front hub wheel shake

[auraslip]

Above ten miles per hour my front wheel oscillates back and forth when I let go of the handlebars. Even if I loosen my grip it does it.

The wheel is true, so I'm thinking it must have something to do with the hub or the way I mounted it in the drop outs.


[youtube]r1oHOlBaeS0[/youtube]

 

[dogman dan]

That's nasty. Nothing crooked looking about how it fits in the dropouts? I've had some bad pogo, up and down shimmy. Usually out of round rim or tire with a boob growing on it for that. Big fat tires can be notorious for out of round when inflated so look at that stuff first.

After that, if nothing is evident, I'd start looking at other issues, like too much weight high on the rear rack, combined with a flexy frame perhaps? Loose headset? Something up with the hub bearings?

It's classic high speed wobble, but to get it at 10 mph, something is definitely bad. Try taking off the batteries and pedaling 10 mph, maybe down a hill, and see what happens.

 

[TMaster]

Flip the bike over on its handle bars and seat, and run the motor like normal. After running it grab the wheel and see if theres any play side to side etc...

 

[AussieJester]

Thats nasty :-| Also...i tho9ught one of the appealing features of the frock
motor was it silence, is this noise normal? My rc motor makes less noise than that at anything less than full throttle

KiM

 

[dogman dan]

9 c's are one of the noisier dd frock motors. The large covers act a bit like a guitar body, and make the low grunting sound at start up louder. It's the sound of the power going from pole to pole or just the magnets clicking from winding to winding at low rpm. Solution, ride faster, they get nearly silent by 20 mph.

 

[Drunkskunk]

Death wobble, Also known as shopping cart wheel.

I had that happen on a jeep on the freeway once. shook so bad it tore the motor off the mounts and pretzeled the frame.

I had it happen some on the first Ebike too, and a few road bikes. It can be a nightmare to solve, but happens when a combination of weight, angles of the front fork, and/or flexability of the frame and fork combine with speed. Unsprung weight and heavy wheels can make it bad. being tail heavy can make it worse.

A fork and head angle combination with lots of trail might raise the harmonic above your maximum speed, killing the wobble.

 

[amberwolf]

Usually out of round rim or tire with a boob growing on it for that.

:lol: *dies laughing* I have had a couple of Kenda Kross and Krossroads herniate; never thought of it like that before but now I will never look at them the same. :lol:

Big fat tires can be notorious for out of round when inflated so look at that stuff first.

Definitely. The cheaper the tire the worse this can be, too. I have also seen them not seat on the rim properly, causing lumpiness on one side but not the other, which will do what you see (but not usually at that low a speed).

It's classic high speed wobble, but to get it at 10 mph, something is definitely bad. Try taking off the batteries and pedaling 10 mph, maybe down a hill, and see what happens.

Yeah, this is what people call a "tank slapper", once it evolves to be enough force to tear the bars out of your hands. Never experienced it, but have seen it, and it is frightening! As said, rake/trail can do it. Did the geometry of the bike change from before putting the motor/wheel in there? Meaning, is it taller or shorter in the front or rear than it used to be? If it's a suspension bike (both front and rear) then after adding the batteries, as dogman says, it could make the front or back end a lot lower than it used to be, changing how it rides. Seems to me it'd be hard to make it different enough to cause *that*, though.

There is one thing I can think of that could also cause it; if the front wheel including tire that's on there now is significantly smaller than the original, like going from 700c down to 26", but the rear wheel is original, the bike would have less rake and trail. I still wouldn't expect that kind of problem at those speeds, though.

 

[AussieJester]

There is one thing I can think of that could also cause it; if the front wheel including tire that's on there now is significantly smaller than the original, like going from 700c down to 26", but the rear wheel is original, the bike would have less rake and trail. I still wouldn't expect that kind of problem at those speeds, though.

Negative trail/too little trail is actually what causes high speed wobbles, you will have very light steering at low speeds but be unbalanced at high speeds. Larger rear smaller front does reduce trail so this could possibly be contributing to the problem...

KiM

 

[auraslip]

I had to read this wiki to get a handle of what you all are talking about.

I let you know after I run some tests.

 

[AussieJester]

I had to read this wiki to get a handle of what you all are talking about.

I let you know after I run some tests.

Something as small as tire pressure could even contribute to changing the geometry, if the trail is
borderline incorrect to start with. Too much trail isn't a big problem when you don't have enough though
this can be extremely dangerous at speed tank slapping is the result its almost uncontrollable. When
you have the wobbles try getting all your weight back off the front wheel sounds silly but applying more
power is how to stop tank slapping the exact opposite to your instinctive reaction to button off the throttle.

KiM

 

[auraslip]

Ran some tests:

With out batteries on my bike, as in pedal power or more accurately hill powered , the wheel still shakes.

I checked my boobs for lumps, er, I checked my tire for bumps and it came up clear.

