Gymkata

I've seen countless threads on here that discuss tire height and there are members here running many different wheel/tire setups. I know that most here say you should try to keep your tire height as close to stock as possible. I haven't seen a lot of detailed explanation as to why.

My question is how does tire height affect the various nannies the ISF has (TC/VSC/ABS/ELSD/etc)? Also, is it more important to keep tires closer to stock height or closer to the same height front/rear?

i.e. which could have a bigger impact of how the nannies work?
255/40R18 (f) 26" - 275/35R18 (r) 25.6" - 0.4" difference front to rear but closer to stock height
265/35R18 (f & r) 25.3" - no difference front to rear but 0.7" shorter than stock

The reason I ask this is, without resorting to ordering custom built wheels, it can be difficult finding wheels in the proper widths/offsets for the ISF necessary to run wider tires.

Thanks in advance

ISFpat

iTrader: (10)

I'm going to go ahead and say I personally did not experience any issues on my other cars when I ran a smaller diameter tire at the track (road course). I didn't have traction control on of course but my times were much faster when the tires were wider. The biggest issue with different height tires is the speedometer. The traction control should not go out of whack if the tire size has changed.. if you're at the track then it doesnt matter since you should leave TC off. On the street you hardly engage TC so it shouldn't be a big deal.

MYISF

Gymkata,

Great "friggin" question as u and I have gone around and around about wheels and tires (PMing)and I am wondering the same thing.

Not to steal your post but if I install aftermarket wheels with this setup:

19x8.5 +35
19x9.5 +38

What would be the impact (per Gymkata's question) besides the affect on the speedometer based on the two different tire setups as follows because each setup below (per the tire manufacturer) works on that wheel setup:

245/35/19 (ft) 275/30/19 (rr)

235/35/19 (ft) 265/30/19m (rr)

ISFpat

iTrader: (10)

Why are you guys making this so complicated, the impact is the wider tires would provide a wider footprint as well as larger sidewall to protect your rim.

The 245 and 275 is more ideal for that wheel setup because 235/35/19 is a very small sidewall

Gymkata

I understand everything you said in your first response, and I know that there will be no serious issues with the setups I am considering. I have been running non EO sizes for years on the ISF without issue. There shouldn't be an issue on the track with everything off, but I am interested if there could be an issue when driving hard on the street w/all nannies active...i.e. would everything work to its full capability if you lose traction while taking a clover leaf too fast in the rain?

I guess I was looking for anybody who has detailed knowledge of how all the nannies work together to weigh in regarding what affects, if any, there are in various changes to tire height, specifically differences between front/rear height.

ISFpat

iTrader: (10)

I'm going to go ahead and say this.. I've never see a GTR or Evo or STi with a very advanced TC system go haywire because of a change in tire size.

The only reason why everyone wants to keep the overall tire height the same is because of the speedometer. The whole TC issue was because someone speculated that this causes a decrease in performance because its not OEM thinking that the TC will not adapt. Then someone else uses the same info and passes that on.

The big picture is that there should be a balance in tire width vs sidewall size. We have wider tires than a smaller economy car is because our cars are heavier and have more power which demands more grip (simple physics). We have a smaller sidewall size because the smaller the sidewall the more feedback is applied back to the driver and vehicle (shorter sidewalls are generally stiffer, stiffer sidewalls have more feedback). Although there is a limit in tire width where more does not mean better because other variables are taken into equation where its non beneficial in overall balance such as tire weight, suspension based limiting grip, turn in feedback, etc. (think diseconomies of scale)

Gymkata

Thank you for your responses. I'm interested to see what Lobuxracer has to say, but what you are saying is essentially what I figured was right...that minor differences don't matter. The ISF has shown it can adapt well to changes...i.e. how well the ecu adapts to I/H/E. I have just learned over the years to ask others and learn from their experiences/mistakes before making unnecessary mistakes of my own.

lobuxracer

iTrader: (2)

How carefully are you measuring the "improvement?"

AFAIK, no one, even FIGS, has measured the chassis, determined the roll centers, and figured out what is being left on the table as far as the IS-F chassis is concerned. Sure, wider rubber offers more lateral grip, but moving the wheel centerlines impacts scrub radius, roll centers, roll axis, instant centers, and all the work the factory did to make the car handle well. Lowering the car to make it look good does not mean it will perform well mechanically.

