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#31
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if you wanted maximum braking, where would you sit?
Bill Putnam wrote:
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 locked up both front wheels on my recumbent tadpole trike. It should be noted that I have Avid mechanical disc brakes that are designed for use on wheels of about 27" (~68 cm) in diameter on wheels that are about 20" (~51 cm) in diameter. Tom Sherman - Quad Cities |
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#32
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if you wanted maximum braking, where would you sit?
You won't be able to get butt far back enough and still reach the bars on
a road bike to skid the front wheel (on clean dry road). True, but braking on a road bike isn't usually as nearly effective as on an MTB because you can't really pull big leverage while on the hoods. -- Phil, Squid-in-Training |
#33
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if you wanted maximum braking, where would you sit?
"S o r n i" wrote in message ... Cockpit of a 747? Bill "landing, that is" S. Interesting choice. Maximum braking forces occur during taxiing before takeoff and have been known to put a wrinkle in the forward fuselage crown skin panels. Braking forces on most commercial airplanes are limited by system design but generally this is the load case (compression buckling) that determines the skin gage of the upper panels. Phil Holman |
#34
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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? Rick "Sounds like fun!" Onanian No, it isn't the case on any of my trikes. Yes, it is fun. -- Ted Bennett Portland OR |
#35
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if you wanted maximum braking, where would you sit?
We skid our front wheels all the time on our icebikes ( studded
tires).. lock up the front wheel and continue to pedal.. the rear wheel has more than enough traction on ice to move the bike forward with the studs scratching the ice while being locked up.. But it takes practice keeping the front wheel straight... a little movement in the handlebars and down you go. Wheelies on ice is another way to put some excitement in your life. charlie |
#36
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if you wanted maximum braking, where would you sit?
Sheldon Brown wrote in message ...
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. I think you are correct because the OP misstated the problem. In reality however Tim M is correct. In practice, when we go over the bars it is not [usually] because the front wheel stops and the bike and rider pivot around the contact patch. More often we go over the bars because the frame and fork and rider and rear wheel pivot around the front axle. If you build a bike like Tim says, you are right that it could still endo around the contact patch. However, such a bike would not endo around the front axle. I have seen riders endo after dropping the front wheel in a pothole. Tim's bike would not do that. The rider would come off the seat in an even more painful way. Doug |
#37
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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. wle. The bikes two tires will have their maximum adhesion sum if weight unde braking is evenly distributed on front and rear tires. The more weigh on the front, the more front braking, the less rear braking and vic versa. The endo issue occurs when the moment caused by the decel of th bike+rider about the front contact patch exceeds the product of distanc the combination cg trails the front patch times the mass- a point a which the weight is entirely on the front tire. If selecting positio for maximum tire adhesion, you’ve already removed endo as an issue The front/rear weight distribution of the bike under constant speed i not the same as the weight distribution under deceleration. Unde decell, a moment is created which is the product of the height of th combined (bike+rider) cg above the road times the mass of th rider+bike. That moment will shift weight rear tire to front tire b the moment/(cg distance behind the front contact patch). The lower yo are, the less forward shift in weight under braking. The further bac you are, the more weight on the rear to start with and you ca distribute the weight more evenly under decel to a point. Beyond tha point, you have too much weight on the rear and the front tire i underutilized in braking The optimal point will vary on a given bike on level ground vs downhill (although your posted question specified level ground)l, an since the traction varies from surface to surface, the optimal poin will vary as well. In fact, the optimal position is not a constant on given bike as you have to factor in the aero drag of the rider at sa 30mph vs. 10 mph If you neglect the air drag, since it will be a minority component i the calculation, the optimal position for a max G decel will be continuous series of points along a path going back and upwards although lower and further back than typical seat positions I started to do some calculations assuming a bike decels perhaps twic as fast as a street car but calculated endos at much lower g’s tha that. So before proceeding further, does anyone know typical best dece rates on bikes in g’s? or times or distance from 20-0mph - |
#38
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if you wanted maximum braking, where would you sit?
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#39
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if you wanted maximum braking, where would you sit?
On the road I've skidded my front wheel in a panic stop in the following
postion... Right foot clipped in Left foot out front lower than the front axel No appreciable rear brake, the rear wheel about 1ft off the ground Front brake locked Rear of truck in front of me going from 45mph to zero, blue smoke as it starts to jack-knife Me encountering the rear liftgate at less than 1mph, no damage to me or the bike Clearly my CofG had most of my weight pressing straight down on the front wheel, my left foot lower than the axel provided additional downward force to counteract a total flipover. My skinny slick sew-ups provided the maximum friction and slip enabling me to skid, stop and not flip over. JeffP.... "wle" wrote in message om... 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. wle. |
#40
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if you wanted maximum braking, where would you sit?
RE/
We skid our front wheels all the time on our icebikes ( studded tires).. lock up the front wheel and continue to pedal.. the rear wheel has more than enough traction on ice to move the bike forward with the studs scratching the ice while being locked up.. But it takes practice keeping the front wheel straight. That sounds like good practice for surviving front wheel washout in regular riding. -- PeteCresswell |
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