Wheel size and tire size specific to performance driving

[BTF/PTM]

Hi everyone,

So I've done the usual runs of searching and tuned my question to what I haven't managed to find yet. I've pretty much narrowed it down to the Konig rewinds since the offset and weight (and aesthetics) fit the car well, and for a street car the cost of Panasports or Minilites just doesn't seem feasible. So...

The car will be a street/autocross car with the occasional trackday thrown in, and will use only bolt-on suspension improvements.

For the performance gurus, is there a notable difference in handling between a 225 and a 205 tire? Based on what I've read, it looks like a 225 tire on a 16" wheel is pushing the limits of fender clearance without fender lips or special coilovers, so my two choices are pretty well narrowed down to using a 205 on a 16x7 or a 225 on a 15x7. It seems like most folks here use one of those options.

Any feedback is greatly appreciated, hopefully this information isn't actually sitting in plain sight already on this forum.

Edited November 12, 2011 by BTF/PTM

[5thhorsemann]

Alot depends on the ride height in our 240's, I run 235/60-14's and had some rubbing issues that required me to add stiffer springs and stay at the stock ride height.

A 225 or 235 tire has a big contact patch compared to a 205 which is good for handeling, but creats more drag whigh is bad for top end and hurts fuel economy.

The series of the tire 50, 60, 70 and so on, reflects the sidewall height in ratio to the width of the tire, the lower the number the shorter the side wall. The shorter the sidewall the better it handels in corners, the taller the side wall the better the ride.

With a smaller rim, I get more sidewall, a 60 series, which helps with the stiff springs, it handels really well in corners, but it is strictly a street car. You are going to have a 15 or 16 inch rim, so you will end up with a 50 or even 40 series which will give you really short sidewalls and your car will have a harder ride, but corner better.

Also the 225 will be harder (stiffer) at the steering wheel, much more so at low or no speed, due to those big contact patches, unless you go with power steering.

Edited November 12, 2011 by 5thhorsemann

[jmortensen]

More tire is better, and lighter weight is better, so there is a bit of a catch 22 there. If you don't have flares or coilovers then you're limited, but I believe you can go with a 15x8. I would put a 225/50/15 on a 15x8. That size tire is a very good option for a stock fendered Z. You will definitely notice the difference between a 205 and a 225. 205's fit 7" rims best, 225's fit better on 8" rims. I know people put 225's on 7's all the time, but when you're autoxing you'll notice the floppy feeling of a tire that is too wide for a rim in the transitions.

If you're autoxing and don't have a lot of money to spend but want to run race tires, you might try getting a set of steel circle track wheels made, and then mount race tires on those. The spun steel rims are only a couple lbs heavier than an aluminum rim, and I can tell you from experience that if you remove the spare tire, you can get 3 15x8's in the hatch and then put another one in the passenger seat to get them to the track, and still have enough room for a jack, small toolbox and a milk crate with spares and rain gear, etc.

[steve91tt]

I'm a fan of a sticky set of 205 50 15s. They don't have the contact patch of the 225's but they have less rolling resistance and they are shorter than stock which give an effective gearing advantage. There are also more high performance options with the 205s as these are commonly used by track Miatas.

[BTF/PTM]

Thanks, guys!! Jmortensen, I was hoping you'd chime in, you and JohnCoffee have been favorites for the threads I dig thru for info specific to improved performance. I've actually poked around for steel wheels becuz I'm one of the wierdos who likes the look of plain black steel, but the mainstream manufacturers I find all seem to only make 5-lug wheels.

225's on 15x8's may be the perfect option if they'll fit without rubbing. As mentioned, it seems like a zero-offset or some other minimum-offset is preferred for our cars, so the Rewind could still be a good bet.

Thanks again, everyone, keep it coming! =)

[jmortensen]

Rolling resistance is not much of a factor for your average Z that sees autox and track days. For autox especially, it is a non-issue. The largest tire allowed in SCCA F Prepared is 16x10. What does John Thomas run? 16x10. For track days it is probably also a non-issue as well, unless your track has very long straightaways and very few corners. If your track is typical and there are lots of corners, you'll do better going faster into the braking zones with more available traction to slow the car down, faster through the corners and exiting the corner with a higher speed than you by working out of the last corner with a slower exit speed and using the rolling resistance to eventually get an extra 2 mph at the end of a long straight, only to have to get on the brakes earlier.

I speak from experience, having autoxed with 205/55/14 DOT's, then switched to a 250mm slick. The slick was much faster in both situations, even though I had them mounted on very heavy wheels (25 lbs each). My new build will have 15x14's front and rear with 13" wide slicks and the wheel/tire combo will be about the same in weight. Should be fun, but to be fair, I will have a V8 so less concern for rolling resistance.

