Trade-offs between wider ander lighter wheels/tires

[polizei]

Guys,

I currently have a set of uber heavy 17" Mille Miglia Cup wheels on my narrow body '87 Carrera:

Front Wheels are 7.5" wide with 225/45ZR tires
Rear Wheels are 9" wide with 255/40ZR tires

I've been doing more track events in recent years, and I'd like to replace my wheels with lighter ones. Lighter options that can run the same size tires are expensive and hard to come by. This has me thinking about going with narrower wheels/tires to reach my goal...

Can anyone help me understand the trade-offs between wider vs lighter wheels & tires? My understanding (right or wrong) is that wider tires are helpful for:

1. Higher Acceleration capacity (need to align width to HP)
2. Higher Cornering Speed capacity (lots of other factors to consider)
3. Higher Brake Torque capacity (need to align width brake caliper/rotor torque capacity)

But doesn't lightness positively impact each of these too? Please help!

Andy

[Bill Verburg]

Originally Posted by polizei

Guys,

I currently have a set of uber heavy 17" Mille Miglia Cup wheels on my narrow body '87 Carrera:

Front Wheels are 7.5" wide with 225/45ZR tires
Rear Wheels are 9" wide with 255/40ZR tires

I've been doing more track events in recent years, and I'd like to replace my wheels with lighter ones. Lighter options that can run the same size tires are expensive and hard to come by. This has me thinking about going with narrower wheels/tires to reach my goal...

Can anyone help me understand the trade-offs between wider vs lighter wheels & tires? My understanding (right or wrong) is that wider tires are helpful for:

1. Higher Acceleration capacity (need to align width to HP)
2. Higher Cornering Speed capacity (lots of other factors to consider)
3. Higher Brake Torque capacity (need to align width brake caliper/rotor torque capacity)

But doesn't lightness positively impact each of these too? Please help!

Andy

a quantitative analysis could be done w/ data acquisition if you have it and have lots of different wheel/tire iterations to compare but qualitatively you want light, wide, low, strong, correct o/s wheels and same for tires but add short/stiff tire sidewalls

the only way to get all of these is to spend some $

some of the best wheels available today that fulfill all of the things you want except cheap are Forgelines

here is a very nicely setup Carrera w/ 18" Forgeline Carrera GA3 wheels 8ET30 245/40, 9.5ET20 275/35 alternatively shorter 245/35 & 275/30 or 285/30 tires could be used or hoosier has some even shorter 'cheater' sizes too

[JeremyD]

The 16" BBS RS wheels are some of the better track wheels out there. Usually 1/2 the price of fuchs - and in my experience - about a pound lighter on each quarter

[Flojo]

BBS are great and astonishingly light.

I remember when I switched from my 7x15 rare mercedes fuchs to BBS RG015 7x15.
It felt half weight.

[Quicksilver]

For an extremely graphic, real world example of the difference between a light wheel package (low unsprung & rotating weight) and a heavy wheel package (high unsprung & rotating weight) go watch episode 3 of Top Gear's season 12.
The are trying to hop up a cheap car to improve its track time. First thing they do is get big brakes. Their large rotors are the same type of weight as a wheel: unsprung rotating mass. (The worst weight on a car.)

Every time the extra mass doesn't follow the irregularities of the road the side distortion of the tread goes away and stops generating side forces. It won't start generating cornering force until it gets back in contact with the road AND distorts sideways back into its loaded configuration. People only think about the first part of this double whammy. It is a huge deal.
The rotating part of this also has a double whammy. You have to accelerate the weight down the road plus you have to spin up the rotation of that same mass. It makes a big difference.

------
BTW - If you read through Bill's (the brakemeister!) posts you will see he is constantly giving people advice on not "over upgrading" their brakes. He will point out repeatedly that people do not need bigger brakes for their various applications. Unsprung weight is the reason...
(It is SO hard to argue against the "If more is better then too much is just right" mindset...)

