Smaller tires? (auto-x/daily driving)

I'm considering getting smaller tires, shorter to be more specific...

I'm currently running 245/50/15s on my 7 inch wide cookie cutters. I've been doing pretty good at auto-xes.

However I don't like the feedback much, it's still not as stiff, and direct as I'd like.

Anyway, I'm considering going to 225/50/15 tires, for this reason only. It would lower the car a bit, not messing up the suspension geometry, but in theory it should provide more torque to the driving wheels, as well as possibly make the brakes more effective...

Ideas, thoughts?
Thanks,
Ahmet

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It's all the driver...

Do you have adjustable sway bars and/or springs? The pics you posted show A LOT of body pitch, both side to side and front to rear. I had an 87 951 with springs, sway bars and 245's and enjoyed great chassis stability.

The way your car pitched toward the outside tire, I would be worried that less rubber would mean more understeer. IF you are just out to change tires, perhaps 245 / 45's will do the job you looking for.

Just my .02 that’s actually worth .00

No, I have stock sway bars (and switch to better bars is not allowed in my class)...

I looked into 245/45s, but only tires made that size are pure racing tires, a no-no for my dual purpose car, on principal.

Thanks for the reply,
Ahmet

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It's all the driver...

Not only will the smaller tires improve your gearing and lower the car, they also reduce unsprung and rotational weight = faster.

A lot of people will say wider is better, but past a certain point wider is just heavier and slower.

You might seriously consider two sets of wheels and tires. The gains you are making for the track might be counter to what you want on the street. Plus sticky racing tires will do more for you lap times than anything else.

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'86 Cab, '77 Targa, '85 toyota 4wd rain mobile http://www.cheaterswayside.com/911/

This is what the 245s look like.





Countering to the suggestions some have given, I think this is too much grip to give up, for a lower center of gravity, and more torque.

Don't get me wrong, you're making valid points, but I am, and will still be traction limited right?
Ahmet

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It's all the driver...

As you well know, the suspensions job is to keep the tire flat on the ground, not to support the car during rapid transitions. Fortunately, it just works out that way.

The way I understand your position, you will gain nothing with more power. Your cars suspension is MAXed out. To reduce your contact patch would be very counter productive. Not only are you pressuring that outside tire, but you are lifting the inside as well. Smaller tires would mean (to me that is) less tire holding the force on the outside and less tire dragging a toe on the inside.

Please keep in mind that I could not get a job changing tires, so the above "points" are only my opinion. I will be curious to hear what your other 944 race buddies are doing with tires... Can you use gumballs in your class?

Yeah, I don't really need more power, already have plenty of throttle oversteer (which admittedly is a direct result of being hard on the throttle, early), but I think that's mostly because of the missing LSD.

Anyway, I'd ask to other 944 guys about this, but I've beaten every single front engined Porsche at auto-xes so far, including the turbos and 968s, as well as S2s, infact the closest one was over 3 secs off me!

The only other person that seems to be doing good (that I know of) is talking about running 205/50/15s, which would lower the car a tad over an inch. That's all fine and dandy, but that's too much traction to give up! (He hasn't tried these tires yet).

Again, the car can be as stable as I want, but if it's cornering limits are going to be lower, why bother?

Racing slicks are allowed, and I know they'd improve my times big time.

However I have this thing where I drive my car on the street, exactly the way it was raced, I don't remove my spare tire, I don't pull out the floor mats, etc. Even the same tire pressures, and alignments.

In the future, if I get really serious, that may change, but not for now.

+I like being able to feel inside that everybody that raced against me (and I got 2nd place at last years regional finals in C-stock mind you) had done pretty much everything they could. This year I'm against much more experianced drivers. Infact, one of them got 1st place nationals last year, another got 4th place, and another 3 or so have 6+years of experiance, and one is an instructor out of the 7. Last sunday I got 3rd place out of the 7, with my car set-up the way it was. Only 2 people were on street tires! Needless to say, I'm feeling good with this...

Thanks for the replies, I may stick to 245/50/15s, at least for the foreseable future, in which the car should remain dual purpose.

Thanks again,
Ahmet

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It's all the driver...

The 245 do look mean. But it would be mistaken to think the smaller tires give a smaller contact patch or less stick. The tire patch will expand until the area of the patch multiplied by the PSI in the tire equals the load on the tire. Period.

The amount of rubber you have on the road has nothing to do with the width of your tires. The only way to increase your contact patch is to reduce your tire pressure, or increase the load on the tire. This is not my hypothesis, it is fact.

