Size Matters

[???]

Hello, I have some questions about sizing and weight. A long time ago, I read an article somewhere about how wheel size affects acceleration, stability, braking, handling, etc. I have been trying to find the article but I can't so I figured I would ask my questions here. I know that some people have a specific set of wheels for the track that are a specific size and they use competition tires on them. I also see many people slam there car to the ground, go staggered (which i have been told causes understeer), and go so wide that they are constantly rubbing the tires/wheels against suspension components, brake components, the fenders, or the fender wells. In the article i read, the author said that most people assume wider is grippier because there is a wider contact patch but it isn't always 110% true. He said increasing diameter has something to do with speed and i know the heavier a wheel is, the slower the car will be. So, my questions are:

1. how exactly to the dimensions of the wheel effect performance on the track

2. how do you choose a wheel size

3. what are some good track-day wheels (I've seen some light enkei's, motegi's, rotas, ssr's and ray's wheels. they must be light and durable. a cool design is great but that is the thing with the least priority)

4. are gram light 57xtreme's good track day wheels

5. how do you choose the tire size (lower or higher sidewall? 1 size wider or slightly stretched?)

6. how do staggered setups affect performance? what about stretched tires?

and i've read a lot of threads where people say the wheels they are running are the best even if they don't even know the weight of the other guy's set of wheels. you can't say something is the best without considering other things especially if you haven't bothered to look at the weight or something. PLEASE don't do that.

thanks

[???]

also when recommending a set of wheels, please put aerodynamics into account if it makes a difference.

7. how does offset affect performance and tire selection?

[???]

By the way, I am fine with having fenders rolled if it is necessary.

[Gary in NJ]

Lots of questions...and a lot of opinions will follow...and here's just some of mine.

First off, you must think of your wheel and tire as a system. On a car that makes only 140-ish pounds of torque the weight of the wheel/tire combination is critical...even on a 2,700 pound car. So many people spend countless hours choosing "the best" wheel; laboring over size, offset, weight, looks, and then slap on a tire that weighs 27 pounds.

Unless you are going to go FI and achieve 180 (or more) ft/lbs of torque, you will be well served to think of the weight and configuration of your tires as part of the cars performance, and less so as "wheel jewelry".

Choosing a wheel and tire is an art of compromises. Wide wheels and tires weigh more...a lot more. All else being equal, the bigger you go, the more power you will need to move the weight.

1. how exactly to the dimensions of the wheel effect performance on the track

Covered above.

2. how do you choose a wheel size

I went with 17's because they are light (16's are lighter still) and I can find good rubber in a rim width of 7 to 8.5 inches. I didn't choose a 16" wheel because in order to achieve the OEM outer diameter of 24.6, I'd have to get a tire with a high 50% aspect ratio. Compromise.

3. what are some good track-day wheels (I've seen some light enkei's, motegi's, rotas, ssr's and ray's wheels. they must be light and durable. a cool design is great but that is the thing with the least priority)

Establish a budget, find a style that moves you...then get obsessive over weight. I think I can now name every 17" wheel under 18 pounds. My short list included 6 wheels that I liked. In the end, I didn't choose the lightest in my budget (Kosei K4R 17x8 et36 @ 15.8 lbs) because it just looked a bit to plain; rather I chose a wheel that was closest to my favorite style. My favorite style was at the top of my "allowed" weight range (TSW Nurburgring Bronze) so I chose the Enkei PF01 17x7.5 +38 @ 17.2 lbs. Compromise.

4. are gram light 57xtreme's good track day wheels

Of course they are. They look good too. Very similar to the Enkei PF01's (I don't know who is copying who, nor do I care). They didn't make my final cut because in a 17x7 they only come in et42 & et50 and I wanted to be in the et35 to et 40 range.

5. how do you choose the tire size (lower or higher sidewall? 1 size wider or slightly stretched?)

Once again, you are power limited, as a result so are your choices. Sure, you could slap 255's on, but you wont be able to move 'em. There's nothing wrong with the OEM size spec. With good track tires you'll be hard pressed to out drive them. With that said, many people also choose 225 section tires for a 7 to 8.5 inch rim. If you want to keep stock final drive ratio and keep your speedometer accurate, then 215's or 225's in the stock 45% aspect ratio is just fine. Also, think of the sidewall height (aspect ratio) as part of your suspension. The less sidewall, the harder the ride. However, shorter or stiffer sidewalls also improve handling (turn-in and recovery).

6. how do staggered setups affect performance? what about stretched tires?

Stretched rubber has nothing to do with (good) performance and is chosen as form over function. A staggered stet up is appropriate when the engine makes enough power that performance is compromised by too little traction during acceleration.

