speed wobble

[Evan Andreou]

Same descent, similar speed and no wobble today. Baffled

[TheLegend]

I have seen a report about motorbikes once where the speed wobbling was demonstrated. In German it's called "Schimming" and can happen with any motorbike so I guess also with bicycles (once wittnessed this of a friend on a MTB going fast downhill on the road, hands away from the handelbar).

The motorbike manufacturer actually tested a small device to reduce the schimming effect. Don't know how successful it became though.

 

If trying to fight it I'd start with the tyres. But wait till THE dude reads this, he'll sort you out - probably you can fight this with some home made paste increasing the friction of your seatpost

 

[Irishj9]

two words " cracked frame"

 

That was the cause of all my speed wobbles, which could be predicted always to start at 50kph.

 

 

 

[Johan Bornman]

The common garden variety speed wobble, or as motorcyclists call it, a "tank slapper" is present in every bicycle and rider combination. Every bicycle and rider developes a natural harmonic at a certain speed. Fortunately most combinations work out at way above what we can travel at - 70kph and above.

 

The wobble is caused by the bicycle oscilating in a long wave with the node at your saddle. In other words, the saddle remains rigid and in front the bike flaps around and behind the saddle it flaps around. You can reduce the speed your bike wobbles at by a) inducing the wobble by shaking - common in winter if you shiver and reach a highish speed and. b) by riding no hands and steadily accellerating downhill until you hit the wobble speed.

 

The latter is a good exercise for getting rid of the wobble. Ride downhill no-hands until the bike wobbles and then destroy the node by standing up and pinching the saddle's cheeks between your thighs. This takes a bit of courage at first, but you'll soon discover that it gives you tremendous confidence because it enables you to arrest the wobble at once.

 

Once you've mastered arresting the wobble at that lowish speed (48kph on my road bike), you can start looking for hands-on situations (i.e. lolng downhills) to test your new-found skills at the high-speed hands-on-the-bars wobble.

 

Suikerbosrand is ideal for this as it has long straight downhill stretches where you can easily reach the speeds required to train your anti-wobble strateties.

 

Other ways to get out of the wobble is to slow down and to change the bike rider combination. Neither of these are options in the normal run of things. Besides, the wobble is present at such a wide range of speeds, that slowing down dramatically is the only way to arrest it once it starts, a little change in speed wont do it once it has started.

 

As someone said, a loose headset can also induce it and as with no-hands riding, it merely brings it on sooner. Tightening the headset merely ups the wobble speed again, often outside your range.

 

Wobbles are extremely disconcerting and often put people off cycling for ever. It makes good sense to learn how to cope with it. Wobbles don't go away, you learn to cope with them.

 

[Johan Bornman]

Road courness and tyre rolling resistance can play a role if the bike is on the twitchy side. As you increase speed the frictional force pushing back on your bikes tyre' date=' oposite to the direction of speed increases. An increase in road roughness will also increase this force.

 

As the gradient increases the amount of mass on the front wheel also increases. The frictional force acting in the opposing direction to travel is also determined by the downward force on the wheel which has now increased.

 

If you put all these factors together it means that there is a greater force actinig on the wheel and thus a greater force acting on the fork causing it to flex backward more than if these factors where not there. Depending on the amount of flex, which is dependent on fork and frame stiffness, the bikes wheelbase shortens making the bike more "unstable". If it is already on the twitchy side, a characteristic of a short wheelbase, then yes, speed wobbles can occurr. But like I say, a well designed bike should take all these factors into account.

[/quote']

 

Are you making this up or have you been reading bicycling magazines again?

 

Road irregularities play NO role. Try it for yourself. Go no-hands and you'll see that irregularities don't induce a wobble.

 

There is no frictional force "pushing back on your bikes [sic] tyres" In fact, on an inflated bicycle tyre there is no friction between road and tyre at all, unless of course you're wheelspinning your back wheel.

 

Going downhill cannot increase the friction as there was none to begin with and the type of scenario you envisage doesn't exist.

 

Actually, now that I've read your comments on a shortening wheelbase due to flex, I know you're not making this up...you're halucinating.

