Tire musings and other random thoughts from my drive home.

Ok, so commuting to work and from work is terrible. I'm stuck in traffic on the highway (494 for anyone that knows MN). No chance to wring the car out, totally straight, boring and lined with concrete. It does afford me one luxury though, and that is the time to think about random car things. Normally it's fulfilled with fantasies about bad things happening to good cars (20B AWD Rx7 anyone?) but occasionally, it wanders to more...practical applications.

This time around, it wandered to grip coefficients. And that begged the question- is there such a thing as too much tire under a car? I'm not talking about attempting to stuff 245's on a 7 inch wheel, or anything goofy like that, I'm talking a properly matched set of wheels and tires (in my case, 245/40/17's on a 9" wheels on my DD) that exceeds the actual need for lateral grip in a more... spirited situation. Now I'm well aware that my current setup is WAY overkill for slogging it out in gridlock, but that's beside the point.

Now, as I understand, grip coefficients come down to friction. And as we all know, friction slows things down. generally speaking, your tires are doing their damndest to prevent a vehicle from going the opposite direction of where you point it, by slowing down the inertia (is that the right word? It's still early) created when you transfer the weight of the vehicle while turning. Car was going left, you want it to go right, tires slow the continuance of moving left, so instead of plowing a barrier, you go right.

And that- is where things get tricky in my head. At one point I was told (it might have been Gran Turismo actually, or perhaps some random car magazine.... maybe even THIS random car magazine) that you're going to see the fastest times at the limits of adhesion. That means, you're not going to go any faster w/o losing control of the vehicle. Anything under that, you could theoretically be going faster. Anything over that, and you're screwed..... or losing. The recent article about the new fitment for the project Mx5 got me thinking... do I really have too much tire under my car? I cannot recall a moment since fitting the new setup where I have actually been on the edge of control. In fact, it's quite the opposite. I've ran the piss out of it deep into a roundabout, pegged the brakes (which in an ms3 is a REALLY effective way to unload the rear and get snap oversteer) and attempted to get the ass end to come around. No dice. I've also run it deep into the same roundabout (we have really nice tracks that feature roundabouts and other highway/local road type turns by my house ) and done my usual trail brake turn in smooth transfer combo. Used to be that the traction control would kick in at about 50mph, indicating that the car was about to go for a walk in the weeds. I've shut it off multiple times, and tried the same dive, and beyond that point, it doesn't feel like I'm losing control at all. Now this could all be ability, or fear, but it doesn't feel like at least for myself, that I have the stones to exceed the ability of the car. I suppose it would take a more professional driver (read: one that doesn't suck eggs at driving) to find that out for sure..... but has anyone really sat and figured this out?

Sorry for the rambling... I'll go back to making useless I want it but it's too far away comments in the 2010 classifieds.

I'm in the same boat. I can't get the Celica to even think about E36 M3ting the bed.

245/45-16 RS2s under a 2300lb car with a completely re-done suspension, and a stock drivetrain.

It's actually quite frustrating and unfilling to drive at anything over 5/10th.

Racecar Engineering had an article about when is too much tire. I will look it up when I get back to the apartment.

I do believe that a car can be overtired. 245s on a Mazda3 or Celica is definitely overkill unless you are tracking the car or autoxing it. For DD duties or even playing on the street, its more than you can (or should) use. Heck, I have 255s on my 300 RWHP RX7 that I use for street and track days.

More aptly sized tires would probably be more fun to drive since you have a chance of reaching the limits. I've actually had a lot of fun driving my 90 Miata with good all season 185/14s simply b/c I could reach the limits of adhesion easily.

the traction control on the ms3 is very much "preventative" in that it kicks in when it thinks you are doing something that could result in a walk in the weeds. basically it sees you cut the wheel and lift throttle or jab the brakes and the idiot alarm goes off. doesn't mean thats what would have happened, just that it could have.

is it just the lack of stones to push the car as far as the tires will take it? you really shouldn't be doing that on public roads anyways

Right- obviously trying to check the limits on a vehicle in a public situation is a bad idea.

