My DT-03 for fun driving and tinkering

[ThunderDragonCy 7877]

22 minutes ago, BuggyDad said:

"a bit too flexy" may be no bad thing as far as material choice is concerned here. Although if it's very rubbery I think I'll need to thicken it up or brace beneath. What I've got there is a 6mm diameter solid, about 50mm long. Reckon that'll wobble about like jelly, or at least hold itself roughly in position in normal running? 

I would put some cross braces or shear plates in. I got some 6mm body posts and they can be almost folded back on themselves. Either that or up the diameter to something like 10mm and make them hollow. 

[BuggyDad 4465]

Quite looking forward to getting the next wing mount tested. Although it'll have to wait till I have other things to print. Might be a long while. 

Next iteration looks like this, to reduce a little the up/down flex which sounds like it might be massive with TPU, I've upped its depth to nearly 12mm at the deepest point:

Previously:

TPU printing looks expensive though at over 3x the cost of MJF Nylon PA12. 

[BuggyDad 4465]

Rainy day but I couldn't resist a lunchtime run having got this running again with 3° rear toe. Rain forecast all week so it won't get better. The car keeps straight under power, of which it has plenty, and the wheels hug the ground through the rough stuff beautifully now. It's better than ever, although it needs that wing mount printing to finish the job. 

What I'd really love for it next would be progressive springs. Keep the softness but ramp it up on the hits. Its belly is on the ground a lot. But I don't think such a thing is doable in a practical way. 

[BuggyDad 4465]

Done a bit more learning on Fusion360 and also had a bit of a rethink after I got some TPU in my hands. It's very flexible. So I braced my wing design in an effort to make it less flexy. Idea is it'll still be flexy, just a bit less so. Aiming for the wing to stay in place running, not move about with wind resistance, but in a crash I want it to bend and spring back rather than break. 

 

[BuggyDad 4465]

So here is Wing Mount MKVI in the flesh:

Finally my DT-03 has its wing again!

It is the above design but printed in nylon (TPU was ~3x the cost). I ❤️ 3D printing. Ability to design and get made one-offs like this has just so much potential. 

There are a few issues with it, but they're minor. It fouls the gearbox where the spur cover top screw goes in, so I've cut it away for that. And the lower mount points need to be shaved back 0.5mm and cut out a little to clear the switch housing. That last one was a disappointing rookie omission from my designs and is the same in MKVII, but is fixable with a knife and then invisible.

Both these issues could very easily be avoided in another design iteration. And the gearbox fouling would go away if my wing holes were closer together - maybe I'd put them at stock width in a redesign and use a new wing. 

This version could be improved by using perhaps 4mm section instead of 6mm, bringing the lower braces in a little to eliminate the fouling issue and perhaps losing the cross bar. 

MKVII is also printed and will follow shortly. Different idea aiming for flex when printed in nylon. MKVI here is rigid in this material, and although I think, subject to the above changes, it'd be the better option aesthetically, I'll want flex for my runner so will likely move to MKVII. So, more to follow... 

[BuggyDad 4465]

And so follows wing mount MKVII:

As intended, highly flexible. In fact it'll move around a lot in the wind it's so flexible, but I doubt it'll break. It can be stretched easily back to hit the gearbox or up to vertical. 

I think if there's a MKVIII it'll have a tiny bit more angle to the wing, a narrower deeper arm profile to reduce flex but retain the idea of flex in a crash, a raised flat section to sit the wing up from the arms and a less pronounced curve, to make it look a little more like a normal wing mount. And of course correct my chassis fitment error that necessitates use of the knife.

But all that is splitting hairs. This'll do the job nicely for now. 

Credit where it's due - the idea of a 10mmx2mm curved ribbon of PA12 nylon I stole from @jonboy1's Astute battery holders and his demonstration of how they flex.

Weather permitting, might give it a rag around the yard tomorrow and see how much the wing flaps around in the wind. I suspect a lot but I don't really mind. 

[BuggyDad 4465]

Well, poor old MKVII did not last the distance I'm afraid! 

However, too much was asked of it. My son forgot himself a bit with the buggy and while I saw neither of the offending crashes I saw a few other nasty moments (including a bad hit on the front shock tower which cracked the tower mount but otherwise my bracing took with nothing but a stylish little bend). So it's probably still a step in the right direction. This may hasten my design of a still bendy but somehow more resilient MKVII. Meantime, MKVI is back on. 

[jonboy1 2943]

10 hours ago, BuggyDad said:

Well, poor old MKVII did not last the distance I'm afraid! 

However, too much was asked of it. My son forgot himself a bit with the buggy and while I saw neither of the offending crashes I saw a few other nasty moments (including a bad hit on the front shock tower which cracked the tower mount but otherwise my bracing took with nothing but a stylish little bend). So it's probably still a step in the right direction. This may hasten my design of a still bendy but somehow more resilient MKVII. Meantime, MKVI is back on. 

You don't want to listen to that @jonboy1 - he's a right wrongun!

