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Mad Ax

Hotshot Custom Chassis Build

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Firstly - it's possible I've already started a thread about this sometime in the past, since I already have some photos online from 2020, but I've searched and can't find it, so, well, here goes again!

I'll start this thread with a little bit of backstory.

Back in the early 90s, a friend of the family called up and said he had a Beatties bag full of old Tamiya parts, and that I could have a complete Tamiya Grasshopper if I was able to make a runner out of the Hotshot that was supposedly in there somewhere.  I didn't really know much about Tamiya's back-catalogue in those days (literally all I knew about Tamiya RC cars came from the inside of the box for the Mini-4WD series, that showed what other Mini-4WDs were available, and I knew some of them had 1:10 RC counterparts).

Well, there was indeed most of a Hotshot in the Beatties bag, although it had been well and truly used and abused, the shocks were bowed like bananas, the drive triangles were cracked and split, and it was clear from the off that it wasn't going to go back together without a lot of work.  Most interesting, though, was the chassis: it was made from natural GRP, with two plates that made the sides, and one for the floor, so the top was open.  I had no idea what it was at the time, but it was probably a TMS Racing or Jim Davis chassis, or a home-made copy (I think the plans were posted in a magazine, so people could make their own).

I was able to make a working Grasshopper, which I kept, but after some time the sorry Hotshot went back to its original owner in pieces, and that, I thought, would be the last I ever saw of a Tamiya Hotshot.

I surprised myself by getting back into Tamiya in the mid-00s, and was surprised by Tamiya when they re-released the Hotshot in 2007.  Somehow I managed to save enough money to buy one, which I built and painted in my own colours.  These shots are from October 2007, when I get it its very first gentle run, before deciding it was better off on the shelf than on the grass.

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There the Hotshot stayed - my fears of banannering shocks and splitting drive triangles prevented me from running it - and I've even got some records that it was listed for sale in 2009 and again in 2010, although thankfully it never left my possession.

I attended a few Iconic bash events around 2014, 2015 time, and that's likely when I decided the Hotshot should get some track time.  I slipped in a Trackstar Turbo brushless system and took it along to Broxtowe, where I was convinced I'd completely smashed the front of the chassis after hitting a concrete post flat-out.  Amazingly, it survived, and went on to be a wildcard entry in the early class as the Revival that year, when I drove for 3 hours from Pembrey in South Wales (where I'd been attending a motorcycle trackday) to A1 Racing Club in Grantham, for my first ever Revival, without even having a place to race.  Revival wasn't nearly so popular as it is now, and the organisers were kind enough to slot me into a spare place in the C heat.  After a long day at my first ever astroturf race meeting, and my first ever vintage race meeting, I secured 3rd place in the C-final.

Since I was in the slowest heat of the slowest class, I was technically the slowest possible driver to actually get a trophy on that day.  Which I felt was an achievement, considering I went all that way without a race booking, half-stripped the spur gear in the practice session because I had the pinion installed too close to the can, and had to run the rest of the day with only half a spur gear.  It also meant that I was in the very first final to run that day, but had to hang around for 2 hours for all the other finals to run, before the trophy ceremony and a 3 hour drive back home to West Wilts.

The following year I would campaign the Hotshot again, and that's where I would modify the front bumper into an angled skid plate using the vent from my gas powered soldering iron, after it got broken in a poorly-landed jump.

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In 2017 I would acquire a vintage Top Force Evo, and that would become my Revival race car for the rest of my Iconic career so far.  At the 2019 Revival I bought a re-re Supershot from the track shop, with the intention of running it the following year, with the old Hotshot as a backup car, but when booking opened early in 2020 (before the pandemic broke), numbers were so high that I didn't get my first choice in the Hotshot class, and had to settle for running the Top Force, which has been the case every year since.

Earlier this year I got my wish, and my entry into the Hotshot class was accepted.  That meant I had to build my Supershot re-release from new (another thread to follow about that later), and also get the old Hotshot back into racing condition again.

Sometime in 2019 or 2020, after browsing showroom entries on here, I had the idea to make a custom chassis for the Hotshot, in the style of the TMS Racing chassis from the 80s.  I figured it wouldn't be that hard, as I'd already been playing around with custom chassis designs for other things at the time.  I'd also bought a 4-shock conversion kit from Radshape RC.  These photos were taken in December 2020, when I had my first attempt to build a custom chassis.

