Flott Leben

Will this work on the 5-spoke cup 993 rims:

8 inch front wheels - 235/40-17
9 inch front wheels - 275/45-17

What, generally, is the maximum permisible tire width to wheel width ratio? Does it differ from front to back?

Thanks!

Viken

235/40-17: Ideal wheel width is 8.5" but 8" will work as a minimum
275/45-17: Ideal wheel width is 9.5" but 9" will work as a minimum

It all depends on the particular tire you are using. Each tire manufacturer should have a minimum, ideal and a maximum permissible wheel width for each tire size. If you look here, you'll see that.

Flott Leben

Viken,

As always, you are an incredible resource! Thanks.

One more question:

If I use the maximum tire width for my wheels, what handling, cornering, sizing, etc. drawbacks will I encounter, if any? Thanks.

Viken

First, you must understand that there are NO benefits from installing wider tires unless you compensate for downforce. In other words, the pressure per square inch from the wider footprint being the same, you will never notice a difference. As to drawbacks, there are two. First, a bigger tire is heavier and more unsprung weight is usually not desirable. Second, and this is strictly personal, the looks. I just don't like a tire that looks like a donut.

Flott Leben

Viken,

Could you please explain the downforce issue a little more?

Viken

On any given tire footprint, there is a certain amount of pressure (i.e. pounds per square inch). If you increase the size of the footprint without increasing the pressure, you are not taking advantage of the extra rubber touching the ground.

Flott Leben

I am sorry to keep asking for more information but are you referring to downforce during forward acceleration (i.e., wheel spinning on takeoff) or lateral acceleration when hard cornering (where side forces and not just down force come into play)? Is there a tech article or thread that discusses these issues in more depth? Thanks.

DJ

Flott,

The answer to your question is Yes. Other than for looks, the reason to put on wider tires is for more grip--specifically, if the car is currently "under-tired". With a wider tire, it will have more rubber in contact with the road surface, and the specific amount of heat generated will be absorbed into a larger area (more rubber). If your tires are overheating and losing grip, then going to a wider tire may solve your problem. But I doubt that this a problem that you are actually experiencing. If you want wider tires because they look good, then have at it, but don't expect miraculous improvements in grip.

Larger front tires generally improve braking. Larger rear tires generally improve grip under hard acceleration. On my 993, I tried various different sizes and combinations from 225f/285r to 265f/315r and everything in between. Utilizing the same compounds, there was not a commensurate increase in grip with wider tires, but, on the track, the wider tires could go longer before overheating.

Just for the record, I found the best grip/handling/lowest lap times running Pirelli slicks in the stock sizes.

To add on to what Viken was mentioning about downward force on a tire, remember that (up to a certain point) more downward force--or weight, if it's easier for you to think of it that way--on a tire means more grip. Now, imagine that the current tire size has a footprint of 10 square inches, and that the new proposed tire has a footprint of 11 square inches. For the sake of argument, let's assume that when the car is placed on a set of scales, one corner of the car weighs 750lbs. So, with the original tire size, there is 75 lbs of downward force per square inch of contact patch (750lbs. divided by 10sq.in. = 75lbs/sq.in.). With the bigger tires it would be only 68lbs/sq.in., so the downward force on each square inch of contact patch has decreased. Like I said before, if the car is originally "under-tired" then increasing the width of the tire *may* solve your problem, but more likely, it won't.

JohnM

For those interested in a bit more technical detail on the effects of tyre width, here is an extract from a post on this general topic over at Pistonheads

The school physics books tell you that the coefficient of friction is a constant, but in reality this is an oversimplification. In the case of tyres, the coefficient of friction drops off as you increase the weight on the wheel. This is why you can tune a car's handling by adjusting the anti-roll bar, and also why you *can't* tune with anti-roll bars if the load on the tyres is too small (the vehicle is overtyred).

Because of this, the bigger the contact patch the more grip you can get. In a drag race, dropping the tyre pressure increases the contact patch area and increases grip. Even on road bikes you will see people dropping the tyre pressure to almost nothing for the absolute maximum grip down the strip.

