Balanced Tire Sizes for road racing

[Damn True]

I do admit I'm at a loss when it comes to the what types of races there are out there I would like to compete in. The Silver state classic is one that I read the rules on but I don't consider it a true competition in the sense that your racing against a clock and not another driver.
This Forum is my education and thank you to thoses that teach.

If you are looking to go wheel-to-wheel racing you may find the opportunities very limited.

In order to race against other cars of your vintage there are only two venues:

a) Historic T/A. The problem there is that they only allow genuine log-booked cars from the original T/A series to compete. You cannot build a car and race with them.

b) SCCA vintage. The problem there is that you have to build to period spec with the exception of modern safety items. So no big changes to suspension, 15" wheels, 4 speed, old school rubber.

You can build into a series like the NASA American Iron or American Iron Extreme, and the AV8SS series. A few older cars have run there. But, you will be running against Camaros and Mustangs 35 years newer than your car.

[sinned]

Serious track whores almost always an "R" compound tire or at least a dual purpose. You'll find that most company’s build 275's and some will do 285's with one doing a 295. The bigger, blingier sizes are not typically built for competition use with the exception of the 315 (Corvette OE size). Keep that in mind when selecting tire size.



I think the biggest rear tire anybody on a race track is running on the back of an early "F" body is 275.

[Ralph LoGrasso]

Agreed looks are not bad, just don't claim you are a function over fashion type person while trying squeeze maximum rubber.

This is something I disagree with. If you've got the room, capability, and a properly designed and tuned suspension to go along with them, why wouldn't a 315/335 combo outhandle a 245/275 combo? If the suspension's been designed around the wide rubber, why wouldn't it perform better? Wider tires should transpond into increased lateral grip, and better handling. I'm sure the car will follow damn near every crack in the road (my 275/295 SS does), but on the race track, it should really shine.

[sinned]

Sure, but at what point do you reach diminishing returns? Fitting a 315/335 combo on an early "F" body will not outperform the same car with 275's enough to justify the work.

[alcino]

I like my wheels and tires on the small and light side( I have 205/50/15 front and 225/50/15 rear). Helps braking and accelerating from the reduced rotational mass. I actually noticed the difference going from 45 pound wheels and tires to 35 lbs when i would launch. Felt like it had a more power and lost a half second on my 1/4 time.

Granted my car is a lot smaller than the f body, so 205 may not be enough ( but 245 to 275 should do it).

IMHO its way more about tire compound than size. If you want to be competitive its easier to get R compound tires than to dump thousands in suspension, wheels, and engine mods. Trust me on this one. The drawback is faster tire wear. My Toyo RA1s are getting me about 10,000 miles. I feel this is a good compromise for the competitive edge.


Alcino

what I have
15x8 bassett spun steel wheels 16.0lbs
toyo RA1 205 and 225/50/15 ~20lbs full tread

[Ralph LoGrasso]

Sure, but at what point do you reach diminishing returns? Fitting a 315/335 combo on an early "F" body will not outperform the same car with 275's enough to justify the work.

I agree with you there--there has to be some point where the marginal gain just isn't enough to justify the work, or cost. That point probably comes when you start going in excess of 275s all the way around. But, if you've got the coin and time to make it work, and you want the look / slightly better performance, there's no reason not to go with it. Fitting 315 (or if not 315s, then likely 295s) on the front of an early F-body will not be so hard in the near future. There's an aftermarket sub in the works that will allow that size tire. The rear will require a mini-tub. It takes a decent amount of work, but the larger tires can still be fitted.

Alcino makes a good point about compound and tires as well. You can't pull 1G if your tires will only hold till .9. When I swapped from 17s to 18s on my SS, the car actually handled better with the 18s. Mostly because the 17s were a cheaper brand, HPZ or something like that. I went from 275/40/17 all the way around to 275/35/18 l 295/35/18 Kumho Ecsta MX. I was expecting a slight decrease in handling, and a definite decrease in traction because of the smaller sidewall in the rear -- but it was the exact opposite. Good tires go a long ways. I've seen people that build high performance cars, put lots of money into the susp, etc. and then go with cheapie tires. It just doesn't make sense to me.

[sinned]

Just a point since Ralph brought up tire sizes....Trans-Am cars pull in excess of 1.8g of lateral traction with 16" tires. Just something to think about.


They also pull 0-60 in the sub 3 second world and 1/4 times ~10seconds. So running 335/18's really isn't required to handle very well.

[rohrt]

One thing that just came to mind on why to go with a biger tire in the back and correct me if i'm wrong. If you wind though a 90 degree turn into a strait away and punch it wouldn't you want the extra meat in the back to get the best grip you can to keep from spinning. Maybe I'm just splitting hairs at this point but I was just wondering if that a valid concern.