With the 4-shock kit installed, and on mismatched Tamiya CVAs - too long in the back, too short in the front.  Wing was added at the Revival after the stock one got damaged.

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It looks like I tried to weigh the chassis.  I'm not sure why - I don't think weight is the best reason for making a custom chassis.  Still, I can see how much I gain or lose once the job's done.

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Stripping the Hotshot down to its core components

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These are the main bits required for the chassis

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And somehow, I need to make the whole cabundle fit together like this

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And that was as far as I got with it.  I don't know if I was lacking in parts, enthusiasm or skill, but I never got as far as creating a chassis plate.  I remember one key mistake being that I took the chassis apart before I'd made a jig, so my measurements would likely be off and I'd end up with a non-standard wheelbase and possibly the kickup and anti-squat angles all wrong too, not to mention one side being longer than the other and it going round in circles.

Fast-forward to April 2023, and it's time to get started on the chassis build again, as it's race time at the end of July and this needs to be finished, testing, working and at least partly set up by then.

Here's what I started with:

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My Supershot chassis is fully assembled.  The wheelbase and track width is the same, so it was perfect to use for making a jig.  I began with some chunky alu L section.

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With careful clamperage, some locating holes are drilled.

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Gonna kinda look like this

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Adding the cross brace at the rear to keep everything 100% square

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And another at the front.  Somewhere along the road it did lose a tiny bit of squareness, such is the accuracy of my drilling ability, but it should be close enough to at least build one side of the chassis.  Then I can use that as a drilling template from the other, so it should be fine in the end.  A miniscule change in wheelbase isn't going to be noticed in scrutineering.

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Added some extra pieces to help hold the transmission cases exactly where they need to be.

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It was at this point that I went for my field of 2mm GRP sheet and found it barren.  Alas, all that work, and I can't start on the actual chassis yet!  What's worse, black GRP sheet is only available from China on a 2-3 week delivery.  I could have gone au naturel and sourced from the UK, indeed natural colour is more period-correct, but I think black will look much better with my existing colour scheme, so I've placed the order and am now doing some virtual thumb-twiddling while I wait for it to arrive.

I figured I could at least start making a template with plasticard, but all I had was 0.5mm, which is a bit flimsy.  Plus I wasn't really thinking straight, and I made the base first, when actually it would make loads more sense to a) make the sides first and 2) use the existing chassis box to get a starting point on where the holes need to be.  Which may even negate the need for a jig in the first place.  :faceplam:

Still, this is what I started with

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So, I guess you could say my custom chassis build has begun.

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The good news is that I have quite a lot of 1mm alu sheet lying around, which isn't really much use to me as it's too flimsy for the bulk of what I fabricate, but will be rigid enough to at least make an initial template.  I've got a long day in the workshop planned for Saturday, so fingers crossed, this thing starts to take some kind of shape...

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Looks like another interesting project! Have seen other custom hotshot chassis, and it's something I'd like to try myself one day. Can't understand why you didn't use the chassis box for a template though? And would there be any benefit to lengthening the chassis?

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51 minutes ago, foz75 said:And would there be any benefit to lengthening the chassis?

Longer chassis adds high speed stability, but with standard steering Hotshots you lose turn in to some degree, it’s give and take

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Mad Ax, these are my suggestions:

- tilts the rear gearbox by 1 degree to obtain the rear anti squat angle.
- 3D print two rear hubs to have 1 degree of toe in.
- Use the Egress steering linkages and make a decent steering system, better if you custom build the steerer yoke to have the correct ackermann geometry and the spacing for the ball connectors.

Max

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8 hours ago, foz75 said:

Can't understand why you didn't use the chassis box for a template though? And would there be any benefit to lengthening the chassis?

The main reason is the awkward shape, it's difficult to use the box to accurately mark the drill holes on the new chassis plate.  That said, I'm probably going to give it a go on some alu sheet soon and see how it comes out.

Lengthening the wheelbase isn't allowed under Iconic rules, so I'll be keeping it stock.

