[jmwilson125]

Hi all,

So obviously swapping wheels will not get you more horsepower from the engine, however it would increase your power to the wheels as far as a dyno could tell.... I think.

Lighter rims accelerate faster, and from what I understand a dyno will calculate HP based on the weight of the roller being spun and how quickly the roller is accelerated.


Has anyone done wheel upgrade where they shed a bunch of weight and dynoed before and after. I would be curious to see what difference it would make, if any at all.

What do you guys think.

[RPM90]

Originally Posted by jmwilson125

Hi all,

So obviously swapping wheels will not get you more horsepower from the engine, however it would increase your power to the wheels as far as a dyno could tell.... I think.

Lighter rims accelerate faster, and from what I understand a dyno will calculate HP based on the weight of the roller being spun and how quickly the roller is accelerated.


Has anyone done wheel upgrade where they shed a bunch of weight and dynoed before and after. I would be curious to see what difference it would make, if any at all.

What do you guys think.

The actual HP/torque should not change based on the weight of the wheels/tires.
What does change is the power to weight ratio, so that the power the engine puts out is still the same but the overall weight is now lower thus potentially improving acceleration.

Also, since wheels and tires are "unsprung" weight their greater advantage is to handling with potentially better steering response and transitional handling.

No, I have not dyno'ed after a wheel/tire change.

[Alpine F31]

Originally Posted by RPM90

since wheels and tires are "unsprung" weight their greater advantage is to handling with potentially better steering response and transitional handling.

Exactly.

[moldcad]

What the title of the thread immediately brought to my mind is increased torque (thus the propelling force) on smaller radius wheel due to the changed overall transmission ratio

[G-Mann]

Weight shedding, rear seat delete, full tank vs half a tank, lighter rims, drag radials, vs no tread run flats are not going to show up on a dyno. They will make a difference on the track where it really matters. Acceleration, 60 ft times. 1/4 mile times and trap speed should improve but your HP will remain the same.

[bluemeanie]

A lighter wheel + tire combination will have less rotational inertia for the engine to overcome, hence more hp is available at the wheel. In addition, a smaller diameter wheel + tire combo will have even less rotational inertia. Thus, tire and rim weight and diameter all factor in to improved performance.

This sort of thing matters less at high hp to weight ratios, but is noticeable when driving momentum cars.

[Killramos]

^ it wont increase the horsepower but would just make it take a little less time to spin up through the rpm range with more inertia.

P=T*w or Power = Torque * Rotational Speed

Different weight wheels affect neither of those parameters...

Lighter wheels do increase performance but not power. This is not even a point that is up for debate with a knowledge of high school physics.

[bluemeanie]

True enough. But if you agree that the car with lighter wheels has increased performance, which it will have, where did that performance come from? The equation you gave does not account for rotational moment of inertia. College physics.

[Killramos]

Still doesn't affect horsepower. Lack of inertial forces, whether they be rotational or directional will affect how a certain amount of horsepower influence rate of acceleration. Which we perceive as performance. 0-60, 1/4 mile, lap times, whatever you are looking at.

You put a 600 HP engine in a 1 series and it goes pretty fast. Same engine in a 7 series making the same power not nearly as fast. That doesn't mean the 7 series doesn't have the same horsepower. Its the inertia and aerodynamic resistance of the bigger heavier car that makes it slower. And a dyno will show ( drive train losses aside which are largely frictional, not inertial) they are the same.

There are many factors in play with a car but just because its complicated does not mean that a reduction in inertia will increase horsepower.

We will call that University Physics my friend.

[jmwilson125]

I am aware that the actual HP would not increase. However a dyno works by calculating a know mass , the starting velocity, the end velocity, and the time it takes to get there. ..... I guess I just realized now that it is the mass of the dyno being accelerated not the mass if the vehicle. Therefore it wouldn't change. Ooooops my bad I'm an idiot haha

[bluemeanie]

Well, let's try to make this as objective as possible. It's not a matter of simple physics, but more a problem of how to accurately measure wheel horsepower as a proxy for expected performance. If you measure horsepower on an inertial chassis dyno, such as a dynojet, you will find that inertial losses of the drivetrain (and any changes made to reduce these losses) can affect the calculated wheel hp. This due to the fact that you are measuring the rate of acceleration of the dynamometer flywheel mass through a wide engine rpm range. Inertia matters under these conditions. If, on the other hand, you use a different type of dyno test to measure steady state conditions, inertia becomes relatively unimportant. So I would agree that that inertial dyno is not a great way to measure hp. I would assert though that such measurements are useful for both engine and drivetrain tuning.

To the OPs original question, which I think really is geared more toward performance than hp--yes, wheel (and tire) weight does affect performance to some degree, especially if you are dealing with a low hp car. Other modifications typically made to track and race cars to reduce inertial losses include lightweight driveshafts and flywheels.

[pikcachu]

here you have a good read.... http://www.w8ji.com/rotating_mass_acceleration.htm

[335BOY]

Originally Posted by bluemeanie

True enough. But if you agree that the car with lighter wheels has increased performance, which it will have, where did that performance come from? The equation you gave does not account for rotational moment of inertia. College physics.

Car , in theory , will have increased performance but that will be virtually imperceptible ....possibly at the limit on a track.

[335BOY]

Originally Posted by pikcachu

here you have a good read.... http://www.w8ji.com/rotating_mass_acceleration.htm

Interesting. Look for lighter tires!

[krhodes1]

Originally Posted by bluemeanie

Well, let's try to make this as objective as possible. It's not a matter of simple physics, but more a problem of how to accurately measure wheel horsepower as a proxy for expected performance. If you measure horsepower on an inertial chassis dyno, such as a dynojet, you will find that inertial losses of the drivetrain (and any changes made to reduce these losses) can affect the calculated wheel hp. This due to the fact that you are measuring the rate of acceleration of the dynamometer flywheel mass through a wide engine rpm range. Inertia matters under these conditions. If, on the other hand, you use a different type of dyno test to measure steady state conditions, inertia becomes relatively unimportant. So I would agree that that inertial dyno is not a great way to measure hp. I would assert though that such measurements are useful for both engine and drivetrain tuning.

To the OPs original question, which I think really is geared more toward performance than hp--yes, wheel (and tire) weight does affect performance to some degree, especially if you are dealing with a low hp car. Other modifications typically made to track and race cars to reduce inertial losses include lightweight driveshafts and flywheels.

Bicycles are the ultimate example of this - lighter wheels on a bicycle make an incredibly dramatic difference in how the bike rides/accelerates. Far more so than the same amount of weight taken off the frame. My bike racing years are FAR behind me, but IIRC the rule of thumb was that an ounce off the wheels was worth 3-4 ounces off the frame.

[CalCarNut]

Smaller diameter wheels/tires will numerically increase the rear end gear ratio and help acceleration, but it doesn't add any HP. It's a 'cheater gear' process. Using a 3.42 ratio with stock 255/50s ..but change the 255/50s to 285/40s..and now your final ratio is 3.55..which will accelerate the car faster in the power band. No added HP..but certainly 'quicker'....