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Tamiya Hot Shot // Wheel Doesn't Spin

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So now I have another issue with another newly purchased car. Well, not really an issue per say, but something odd that I've never seen happen on an R/C car.

I built my new Hot Shot the other day... and it wasn't fun at all, so many things went wrong! Even my Konghead build went smoother.. anyway, I've noticed that if I lift the car from the car and pull it on it's rear wheels, one of the wheels doesn't spin. I think it's the front right if looking at it facing away from you.

It also doesn't spin when under acceleration if I'm holding it. BUT... it does spin if I compress the suspension a tiny bit. It's like the pressure of the suspension is preventing it from spinning. This means it SHOULD be spinning when on the ground as the suspension will be slightly compressed. But I cannot fathom why a wheel won't spin. It doesn't feel tight or jammed when I turn it manually.

If I spin either of the front wheels, other wheel does turn in the opposite direction as it should, so it's definitely connected properly. But why wouldn't a single wheel spin when the suspension is fully decompressed?

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The first thing I would check is to make sure the dogbones are engaging properly.  The stock dogbones do require something on either side of them to keep them "in place".  I've used cut ball point pen springs but ended up using some cut pieces of automotive silicone vacuum tubing jammed in there to keep the dogbones in place.

 

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It's probably the ball joints aren't screwed in fully into the red upright at the bottom. If they protrude slightly the 2mm screws that hold the ball retaining plates in place dig into the wheels. If you look at the inside of the wheel you will be able to tell if this is happening as there will a ring of scratches round the inner part of the wheel. When you compress the suspension, the angle of the lower arm changes and lifts the screws away from the wheel. 

 

 

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Tamiya has pink urethane sponge thingies.  But I stopped using them because these are as hard as rubber and they stay compressed, instead of bouncing back.  

RndjWVF.jpg

Below is what I did on Bigwig.  As @Mahjik said, ball point pen springs may not be as durable.  You can use anything that's springy enough to keep it "floating" between two cups, a piece of sponge, styrofoam ball, vacuum tubing, o-ring, fuel tubing cut to about 2-3mm, pencil shavings, ball of cat hair, etc. (I'm not serious about some materials here B)

Dog bones work best if they are not jammed to one side.  Check if the dog bone moves side to side and has some slop.  Usually, gravity tends to move the dog bones to the wheel-side cups.  Something in those cups would help push it away just a bit, and keep the dog bone nicely in the middle.  If you stuff too much, though, it could bind at the midway point.  (if I remember correctly, one O-ring each on both ends of the dog bone ended up binding at mid point of the arm-swing)  Anything that would push 2-3mm would be good.  (I have springs on both ends so they push against each other to midway point. The springs are tapered, so they can compress down to 2mm. If it doesn't, it will bind)  

Also check the metal plate @MadInventor mentioned.  Hot Shot has a lot of slop in the suspensions, but the ball joints right below and above the uprights shouldn't be sloppy.  (don't over-tighten those 2mm screws either)

uh1ZnDk.jpg

 

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MadInventor has probably got it spot on, it sounds like everything is working properly but the wheel is binding on something.  If that doesn't help, check the drivetrain from one end to the other.  The opposite wheel spins, so we can probably assume power is getting as far as the diff, so you want to check between wheel and diff.

I'd start at the wheel.  Is it properly mounted on the drive hubs?  Is it firm but not too tight?  Is it rubbing on anything?

Then remove the wheel and check the drive hub.  Is it properly mounted on the drive pin?  Is the pin missing?  Is the hub touching anything?

Then remove that and check the axle.  Does it move freely in the bearing?  Are the bearings seated properly?  Is it properly engaged with the dogbone?  Are there any witness marks that suggest the dogbone has been skipping?

Then move on to the dog bone.  Is it properly engaged at both ends?  Does it look like it's been skipping?

After that, check the drive cup.  Is it seated?  Does it move freely?  Can you feel the diff action?

It's hard to check the diff with it installed, but if you turn one wheel the other should go the opposite way with little resistance.  If the motor turns instead then there's something stiff or binding in the diff (or you have it adjusted too tight, which probably doesn't apply to the Hotshot - IIRC it's a gear diff?).  If the wheel spins freely with neither opposite wheel nor motor turning then you probably have a diff problem.

If you grab the opposite wheel and turn the problem wheel, the motor should turn.  If the wheel moves without the motor turning, you have a diff problem.  If the motor turns, everything should be OK.

Usually diagnosing these problems just takes a methodical approach of looking at each bit in turn and eliminating it as a problem :)

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Mahjik: It looks like the dogbones are engaging correctly. Everything spins and turns smoothly, everything interlocks with everything else. The wheel does feel very, very, VERY slightly stiffer than the other wheel, but barely noticeable.

MadInventor: You may be on to something there. It seems as though the lower suspension arm is rubbing ever so slightly on the inside of the wheel. Everything is screwed in tightly, so I just used an X-acto knife to cut away small amounts of the suspension arm to make it clear the inner rim. Still rubs a bit, but now the wheel turns better than it did. Thanks for the assist!!

Juggular: That's not a bad idea with the small springs to keep the dogbones in place. I may have to try that if I can find a sacrificial ball-point pen. And we have two cats, so cat hair isn't a problem (lol). Yeah, the dogbones all move nice and freely, nothing is tight or catching on anything. The problem seems to have been discovered by MadInventor up there.

Mad Ax: He did indeed pretty much get it spot on. The lower arm rubbing on the inside of the wheel was the issue all the along, I didn't even think to check that out. To answer all your questions, the answer is YES to all, lol. Which is a given since the problem has been isolated :)

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Instead of cutting and mangling things, why not just do a proper fix rather than a bandaid?

Get a nice pair ally hexes that are 1-2 mm thicker than stock. Also, have you tried to shim the stock hexes? Might be as simple as putting a 0.5 mm shim behind each hex to get rid of all rubbing and flex.

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