What am I missing with all the ultra-wide wheels & tires?

[Gunman]

Originally Posted by Silver Supra

If you lower the pressure, it is larger. Otherwise, it just changes shape.
( I guess there might be some slight differences at very high lateral g's).

I've seen lots of internet/bench racers argue the patch gets larger, even on equal air pressure, and tire carcass.

[Spuds]

Originally Posted by Wise

Some people do it because they plan to go forced induction in the future and in some states in Australia having a wider wheel and tyre for the increased torque and power makes it easier to get the car engineer certified so you can drive it without having to worry about getting pulled over by the local police and slapped with an unroadworthy certificate.

Wait... WTF? That's a thing!?!? I guess it is similar to inspections in the US, but tire width vs power checks?



In other news...
As for contact patch, in general I would think you would need to run lower pressure to prevent uneven wear and other problems associated with too high tire pressures. Which has the results previously described.

[Silver Supra]

Originally Posted by Gunman

I've seen lots of internet/bench racers argue the patch gets larger, even on equal air pressure, and tire carcass.

Physics >>> internet/bench racers......

[fatoni]

i think they have done tests to show it actually does grow contact patch some. i think due to the fact that it isnt just gas pressure keeping the car off the ground. the reason 9 inch wheels are nice is because they are faster and they can fit the bbks.

[Silver Supra]

Originally Posted by fatoni

i think they have done tests to show it actually does grow contact patch some. i think due to the fact that it isnt just gas pressure keeping the car off the ground. the reason 9 inch wheels are nice is because they are faster and they can fit the bbks.

Static contact patches do not change area - that is the physics. They can change shape, but all else being equal, they are the same area.
If someone goes around changing multiple parameters, yes, area can change some, but it can shrink as well as grow under multiple changes. If you consistently want a larger contact patch, you must reduce tire pressure - but even that within limits.

[Yardjass]

"Physics" assumes a tire is a perfectly deformable air vessel that allows the pressurized air in the tire to react perfectly to the normal force on that corner of the car. The fact is that tires have structural rigidity, especially in the sidewalls, and this affects the contact patch. Assuming that you properly set up the suspension to account for the new width of tire and new wheel offset, you absolutely do get a larger contact patch from a wider tire. That is not debatable.


What is debatable is how much of an effect this change has on the vehicle's ability to accelerate and brake, as well as its cornering stiffness. This is mainly debatable because it in fact, has yet to be fully understood. To get close, you need equations like this, and a whole lot of experimental data from your vehicle in order to fill in all of the constants: https://en.wikipedia.org/wiki/Hans_B._Pacejka
Since I doubt any of the people on this forum have plugged through this with any of their setups, let alone a large number of different ones, I leave you with this advice:
Get ahold of your local 86 club and if the owners are so kind as to let you, try as many different setups as you can so that you know how they feel to you. The theoretical fastest is not always the fastest in reality. Race car drivers make their engineers make changes to "less ideal" setups all the time, and then end up running faster simply because they are able to work better with the new setup.
Also, look at what autocrossers and road racers are saying. I wouldn't take their lap time input as gospel because there is a lot of margin for error with different skill levels but trends can still be noticed in terms of wider vs. narrower, and also things like tire size in relation to wheel diameter and width. For example, we all hate on stretch and camber for obvious reasons but the absolute widest tire that you can squeeze onto a wheel is also not the ideal setup. If you can find information about the same driver running two different setups and comparing them, that would be particularly useful. The classing that your setup fits into is also important if you are going to be doing anything where that matters.


Use the information that you have collected to make an informed purchase based what you prefer and what you want to do with the car.

[LOLS2K]

http://www.ft86club.com/forums/showp...3&postcount=11

[Silver Supra]

Originally Posted by Yardjass

"Physics" assumes a tire is a perfectly deformable air vessel that allows the pressurized air in the tire to react perfectly to the normal force on that corner of the car. The fact is that tires have structural rigidity, especially in the sidewalls, and this affects the contact patch. Assuming that you properly set up the suspension to account for the new width of tire and new wheel offset, you absolutely do get a larger contact patch from a wider tire. That is not debatable.

...

