For those who wonder if higher power output will provide them higher speed or...

For those who wonder if higher power output will provide them higher speed or velocity:

Just shows max. speed (not acceleration) is not dependent on the power you draw out. Speed of a vehicle (this time a motorbike) is a function of engine radial speed (rpm), tire specs and gearing....

Dyno run from a modified CR500:

- Max power at the wheel = 89 hp, or 66.5 kW

- Max torque = 66 lb(force)-ft, or 89.6 Nm

- Max velocity = 136.5 km/h

A stock YZ250 makes around 47 hp with 31 lbf-ft of torque. With the stock gears on at 9600 rpm just before signing out it makes 120'sh km/h. Change the gears to a wide-ratio, it will make above 140'sh km/h.

  • I don't know how they got it but a bike with 89 hp at the wheel that's only geared for 136 kph sounds positively frightening. Actually Kiko, we have a famous desert race out here that you'd probably know about called the Finke where they gear the bikes for ridiculous speeds usually limited by how big of front sprocket they can fit on before the chain hits the case and how small of rear sprocket they can run before the chain's running on the hub and in their day up there CR500's were clocked at 220 kph on dirt with radar guns.

  • Tyler, Johnny Letti is in your friends list. His bike i heard is from the European MX champ in the 80's-early 90's George Jobe ;)

  • I think he's making 90hp

  • Geoff, prolly you'd seen his CR500 - he's even using a smaller rear sprocket when he dynoed it, where he got that 136.5 kph speed.

    i have to admit that i might have interpreted it incorrectly (not explicit enough); that 136.5 kph he has gotten could have been reckoned when he got the peak-hp of 89 hp, which is at a bit lower engine-rpm, long before the bike actually signs off. So, meaning, his maximum speed is higher than what is reflected here, though we have to see the full dyno graph to verfiy its true max-speed before the rpm signs off, which we can't check here in this photo mate....

  • About 92 horses at the motor. Deduct 3 horses (parasitic loss from motor tranny to the wheel and all the surface resistance in that dyno-roller), they got approx. 89 horses registered Tyler....

  • Yeah, I just had a good look at the data Kiko and even that 136.5 you quoted was at max torque, not max power. States 138 at max power though that doesn't seem to make sense because that would mean peak torque and peak power happen at nearly the same rpm. It also quotes a V max of 160 km/h which sounds more realistic. The way that whole printout is done is pretty confusing but I've actually had a heap of experience with bike dynos and dynos in general and on a dyno the bike or car doesn't necessarily have to be run in top gear. in fact we very rarely do dyno them in top gear because it causes unnecessarily dangerous high wheel speeds. We generally only run cars in 3rd gear whether they're 4 or 5 speeds. In a 4 or 5 speed using 4th gear might result in a marginally higher power reading because you're not driving the layshaft but from experience there's not much in it. And in a bike because all the gears are indirect it won't make any difference to the power reading if you use 1st or 5th it'll just draw the graph over a longer period of time and gather more samples. And on the car dyno we other have to run autos in 1st so they don't change gears on the dyno or only start the run at a really high speed where they won't try to kick down.

  • Still not buying the 90hp from a CR

  • Tyler, I didn't either but he has the proof. Johnny Lehti bike is wicked. He proved me wrong

  • Oh yeah, i didn't look enough to see there's 138 kph outright Geoff LOL! :D

    What could be the size of Letti's rear sprocket base on what you see. That is really small for a MX bike.

  • Yeah, on the Finke bikes they used to machine a bit off the sprocket mounting lugs so the chain wouldn't hit them to run even smaller sprockets. 36 teeth comes to mind and 16 or 17 on the front.