7 hours ago, kontemax said:

- tilts the rear gearbox by 1 degree to obtain the rear anti squat angle.
- 3D print two rear hubs to have 1 degree of toe in.
- Use the Egress steering linkages and make a decent steering system, better if you custom build the steerer yoke to have the correct ackermann geometry and the spacing for the ball connectors.

Great ideas - thanks!  Tilting the rear gearbox would be an easy mod to incorporate into the chassis design, I think.  I'm not sure if racers were really thinking about anti-squat in 1986, but it's not like it was an impossible mod back in the day.  Iconic rules say mods have to be "period correct" - making your own chassis from GRP was definitely a thing back then, and making the holes in slightly different places to alter geometry would be something anyone could do (the question is, was it common knowledge that this could be done back then?)

I don't know if toe-in rear hubs were available for the Hotshot in the late 80s, if not then they wouldn't be allowed under the rules.  We're allowed to 3D print parts where the original parts are fragile or hard to find, but we're not allowed to 3D print parts for a performance gain that wasn't available BITD.  I may technically have bent the rules slightly when I used 3D printed toe-in rear hubs on my Blitzer Beetle 2 years ago, but while toe-in rear hubs were never an option on the Blitzer, they were a common option on the more advanced racing trucks in the same class, so as far as I was concerned I was still well within the spirit of the competition.

Using parts from later cars is definitely prohibited (well, technically I could use Egress steering linkage, but I'd have to run the car in the Egress class).  Again, I don't know if anyone was making proper cranked steering linkages for the Hotshot in 1986.  I might have to look at all the eligible cars in the 83-86 class to see if any of them had a cranked steering linkage.  If they did, making my own would be legal, but possibly beyond what I'm capable of right now.

Thanks for the hints, though :) 

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What a great project! Love a MadAx custom job :D

I'll be following this in earnest, looking to pick up as many workshop tips as possible.

I've had a custom chassis project in mind for years now, I even have the donor kit in the loft. Maybe one day....

Oh, nice sponsored mug too! 

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1 hour ago, Badcrumble said:

Oh, nice sponsored mug too! 

My wife sells stuff on Inkthreadable, so she let me upload my own designs to get some mugs for myself.  I realised out of all the hundreds of mugs we have in the house, I only have either generic crockery set ones or freebies given away at conferences.  Given we were all working from home and our sense of individuality was being hidden away behind a Teams chat window, I started putting more customised and personal stuff around my desk to prompt conversations.  Or maybe I'm just more of a narcissist than I thought and I was craving attention :lol:

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Slightly OT question, but do any other cars, apart from the hotshot series, have front and rear ends that mount on the sides, like the hotshot? From what I can see, most if not all modern cars have the gearbox mount through the bottom of the main chassis. Just thinking that sideways mounted might be easier for a custom chassis build?

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9 hours ago, foz75 said:

Slightly OT question, but do any other cars, apart from the hotshot series, have front and rear ends that mount on the sides, like the hotshot? From what I can see, most if not all modern cars have the gearbox mount through the bottom of the main chassis. Just thinking that sideways mounted might be easier for a custom chassis build?

The Thundershot/Dragon series that immediately followed and replaced the Hotshot series uses the same side mounting idea. These were replaced by the DF01/Manta Ray/Top Force series which were the first Tamiya to use the more modern, conventional horizontal deck arrangement. 

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Finally got a bit more work done on this one last week.  This 2mm FR4 sheet arrived from China about a week earlier than planned, so I was able to get right into the build.

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The first thing to tackle was getting the front mount in the right place.  A question was asked earlier - why not use the box chassis as a template?  Well, this is why - unless I want to trim the chassis down until its sides are flat, there' no reliable way to trace the holes:

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The underside isn't exactly smooth either, so it's not easy to work out where the chassis base should go and ultimately where the flat floor should be.

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In the end, I went back to the tried and tested combination of guesswork and deduced reckoning.  I knew the maximum possible depth of the floor based on the measurements from the front gearbox mounting holes in the side of the chassis to the lowest part of the floor, and from there I could work out where the holes for the gearbox mounts should go.  I also wanted to use the front sway bar on this car (although the Supershot doesn't have it and it doesn't fit with the Radshape front shock towers without some modification) so I used the stock sway bar locating plate as a template to judge how long the chassis needed to be (this build will incorporate the front sway bar plate into the chassis side plate).

