Type R Club of America

aklackner wrote: ↑Mon Sep 24, 2018 10:28 pm I’ve spent 0 time in on the track but isn’t geometry a concern? Stretching means the tread is smaller than the rim cross section (think trapezoid)? If so the anytime there’s force where the tread is pulling against the cross-section you’re only pulling on a single bead vs where the tread section is at least the width of the cross section you’ll always be pulling against the bead on both sides right?

Interesting line of thought but I don’t see that there’s any difference (within reason) in the load paths for stretched vs. square vs. pinched. In all cases, the wheel is laterally displaced until there’s sufficienct tension across the tire to stop the movement. I’ve got pics of one of my EF’s on 15x9’s with 225’s and you can see the tire deformation is similar to what you see with a pinched tire (like a 225 Hoosier in a 7” wide wheel)... the advantage to a slight stretch or square fitment is that there’s relatively “less” time/displacement before the tire loads up. The result is better feel and peak grip.

That makes sense but at least with a pinched setup the limited sidewall is pulled into the lip, with a stretch it’s also going to be pulling away from the seat.

Clearly from the number of people running stretched setups and lack of failures attributed to it the forces may not be sufficient to actually present an issue.

I also assume it’s going to have an affect on cornering force, so understeer to some degree, unless also running a taller tire.

aklackner wrote: ↑Tue Sep 25, 2018 12:04 pm That makes sense but at least with a pinched setup the limited sidewall is pulled into the lip, with a stretch it’s also going to be pulling away from the seat.

That’s not how it works... the loads are the same. The only difference is going to be the amount of wheel to tire deflection before you hit whatever limit is dictated by the dimensions and construction of the wheel/tire assembly.

Clearly from the number of people running stretched setups and lack of failures attributed to it the forces may not be sufficient to actually present an issue.

There’s a relatively large number of folks (think of all the Autocrossers, track day folks, and LeChump racers) across the last ~10-15 years of street tire development and literally tens of thousands of sets of tires. If there was even a low rate of failure on “stretched” setups, the feedback would be out there. This stuff has a ridiculously high visibility in the Autocross community due to how big an advantage a couple tenths are and how much information sharing goes on.

I also assume it’s going to have an affect on cornering force, so understeer to some degree, unless also running a taller tire.

A stretched tire will lead to understeer? 100% wrong. If anything, it’s the opposite. The better supported tire stays more “square” to the track/road surface and tends not to rollover like a pinched tire. The result is better grip and less likelihood of understeer.

I appreciate the discussion and commentary... keep the questions coming.

PS
I’m not a tire engineer so maybe there’s an underlying concept/idea that I’m missing.... just my $0.02.

http://what-when-how.com/automobile/veh ... utomobile/

Question on the effect on understeering was based some reading around the effect of a tire’s cornering stiffness on slip angle. Just thinking the stretched sidewall would resist the deformation of the contact patch a bit more?

aklackner wrote: ↑Tue Sep 25, 2018 7:34 pm http://what-when-how.com/automobile/veh ... utomobile/

Question on the effect on understeering was based some reading around the effect of a tire’s cornering stiffness on slip angle. Just thinking the stretched sidewall would resist the deformation of the contact patch a bit more?

Yep, I’d agree that a stretched tire should resist contact patch deformation better... which means a higher grip potential for that axle. This shouldn’t mean understeer unless you’re thinking that the rear would pick up relatively more grip than the front of the car? Even if this were the case, it’s easy enough to get the balance back with tire pressure, alignment, and spring/bar.

Interesting to see the profile difference of the 205/50/15 RE71R on 15x8.

I have just about finished off my first set of 205/50/15 RE71R on 15x7. 7 auto X with one more next weekend. 2 lapping days. Plenty of cruising. I did have a shop flip the tires as the outer edge was wearing fast. Stock suspension and alignment.

Two weeks ago I ordered a new set from Costco.ca $670 CAD plus a $70 CAD Costco gift card that comes in the mail instead of the instant rebate they did last year. $600 CAD ($465 USD) mounted and balanced. The price can't be beat up here in Canada. They just arrived and I will get them mounted in a couple weeks. They will just sit until next season anyway but I wanted to take advantage of the rebate.

The only downside for me is the tread wear is too soft for lapping days. A friend with an AP2 S2000 is already on his second set this season and they are looking worn. I think I would prefer the Toyo R888 for lapping (HPDE) since they will last longer.

I just want to say thank you so much for your help, Xian.

g3teg97 wrote: ↑Mon Oct 01, 2018 1:14 pm I just want to say thank you so much for your help, Xian.

Anytime dude! Just need to find a reasonably priced R (preferably with a K-swap) so I can be back in the cool-kids club with you guys.

Xian wrote: ↑Mon Oct 01, 2018 6:18 pm

g3teg97 wrote: ↑Mon Oct 01, 2018 1:14 pm I just want to say thank you so much for your help, Xian.

Anytime dude! Just need to find a reasonably priced R (preferably with a K-swap) so I can be back in the cool-kids club with you guys.

You never left sir. Bring the EF or whateversledyouhavetodayitchangesdaily to Expo/ITRCA gatherings anytime!

Looking to replace my Toyo R1Rs next spring and am looking for a tire that has a good mix of grip and tread wear. The car will be doing a mix of street driving, hpde/lapping, and autocross. As my car is JDM it puts me in the hardest pax group for autocross, to a point where it will not be all that competitive without major upgrades, so the fastest tire is not really on the top of my priority list. Another issue is that lapping track surface is pretty rough and is known to chew up tires, especially the right side. While they are in the process of repaving the worst sections of it I think a very soft tire will still get pretty beat up out there.

There seem to be only 2 tires available in the 215/45/16 sizing; Falken Azenis RT615K+ and BFGoodrich G-Force Rival S 1.5. From my understanding the Azenis are old tech and aren't as fast as some of the other tires out there (RE-71R) but will wear better. I've ran the original Rivals on my STI and they had decent grip but wore out too quickly.

I'm not opposed to switching to a 205/45/16 tire for more tire selection. The RE-71Rs seem to be the most popular option within the autocross community but the biggest complaint is fast tire wear.

I've ran Direzza Z1* on my previous GSR and loved them but that was a long time ago. I'm told the new ZII* are not on the same level and are almost identical compound to the Azenis.

Are there any other tires I should be looking at that I might be missing? Anyone have any experience with Hankook RS-4s?