So why do fast race cars use big tires? Heat dissipation. High speed race cars produce lots of tire heat, and if they don't dissipate that heat the tires wont last. F1 cars see tire temps that approach 300 degrees F.

Big tires can dissipate more heat than small tires. And for a given tire pressure will experience less sidewall deformation, therefore produce less heat. That is why high speed race cars use them.

But autocrossers dont run fast or long enough to generate much heat. So the big tires are really just dead weight that slows your acceleration, braking and adds unsprung weight.

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'86 Cab, '77 Targa, '85 toyota 4wd rain mobile http://www.cheaterswayside.com/911/

I would have thought that 245s are too wide for a 7" wheel as they normally go with 8-9". Do you get any feeling through your steering that the tyres are peeling of the wheel? I say go for the 225s. Btw, if it's not a trade secret, what tyres and what pressures do you run and also what were the guys that beat you using?

Cmorelan – With all due respect, I believe your information is erroneous. Perhaps you should you look into some books of auto handling such as Vic Elfords book on how to drive a 911, Paul Frere book on Sports Car and High Performance Driving and another book titled Engineer to Win. While I will admit that reading is no substitute for training with an expert, it does offer the mental data necessary to make experience more meaningful.

As far as the statement about wide tires being for heat dissipation and not more grip, I do no know how to politely say this – but that is incorrect. If the idea that “The tire patch will expand until the area of the patch multiplied by the PSI in the tire equals the load on the tire. Period.” Means more grip, why not run 105 / 85’s at 75 PSI?

Facts: there are two ways to increase your tires contact patch to the road – go with a wider tire and / or a bigger wheel. Decreasing pressure (slightly) will only bring in a little more sidewall into play. There is a very fine line here, and that is why experienced drivers and crews will mark the sidewall so as to know if the “roll over” is too significant (in lieu of a pyrometer). Aside from the tire size, the next factor in a tires grip is the compound from which it is manufactured. Sticky tires hold the track better, period. This is due to increased friction between the wheel and the road surface. And friction equates to heat.

Heat is the killer of tires and as far as heat [I]in[/I[ tires goes, larger tires = more heat retention, not more heat per say. The best judge of tire performance (and proper pressure) is a pyrometer: the outside tread edge of the tire should be close to the middle in terms of temp. If the inside is hotter – too much pressure – if the inside is cooler, too little pressure. TREAD dissipates heat (and allows for various conditions such as water dissipation too) and the rubber compound used also determines the amount of heat SOAK. High performance tires will get hotter so they can stick to the road better. High mileage tires are harder, resisting heat soak and thus returning longer life.

I hope you will read my comment with the spirit that it is offered. There are hot debates over varying opinions these days, and our posts disagree greatly. Perhaps I am wrong, but I believe in my research. Many years of Vintage racing have successfully proven the above statements as fact.

Gosh – I just got up to get a drink of water too….

Nobody's sleeping tonight.

I'll be slightly less polite. Wider tires don't put down a larger patch? Huh?

To use an exaggerated example, a bicycle tire on a 16-inch wheel would offer significantly less grip, and a significantly smaller contact patch with the track than a 16x10-inch wide slick. I can lock up the wheels on a bicycle with a pair of cables and a levers. I wouldn't be able to do that with 10-inch tires on a 16-inch wheel, would I?

Now, wait a minute, I just re-read your post, and I think you're just using a word in an unfamiliar way. By "width", you're referring to larger-diameter tires, not wider (as in tire width) tires. You mean 14-inch diameter, as opposed to 15-inch or 16-inch, right?

It's true that larger-diameter tires don't guarantee a significantly larger patch than smaller-diameter tires. Wider, tires, however, (the "245" in the 245/45/XX sizing) you guys keep talking about, do. Viprklr is absolutely correct about that, but I get the feeling you were talking about 'bigger', or even 'taller' tires, when you said width, but not narrow versus wide.

All right, now I'm going to sleep.

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Jack Olsen
1973 911 T (3.6) sunroof coupe
jackolsen@mediaone.net

[This message has been edited by JackOlsen (edited 03-16-2001).]

Wow, just got into about the same argument on the 944 side of the board!

Anyway, JackOlsen, and VIPRKLR are correct.

Let me explain:

The grip (or friction, or traction) a tire provides does not increase linearly to the weight on it.

Another way to put this would be:

The traction a tire provides is NOT proportional to the weight on it. More weight =more traction, BUT, it's not that simple.

Example: (purely made up #s)

Let's say a 245/50/15 tire, with 100lbs on it, can provide 100lbs of ressistance to movement, in a horrizontal direction, applied from the wheel's center.

