Effect of wheel/tire size on acceleration

[hughc]

I think I always realised that a somewhat smaller rear tire would provide better (quicker) acceleration but it wasn't till I drove buddy's '77 911.

I always thought my '83 (with 17" wheels) was running quite nice and then I drove buddy's '77 (with 15" Fuchs) for only a short distance and soon realised there was a noticeable difference in acceleration. Also, I might add, his engine was much more willing to rev up quicker. Probably a result of the smaller tires

The difference is that noticeable that I'd like to consider swapping my 17's for 15's.

Of course maybe his car is in a better state of tune but I doubt it as he's not the tinkering type.

Last I checked my timing was 22 degrees at 3000rpm, so I don't know if there is any improvement to be made in that area. My A/F ratio is always within spec.

Do you think this is something I have to live with until I get a whole new set of wheels and tires?
Thanks
Hughc

[Len 911]

What is his tire circumference compared to what you have..if they are both 25" then I would think it would not matter the size of the rim..your buddy will just have a higher side wall.

[HarryD]

I would check to spec for your timing advance. Mine is around 30+ at 3000 rpm.

The problem with 15's is getting decent grippy rubber. I had to go to 16's for my daily driver to find something suitable that was not a track tire.

[hughc]

Originally Posted by Len 911

What is his tire circumference compared to what you have..if they are both 25" then I would think it would not matter the size of the rim..your buddy will just have a higher side wall.

Good point. Next time I run into him I'll check his tire size and compare the diameter.
I believe my new Falkens are 24.9" but I can verify that as well.

I'll also have to verify his timing to see how it compares to mine, 'cause his car sure seems to have more skat than mine.

Thanks Len

[Arne2]

15" Fuchs are much lighter than almost any 17" wheel. Rotational inertia of the heavier wheels will be noticeable. So even if the overall diameter of the tires is the same the 15" will feel more eager to rev.

[Cory M]

Bill response with spreadsheets and plots in 3...2...1....

[HarryD]

Originally Posted by hughc

Good point. Next time I run into him I'll check his tire size and compare the diameter.
I believe my new Falkens are 24.9" but I can verify that as well.

I'll also have to verify his timing to see how it compares to mine, 'cause his car sure seems to have more skat than mine.

Thanks Len

That is approximately the Stock OD. If his are 205/60-15's it would be the same.

As noted above, a smaller wheel with a tire of a similar outside diameter will be lighter. It will then be easier to spin up. This is the basis of the argument for reducing unsprung weight.

[Trackrash]

Seems you are comparing apples and oranges. I wonder how much lighter the '77 is compared to your '83.

The overall height of the tire can make a difference in acceleration. A lot of guys put on 15" tires and wheels for autocross. But they are shorter tires than stock size.

The only way to know for sure is to swap tires and wheels with your friend.

[JCGMS]

Originally Posted by Trackrash

A lot of guys put on 15" tires and wheels for autocross. But they are shorter tires than stock size.

Seems logical that the only way to know is to swap tires with your buddy on your own car.

By the way here’s a set of small 15’s (205/50/15) compared to stock diameter 205/55/16. Really wakes up the car for autocross but revs too high at highway speed.





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[HarryD]

Originally Posted by Trackrash

Seems you are comparing apples and oranges. I wonder how much lighter the '77 is compared to your '83.

The overall height of the tire can make a difference in acceleration. A lot of guys put on 15" tires and wheels for autocross. But they are shorter tires than stock size.

The only way to know for sure is to swap tires and wheels with your friend.

I some ways you are correct.

The 1977 has a curb weight of 2,469 and the 1983 is 2,855. So the 77 is a bit lighter so that helps.

The lighter wheel tire combo has a greater effect than overall weight reduction alone since the lighter wheel/tire combo reduces unsprung weight.

It is said that every pound of unsprung weight has the same impact to handling as 10 pound of sprung weight.

Unsprung weight is the total weight of the tire, wheel, brakes, and everything else that is attached at the bottom end of a spring. So why is it important?

If you can reduce "unsprung" weight, it is easier for your motor to accelerate the car. Plus you improve the ability of the suspension system to respond to bumps and potholes with minimum impact on the passenger compartment. It's all about inertia. Both to speed up the car (and brake for that matter). If the unsprung weight is lower, then the suspension can more easily adsorb the impact of bumps and ruts, so that the passengers don't feel the road irregularities.

So if you double the unsprung weight, then the suspension needs to work twice as hard to adsorb the impact of these obstructions. If you reduce that unsprung weight then much less of the energy of these impacts are felt by the passengers.

JCGMS-I do the same when I Autocross. The smaller diameter tire gives better gearing for initial acceleration at the expense of top speed.