BUT, when you look at lateral grip (side force) other factors start to matter. The tyre develops side force because of the slip angle between the tyre and the road. This slip angle means the tread is being pulled sideways by the road surface. At the front of the contact patch the deflection is relatively small. As you move back along the contact patch the deflection increases steadily. At some point, the sideways forces in the tyre exceed the friction between the tread and the road and the tread starts to slip relative to the road. When the tread is slipping like this it produces less grip on the road. Now imagine increasing the slip angle and imagine what effect this has on the side force. As the slip angle increases the sideways deflection builds up quicker so the front of the contact patch works harder. But more and more of the back of the contact patch is sliding and losing grip. At some point you reach a maximum point where more slip angle means less side force because you are losing more grip at the rear of the contact patch that you are gaining at the front. This is often referred to as 'breaking away' where you ask the tyre for more grip and end up getting less.

The longer the contact patch is, the more gradually this break away occurs. If you shorten the contact patch, the break away occurs more abruptly but you get more absolute grip at the peak. This is because there is less variation in sideways distortion between the front and back of the contact patch, more of the contact patch reaches maximum grip and starts to slide at the same point. Having a shorter contact patch also means you get less self-aligning torque so there is less feedback through the steering about how close the tyres are to breaking away.

When you fit wider tyres, what you're doing is making the contact patch wider and shorter for the same tyre pressure. This means you get a more abrupt breakaway but more grip right on the limit. The disadvantage is more expensive tyres, more tramlining and steering kickback, more wind and rolling resistance and noise, less grip in slippery conditions, a more abrupt breakaway to catch out the unwary driver and less warning through the steering about how close the tyres are to breaking away.

buckdr1

Guess that sums things up!
Bill

Rob993

Excellent information guys, I am learning a lot over the past few weeks (a hundered times more than reading Porsche Magazines the past few years).

I have a C4 993 (1995) that I will be replacing tires on the back, as the front are still good. The current rear tires Toyo TS1's are 255/40-17's.
I love the look of wider rubber but not the donut shape as you mention Viken. Can I increase to either a 265/35-17 for a little more width or is there a larger size that still looks ok?

Thanks
Rob

Viken

Yes, the 265 will work fine on a 9" wide wheel. However, consider the fact that there are no real benefits as discussed above.

JPS

Kind of resurrecting this...

So I have 7/9 f/r 17". Stock is 205/50 & 255/40. Of course for understeer I have shifted to 225 up front which has made a large difference.

Q: I am seeking to put a set of track tires (still DOT) on them. My original thinking was go as big as I can, 225 or 235 up front, 275 in the back. Not lots of mfgrs available in that size (Kumho only). Am I to understand that I won't gain by putting the 275 on? By that logic, could I go 245 in back which has more mfgrs to choose from, and still get similar performance, though perhaps slightly hotter tires?

Thanks,

Matt H

Loads of manufacturers make the 235/40R17 and the 275/40R17. What sizes are you talking about that you cannot find? Or are you referring to track tires only?
__________________
Best Car Insurance | Auto Protection Today | FREE Trade-In Quote

JPS

Sorry, I wasn't super clear. Yes, for track tires. I think the 235 on a 7" is too big, but 225 I think is helpful for the understeer.

Toyo, for example has a 205 (I feel too small), and a 235 (too big for rim) in the RA-1 or whatever they are. The new Dunlops for racing have 225 up front, but only go to 245 in a 17. Kumho has 225 and 275, but some are telling me the 275 will actually hinder cornering on a 9" rim simply because there will be so much roll since the tire will be so fat on the rim.

So, perhaps a better way to ask this: I have 7" and 9" rims f/r. What is the optimum size tires to put on that for DEs, trading the stick and lower temp of a fat tire against the lower roll & higher temp of a more narrow tire?

The new Dunlops are a bargain intro price, and I'm also wondering if the 245 would be worth the $ savings.