Bicycle Mechanics - The shakes........

Help. What would cause a bike to start shaking (has happened on new, old and many) when coasting at least 12mph, riding with no hands, sitting up straight. Shaking to a point where I must grab the handlebars or shake out of control. Appling pressure to the side of the top tube with a knee helps.
I have been told everything from geometry, size, seat too high, etc

Any help, ideas, thoughts or solutions greatly appreciated.

Bob

coast at or less than 11.9mph to avoid resonance issues?

Bob, There is a gyroscopic event that happens around this speed. I don't remember the correct name, but it is during the transition range from steering normally to using countersteer. The design of the bike, very roughly say a race frame vs. a touring frame, headset tightness, tires and possibly anything else will determine the severity of the unstability and at what speed it happens.

Brad

PS With motorcycles a second transition happens at a much higher speed, over 100 MPH.

It's a clue that bikes are meant to be ridden with hands on the bars and weight on the front wheel. So when you do ride hands off rest one knee on the top tube.

It's caused by harmonic resonance. Schwinn did a big study in the 60's to study the cause of this. It's a combination of the bike's geometry/size, tubing thickness and load. I have a 24" Schwinn Super Sport, it has a big issue with HR, the 26" frame is much better. The 26" frame has a larger diameter top tube, that may be the big difference for this particular case. Most famous case of harmonic resonace is that bridge in Washington, the black and white video of the bridge bucking and swaying in the wind.

Its called shimmy, and Dave Moulton has a very good article (http://davesbikeblog.squarespace.com/blog/2006/8/18/shimmy-re-visited.html) about it.

Basicaly as the front wheel rolls forward, physics makes it also want to move side to side at certain speeds. The rear wheel acts as a pivot, causing the whole bike to move side to side.

The more weight there is on the rear wheel, the stronger it acts as a pivot. Which is why the shimmy is worse when you ride with no hands and most of your wieght is on the rear wheel.

Some bikes get it at 12, some bikes get it at 30.

http://www.sheldonbrown.com/brandt/shimmy.html

Also check your headset for proper adjustment. I know that loose steering head bearings in motorcylces can also cause this shimmying at certain speeds.

Chombi

Since we do not balance our wheels with bicycles they are all out of balance. When riding with no hands there is nothing to damp out the resonance that occurs at certain speeds where the out of balance condition and the mass of the wheel reach this resonant point. The cure is simple. Put your darned hands back on the bars. Your hands on the bars produces a damping effect where the energy of the resonance is absorbed by your hands and forearms and the bike doesn't shimmy anymore.

THis happens regardless of loose, tight or perfectly set up headsets and wheel bearings but having loose or tight headsets or loose wheel bearings will certainly make the effect more noticable.

On motorcycles where the wheels are balanced we can ride hands off at almost any speed. But the weight of the motorcycle is so much more than the weight of the rider that it's near impossible to steer by bumping our butt around so not many riders will do this other than some stunt riders that have worked hard to master riding that way.

Since we do not balance our wheels with bicycles they are all out of balance. When riding with no hands there is nothing to damp out the resonance that occurs at certain speeds where the out of balance condition and the mass of the wheel reach this resonant point. The cure is simple. Put your darned hands back on the bars. Your hands on the bars produces a damping effect where the energy of the resonance is absorbed by your hands and forearms and the bike doesn't shimmy anymore.

THis happens regardless of loose, tight or perfectly set up headsets and wheel bearings but having loose or tight headsets or loose wheel bearings will certainly make the effect more noticable.

On motorcycles where the wheels are balanced we can ride hands off at almost any speed. But the weight of the motorcycle is so much more than the weight of the rider that it's near impossible to steer by bumping our butt around so not many riders will do this other than some stunt riders that have worked hard to master riding that way.
I dunno, but I think wheel balance on bicycles have very little effect on bike stability. I never had a wheel balanced all the bikes that I've owned and I know how they all are heavy at certain sections, specially where the seam is on the rim where there are bridged together with a piece of metal. This never affected the ability of my bikes to be ridden without hands, ever. The only time I detected a problem was when the headset on some of the bikes got loose or one of the wheels might have been mounted a bit crooked on the dropouts., then I could not ride the bike hands-free. Heck, even the old English bike we had as kids with ancient looking steel rims rode beautifully without hands on the bars, and there was no way those wheels were balanced. The only time I suspect that it could be a factor is maybe at much higher speeds than a "regular" cyclist might encounter (70+ MPH going downhill?).
JMOs

Chombi

Bob, There is a gyroscopic event that happens around this speed. I don't remember the correct name, but it is during the transition range from steering normally to using countersteer. The design of the bike, very roughly say a race frame vs. a touring frame, headset tightness, tires and possibly anything else will determine the severity of the unstability and at what speed it happens.

