what do bigger wheels/tires do to lap times?

[Brian Cameron]

I've done some searching for this but maybe haven't been using the right terms, because I haven't found a related thread yet.

I'm looking at going to larger wheels (and therefore tires) on my 993, and wondered what the correlation between size and lap times was - eg for time trials.

Lots of folks want bigger for (at least partly) cosmetic reasons, but what's the practical benefits? If anyone has anecdotal info on the effect of more rubber on the road to road course lap times, I'd love to hear it.

BTW my experience with moving to R compounds knocked off about 2 seconds on a road course where typical lap times ranged between 1:30 and 1:40 for similar cars. So if you changed to R's along with changing tire size, that would factor into the equation.

many thx

[ZOA NOM]

The math sez you would just change the RPM range a little bit. Taller tires should mean lower rev's at launch, and throughout the RPM range for the speed you are travelling. It can help on a longer straight, but might hurt in a tight turn where you want higher rev's at exit. You could almost gear your car for a specific track with different size tires. I would say, on a course that you never reach top end, a taller tire would hurt more than help, all else being equal.

[rexav8r]

That's a VERY complicated question really. (I think) & I'm sure there's a lot of guys that can do a better job than me at answering...

It depends on engine power, suspension setup, and what type of course you're running.

I've seen a lot of guys throw big rubber on for looks but lack big power so they actually get slowed down. My car is a narrow bodied early car. Street tires are 205 50, they look OK but I REALLY like the look of my 225 45 R15 track rubber. I run the 225s all the way around and I love the grip I get when I auto-x. I am doing my first DE @ Mid-Ohio next weekend. I've been asking the same question and most guys with experience tell me that I'd probably be able to better max-perform my high speed turns with slightly LESS rubber up front. Which makes sense when you discuss the dynamics of it all.

Maybe Grady, JackO, or Tyson will respond. I know they have experimented with different setups and know more...

[aigel]

You need to clarify if you are adding larger diameter or larger width or both! What size wheels and tires are you running now and what will you run in the future?

The width is almost trivial, but diameter is not. You may have larger wheels but add low profile tires, hence barely increasing diameter. Or you may actually increase diameter significantly ...

George

[island911]

911 tires get bigger for one reason - softer compound.

soft compounds on too skinny will overheat, and rip right off.

for various reasons, going wider, keeps the tire temps in check.

[Jack Olsen]

There are two ways to go 'bigger.' Taller tires -- meaning tire/wheel combinations with a larger overall diameter -- will reduce your effective gearing, but increase your potential top speed in each gear (if you're currently hitting redline, but wouldn't benefit from shifting to a higher gear, or if you're in your top gear). In case that sounds like a benefit, I'll put it more simply: if your car is low on torque, a taller tire will make the situation worse. But if there are particular points on a particular track where your gearing is too short, taller tires can sometimes be a cost-effective way to address the problem. Generally speaking, though, it's almost always better to go to a shorter tire so that your gearing is reduced, and therefore gives you more effective torque.

(A note for future thread readers who might not understand tire sizing: A tire's overall diameter (23-27 inches, or thereabouts) does not correspond in a linear way to what diameter of wheel you're mounting the tires on (15, 16, 17 or 18). The tire's overall diameter is determined by the second number in a tire size, which indicates the percentage size of the sidewall, relative to the tire's width. With a 245/45x16 tire, the '45' is the sidewall size -- in this case 45% of the tire's 245mm width. A 245/50x15 tire would fit on a smaller-diameter wheel, but would be a larger-diameter tire, because 50% of 245mm is larger than 45%. A 275/45 tire would have a much larger overall diameter than a 245/45, because 45% of 275mm is larger than 45% of 245mm.)

The other factor in tire size is width. This is the big one. Wider tires change the shape and mechanical properties (and to a limited extent, the size) of your contact patch. They also allow you to lower your tire pressures (and increase the size of your contact patch) without running into problems with heat dissipation as quickly.

