel width). This proved to be impractical. So I used a 3mm hollow punch (for rivetting) (a flat head 3.5mm punch would be ideal) and hammered but very little movement was created. BTW a smaller punch would have only widened the axle (squash rivet effect) and made it harder to remove. So I used a larger hammer (see photo below) and found that I needed moderate blows to extract the axle, which I was suprised at. I was expecting to crack the ABS wheel but those wheels are tougher than you think and the MDF helps absorb the blows. I found that the axle had a small black plastic wheel bearing which slid off, which would probably melt when transmitting a lot more power, so they'll have to be replaced with ball races.

Inside the gearbox I found that the JH motor has a 10 tooth pinion and a 43 tooth spur gear. Thus the gear ratio is 4.3:1 on the 1st reduction. Then it has a 10 tooth small gear moulded onto this Spur which engages with a 39 tooth large output gear which is locked on the rear axle, a 2nd stage reduction of 3.9:1, resulting in an overall gear ratio of 4.3 x 3.9 = 16.77:1 on the SLOW gearbox setting. This is the setting I plan leaving it on when I add a new upgraded motor with a lot more power.

BTW The other side of the 43 tooth Spur gear also has a 17 tooth gear moulded into it, which is for when the spur gear slides across to engage with the "Fast" gear setting on the gearbox selector. That has a different overall gear ratio but I'm leaving it on the slow setting. This 17 tooth gear is what you can see on Mark's brushless JH motor page.

It seems that 16.77:1 is quite a high overall gear ratio and thus should be suitable for the highest RPM brushless or brushed motors available in the 280 can size.

Cheers,

Alistair G.

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Today I managed to machine the Spur gear to remove the plastic bush which is moulded into it, ready for inserting ball races.

This is shown nicely in Mark's brushless JH topic. I followed some of his instructions but because of a lack of a Dremel I did it a different way. I initally thought of using an end mill in my bench drill chuck but of course I haven't got any end mills. So instead of spending money I looked through my tools and found a Letter 'G' drill bit (0.261" or 6.6mm) which was rather long but had something like a 100 degree point on the end of it instead of the normal point angle. Methinks someone has done that with a drill bit grinder in the past. OK so it's a few degrees of bevel and not flat like an end mill but I thought it would do, so I set up my 8" bench drill to use it.

I grabbed the 3mm shaft that the spur runs on and put it into the chuck with about 30mm protruding. Then I put the spur gear into the end of a set of 4" approx. parallel clamps. Pictures below show this. Then I rotated the drill table left and right and pulled it up and down until the 3mm shaft entered into the spur gear 3mm central hole by about 1/4 inch or so. The table had to be this low to allow the large long drill to clear the gear when I put the large drill in later on. Then I tightened up the drill table and the clamped the 4" parallel clamp to the drill table with a small C clamp. Then I needed something on the other side of the spur gear underneath to support it as when you drill away the plastic moulded in bush it excerts a bending moment on the spur causing the spur to be drilled at an angle instead of orthogonal to the table. So I placed a scrap of wood about 1/2 inch or so under the gear. By luck it was the exact right thickness of wood. Then I clamped the wood to the table as well. Then I inserted the long drill bit by opening the chuck to max. capacity and there was then enough clearance to get the long drill up into the chuck past the spur gear.

I set the depth gauge on the drill (so as not to drill out the 'bearing seat' needed for the later ball races) so as to drill 6mm down. Since the drill started too low (long drill) for the depth gauge to be set in the normal way, I instead pulled the chuck using the handles down until the drill was touching the work. Then I got my digital calliper and measured 6mm. I adjusted the 1st depth gauge nut to be this distance above the stop, using the 2 'inside' calliper blades. Then I did up the 2nd nut (lock nut) against it. Then I gritted my teeth and commenced drilling.

As I was drilling there was a small amount of left over material on the very outside of the bush, on 1 side, protruding, but I found that I could remove some of it by drilling with multiple passes. I went down virtually the full 6mm and kept checking with a very bright torch (flash light) to see if I had gone far enough. Eventually I reached the point where I was level as best as I could judge by eye, with the surrounding recess in the bottom of the spur gear.

Later on I used a pair of pliers to scrape off the small amounts of material that the drill missed (as I mention above).

I now have a hole 9mm internal diameter and has to accomodate a 3mm shaft, and I have a suitable flat (OK with slight bevel) bearing seat. Either I can do what Mark suggests and bond in a piece of Nylon tube, or use two off 3mm ID x 9mm OD x 3mm wide bearing to fit the hole. More later.