The same tire with 200lbs of force on it, will NOT be able to provide 200lbs of ressistance in the same direction, applied at the same spot. For comparison's sake, let's just say it can provide 180lbs of ressistance.

Now, the same tire with 300lbs of force pushing it down (let's say weight) will NOT provide anywhere near 300lbs of ressistance, nor 280, something more like 240.

The more the weight increases, the less the grip of the tire increases. But in theory more weight =more traction.

The same principal applies to the wider tire:

Given that a 245/50/15 tire can provide 100lbs of ressistance, to movement in a horrizontal direction, when loaded with 100lbs, a 490mm wide tire of the same construction, loaded with the same 100lbs will provide MORE friction, but again nowhere near exactly twice the traction. This is again because of the first law I stated, the friction a tire provides is not directly proportional to the weight on it. That is traction increases with weight, but tapering off as weight increases, and never as much traction is gained, as the weight increased.

HOWEVER,

Jack, you'd be able to lock a 245/50/15 wheel with only cables, if the only weight on it were the weight of a bicycle. Likewise, you won't be able to lock up a bicycle wheel/tire if it were loaded with the weight of the back of your 911!

cmorelan, I can see that you have an engineering background... Perhaps the example you give would work with a baloon, or something to that nature, that doesn't ressist being shaped, by pressure. A tire sits flat against the pavement, and when inflated properly, a wider tire will have a wider (bigger) contact patch. (Keeping everything else constant).

Hope this clears it out for everybody, let me know.
Ahmet



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It's all the driver...

Boy, I knew that would get the hair up on some backs. We are all having fun here right? Jack, I am talking about width not diameter.

But here is the deal, wider tires do not increase the contact patch area unless they are running a lower tire pressure. Follow this 'splaination -

The tire has a load on it, say 500 lbs, pushing it down caused by the weight of the car. But the tire does not move in response to that downward pressure becuase there is an opposite force of 500 lbs pushing up on the tire from the ground. That 500 lbs is the area of the contact patch multiplied by the pressure in the tire. That contact patch area will expand until the force is exactly 500 lbs. Basic physics, and the width and diameter of the tire aren't even variables in this equation.

I am sure we can all agree that lower tire pressures cause bigger patches, thats why we can tell really low pressure just by looking at a tire. Over pressure and the tire deforms less, and the contact patch gets smaller. These observations reinforce my points.

A wider tire with the same load and pressure will have a WIDER contact patch than a narrow tire. The narrow tire will have a LONGER contact patch. But the AREA of the patches will be identical.

In the extreme example Jack gives about a bicycle tire, if pressure in the tires is equal and the sidewall was tall enough to allow this much deformation yes the contact patch would expand to have the same area as the wide tire. It would be a very long skinny patch. Laws of Physics dictate that it must else the forces on the wheel would be imbalanced and it would be accelerated. In practice the bicycle sidewall is not tall enough so you would be resting on the rim.

Okay guys, have at it. I am putting on my fire suit now.



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'86 Cab, '77 Targa, '85 toyota 4wd rain mobile http://www.cheaterswayside.com/911/

Originally posted by 1.2gees:


Perhaps the example you give would work with a baloon, or something to that nature, that doesn't ressist being shaped, by pressure. A tire sits flat against the pavement, and when inflated properly, a wider tire will have a wider (bigger) contact patch. (Keeping everything else constant).

What you say would be true, again if a tire didn't have ANY structural integrity. Sidewalls ressist being deformed (fact).

"Lower tire pressures cause bigger patches", this is true, however we don't want any of the sidewall touching the ground! I'm talking about maximum tread touching the ground. Again, we're talking physics, yes but this is in case of a tire!

PS: Why do race tires not have tread? (intended for the dry), or Yokohama advertises that for the same load, same size of tire, etc. with the same pressure, that their tires have more "contact area"? Well, simply because on race cars, you want maximum contact possible, (for more traction, and less tread/rubber deformation under load). Yokohama advertises, just as put it, that their tires will have "more contact area", with the same pressure, same load, etc. then any comparable non-competition tire.

If what you said would hold true, than Yokohama is lying, as more "contact area" would be impossible, between different tires of the same size, with same load, etc.

This is also why many racers shave their tires, to increase contact area. (Keep in mind pressure is kept the same, and so is the compound, the dimentions of the tire, etc).

If you don't agree with me, please contemplate.
Ahmet

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It's all the driver...

