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Fixed gear skidding and pedal force
Hi All,
I have been riding my fixed gear bike exclusively for the past few months. I have a brake which is reserved for emergencies. I have not had any emergencies yet, but out of laziness I have used the brake a few times. Otherwise I just slow down by resisting the pedals. I don't live somewhere where I need to make short stops, so I don't "skip" or skid my wheel. Just for laughs I have tried, but I can't really do it. Even back in the days riding a track bike around in NYC, I never really could do it. So I am wondering about how my weight and gearing affect my (in)ability to skid. I can of course effect a skid by leaning all the way forward, but I am more concerend about stopping than seeing how far I can skid. Traction is a function of downward force and friction. Assuming normal road surface, with a weight of 100kg, a speed of 25km/h and a 48x18 gear on 165mm cranks, how much force is required at the pedals to stop the wheel? What difference would it be with say 70kg, 175mm, and 42x18? Tips on how to figure this out appreciated. I don't really care about the answer, I am just interested in the physics of it. Joseph |
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Fixed gear skidding and pedal force
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#4
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Fixed gear skidding and pedal force
Werehatrack wrote: On 19 Nov 2006 11:33:11 -0800, wrote: Tips on how to figure this out appreciated. I don't really care about the answer, I am just interested in the physics of it. Before wasting your time trying to figure it out, consider that for a conventional bicyle as well as most other wheeled vehicles, when braking force is applied and deceleration begins to occur, the downward vector shifts forward; there is "weight transfer" to the front wheel. If enough braking effort is applied to the rear wheel, there will be a point reached at which sufficient forward weight transfer will occur to reduce the traction of the rear wheel to the level at which it goes into a skid...and at that point, the weight transfer is partially reversed *but the braking effectiveness is still lost because the wheel is in a skid*. Thus, the place that braking needs to be able to occur for safety is in the front wheel, *no matter how effective the rear brake may be*. Rear braking capability is not the key to stopping safely in an emergency. Front braking capability rules the day in this. While the static coefficient of friction between the tire and the pavement is greater than the dynamic, the latter is not zero. Braking force is not decreased that much at the rear wheel due to skidding. The real issues are controlling the bike directionally and maintaining balance once the rear wheel starts to skid. -- Tom Sherman - Post Free or Die! |
#5
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Fixed gear skidding and pedal force
Werehatrack wrote: On 19 Nov 2006 11:33:11 -0800, wrote: Tips on how to figure this out appreciated. I don't really care about the answer, I am just interested in the physics of it. Before wasting your time trying to figure it out, consider that for a conventional bicyle as well as most other wheeled vehicles, when braking force is applied and deceleration begins to occur, the downward vector shifts forward; there is "weight transfer" to the front wheel. If enough braking effort is applied to the rear wheel, there will be a point reached at which sufficient forward weight transfer will occur to reduce the traction of the rear wheel to the level at which it goes into a skid...and at that point, the weight transfer is partially reversed *but the braking effectiveness is still lost because the wheel is in a skid*. Thus, the place that braking needs to be able to occur for safety is in the front wheel, *no matter how effective the rear brake may be*. Rear braking capability is not the key to stopping safely in an emergency. Front braking capability rules the day in this. I am quite aware of this weight transfer. In a thread called "Heat resistant tubular glue?" a while back I figured that riders have easily over 90% of their weight tranferred under heavy braking. But I don't know anything about the friction of the surface, and what sort of limitations this imposes. Front-wheel braking is more or less limited to about .5G due to weight transfer and the size and configuration of regular bikes. Much more than half a G and weight transfer goes over 100% and you flip over the bars. Rear braking is limited by traction. I wonder how close to .5G one can get with a back brake? This is all theoretical for me. I have a front brake on my bike. These are just things I ponder as I ride around with nothing more exciting than the occasional cow in the road to distract me. Joseph |