Cheers,

Alistair G.

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The only bearing I could find in a 3x9 x3 had a flanged outer casing, hence why i used the ones I did. You may have more luck on the bearing search than I did.

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The only bearing I could find in a 3x9 x3 had a flanged outer casing, hence why i used the ones I did. You may have more luck on the bearing search than I did.

Hi Mark, sorry for the delay, a busy week beyond belief LOL. I looked up bearings and found this ;-

http://simplybearings.co.uk/shop/p2566/603...oduct_info.html

...these are open (no metal or rubber shields either side) but are not flanged it seems.

I presume that you wanted these for the 3mm shaft that the Spur runs on?

I haven't considered that yet, I was going to start with replacing the plastic wheel / back axle bearings, these are 4mm Inside Diameter (ID) by 7mm Outside Diameter (OD) by 4mm width. Unfortunately there is no such bearing in commercial production, the nearest I can come up with is 4mm ID x 7mm OD x 2mm wide ;-

http://www.bocabearings.com/docs/pdfs/Engi...ing%20Guide.pdf

...see page 5, they are coded MR74 , this is an open bearing (no shields on either side) like the one above, or MR74ZZ ( MR742Z ) which is 4mm ID x 7mm OD x 2.5mm wide ;-

http://www.brockmodels.com/product_info.ph...9edc3kiu7r8cpe7

These 2 types (2mm and 2.5mm wide) are also available on Ebay, run a search for " 4mm 7mm Bearing ".

So, to equal the width of the original plastic bearing I would use two of the MR74 bearings, with some risk of bearing damage due to the 2mm wide bearings having no shields.

Alternatively I could use 2 of the 2.5mm wide bearings. Even though the plastic bearing is only 4mm wide, the hole it goes into is 5.3mm deep with a 1mm approx. recess near the wheel end of it. So two of the 2.5mm bearings would fit, albeit with a little of the outer bearing being unsupported. They have 2 shields each (one either side) to keep the dirt out. This seems like the most logical solution. A 4mm x 7mm x 5mm bearing would be the ideal solution but there is no such item commercially available it seems.

Cheers,

Alistair G.

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I used '4x8 x3mm Metal Shield Ball Bearings' on the rear axle from Walawala eBay Store. I bored out the bearing seat slightly larger so I could use a larger bearing size on the axle. There's no need to remove anything from the rims to fit them in.

On the Spur gear shaft I used 3x7 x3mm Metal Shield Bearings and a nylon ring. Gearbox is very smooth now with all bearings. I gave the sides if the gears a sand with 600 grit sandpaper to make sure they ran smoothly against the other gears too.

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I used '4x8 x3mm Metal Shield Ball Bearings' on the rear axle from Walawala eBay Store. I bored out the bearing seat slightly larger so I could use a larger bearing size on the axle. There's no need to remove anything from the rims to fit them in.

By bored out, do you mean by using a bench drill with 8mm drill bit and using the depth stop to drill down only 3mm into the bearing seat?

Alistair G.

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I cut them out by hand first with a sharp 7.5mm drill bit then a sharp 8.0mm drill bit. Turning at high rpm in plastic tends to melt it slightly where as turning by hand with sharp tools cuts the plastic clean and leaves a smooth finish.

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Whilst I was looking at the gearbox I had a chance to examine the rear axle and found that the gears were "keyed" to the shaft by using "slots" which have been "machined" or pressed into the axles. You can see this on the photo's below. The gears have a metal nut at the centre which is just like the ones used on the wheels (no threads).

The shaft is of course 4mm diameter.

Cheers,

ARG.

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Thanks Mark, I took your advice and drilled out the rear gearcase bearing seats, an excellent suggestion, it allows us to use more bearing thickness for higher duty.

I started with the larger gearbox half first. I tried gripping it in my drill press vice but it didn't fit as it's max. jaw opening is 54mm! Curses! So I went and got my Myford machine vice # 78217 which has a 2.23 inch jaw opening (max.) and as luck would have it, it was just big enough to accomodate the gearbox half, plus two 1/32" plywood protective pieces to prevent marking the gearbox. I also saw that this larger gearbox half has a protrusion (shown below in the pic's) which stops it laying flat normally, so I put it on top of a piece of gauge steel (a flat steel bar about 1 1/4 inch wide and a few inches long and about 1/2 inch thick) that I had lying around, this allowed the gearbox to lie flat.

