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Old 02-28-08, 11:08 AM   #1
makeinu
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small wheels and "bump steer"

I think it's generally agreed that, for a properly designed bike, smaller wheels do not necessarily imply less comfort or poor handling. Unfortunately, it seems that it's often the case that smaller wheeled bikes aren't very carefully designed, often leading to poor handling and uncomfortable riding. However, what about the tendency for large bumps, potholes, etc to unexpectedly steer the bike. Does anyone think that this "bump steer" phenomenon is increased with smaller wheels?

I personally am a bit undecided. While, on the one hand, I feel that my 20" Downtube definitely experiences more bump steer than my 26" junker, on the other hand, I often get the impression that the 8" wheels of my Carryme stick to the ground, almost impervious to bumps, like suction cups. I suppose one way to rationalize my feelings is to attribute the sensitivity to bump steer of the Downtube to less stable geometry and consider smaller wheels as actually being superior in terms of bump steer due to the larger percentage of the wheel being in contact with the ground (so hitting a bump does not change the contact point much). On the other hand, perhaps I only perceive the Carryme as being more stable over bumps because I don't ride it as fast as the Downtube.

So I ask, what are the factors which affect handling over bumps? Does suspension make any difference (whether from a distinct suspension or simply pneumatic tire pressure) or does that only affect comfort? Would wider handlebars make a big difference? Could geometry considerations like trail really have an affect despite the fact that trail is relative to the contact patch (which is a moving target when bouncing over large bumps)? What about higher/lower center of mass? Although all ideas are welcome, has anyone actually studied this?

Last edited by makeinu; 02-28-08 at 12:04 PM.
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Old 02-28-08, 11:36 AM   #2
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No formal studies here but just thousands upon thousands of miles of riding on a wide variety of bikes, among them, my Phillip's Twenty.

It would seem that whenever there is a discussion on the deficiences of folders that the venerable Twenty and bikes like the Brompton receive very few complaints unless we talk about weights

Part of this must stem from both bikes being well designed and this probably has the most to do with my 20's lack of twitchiness and amazing stability on he road... I run 20 by 2 tyres at moderate pressure which helps the bike absorb shock and keep it from being skippy.

Running my tyres at higher pressure does have a negative effect on the bike's ride and handling characteristics just as it effects my full sized bicycles.
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Old 02-28-08, 12:20 PM   #3
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No formal studies here but just thousands upon thousands of miles of riding on a wide variety of bikes, among them, my Phillip's Twenty.

It would seem that whenever there is a discussion on the deficiences of folders that the venerable Twenty and bikes like the Brompton receive very few complaints unless we talk about weights

Part of this must stem from both bikes being well designed and this probably has the most to do with my 20's lack of twitchiness and amazing stability on he road... I run 20 by 2 tyres at moderate pressure which helps the bike absorb shock and keep it from being skippy.

Running my tyres at higher pressure does have a negative effect on the bike's ride and handling characteristics just as it effects my full sized bicycles.
A lot's been said about twitchiness and stability on the road, but what does that have to do with a bike bucking left or right when hitting large potholes? I'm talking about situations where the wheels leave the ground. Are you saying that you feel that frame geometry and/or wide tires can help keep the bike straight when being launched into the air? If so how?
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Old 02-28-08, 05:51 PM   #4
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I'm afraid I'm ignorant on the issue. I thought bump steer was the effect that you get while riding with one hand on the bars and hitting a bump. Since the front of the bike moves up but your upper body does not due to inertia, the effect is to lean forward wrt the bars, pushing the end you're holding forwards, resulting in unintended steering movement.

I suppose the same might happen while riding both hands if the geometry is imperfect, such as handlebars not aligned or if the front wheel contact patch is not exactly in line with the steering axis.

Bump steer in cars is a commonly used tech term, arising from steering geometry issues.
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Old 02-29-08, 08:22 AM   #5
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I'm afraid I'm ignorant on the issue. I thought bump steer was the effect that you get while riding with one hand on the bars and hitting a bump. Since the front of the bike moves up but your upper body does not due to inertia, the effect is to lean forward wrt the bars, pushing the end you're holding forwards, resulting in unintended steering movement.

I suppose the same might happen while riding both hands if the geometry is imperfect, such as handlebars not aligned or if the front wheel contact patch is not exactly in line with the steering axis.

Bump steer in cars is a commonly used tech term, arising from steering geometry issues.
On cars the term "bump steer" is related for the tendency of the suspension to turn the wheels as the wheels move vertically. Here I'm talking about the tendency of bumps to turn the steering by whatever means (whether it be suspension geometry, or even just angle of impact). Please forgive my sloppy terminology.

What I'm trying to get at here is that when most people talk about a bike having "stable" or "solid" handling they usually seem to be referring to its resistance to leaving a straight path due to input at the handlebars. While I understand how classical engineering analysis attributes the importance of this measure to maintaining control of the vehicle in the face of nonidealities, it's not clear to me that stability on the road necessarily relates to stability over bumps which result in losing contact with the ground. While the "twitchiness" of smaller wheels can be corrected by adjusting the steering axis relative to the contact patch, I'm skeptical as to whether such adjustments can make any difference in scenarios where the contact patch ceases to exist.

In any case, I'm not particularly happy with the way my bike handles over large bumps and I'm trying to understand what I should be looking for if I wanted a bike which handled better over large bumps.

Last edited by makeinu; 02-29-08 at 08:39 AM.
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Old 02-29-08, 04:34 PM   #6
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I have been thinking about it for a while.

If you have fat tyres, then when hitting a bump edge-on, the edge of the tyre contacts first so this imparts a steering torque.

It would be very interesting to mount a skinny on the front and see how the behaviour changes.
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Old 02-29-08, 04:45 PM   #7
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Factor in that afatter tyre rinning lower pressure will absorb a lot more shock than a skinny tire at high pressure... the skinny tyre will also contact the bump edge first and impart more shock as well as some steering torque.

There is a reason why mountain bikers don't use skinny tyres and why they often mount a wider tyre up front.

When you ride on the road or commute it is always about finding that tyre that gives you best balance of ride, handling, and speed and when you improve on one area, you often lose an another.
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Old 02-29-08, 07:13 PM   #8
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The frictional resistance to change of direction is a stabilizing factor that will vary with tire pressure and tire diameter

It will increase with lower tire pressure (wider tire) due to the larger contact patch

It will also increase with larger wheel diameter due to elongation of the contact patch

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