My (partially Tamiya) Rally chassis

[Honza]

So, I already posted a few pictures of my rally chassis before, but since I needed to rebuild the chassis because of new chassis plate, I thought that I could make a proper build thread with some more details

I've been experimenting with 3D printed parts for a few years, with my last car being MF-01X with a custom middle section, containing motor, and some parts around the axles. I also tried feasibility of 3D printed gears there. The chassis worked pretty well, but there were still some issues, and I wanted to experiment with a center diff, which couldn't fit in the chassis. So, I decided to make a fully printed chassis.

well, fully.. almost The bottom plate is 3mm thick aluminium plate. A bit overkill, but I wanted a rigid "backbone" that will take forces from the bumper and axles. 

Here with suspension and steering. The suspension is short reversible type, it so far worked the best for me. However, I used MF-01X knuckles to get a bit better steering geometry withou collisions with the arms. In the future, I plan to get TRF420 knuckles and make a custom arm. Steering has 50 degree throw - well beyond capacity of Tamiya CVDs (future proofing 😉) .

The servo is mounted on a floating mount. I wanted to mount all components to the aluminium plate, however it backfired a little here, the servo mount is a bit too flexible, so I need to come up with a better solution.

The battery is mounted in a little frame, which also makes connection point to the top deck. The connection was necessary for the temporary printed deck, aluminium plate doesn't flex as much. The honeycomb pattern on the bottom should help with debris, that could get between the battery and plate, so battery won't jam in its slot.

 

[Honza]

Now motor mount. I really liked design of last shaft driven tamiyas, with easily removable center gearbox - so I made my own version. It makes servicing much easier. The green part on the spur is a center differential. More on that later 😉. Motor plate is printed from PETG, as PLA, used on the rest of the chassis, has too low melting point and tends to creep.

The mount has embedded M3 nuts to make sure that it will survive multiple disassembles.

And here is the chassis with all central components mounted. The picture demonstrates that there are no need for additional braces to hold it together...

..so these side guards are there purely to protect the components from a direct hit. It's not really visible from the photo, but they are angled upwards at around 10 degree, gaining additional 5mm of ride height.

[Honza]

While the right and front section of the left guard are screwed directly to the chassis, rear left part also serves as a battery clamp held by clips - as there will be driveshaft above the battery, I needed it to be removable on the side, but I didn't want any holes in the chassis side.

 

Rear gearbox uses standard TL01 gears and gear differential. I chose this drivetrain to raise the driveshaft above the chassis. You can also see one of the few non-tamiya parts (apart from 3D printed ones) - the outdrive comes from Maverick/Himoto chassis.

As I didn't need to do anything with it, it went in assembled, including YeahRacing shocks.

The top link mounts might be critical part of the gearbox, as they contain holes horizontal with printing plane. On one half of the gearbox, they delaminated during assembly, but after reprint, they were able to withstand some crashes, so I'm not sure whether I should change the design or not..

 

[Honza]

The front gearbox is a bit more interesting. I needed to open it to clean it, so I made a few pictures in the process.

You can see 3D printed gears there. While I wanted to use TL01 gears, like in the back, it'd make the front driveshaft run under quite extreme angle and in possible collision with the motor tabs, so I decided to print 7mm shorter variant of the gears from PETG, which allowed me to move the outdrive closer towards the centre of the chassis. The differential is taken from my MF-01X - FF03 ball diff outdrives combined with 3Racing diff gear. Like in the MF-01X, the front gearbox can fit 1510 bearings to make this work.

Here you can see why I printed the gears from PETG - during initial assembly, I put temporary 3D printed shafts in the gearbox - and then forgot about that until a test drive. Of course the shaft broke and gear got in contact with the gearcase. PLA melted away, but there is no sign of wear on the gear.

Just like in the center gearbox, gearcase are held to the chassis through embedded nuts - in this case it's because of print orientation and high loads from the dampers.

The two halves are held together with two 3x15mm screws - they are there mostly to bear axialnload from the bevel gear. Additional support is provided by damper mount, chassis, top plate and ARB mount.

[Honza]

And here it is mounted in the chassis. Councan notice thet the ARB mount is higher than in the rear gearbox. That is to make more clearance for the suspension mount, which can be moved upwards to create a kick-up angle. 

Here is the very short front driveshaft, 20mm long. You can imagine angle that would've been there, if the gearbox outdrive was 7mm off-centre.

The top deck looks massive, but it's actually hollow, with added support in the center section - as a result, it's quite flexible, but only in the straight sections.

And completed chassis. At least for today 🙂 Please let me know what do you think, any idea will be appreciated I plan to release the files in the future, once everything's sorted out.

[Grumpy pants]

Very clever 👍

[Honza]

Getting ready for the weekend race  The chassis is now ready to run. I also added chassis cover mounts on the top chassis plate, but I underestimated motor cable thickness, so cover doesn't fit as tightly as I would like - but I don't have time to reprint it.

And installed body. 