They are almost always faster on a track than skinnier tires, everything else being equal.

The exception would be on very low horsepower cars, where you're going to hit a trade-off point between the increased drag from the tire's contact with the track, as well as an increase of aerodynamic drag, since the car's frontal area with the larger tires is bigger, and more significantly: the increased rotating (and unsprung) mass of heavier wheels and tires. There's always a trade-off, isn't there?

But the general rule with tire width remains: wider is faster. If your car is 3000 pounds or less, and making 250 hp or more, then my opinion is that there aren't any tires in commercially-available widths that are going to slow you down. You're limited by what will fit on your wheels and in your flares (and also by the need for proportionate widths relating to your car's F/R weight bias), and that's about it.

Bigger can be worse. But wider is almost always better.

[ZOA NOM]

Duh, I just assumed that everybody understood "wider, eh, better, eh". Certainly, wider tires should enhance the car's performance on the track. I don't know why I jumped on the diameter issue. Nice explanation, Jack.

[Bill Verburg]

A bigger wheel doesn't necessarily mean a taller tire.
Typical 911/964/993/rear fitments might be
8x16 225/50 @25.1x9.2" 835rpm
9x17 255/40 @25.2x10.7" 831rpm
10x18 265/35 @25.2x11.1" 831rpm

As you can see they all have about the same height(revs per mile is a more accurate measure)

What you get w/ the bigger rim is a wider tire w/ a shorter sidewall that runs at smaller slip angles and runs coooler longer.

[Don Plumley]

Another consideration: Bigger wheels -- primarily diameter but also width -- are generally heavier. That translates to more rolling mass and more weight on the suspension. Wider tires are of course, also heaver. That's one of the big benefits of forged wheels, in addition to their strength they are lighter than cast.

Another consideration of different wheels is tire size availability. For stock 993 17" wheels, my favorite tires (RA-1's) are not made in an optimuum front wheel size. In 18" wheels they have more options. When you are in Jack's league and run super wide tires, you have even more fun trying to source the right rubber.

Here's the math:

wide sticky + effective diameter + tire availability + wheel weight = complicated equation

[Bill Verburg]

Yes, there are always - to offset the +, but in the case of wheels and tires and brakes(to a point) the + outweighs the -

RA1 actually has some really poor choices in 17 or 18 for our cars. The problem is getting fronts and rears that go together well .

[Jack Olsen]

Originally posted by Don Plumley
Another consideration: Bigger wheels -- primarily diameter but also width -- are generally heavier.

This often isn't the case with lightweight wheels, at least with regard to the wheel diameter. For a 25-inch wheel/tire combination, if the widths are the same, a 17-inch wheel will have an extra inch of alloy in the combination, but an inch less of sidewall (two sidewalls, actually). So while the wheel itself will weigh more, the tire that goes on it will weigh less -- and the wheel/tire weight sometimes goes down. Put another way, with wheels like Fikses or Fuchs, the additional 'inch of wheel' will often weigh less than the additional 'inch of tire.'

Of course, wider wheel/tire combinations will weigh more, and overall decreases in tire diameter will reduce overall weight.

[Brian Cameron]

Wow, I got both Bill and Jack to weigh in on my thread! I will never wash my keyboard again!

So we've established that wider tire and lower sidewall, is more contact patch and better cooling, overall height being the same.

So what did it do to your lap times to go 1 or 2 inches wider, gentlemen?

[Mahler9th]

I see some good information here. However, I encourage all to go to trusted, "old school" racing resources... including some of the basic stuff by Puhn and Smith. If your purpose is optimal lap time everything else being equal, study and understand the basics.

Of course, gearing is an important consideration. In our area (NorCal) we don't see many folks with 964 or 993 cars at the upper levels of time trialing or racing. There are reasons for that. There are a few newer cars we see from time to time, and my point is that most don't change gearing because it is really expensive. So they pick wheels to go with the stock gears they have. Of course the exception being those with factory cars.