[Driven97]

How about 15s? There's less weight and since the heavy part of the wheel (the rim) is more inboard there's less rotational inertia as well. Plus you can run a smaller diameter tire for lower cg, better gearing too.

15" Hoosiers are cheaper than 17s to boot, and come as wide as 275mm.

[polizei]

Guys - Thanks for the great comments. It sounds like everyone is in agreement that I need to "lighten up", even if that means sacrificing a little width because i'm on a budget. Agreed?

2 options:

1. 18" wheels with shorter side walls --> issue is fitment. fenders are rolled up front to accommodate 225/45. Would I need to do more work to fit 225/35/18 up front? How about going from 255/40/17 to 275/30/18 in the back?

2. 16" or 15" wheels with narrower tires --> issue is front calipers. I have Boxster S calipers on the front which require 17" wheels. Could I shave the calipers to fit 16" wheels? Might be economical...

Thanks!

[T77911S]

would the money be better spent on some other upgrade?

if you have the widest tire/wheel you can fit personally i would look at suspension or engine upgrades.

[Bill Verburg]

Originally Posted by polizei

Guys - Thanks for the great comments. It sounds like everyone is in agreement that I need to "lighten up", even if that means sacrificing a little width because i'm on a budget. Agreed?

2 options:

1. 18" wheels with shorter side walls --> issue is fitment. fenders are rolled up front to accommodate 225/45. Would I need to do more work to fit 225/35/18 up front? How about going from 255/40/17 to 275/30/18 in the back?

2. 16" or 15" wheels with narrower tires --> issue is front calipers. I have Boxster S calipers on the front which require 17" wheels. Could I shave the calipers to fit 16" wheels? Might be economical...

Thanks!

the choices of tires in 15-16" sizes is pretty poor

when fitting wheels and tires shorter tire is always easier to fit but you also need the right ET and width wheel.

numerically higher ET gives you more room at the lip and less on the inside
for an SC/Carrera chassis 8 ET25-31 is the usual fitment w/ 225/50 to 245/35 tires
225/50x16 is ~24.8" tall as is a 245/35 x18(there is some variation among brands w/ these sizes down to 24.7" or up to 25") so using a wheel w/ close to ET30 is the ticket, the narrower the wheel and tire the Lower the ET you can use(to a point)

in back there is zero wiggle room for a 9.5" wheel in 17 or 18" sizes ET19-20 or use a 9" wheel

as I said Hoosier has a really nice cheater 255/35x18 that is 24.8" x 10.8" that matches well w/ a front 245/35 x18

or use the 245/35 x18 w/ a 285/30 x18 @24.9"

or do as the Carrera owner above did and use 245/40 x18 @25.7" w/ 275/35 x18@25.5"

in other brands you'd have to make similar but different choices.

On my Carrera I use 8x17ET25 235/40 and 9.5x17 255/35 tires, I've used other larger and smaller sizes but this is a sweet spot. I originally designed the car to use 245/40(or/35) and 275/ 35 x17 but availability dried up as it had previously had in 245/45 x16(another sweet spot for tires size)

If you don't particularly care about wider widths you can used 8 & 9(or 9.5) wheels w/ the same o/s as mentioned above but then you are limited to a 205/50 or /55 front or possible a 225/50 or /45 and a 225/50(or cheater 275/35 x15 Hoosier)

As you mentioned in the OP there are compromises to be made ie width vs overall height vs sidewall height vs weight vs availability etc.

It's definitely worth it to get as light a wheel/tire set as possible, w/ as short a sidewall and overall height as possible, such that the tire is at the narrow end on the range for the wheel(ie if a tire is spec'd for a 9-11" wheel it is better to use a 10.5 or 11 than a 9 or 9.5)

[Cory M]

Just keep an eye on the classifieds and pick up some decent used 17" 8/9 wheels. You can run up to 245/275, or size it down if you want more clearance. Plenty of proven options out there, "no need to "reinvent the wheel".