danglerb

I am poking around looking for cheap track tires and lower profile (shorter sidewall) tires on stock size 16" rims has me curious. The smaller diameter tire raises the effective rear end ratio (same rpm, makes less forward motion), and slightly lowers the car without changing the suspension geometry.

Anybody try it with? results?

IcemanG17

interesting....it seems with most modern performance tires being 17-18-19" or more...you end up with TALLER diameter tires, which makes the ratio taller..... 928's were designed around a 25" diameter tire...but some tall 19" wheels can be over 27"....

the lemons racer with stock 474 bilsteins and cut springs handles quite well on stock 225/50-16 tires....

James Bailey

Yes Mike I run 15 inch rims with 225x50x15 tires but according to Kibort it makes no difference

dr bob

Jim, it really depends on what gear you are in, how fast you are going, and where you are on the torque curve.

I think that's how it works anyway...

RyanPerrella

Hi Jim!

James Bailey

I think we ALL can agree it lowers the center of gravity.....which is not a bad thing ! reduces the weight of the car...which I really need What's torque anyway.......

Imo000

Tell Mark that the decreased roatational mass make a significant difference too. I'm glad he's too busy with his car or else this would be another pi$$ing match.

mark kibort

Jim, at the risk of sounding foolish, even though I know you are just joshing me, you are. dont fill the world with mis-information.

I never said it makes " no difference" i said it depends. AND, you dont gain HP by changing gearing for ANY operating speed range, you ONLY shift that speed around.

a final challenge. find me ANY speed with ANY tires of ANY size that dont create the exact same accelerative force at the rear tires, IF ,the HP is the same, and Ill buy you that car! what this means is that the speed range is what determines what size rear tire you want to have for a given set of gears, OR what gear ratios you want for a given set of tires.

mk

dr bob

Has the added advantage of lowering the CG of the car as Jim reminds us, while maintaining better suspension geometry.

It will affect the shape of the contact patch, and possibly the size depending on the tire and the air pressure in it.

Jim's 15" rim option is pretty cool provided you don't need the space inside for S4+ brakes.


Mike--

What 'cheap track tires' are you looking at?

mark kibort

Never too busy for a hp -torque discussion .

Significant? define , "Significant".

remember, the weight saved on the tires, is about equal to 1.5x if it was in the car for an 18" rim. (less for smaller) and around 2x around the tire sizes we work with. in other words, you save 10lbs off the tire, you might gain near 1hp. you reduce the diameter from 25 down to 22", and you might gain .5 hp with no weigh change.

chew on that!

mark kibort

There is a reason that the 18" wheels are more desirable. we could run little slicks on 13" wheels if we wanted to, but why dont we?

there is also an advantage of a little more inertia to fight the effects of tire lock up. gives more of a stable ride, and you can have more rubber to wear and dissapate heat when a tire is larger. they spin slower for a given speed, which is easier on bearings, as well as having more mass to dissappate heat from the brake rotors.

Again, lots of trade offs.

mk

Imo000

What about the weight saved on the rims? Chew on that!

I've read a test where the only change to a car (nto Porsche) was to ditch a set of very large "bling" rims/tires and go to a set of stocks, the gain in the 1/4 mile was a tenth (0.1) of a second......that IS a significant amount, don't you think?

danglerb

I'd like to find a tire that some common track vehicle uses briefly and then sells cheap or gives away, and that would be suitable to use on a early 928. Specifically I saw some MiniCooper track tires on CL in a 215/30/16 (I think), which I know are too small, but it started me thinking.

Depends is the wrong word, it has the ring of even chance of good and bad, and I suspect "most" of the time the higher numerical rear end ratio when applied to a street car will be an advantage, except on some turns on some tracks where it might be awkward, but there is that chance with any combination of gears. Its like 0-60 times, you really really don't want to have to shift at 58 mph.

Little bitty slicks and screams around a track, its called a go cart, and works pretty well.

odurandina

mark is surely right, but for there is also something to be said for running 15 and 16 inch rims for the track;

the larger rim tires cost isn't so great... (unless you require cup tires or slicks), the cost savings in used tire market for 15 and 16 tires is considerable. you can run smaller than stock diameters (24.5") on stiffer tires to improve gearing... look at Nascar rims/tire sizes and see why having all that air/load capability factors in.

we run 17, 18, 19 inch rims for street not just for the performance advantage... sure that's part of it, but the SAFETY advantage of HAVING a tire failure and not having a big hunk of rubber flapping around is HUGE. the load disadvantage is compensated by stiffer, y-rated tires under higher pressure.

I had a front tire go down at 155 mph on an empty highway (late at night) running y-rated dunlops and didn't lose control of the car because i was running a 18 inch rim... on 245-35-18s. i had plenty of time to slow down before the tire was shredded.

mark kibort

let me give you something to chew on. the weight savings on the rims is only about 1.4 times if the weight was IN the car. again, lose 10lbs on the rims, its like 15 is in the car. about 1.5hp overall as an effect on overall performance.

The drag strip numbers of .1 seconds, for a "very large " set of bling to save weight, is significant, but what are the values for weight and diameter. overall gearing could have been changed and could have been 10lbs per tire and wheel saved. (like when I went from 28lb C2 aftermarket rims to 18lb kinesis ), and another 5lbs from a hoosier vs a toyo. In that situation, 15lbs per wheel and tire, x 4 corners would be "Significant". thats 60lbs of rolling weight, which could have the effect of near 12hp! thats worth .1 or .2 seconds in a drag, depending on many other factors there too.

mk