Well I'll be damned..

[HaLo]

I just test fitted my 245/45R16 on my 16x7+32 Advan wheel... on the front of my S13 with D2 Coilovers... And you know what? No spacers are required... I just got some H&R 15mm spacers just in case and I won't need them... It doesn't even touch the inner fender. I am really surprised..

[hurleyboi514]

thats good to hear! now put a 275 in back...

[019]

really? wow... how much camber are you running? i have some 16x7.5 with 205/55 and they won't fit with my coilovers (not d2). i got some 5mm spacers and they barely clear. oh, the offset is +40 +35 with the spacer, obviously. i've been meaning to get a hold of some 225/50 to see if they'd clear. maybe i'll get lucky then

[Flybert]

Originally Posted by 019

really? wow... how much camber are you running?

Camber has nothing to do with it. Shock bracket length and wheel offset has everything to do with it.

[nlzmo400r]

my 245/45 clears in the front with my d2s with a 16x7.5 with a 22 offset. Whattaya know, d2's are good at somethin eh? haha

[98sr20ve]

Originally Posted by HaLo

I just test fitted my 245/45R16 on my 16x7+32 Advan wheel... on the front of my S13 with D2 Coilovers... And you know what? No spacers are required... I just got some H&R 15mm spacers just in case and I won't need them... It doesn't even touch the inner fender. I am really surprised..

I wonder what that would mean for the Kosie wheels in 16X7.5 same offset. Sometimes I can't seems to wrap my brain around the wheel offset thing once you throw larger wheels in but the same size tire.

[jmauld]

Originally Posted by HaLo

I just test fitted my 245/45R16 on my 16x7+32 Advan wheel... on the front of my S13 with D2 Coilovers... And you know what? No spacers are required... I just got some H&R 15mm spacers just in case and I won't need them... It doesn't even touch the inner fender. I am really surprised..

Hold on to the spacers until you get a chance to race on them. They may rub under load. Of course, spacers may not help with that kind of rubbing.

[HaLo]

I will hold on the spacers just to widen the front track to reduce understeer.

[019]

Originally Posted by Flybert

Camber has nothing to do with it. Shock bracket length and wheel offset has everything to do with it.

i beg to differ. when i first put on my coilovers, they were so low that i was running about 4-5* of camber and i rubbed the shit out of my tire. i'm running 2* of camber now and it clears fine. that was on stock 15x6 rims with 205/55's on.

[98sr20ve]

Originally Posted by Flybert

Camber has nothing to do with it. Shock bracket length and wheel offset has everything to do with it.

It depends if you are adjusting camber at the knuckle or the tophat. If you adjust at the knuckle then you can easily gain or loose a lot clearance thru camber adjustment.

[turtl631]

Why do autocrossers tend to put such huge tires on wheels? A 245 seems more at home on an 8-9" wheel than on a 7". For example, I would think that for a 7 F 8R combo, 225 and 245 would be good healthy tire sizes. Yet I routinely see people go even wider than this. Is there a benefit to cramming so much tire on seeing as grip is mostly a function of compound?

[ghambino]

I would guess to increase the amount of tire that makes contact with the road, which results in more grip.

[yudalicious]

Originally Posted by ghambino

I would guess to increase the amount of tire that makes contact with the road, which results in more grip.

too bad grip is a function of compound and weight, and bigger tires don't necessarily mean more contact patch, in other words, your answer is wrong.
I am with turtl, I'm also curious why people run such big sizes, is there a benefit that we're not seeing here?

[HaLo]

Originally Posted by yudalicious

too bad grip is a function of compound and weight, and bigger tires don't necessarily mean more contact patch, in other words, your answer is wrong.
I am with turtl, I'm also curious why people run such big sizes, is there a benefit that we're not seeing here?

It works.

225/245 combo doesn't.

I guess you can't apply some road racing/lapping rules on auto-x.

[Hikarispeed]

Clearance also depends on the specific tire you're running. My ES100's don't have the same shape compared to my Avon's

Well the results from autox in my region tend to give better results to the guys with the wider tire. There are so many variables in there though as you all know.

You can go too big; too much rotational inertia, affects to your suspension, and not being able to get them to optimal tire temperatures... Tires are very hard to analyze so it is difficult to determine the best width. The best way is to test.

