Light Weight Racing Wheels

[CONAN]

Hi skyliners

I'm was just wondering people with light weight racing wheels does it affect the performance of the car compare with stock wheels?

Thanks

[scathing]

Light weight wheels reduce the unsprung weight of the car. This reduces the amount of effort required by your suspension to control its movement, and so your suspension can spend more time doing what its meant to (i.e. absorbing bumps).

An important concept is "unsprung weight." This is weight that is not supported by the suspension of the car. This usually includes the the weight of the wheels and tires as well as a percentage of the weight of the suspension itself, including control arms, anti-roll bars, shocks, and struts. Reducing unsprung weight is the key to improving handling. The lower the unsprung weight, the less work the shocks and springs have to do to keep the tires in contact with the road over bumpy surfaces.

Source: Muscle Car Club

Alternatively, you could run larger diameter and/or wider wheels. This reduces your tyre's sidewall height and increase the car's track, for better handling, while not increasing your unsprung weight and decreasing your suspension's effectiveness.

Lighter wheels also reduce your rotational inertia, which should bring gains to acceleration (in a similar fashion to reducing the weight of other driveline items, such as your flywheel).

From what I understand, reducing the unsprung weight has a much greater effect (per unit reduction) than reducing sprung weight in terms of handling. Same with reducing rotational inertia from the driveline.

i.e. pulling 10kg out of the wheels will do a lot more for handling and the car's acceleration than, say, ripping out 10kg worth of interior trim.

I could be wrong on this last point though.

Edited October 27, 2005 by scathing

[CONAN]

Thanks for that information scathing.

I'm looking to replace my stock wheel with

bigger and wider Jap light weight racing wheels.

Hence thats why I asked.

[scathing]

What car have you got, and have you considered what rims you're getting? Have you considered what tyres you're getting as well?

If you're going to get a very sporty (or Comp-R) tyre, they tend to have reinforced sidewalls. As such, going for a larger diameter may not necessarily bring you handling improvements over a smaller diameter with good tyres. Generally tyre weighs less than metal, so keeping a smaller diameter wheel with good tyres might "handle" just as well, but with the benefit of having a lower unsprung weight.

Lower profile tyres inherently have reinforced sidewalls (since they can't be permitted to flex as much), and since high profile street tyres generally go on econoboxes designed for people who want asoft ride, you don't get as high a percentage of what's available with good sidewalls.

If you're going for a proper semi slick, however, your need for a big rim with low profile rubber isn't as pressing. I ran Falken Azenis RS-V04 semi slicks on 17" rims for a while. They felt stiffer than the Bridgestone Potenza RE040 street tyres I had on 18" rims (in terms of the steering response and feedback when cornering on smooth road / track), but with the lower unsprung weight the car's ride comfort and bump absorption was better on Sydney roads when using the semis.

Its also a lot cheaper to go for a smaller tyre, which is definitely a concern to me with the rate at which I chew through them....

Edited October 27, 2005 by scathing

[CONAN]

Ive got 15x7 on at the moment.

Looking to get either 17 or 18 ( but mostly 18s) and

8 or 8.5 inches wide.

I looking at mainly Jap lightweight racing wheels

like ray volk, work, etc.

I dont want to go lower than 40 profiles for my tires,

because I was told that any lower you will fill the bumps

in the road more.

[scathing]

OK. Are the 15x7's the stock size? What size tyres have you got on it?

[CONAN]

205/55/15

[scathing]

OK, if your stock rim is 7" wide then you can run an extra inch without getting an engineer's cert.

On an 8" wide rim you can go anywhere from 215-255 width. I'd probably opt for a 225 or 235, just because those widths are easy to find rubber in.

You could run a 245/35 R17 tyre, and its rolling diameter would only be 0.5% off. On a 235 width you'd have to run 235/45 R17 and you'd be 1% off.

Go up to 18" and you'd be looking at a 215/35 tyre and you'd be 0.1% from factory.

(Recommended widths for an 8" rim thanks to Falken Tires. Calculations courtesy of Miata.net's tyre calculator.

Doing the numbers, if you want to keep it over a 45 profile without having to do a speedo /diff recalibration you're going to have to stick with 16" rims. That's assuming you want the tyres as wide as possible. You could stick with 7" wide rims, run narrower rubber (for example a 195/45 R17 tyre will have the same rolling diameter as your stockers) and get your profile back.

Mind you, the actual sidewall height difference between a 245/35 R17 tyre and a 195/45 R17 rim is only 2mm, so in terms of how much sidewall you've got to absorb bumps there's nothing in it.

I haven't tested it myself to see if an 88mm high sidewall on a 195mm wide tyre will absorb bumps significantly better than an 86mm high sidewall on a 245mm wide tyre, but I can't see there being much in it. Nor do I know if having a 55mm wider tyre (and so a 55mm wider track) gives you better handling either, but it seems likely that it will be noticable.

What I do know is that the lower profile tyre will cost a lot more, so that's something else you should consider. If you're serious about replacing your wheel / tyre combination for performance then you're not going to scrimp on tyres, so you need to work out what you can afford and buy rims to suit.

There's no point having LM GT4 Magnesium wheels if you can only afford to put retreads on them.

[CONAN]

I'm gonna be aiming for 18 x 8 or 18 x 8.5 all round.

I want a 225/40/18 tires.

So according to that calculator I'll 5.1% slower.

but if I go 35 profile it is 0.1 %.

I want to go 40 though, 35 is too low plus I'll feel the

bumps in the road more.