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#21
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if you wanted maximum braking, where would you sit?
wle wrote:
if you could position your center of gravity anywhere, to ensure maximum braking power, where would it be? obviously over the front wheel is no good, you would flip. there is a point, leaning either ahead of the front wheel, or behind the back wheel, that the opposite wheel is off the ground. clearly those are 2 limits, the answer must lie between them. if there were very little friction, it would hardly matter. assume a level road, brakes that can cause a skid no matter what. ok, so where do you sit? state assumptions, like coefficient of friction between tire and road, weight of bike and rider. show your work. now for extra credit, make it a function of road slope. As you still have to hold on to the bars, throwing yourself back off the saddle is the best you can do. The back wheel still skids but the bike shouldn't cartwheel if you go back and low enough. Failing that, slide the bike sideways at 90 degrees to the direction of travel. This may hurt. |
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#22
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if you wanted maximum braking, where would you sit?
Sheldon Brown wrote in message If the angle is shallower than the critical angle, locking up the front
wheel will cause it to skid. This is the case for tandems and many recumbents. Have you ever skidded a front tire on clean dry pavement with a tandem? Or has anyone else? I have never been able to skid the front tire on my tandem on clean dry pavement despite practiing emergency stops to the point where I worried about the fork failing, though I haven't bent a fork or observed any obvious damage from this maneuver. I know Chalo's bent plenty of forks but I'd like to see him do that with my Santana. Bill Putnam, who on second thought wouldn't like to see Chalo bend the fork on his tandem even if he could. |
#23
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if you wanted maximum braking, where would you sit?
David Damerell wrote:
It is extremely unlikely that front brake will cause a skid no matter what. If tyre traction is that poor then that is evidently the limiting factor on braking, and the CoG may roam freely. I've done long 2-wheel skids on dry tarmac. A panic snatch of the brake will get it sliding, after which the friction is greatly reduced. Steering is *difficult*. |
#24
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if you wanted maximum braking, where would you sit?
On Wed, 07 Jan 2004 19:54:47 -0600, Tom Sherman
wrote: I would sit near or below the level of the front axle to reduce or elminate the tendency for the rear wheel to lift off of the ground, and I would also be near the rear wheel for more favorable weight distribution for braking. This describes a recumbent lowracer. Braking can be improved by adding a second wheel in front (tadpole trike) as weight transfers forward to two contact patches during braking, and locking one or both front wheels under braking will not lead to a loss of balance. I imagine that all of those conditions would result in fishtailing under very hard braking. Is that the case? Tom Sherman - Quad Cities -- Rick "Sounds like fun!" Onanian |
#25
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if you wanted maximum braking, where would you sit?
Zog The Undeniable wrote:
It is extremely unlikely that front brake will cause a skid no matter what. If tyre traction is that poor then that is evidently the limiting factor on braking, and the CoG may roam freely. I've done long 2-wheel skids on dry tarmac. Dry tarmac coated with dust or gravel? Otherwise I find it hard to believe. A panic snatch on my bikes will result in me going over the bars. ~PB |
#26
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if you wanted maximum braking, where would you sit?
Zog The Undeniable wrote:
David Damerell wrote: It is extremely unlikely that front brake will cause a skid no matter what. If tyre traction is that poor then that is evidently the limiting factor on braking, and the CoG may roam freely. I've done long 2-wheel skids on dry tarmac. With slick tyres? That seems highly unlikely. [Note the words "no matter what" copied directly from the original post, so I think I can specify tyre design.] If I panic snatch the front brake I get a free flying lesson. -- David Damerell Kill the tomato! |
#27
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if you wanted maximum braking, where would you sit?
RE/
ok, so where do you sit? I can't imagine sitting when braking as hard as I can. I'd be afraid of endo-ing. It's more like crouching with my butt somewhere between the back of the saddle and the rear axle. -- PeteCresswell |
#28
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if you wanted maximum braking, where would you sit?
Rick Onanian wrote:
On Wed, 07 Jan 2004 19:54:47 -0600, Tom Sherman wrote: I would sit near or below the level of the front axle to reduce or elminate the tendency for the rear wheel to lift off of the ground, and I would also be near the rear wheel for more favorable weight distribution for braking. This describes a recumbent lowracer. Braking can be improved by adding a second wheel in front (tadpole trike) as weight transfers forward to two contact patches during braking, and locking one or both front wheels under braking will not lead to a loss of balance. I imagine that all of those conditions would result in fishtailing under very hard braking. Is that the case? Fishtailing only happens on the recumbent lowracer [1] when the rear wheel is locked up, and this takes considerably more braking force than it does to lock up the rear wheel on an upright. My trike [2] has two front brakes (Avid mechanical disc on each wheel - one lever for each brake) and no rear brake. Stopping quickly takes as much skill as stopping an automobile without ABS and the brakes biased towards the front. Subjectively, the stopping power does not feel as great as it is, since the rider is naturally braced by his/her legs being on the pedals, unlike an upright bike where bracing is done with the (much weaker) arms. [1] This does not apply to all recumbent bikes. Some are poorly designed with a high seat and too much weight on the front wheel, and may have braking performance that is worse than an upright bicycle. [2] http://home.mindspring.com/~kb7mxu/i...ragonFlyer.jpg This is the same model as my trike. Tom Sherman - Quad Cities |
#29
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if you wanted maximum braking, where would you sit?
stu wrote:
That can be circumvented. Build a bike that positions your center of gravity below the front axle. You could lock the wheel up tight and still not do an endo. No. The important dimension is where the center of mass is in relation to the front wheel's contact patch. An endo is possible with the weight below the axle, although not likely because of insufficient traction. short version a locked front wheel does not undo you, you skid and fall of sideways. As an owner of a bicycle with a 12-inch (30.5 cm) seat height, I can verify this (at least on a dry, broom finish Portland cement concrete surface, which is certainly a high traction situation). Something beyond friction between the tire and the pavement surface (such as wedging the wheel in a storm drain grate) would have to occur to get the rear wheel off the ground. Tom Sherman - Quad Cities |
#30
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if you wanted maximum braking, where would you sit?
Sheldon Brown wrote:
Tim McNamara wrote: Under "maximum braking" the front wheel stops dead, the bicycle flips, and the rider is ejected. If you are talking about keeping both wheels on the ground that is far less braking force than maximum. That can be circumvented. Build a bike that positions your center of gravity below the front axle. You could lock the wheel up tight and still not do an endo. It actually has nothing to do with the axle, since a locked-up wheel doesn't rotate, so it effectively stops being a wheel. The critical thing is the angle of a line drawn from the tire contact patch to the center of mass of the bike-and-rider. If this angle is steeper than a critical value, locking up the front wheel will cause an endo. This is the case for typical upright bikes. If the angle is shallower than the critical angle, locking up the front wheel will cause it to skid. This is the case for tandems and many recumbents. The value of the critical angle depends on the coefficient of friction between the tire and the road surface. For vehicles where the angle is steep, the rear brake is useless at maximal braking. For vehicles where the angle is shallow, both brakes are needed to achieve maximal braking. Note that the _length_ of the line doesn't matter, only the angle. A good example of this would be an ordinary bicycle, which has a long line from the contact, patch to the combined rider/bicycle center of mass that is at a steep angle (from horizontal). It is the general consensus that these bikes are relatively easy to do "endos" on. Tom Sherman - Quad Cities |
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