I'm running 225's, will go to 245 next season to know how they differ for my setup.

[ghambino]

Originally Posted by yudalicious

too bad grip is a function of compound and weight, and bigger tires don't necessarily mean more contact patch, in other words, your answer is wrong.
I am with turtl, I'm also curious why people run such big sizes, is there a benefit that we're not seeing here?

How is that wrong? and stop repeating what we already know. Why do you think people upgrade to a wider tire. If you take a tire that is of the same compound in a 225 and a 245 which do you think will give you better traction. Of course you can go overboard if you put an excessively wide tire that will hinder performance ( for example a 255 tire on a underpowered AE86), thats why you test different sizes to see which one gives you good grip without creating too much tire weight and rotational inertia. Go to an autocross event and look at the rear tires of a porsche 911.

[ghambino]

Originally Posted by HaLo

I just test fitted my 245/45R16 on my 16x7+32 Advan wheel... on the front of my S13 with D2 Coilovers... And you know what? No spacers are required... I just got some H&R 15mm spacers just in case and I won't need them... It doesn't even touch the inner fender. I am really surprised..

Do you get a lof of tire flex with the 245s on a 7inch rim?

[jmauld]

Originally Posted by yudalicious

too bad grip is a function of compound and weight, and bigger tires don't necessarily mean more contact patch, in other words, your answer is wrong.
I am with turtl, I'm also curious why people run such big sizes, is there a benefit that we're not seeing here?

That argument may hold some truth in the theoretical world. But here in the real world where cars are limited by the width of tire they can run, it really isn't possible to run too wide of a tire.

[hurleyboi514]

Originally Posted by ghambino

Do you get a lof of tire flex with the 245s on a 7inch rim?

the tires he's running probably are R compounds which have very little flex...

[nlzmo400r]

^^this is true, on a street tire, sure, there would be too much shoulder flex, but on ao32's, doubt it

[98sr20ve]

Originally Posted by HaLo

It works.

225/245 combo doesn't.

I guess you can't apply some road racing/lapping rules on auto-x.

So true, Autocrossing has so much more understeer then roadcourse stuff.

[HaLo]

Thanks a lot guys for explaining what I could not.

I am running A032R, which are Race tires. I also ordered some Hoosiers and will see what I can do with those.

[yudalicious]

Originally Posted by HaLo

It works.

225/245 combo doesn't.

I guess you can't apply some road racing/lapping rules on auto-x.

obviously it works seeing how many ppl opt to use this set up, I was just curious on WHY it works, as it seemingly goes against physics
(and ghambino, what you said is still wrong, more patch DOES not mean more grip, think an R compound tire w/ a smaller contact patch vs. Wal mart tire w/ bigger contact patch. AND wider tires DO NOT always mean more total patch area, maybe more "horizontal" patch that helps w/ lateral stability.)
I'm not arguing against you autocrossers that run this set up, obviously there's alot more to it then simple physics, I am just simply wondering why.

[Hikarispeed]

Originally Posted by yudalicious

obviously it works seeing how many ppl opt to use this set up, I was just curious on WHY it works, as it seemingly goes against physics
(and ghambino, what you said is still wrong, more patch DOES not mean more grip, think an R compound tire w/ a smaller contact patch vs. Wal mart tire w/ bigger contact patch.

Compare apples to apples dude. A good book to read is Race Car Vehicle Dynamics by Milliken.

I work on some club racer's cars on weekends from GT3 to FF1600's. They are limited on wheel width, but they cram as much tires on there as possible. These tires are specially made to be so very wide on a skinny 7" rim. Sidewalls are so stiff that if you get a flat, it is hardly noticable statically.

I heard Hoosier's should blow A032R away easily, tell me if it really does.

[yudalicious]

I am comparing apples to apples, just merely informing ghambino that not only did he NOT answer the question at hand, he missed 2 critical truths of the laws of physics...
with that said, I am not arguing that bigger tires won't give you better times, you guys are the experts here, I am merely asking why they do, even though doing so seemingly goes against physics...
so far 3 people have reaffirmed the fact that most people run as much tire as possbile, but no one has replied why.

Originally Posted by Hikarispeed

Compare apples to apples dude. A good book to read is Race Car Vehicle Dynamics by Milliken.