7. how does offset affect performance and tire selection?

Offset has an effect on scrub radius. When extreme, offset can effect initial turn-in, how the vehicle rotates and how it recovers. It can also lead to tram lining (following low spots in the road). All of this is true for too wide or narrow of a section width as well. None of this should be an issue if you keep the offset reasonable (et50 to et35) as well as the section width (205 to 235). Personally, I chose a +38 offset because I like the flush look. I would have been just as happy with anything in the +35 to +40 range. Remember, the wider the section width, the more off set (higher numerically).

Last thing. A lot of people choose tires based on some magazine shootout or what a friend recommends. Nothing wrong with that...but you really need to be a little weight obsessed here too. One thing is for sure, any performance tire you choose will have a transformative effect on this car. Even the *worst* performance tire will be light years ahead of the OEM hoops. Look at tirerack when choosing tires as they include tire weight. Assuming a 225/45-17 tire, Dunlop Sport Maxx RT's weigh 18 pounds where Dunlop SP Sport 01's weigh 27 pounds each. You need to look at specs like Loading and Speed range with a analytical eye. Sure, a 94 load rating is "better" then a 91...but you don't need it on a car weighing 2,700 lbs. That difference in rating adds weight. The aforementioned Sport Maxx RT with a 91Y rating is 18 pounds, with a 94Y rating it's 21 pounds. All of that money spent on light weight wheels lost on the tires. Oh, you paid more for the heavy tires too.

A lot of what I provided above is generalized, but serves as a good rule of thumb. Think about everything you want out of your "system", and then adjust it based on your budget and what is commercially available.

[???]

thank you that was very helpful. There is just one thing that I'm not 100% clear on: Is a wider tire grippier?

Also, where is the thank you button?

[Gary in NJ]

Originally Posted by ???

There is just one thing that I'm not 100% clear on: Is a wider tire grippier?

A wider tire isn't necessarily gripper. "Grippiness" has as much to do with tire compound as it does width and thread pattern. But all else being equal, the same tire in 215 will be gripper in 225 due to its greater surface area. This greater surface area creates additional friction.

A tire creates "grip" through friction. A wider tire has a higher coefficient of friction. Of course, you need more power to overcome that friction; even while driving in a straight line.

There comes a point of diminishing return with regard to section width. Like I said above, a summer (max/ultra) performance tire width in the 215-225 is a very good size for our car. If you go wider you will add weight, no way around it. You will also add friction. Any wider then 225 and you'll need to reduce the aspect ratio, further adding weight (to obtain a stiffer side wall) and compromising ride comfort.

[fatoni]

Originally Posted by Gary in NJ

A wider tire isn't necessarily gripper. "Grippiness" has as much to do with tire compound as it does width and thread pattern. But all else being equal, the same tire in 215 will be gripper in 225 due to its greater surface area. This greater surface area creates additional friction.

A tire creates "grip" through friction. A wider tire has a higher coefficient of friction. Of course, you need more power to overcome that friction; even while driving in a straight line.

There comes a point of diminishing return with regard to section width. Like I said above, a summer (max/ultra) performance tire width in the 215-225 is a very good size for our car. If you go wider you will add weight, no way around it. You will also add friction. Any wider then 225 and you'll need to reduce the aspect ratio, further adding weight (to obtain a stiffer side wall) and compromising ride comfort.

i think the issue at the root of this is that tires are meant to operate in certain conditions. if a tire gets too wide, it can become impossible to warm the tires up because the car cant generate force faster than the tire can dissipate heat. it used to be the case where tires were often too wide but with modern tire compounds it becomes less of an issue i think were starting to see that the fastest tires on a car are growing in width.

[wparsons]

To add to what's been said already, going to a staggered setup will 100% increase understeer unless you change the spring rates to compensate for the change in grip balance.

[chadstyle]

Originally Posted by wparsons

To add to what's been said already, going to a staggered setup will 100% increase understeer unless you change the spring rates to compensate for the change in grip balance.

I disagree fully. I have a staggered setup with 9.5" rears and 8.5" fronts and I'm running 255/235 respectively. It didn't change the overall handling characteristics and my car doesn't "push" or under steer in the corners at all. In fact, it has far more grip and control without making the steering heavy and I can still toss the rear end around with a simple flick.

I also worried that a staggered setup would cause poor handling results but that's not the case. I've added a decent amount of TQ and HP and the added tire in the rear helps get it to the ground. I plan on going 275 rears after going FI but I'd say 255s are sufficient for NA builds and track days.

If you're keeping these wheels as your only set I'd go with some 17s and keep them as light as possible.