Haven't you guys ever heard of the experiment where you can find the weight of a car by measuring the contact patch of each tire and multiplying by the air pressure in each tire. cmorelan is correct. The shape of the contact patch changes, but not the area. It's hard to argue with Newton's Laws.

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Bill Krause
'79 911SC Euro

wckrause, the example you give is used to give a rough estimate to the weight of the car.

Also, along the area covered by the tire, pressure is assumed to be the same +the thread make up is not considered-->it's a rough guess.

Also, air pressure is a factor here, in all examples I've given, the pressure was to remain constant.
Ahmet

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It's all the driver...

1.2gees, racers shave their tires for two reasons:

1) Improved tire life. Under racing conditions full tread blocks will get ripped right off the tire from the heavy load and all the deformation.

2) Shorter tread blocks = less tread block deformation on side loads = more responsive tires.

Yoko's ad may be true, but there is a subtly here. A slick will transfer tire pressure to the ground in a 1:1 ratio. Now cut grooves in that slick, say reducing the tread area by 50% and that treaded tire is transferring pressure to the ground in a 2:1 ratio, this is because the tire does have some rigidity and does not droop over the tread blocks. So a tire with very narrow tread grooves will put more rubber in contact than a tire with wide tread grooves. But if you measure the area of rubber contact plus the area of the grooves between that rubber, this total area will remain a constant if it is a slick or an aquatread.

so we won't win a class action against Yoko.

I am not advocating the skinniest possible tires. There are limits to these things. Too much sidewall deformation and you have sidewall contacting the ground and a deformed contact patch, not a good thing.

What I am saying is Big enough is Big enough and going any bigger is just adding weight and slowing you down.

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'86 Cab, '77 Targa, '85 toyota 4wd rain mobile http://www.cheaterswayside.com/911/

I vote for cmorelan. His explanation totally makes sense to me.

OK, I think this is making sense now...

So what would YOUR vote be, am I wasting money, and power by using 245s on my car?

Would you go to 225/50s, or 205/50s then?
Ahmet

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It's all the driver...

Read "Engineer to Win" & “How to Make Your Car Handle” before believing anything.

cmorelan said: A wider tire with the same load and pressure will have a WIDER contact patch than a narrow tire. The narrow tire will have a LONGER contact patch. But the AREA of the patches will be identical.

I respectfully say BUNK to the above. Why not take a Pro Stock Dragster and put those skinny tires in the back and big meats up front? LOL The ONLY way to make a longer patch is to buy a larger wheel. There is no way in hell you will "stretch" any tire, either buy down force or by magic. The only way a 225 mm wide tire would have the same contact patch as a 245 mm wide tire is if you were to melt it down an pour it in the ground. The larger tire has no less force acting on it than a narrow tire. It is, however, better able to RESIST those forces. What is being overlooked is the fact that the tire is moving with several forces acting on it at the same time: inertial movement, steering, and in the case of the drive tires, forward rotational force.

Having discussed this post with Roy Gower, race car engineer for Ford and consultant for Panoz, he offer the following example: Take a manhole cover and place it on a 4" round, 2" tall rubber puck. Secure a string to the puck (simulating steering resistance) and using the string, pull the assembly toward you on pavement. It will take force, but it will happen. Now try to pull it toward you quickly. When it is half way to you, change the direction of your pull. There will be some, but very little resistance to the change in motion.

Do the same thing with an 8" wide, 2" tall rubber puck. It will not only take more energy to move it, but it will also have far greater resistance to the direction (steering) change than did the small puck.

All this is even better thought of in terms of braking. More tire = shorter stopping distance.

Greater surface = more resistance. If you want to want to push through turns, or have the rear end come around more easily, use narrow tires, OR over inflate your wide tires. IF you desire less of these attributes, increase your contact patch with wider, and higher (17" wheels as opposed to 15") rolling equipment. BTW: if you decrease a narrow tires air pressure too much, the side walls will bare the weight and center tire section will share less of the work.

Why do you think all the modern race cars use 18" tires? CLUE: its not because they look neat.

Also, with the added weight of the Turbo 911's motor, why do you suppose PORSCHE decided to use such wide wheels and tires that they had to modify the bodies? CLUE #2: it gave the high performance car a chance to put the power and handling to the ground.

DO NOT TAKE MY WORD FOR THIS – do what I have done: go to the book store and read what the tried and true formulas are. Read what years of experience have taught the experts. Then, having gathered the proper, accurate information, you can make your own decision.

If one formula applied, then there would be no competition. Read "Engineer to Win" & “How to Make Your Car Handle” before believing anything.



[This message has been edited by VIPRKLR (edited 03-16-2001).]