Keep the 275/ 315 tire setup or trade off for less weight?
 

I finished my track car last year, but did not get to make it out to any large tracks. Only did a autocross and a couple little lapping sessions on a banked oval. So I do not know how well the current tire setup is really working. I am eager to shed weight off the car, as each rear tire/wheel combo weighs in at a hefty 55lbs.

Here is a quick spec of my car-

Weight- 2520lbs w/o driver
Power- 428 rwhp non-turbo 20b
Downforce- APR GT wing
Custom front splitter with undertray
Custom rear diffuser
Suspension- Showa custom coilovers
Sway bars- stock
Front tires- 275/40 -17 nitto Nt-01
Rear tires- 315/35 -17 nitto Nt-01
Front weight 48%
Rear weight 52%
Crossweights- see pics

http://i254.photobucket.com/albums/h...DSC01353-1.jpg
http://i254.photobucket.com/albums/h...DSC01354-1.jpg

So my question is, will a chassis with my current weight/ power, and downforce be able to fully utilize a 315 wide rear tire? Can it get the tires up to temp without issue? Would the drop in unsprung weight and rotational mass of going to a 285 rear tire in a 18" size make for a faster lap time, even with less rubber? Switching tire/rims would reduce 15lbs a corner.

Anyone have experience with trying these sizes on a road course?

Thanks for any input!

My experience is limited track-wise, but I would opt for the lighter wheel/tire combo. This is why I went with the wheels I did with my setup. I'm shooting for ~2600lbs, and went with a 255/40r18 on all 4 for my street setup. With a good tire compound you shouldn't have any traction issues, but its a bit up to you what you are willing to trade off for lateral grip. If youre shooting for an all-out grip monster, Id do the 18x10/10.5 all around whatever size (~275-295) R-compounds you favor. I know your car is making more power than mine will, but I see no real need for that large of a rear tire in your application. Ive dealt with ~450rwhp single turbo cars running 265 rears in something like the direzza star specs for Advan AD08s with little traction problems. Im assuming since this is your track car you are referring to R-comps, which I have much less experience with, so my "advice" may be a bit skewed. 15lbs per corner is a big deal though (as Im sure you know), and Id gladly trade off a slight bit of grip for the advantages that come with lower weight. I also question if you can properly get those tires up to temp. Have you been checking with a pyrometer at your track days? Really the only way to know for sure whats going on.

With 400 plus rwhp go even bigger if they'll fit and also your car must be pushing with 275s in the front unless you have a tight rear and loose front.

C5 corvettes that weigh more and make 100 less horse power run 335s and they have massive rear wings.

I have 333 rwhp and run 275 40 17 all the way around and over heat tires after about 4 laps. My car could easily use 315s and I have 100 less hp.

Rotational weight is huge though so don't go cheap on wheels but again you could use more tire if it will fit.

You might have an issue if you were talking about a street car with conservative alignments that might not take advantage of the full width of the tire and cost you in weight/rolling resistance, but on a track car like yours, you're probably able to use all you can stuff in there.

Yeah, I am not very concerned with straight line grip as like you mention R-comps grip pretty well in that department. Once in a while I can loose a little bit of traction in 1st gear, but on the track it will be 2nd gear and up. Didn't have a chance to check tire temp, as I went to those track events mainly to monitor the engine temps and reliability of all the fittings.

This makes me happy to see, as keeping my current setup is easier and cheaper :) I do not have a ounce of understeer surprisingly. Only time I could provoke it was by being overzealous with the steering through extremely tight low speed turns. Still was very little. With the engine so far back and down in the chassis, the front end wants to change direction without fighting. Current wheels weigh 19lbs each in the rear and 17lbs each in the front. Would like to buy lighter ones, but not sure I can justify the cost for 4lbs a corner


Looks like for this season I will try out the 315's. Will buy some kumho v710's though :)

Kumhos are good for autox but get greasy in a hurry on road courses and consequently will wear very fast while you run slow laps sliding through the corners.

Good to hear on the turn in. Nothing worse than waiting for the car to turn before applying the go pedal.

What R-comp is doing best on road courses? I heard some positive things about the new BFG, any truth to that? Also have a few guys telling me the R888's are junk, which the NT-01 is virtually the same tire and compound.

Hoosier R6's are a popular tire among the tracks guys. I heard that the NT01 was very good for what it is (not as sticky as Hoosiers, Hankooks, etc...but last longer). I've also been told the compound was the same at the Toyo RA1 which has proven itself a very good r-comp that lasts a long time.

With that HP if you actually use the right pedal nothing but a Hoosier or race tire/slick is going to work. 888s, NTs, RA1s etc.... will get hot and melt. Cheap tires are like cheap brake pads in the end you're better off just spending money than dealing with the issues of a poorly performing product.

RA1s and likely NTs are fine for low HP light weight slow cars that won't heat them up like spec miatas but a car like yours if driven will chew those tires up in no time.

