Need help with differential for Monster Beetle

[btsai]

Well, I decided to dust off the old Monster Beetle (bought it new more than 20 years ago...). Had to buy new differential gears but was happy to find them at Tower Hobbies -- I guess Tamiya still produces them.

Took me a while to figure out how to put the diff back together (had to look up the manual here on the site -- thanks!). I thought I had it figured out but I was wrong.

Here's the deal:

I had to grind/sand down the new diff gears just a bit, otherwise when I tightened down the two metal sides, they wouldn't move at all. A little Dremel and sandpaper action and voila, things spin freely.

So I put it all back together and I try it out in the back yard (sure hope those dog bones are hanging in there...). Motor and battery are good -- but the rear end doesn't "engage" fully. I get it going a bit -- it goes a little -- but mostly I hear some sort of spinning/plastic gear sound. I stop messing with it for fear that I have re-stripped the diff gear I just replaced. I disassemble it enough to check it out -- phew -- it's okay.

So....... what am I doing wrong? The motor spins the internal diff gear it is connected to, but the half-shafts don't turn. I just hear the spinning of gears in the diff.

Oh -- one more thing -- when I spin the half-shafts by hand, they feel smooth. But with the wheels on and when I turn one wheel, the other wheel doesn't spin the opposite direction...

Help!

[mymonsterbeetleisbroken]

hi btsai and welcome to the site!!!

hopefully we can give you some help to get your beetle running well []

personally, it sounds like something went wrong during the rebuilding of the diff assembley, as you shouldnt have to dremel anything to make it all fit - unless the parts you ordered were made wrong from the factory[?] thats a bit of a long shot but it could be possible.

heres a little check list:

make sure you have the diffs 2 plastic bearing carriers located correctly in the gearbox sides.

make sure the gearbox sides havent flexed out of shape, they should sit flat on a flat surface, if the bend inwards or outwards they may be causing the diff to bind or slip.

make sure the little metal rod that goes through the middle of the diff is in place, if its missing the diff will not work properly.

check to see if the splines on the drive cups that insert into the diff are not worn, it sounds like your problem could be caused by the drive cups popping out of the diff, causing you to loose drive to the rear. if this happens the truck will just make a motor and gear sound. this can also be caused by a missing central shaft in the diff.

did you replace all of the diff gears? or just the bevels or planet gears?

hopefully something i've mentioned may be of some help []

a fully working diff will be a little tight, but when turning a rear wheel, the opposite wheel should spin in the opposite way. if it doesnt spin at all the diff is still ok, just a little "run in". if the rear wheel spins in the same way, the diff is locking - that means its worn or broken!!!

[btsai]

MMBIB -- thanks for your reply!

So tell me about the "bad" gears. Were they too wide???

Anyway, here's where I am so far:

The diffs 2 plastic bearing carriers are located correctly in the gearbox sides (with the little key in the slot).

The gearbox sides seem flat and not out of shape. But I suspect this may be the area I have to mess with... (see below).

The little metal rod that goes through the middle of the diff has been in place.

The splines on the drive cups that insert into the diff are not worn. (I tried them separately, by hand to double check.) I don't believe the drive cups are popping out of the diff. But I did hear the motor and gear sound as you described...

I actully replaced only the one nylon gear that has the teeth around the outside and then the smaller teeth that mesh with the other gear that holds the three litttle bevel gears.

The (original, old) gear that holds the three smaller bevel gears is the one I made a little narrower. Reason being is that when I put the new one in and then tighten down the entire thing (with the two metal sides), it entirely freezes up the gears -- they won't move at all. So, it's sounding more and more like the metal (especially the right side) is not as flat as I think?

With the ground down gear, it was too loose and was acting way too run in. With the new "full width" (stock size/unmodified width) gear, it is too tight and won't move at all. Also, with that new gear, before I tighten it all the way, the diff DOES work correctly when I turn the half shafts by hand.

Of course, Murphy's law hit this morning on something else. You know the area on the "frame" (red plastic) where you screw in the three screws that hold the cover of the leading part of the rear trailing arm? Both sides broke a piece off so now there will only be two screws holding those pieces on. Grrr. I guess maybe it's just too brittle from being 20 years old. I wonder if the frame from the new Frog will fit -- and be available separately...

Have any further thoughts before I put the whole thing on E-bay and buy a Stampede or RC10BT instead???

[mymonsterbeetleisbroken]

it's sounding like a very odd problem!

try bending the side plates a little and see if that helps.

double check everything in the diff assembly to make sure its exactly as the manual shows it. apart from that i cant think of anything else that may be causing the problem!

the area of the frame where the trailing arm locating parts screw in does become weak over time. the ones on my runner monster beetle are a bit "dodgey", but they are still in place.

chassis frames do turn up on ebay, as do most parts []

the re re frog chassis will fit, its just a different colour.

when i found this site a few years back my monster beetle was old and busted (hence the name[]) but through this site and ebay i managed to get it working perfectly again []

breaking old bits and replacing old worn out, brittle parts is all part of running these old tamiyas. []

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just had a thought - are the bearings each side of the diff the correct size? if they were somehow wider bearings they would make the diff to tight, just a thought []

[btsai]

I guess I'll try bending the plates just a bit so that they don't squeeze the diff gears so much, but still keep it tight enough to maintain diff "action".