#6
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Fixed gear skidding and pedal force
wrote: On 19 Nov 2006 11:33:11 -0800, wrote: Hi All, I have been riding my fixed gear bike exclusively for the past few months. I have a brake which is reserved for emergencies. I have not had any emergencies yet, but out of laziness I have used the brake a few times. Otherwise I just slow down by resisting the pedals. I don't live somewhere where I need to make short stops, so I don't "skip" or skid my wheel. Just for laughs I have tried, but I can't really do it. Even back in the days riding a track bike around in NYC, I never really could do it. So I am wondering about how my weight and gearing affect my (in)ability to skid. I can of course effect a skid by leaning all the way forward, but I am more concerend about stopping than seeing how far I can skid. Traction is a function of downward force and friction. Assuming normal road surface, with a weight of 100kg, a speed of 25km/h and a 48x18 gear on 165mm cranks, how much force is required at the pedals to stop the wheel? What difference would it be with say 70kg, 175mm, and 42x18? Tips on how to figure this out appreciated. I don't really care about the answer, I am just interested in the physics of it. Joseph Dear Joseph, These might help you get started: http://www.glenbrook.k12.il.us/gbssc...energy/cs.html http://www.physicsforums.com/archive.../t-120420.html http://www.stoptech.com/tech_info/Th...%20Systems.pdf http://www.msgroup.org/forums/mtt/to...p?TOPIC_ID=172 Cheers, Carl Fogel Ok, so traction (friction) is linear. This means my 100kg stick to the road twice as much as a 50kg flyweight. As is evident by said flyweight dropping me in the hills, I cannot produce 2x the power and by extension 2x the force of said flyweight. A lighter person has therefor a greater chance of being able to lock a fixed rear wheel than a heavy person. Does that sound right? I think I could figure it out, but I don't know a friction coefficeint to use as a starting point. I assume under rear-wheel only locked braking, an equilibrium is reached where weight transfer balances friction. Does this sound right? Joseph |
#7
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Fixed gear skidding and pedal force
Johnny Sunset wrote: Werehatrack wrote: On 19 Nov 2006 11:33:11 -0800, wrote: Tips on how to figure this out appreciated. I don't really care about the answer, I am just interested in the physics of it. Before wasting your time trying to figure it out, consider that for a conventional bicyle as well as most other wheeled vehicles, when braking force is applied and deceleration begins to occur, the downward vector shifts forward; there is "weight transfer" to the front wheel. If enough braking effort is applied to the rear wheel, there will be a point reached at which sufficient forward weight transfer will occur to reduce the traction of the rear wheel to the level at which it goes into a skid...and at that point, the weight transfer is partially reversed *but the braking effectiveness is still lost because the wheel is in a skid*. Thus, the place that braking needs to be able to occur for safety is in the front wheel, *no matter how effective the rear brake may be*. Rear braking capability is not the key to stopping safely in an emergency. Front braking capability rules the day in this. While the static coefficient of friction between the tire and the pavement is greater than the dynamic, the latter is not zero. Braking force is not decreased that much at the rear wheel due to skidding. The real issues are controlling the bike directionally and maintaining balance once the rear wheel starts to skid. How much is it decreased, and how much was it to begin with? Joseph |
#8
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Fixed gear skidding and pedal force
On Sun, 19 Nov 2006 13:54:07 -0800, joseph.santaniello wrote:
Rear braking is limited by traction. I wonder how close to .5G one can get with a back brake? How high is your CoG? If you're not in lowrider recumbent territory then forget it. Mike |
#9
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Fixed gear skidding and pedal force
Mike Causer wrote: On Sun, 19 Nov 2006 13:54:07 -0800, joseph.santaniello wrote: Rear braking is limited by traction. I wonder how close to .5G one can get with a back brake? How high is your CoG? If you're not in lowrider recumbent territory then forget it. The correct term is "lowracer". Lowracers are for riding, "lowriders" are for show. -- Tom Sherman - Post Free or Die! |
#10
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Fixed gear skidding and pedal force
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