I also made the assumption that the line across the holes should be parallel to the ground, since they pretty much look like they are and I've no way of measuring unless I guess at what the parallel line of the chassis is, since it doesn't sit flat on the ground (see above) and its final resting point when on the springs depends on F/R weight balance and spring settings.

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One hole drilled, using a screw to secure the plate so the next hole goes in exactly the right place.  I allowed myself a little extra on the chassis plate, since I wasn't sure if I might have to move the front sway bar forwards to clear the shocks.

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Next trick was to locate the rear gearbox.  The gearbox actually has some nice wide mounting points, but these don't touch the stock chassis.  Instead the chassis bolts on through the narrower mounting holes at the top.  Those nice wide points are for the rollcage, but the rollcage is too flexible to get an accurate location measurement from.

I didn't want to cut up my stock chassis, since there's a chance I'll need it on the day (if my new one breaks or the scrutineers find something illegal about it).  So, to get a measurement for the rear gearbox mount, I used two identically-sized brass tubes to space the stock chassis away from the sheet.  In theory, this will properly locate the plate against the chassis in exactly its correct final position.

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Then I passed a drill bit through the rear mount to get a final position.  Naturally there will be some flex in the chassis and some play in the front bolts and possibly some twist in the drill bit, but this it as close as I can get it without either more advanced or precise measuring equipment, more engineering knowledge, or a better brain.  As long as the wheelbase is reasonably close to the original car and the front kickup angle isn't wildly out of spec, it should be fine.

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However, I wasn't entirely happy with that technique.  I couldn't quite get enough force on the drill bit to mark the locating hole without moving everything, so I came up with this idea instead.  I reattached the rear gearbox to the chassis, then screwed in a piece of threaded rod (actually an old steering link).

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From there, I could get a more accurate location for one of the holes, and draw around it with a sharpie.

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Then, I was able to locate the chassis using that new hole and a longer screw, so the front and rear ends are pretty much locked where they need to be for stock geometry, and repeat the process for the rearward locating hole.

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On @kontemax's advice, I measured around 1 degree from the first hole to the second one in order to tilt the rear gearbox for anti-squat.

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Eventually I ended up with this.

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which I roughly trimmed down into this

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Then, with dust mask and extractor, I started the laborious task of filing it all down to somewhere roughly approaching a final shape

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before marking and cutting the other side

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With a bit more hackery, I had a very rough rendition of a chassis

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to be continued - I have more photos to share but my allotted RC update time has elapsed

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OK - over a week later and I'm finally getting around to finishing the last bunch of updates :o 

So, with the sides of the chassis taking shape, I had a solid template from which to make the floor.

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There isn't much to mount the floor to the chassis on a hotshot.  Under the standard car, the battery tray makes part of the floor, and uses a plastic hinge at the back.  This hinge will take up the tiny bit of rotation in the chassis where I added some anti-squat - although the angle is tiny enough to not really need this.

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The sides of the chassis cause the floor to be aligned.  As I discovered before, with only one screw securing the front bumper plate, there's nothing to centralise the floor or bumper in order to get a square car.  This has turned out to be a better way of doing things.

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As you can see in this photo, I had to space the chassis sides out a little to go around the floor plate.  The width of the floor is fixed by the width of the front bumper mounts, but this is wider than the chassis mounting lug width.  This was only temporary until I could measure everything to be cut.

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One side of the chassis is trimmed

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Needed to cut a little section out here as the bumper mounting plate nut is too close to the vertical plate

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Before I got too heavily into the other chassis plate, I decided to locate the battery.  I'm using a shorty as a template because I'd like to run shorties, but I'll have the option of running a stick pack in here.  Revival has rules about batteries, but I think they're more guidelines, since virtually everybody runs LiPos and there's rules on how much additional weight you can add.  It doesn't add up.

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Chassis plate trimmed for battery

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LiPo stick pack also fits

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Exposure is poor, but I cut some of my favourite L-section to make the chassis joining part

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And here it is!

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I figured out I could get the original sway bars in if I was careful with my shock mounting.  I also figured I'd use the original sway bar plates for a bit more rigidity.  Feet not to scale.

