Hands off oscillation - what gives?

With my brushless front hub motor locked into a constant speed, be it fast or slow, the front wheel will start an oscillation when both hands are removed from the handle bar. The torque creating this oscillation cannot be felt in the bar at any time during normal riding conditions. This happens almost immediately after I let go, and it quickly gets out of control with the wheel violently sweeping left and right equally.

I first noticed this when I sat upright for a breather (I pedal hard a good deal of the time) and placed my hands on my hips. I’m sure someone got a good laugh watching me pull in the reins, trying to recover from what would have been a nasty toss.

Though I do not consider this a problem that needs to be rectified, I am curious as to what may be the cause. Considering the front end of a bicycle was never designed to propel the frame and rider, could the frame and or fork geometry somehow be responsible? I should think that the fork rake would tend to make the system more stable when a force is applied at the dropouts. Another thing I’ve considered are my torque arms. Since my suspension fork (perhaps another contributor) is aluminum, I have made some serious torque arms (one on each side) with anti spreading measures, that lever force a good 5 inches up. Could they be batting reaction forces back and forth? Perhaps this is an inherent condition of a hub motor as tangent forces pulsate off the circumference of winding diameter to complete a cycle.

Any thoughts? Can anyone duplicate this phenomenon?

Resonance.

Tail wag? Where and how securely are your batteries mounted?

It's a pretty tight ship. A 13 pound NiMH pack is strapped to solid 4 point rack just above the rear tire. With your hands on the bar, it tracks nice and true.

99,9% (as Lenk said) resonance between inbalanced wheel and fork springs. No help IMO.

An imbalanced wheel seems quite plausible. Perhaps I threw the baby out with the bathwater!
I had a shop work on truing the wheel after I received it because it had a bad hop. This was not fully worked out due to some poor manufacturing.

HAL9000v2.0 said:

99,9% (as Lenk said) resonance between inbalanced wheel and fork springs. No help IMO.

Click to expand...

Yep, what they said. Adding the weight of a motor just makes it worse, and a springy Aluminum fork, too. Nothing you can do for it, a perfectly true wheel would do it too, eventualy.

Is this a motor that actually operates when the hands are off the bars?

If not, the change in the hub's resistance to forward motion as the permanent magnets pass by the would seem to be a likely explanation.

If not - the contact patch (where rubber meets road) is actually BEHIND the spot you would find if you drew a perfectly straight line down through the head tube. What this means is that if something exerts a forward force on the bottom of the tire* that portion of the tire will try to find the easiest way to move forward. Flopping the handlebars is easier than moving the whole darn bike.


(*wheel pushes on ground, ground pushes back i.e. exerts equal-and-opposite force, that's how the bike moves)

Weird. I could do a no-hander forever with my hub in the front it was so easy. I suspect (like cerewa) it has something to do with the point at which the wheel contacts the ground in relation to the head tube.

*drips ice cream on keyboard*

...

Dammit.

I would run the motor with the wheel up... if it don't wiggle, csmarr is likely correct.

The traction issue has merit. When I lift the front end and leave the handle bar free, the wheel spins rock steady at top RPM. I went out and rode the bike with no hands under pedal power only and there was no oscillation.

cerewa said:

(*wheel pushes on ground, ground pushes back i.e. exerts equal-and-opposite force, that's how the bike moves)

Click to expand...

I know from my RC helicopter days that when a force is applied to a rotating mass, the effects are witnessed approximately 90 degrees ahead of rotation. If lateral forces are present during traction, where the rubber meets the road, the force would appear on the back of the tire about midway up, and serve to rotate the fork, adding to the misery. This could go far to explain why it occurs almost instantly when I let go under motor power.

ahend said:

With my brushless front hub motor locked into a constant speed, be it fast or slow, the front wheel will start an oscillation when both hands are removed from the handle bar.
...
Any thoughts? Can anyone duplicate this phenomenon?

Click to expand...

I have the same problem, but my bike has a rear-hub motor. It happens at very low speeds; like single-digit mph.