[wparsons]

Originally Posted by chadstyle

I disagree fully. I have a staggered setup with 9.5" rears and 8.5" fronts and I'm running 255/235 respectively. It didn't change the overall handling characteristics and my car doesn't "push" or under steer in the corners at all. In fact, it has far more grip and control without making the steering heavy and I can still toss the rear end around with a simple flick.

I also worried that a staggered setup would cause poor handling results but that's not the case. I've added a decent amount of TQ and HP and the added tire in the rear helps get it to the ground. I plan on going 275 rears after going FI but I'd say 255s are sufficient for NA builds and track days.

If you're keeping these wheels as your only set I'd go with some 17s and keep them as light as possible.

Then you have a car the defies physics... adding more tire out back increases grip in the back which means the rear now has more grip than the front where stock is equal.

How hard do you drive your car, have you taken it to a track or auto x? I can guarantee that if you drove your car with a square tire setup on a track (same tires as you currently run, just same width all around), then drove it again with your current tires you would definitely notice that the car is less willing to rotate (rotate != spin/oversteer...) and more willing to push slightly through corners.

[Gary in NJ]

Originally Posted by fatoni

i think the issue at the root of this is that tires are meant to operate in certain conditions. if a tire gets too wide, it can become impossible to warm the tires up because the car cant generate force faster than the tire can dissipate heat. it used to be the case where tires were often too wide but with modern tire compounds it becomes less of an issue i think were starting to see that the fastest tires on a car are growing in width.

Good point. And while this is off topic, its worth mentioning.

Never run brake pads designed for the track on the street. Race pad are meant to operate at a temperature range that you will never operate on the street. The result of using race pads on the street; longer stopping distance, less feel and less money in your pocket. People see the word "race" and just assume its better.

[ZDan]

The amount of extra lateral grip from 20mm greater section width isn't enough to radically transform the handling balance of most road cars. Running all manner of different tire stagger on the s2k (205/225, 225/245, 205/245) didn't really affect the car's balance at the track. On my FD I ran 245/275 and then 255/265 on back to back track weekends, no big change in its handling balance either. That said, on a 200hp 2750 lb 54/46 F/R car, I don't see any benefit to staggering tire widths. I'd probably try go 235/40 square.

[chadstyle]

Originally Posted by wparsons

Then you have a car the defies physics... adding more tire out back increases grip in the back which means the rear now has more grip than the front where stock is equal.

How hard do you drive your car, have you taken it to a track or auto x? I can guarantee that if you drove your car with a square tire setup on a track (same tires as you currently run, just same width all around), then drove it again with your current tires you would definitely notice that the car is less willing to rotate (rotate != spin/oversteer...) and more willing to push slightly through corners.

I would normally agree with you here. I have tracked the car once (waiting on seat lowering brackets for future track days) and I have raced Shifter Karts and dirt bikes since my early childhood to present. I will be the first to admit that I thought a 235/255 setup would cause a slight push in the corners and nothing. In fact, for the past few weeks, we have had serious rain in South Florida and behind my office we have a HUGE open lot. I have setup a small course and run through it with full grip and other runs with no grip at all. Never once have I felt it push and I was blown away by the grip in the wet conditions. When it does finally start to let loose its extremely controlled and it happens so slow that you can take a moment to decide to reel it in or let it all hang out. Really a fun little car.

One thing you did mention in your original post that I kinda skimmed over was the spring rate equation.. You're right and when I was shopping springs I looked for a stiffer setup with a close ratio to the factory front/rear springs. I also installed sway bars (the lightest one offered in the rear on lightest setting currently) so I must admit that this all plays a roll in my setup.

I would venture to say that if left bone stock on the suspension but add the wider rear wheels I would have noticed a major difference in turn-in and steering response. I fully agree with you if the springs/sways are stock then you will disrupt the handling.

I personally have a end goal in mind. I've seen tests where they lap the car on stock rubber, then upgraded rubber at same tread width, then +1 rubber all the way around. The fastest lap was the one in the middle with upgraded rubber on stock width tread. They did turn some higher corner speeds on the +1 setup (I'm 95% sure it was 235s all the way around) BUT, the car isn't strong enough to move the added mass down the straights. My goal is 300 whp on 93 and 215+ TQ. I don't want to do everything twice so I'm building the car with that in mind.

There's a lot of factors to this equation that the OP must take into account when deciding on how far he's going to go with this.. Is it just wheels? Suspension too? Gonna start adding power? How much power?

Hopefully he can read my post here and ask himself the same questions and get a better idea on what will suit him best.

[???]

are there any benefits to a larger diameter?

thanks for all the responses by the way