PS Hankooks are great for the money but they won't come in your size.

the R888's get compared to the Ra1's. people love the Ra1's, and supposedly they are the same compound, but the construction is different.

r888 vs ra1 needs a different alignment, AND the Ra1's like to slide around a little, and the R888's do not, so it takes a different technique too.

we were faster on the R888's vs the Ra1's, not by a lot, but its the same compound. we have then switched to the hankook slick, and dropped 3 seconds without doing anything else. again the hankook takes a different chassis setup than the R888.

and we're about to switch to hoosiers, which should need a different setup.

as far as wear goes, we're running a honda, and we've run the 25 hours of thunderhill with the Ra1, R888, and the hankooks. the R888's might last a little longer than the hankooks, but not much. the hankooks were going about 6 hours on the front...


um the other data point, are the imsa GTU cars, they were running something like 305/16 and 335/16's at a weight of 2250lbs, with less power than you....

I cooked some new 888s in 4 hours of track time. 333 rwhp and 2650 weight.

IMO 888s suck.

incomplete data point. we've cooked tires too.

although price vs performance the hankooks are WAY better than the R888's

The RE Amemiya Hurricane 7 is running 18x10 all 4 corners and Yokohama Advan A050, 265/35/18

That turbo 20b engine is making over 600 rwhp so they must know something we don't running those small tires with so much power.

http://octanereport.com/tuned-cars/2...mazda-rx7.html

Excellent information. In SPO class if I run race slicks, then the power has to go down. Which I actually like the idea as it will allow for a lower redline/ less abuse on the engine and drivetrain.


Definitely a good point on GTU race cars.


I keep seeing that as well. I know time attack in japan uses a spec Yokohama tire, wonder if they are limited in tire width too? Kinda ironic all of them seem to be 255 or smaller on RWD cars....

That's pretty much a show car, but I have noticed Japanese tuners running smaller tires—perhaps due to regulations for whatever race/time attack classes they run. It could be for a time attack format where you just get a couple laps, the small tire will get up to temp quickly and not have as much rolling resistance, and you're not worried about overheating and longevity because you'll always run fresh ones.

I wouldn't say they know anything we don't know, and I'll go back to my original premise: I've never seen anyone in racing run less than the maximum amount of tire they can under regulations. if I could stuff more tire under mine than the 18x10, 285/30/18 all around setup I have, I'd be all over it.

I'm not looking for outright speed at this point but something that will last me for a several track days and wont break the bank. Do you think the NT01's last longer on track then some R6s? I would think so but I figure I'll check who knows a bit more.

that's actually a good point, we're running PT and TT this year, and we're limited on tire size, in both classes. actually come to think of it, PS1, honda challenge, E2 and specE30 are all either on spec tires, or limited by tire size

so logan, are you running in a particular class?

Yeah, Nasa ST1 class. They have a great set of rules, that either subtract or add to your power/weight ratio.

yeah i just penciled the numbers, your car fits really well.

Tires
 

After years of playing with tires we went to slicks three years ago. I'll never go back. The performance is so superior and the life is the same if not better. I buy one set a year of Yokohama A005 slicks and they last a season of time attacks and one major race event (Edmonton Indy support race). The slicks are good for about 2 seconds a lap on a 1:05 course over the RA-1's with no other changes.
I run a 260/640/17 up front and a 280/650/18 out back on Enkei RPF1 rims and 400 rwhp. The whole assembly is half the weight of my r-comps and the tires last the entire season. We also run a third gen with the Yoko/Enkei combo, but he runs the 280's all around and 440 rwhp.
He has a site at:
www.4kruzn.com

My site is
www.rxracing.com

after thinking about it, i'd take the points, and run the biggest stickiest tires i can cram in there.

my experience is a little limited, as the hoosiers get the first break in next weekend, but yeah the hankook slick was 3 seconds faster for us, with the same tire life as the R888's, and they even cost about the same, so it's a pretty easy choice.

if you have to add weight, in our experience 50-100lbs isn't as significant to lap times as sticky tires.

So do you think if I go 315's rear and 285 front slicks that it would be too much? I am eager to run slicks, because as you stated they are much lighter and much stickier.


Yeah, I am extremely interested in the what the result of hankook vs. Hoosier is for you. Hankooks price is really attractive.

Something to consider...I know that Bill (blackey) had a serious push with his car last year when he was running 285/30/18F and 335/30/18R. (4 inch total stagger)

see Bill's build thread

I would take a look at his thread and send him note. Bill is a nice guy and will tell you what he has learned about running high HP cars with wide rubber.

Guy

I did read his thread a while ago, super nice car. I will get in contact with him sometime to gather his thoughts. One thing to remember is my weight distribution, which is 52% heavy in the rear, and the engine is way back and low to the chassis. So far this has resulted in 0 push, even with large stagger.

Searched around and it doesn't look like the hankook's come in 315/35-17?