 

And please don't cry about my tone, when you dish out fairy tales on the tech site you better have a flame suit.

 

[Guest Big H]

The common garden variety speed wobble' date=' or as motorcyclists call it, a "tank slapper" is present in every bicycle and rider combination. Every bicycle and rider developes a natural harmonic at a certain speed. Fortunately most combinations work out at way above what we can travel at - 70kph and above.

 

The wobble is caused by the bicycle oscilating in a long wave with the node at your saddle. In other words, the saddle remains rigid and in front the bike flaps around and behind the saddle it flaps around. You can reduce the speed your bike wobbles at by a) inducing the wobble by shaking - common in winter if you shiver and reach a highish speed and. b) by riding no hands and steadily accellerating downhill until you hit the wobble speed.

 

The latter is a good exercise for getting rid of the wobble. Ride downhill no-hands until the bike wobbles and then destroy the node by standing up and pinching the saddle's cheeks between your thighs. This takes a bit of courage at first, but you'll soon discover that it gives you tremendous confidence because it enables you to arrest the wobble at once.

 

Once you've mastered arresting the wobble at that lowish speed (48kph on my road bike), you can start looking for hands-on situations (i.e. lolng downhills) to test your new-found skills at the high-speed hands-on-the-bars wobble.

 

Suikerbosrand is ideal for this as it has long straight downhill stretches where you can easily reach the speeds required to train your anti-wobble strateties.

 

Other ways to get out of the wobble is to slow down and to change the bike rider combination. Neither of these are options in the normal run of things. Besides, the wobble is present at such a wide range of speeds, that slowing down dramatically is the only way to arrest it once it starts, a little change in speed wont do it once it has started.

 

As someone said, a loose headset can also induce it and as with no-hands riding, it merely brings it on sooner. Tightening the headset merely ups the wobble speed again, often outside your range.

 

Wobbles are extremely disconcerting and often put people off cycling for ever. It makes good sense to learn how to cope with it. Wobbles don't go away, you learn to cope with them.

 

[/quote']

 

What happens with Tandems..... is it double the effect??????

 

Could it also not be ascribed to wheels being unbalanced. It always happened to my Mother's old pre 1971 Toyota Corona?????? Sheesh daai ding het die hele pad volgespring hier by so 95km/huur!!!!!!

[Kobie]

Had the same experience down Ertjies berg as well as Kranspoort. The wobble continued till I came to a dead stop. The problem was that I raised the handle bars too high out of the steering tube, when I returned it to its original position, I never had the problem again.

[Johan Bornman]

The wobble continued till I came to a dead stop. 

 

Not likely. The speed wobble would have stopped long before you reached a dead stop. However' date=' a rush of adrenaline takes a while to subside and that made your mind wobble until you calmed down. It is not pleasant when a speed wobble suddenly pounces on you at high speed.

 

 

The problem was that I raised the handle bars too high out of the steering tube, when I returned it to its original position, I never had the problem again.

 

Yup, that would have done the trick and make the bike resonate at a different frequency. However, at the new height, the wobble was simply moved to another speed. Hopefully well out of your normal range.

 

 

[joevan]

I agree with you to some extent Johann, the explanation offered was a bit overboard.

Luckily I've never experienced this on a pedal cycle, but many years ago when I was a serious motorcyclist, experienced what in m otorcycling terms is commonly known as a tank slapper. This happened at 180km/hr, so I experienced a rapid aging process as well.

After many visits to the "experts" and a lot of experimentation, traced the problem to two areas:

 

1. Fork walk - the front forks were flexing at high speeds.

2. Irregularity in the front tyre.

 

After changing the tyre and bracing the front fork, never had this happen again.

 

The rolling mechanics of a two wheeled vehicle whether powered by an engine or by two feet must be pretty similar.

 

I'd start by looking at the same areas on a bicycle, -- forks, cracked or flexing, loose steering head, (In fact in the long gone days, many motorcycles were fitted with an adjustable head bolt that could be tightened up by hand if you wanted to go fast), and change the front tyre to see if this makes any difference.

joevan2008-02-21 14:32:45

[Johan Bornman]

I agree with you to some extent Johann' date=' the explanation offered was a bit overboard.