I never changed the bearings (they're the same ones that have been in there), so I'd be surprised if there is a problem in that area.

Thanks again -- I'll try to get back to it tomorrow or Tuesday. I'll let you know if I have success.

One last thought -- use the old gear that I made thinner so that it would spin freely, and put a spacer (thin washer) on each side of the diff to keep that part of it together tighter. Think that'll work?

[DJTheo]

quote:

The (original, old) gear that holds the three smaller bevel gears is the one I made a little narrower. Reason being is that when I put the new one in and then tighten down the entire thing (with the two metal sides), it entirely freezes up the gears -- they won't move at all. So, it's sounding more and more like the metal (especially the right side) is not as flat as I think?

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id="quote">id="quote">

Could it be that your 3 little diff gears or bevel gears are worn/warped and only work in the old worn out gear? Maybe you need to replace those too?

Cheers

[btsai]

DJTheo -- I checked the gears -- they looked okay.

Basically re-did everything I had done before but tweaked the metal plate to adjust as MMBIB suggested. And -- it WORKS!!! Woohoo! Thanks for your help! The diff is definitely sensitive to the amount of space/play that it has. I think if I had to use the thinner gear I could make it work with some shimming...

I'm still a bit tentative about driving it -- I'm not quite convinced it is as tough as it once was. (And I'm still worried about the chassis mounting points that I mentioned before...)

At any rate, I'm a happy camper. Fun to make it work again. I had all three of my kids try it out - they haven't seen it work for a couple of years or so. (I take them to the local tracks once in a while to see the races -- both on-road and off -- they like off-road better (no surprise), and I've always loved on-road. (I used to race a long time ago...).

[HunterZero]

With the Frog re-release coming, I wouldn't worry about drivetrain parts availability for the Monster Beetle, since you'll soon be able to get pretty much all gearbox parts and the driveshafts for it again. I recently restored my Frog to running condition, and it still has all the original 20 year old driveshafts and gears in the gearbox. I had to add some 5mm shims (used parts from the Tamiya #53587 5mm shim set) to stop the diff 'crunching' on the right side, but now it works great. Very fast, very solid.

You can still get all the gearbox internals including the spider gears and bevel gears new quite easily.

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXGP49&P=7

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXGP48&P=M

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXGP38&P=7

The large bevel gears appear to be discontinued from Tower, but they're easy enough to find on eBay.

- James

[mymonsterbeetleisbroken]

glad to hear you fixed it!!!

[jims hobby shop]

also when you tighten the gearbox down and it binds , check that you are not using the fast attack diff gear , it will be inscribed on the face of the gear.blackfoot will say blackfoot,

some people sell them as the blackfoot beetle etc. gear but it thicker and will only work by grinding it down like you stated.

jim.

[btsai]

Hunterzero -- good info! Glad to hear that my shim idea may work -- if needed, too...

MMBIB -- []

jsourbeck -- also good info. If I need to replace those gears, I'll keep that in mind.

It was fun to drive again. I have to admit my driving skills are rusty. Of course, running a Monster Beetle around the back yard will never qualify as "precision" driving... LOL!

[NYDriver]

Btsai, I have a NIP vintage Red Monster Beetle/Blackfoot Frame that I was going to use but have decided not too ast I have about a dozen projects on hold, if interested please contact me at oldjoesjoes@aol.com

[btsai]

One last thing -- thought you guys would enjoy. Since the monster beetle is running now I've been trying to get the cob webs out of my driving skills.

I had it on some smooth concrete this afternoon and started pulling some drifting type moves (on accident at first -- then did some more for fun).

Cool!

[mymonsterbeetleisbroken]

excellent stuff [8D]

i took my monster beetle around my uncles house the other day

he has randomly dug up all the grass and plants in his garden over the last year or so, and still hasnt got round to sorting it out.

so the ground there if full of mud, bumps and hills. plus it's been raining so the hole thing was like a swamp!!! [^]

the beetle made easy work of the puddles, bumps and slopes, even with only 2 wheel drive it didnt get stuck once []

i've asked my dad if i can dig up the grass at home to make an off road corse, guess what he said.....

no [] []

[Spatzel]

Hi,

What!!?? These bevel gears was available on tower?

you dont want to know how much i paid for them on ebay (2 sets for supershot).Never read anything about these hidden stock on tower here.

Was it a conspiracy or what?Are there any more such treasures on tower?

[btsai]

jsourbeck -- I just re-read your note. My new gear does indeed say "blackfoot" on it. No wonder I had to grind it down. Guess I got lucky figuring that part out on my own, eh!

[HunterZero]

quote:

Originally posted by btsai

jsourbeck -- I just re-read your note. My new gear does indeed say "blackfoot" on it. No wonder I had to grind it down. Guess I got lucky figuring that part out on my own, eh!

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Blackfoot uses the exact same differential spur gear and counter gear as the Monster Beetle. Not sure why you'd have to grind down that gear to fit if it's marked 'Blackfoot'.

- James

[btsai]

NYDriver -- sent you an e-mail...

Hunter -- I think I must have gotten one of the "fast attack" gears. I measured the old and the new with a caliper and they were NOT the same width...