From a wheel weight issue, there really aren't any really tricky decisions once you consider the weights of those later cars, and the typical brake packages they run. Most choose a forged wheel with a good track record, like the Fikse.

I have my own experience with a heavy car and a wheel/tire change. Years ago I was running a 3000 pound 951 with about 300 bhp. Started with 16 inch factory forged wheels in the 245 width and standard ar. When I decided to move up, I chose Fikse 9s and 10s in 17 inch size. I used a variety of tire brands and sizes through the years (eg, BFG R1, Toyo, Kumho, Hoosier). Up front I ran as skinny as 235 and as wide as 275. In the back I ran as skinny as 255 and as wide as 275.

The cat's pajamas at the time would have been a Hoosier product that as I recall was 285/30 in an 18. It would have fit the body work, and the gearing would have been great for my local tracks. I did not run it because I did not want to buy 18s-- they were about a grand a wheel. 17s were cheaper. That was all 6-7 years ago. I am not sure whether Fikse and others are still getting a premium for 18s...

I suspect that the tires getting the most attention from a development standpoint are the 18 inch sizes. In fact, I believe that it is quite possible that a hot 18 like the current Hoosier is actually superior to my relatively old-tech Goodyear bias plys. So choice of yire type and brand should of course factor in.

When I was going through all of this years ago, I sought info from pros like the guys at KMR, and they were very friendly and helpful. So my advice is to consider things from an appropriate, informed technical point of view. You can go faster with the right choices... and you can make changes that cost money that have no net benefit. And in between. Being informed is key.

[Jack Olsen]

Originally posted by Brian Cameron
So what did it do to your lap times to go 1 or 2 inches wider, gentlemen?

That's a hard question to answer. If you're still in the first few years of tracking your car, then your lap times are going to continue to drop, no matter what changes you make (or don't make). You're still learning to track the car, and your absolute best time one weekend will just be your baseline the next time you're at the same track. It will drop, considerably, from visit to visit. At least, that was the case for me as I went from 7x8 wheels all the way up to 9.5x11 with the same drivetrain. My lap times kept dropping, but they'd drop on the weekends where I hadn't done anything to the car just as reliably as when I'd made wheel and tire changes. I was simply getting better at what I was doing. What's more, alignment and suspension changes (even settings) go into the same mix, and are going to have a huge impact on lap times. As you dial in your driving and your suspension, you're going to get quicker, period.

Mahler9th's points are well made, especially if (like him) you're able to drive at your car's actual potential. Look at what your class winners are doing, and start from there. Tire technology keeps improving, and the guys at the head of the pack have probably done most of the current work already -- that's why they're winning.

[rexav8r]

Wow, I only had time for a quick post before... I'm gone over night and WHAMM-O look at all the input here... !!!

I think Jack's last post was a BIG key.

As YOU get better with YOUR car and learn how to set the balance of THAT car, entering and exiting high speed turns, or rather rather high performance situations, you may WANT the car to oversteer or understeer more to allow you carry more speed into or out of a turn. Different setups for different applications. You can do that (set the balance) by transferring weight forward or aft with power and brakes which finally comes down to pressure applied on your tires. The suspension setup helps you direct how that pressure is applied to your tires.

I know most of that is pretty fundamental, but it's also very dynamic. Tires; their size and compound make a huge difference in your final lap time.

High performance tires can also be less forgiving. I've talked with several instructors who actually recommend that drivers new to big track speeds use lower performance or even street tires, to learn the performance capabilities of their car and let the car give more feedback before they get into a bad situation (over their capabilities). High performance or tires with more rubber/grip are less forgiving and might not give as much room for recovery.

[Mahler9th]

My advice: as you learn, it almost never pays to focus on carrying more speed. It almost always pays to focus on exit speed.

Disregard if you are racing against me in my class.

- MM