I'm just careful to never remove both hands from the handlebars. Sure wish I could fix the problem though.

I'm not a stunt-clown that rides no-handed all the time, but it would be handy to remove both hands from the handlebars ocasionally. Sometimes I'm carrying something in one hand on my bike, and I need to use my other hand for something else.

I think this could be a couple things. First, bad oscillations of the front tire can often be caused when the angular momentum of the front tire combined with steering actions make the front forks flex. If the sliders of your front forks are loose at all this will make an oscillation like this 100 times worse. Btw, everything in a bicycle steering system is super sensitive to looseness. Also, I'm pretty sure powering the front wheel will make the front tire more aggressively try to keep the bike upright. If the steering is already close to oscillating this will push it over the edge. I'm not quit sure what the solution would be. I can only suggest that something about the steering system needs to change. A summary of the math that describes this all http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics details are over my head, but the qualitative conclusions are quit useful.

Marty

is you bike a cheap chinese generic frame? i have this problem on my bike and i put it down to the poor quality frame. too much play in the headset.

I have it in my two wheel bike but none at all in both three wheel bikes and they are alu frame with steel forks. My two wheel is all steel and it will throw you off the bike if you let the handle bars go

You might try balancing the wheel.

You can remove the wheel from the bike. You will have to be careful with the hub motor's wires as not to break them--especially the hall-effect sense wires. You can prop the axle on both sides using smooth wood blocks. I suspect that the wheel won't spin because of the magnet's grasp on the hub, but the wheel may try to turn one way or another, rolling on the blocks, so that the heavy spot is on the bottom.

You can use either stick-on wheel weights, available at a tire store, or lengths of solder wrapped around a spoke to weigh down the part of the wheel that turns to the top. If you use stick on weights, they can't go where the rimbrake rubs.

There could also be something wrong with the geometry, such as backwards front fork, or one without enough trail. If look at the side of the bike and imagine a line going from the steerer-tube to the ground, and another line going from the front axle down to the ground, the axle line should "trail" behind the steering axis.

Some of the other comments may be alluding to this, but I'm guessing that the oscillations are due to the fact that upright bikes have a very small rake angle. This gives them a highly manuverable, yet very unstable CG. Without a feedback mechanism (ie YOU), the handlebar/motor assembly balance is at the whims of the motor/wheel's force differentials. Magnetic field oscillations, tire friction changes and path distance changes (ie the front tire's path changes seeking the path of lowest resistance) etc, all add up to a harmonic that is revealed in the side-to-side motion you experience. It's like an unbridled horse without a rider, it just goes where it wants to. In the classic-unpowerd-bike-hands-off-scenario, your body is easily able to steer because your CG changes overide any other force that's currently acting on the front wheel. In your scenario, the motor is "stronger" than you, or put another way, it's influences on the front wheel's balance dynamcis have a mechnical advantage over your CG change transmitted through the frame to the front-wheel assembly. Therefore it's oscillating forces have a larger affect on the actual movement than yours and are allowed to find their own harmonic effectively overcoming the input you are attempting to give by changing your center of gravity. One way to reduce the problem is to use a bike frame with a greater rake. This gives it a slower response, but adds stability (IOW, it gives you a greater mechanical advantage by increasing the angle through which your CG changes are passed to the front-wheel and increasing the side contact patch size of the front tire to the ground).

ahend said:

With my brushless front hub motor locked into a constant speed, be it fast or slow, the front wheel will start an oscillation when both hands are removed from the handle bar. The torque creating this oscillation cannot be felt in the bar at any time during normal riding conditions. This happens almost immediately after I let go, and it quickly gets out of control with the wheel violently sweeping left and right equally.

I first noticed this when I sat upright for a breather (I pedal hard a good deal of the time) and placed my hands on my hips. I’m sure someone got a good laugh watching me pull in the reins, trying to recover from what would have been a nasty toss.