I work on some club racer's cars on weekends from GT3 to FF1600's. They are limited on wheel width, but they cram as much tires on there as possible. These tires are specially made to be so very wide on a skinny 7" rim. Sidewalls are so stiff that if you get a flat, it is hardly noticable statically.

I heard Hoosier's should blow A032R away easily, tell me if it really does.

[yudalicious]

so far your post has given the most info on the question, there's probably many variables that come in play, and I do see autocrossers running wider tires alot, just trying to understand the physics behind it.

Originally Posted by Hikarispeed

Clearance also depends on the specific tire you're running. My ES100's don't have the same shape compared to my Avon's

Well the results from autox in my region tend to give better results to the guys with the wider tire. There are so many variables in there though as you all know.

You can go too big; too much rotational inertia, affects to your suspension, and not being able to get them to optimal tire temperatures... Tires are very hard to analyze so it is difficult to determine the best width. The best way is to test.

I'm running 225's, will go to 245 next season to know how they differ for my setup.

[jmauld]

How does it go against physics?

[yudalicious]

Originally Posted by jmauld

How does it go against physics?

well, it doesn't really go AGAINST physics to run bigger tires, but since ffriction (grip) between the tire and the surface is based on coefficiient of friction of the tire (compound) and the normal force (weight), contact area doesn't really come into play. My other point is sometimes, running wider/taller tires does not always increase TOTAL contact patch.

Now we all know that's only part of the game, as obviously wider tires DO do better, and so far I've only found 2 possible explanations:
1. the wider tire offers more horizontal/lateral patch
2. better heat dissipation

I'm merely looking for some more explanations, it'd be nice if I could know why doing something helps.

[Rennen]

Originally Posted by yudalicious

but since ffriction (grip) between the tire and the surface is based on coefficiient of friction of the tire (compound) and the normal force (weight), contact area doesn't really come into play.

This isn't a classroom block drawn on a chalkboard, its a tire. And tires are deformable, tires also get much of their force from getting in-between particles of concrete and creating non-normal forces. If you are a running a wider tire, you can run at a lower pressure because the cars weight is spread out over a larger contact patch, this lower pressure allows the tire to deform more, and create more forces.


Let me see if the intranets has anything useful...



BINGO!: http://hyperphysics.phy-astr.gsu.edu...rict3.html#are

Then click the "coefficient of friction" in the last paragraph.


So there you go, those tricky physicists are making assumptions that aren't quite valid for autoracing.... or you can just tell all your friends that your car defies physics.

-Matt

[yudalicious]

I did some research and came up with this:
http://forums.bimmerforums.com/forum...d.php?t=328261

Quick summary:
"For anyone who liked the "wider tires aren't any faster" debate...
I would suggest running out to Barnes and Noble and picking up the March issue of Racecar engineering. The very last article in the mag (last 2 pages actually) deals with this exact topic. The author admits that everyone 'knows' that wider tires are faster, but also that, scientifically speaking, they really shouldn't be. That is a huge and horrible oversimplification, but in case anyone was looking for a definitive answer I want them to know this isn't it. What is in this article is a couple of theorys as to why wider tires appear to be faster and also a quick explanation of why they shouldn't be. There is also supposed to be a second article in the series in the next issue so keep an eye out for that as well.

I'm not trying to start the whole debate again, but the article was a good summary of the whole tire paradox and at least one of the why-wider-is-better theorys was something I'd never heard mentioned before. Anyway, just go read it, it's quick and interesting."

And, this from a guy who read the article and summed it up:
"Here's a brief synopsis though. (of my posts, or the article...? hard to tell... )

- Columb's law for friction
- Tires contact patch area does not vary with its width.
- Tire width, construction, and inflation pressure change the contact patch shape, not size.
- If we take a narrow tire and make it wider, the contact patch stays the same size, but gets wider and shorter.
- A wider tire has a greater surface area in contact with air to surface area in contact with the ground ratio.
- Wider footprints (contact patch shape) have smaller region of acceptable slip angles that are within the grip zone of the tire."

Basically I think this is very interesting, as it seems zilvia.net isn't the only forum having trouble to explain WHY wider tires are faster, hope this helps.