Luckily I've never experienced this on a pedal cycle, but many years ago when I was a serious motorcyclist, experienced what in m otorcycling terms is commonly known as a tank slapper. This happened at 180km/hr, so I experienced a rapid aging process as well.

After many visits to the "experts" and a lot of experimentation, traced the problem to two areas:

 

1. Fork walk - the front forks were flexing at high speeds.

2. Irregularity in the front tyre.

 

After changing the tyre and bracing the front fork, never had this happen again.

 

The rolling mechanics of a two wheeled vehicle whether powered by an engine or by two feet must be pretty similar.

 

I'd start by looking at the same areas on a bicycle, -- forks, cracked or flexing, loose steering head, (In fact in the long gone days, many motorcycles were fitted with an adjustable head bolt that could be tightened up by hand if you wanted to go fast), and change the front tyre to see if this makes any difference.

[/quote']

 

Well, either you concur or you don't. Which part of the explanation don't you agree with? From your explanation it seems as if you attribute speed wobbles to different mechanisms altogether.

 

How did the "experts" trace the problem on your motorcycle? How was the front tyre "irregular"?

 

Most motorcycles come with a damper on the steering, exactly for this reason. Whether the damper is a friction mechanism as in a bolt-down headset or a gas strut, makes no difference to the way it arrests speed wobbles.

 

To make a wobble go away you can change things around. Start with the particular rider. Your sister on your bike won't find the wobble at the same speed. Likewise, a different tyre or higher handlebar or lower seatpost or more lunch in your back pocket will put the wobble elsewhere. Point is, it is not faulty components but an unfortunate combination of components and rider.

 

 

 

 

[joevan]

 Sorry Johann, maybe I didn't make it clear enough.

 I concurred with your disagreement on the explanation from The Break regarding friction, frame shortening because of the speed etc.

Certainally at higher speeds, I would expect the "friction" on the front tyre to decrease, not increase, unless of course the rider is stiffenig up and putting all his weight on his arms and thus the front wheel. This supports your contention of rider involvement.

In my motorcycling case, we firstly found that the idiots who assembeled the bike had not tightened up the steering head, and that was pretty loose. That partly sorted out the problem. Next we fitted a brace between the two sides of the fork, matters improved further, but the "hint" of a potential wobble was still there all be it mild. It was one of the SA motorcycle circuit racers who raised the tyre issue. He went into a long and detailed explanation of tread patterns, direction of rotation, irregularities in the tread pattern etc.

We fitted a new tyre, and no more wobble!!!!!

Yes I also agree items carried on the bike which can affect the balnce of the rider, and centre of gravity can also play a role.

 

 

[joevan]

Oh and by the way, couldn't find any mechanics who were prepared to test the rider theory since it came on suddenly at 180km, and then remained there until until the speed had dropped to about 100km/hr.

 

Can't say I blame them. The tank slapper I had was so bad, that I was literally bouncing off the armco barriers on either side of a three lane freeway!!!!!!!!!!1

joevan2008-02-21 15:21:21

[The_Break]

Hi Johan

 

I think you need to study Newtons laws again and then reread my comments as well as your comments about my comments. You may just find yourself a little embarrassed.

[ewep]

 

 

Wat as die wobble agv selluliet is? Moet ek nog die top tube vasknyp?

 

OK, nou my vraag, HOE gaan ek dan die trappe draai as ek die hele tyd die top tube vasknyp?

 

[lowracer]

you might also check out the frame alignment.

 

but as JB said, all the parts(including rider) work together. change anything, and the problem most likely will disappear.

[Guest Big H]

Hi Johan

 

I think you need to study Newtons laws again and then reread my comments as well as your comments about my comments. You may just find yourself a little embarrassed.

 

This IS going to be interresting...... Boreman is now furiously looking for his Std 8 Science handbooks and WILL retaliate with a lenghty four page ode to the virtues of why Newton is wrong and Boreman IS right. ...... yawn..... popcorn time!!!!!!!!