[Walter Moore]

Jon,

I have always wondered if a stock Z, particularly one with an open differential would see any advantage to a front to rear "stagger", that is with wider tires in the rear. Most modern sports cars do this, at least on the high performance packages. I have just wonder if having a little more tread in the rear would help coming out of the corners.

Do you have any words of advice on that point?

My interest in tires is growing because my existing tires are pushing 6 years old, and the "experts" now recommend replacing tires at that age. Some people in the local club give me a hard time about still having tread on tires that old, but to be fair the existing tires are supposed to be 80k mile tires.

I guarantee that the next set will NOT be rated for high mileage. I want something sticky this time.

[jmortensen]

I think if you have a REALLY high hp car or a car with more weight in the rear, then bigger rear tires makes sense. If you were drag racing, it also makes sense. For a road racer or autoxer I'd be more inclined to run bigger tires in the front on a fairly stock Z, since understeer seems to be a bigger problem in Z's (wider front track also helps drive off the corners). I believe some of the stock class 350Z guys were doing that a few years back, running the rear rims on the front. Stickier tires will help too, but really the solution to your problem is an LSD.

[WW2Winger]

Jon, what calculated front roll couple range do you run ?

[jmortensen]

I wasn't calculating it previously, and I really wish I knew WTF that meant before. I had a pretty unscientific method of doing what my friend who was faster than me told me to do until I got to be as fast as him, then I started talking to different people, taking tire temps, which led to running way more camber than he suggested, more caster, different toe settings, even engine mods like going bigger on the cam, etc. My plan next time around is to run somewhere between 55 and 60% front, now that I know what calculated front roll couple means.

[WW2Winger]

Thanks Jon, wouldn't you agree that autocrossers would tend to run those numbers while road racers would have to run much more front couple? I run between 69 and 79 % depending on the track. On very fast tracks, the only way to get around with an oversteering car is to slow down. When attacking a fast corner the last thing you want is for the rear end to swap places with the front. I would imagine in autocross, you could use some oversteering to help rotate the car around very tight corners. Are we speaking somewhat the same language? Some folks on this forum seem to be setting up for some pretty serious oversteer but I have concluded that they must be autocrossing and needing the turn-in.

[jmortensen]

Just on its face that number seems very very high. The 55-60% number suggests neutral handling or slight understeer. If the numbers or calculations are off to start, then that can dramatically effect the output of the calculations and it's better to use the numbers as a guideline than an absolute, but what you've suggested would predict lots of understeer.

Here's a thread with more technical info: http://forums.hybridz.org/index.php/topic/63492-suspension-tech-motion-ratio-unsprung-weight/page__st__80

And here's one of Dan McGrath's posts on the subject:

The weight transfer worksheet is an implementation of the Total Lateral Load Transfer Distribution (TLLTD) material presented in Race Car Vehicle Dynamics (RCVD) by Milliken.

In the text (chapters 16 and 18) Milliken states the basic relationships between spring rates, motion ratios, static weight distribution, track width, and other vehicle variables to the TLLTD in a steady state corner.

Within the text, are some suggested starting points for setting up a car that will be driven on a road course type circuit.

On page 605, the following recommendations were given: (begin quote)

Ride Frequencies:

Non-Aero Sedan________________1.6 to 2.0 Hz (with front higher)

Aero Cars _____________________3.0 to 5.0 Hz (with front higher)

Roll Gains:

Sedans________________________1.0 - 1.8 deg / g

Aero Cars______________________0.25-0.5 deg / g

TLLTD's

To insure initial understeer, calculate the TLLTD to be 5% more than the weight distribution at the front.

(end quote)

What this is telling you is that if you have 50% of your weight on the front tires, then the front suspension should supply ~55% of the total roll stiffness. Remember, these are only suggested starting points for road racing cars. Cars intended for other purposes (like autocross) will use slightly different values (My car is set up with 2.5Hz frequencies, and Cary and others are exceeding 3 Hz frequencies).

There are a couple of weight transfer worksheets available. The one that we keep referring to is marketed by Smithees out of Australia. As far as I can tell, this worksheet directly implements the gospel according the Milliken. The WTW provides a very user friendly worksheet to input the vehicle parameters. The worksheet then provides a "magic number", which is the difference between the static front weight percentage and the percentage of the roll resistance provided by the front suspension. If the number is greater than 5%, the prediction is that the car will tend more toward understeer, and if the number is less than 5% then the car will tend more toward oversteer.

Although this tool isn't exact (because of inaccuracies in user measurements), it is a great tool for visualizing the effect of suspension changes like spring rates and roll centers.

Edited November 13, 2011 by jmortensen