One way to resolve the perceived "midget problem"
in [Racing]
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From: Brutus on 13 Oct 2007 22:01 "sturd" <mikesturdevant127(a)hotmail.com> wrote in message news:1192320650.214020.100560(a)q5g2000prf.googlegroups.com... > Bruce Hartweg says: > > > no, wrong lesson. drag will be factor in achieving top speed, and > > a minimal force in braking. > > That's wrong, as I've calculated and posted here months > ago. Aerodynamic drag force is greater than braking force > over some speed, Like a parachute slowing down a dragster at 330 mph? When I get out of the high speed tuck at, say160mph, the braking force created by the additional wind drag is almost negligible compared to using the front brake... depending on weight, speed, frontal > area, coefficient of drag, etc. Can you, or anyone else show the difference in drag coefficientcy between a 120lb rider and a 160lb rider on a motoGP bike. I doubt that it's even .35 verses .37...Another-words a very small differential clearly offset by the mass reduction (while braking) as Mark has pointed out. But the speed for this to > happen is well within the capabilities of even a 125. > > > Go fast. Take chances. > Mike S. >
From: sturd on 14 Oct 2007 20:44 Brutus says: > When I get out of the high speed tuck at, say160mph, the braking force created by the additional > wind drag is almost negligible compared to using the front brake... That's wrong. I'm not going to do these back of the envelope calcs again, if you've had a college physics class I'd love to see your analysis. Hope this pointer works: http://groups.google.com/group/rec.motorcycles.racing/browse_frm/thread/40235cb71b88a97f/2a89796b0a6a332a?lnk=gst&q=frontal#2a89796b0a6a332a > Can you, or anyone else show the difference in drag coefficientcy between a 120lb rider and a 160lb > rider on a motoGP bike. I doubt that it's even .35 verses .37...Another-words a very small > differential clearly offset by the mass reduction (while braking) as Mark has pointed out. The difference in coefficient is probably negligable. The difference in frontal area is probably not, based more on height than weight. I think. Mark N won't even go through the calcs and tell anybody where they are wrong. Will you? Go fast. Take chances. Mike S.
From: Bruce Hartweg on 14 Oct 2007 21:24 sturd wrote: > Brutus says: > > >> When I get out of the high speed tuck at, say160mph, the braking force created by the additional >> wind drag is almost negligible compared to using the front brake... > > That's wrong. I'm not going to do these back of the > envelope calcs again, if you've had a college physics > class I'd love to see your analysis. Hope this pointer > works: > > http://groups.google.com/group/rec.motorcycles.racing/browse_frm/thread/40235cb71b88a97f/2a89796b0a6a332a?lnk=gst&q=frontal#2a89796b0a6a332a > > >> Can you, or anyone else show the difference in drag coefficientcy between a 120lb rider and a 160lb >> rider on a motoGP bike. I doubt that it's even .35 verses .37...Another-words a very small >> differential clearly offset by the mass reduction (while braking) as Mark has pointed out. > > The difference in coefficient is probably negligable. The difference > in frontal > area is probably not, based more on height than weight. I think. > not much. go to the riders page on motogp Nicky & Dani sitting right next to each other. nicky maybe has an inch or so bigger silhouette but much of that is behind the bike - when upright, probably just friomt he logo up (fairly even) and the arms/shoulds (where Nicky is wider) so take the bike - the majority of the area - same for both, and the body (already a lesser percentage of total) and the differnec is maybe (pure guess) 10% (and that over mayber 30% total area). > Mark N won't even go through the calcs and tell anybody where they > are wrong. Will you? > skip the calcs for a minute and think! when you are stopping hard, all the wight transfer is forward - your hands and arms *pushing* on the handle bars to keep from putting your face through the windscreen. the most force is on the front wheel and the back wheel gets very light and even off the ground in extreme case (either intentionally or not). if the riders aero differences were the factor, that wind resistance is pushing back on the rider head/chest the force to slow the bike would be pulling back on the bars, s so the front of the bike would rise not dive. have you ever seen/experienced a wheelie while stopping? didn't think so. Still not convinced? OK - try this for absolute proof - no equations to argue relative weighting factors, no guessing about what the actual drag for each rider is, etc. - a pure experiment that will answer this. get on your bike, go out to some isolated area with a fairly long straight, mark a fixed point as your brake marker. then get going to say 125 mph when you get to the marker brake and determine how long/far it takes to get you speed down to 60mph. do 3 runs as follows: A) aero only - at the marker - popup and make yourself as big as possible, do NOT apply brakes (and keep the clutch in so there is no engine braking) B) brakes only - stay as tucked as possible - but brake (front/rear/engine) as hard as possible. C) both - stop like normal, hard braking and sitting up. I bet you will find B far far shorter than A and the C is best but not that much off of B. Bruce