Surprisingly it broke exactly where i thought it would! Right at the intersection of the two parallel bits. It looks like there might have been a sharp edge there on the inside surfaces

which are big stress crack propagators, so just whacking a big radii in there might be enough to solve it. One other thing I'd suggest trying is tapering the thickness as you go round into the curve section - so instead of it being say 2mm thick all round, start at 3 down by the bit where it broke and gradually get thinner as you go around the curve - as this will not only beef up the current weak point, but also move the bend stress away from the same place and up into the curve where you actually want it to bend. A final thing to try would be to make the upper part one long curve (so not have the upper straight section leading up to the wing mounts) as you again enlarge the area it can bend rather than focusing it all in the small curved section currently. 

Love what you are doing here - keep it up!

 

[BuggyDad 4465]

1 hour ago, jonboy1 said:

You don't want to listen to that @jonboy1 - he's a right wrongun!

Surprisingly it broke exactly where i thought it would! Right at the intersection of the two parallel bits. It looks like there might have been a sharp edge there on the inside surfaces

which are big stress crack propagators, so just whacking a big radii in there might be enough to solve it. One other thing I'd suggest trying is tapering the thickness as you go round into the curve section - so instead of it being say 2mm thick all round, start at 3 down by the bit where it broke and gradually get thinner as you go around the curve - as this will not only beef up the current weak point, but also move the bend stress away from the same place and up into the curve where you actually want it to bend. A final thing to try would be to make the upper part one long curve (so not have the upper straight section leading up to the wing mounts) as you again enlarge the area it can bend rather than focusing it all in the small curved section currently. 

Love what you are doing here - keep it up!

 

This process of thinking and designing is surprisingly turning out to be one of the more fun aspects of RC for me. I really enjoy this type of thinking.

It did also occur to me that in fatigue terms my couple of packs is probably a whole lifetime worth of one bend per pack on your same profile plastic! 

Thank you though. I think your ideas are bang on, and are broadly where I was headed if I read them right. I also want to take a more direct line from chassis to wing for aesthetic reasons, in place of the very obvious vertical then curving back shape. Interface between those arms and the chassis attachment area is still going to be the obvious breaking point, so care in design and tapering down gently from there will be key. The challenge is to do just enough, allowing it still to bend.

The break is where I'd expect it but there's no sharp edge at that point as such (bad photo), rather I think just that bending force increases the closer to the chassis you get, and I hadn't tapered the profile accordingly. I agree though - my first iteration was in part driven by simplicity of drawing it, with as large a radius curve as possible followed by a straight section, and using essentially simple constant profiles everywhere with just fillets to try to reduce point stresses. A next iteration would benefit from a bit more sophistication in my approach.

Another thing I might look at is notching the post so the wing pops into place, or sitting an o-ring in a notch above to retain the wing but enable it to pop off with a bit of force. Two protections in a crash - wing pops off and mount bends. I think that principle is good but the balance between the two is out. 

There is also of course the somewhat obvious fact that I am sticking a big lever out the back/top the car, so in a tumble the car is going to apply a lot of bending force on that lever, and vice-versa. Somewhere in the middle of this something's gotta give, and I'd rather it was my 3d printed part than shock tower or chassis. It should remain deliberately sacrificial, and it should be noted that the wing itself remains in perfect condition, which is valuable since it's quite time consuming to paint and exists in large part to display the single most important decal on any Tamiya: "NO GUTS, NO GLORY!" 😜 

[BuggyDad 4465]

Gave the DT-03 a run at my other location and I have to say it has really come on more than I realised. Gravel, concrete, tarmac with dirt on top etc. Less rough than home, where really a bigger wheeled machine is more suitable. A place where I can run a rally car but on some of it that bobbles about a lot and is out of its depth. The DT-03 now stays serenely flat and straight as a die under hard acceleration. At the back I think the toe in is the transformation. At the front the softened up shocks are just tracking the ground beautifully. I've little to no experience of good cars well set up so I don't know what I'm talking about but the transformation from kit DT-03 to this is night and day. 

But the back tyres took a bit of a beating 😬

 

[BuggyDad 4465]

Wing mount MKVIII.... 

 

... is the same design as MKVI but printed in TPU for flex. It comes up with a very rough surface, for some reason. I think maybe I could've chosen a polished finish for a little extra but I didn't. Anyhoo, that doesn't matter. It has plenty of support to hold the wing steady, so let's see how it lasts. 

I've also spent a quiet Friday afternoon servicing shocks, motor, CVDs and replaced the troublesome sensor wire. And replacing bent M3 rod struts at the front, which I think have saved quite a number of shock towers in the 18 months they've been on. Not a single tower in that time, after being a serial tower destroyer. Admittedly, not a huge amount of driving. And I will get it back to buggy wheels at some point. With these on and a 19t pinion the motor gets hot on grass. 

Recording for my own info, 150 oil front, 500 rear. The old oil was very grey and the red o-rings expanded. 