And this is what development looks like. Several iterations of prototypes and damaged/misprinted parts

 

 

[Grumpy pants]

It will be interesting to read how this performs and what it weighs as well 👍

[Honza]

@Grumpy pants Thank you Performance will be very limited by me, it's my first race after 2 years 😀 So even finishing the race will be success 😀

[TwistedxSlayer]

That is very cool

[Honza]

Report from yesterdays race  Well, results were.. underwhelming - as expected  But most was due to drivers errors or electronics issues, mechanics mostly worked well. And nothing broke  I even got some nice comments and ideas from fellow racers, so the first race was success. I identified some issues, around steering (I definitely need more steering lock) and the center diff, so these areas will be modified, soon. 

I have a few photos, but my phone's camera wasn't up to task, so quality is pretty bad...

 

 

 

[Honza]

 

PS: @Grumpy pants I also put the car on scales, the car weights 1630g with body 

 

[Grumpy pants]

Great pictures and update 👍

Looks like you had fun as well 💥

[Honza]

A little update  The most problematic part during the race was a steering, dirt could get stuck between arms and chassis. I did a quick hotfix, which helped me to finish the race, but I needed to do it properly  So I made a new bridge and steering spacer

 

The purpose of the spacer is to make a flat plane under arms. hopefully, that should help to sweep debris out of the mechanism instead of getting stuck.

 

Now I'm waiting for some parts to improve steering angle...

[EsotericRC]

Love the heavy use of 3D printing, even going so far as to 3D print the gears. Impressive. I have Prusa MK2S and recently got back into RC after a 30 year hiatus to introduce my 10 year old son to the hobby. Was looking to use 3D printed parts (custom arms/wishbones) in the resto-mod build of my vintage 1986 Supershot, so seeing this was definitely rather inspiring. Following... 

[BuggyGuy]

Great work @Honza looks like a very professional build

[Honza]

@BuggyGuy Thank you, I wanted to make it look like something that could be made by Tamiya  as a side effect, it also saved some material, since most parts don't need any infill.

 

[TwistedxSlayer]

Still very cool.

[Honza]

The project is not dead. I just took a break and focused on another project. Anyway, got new Square wide angle driveshafts, TRF420 knuckles and redesigned the steering again to reach full 50° steerinng angle. Now I'm limited by dampers, but turn radius is already tight enough to keep me on the track even after bad entry.

I didn't take photos unfortunately - and now I have some cleaning to do 😀

 

[Novamadcars]

Love that colour green 

[Superluminal]

Looks like it kept most of the gunge outside of the chassis workings

[Honza]

14 hours ago, Novamadcars said:

Love that colour green

Me too! It's not very common in RC industry and it also highlights the 3D printed parts.

13 hours ago, Superluminal said:

Looks like it kept most of the gunge outside of the chassis workings

That's advantage of designing the entire car in CAD - the cover fits perfectly. It worked well despite the large openings in front and rear - nothing got into electronics.

[Honza]

Wow, it's been a while since I posted in this topic. Well, maybe because there are not many updates, the car just works. The only update were XV-02 shocks - I got them cheaply, so why not?

But over the course of last season I noticed, that the center diff started to act weirdly - it needed significant effort to get it unstuck and rotate. Last week, I found out why. Well, turns out that it's not the best idea to use 3D printed parts as friction pads 😁 

I reprinted the parts and added a teflon shim as a quick fix - but I'm aware that it needs a complete redesign - something I had in mind for a while.

The new design requires helical gears, so I tried to do a little test of design and printing - it turned out great! They are just a little hairy because of wet filament.

For reference, those gears are 9mm in diameter, module 0.8.

 

[Honza]

I realized that I didn't properly document the center diff. So I'll do it with the new version  It's based on planetary gearbox. But instead of driving the pinion in stationary ring gear, the cage is driven and ring and pinion are outputs - that results in classic differential action, but thanks to difference in size, with different gear ratios to each output - in my case with roughly 2:1 split.

This torque split is able to reduce wheelspin under acceleration, but still, some extra friction is needed - the original plan was to use one slipper pad to lock the front output to the spur. Unfortunately that was too much friction, so I threw the pad out and relied only on friction between the pinion and cage - which resulted in damage above. Also, the locking force was different due to gear ratio between front and rear.

I wanted to make a new version, which would lock the outputs together for smoother operation, but never got to finish it... Until now. I found some information about Torsen typ C and noticed similarity with my design. The big difference is, that satellites don't turn on axles, but they are instead enclosed within cage, using friction against the cage to lock under load. Additional friction is created due to axial load from helical teeth.

Today, I successfully finished the early proof of concept. Still a long road to the working part, but at least the gears run more smoothly than the previous attempt (I learned a lot about gear printing since then) and the locking action is already noticeable. 

 

[Honza]

And this concludes this year's rally season. I've been putting a lot of effort into my other rally car, which took its toll. DNF due to drivetrain failure in the previous race has shown that the car is due to big overhaul. While it worked quite well in the last race, there are numerous issues that need to be addressed. Starting with the center diff, which didn't see any development since march. Luckily, MF-01M is nearing completion, so hopefully I'll be able to move forward with new developments for this chassis