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#11 Re: Dong Fang DF 250 RTB Bobber Information
07-11-2014, 06:03 PM
got sort of tired of the "dont seem to realize arguments" everyone on the internet has all sorts of opinions and ideas, so i thought i would demonstrate the theory about all this.
Total power required
P(total) = P(rolling resistance) + P(wind) + P(gravity) + P(acceleration)
First Pg, gravity is eliminated as we don't care about descending a hill as I mention even a rock can hit terminal velocity, and of course these speeds are not attainable climbing, so don't even try it...
Secondly Pa, acceleration also drops for its not really a question about how long it will take dan to reach these speeds, only that it can be reached....
So then we are left with:
1 Pr Rolling resistance.
The power required to overcome rolling resistance can be described by the formula P = Crr x N x v, where
P is the power required.
Crr is the rolling resistance coefficient. define this based on the type of bike (road, mtb, cross) you used.
N is the normal force of the bike and the driver against gravity.
v is the rider velocity
As exact factors don't matter as long as they are consistent, and frankly we don't know them, we use 1.
@60mph
Pr = 1 * 1 * 60
@80mph
PR = 80
Here we can see this measure is linear vs speed, so the power required for rolling resistance alone is 33% greater for dan than others. (80-60) / 60 = .333333
2 Pw Wind resistance
The power required to overcome wind resistance (drag) can be described by the formula P = 0.5 x ρ x v3 x Cd x A, where:
P is the power required.
ρ is the density of air.
v is the rider velocity, relative to the wind.
Cd is the drag coefficient.
A is the the surface area of the rider facing the wind.
again keeping this constant for unknown factors:
@60mph
Pw = .5 * 1 * 60³ * 1 * 1 =
½ 60³ = 108000
@80mph -> 256000
Here you can speed is the factor cubed, so dans power contribution requirement is 137% greater than others. (256000/108000) / 108000
wow
Conclusion:
Without knowing all of these other coefficients I can not plug in the numbers to get the real power dan claims his higher gearing has compensated for...
But I only suggest that at highways speeds, wind is the greatest resistance to be overcome, and he has at least enough power to overcome 137% MORE of it than i do...
great hope for the argument that there is still power to be had by lifting the float ceiling. but still, who wants accelerate at a snails pace...
that's why i believe the real discussion is not about how to reduced gear ratios, but how to increase power.2012 DF250RTB_B with Yamaha XV250 V-twin
1998 XVS650 (For Sale)
2003 LS650 with Voodoo Vintage hardtail

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