Brad

PS With motorcycles a second transition happens at a much higher speed, over 100 MPH.

Not to try to start arguments but this major misunderstanding of how to steer two wheel vehicles is a pet peeve of mine.

We steer two wheelers by counter steering at all speeds. And every turn has three parts. First there is the pressure on the bars away from the direction we want to go to make us fall into the turn. Then we turn the bars into the turn to stop us from falling into the lean and establish the balance point for the turn lean angle. And finally we recover back to upright by turning the bars further into the turn to steer the bike back in under our body. This works at all speeds There's no magical cross over points where normal steering is used and counter steering will dump you on your head. Instead what does occur is that as the speed drops we need to use more inward angle of the bars to catch the balance point than we require at higher speeds and lean angles. But all turns on a bicycle have those three occurances. The push to the outside to initiate the lean into the turn, the balancing at the new angle in the turn and finally turning into the turn to lift the bike back up.

As for the 100 mph second transition I again have to wonder who told you that bit. As a motorcycle racer for about a decade now I can assure you that there is no second effect at 100 mph or anywhere near it. The only thing that happens is that as the speed climbs the steering gets extremely heavy thanks to motorcycles using such heavy wheels which make great gyroscopes.

Anyhow, I"m sorry to jump on your post so heavily but like I said the whole steering thing is a big pet peeve of mine. Too many motorcyclists have either dropped their bikes in parking lot prat falls or have lost their lives because they didn't understand how to steer a two wheel single track vehicle properly and tried to steer "normally" when they should have been using the countersteering.

Need more proof? As you come up to a stop and you're barely crawling set your left foot out for the stop and push hard on the left hand grip to point the wheel strongly to the right. The bike will firmly lean over to the left. So... you turned the bars to the right and the bike fell to the left. If that isn't counter steering then I'll each my riding jacket.

The whole NAME of counter steering is confusing. Locally the motorcycle schools are now calling it "push" steering. It's far easier to understand for new riders and it works well. You want the bike to go right? Push on the right grip. To go left push on the left grip. This works at all speed and lean angles. You're in a right turn and want to straighten up? That means you want to lean the bike "up" to the left so push on the left grip. You're in a right turn and need to tighten the line? Then push on the right grip long enough to increase the lean angle and tighten the line.

Obviously on a bicycle the idea of "push" is going to be measured in oz instead of pounds and the amout of angle will be small. But this method works and is easy to understand.

OK, I'll get down off my soap box now.

The idea of countersteering to initiate a turn is probably very hard to convince somebody who doesn't realize they do it innately.

Some people often mistake so called "body steering" (which does not really work) to countersteering. the sensation of countersteering with a two wheeled vehicle like a motorcycle is you "dive" or lean into a turn, when you are actually pushing on the bar (specially on race bicycles and motorcycles where you have some weight over the bars and the front end) that is at the direction you are turning....thus the countersteering effect. It's hard cause even though it's intuitive on a bike at speed, many people could not connect it with what they picture when you're turning the bar the opposite of the direction you are going.

Chombi

What happened to the OP's question? LOL
Anyone who doesn't think you need to push to counter steer try do about 10-15 mph and take your right hand off bar and try making a 90 deg. right turn in a fairly sharp radius and see what it fills like, come back and let us know how it felt. :)

And OP I have never had a bike or motorcycle that has speed wobble issues so can't help.
My bike I have no trouble removing hands even at speeds over 35 mph so I can't help you with that.

BCRider, The second transition is in the 140 MPH (225 KPH) range for a ZX10. It's somewhere above 145 MPH for my 955i Daytona as I haven't observed it yet.

At a walking pace if you turn left using just the handlebars, you steer left. Depending on where the transition occurs for a specific bike, at a bit faster if you initiate a turn with just the bars to the right, one pushes on the right bar.