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Here's an over-exposed mockup with the cage on.  I decided, I while I was filing down the chassis plates, that I'd cut them way too tall and had loads of meat left to remove.  It turns out I didn't have as much meat as I thought, and the roll cage mounts are now above the chassis sides.  Dangit.  Still, that's a problem for another day.

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Roll cage mount here has been stretched over the side of the chassis plate.  It's soft plastic, so it doesn't really affect it too much.

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Body loosely plonked on.  It needs a lot of trimming - the shock towers and sway bar mounts are much thicker than before, so they have deformed the body in this pic.  Probably that entire section with the Speed Disc logo on can go.sm_P4220039.jpg

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And that's as far as I got since the last workshop day.  I have done some messing around with the front shock tower position, because the Radshape RC towers don't offer as much height as the Supershot towers.  There's a simple fix for that.

When I last started this project, I'd bought some rather tasty-looking Turnigy shocks that perfectly match the black, red and silver colour scheme, but the sizes aren't spot on and like a lot of shocks these days, they come with no pivot balls and I have no way of telling what size they should be (apart from, not the same size as any of the balls I've already got).  So a couple of days ago I ordered some Absima shocks, also in red and black.  They look fab but probably aren't brilliant performers.  Still, with careful rebuilding and the right oil, they should do the trick.  The hardest part when buying aftermarket shocks (after finding the right balls) is finding spring options that will fit.  I hope they're similar enough to Tamiya springs - it's annoying to be stuck with just one spring rate for the whole event, as IMO that's one of the most important thing to get nailed on setup day.

I was hoping to get more done on this one today, but I wasn't happy with my last round of mods to the SRB Superlight, so I took that apart instead and completely remade the back end (3 times, in fact).  I'm still not 100% happy with it and I'm not entirely sure it will be Revival legal, but that's a problem for another thread.

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OK - more updates!  With the evenings getting lighter and just a little bit warmer recently, I got myself a bit of spare workshop time to look at the new shocks I purchased for this beast.  They are made by Absima, they are about as cheap a shock as I have ever seen, and they colour match the car perfectly, which is obviously the most important thing.

The problem is, they're a little too long at the back.

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Solution - pull apart the rear suspension to modify it.

The bonus of having long hair, besides looking like a Viking god (albeit a rather short one who was left somewhat lacking in the beard department), is that one always has a handy elasticated tying-up thing handy.  The drawback is my hair got in my eyes for the rest of the night.

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These are the shock brackets that came as part of the conversion kit from Radshape RC, many years ago.

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This is me using a flange nut to work out the lowest point I can drill a new mounting hole at and still secure it with a flange nut.

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This is the hole drilled

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By securing the opposite mount upside down, I was able to accurately drill the holes on both sides in exactly the same place, using the stock holes as a template

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Mounted

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I then turned my attention to the front end.  The Radshape towers are way shorter than the Supershot towers, and to be in with the fairest chance of getting these mods accepted for Revival, I want my geometry to be as close to the Supershot as possible.  By canting the front towers backwards, I was able to get closer to the Supershot geometry, and also make it look seriously cool and aggressive.

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Added a motor, ESC and battery to get the static sag.  Note these will not be their final positions

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With the suspension sort of done, in a roundabout sort of way, I turned my attention the steering.  I had kind of forgotten a fairly important thing about this, and overlooked the amount of space I'd need to leave under the front gearbox housing to get the servo in...

For steering duties, a basic Savox will do the trick

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It just - literally, just! - fits

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Last Sunday rolled around quickly, and once again I was back at my dusty desk to have another long day's work on the Hotshot.

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Next point of attention was the battery holder.  I had a few ideas, but in the end decided to stick with, well, a stick pack, on the basis that there's some rule about battery size and location not being changed from stock.

I also decided to stick with the standard mounting bars to make changing the battery easy.

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I cut these little spacers from aluminium tube from B&Q

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countersunk the battery plate so the bolt heads are flush and don't annoy the battery

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fixed side

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For the opening side, I used a locknut with a shorter spacer

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That'll do

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To mount the tray, I cut some more bits of alu L-section

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it will fit something like this...

sm_P5140102.jpg

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