Mohawk (nee Project X)

[BuggyDad]

A slightly mixed bag at Tamiya Junkies today. I had not made my carbon front shock tower because of lack of time, and thinking the 3d print template would probably handle the day, being 3mm thick PETG. Bad choice. It failed in a very light encounter with the dreaded indoor rope. So I fitted the old CF one - wrong (well, I had designed them for different arms, but what do I know?) geometry but my inboard second pair of holes were close enough to just nearly allow chassis bottom out, and actually the car was noticeably more responsive after the change, which I think has to be geometry rather than tower stiffness. I then broke a C hub in a not particularly hard crash not long after, but remembered I had the previous iteration of c hub still in my box, and since they're the same in the key fitting dimensions I could fit just one. And so I also cogitated over ideas to increase, substantially, C hub strength in a redesign, and reduce wear-induced slop. However, because of critical screws/pins at 90° and a major space constraint, C hubs really can't rely on a material with a significant weakness in one axis, so I do think they're a part to get printed commercially. 

From there though, things were looking up. The car did a lot of laps and I absolutely loved how it drove round the outdoor track. Good cornering, responsive, and it's very light which I think helps it in perhaps loads of ways. I think it has a low CoG despite its tall appearance. Jumps quite well too. A few screws came loose through the day, which says I need to use threadlock (they were all to metal, and they were most of the ones I have screwed to metal) but also I think is a product of it getting a really good run out. But otherwise all good until I snapped the steering bridge. However, it was pretty much home time by then, plus I had printed this bridge yesterday in a rush after identifying a new (fairly minor) binding problem caused by my arm change. I could even have swapped the old one back in. Bridge should really (and can easily) be 2mm carbon fibre sheet. 

Addendum: I may have been wrong about wing mount print orientation. My orientation is bad for the screw holes, which was my concern, but perfect for the main structure (with the bonus that this way they print without supports - the other way they need a vast amount - and have a much better finish), and they lasted many many tumbles. Printing them the other way would sort the holes but be a little less good in other ways.

[ThunderDragonCy]

@BuggyDad Mohawk was mega today. Looked brill. Chapeau

[BuggyDad]

Looking at the Schumacher C-hubs @ThunderDragonCy has used on one of his 2WDs, I realised that trying to design these using the same part for left and right is a more serious strength compromise than I'd appreciated. My knuckles have the axle behind the kingpin, as is common, and this means that the knuckle swing is all behind the kingpin or out wide, leaving a lot of space forward to add extra strengthening material. We are used to seeing a deep "C", which is fundamentally a weakness, but this is only required at the back. I know this isn't where mine broke but it is still worth tackling I think.

Where mine broke was around the lower pivot. Because I'd used a pill insert, I only had the material outside that pill diameter to rely on for strength. A bad choice in hindsight.

So considering all the above I started again from scratch (anyway this is often the easiest way). Then I started again again. Then I slept on it and started again today once more. I got there in the end. The new C-hubs:

1. Are not "handed", so they can be asymmetrical front to back, with a lot of extra bracing forward of the kingpin
2. Are a lot wider - my new arms have all their strength in the GRP strips, so the internal recess for the c-hub could be widened a lot with no significant strength compromise and I just have to print a pair of new inner strips at home. Wider means much much stronger, plus a corresponding improvment in slop, which was a problem here.
3. Are further beefed up around that lower pivot and in a few other areas as well as losing the pill inserts.
4. And finally I took the opportunity (which I had previously designed my new arms to enable) to bring the kingpins in relative to the pivots, narrowing front track by 8mm.

I also slightly amended the uprights to lose the inboard Ackermann hole and longer arm end which was unnecessary and could just touch the new arms.

These parts won't be any good on my home printer because they need good strength in all three axes, but it's difficult to envisage a c-hub that wouldn't. Although I have a conceptual idea (to design the main structure lying flat so the pivots run perpendicular to the layers and the "C" is in-layer, and then to print separate bands to wrap around the upper and lower arms of the C to bring kingpin hole strength, because the kingpin holes are the layer weakness), it's hard to implement in a small package, not without its issues and might well not work anyway. I probably won't head down that track because the idea for me of having my own printer was never necessarily that it would completely eliminate my use of commercial printers, rather that it would do some of that work for nearly nothing and enable a lot of design work and prototyping. Already though it's produced more/better running parts than I thought it would.