At almost any speed over a walking pace a bicycle's turn is normally initiated by leaning. I understand your reply and I'm not offended, no apologys are in order. Sadly counter, or push steering when not understood has cost lives. It was one of the major points when I taught my sons how to ride a motorcycle and I used a bicycle to demonstrate.

Brad

I've had my old ZX9R up over 250 kph a few times at track days and the rare time on roads where it was safe to do so. I'm not sure what you mean by a second transition but if you mean that the bike suddenly starts to steer differently then it would be due to aerodynamics of the air over and around the fairing overpowering the bike's normal steering action and certainly not due to the manner in how two wheel vehicles steer.

Here's a good video that shows low speed push steering in action.
http://www.youtube.com/watch?v=C848R9xWrjc

If you can still claim that the bike steers "normal" at low speeds after watching this then you've closed your mind. Besides. If there is some magical cross over speed where it suddenly flips then tell me why folks don't fall down when accelerating into a right or left turn from a stop. If there was some magical transition point there would be a very noticable change in how you have to steer as you cross that speed. But that just doesn't happen. And what about my challenge to put out your left foot and push on the left bar just as you are rolling to a stop? If our bike and bicycles steered "normally" at low speeds pushing on the left bar SHOULD make the bike turn to the right. But trust me, I said to put your foot out so it would be ready to catch the bike when the push on the left bar firmly tips the bike to the left even at less than a walking pace. Don't take my word for it, go try it.

As the others have suggested you ARE push steering even at low speeds. But how you move your body can mask it to your senses. But sit still on the bike and only move your hands and you'll soon realize that you are still push steering or counter steering. Take your pick on what to call it.

For me the light dawned in a big way when I was learning to ride a trials motorcycle. To get the bike to turn through the extreme corners I found myself pulling the bars around almost to full lock to get the turn started and then ramming them over the other way to catch the tip and balance through the turn. Similarly at those speeds to lift the bike back around to straight took a very coarse movement of the bars into the turn to lift it back up.

BCRider, The second transition is in the 140 MPH (225 KPH) range for a ZX10. It's somewhere above 145 MPH for my 955i Daytona as I haven't observed it yet.

At a walking pace if you turn left using just the handlebars, you steer left. Depending on where the transition occurs for a specific bike, at a bit faster if you initiate a turn with just the bars to the right, one pushes on the right bar.

At almost any speed over a walking pace a bicycle's turn is normally initiated by leaning. I understand your reply and I'm not offended, no apologys are in order. Sadly counter, or push steering when not understood has cost lives. It was one of the major points when I taught my sons how to ride a motorcycle and I used a bicycle to demonstrate.

BradThink about it in terms of Newton. You cannot lean unless there's a force to push your weight in that direction. The force is your tire pulling your bike out from under you (thanks to countersteering) to initiate the lean, which you then steer into to maintain balance while completing your turn.

Countersteering is inherent in steering any 2wheel vehicle whether you perceive it or not.

What happened to the OP's question?


Poof! Gone...

In reading my last post over again I realize that it's got far too much of "my way or the highway" for the tone of it. If it were me reading it I'd find it disrespectful. For the rather blatantly confrontational manner in which I wrote it I apoligise.

However the ideas and the urging to try my examples to help anyone prove that this works for yourselves is still very much encouraged.

If I seem a bit overboard about this I was talking to one younger 20 something one time while we set the sag height of his sportbike for his weight. As we talked it became apparent that he thought counter or push steering was something to be used only occasionally and that he could lean steer a bike with the best of them. Not two days later I learned that he smeared himself over a cliff on our Sea to Sky highway when he ran wide off a turn and into the cliff. Fellow riders said that he wasn't even going all that fast for the turn. To this day I can't help but think that he was fighting with his steering while his mind argued over which technique to use. That is just one of the factors that makes me pull out my soapbox when this topic comes up. But it's likely one of the more influential ones.

Anyhow again I apoligise if my tone in all this has come across as too harsh due to my enthusiasm.

OK so as the "OP" I will sum it up as such.....very thought provoking and learned some new physic and gravitational steering theories. I really do appreciate all the entries and I will keep either my hands on the bars or knees on the frame as I ponder the resonating values of countersteering, push and looking forward to the second transition as I pedal like hell to get it up over 100 mph.........

Thanks to all