Hey GRM!
I'm a regular HPDE attendee and always looking new ways to gain a few mph and / or drop some lap time. I was chatting with a few friends the other day and we were discussing the merits of running smaller vs larger wheel and tires. Obviously certain things are highly dependent (power, weight, etc) but I'm curious if I might be able to squeeze out a few more mph into corner speed and exit as well as diminish braking zones by switching to the larger set on higher speed sweeping courses.
Vehicle is 2009 RX8 r3 with light aero, custom valved Ohlins DFV 14kg front (3kg helper) with 8kg rear (.8kg helper), tuned OSG diff and 5.12 r&p. Power = approx 200whp / torque 150wtq. Last street curb = 2942 lbs (1/2 tank with no ballast, I'm approx 185-190 lbs) new curb should be less but waiting for balancing appointment). Current track wheel setup is 17x10 w 255/40 and considering 18x10 w 265/35.
What are your guys thoughts and experience? Anyone ever have an "ah-ha" moment? Too circumstantial to answer? Think it's worth the investment of keeping two sets? Carrots?
Thanks in advance!
Not claiming to be a expert but tire patch size, seems to me would, be what you are looking for. If you are going to keep the same rim width it makes no sense to me to get a bigger rim. All that would do is add weight. Better to get a wider 17" tire in the rolling diameter you need.
Now going to a 18" wheel does get you a bunch more choices. To me going with a 295 30 18 will give you a 1.6" wider tire patch than what you currently run. It would also be 1.2 larger than what you were looking to do. That is were you will get the grip you want also along with tire compound.
Just my humble opinion. Good luck.
84FSP
PowerDork
7/25/24 3:18 p.m.
Similar thoughts to brad121a4 - hefty contact patch and the lightest wheel you can hang em on. YMMV
Thanks for the input guys. Yeah the 18x11 would definitely offer a lot more contact patch compared to the 17x10 (or 18x10). Hindsight I should've kept the set of 18x11 I ordered for a project that didn't come to fruition; maybe one day I'll get lucky and find a high quality used set somewhere.
Imo it would be interesting to compare the relative wheel/tire weight of 17x10 and 18x11 plus their lap times.
84FSP
PowerDork
7/26/24 2:42 p.m.
Is the 18" move about available tire width? I would be amazed if like for like the 17
wouldn't be significantly lighter.
I have played around ALOT with tire width, wheel width, etc...
I haven't found marginally wider tires to be worth much measurable time over 1 fast lap. However, the added tire CAN help with keeping the laps consistent through more thermal mass and keeping the tires in their operating window.
My biggest time dropper on track (other than getting stickier compound tires) was a tire pyrometer to help me get the alignment dialed in a bit better. This told me I needed more camber and helped dial in tire pressures. The usual caveat of "the stopwatch is the true judge of what changes are helpful" applies to a pyrometer.
What I mean is that if the stopwatch says it's faster a certain way, it over rules the pyrometer
Edit to add.... I would love to see how much tire contact patch area increases with wider tires. Assuming you run the same pressures, and the tire has a similarly stiff sidewall, and the weight didn't change, the contact patch area shouldn't change that much. It should get "shorter" and "wider".
I haven't seen this explained to my satisfaction, but that's how it makes sense to me.
I just used GTS (Googled That SH!T) and found this website:
Contact patch calculator here: https://bndtechsource.ucoz.com/index/tire_data_calculator/0-20
My 17x10 ce28n weighs 16.8 lbs with 255/40 R888R at 26 lbs = 42.8 per corner.
The 18x11 advan r6 I previously owned weighed 18.2 lbs (iirc) with hypothetical 285/30 (both the AD09 and RE-71RS are 28 lbs) = 46.2 per corner.
Both setups are reasonably lightweight but I'd be curious to see how that extra weight played into the vehicles' driving dynamics in regards to rotational mass (and if that translated into better lap times, more consistent sessions, confidence, etc), being that my car is generally under powered to others. I bring a pyrometer to every event and I check and set pressures before and after every session.
ps- the contact patch calculator is pretty cool. I've never seen that and thank you for sharing!
myf16n said:
I just used GTS (Googled That SH!T) and found this website:
Contact patch calculator here: https://bndtechsource.ucoz.com/index/tire_data_calculator/0-20
Is it real? I can't get results from the site on my phone, but I do know normal force and resistance. If sidewall and pressure and Norma force are the same, the surface area should be the same.
dps214
SuperDork
7/27/24 9:37 a.m.
Driving4fun said:
My 17x10 ce28n weighs 16.8 lbs with 255/40 R888R at 26 lbs = 42.8 per corner.
The 18x11 advan r6 I previously owned weighed 18.2 lbs (iirc) with hypothetical 285/30 (both the AD09 and RE-71RS are 28 lbs) = 46.2 per corner.
Both setups are reasonably lightweight but I'd be curious to see how that extra weight played into the vehicles' driving dynamics in regards to rotational mass (and if that translated into better lap times, more consistent sessions, confidence, etc), being that my car is generally under powered to others. I bring a pyrometer to every event and I check and set pressures before and after every session.
ps- the contact patch calculator is pretty cool. I've never seen that and thank you for sharing!
Well there's one easy solution - ditch the r888rs for any modern 200tw and you'll go noticeably faster.
dps214 said:
Driving4fun said:
My 17x10 ce28n weighs 16.8 lbs with 255/40 R888R at 26 lbs = 42.8 per corner.
The 18x11 advan r6 I previously owned weighed 18.2 lbs (iirc) with hypothetical 285/30 (both the AD09 and RE-71RS are 28 lbs) = 46.2 per corner.
Both setups are reasonably lightweight but I'd be curious to see how that extra weight played into the vehicles' driving dynamics in regards to rotational mass (and if that translated into better lap times, more consistent sessions, confidence, etc), being that my car is generally under powered to others. I bring a pyrometer to every event and I check and set pressures before and after every session.
ps- the contact patch calculator is pretty cool. I've never seen that and thank you for sharing!
Well there's one easy solution - ditch the r888rs for any modern 200tw and you'll go noticeably faster.
Aww no way, I literally just bought a set. Has the R888R really aged that poorly?
CAinCA said:
Have you seen the Track tire buyers guide?
https://grassrootsmotorsports.com/articles/track-tire-buyers-guide/
I have now and looks like I'll be ordering a new set. At this point I'm leaning towards the CR-S (if I can find some) or the RE-71RS. Ideally a fast pace tire with good heat tolerance and wear. Thanks for the heads up.
In reply to Driving4fun :
I've had two sets of the CR-S V2. I think you'll really like them.