[Hi_Fi_Guy]

There are a handful of online calculators that can tall you acceleration times based on variables including tire and wheel size. Can really make a difference if you want to say, shave a few tenths off your 0-60 time on one hand or lower your RPMs while cruising on the other end of the spectrum.

[RSBob]

When I changed from 16” 6s and 7s to 17” 7.5s and 9s the feel moved from lively and toss-able to more of an E class Benz even with lower profile tires. I was looking for a better tire selection and more grip. I got both but lost a bit of it’s soul.

[tirwin]

Originally Posted by RSBob

When I changed from 16” 6s and 7s to 17” 7.5s and 9s the feel moved from lively and toss-able to more of an E class Benz even with lower profile tires. I was looking for a better tire selection and more grip. I got both but lost a bit of it’s soul.

Sorta makes you wonder why the 16” Fuchs were considered an upgrade back in the day.

[HarryD]

Originally Posted by RSBob

When I changed from 16” 6s and 7s to 17” 7.5s and 9s the feel moved from lively and toss-able to more of an E class Benz even with lower profile tires. I was looking for a better tire selection and more grip. I got both but lost a bit of it’s soul.

Feel the same way when I moved from 15's to 16's for my daily driver. Still use the 15's for Autocross.

[Cajundaddy]

Wheel weight alone matters a little bit. Wheel and tire combined weight matters a lot (weigh and compare rears). Wheel diameter matters for acceleration but there is always a tradoff between low gearing for AX and higher gearing for highway cruising. Finding the sweet spot for an all purpose car depends on overall wt/hp and transmission ratios.

Car weight matters a lot and a 400lb difference in race cars would be 1-2 seconds per lap on many tracks. The rule of thumb at the drag strip is 100lbs = 10hp, so a car 400lbs heavier is like adding a 40hp restrictor plate to their motor. A very big deal.

[JCGMS]

Originally Posted by HarryD

JCGMS-I do the same when I Autocross. The smaller diameter tire gives better gearing for initial acceleration at the expense of top speed.

Yes. Having to shift to 3rd On a fast course cramps my style.

For me staying on 15s for “daily” driving would be ideal if there was a reasonably priced stock diameter Pilot Sport 4S or Potenza S-04 equivalent. I really don’t like the Pirelli P600s.




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[Driven97]

Originally Posted by JCGMS

Seems logical that the only way to know is to swap tires with your buddy on your own car.

By the way here’s a set of small 15’s (205/50/15) compared to stock diameter 205/55/16. Really wakes up the car for autocross but revs too high at highway speed.

How do you like those Federal 595-RS-RRs? I had the old RS-R on my AMG and loved those things for the price, and I am a total tire snob. Was thinking of trying the RS-RR on my 17s.

[IROC]

I've run everything from 225/50-15s to 275/40-17s on my car and the difference in acceleration is very noticeable. The 275s ultimately were better for lap times, but the acceleration was shocking with the 225s.

[JCGMS]

Originally Posted by Driven97

How do you like those Federal 595-RS-RRs? I had the old RS-R on my AMG and loved those things for the price, and I am a total tire snob. Was thinking of trying the RS-RR on my 17s.

For my level of expertise I don’t notice a major difference between the RS-RR and the RE-71R. At about $250 all in for 4 tires they’re hard to beat.


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[Bill Verburg]

There are multiple effects imposed on a vehicles dynamics from multiple sources discussed here

lets try to separate them out
1) effects of tire OD

this is very straight forward, from multiple dyno runs w/ the same chassis and different wheel/tire combos it has been determined that 1" of additional tire OD is roughly equivalent to 10 lb-ft of torque, this is called gearing cost

2) tire and wheel weight
I chose this tire because it is available in 15, 16, 17, & 18 in sizes that would be appropriate for use on a 911 SC or 3.2 Carrera

notice that the inertial cost of all 4 is w/i a range of 2 lb-ft but the OD range is 1.2" or a bit more than 10lb-ft, gearing cost the most advantageous for acceleration is the 15 followed closely by the 18 w/the 16 & 17 trailing behind. These effects are also seen in braking where the larger the OD the more energy goes into the wheel than the stop. Normally the inertial effects aren't seen o much in cornering but they are there, usually these effects are seen when it all goes wrong as there are huge gyroscopic forces involved.


wheels have less effect than tires on dynamics because of 2 factors
1) they are generally lighter
2) they have a lessor radius than the tire

Same for brake rotors

Besides acceleration weight and tire OD also affect the ride, a lighter rotating assembly is easier for the shock to control and thus gives a better ride and the larger the tire OD the better the ride because of less effect of road irregularities. It's difficult to quantify these effects.

Chassis weight also affects acceleration as well as braking and cornering

Here's a 3rd gear acceleration graph for a US 3.2 Carrera

what's interesting is that a change from 225/50 x16 to 245/40 x15 is roughly the same as a 500# loss of all up chassis weight