I measured the trail at 2 inches. As in 2 inches from where the tire meets the ground from where the fork axis intersects the ground. This article is much better than the wiki link for discussing bike handeling.



I flipped it over ran it and it shakes, but not nearly as bad.

[youtube]cA8QAwTPcPQ[/youtube]
Sorry for the low light, and youtube quality.

The wheel is slightly out of true, but not enough to make the brakes rub. I think maybe some of the spokes are not as tight as they should be. Ima try to true it and see if that fixes it.

 

[AussieJester]

Tiz a strange one indeed mate, have you got a spare wheel you could pop in the front
and try downhill to see if this rectifies the issues, will at least isolate the problem.

KiM

 

[Mark_A_W]

The key words are:

"when I let go of the handlebars."

My front hub ebikes (all 4 builds of them) all do this.

Don't ride no-hands.


There's nothing "wrong". That's just how they are. Keep a hand on the bars and everything is fine.


My brother's rear hub build is like this too, and my rear hub build can be ridden no hands, but sometimes it does start oscillating.

 

[dogman dan]

Now I gotta go out tomorow and see if mine doesthis. I never felt like I needed a death grip on the bars on my bikes.

 

[Mark_A_W]

Well, one finger was enough to keep it under control. You'd never know it did it till you let go on my bike - and it was 10x in amplitude than the video above.

With one or more fingers on the handlebars, my bike handled very well - it was balanced front to back.


But no hands = shudder.

 

[AussieJester]

Do you know why it does it Mark?

KiM

 

[biohazardman]

My 125 Honda nearly yanked the bars out of my hand a 60mph on the freeway once. Should have seen the ladies face in the car in the lane next to me I think it scared her more than me. I let off the throttle gently and it mellowed out by 35mph so I limped it home. Found the only thing holding the rear axle in place had been the tiny chain adjustment bolt which had broken. What used to be a slot in the axle was now a whole large enough for the axle nut to fit through. Probly why they sold it to me so cheap.
I have a rear motor and when I let go of the bars it wobbles also but not as bad as yours still with one finger on the bar one would never even know it has a problem. Hope you can fix it or at least make so it's it not as bad.

 

[Mark_A_W]

Do you know why it does it Mark?

KiM

Not really.

I suspect it's something to do with the procession of the rotating mass (you know that trick where you try and turn a bike wheel with a handle on it at a science exhibit for kids).

 

[enoob]

that is something to fix . not live with

start by using a long true strait edge (pole or 2x4) hold it against the rear tire with the pole projecting past the front tire . you want the strait edge to touch the front and rear tires in 4 spots . with the leading edge and the trailing edge of the rear touching can you see the front doing the same or can you turn the bars and only get the front side or back side of the front tire to touch at one time ? not to clear an explanation but ill show you with some photos come morinin if you need.

does the problem change if you aggressively change your stance on the bike ? ie: move weight back and forth

after that try posting a shot of your rake an trail and see if we can help.

is the bike in your sig the one your riding in the vid ?

big hills and hard braking should be avoided till you get a handle on that one

see this john in cr ? this is precisely what you may have to deal with.

 

[Mark_A_W]

You are all worrying about nothing.


Betcha can't fix it.

 

[adrian_sm]

Sounded like something was loose in the first video.

Looks like the natural frequency of the front wheel/forks/handlebars gets excited at 10mph +.

Some solutions
Either change the exciting source,
- find/fix something out of balance
- find/fix out of true etc., or
Change the stiffness/mass/dampening,
- makes sure everything has no play. ie. headset tight, axle tight, no slop in bearings
- change mass/load on front wheel
- different steering rack/trail

So I would:
1) check for slop in front end
2) re-true wheel, whether it needs it or not.

Good luck.

Adrian

 

[enoob]

You are all worrying about nothing.


Betcha can't fix it.

carefull now . us carpenters dont like hearing " dont worry , bet ya cant fix it"

i tell ya some roman said that to jesus and look where we are now

Im not gonna try and pretend like i know what the exact problem is but i can tell you its tuneable . thats the key word . it can be altered so it happens outside his speed range . i have no idea what one single thing can be changed to fix it but my experience is its always a few things that need fixing or changing to make it go away .

some geometry is just prone to it but then these are the ones that turn fast and easy . thats where steering dampeners come in . i dont see a sub 50mph ebike getting a steering dampener anytime soon.

 

[amberwolf]

i dont see a sub 50mph ebike getting a steering dampener anytime soon.

CrazyBike2 had one for a while, but it was old and had too much slop in it so I took it off. I had it on there for a different problem, since I have remote steering I didn't want it to be as sensitive as it was to every little twitch of my hands/etc. Eventually I changed the "gearing ratio" between the bars and the wheel, by moving the pivot points on the rod, so it's not 1:1.

