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09/27/2012

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Geoff

Centrifugal force of the rotating pedals and wheels pushing the bike down and making it feel more stable and in control.

Similar thing to my motorcycle.. at 120kmh (70mph)my bike does not get buffeted by the wind enough to make it move about, at 100kmh(60mph) I get blown all over the road. The centrifugal force of the wheels spinning pushes down on the road. IIRC the heavier the wheel, and the larger, the more the effect as well.

Anyway, that is what I was told when I asked about the same thing - my bike feels more stable when the pedals are rotating even if force is not being applied to the drive, centrifugal force is being applied to the whole bike (so perhaps, more effect with a 175 crank than a 170?)

fiera

What is most frustrating is the signs they unfold for road work and stick them right in the middle of the sidewalk. This will be for a quarter mile stretch where there is no room for a bike on the street. I always wonder what someone in a wheelchair or power chair would do. There is no way a wheel chair could get by.

Ross

Basically what Geoff said. The motion of your cranks spinning and your legs moving up and down linearly has the same effect as your wheels spinning. It's also why riding your bike with no hands is easier the faster you go. The the force of your wheel spinning is projecting out from the center of your hub. Imagine your spokes are arrows pointing out forming a vertical plane. Your cranks and legs are also forming these planes of force. The more "vertical force planes" you have, the easier it is to stay upright. Stop pedaling, and you take away 3 out of 5.

Wil

Geoff and Ross, I hate to disagree but there really isn't any force called centrifugal force. What people call centrifugal force isn't a force at all but an effect of all points on the wheel attempting to travel in a straight line (radial acceleration). However, some of the force we do feel that keeps us upright more easily when going faster is gyroscopic force(this is more true with motorcycle wheels that have more mass). Gyroscopic force has limited affect on the stability directly (of a bicycle) but does provide varying levels of feedback to the rider so that they can adjust (w/o reading Mike's link, this is probably what is being referred to as microsteer). Therefore, the faster you go, the more gyroscopic force, the easier it is for the rider to gather feedback and correct. You can try this riding no hands slowly then quickly.

John h

This is really a very fascinating topic but it's beginning to hurt my head, thinking about all the physics involved. All I know is gravity sucks and no matter how fast I pedal my spinning trainer, I think it will still fall over it I take the legs off of it. Any body want to post a video proving me wrong and I will reconsider my opinion. =)

Doug

Mike Freiberger has the right answer here. Note that rotating wheels aren't needed for a bicycle to stay upright at all -- people have made bicycle-like-devices with skis rather than wheels and they could still stay upright (with a rider) just like a bicycle would just fine.

(They've also made bicycles where they have two more wheels that exactly counteract the gyroscopic forces of the main wheels and they stayed upright too, but that's not really as convincing as the ski bike as people still see wheels.)

Side note: claiming that "there really isn't any force called centrifugal force" is silly. While the statement may be technically accurate, if one is in the rotating frame of reference the force certainly *seems* real, and in general one can do calculations and such with it as if it were real. In most cases, making a big deal out of the distinction is a pedantic thing to do.

Tom Wald

Where is the path that you show in the photo? Our city is considering a path that looks like the one you've posted. However, some local bike advocate leaders think it wouldn't be worth it to build a physically protected path next to a freeway, since there are other parallel routes where kids can share the lane with cars going 35+ mph (no, I don't get their reasoning, but important people respect their opinions).

Wil

Doug, I hope everyone else caught the irony.

Jonathan

I endorse this. I pay attention also to the riders ahead of me, and if they stop pedaling, I assume they are going to slow down and I should begin preparing for that.

The converse means that I often continue to pedal, even without applying force, in order to let riders around me know that I am NOT planning to stop.

Matt

Tom,

The bike path follows Storrow Drive on the south side of the Charles River. There is another path on the north side of the river on Memorial Drive. Waterways always seem to be a good draw for pedestrians and non-motorized traffic and my guess is that these paths have actually been here long before Texas was a state =)

The speed limit for traffic, I think, is 50. The roads handle a lot of traffic, but they are not high speed freeways.

Matt

Ross

Caster (http://en.wikipedia.org/wiki/Caster_angle) is more a factor than gyroscopic force. But the faster you go and the heavier the wheel, the greater the gyroscopic force. As i explained before, the movement of your legs up and down, and your cranks rotating, do have a gyroscopic effect.

http://science.howstuffworks.com/gyroscope1.htm


http://sheldonbrown.com/brandt/gyro.html

Adam

Matt, I know this path and commute on it when I'm working in Allston. It really does get narrow right there and I think I'm guilty of exactly what you mentioned. I'll keep it in mind the next time I'm down there!

Gary Laugen Jr.

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