Light rims question!!

[mellowboy]

OK my friend told me that light rims will cause alot of wheel spin? I mean i want to get those 15 inch Racing Harts CPO-35's? What is the purpose of having light rims? Those rims are barely 8lbs each!

[buh_buh]

the purpose is probably not to weigh your car down...

[SportSC4]

The purpose of lite wheels are to reduce your rotational mass. Light wheels definitely improve acceleration! If you wanna go all out, Toyomoto makes some of the litest tires.

[Dezoris]

Wheelspin with light rims is relative to the tires and your alignment.
There is a slight performance difference when going with an 8lb rim, mainly in your 0-70 times, your 5-30 will increase slightly.

If you don't have sticky tires and your camber and toe are off you will get wheel spin, which you would get with any wheel.

You are looking at maybe a 2-5 HP gain
Integra Steel wheel and Civics weigh 18lbs the tires weigh 19-23 lbs
Most performance 55/50 series 15" tires are anywhere from 17-22 lbs.

So you would be looking at
152 lbs stock in wheels and tires
112 lbs for the new rims and tires

The actual difference is in here
stock=72lbs (FWD)
Mod=56 lbs (FWD)
which means you are reducing unsprung weight by 16 lbs
good for 2-5 HP
for automatic it is less.

[texan]

Just to clarify, lighter rims do not increase horsepower output. It's the same as removing weight from the car, it doesn't make power but it does increase performance. And there are plnety of 15" rims that weigh much less than 17 lbs, the 17" rims I am buying weigh 13 lbs a peice.

[Dezoris]

Originally posted by texan
Just to clarify, lighter rims do not increase horsepower output. It's the same as removing weight from the car, it doesn't make power but it does increase performance. And there are plnety of 15" rims that weigh much less than 17 lbs, the 17" rims I am buying weigh 13 lbs a peice.

MMMmm, Depends, I have seen several dynos, where the weight and "size" makes a definite difference in wheel HP.

I am not talking 3 lbs but usually when it is over 7lbs per drive axle, you see some wheel HP.

I have some dynos to prove

But you are right, don't expect any gains, but there technically is some HP gained by reducing unsprung weight, such as stock to 8 lb rims.

My buddy with his Integra GSR went from stock wheels to 15" Volk TE 37s and saw the gains I wrote.

I have also seen dynos in SCC, which I pray I still have at home showing this on a Civic DX where there was a loss of 4 HP from going to
17" that were 20lbs heavier up front.

Let me know what you think Texan

[texan]

Any dyno showing an increase in power output from the engine by reducing rotational inertia is an inertia based dyno. An engine makes power by converting chemical energy into heat, then converting the pressure from that energetic heat reaction into rotational force. Changing something at the tail end of the drivetrain will have no effect on this process, therefore it truly has no effect on engine power. BUT, and this is a big but, an inertial dyno can "see" these effects to a limited extent.

Dyno's that measure power output through acceleration based events (such as Dynojet's dyno systems) work as follows: a drum of specific weight is accelerated by the drive wheels of a car. Using simple math, a rotational mass of given size (the heavy drum), accelerated at a given rate has a certain amount of force being applied to it. This is how Dynojet's inertia dynos work, and why they can "see" power differences when changing rotational inertia to the drivetrain. In a sense we are both right, the engine doesn't actually make more power, but the decreased rotational inertia of the drivetrain allows more of this motive force to be available for accelerating the dyno's drum. If it were instead hooked directly to the engine, or was a brake dyno measuring power output at specific RPM points by finding equilibrium between engine output and braking force, you would not see a difference regardless of how much rotational inertia was removed from the system. The point I am making is that decreased inertia in the drivetrain most certainly has a positive effect on acceleration, but not actual engine output. At the same time though, it's kind fo nice that we can quantify the changes in rotational inertia as power increases, and though this is generalizing and purely dependent upon speed, Dezoris's numbers do show how much (or little, depending upon your viewpoint) an increase in performance one can expect from lighter rims.

[Dezoris]

Originally posted by texan
Any dyno showing an increase in power output from the engine by reducing rotational inertia is an inertia based dyno. An engine makes power by converting chemical energy into heat, then converting the pressure from that energetic heat reaction into rotational force. Changing something at the tail end of the drivetrain will have no effect on this process, therefore it truly has no effect on engine power. BUT, and this is a big but, an inertial dyno can "see" these effects to a limited extent.

Dyno's that measure power output through acceleration based events (such as Dynojet's dyno systems) work as follows: a drum of specific weight is accelerated by the drive wheels of a car. Using simple math, a rotational mass of given size (the heavy drum), accelerated at a given rate has a certain amount of force being applied to it. This is how Dynojet's inertia dynos work, and why they can "see" power differences when changing rotational inertia to the drivetrain. In a sense we are both right, the engine doesn't actually make more power, but the decreased rotational inertia of the drivetrain allows more of this motive force to be available for accelerating the dyno's drum. If it were instead hooked directly to the engine, or was a brake dyno measuring power output at specific RPM points by finding equilibrium between engine output and braking force, you would not see a difference regardless of how much rotational inertia was removed from the system. The point I am making is that decreased inertia in the drivetrain most certainly has a positive effect on acceleration, but not actual engine output. At the same time though, it's kind fo nice that we can quantify the changes in rotational inertia as power increases, and though this is generalizing and purely dependent upon speed, Dezoris's numbers do show how much (or little, depending upon your viewpoint) an increase in performance one can expect from lighter rims.

That is such a good point and actually I did not really see your angle.
Yes, you will see more HP at the wheels on an inertia dyno, your engine output however is not increased, for the reasons you stated.
Real world you will see a minor, very minor accel difference, why?
only due to mechanics and weight. Obviously no one will have an engine dyno run on thier car, not practical, nor would it be an accurate result, secondary to drive train loss. (unless you really want to know) but IMO, that does not even matter, it is the power to the wheels that tells the tale.

But I guess, it is like porting your head, just as it makes the engine breathe, more efficiently the lighter wheels eliminate, a minor percentage of power loss due to the drivetrain.

That we are in agreement on.

[mellowboy]

....DAMN ...thanks guys?

[CivicSiRacer]

When I switched from my 17s to my 16s I noticed a big difference. Also going from 205/45x16" (23.5") to 215/40x16" (22.9") my gearing improved and was able to accelerate off the line quicker.