Next I did up the vice but not tight by any means, just enough to hold it firmly and stop it moving. Then I had to swivel the drill press table out of the way as there wasn't enough room to get an 8mm standard series drill in (which is quite long) otherwise. Also I found that I had to rest the machine vice on top of a few pieces of flat steel to prop it up as otherwise the drill did not have enough travel to reach the top of the gearbox half where I wanted to drill! Then I put a 7.25mm approx. (nearly all my stuff is Imperial!) drill in the chuck and went into the end on the slowest speed that the drill would do (about 500 RPM) and drilled down to 3mm depth using the depth gauge on the side of the drill press. I had an 8.00mm Tamiya metal plain bearing (Phosphor Bronze bush) to hand to try for test fit. Then I used 7.5 approx. , 7.75 approx. , and finally 5/16", but I found that at around 7.94mm the 5/16" drill was too small and the press fit was way too tight. So I tried an 8mm metric drill which was 8.00mm dia. on the very end of the flutes and this created a press fit which was just right (plastic tends to spring back a little when you drill, just like wood). Don't press the "test" bearing in more than a little (0.5mm) or you'll never be able to get it out! I used an Allen key to pop it out after testing. Presumably this larger gearbox half will now accept an 8mm OD x 4mm ID x 3mm wide ball race with a press fit.

With the smaller gearbox half I drilled straight into it with the 8mm metric bit, and didn't bother with the others. The fit was just nice with the 8.00mm test bearing. There are 2 protrusions on the bottom of this half which make clamping it in a machine vice a challenge but I was able to do it without the metal bar being put underneath this one this time. Watch as you tighten the vice a little that the upright section does not bend as you tighten. This happened to me a few times before I realised that you have to make sure that the gearbox half is supported fully underneath. All went well and now that half is also ready for it's 8x4x3 ball race as a press fit.

BTW I found that you head to drill really quite slowly or else the plastic heated up fast and started to smell!

Also BTW PLEASE NOTE that the 850 sized Tamiya ball races ( 8x5x2mm) will NOT be suitable since the rear axle is 4mm diameter, so don't press an 850 in by mistake or you won't be able to get it out!

Cheers,

Alistair G.

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OK I just ordered " 4x8 x3mm " bearings (6 off, enough for 3 Jet Hoppers) for the rear axle, and " 3x7 x3mm " bearings (again 6 off) for the spur gear shaft, from " hkdrpro " on Ebay. Walawala store had only the larger ones, and Fusion the UK seller wanted 2x the money for them.

The latter bearings are so that I can do what Mark has done and put a tube into the Spur gear to reduce the internal diameter, ready to take 7mm OD bearings instead of 9mm ones, since I just measured the internal diameter of the Spur and it's 8.84mm which means the press fit would be too tight for a 9mm bearing and I can't drill it out with a 9mm drill as the end of the drill would drill into the bearing seat (I don't have an almost flat topped 9mm one like I used to remove the existing plastic "bearing" in the Spur) and I don't have a 9mm reamer.

So I'll have to buy some 9mm approx. plastic tubing to reduce the internal diameter like Mark did. Plastruct make ABS and Butyrate tubing which is 9.5mm OD x 6.3mm ID (they are American and based on Imperial sizes) which would suit as the ID can be drilled out to 7mm to take the " 3x7 x3mm " ( " 730 " ) bearings.

I just measured the depth to the bearing seat in the Spur gear with a depth gauge and it's 5.5mm deep, which is enough to accomodate two 730 bearings (each has 2 metal shields, one on each side of each bearing, and each bearing is 3mm wide) with 0.5mm sticking out.

Cheers,

Alistair G.

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Today I fitted the new servo to the JH. I grabbed a used servo out of a car I got a while ago and I don't know anything about it other than I just noticed that it says "4.8V" on it and it is a JR (Japan Radio) servo imported by Graupner. Looks like it's a standard sized servo for 1/10 scale cars, 54.3mm at the widest part (across the "ears" on the edges of the servo), body is 38.8mm wide, 38.3mm deep (from bottom of servo to the top of the servo, measuring to the black part on top not the white splined part), 18.7mm thickness. I assume that it's non ball raced, has only Nylon gears, and is not coreless or digital or anything fancy, and that it's torque is around 3 Kg.cm .