Though I do not consider this a problem that needs to be rectified, I am curious as to what may be the cause. Considering the front end of a bicycle was never designed to propel the frame and rider, could the frame and or fork geometry somehow be responsible? I should think that the fork rake would tend to make the system more stable when a force is applied at the dropouts. Another thing I’ve considered are my torque arms. Since my suspension fork (perhaps another contributor) is aluminum, I have made some serious torque arms (one on each side) with anti spreading measures, that lever force a good 5 inches up. Could they be batting reaction forces back and forth? Perhaps this is an inherent condition of a hub motor as tangent forces pulsate off the circumference of winding diameter to complete a cycle.

Any thoughts? Can anyone duplicate this phenomenon?

Click to expand...

My front hub bike does EXACTLY the same thing. It's caused by the hub motor.

Don't ride no hands.

If you fix it by playing with the Rake Angle (I can by adjusting the triple clamp forks) your handling will get heavier/slower.

Other than this my bike handles pretty damn well, so I just keep at least one hand where they should be!

Mark

ahend said:

... Considering the front end of a bicycle was never designed to propel the frame and rider, could the frame and or fork geometry somehow be responsible?

Click to expand...

Yes, I believe it is at least partially, if not completely responsible. The following statement is a clue:

ahend said:

I went out and rode the bike with no hands under pedal power only and there was no oscillation.

Click to expand...

Simplified...

Most single track vehicles incorporate positive 'Trail' for stability. 'Trail' is the distance between the front tire's contact patch, and point where the virtual steering axis meets the ground. If the contact patch is aft the virtual steering axis, then it is said to be 'positive trail' - as is the case with most bicycles. Problem is, positive 'trail' only works when the front wheel is being pushed (think grocery cart). If the front wheel is driven, then the tire's contact patch tries to 'pass' the virtual steering axis whenever the handle bars are turned slightly, OR, if the bike is leaned. Loose parts will undoubtedly exacerbate the situation, but simply cinching everything down won't, IMO, cure it. The fix will probably require reducing or eliminating 'trail' - an easy fix, assuming a rigid fork - you just bend each fork blade forward a little. The down-side to eliminating 'trail' completely, will be increased steering sensitivity and the loss of hands-free-riding (which doesn't pose a problem for me personally... because most of my bikes have little or no 'trail' anyway).

Unless you have cruise control...how do you ride no-hands under power?

Mark_A_W said:

Unless you have cruise control...how do you ride no-hands under power?

Click to expand...

This problem showed up soon after I built, and started using an electronic cruise control for my controller. Guess I will have to put it into coast mode before surrenduring control of the handle bar, not that I make a habit out of it.

Thanks for all the great feedback, it's good and thought provoking stuff.

Andy

That's all well and good, but from a practical experiment (1 test is better than 1000 theories )...

the only handling issue of my ~550w front wheel drive (all wheel drive if I pedal) is the no hands oscillations.

I can spin the front wheel in the wet, you have to be careful of that, but the bike handles pretty well overall.
Certainly better than all the weight at the back (wheelie time!).

All that stuff about steering issues in the above is a total load of bollucks.


I do plan on building a rear wheel drive e bike, but only if I can get the batteries in the frame area. Once you go over a kilowatt I think rear would be safer (assuming bike is balanced), but front works well enough.

The most important thing is balance, not FWD or RWD.

Mark_A_W said:

All that stuff about steering issues in the above is a total load of bollucks.

Click to expand...

IMO, it is probably safe to say that the issues Tony Foale raises, impacts lighter, slower, lower wattage vehicles to lesser degree. None-the-less, I believe his knowledge, theorie(s) and pratical experience possess relevance and genuine merit - many, many others certainly think so. You are, of course, free to accept, reject or believe whatever suits your fancy. However, I would encourage you to crack the cover on one of his *books before you ridicule.

* available at many libraries world wide.

Ok, Papa, your point is made.

My point is a front wheel drive bike exhibits none of the steering problems described (other than no-hands). I know because I built one, as did many others.