Ideally, you keep the OEM diameters and keep the wheel centerlines where they designed the suspension to perform well, then make changes and measure the performance. Or you do what the real pros do and measure the chassis, see what the impact of your change(s) is(are) and decide if the trade offs are worth it. It isn't easy. It's painstaking work. It's also why, IME, changes to the OEM setup are often worse than stock when you put a watch to the car's performance. I've done enough of this to be very skeptical of any aftermarket part claiming to be an improvement, only because I've bought this kind of stuff in the past and it delivered slower measured performance after install and took a lot of painstaking work to actually deliver on the promises made by the aftermarket part manufacturer.

MYISF

Thanks and I understand this and that the 245/275 works better (???) based on the diameter and how it affects the speedometer...but I don't think that is necessarially what the OP is after...nor am I.

Lets start off by asking what is wrong with a smaller sidewall (235/265)...in a post u mentioned somehting about smaller sidewall providing more feedback...I agree but to the OP's question is does this affect or would it affect TC/VSC/etc...?

I also would like to "stir the pot" by saying why is 275 wide better than 265 wide...per "lobuxracer" keeping everything close to OEM would cause the car to work better within the original design...so if I just want to bump my tire sizes up one to say 235/265...again besides the affect on the speedometer...what would one be losing?

For instance if I compare the specs on a 265 & 275 Toyo T1 Sport tire (see below)...



The main differences are
diameter (which we have discussed)
side wall height (we have discussed that)
load (could this be an issue?)
Rev's per mile (which this is because of different diameters)

These two sizes r the same as follows:
approved rim width
weight
inflated overall width

So do these differences affect the cars ability to optimize TC/VSC/etc...I think to "lobuxracer" post that unless u r planning to measure this somehow by measuring the imapct on the chassis, etc...one would not really notice an impact especially on the street...so then it comes down to two things (comparing these two tires)...do you want your speedmeter to be around 2 miles pe hour off and/or do you want smaller sidewalls (for feedback, appearance, etc...)...sorry but that is my 2 cents and BTW cool discussion!

flowrider

iTrader: (9)

^^^^My postfrom another thread:

Lou

MYISF

Interesting and I hear u as I have read this in other places but why is that important to the performance of the car or how would the load rating affect the performance of the TC/VSC/etc...?

lobuxracer

iTrader: (2)

How deep do you want to dig into this? Changing tire diameter changes scrub radius, instant center, roll center, roll couple, and roll axis. It's unlikely it will be mechanically better than stock. If you add offsets different from the factory design, you throw salt in the wound. Without measuring and modeling the changes you want to make, it's impossible to determine if it will actually be better than stock. Most often it is worse even with the best intentions.

That said, if you're interested in visual performance, none of this matters. If "closing the wheel gap" is a big deal, just do what it takes to make this happen and ignore the rest of the mechanical performance concerns.

daedalus

iTrader: (2)

Only way this affects the car is if the rev/mile were calculated by the TPMS that is use to control the vsc/tc. Only car I know of that does this is the Z06/C6. On that car if you run the wrong tire combinations the cars go into limp mode and do not function correctly. But for the ISF I cannot imagine any appreciable difference in the functioning of these systems in comparison to the change in mechanical grip and performance from running stickier and wider rubber or vise versa.

Gymkata

Thanks for the responses everybody.

I am very close to finally pulling the trigger on new wheels/tires. At the moment, I am fairly set on getting the following squared setup:

18x9.5 +45 Wedssport SA10R wheels (20.4 lbs)
275/35R18 Michelin PSS tires (25 lbs - 25.6")

I also plan on ordering the 2 piece rotors from FIGS soon (and maybe the OS Giken LSD & BC coilovers w/swift springs).

This setup, between wheel/tires/rotors, would shed over 41 lbs of rotational mass while increasing the footprint +50mm front and +20mm rear. If I do get the OS Giken LSD, I imagine 275mm rear tires would be plenty considering how well my current 275mm PSS tires grip without a LSD. What do you guys think? I'm not going for looks, but I do think it will look good.

BTW, is anybody interested in buying 2008 ISF wheels with a few minor dings and 245/35R19 & 275/30R19 Michelin PSS tires with good tread left? I won't need them anymore.

EDIT: I was considering Nitto NT05s as well. I looked a bit to try to find Nitto tire weights, but they aren't posted anywhere I could see. I finally found a posted tire weight on a Corvette forum for Nitto NT05s in 275/35R18...27.4 lbs...basically 2 lbs heavier than a Michelin PSS in the same size. Not bad, many tires are heavier than that.

MYISF

It sounds friggin bad azz to me...I can't wait to see your car all set up!