optimal street tire size for RTR on 17x7

[mrazny]

Originally Posted by 7thgear

the thing about slalom, is that you're limited by rules in terms of rim size and tire

changing the wheel costs you points, changing the tire doesnt

so you're at an advantage stuffing as large a tire on the car as possible and focus on keeping the car flat (like putting on huge swaybars)

this WORKS very well for AUTOCROSS

In SCCA stock class rules, you can change the front, or you can change the rear (new this year), but you cannot change both. Just wanted to be sure this doesn't lead to anyone getting confused rules wise.

There is an eventual disadvantage going wider with "street" style tires, which for SCCA is 140 UTQG or higher. With Hoosier A6's their construction handles pinch very well. With say Dunlop Star Specs, though people do think their sidewalls are stiff, the pinch change the shape (limits contact patch), and the flex under cornering is definitely there. I'll try to find the video of an S2000's rear tires and how much the tire slides.

After a certain point, all you are doing is making the tire more vague without truly gaining lateral grip. Finding that certain point though is always the key. No amount of thought replaces testing though.

[des]

Originally Posted by mrazny

After a certain point, all you are doing is making the tire more vague without truly gaining lateral grip. Finding that certain point though is always the key. No amount of thought replaces testing though.

I've driven Toyobarus on 215 and 235 Dunlops -- not on the same surface on the same day -- but the 215s felt overmatched in the rear to me.

In comparison, the 235s don't feel especially vague on turn-in (and I'm on the 45-series). I do notice a hit on gearing.

A 225 might be a nice compromise -- who knows, until someone tests for that -- but I'm going to stick with 235s (only 40 series this time) for Nats.

[simpleisbest]

Is the pinch something that could be lessened with more psi? I imagine running more psi is standard for RTR to provide more overall spring rate. (while trying to not run too much to adversely decrease contact patch/grip)

Also, what would be the best/easiest way to measure tire flex while in grid (ie, returning from a run)? Pyrometer? Tread wear/sidewall scuff? Chalk sidewall?

[bneale]

Originally Posted by simpleisbest

Nice! Do you notice any significant loss in acceleration with the taller tire?
How is turn-in and feel in slalom transitions? What psi were you running?
(sorry for all the questions!!! LOL)

Sorry I'm late in responding. I was on my lunch break. I don't mind if you ask me questions at all. I'm glad to help but I must admit, I'm just starting to get into Autocrossing. I've only competed in 2 events to my experience is limited.

However, that being said, the person training me is the guy driving for Team VCMC in the Scion Tuner Challange. I respect his advise WAY more then anyone else as he was the 2003 Canadian C Stock Champion and I'm telling you right now, he's going to win the tuner challange as long as it comes down to driver skills.


When I started the day, I had 42.5 PSI in both the front and rear tires. I took them both down to 40.5 PSI and started my first run. After completing the course, I checked and they had come back up to 42.5 PSI.
I kept bringing the PSI down to get the front end to grip a little more. I found that I like to have around 40PSI on the fronts when the tires are warm and about 39PSI in the back.

At the end of the day I found I like a little bit less pressure in the back then I did the front. I used chalk on the sidewalls of my tires to see if the tire was rolling over or not and it looked perfectly fine.

The tires themselves performed well above my expectations. As someone mentioned on here, they are slightly taller then the Primacy's I had on before but you almost want this. Because they are taller, I could hit over 100kph in second gear. It does hurt accel slightly, but remember, they are only 8 tenths of an inch taller then the Primacy's.

Turn in was REALLY good with these tires once they scrubbed in. Turn out was also really good. I got a little sideways more then once but the tires basically just said "thank you for the slip angle... can i have some more" and kept going right a long without even a hit of losing control. Compared to when I ran the Primacy's both track days were STUPID hot outside but the Dunlops were 10 times better. Huge difference compared to the stock tires and worth every penny I spent.

Over all I came 2nd out of everyone on street tires. I'll be competing at the VCMC event next in the RTR class and I'm trying to get Scion to pay me for racing.

For those that are not aware, check this out. First = $500 bucks or more then half the price of my tires.

http://www.scion.ca/scion/en/lifestyle/racing


bneale

[simpleisbest]

Thanks! Awesome feedback!!!

[bneale]

Originally Posted by simpleisbest

Thanks! Awesome feedback!!!

The best part about the Dunlops is that even though they are a wider tire, they are cheaper over all to replace then the Primacy tires I had. I think Tirerack has the Primacy's for 188 but my Dunlops were only 166.

bneale

[UncleFester]

Too bad they don't offer that contingency in the US.

[mrazny]

That's nice that Scion Canada does that for you guys. It doesn't look like a similar program exists (at least yet) for the US.

National style autocrossers take every tenth they can get for short tires. "Only 8 tenths" is not something you'd here very often from Jacket wearing National Champions. Doesn't mean it isn't the right compromise, but it is a compromise. .8 is a lot.

[simpleisbest]

Hmm...I might try the 235/40-17 Dunlops out once Tire Rack gets the stock class sized RPF1's in.

According to an online tire calculator, with the 235/40-17s it looks like you loose about 0.5mph at the top of 2nd and "lowers" the height of car by 0.1" vs stock and by almost 0.46" vs 235/45-17. Not sure if I want to raise CoG by almost 1/2" as a trade off for more lateral grip.

http://www.1010tires.com/tiresizecal...?action=submit
(actual measurements will most likely be a little different due to differences in tire construction)

[AlanLGuy]

Originally Posted by bneale

The best part about the Dunlops is that even though they are a wider tire, they are cheaper over all to replace then the Primacy tires I had. I think Tirerack has the Primacy's for 188 but my Dunlops were only 166.

bneale

True, but look at the treadware ratings. The Dunlops are going to wear quite a bit faster, especially if you are autocrossing them regularly. Most people go through a set a season.

Also not that it matters a whole lot, but I daily drove on my Dunlops the week I got them to put some miles on and I saw a significant drop in gas mileage vs the Primacy's (from 32-26, though I expect it's really closer to 3-4 miles per gallon). The primacys are a low rolling resistance tire while the Star Specs are not.

[UncleFester]

Yea it's not fair to compare them. Apples to Oranges.

[mrazny]

Why lose gas mileage? Because Racecar

[Gearhd79]

Originally Posted by simpleisbest

I'm looking to use 235/40-17 in a couple months, once my RPF1's are available.
Most likely go with the Dunlops.

Good to see this thread!!

I was thinking exactly the same setup for CS

[funbeatsfast]

http://www.ft86club.com/forums/showthread.php?t=12609

Looks like Star Specs or maybe Yoko Neovas for 2013