It is an old clip , but just more proof that larger, heavier wheels ALWAYS slow the car down.

 

even larger lighter rims slow the car down , same with lighter larger brakes . more mass further out .

 

That is a known fact.

Tire and Rim size combinations goes hand to hand with road conditions.

IMHO the G8 right size for all road conditions is 18 to 19 in with the stock suspension.

 

Here is a video I found that is pretty interesting how much wheels affect performance.

http://www.youtube.com/watch?v=-_NYWyVHcl0&feature=youtube_gdata_player

Interesting,good to know! BR.

 

Okay call me crazy, but how does a 20in rim that is lighter than a stock 19, but has the same outside diameter tire as a stock G8 going to slow it down?

If anything I would imagine that have a wider track on a would slow the car down because of the added roll resistance of a 285 vs a 245... Some one school me on this because I don't know.

 

Okay call me crazy, but how does a 20in rim that is lighter than a stock 19, but has the same outside diameter tire as a stock G8 going to slow it down?

What people do not see is not about the weight, is about the tire. The bigger the rim the smaller profile the tire has to be to keep with the same outer diameter. The suspension has to contour to the road in order to have some grip. A wheel on the air, does not have any grip at all. The sidewalls of the tire acts like a cushion absorbing the smaller bumps. Harder sidewalls makes the tire skip and hop on smaller bumps.


To prove how is, try to push a cart with hard plastic wheels and one with softer wheels, the one with the softer wheels has a better response.

If the surface is smooth as glass the lower profile tires will perform better but that is not the case on real world.

The other way around more sidewall gives smoother ride but the sides flexes more. The key is to find the right combination for the surface that are you driving.

 

My take is that when you go to a lower aspect ratio than 50 that the tire side wall doesn't have enough give, and it tends to break loose in the corners.

So now when you go to a 20" plus rim, in order to maintain the same tire diameter you go with a extreme low profile tire.

And also aluminum weighs more than the rubber of the tire, you get an increased flywheel effect, that effects acceleration and braking.

 

My take is that when you go to a lower aspect ratio than 50 that the tire side wall doesn't have enough give, and it tends to break loose in the corners.

The aspect ratio of a 50 is all relative. it's 50% of the tires width. Therefore a 355/50Rxx tire is still gonna have a pretty big sidewall, while a 165/50 Rxx gonna be a rubber band.

 

I found the science of this questions on another forum...

But from personal experience - Large High performance brakes (cross drilled and same weight as oem), Larger wheels (same weight) and any other components (you wouldn't put a larger diameter flywheel on - but if you could) - they ALL increase the rotational mass necessary for your car to overcome at acceleration. It's simple physics.

The power to accelerate a rotating system is given by P = (torque)*(da/dt) where a is angular position and t is time. Torque is equal to the product of the system's rotational inertia (I) and the system's angular acceleration (dw/dt) therefore this can be rewritten as

P = (I)*(dw/dt)*(da/dt)

Thinking of the brake/tire/disk system as being one solid disk and, since the rotational inertia of a solid disk is given by

1/2(M*R^2) (where M = mass and R = radius) this can be rewritten as

P = [1/2*(M*R^2)*(dw/dt)*(da/dt)] = 0.50(M*R^2)*(dw/dt)*(da/dt)

If the mass of the disk is reduced by, for example, 25% but its radius is increased by 20%, the power to accelerate the new system would be given by

P = [1/2*(0.75M)*(1.2R)^2*(dw/dt)*(da/dt) = 0.54(M*R^2)*(dw/dt)*(da/dt).


An engine will have P flywheel hp available to accelerate the brake/tire/disk system, therefore (at the same rpm)

0.50(M*R^2)*(dw/dt)*(da/dt) = 0.54(M*R^2)*(dw/dt)*(da/dt).

For a given rpm, da/dt are equal on both sides of the equation therefore

0.50(dw/dt) = 0.54(dw/t), dw/dt = (0.50/0.54)*(dw/dt) = 0.93 (dw/dt).

That is, for the flywheel power available at any given rpm, the brake/tire/disk system with the reduced mass but increased radius will have an acceleration of only about 0.93 that of the unmodified system.


The dyno roller which is accelerated by the car's tire has its own rotational inertia (I) and the power it measures is given by P = (I)*(dw/dt)*(da/dt). Since for a given rpm, its I and da/dt are the same, it will measure a lower hp reading from the lower acceleration of the modified brake/tire/disk system, i.e.,

P(1) = (I)*(dw/dt)*(da/dt) > P(2) = (I)*(0.93*dw/dt)*(da/dt).

 

I now have a headache

 

The power we lose by using big brake kits (lighter or not) is often recouped by better brake modulation, less fade and later trail braking. Larger wheels and brakes are USELESS in a drag race for example. (unless something goes terribly wrong!) In many cases (such as the video above) if the wheels are too large and rubber is too small - power can not be applied to the track/road properly.

I love the looks of a lowered car with big wheels - but I don't drive one.

 

All of this science & larger heavier = slower bit is not what's going on in the video. The car in the vid has more than enough additional power to compensate for the slight difference in weight/rotating mass of the bigger wheels.

The problem here is that it can't get the power down to the ground 'cause the ultra low profile tires just wont grip. & he's having the same problem in the corners which is what is causing his lap times to be slower than the non supercharged version of the car. Sometimes bigger wheel/tire combos will give better handling, like moving from a 16" to a 17". But there needs to be a balance. Overdoing it & not having the right amount of rubber on the ground will result in slower lap times every time.

 

kind of taking the fun out of things applying all these mathematical formulas and researching video aint it?

I bet less then 3% of car owner drives his car to the potential that a larger brake or rim package will hinder its performance

 

I love this discussion; this sorta stuff always fascinated me ever since I learned how to decode the not-so-obvious

245-45-r18

nomenclature.

Well done on the discussion everyone!