front hub wheel shake

[amberwolf]

I noticed that in my fork the stock torque arms that came with my ebike kit are not completely solid, and the axle moves a little bit in between regen braking and acceleration.

*That* you definitely ought to fix, and be absolutely sure it *is* fixed.

Every time the axle can rotate a little in there, it can pry the ends of the dropouts just a little farther apart. Eventually they'll split, and the wheel will come out--depending on exactly how that happens you might get to play hugsy with the road. If you're lucky, the torque arms will prevent that last bit by holding the wheel to the dropouts even if the axle spins out (ripping the wires loose, possibly), but if at all possible, make it so the axle cannot rotate at all, regen braking or hard acceleration or alternating between the two.

Just remember that (especially in front) dropouts were generally not made for the forces of axles torqueing against them, only the upward force of the axle against the fork itself. So without being totally tight and not being able to rotate against the torque arm, the dropouts are being asked to absorb forces they were not designed to take. Sometimes they're strong enough to take that, sometimes they're not. Experimentation on a ride sucks as a method of testing that. Let Justin do all that stuff up at RBC in the lab.

Thankfully I have never experienced this or even seen it in person, but I have certainly seen enough vids and pics of it happening to never ever want to.

It makes me wonder if I could go pick up a motorcycle fork from the junk yard

I got one from Freecycle.org. It's heavier than my whole DGAmkII bike, including the motor and maybe the batteries, but it was free. Only thing wrong with it is a slightly leaky seal on one side for the oil shocks.

[dogman]

I think it's a little bit of everything, Big tire, front motor weight, etc combined with a lightweight flexy fork and a flexy frame. Then the battery placement is amplifying it. The front wheel is responding to frame flex coming from the heavy batteries high on the rear. Take the batteries off, and see if it still happens with just the motor on the bike.

My best experience with death wobble occured in Santa Fe, when a car ran me into the gravel on a 50 mph descent. The frame flexed like crazy, looking like it was made of wet spagetti. I really think the whole thing is from the battery weight.

[auraslip]

Slowly as things go wrong I'm learning how different parts of the bikes function and more importantly how they need to be maintained. Today I learned how to adjust a threaded headset.

The LBS did it for me three months ago, so I didn't think to check it again. When I did I found that the adjusting nut took a whole turn before the bearings put up resistance to being turned.

I can now ride at all ranges of speed with out speed wobble, death wobble, tank slap, and bucking buckerooskies.

If I take my hands off the bike it will still wobble a little, but as long as I keep the tiniest pressure on the handle bars it stays straight =)

Also I redid my torque arm setup:



Notice that with out the C-washer the stock torque arm is flush with the drop out.

To remove the C-washer on the otherside I had to either undo all the wiring from the controller or cut it. I figured now was a good time to make a quick release system for my wheel.


Two pieces of bike tube. I was skeptical that it'd be water tight. However the tube was a 1.75-2.25 size and it was still nice and snug.

I may try I smaller tire size to see if it completely eliminates the wobble, but as of now I'm gonna call this one solved.

[liveforphysics]

So, if I understand correctly you adjusted the headset bearings and the wobble is gone? And you've got the axle secured so it can't wiggle now as well?

These are things that MUST be done before you ride a bike bro. It's not fair to yourself, of the cars driving behind you to be involved with a death/injury over not prepping a bike to be road-worthy.

[auraslip]

So... I'm reviving this topic. To be completely honest, even after tightening up the headset the shimmy was still there. If I recall, I just didn't want to make you guys think you weren't helpful. The headset wasn't too loose to begin with sadi the guy at the bike shop.

What did fix it was replacing the hub motor. But anyways

On my new bike I have really bad shimmy. The whole bike feels tweaky in the handling department.


Here is the new bike. Here is build thread -> viewtopic.php?f=3&t=37204

If I take my hands off the bike, it immediately starts to shake. I don't know why. The headset is nice and tight. The forks are good and strong. Maybe it's something to do with 24" wheels?

The handling is also very poor. It turns way to quick. It's hard to keep it going straight while pedaling. It bobs back and forward with each pedal stroke. I've tried adjusting the pre-load and rebound on the fork to stiffen it up, and it's seemed to help a bit.

[auraslip]

I let all the air out of the fork and now it handles fine with no wheel shake.

WTF. I thought more travel = more trail = more stability.

[dogman]

Was that frame built to have a suspension fork? Or is it's geometery intended for a rigid fork that is much shorter?

When I had a similar problem, the cause was obvious.

Mongoose blackcomb commuter.JPG (100.67 KiB) Viewed 114 times

But not as obvious as you might think. I had slop in the braces that support the rear rack. When I tightened that up, I could resume riding this bike no hands. Still has horrible balance, but the shimmy went away.

So look for anything moving around. How tight is everything inside that battery box?

[auraslip]

80mm travel or 100mm travel and the suspension is 120mm travel so that right there is a problem.

The zip ties holding everything down are cranked hard enough to eat into the balsa wood, but that's actually something I haven't thought of. I'll check everything to make sure it's all tight.

[D-Man]

I have a 100mm aftermarket fork which seems too long. The frame was too small anyway so it ended up right with a longer reach stem. Never did solve the ebike headshake thing. Its a geometry thing that only the bike designers know. Got to hold the throttle anyway so why let go of the handlebars? Maybe to give someone the double finger?

[auraslip]

So you can ride with your arms crossed while sipping on your eciggerette like some space edged Buddha floating over the texas prairies. duh.

I used to do that while towing my trailer home on my old bike.