[sinned]

Well, you don't ever "punch it" on a road course. O/T, HPDE, road racing are all finesse driving skills events. Go punch it on the drag strip. On a road course there is such a thing as having too much HP and most PT cars being built have way too much for a road course car. If you can easily overpower a 10" dedicated track tire you need to turn it down a pinch. Tires should always be balanced, if you have an over steer issue than there is something wrong with the driver or the set-up that needs to be corrected.

[rohrt]

Thanks Dennis

This thread has helped me figure out a few things. One of wich is that there just are not many road race type events out there to compete in in the Mid West. I did find this one

http://www.roadamerica.com/2006/index.htm

There just isn't the Buttonwillow type tracks around hear.
So instead of thinking how to make the best corner carving car I can, I might as well lean more on just making it comfortable, fun and safe. I figure once I get the car built I will be lucky if I can get to two road race events in a year. I go to the 1/4 track more often since they are close. My original goal was to run in the Silver State and I still plan on doing that.
http://www.silverstateclassic.com
So I'm going to look into what other events are out there and try to decide what looks fun.

[Steve Chryssos]

Now turn this whole thing around. The real question here is if most of us are NOT performing the mods necessary to add 275 tires up front, what is max rear tire size that is appropriate for a 255 front? That 245 or 255 front tire is the true limiting factor. Way too damn many 255/335 cars out there now and coming together. Folks see the Mule's 335's and overlook the 275's up front. And they forget or just don't know how much work went into fitting those 275's AND maintaining proper turning radius (the frame rails were cut)

As rear wheel power output increases, a wider rear tire will be beneficial (up to a point-700RWP will still fry a 335/30), so greater tire size differential can be justified. The numbers change with other variables as well such as overall weight, weight bias, etc. AA small light car needs less tire than a big heavy car. A mid-engined F40 really does need big ass rear tires. More differential is appropriate. So a hard and fast 1.0:1 rule is a bit oversimplified. Maybe 1.2:1 is better as a guideline.

The only hard and fast rule I can think of is as follows:
The big doggies (the cars that can make a thudding sound when they whip it out and lay it on the table) are NOT sporting a significant front to rear tire size differential. And they're NOT running steamrollers that don't want to turn. If you're stuck with a 255 or smaller tire up front, the only reason to run 335's is to overcompensate for something. Hopefully it's just too much horsepower.

335's look cool. Mini-tubs are cool. If you have a typical 3300lb car with 500RWP and a 255 up front, draw the line! Buy those mini-tubs and then fill em with 295's or 305's. 600HP? Go 315mm. It's not like the mini-tubs will shrink over time. You'll always have room for 335's if you decide at a later time that you absolutely, positively just gotta have 'em.

And if you come across someone with 235/335's and a bone stock ZZ4?

Snicker.

[redss86]

335's look cool. Mini-tubs are cool. If you have a typical 3300lb car with 500RWP and a 255 up front, draw the line! Buy those mini-tubs and then fill em with 295's or 305's. 600HP? Go 315mm. It's not like the mini-tubs will shrink over time. You'll always have room for 335's if you decide at a later time that you absolutely, positively just gotta have 'em.

So, if I were to have a 550-600hp engine, and can only fit a 255 front, then I should be ok w/ a 295 rear? These will be on an 86 Monte Carlo so it will be little be heavier than your f bodies. Also, could I get away w/ the 255 on an 8" rim, or do I need a wider rim? TIA

Joe

[Ralph LoGrasso]

Joe, you would probably be better with the 255 on an 8.5" rim, but an 8" rim should work.

As for the 295s with 550hp, and the entire tire debate really. It's all subjective. I can spin my 295/35/18s with 360hp in my 4th gen SS. Mat the gas from a stop or a 5-10 roll, and they'll spin plenty nice. I'm sure you'd be able to spin them through 2 or 3 gears with 550hp. Once you start talking about excess of 500hp, and "low profile" street tires (not drag radials) traction is always going to be hard to come by, no matter the size. A wider tire will likely grab more than a thinner one (especially coming out of turns), but don't expect a 335/30/18 to dead hook a 500rwhp car. You'd have a much more balanced car with the 295 rear, so that would be your best choice, IMO.

[Damn True]

Serious track whores almost always an "R" compound tire or at least a dual purpose. You'll find that most company’s build 275's and some will do 285's with one doing a 295. The bigger, blingier sizes are not typically built for competition use with the exception of the 315 (Corvette OE size). Keep that in mind when selecting tire size.

Not many offerings in that size (315-35-17) is there?

Plenty up through 275/40, but after that.....not a lot.....other than "R" compound.