From: Mark N on 14 Oct 2007 23:28 Bruce Hartweg wrote: > sturd wrote: >> Mark N won't even go through the calcs and tell anybody where they >> are wrong. Will you? > > skip the calcs for a minute and think! > > when you are stopping hard, all the wight transfer is forward - > your hands and arms *pushing* on the handle bars to keep from > putting your face through the windscreen. the most force is > on the front wheel and the back wheel gets very light and even > off the ground in extreme case (either intentionally or not). > if the riders aero differences were the factor, that wind resistance > is pushing back on the rider head/chest the force to slow the > bike would be pulling back on the bars, s so the front of the bike > would rise not dive. have you ever seen/experienced a wheelie > while stopping? didn't think so. > > Still not convinced? OK - try this for absolute proof - no equations > to argue relative weighting factors, no guessing about what the actual > drag for each rider is, etc. - a pure experiment that will answer this. > > > get on your bike, go out to some isolated area with a fairly long straight, > mark a fixed point as your brake marker. then get going to say 125 mph > when you get to the marker brake and determine how long/far it takes > to get you speed down to 60mph. do 3 runs as follows: > > A) aero only - at the marker - popup and make yourself as big as possible, > do NOT apply brakes (and keep the clutch in so there is no engine > braking) > > B) brakes only - stay as tucked as possible - but brake (front/rear/engine) > as hard as possible. > > C) both - stop like normal, hard braking and sitting up. > > > I bet you will find B far far shorter than A and the C is best but not > that much off of B. Try as you might, I don't think you'll get anywhere with Mike using real-world evidence, he's too buried in his physics text to look up and think. In his linked post he said this: "The heavier guy also has a bit more braking power, assuming braking is limited by the rear tire coming off the ground. I have no clue how much braking force is available." To me there's where the issue lies. Straight line braking is limited where the rear end is unweighted, maybe off the ground but barely. The increased weight on the front wheel might give the heavier guy an edge in terms of traction, but he needs it because he's also trying to stop more weight. And that added weight is up high, so the lever effect on the bike is greater, lifting the rear easier. If the lighter rider can use a stickier tire because of reduced wear from the lighter weight, he can also apply that to braking. In the end, it seems like that larger rider is at a disadvantage, if perhaps a small one, unless the smaller rider doesn't have the strength to apply his advantage to its maximum. That seems very unlikely in a straight line, but possible, I suppose.
From: Brutus on 14 Oct 2007 23:29
"sturd" <mikesturdevant127(a)hotmail.com> wrote in message news:1192409093.125127.58800(a)k35g2000prh.googlegroups.com... > Brutus says: > > > > When I get out of the high speed tuck at, say160mph, the braking force created by the additional > > wind drag is almost negligible compared to using the front brake... > > That's wrong. I'm not going to do these back of the > envelope calcs again, if you've had a college physics > class I'd love to see your analysis. Hope this pointer > works: > > http://groups.google.com/group/rec.motorcycles.racing/browse_frm/thread/40235cb71b88a97f/2a89796b0a6a332a?lnk=gst&q=frontal#2a89796b0a6a332a Mike, there's a miscalulation in the application of your formula. Fdrag = 1/2 rho * v^2 * Cd * A where rho - air density, around 0.073 lbm/ft^3 v - velocity, let's go from 200mph to 60 Cd - drag coefficient, assume it's around 1 and the same for a big guy and a little guy as shape of the system is only a little different The drag coefficient is NOT 1, I'm sure it's under .4 A - area. If the guy makes the system heavier by 7%, let's for the sake of arguement say he makes the system frontal area bigger by 5% when he is trying to be big and stop In your equation you're not displaying a figure for the area??? Big guy Fdrag (200 mph) = 3287 lbf Fdrag (60 mph) = 296 lbf small guy Fdrag(200 mph) = 3130 lbf Fdrag(60 mph) = 282 lbf You are also not taking into account the effect of the reduced mass (rider weight) while applying the brakes.That figure is, of course, not reflected in your formula. > > Can you, or anyone else show the difference in drag coefficientcy between a 120lb rider and a 160lb > > rider on a motoGP bike. I doubt that it's even .35 verses .37...Another-words a very small > > differential clearly offset by the mass reduction (while braking) as Mark has pointed out. > > The difference in coefficient is probably negligable. The difference > in frontal > area is probably not, based more on height than weight. I think. > > Mark N won't even go through the calcs and tell anybody where they > are wrong. Will you? Yup :) > Go fast. Take chances. > Mike S. > |