As noted before, if you change more than one thing, results may differ. Given the same ID and OD and sidewall, the contact patch stays the same. Change it and your contact patch may change, but not necessarily larger.
The original point was to not automatically assume a wider tire yields a larger contact. As you noted, there are different conditions that benefit from a change in tire width, and there is a "better" tire width for each condition. There are also worse tire widths!!

My post probably focused too much on the effect of width given the same tire pressure and carcass (in the later quoted post).

As you noted, unless I find a test of the conditions I am interested in, or am willing to spend the money to test various combinations, I will not assume wider is better or worse.

I have not read all the posts here, but perhaps there are some that describe enough measured data to make a good decision. I know I use a wider tire (on my Supra) because of other benefits related to road racing because I did find significant data indicating wider was better due to the other aspects not related to size of contact patch. YMMV!

Carry on.

[Stang70Fastback]

There are other unspoken benefits to a wider tire as well.

I would expect a wider tire to wear longer, as the load is distributed over a larger area, and it also does better on crappy roads, as small potholes and cracks are less likely to suck the entire wheel in.

Of course, there are also downsides, such as higher cost, and arguably a higher propensity to hydroplane, though I've done 80 in torrential rain on my 255s without any issue (Michelin PSS FTW!!!)

[churchx]

It's also harder to get enough heat for wider tire, if it's compound grips best only starting from specific temp range. Just like it's harder to bed pads on some of biggest BBKs, when pads of very same compound are easy to bed on smaller stockers. Same story of much more heat needed. And it's not just standing water "aquaplaning" matters at. Throw in liquid dirt or snow slush, and that difference rises quickly. Everything is compromise, and for most uses imho very wide tire cons outweight pros.

[14stu]

I've tried out a couple of combinations on the track and at autox, I have 2 sets of wheels: 17x9 with 245 RE71r and 17x8 with 225 RE71r both of which weigh the exact same (the wider wheels are slightly lighter but tires are heavier and the combos come out the exact same weight).

The wider tires are consistently faster at both autox and on the track for me (1-2s). I run at MSR Cresson, which is a 1.7mi track that has few long straights and a healthy amount of low speed elements.

I thought the skinnier tires might be faster after reading a few of the threads; however, I believe that since my two setups are the same weight I am only getting the wider tire benefit and not seeing any pickup on the smaller setup. My experience leads me to the conclusion that the smaller setup is only going to be as fast or faster with lighter weight.

I know that the R-comp tires all run much wider than street tire equivalents (a 225 Hoosier is as wide as a 245 street tire), and I didn't see any benefits trying a 255 over a 245 street tire for autox (the 255 cost more and didn't seem any faster and didn't show any additional grip under analysis).

[paiceyfan]

I started reading this in the hope that it may shed some light on what I should be running for a DD and the occasional track day, but nope...even more confused now.


Running a supercharger, I want to figure out how I can get better speed through the corners on standard OEM wheels. I have 215 Continental DWS right now and I am very disappointed in their performance. I don't think anyone has mentioned spacers...or is that considered a no-no for the track. Driving in the wet, I might as well have been driving on ice, but the pressures might have been too high at around 35 psi. Nothing's simple is it?

[Pat]

Originally Posted by paiceyfan

I started reading this in the hope that it may shed some light on what I should be running for a DD and the occasional track day, but nope...even more confused now.


Running a supercharger, I want to figure out how I can get better speed through the corners on standard OEM wheels. I have 215 Continental DWS right now and I am very disappointed in their performance. I don't think anyone has mentioned spacers...or is that considered a no-no for the track. Driving in the wet, I might as well have been driving on ice, but the pressures might have been too high at around 35 psi. Nothing's simple is it?

You want to be faster through turns on OEM suspension with one set of tires for street and track with respectable tire life? 225/45-17 Pilot Super Sports.

[Azzudien]

Feel your pain, I have been going back and forth for weeks trying to decide on tire/wheel combo. Finally just said screw it and placed orders. We will see if I was right or wrong once they are on the car and the car is on the track.

Advan RGIII 17x8 +48 Gloss Black
Nitto NT05 235/40zr17

Now I am just burning up the stock Primacy's while I wait. Watkins Glen coming up Monday and Tuesday next week, those poor poor Primacy's will be screaming at me.