My rear arms and uprights are another area I am not driven to redesign for home printing right now, because I like the ones I have (and uprights have a layer/orientation problem a little like the c-hubs), although I could fairly easily take the same approach as I did for the front arms. So I'll just take this opportunity to print spares.

[BuggyDad]

Introduce my 10yo to Mohawk today (it doesn't live at home so he hadn't seen it. He absolutely loved it. So I left him playing with it on some old road wheels (it looks great with black wheels - need some better ones in its future) but after some time he returned rather sheepishly with this:

It'd had a high speed smash straight into a steel fence. One of those sturdy old style bits of steel fence at that. Not wire. No car I have would survive that.

Anyhow, that's kind of the point - "let's see what breaks and how, and then change it" was a big part of my intended approach, so there's a bit of interest here in how it failed. And I think a bit of success too. 

I think what's happened is a frontal and slightly from below impact has triggered an upward rotation, with the bottom part in tension almost shattering and then the top part a bending break (there's evidence for the latter although not clearly pictured). In both cases I don't think the failure is linked to an obvious overly weak spot, which is very pleasing, rather I think it's a "something's got to give" hit. 

So I've learned a bit from that and I already had most of the necessaries printed, in my next version which is different but compatible. Different because design changed to use home printing, which meant making multiple parts to optimise orientation, and to use some reinforcing CF sheet (which I haven't got done - using dummy 3d printed placeholders for now) in places which interestingly didn't break. Still some work to do so it's not running again yet but it's 90% back and will be with one more printed part and a few more minutes spannering. 

In my redesign I had also added perhaps 20% more meat to the area of the lower break, so that's pleasing. Long term, that part might need to be commercially printed I guess but I don't mind a few of those, it's far from the only one. 

Bit of body damage too but I don't mind that either. It's a runner and badly cut, so a new one was always on the cards and this one is perfectly serviceable still. 

[Andreas W]

I like your approach and that's the sweet thing about 3D printing and when you have the equipment. Always having endless access to spares is a relief.

 

I know I would not have been so relaxed if one of the kids had turned my Tomahawk into a banana.

[BuggyDad]

Mo is back! 

Gotta love how a big breakage can be fixed overnight when you've got your own manufacturing facility!

Son involved in reassembly. It is a little finicky, with tool access a bit bad in one area and printing strength/orientation constraints making for a larger number of screws than would be in a kit. I am getting a big buzz out of his enthusiasm for us having our own designed buggy and the thoughts and discussions about how we might improve the design.

It turns out we also broke the middle part of an arm, which would've gone unnoticed had we not removed  it.

I conclude/hypothesise from that:

- arms may now be a little too strong for the bulkhead (I kind of knew that) and so at least part of the bulkhead will probably benefit from commercial printing and/or CF/GRP reinforcing strips

- but I think there must've been a fair bit of flex in the overall arm structure, including GRP strips, on impact to break this part, which I think is a good thing. Or, it cracked through shock rather than bending past a limit, which is a material issue. 

- this black PETG is not that great, I like the flexier orange stuff much more. They're supposed to be the same stuff but in reality they're not. It is noticeably more brittle.  

Oh and I noticed also that currently we don't have chassis bottom out at the front, which is because my tower is wrong (it's pre arm redesign, new design awaits CF cutting capability). I'd forgotten this and it's markedly worse with these wheels. Known issue, may have contributed to failure but in this case I'm guessing not (much). 

[BuggyDad]

Poor Mo got sent straight back out to the RC frontline with the boy while I got down to some housework. An apparently glancing blow with a wall later (unclear how this happened, about 4m off the track we'd marked out 😉) and we have the same (but this time home printed) two chassis/bulkhead parts broken again. Replacements printed so that's no bother but it's time to look at the design with a view to incorporating some bracing material! 

He has however also come up with our project idea for the summer. Some refinement required but it's along these lines and I love that he wants to go down the homemade/printing etc route. 