On a quick test run, I've got a new issue with the wing mount, which is that in TPU it pulls off the screws in a couple of tumbles. Potentially useful damage-saving feature if I get it right but it needs longer screws and a smaller drill size. Easy fix in a redesign and I can probably improve it without. I could drill deeper for longer screws or drill through and use screw and nut. 

This car steers better than the Falcon, which has more understeer under power and more oversteer under (or after) braking. I think that's about weight transfer which is affected on the Falcon by shorter wheelbase and more suspension travel. It maybe also has too much droop in the rear. And surprisingly I think the Falcon chassis has an even more rearward weight balance than the DT-03. 

[BuggyDad 4465]

A running update. With two zip ties to keep it on, the new wing mount has survived quite a few tumbles and is still going strong. I designed it so the wing pops off. If i need another I'll adjust its shape to clear the spur cover properly but otherwise I'm happy with it. 

Over time (well over a year in there untouched) my stiff (500k oil) diff has freed up to a point that it is perhaps now in the sweetspot. And, with buggy wheels on, the car is well mannered. Probably more neutral and easier to drive than it's ever been. I suspect this means the diff has leaked but I haven't opened the gearbox up to have a look!

Before this last run we upgraded my son's to the same 3 degree rear toe and serviced shocks etc, as well as put 500k oil in the diff, and his is rather pointedly not so well mannered, it's an oversteering monster. They'd need a side by side test with same tyres and motors to compare apples with apples (and new tyres should help him - his are pretty worn, and I suspect my abused motor has slowed a fair bit) but from there I think I'm homing in on a desire to get the right amount of slip from a diff on this buggy. For his I will next try AW grease. For mine, I might then try the ball diff I've had in stock unused for nearly two years. I'd do this when replacing the CVDs (they're pretty worn) because it uses different outdrives. 

I really wish there was a sealed gear diff for these. Why so many different diffs, Tamiya? Couldn't you dimension lots of gearboxes for one diff size and then make a basic open diff, a sealed gear diff and a ball diff for the different price points? 

[BuggyDad 4465]

Continuing my little mission to improve all my old shocks, I stripped these and replaced the red o-rings with x-rings all round to create the best setup I know of in terms of shaft/seal friction. I've concluded it makes a real difference to work through the hardware first to minimise inherent friction before starting on spring rate and damping. These are the DF-03 Aluminium shocks. They have a nice feel to them even though they've seen a fair bit of action. Although they're nowhere near the top of the line option, I think they are probably in function terms very good, much better than CVAs. Whether that's precise machining of the bore or the arrangement of seals and white plastic shaft guides, or rigidity of aluminium over plastic or what, I don't know. 

I also increased oil weight to 200 front, 600 rear. The rear remains under damped, the front could probably now take a shade more damping too, but it's fairly close. And the front of the DT-03 is so light that it's a bit different than others to set up - very sensitive. Both front and rear have white 3 hole pistons (I think from the DF-03 shock set) in. The holes look quite large (but I forgot to measure them). I conclude therefore that I would be better to change pistons than further increase oil weight. I think rears would benefit from 53573 (or even 53572 - 2 hole) pistons for sure (assuming the holes are smaller as I think they are) but I don't have any with me. Perhaps the front too for pack. 

Interesting that these upgraded shocks have a smaller bore (the rears) than the kit shocks. 

[BuggyDad 4465]

Feels a bit like sharing your breakfast on instagram but I find it very useful to record changes here, even if it's just for my own use. It'd be nice to get scentific and change - test - chenge - test in one session but I just can't find the time for that. 

So further shock tinkering while I'm banished from the kitchen as the family make my birthday cake. I put the 53573 pistons in the rear. Much smaller holes. With 300 oil they feel pretty good. A bit more damped than big holes and 600. I also changed the front oil up to 300. I intend to order a piston drill set so will use that to measure the holes for the sake of a bit of science as well. 

Front still returns quicker than the rear on a drop test but I'm not sure how relevant an observation that is, given the heavy rear weight bias. I have a feeling it makes sense to err on the lighter side in damping on the front of this, although I might revise that later for jump landing. I'd still like to see if I can get them to pack a bit, so might at some stage try new pistons on the front too. 

My son came in and said "ooh, you're doing suspension. Can we do suspension next week?" which is ace, obvs. So I hope we'll work together on his DT-03 during half term, and maybe go and undo it all with the sand, salt and fun of a beach run. 

[BuggyDad 4465]

Stumbled upon a potentially useful part. Lots of folk buy the Mad Bull steering set as a cheap way of getting knuckles for truck wheels but I spotted the steering links are the perfect length to be repurposed as more flexible/easier to make bracing for the threaded rod appreciation society shock tower brace mod. 

Dunno whether we'll run it, since the steel rod method has worked so well, but I thought I'd chuck it out there. It probably knocks over 1/2 off the high up weight penalty of my mod, if you care about such things (which I don't on this car).