I'm just more curious as to the science of tiring a car right, as opposed to "overtiring" it.

I think for the track (ulitmate traction) there could be a point at which the tire is wide enough that it doesn't heat up sufficiently and that a smaller hotter tire could have some advantage, but I don't know if that would happen within the range of tire widths and compounds generally available.

For the street though, factors other than utlimate trraction come into play. When I ran 225/50-15's all around on my Corvair, I abosolutely hated the way it drove. When I dropped to 205/55's for the front tires (same rims, same make of tire, same tires in the rear) the car was immediately transformed.

On the track, the 225's would still have likely been quicker. On the street I'd never consider going back.

Bill

On a track, could rolling resistance come into play? e.g. a wide sticky tire which will grip better will also weigh more and grip more and create more aero drag, creating more rolling resistenace vs, say, a skinny a/s tire, with very little rolling resistance. Obviously then it starts to depend on track layout, power of the car, etc.

I suppose too, this comes into play when I'm considering what tire combo to go with for my next time around. I'm REALLY happy with the Star Specs I have on it now, and I do not plan to be rid of them. Running anything smaller than a 245 on the wheels on it now seems silly IMO. However, as I'm a bit of a hoarder, I do have yet another set of wheels for the car, that I'm thinking of going all out aggressive on the tires for.. something along the lines of an re11 or AD08. I know they'll wear like crap, but I already have 3 sets of wheels that are shod for this car, so I'm not really worried about burning up tires too quick. I'm kinda wondering if a 225-ish tire on a 7" wheel can offer me better performance than a 245 on a 9" wheel like I have now.

Little narrower and a little taller sidewall makes for less harsh. Also tends to reduce tramlining a bit, I think.

On the other hand, big tires be cool.

I could care less about harsh ride. I took a perfectly good, powerful, comfortable turbo car and make it a trainwreck, and that's how I like it. The ragged edge of undriveable is where I prefer my cars to be.

I think being "overtired" is simply another way of saying "overweight".

You're at a point where the increased raw grip of a wider tire is being overcome by the additional inertia, because a tire is both unsprung and rotational. Plus it has some small degree of additional friction.

I'd bet that if you found 2 matching sets wheels/tires with the following attributes, the wider set would outperform the narrower almost 100% of the time:
- Same tire diameter
- Same rim diameter
- same difference in width between rim and tire
- same tire compound
- exact same overall weight <-- here's the kicker
- 1 set is 1" wider than the other set

Ooooh, I like that rationale. So really it changes it from, too much tire for the car, to the optimal combo of weight/width. Because 300lbs is 300lbs, until you try and fit her fat behind in the back seat.

Or something like that.

scardeal wrote: I think being "overtired" is simply another way of saying "overweight". You're at a point where the increased raw grip of a wider tire is being overcome by the additional inertia, because a tire is both unsprung and rotational. Plus it has some small degree of additional friction. I'd bet that if you found 2 matching sets wheels/tires with the following attributes, the wider set would outperform the narrower almost 100% of the time: - Same tire diameter - Same rim diameter - same difference in width between rim and tire - same tire compound - exact same overall weight <-- here's the kicker - 1 set is 1" wider than the other set

Not always. As mentioned before it can be hard to get a tire up to a high enough temperature to really work if is to wide also wider tires equals a larger frontal area equals more drag.

So... too much then?

I've always been more pleased with less weight on the tire than more grip. Even the tiny 12's on a Metro have enough grip to drive, and less grip just means more fun. Less weight on the other hand makes the ride more enjoyable at 1/10th or 10/10ths.

amg_rx7 wrote: I've actually had a lot of fun driving my 90 Miata with good all season 185/14s simply b/c I could reach the limits of adhesion easily.

fun enough when its dry, endless fun when the pavement gets a little wet.