The original servo was frankly pathetic so this one can only be an improvement, I hope. First thing to do is undo the 4 off 12mm long self tap black screws holding the top of the battery holder section to the bottom of the chassis. I removed the main PCB of the old radio control for the JH toy a few weeks ago, and also removed all of the wires, only leaving the red LED light with it's wires intact (of course I had removed the old stock servo and servo saver). Then I tried pushing the new JR / Graupner servo into place but it didn't fit, as it's mounting lugs / ears are a bit too wide. So I got a small flat file out of my Tamiya 3 Piece Basic File set and filed off the ends of the lugs at an angle to fit the slanted sides of the JH chassis. I tried it a few times and eventually got a tight push fit of the servo into the chassis. It seemed to fit flat just nice. However one side's lug (right hand side) was a little too far forwards (maybe by 1mm or so, the photo's below show this I think) so the servo was slightly slanted with respect to the chassis but later on I corrected this (see photo's).

So I proceeded to put the top half of the chassis (battery holder) back on and then found that the LED mounting fouled the servo. Again, see photo's. So I used a small razor saw (usual disclaimer) with very thin blade to cut away the relevant section (see photo's) i.e. the diagonal bit under the LED mounting. There was still plenty of strength left in the LED mount. I tried to put the top half back on again but it still fouled by a little on the top flat section of the LED mounting, so I filed a little more off it's thickness in the place where it overlapped the servo body, until it cleared sufficiently.

All went well and the top section was able to be screwed down onto the chassis properly (albeit with the chassis bottom sides sticking out very slightly as the servo was quite a tight fit, width wise, I think I need to at some point file away a little more on the servo 'lugs'). The red LED was still able to go back in it's place (OK so it no longer functions at present as it's not really needed anymore). There is quite a gap between the servo body and the top of the chassis / battery holder where I could put in some wood to hold the servo in really well. However the servo is quite solidly held as it is.

A little later I tried pressing on the servo on the right hand side with moderate pressure and blow me it went "click" (friction releasing) and jolted backwards into place by 1mm to match the left hand side and now the servo is not slanted anymore across the chassis.

Next step is to add a new servo saver. The lug that you can see in the photo's on the front section that is width ways between the 2 front wheels that is protuding into the space where my new servo saver will go, will need to be filed off. You can apparently unscrew this section to get at it with the razor saw. That will come another day. BTW in the photo you can see how some kid has almost set fire to my rear arms mounts on my JH!

Cheers,

Alistair G.

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Well I had a look at the steering lock available and the geometry (wheels with too much toe out, not enough lock, steering links not low enough) and I took out the new servo and measured the distance from the centre of the servo saver to the link and it was about 13.5mm, whereas the one on my Grass Hopper was 17mm (3.5mm longer), and I determined that this GH servo saver would lower the steering links and make the geometry much better, so I had 2 choices, either lower the servo, or use a GH style servo saver. A longer servo saver reduces available torque, so I decided to lower the servo. In order to do this I had to start cutting the Jet Hopper chassis at the point where I had "mounted" the servo.

This can be seen in the photo's below. The chassis at this point has some strengthening ribs, that's what my servo was previously sitting on, so I decided to cut them away by 5mm or so and lower them, whilst when finished making sure that the ribs are all about the same height in this section so that the servo would sit flat. I did this by rough cutting them with Tamiya's Modeller's Side Cutter (the one with the straight blades). I could have used a razor saw but I couldn't see a way to get the blade in. I did the cutting of the ribs quite carefully, it took quite a while, you have to be VERY careful to not bend the various plastic support posts around the sides and rear end of the chassis when you close the handles on the side cutters! There is very little room to do this in, but it is possible as you can see in the pic's. Then I used the end of a quite small flat file (the one out of Tamiya's basic 3 files set) to scrape the tops of the ribs down to be roughly flat (easier than you would imagine), the problem here being that you can't get the file flat to do this work as the sides and end of the chassis prevent it. It's possible to do though as you can see. Now I had a flat surface to put the servo in. I also made the servo a better fit (took a little more off the servo 'ears' / mounting lugs).

Finally I refitted the servo by pushing it into place and it fitted very well and finally the geometry was fine. There is full steering lock available (to the limits of travel in turning, on the JH front wheel axle holders) and I will shortly test the torque available on short grass and on carpet, with this replacement, standard 1/10 scale servo. BTW the servo saver I used is off what I seem to remember being my Kyosho Raider? It uses a quite strong coil spring. Suspension has full travel available. I still need something to hold the servo in place i.e. to stop it popping out of place vertically under extreme conditions. A rectangular piece of wood put between the top of the servo and the top half of the chassis (battery holder) would suit, I'll look into this.

Cheers,

Alistair G.

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More pic's showing the result of a new servo in the JH, mounted lower for much improved geometry, as described above ;-

Cheers,

ARG.

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