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#1
Tyre Size and Horsepower
Datsun1000
Posted on: 2006/8/9 0:56
Hi Guys,
Can tyre size affect rear wheel horsepower? The basis of the question If you put a car on a Dyno and perform a run and then chage the tyre size up to a larger overall size, will the rear wheel horsepower change. excluding the very small fraction of a friction increase by the larger tyres. Cheers Guys, A link to proof or actual mathmatical proof for a yes or no answer would be appeciated.  
#2
Re: Tyre Size and Horsepower
Tyrie
Posted on: 2006/8/9 2:31
surely not... or else all the dyno pigs would have custom 10" wheels with low profile tyres.
#3
Re: Tyre Size and Horsepower
B120dat
Posted on: 2006/8/9 2:38
well on the dyno would it not be the opposite?... because there is a no need for traction because the wheels are turning on the rollers, a heavier larger tyre would actually take more to turn and add wieght, actually taking away from minimal amounts of horsepower...talking about tiny amounts of horsepower.
#4
Re: Tyre Size and Horsepower
ddgonzal
Posted on: 2006/8/9 3:08
Quote:
Can tyre size affect rear wheel horsepower?
#5
Re: Tyre Size and Horsepower
benny
Posted on: 2006/8/9 3:24
The horsepower should remain unchanged, however the effective rate of accelleration will change. The larger diameter tyre [increased circumfrence] The effective distance travelled will be longer for 1 revolution. More work would be required to turn the larger diameter tyre at the same rate as a smaller one for one revolution.
I think I got that right. maybe.
#6
Re: Tyre Size and Horsepower
Datsun1000
Posted on: 2006/8/9 3:55
This is my belief that a lager tyer size will use the horsepower for higher top end speed and a smaller tyer size wilol use it for acceleration.
However i am after some way to prove this as it has been an ongoing argument with 2 of my friends that are adament that a larger tyre size will reduce rear wheel horspower. id like to prove it without going down and putting a car on a dyno. and then changing the tyre size
#7
Re: Tyre Size and Horsepower
L18_B110
Posted on: 2006/8/9 3:58
Quote:
no, tyre diametre will not affect bhp on a dyno. tyre diametre is just another form of gearing. and gears are torque multipliers. a taller tyre (or gear) will mean less torque (tractive effort) at the wheels, but they will be turning at a proportionately higher speed, so no net change to the power. eg change to a 10% larger tyre will make 10% less torque, but it will be going 10% faster at the same time, so it all balances out. power = torque * speed / a constant
#8
Re: Tyre Size and Horsepower
Datsun1000
Posted on: 2006/8/9 4:16
Quote:
Below is what the retorted with @ says: so your saying that bigger tires need computer adjustment to multiply the torque change? # says: no that is not what is being said @ says: that is exactly what was said "torque multiplier" @ says: you need to change to settings on the dyno to adjust for the change in ratio @ says: so the ratio DOES make a difference @ says: otherise you wouldnt need to tell the computer the difference in gear ratios
#9
Re: Tyre Size and Horsepower
L18_B110
Posted on: 2006/8/9 5:14
@'s a PITA!
the dyno doesn't care what car is on it, or what tyre its got. the dyno simply measures the force seen at the rollers (torque or tractive effort), multiplies that by the roller speed, divides it by a constant (5252 for imperial bhp) and then it has the bhp figure. no adjustment is needed to the dyno if you change tyre size. eg a 10% larger tyre would produce 10% less torque measured by the roller, but the roller will also be going 10% faster, so the mathematical equation power = torque * rpm / 5252 remains unchanged. The dyno will give the same power output no matter what tyre you put on a car.
#10
Re: Tyre Size and Horsepower
Datsun1000
Posted on: 2006/8/9 6:22
I have been argeing with one of them for an hour now and they still will not admit that tyre size does not effect rear wheel horsepower
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