[BuggyDad]

Holy moly, it's a while since the last Mo update. I hadn't realised this crash was so long ago. Anyway, today I finally got around to replacing the front structure parts. 

From:

To:

 

We are back to a mk1 lower front Bulkhead piece, which is the one-piece version. I had redesigned into a multi-part piece for home printing but this is a commercial print. A new material too - PA11, which is apparently stronger and more flexible than PA12 although I can't feel the difference. The replacement top piece is home printed as per the last one. 

I have also PA11 prints of the steering cranks and bridge, which really I should've fitted in place of the home prints currently fitted, but didn't. So I'll keep them in the Mo box as spares. 

Finally I had substantially beefed up the c-hub and front knuckle design and got prints of those too. I haven't fitted them yet, but I might.

Anyway, Mo is Go. Bring on the track. When, I don't know, but I hope for an opportunity early summer. 

[Andreas W]

Good to see it's back again.  Fingers crossed that it stays a reliable runner- which it probably will do wuth normal, grown-up running . 

[BuggyDad]

2 hours ago, Andreas W said:

Good to see it's back again.  Fingers crossed that it stays a reliable runner- which it probably will do wuth normal, grown-up running . 

No grown ups were involved in the making of this! 

😁

That crash way back last year led to a nice example of the kind of thinking that this stuff gives the opportunity for. When he sheepishly came into the house with two separated halves of Mo, we talked about design and why it might've been that area that broke and we asked the question "how might we improve it?" I think he had to have had the crash to be able to focus his mind for a time on that. Every cloud has a silver lining! 

Although, I haven't so far designed a really good improvement in, and, for the reason only that time marched on and my thinking was on other things, my reprint is of a mk1 part, albeit in apparently better plastic. I tried for ages to design a way to incorporate a carbon fibre spine into this part, to add strength in the link between chassis plate and the block and tower support portion (which itself is strong) but never came a satisfactory answer. 

It does take quite a hard crash to break this but it will generally be the first part to go, and I'd rather it wasn't because it's fairly time consuming to replace and more expensive than most parts to print (commercially) due simply to its size. Not the ideal sacrificial part. 

[Andreas W]

Hehehehe. All in all it's good that you both got the small grey to work and an improvement came out of it. It's a never ending work in progress anyway . 

[BuggyDad]

Well, we got an opportunity to run Mo at the track and it drove well. Ultimately, the front lower Bulkhead piece discussed above did fail. I did always know it needed some design work. @ThunderDragonCy suggested extending the rear of it to capture the ends of longer suspension pivots. There would be room for that, so it does make a lot of sense. It made me wonder actually - I've obsessed about exactly this point at the rear of multiple cars before and modded both Blitzer Beetle and DT chassis to do exactly this. There was no way I was ever designing Mo without external suspension mounts at the back. So why had I not done similar at the front where there are more impacts? Who knows... The bumper would even enable nice easy assembly. 

Before that failure though, the driving was good. 

It does squat down at the rear and its nose lifts, and it's more prone to a wheelie than it should be. So it could be improved for sure. I think I need to harden up the rear shocks a bit and I may alter the geometry at the front because it's actually got a mishmash of parts still, with a tower from a earlier iteration. Some geo change at the front to drop the ride height and/or reduce droop a shade might really help.

Anyway, it's a pleasure to drive and has scope to improve, which is what we want.

There were some other breakages too. Early on it stripped out a rear upper arm screw (design fault - why didn't I screw through to a ball nut? 🤷 Time to design a new rear Bulkhead with this in mind) and snapped a wing mount but I could solve those on the fly. 

[ThunderDragonCy]

@BuggyDad Mohawk looked so good on track! 

This the my Ultra G Evo arm mount. I managed to get tabs on the outside at both ends

I reckon a little ride height out of the ront will help. I would be tempted to try heavier oil in the rear shocks before more springs. The rear looked very "right" in motion, so some heavier damping would push the balance forward without more spring. 

It's such a cool thing.

 

 

[officepizzaparty]

this whole thread is so cool and valuable, and the resulting buggy looks sensational! even the first iteration of the car looked great. your modelling skills are excellent and inspiring to someone like me. thanks for the great read!