 

[auraslip]

Wobble, shimmy, tank-slapper,[1] speed wobble, and even death wobble are all words and phrases used to describe a quick (4 - 10 Hz) oscillation of primarily just the steerable wheel(s) of a vehicle. The rest of the vehicle remains mostly unaffected. Vehicles that can experience this oscillation include bikes, both motorcycles and bicycles, skateboards, and in theory any vehicle with a single steering pivot point and a sufficient amount of freedom of the steered wheel; this does not include most automobiles, however, coil-sprung vehicles with a track bar setup such as the Jeep WJ, XJ, ZJ, TJ, and JK with both stock and after-market suspension lifts may have this problem also. This instability occurs mostly at high speed and is similar to that experienced by shopping cart wheels and aircraft landing gear.[2][3]
Wobble or shimmy begins when some otherwise minor irregularity accelerates the wheel to one side. The restoring force is applied in phase with the progress of the irregularity, and the wheel turns to the other side where the process is repeated. If there is insufficient damping in the steering the oscillation will increase until system failure. The oscillation frequency can be changed by changing the forward speed, making the bike stiffer or lighter, or increasing the stiffness of the steering, of which the rider is a main component.[2] While wobble or shimmy can be easily remedied by adjusting speed, position, or grip on the handlebar, they can be fatal if left uncontrolled.[4]

Since shimmy frequency is independent of bike speed, gyroscopic effects "are clearly not essential to the phenomenon."[2] The top five influences on wobble have been found to be lateral stiffness of the front tire, steering damper, height of bike center of mass, distance of bike center of mass from rear wheel, and cornering stiffness of the front tire.[3]

An academic paper that investigated wobble through physical experimentation and computer modeling, concludes "the influence on wobble mode of front tyre characteristics, front frame inertia and chassis stiffness were shown. In particular, it shows that increasing front tire inflation, stiffness chassis, and front frame inertia about steering axis and decreasing sideslip stiffness of front tire, wobble mode damping is improved, promoting vehicle stability."[5]

What do these mean:
Front frame inertia about steering axis, sideslip stiffness of front tire, lateral stiffness of the front tire, and cornering stiffness of the front tire???

Shimmy is caused by the gyroscopic force of the front wheel whose tilt is roughly at right angles to the steering axis, making the wheel steer to the left when it leans to the left. This steering action twists the toptube and downtube, storing energy that both limits travel and causes a return swing. Trail (caster) of the fork acts on the wheel to limit these excursions and return them toward center.

Shimmy is not related to frame alignment or loose bearings, as is often claimed. Shimmy results from dynamics of front wheel rotation, mass of the handlebars, elasticity of the frame, and where the rider contacts the bicycle. Both perfectly aligned bicycles and ones with wheels out of plane to one another shimmy nearly equally well. It is as likely with properly adjusted bearings as loose ones. The idea that shimmy is caused by loose head bearings or frame misalignment seems to have established currency by repetition, although there is no evidence to link these defects with shimmy.

We have found that a small percentage of carbon forks by various makers were molded with asymmetrical fork blades. Some are off by a little over a millimeter and others are off by two or more. Forks that are off by over 1.8 millimeters in symmetry have a good possibility of being prone to speed wobble.

Based on that last link my thinking is thus: I had to file the dropouts to get the motor to fit. I tried to file as little as possible. Out of lazyness and fear of taking to much metal off the drop outs I didn't get the motor axles to seat completely. On each fork there are tiny gaps between the "floor" of the drop outs and the axle. Perhaps being even slightly uneven gives the wheel enough lean to start oscillating? I will try to get the wheel fully seated in the drop outs and report back.

is the bike in your sig the one your riding in the vid ?

Yes. Around the time I first noticed the speed wobble I had just installed giant 2.45 inch cst cyclops tires and my hub motor.

I found this here.

 

[liveforphysics]

I ride no-handed very often, on motorcycles, and bicycles. I ride 100+mph with no hands on my gsxr on the freeway, just reaching up to grab a handfull of throttle as needed to keep the speed up. I take both hands off the bars at 40-50mph on my e-bike when I'm coasting or slowing down, just sitting back and letting the wind slow me down. They always track as stable and true as can be for me.

I don't normally encounter this effect at any speed on any 2-wheel vehicles that I own. I've had it happen on the GSXR once or twice comming down from wheelies on the freeway a little crooked, and on dirt-bikes once in a while if I over-brake and lock the front tire for a split second. It's a strong enough effect that no amount of force with your arms can stop it, it shakes your arms like toothpicks, and the only way to stop it is to grab some throttle to lift the front end again. Your effect seems to have a similar symptom, but I think it's entirely different in cause and cure.

I read through all those sites talking about it, and I'm just not buying that it's something you can't eliminate. My bikes/motorcycles have never had it, so I inherently know it's not something un-fixable. I strongly suspect excessive flex somewhere in the system, maybe a cracked fork leg? Maybe some junk bearings? Improperly seated axle? Maybe a cracked head-tube? I wouldn't ride a bike that shakes like that until I had throughly inspected and/or replaced all components of the steering assembly with known-good parts, and tried various front tire options.