Rear tires

[gtforme]

Hi guys

Everyone seems to have their own favourite angle to view their FORD GT from and my favourite angle is directly from the back

In saying that I would like to see wider tires on the back such as 345 or 355 instead of the stock 315 , I have been told that you can install 345 on the stock rims and I have seen it done , however what happens is the tires dont give the presence of being much wider as they are restricted by the size of the rims

And that brings me to my next issue and that is I love the BBS rims and as far as I know they dont make them any wider , I have heard that you can have the rims cut in half and have a spacer installed to make the rims wider and take advantage of the bigger tires

Has anyone tried this or does anyone know if you can get wider BBS rear rims , or are there rims out there that look very similar to the stock BBS rims

Thanks for any responses

Rob

 

[paul b]

I don't know about spacers but I was thinking about wheel adapters and using Viper wheels and tires. I have a new set of the lightweight 6 star Viper wheels that should look pretty good on the GT.

 

[cobrajeff]

Wheel modifications

If you decide you want to widen your stock rims, contact James at Weldcraft Wheels in Plymouth, MI:

http://www.weldcraftwheels.com/index.html

These guys can widen, narrow, or repair just about any rim. I have had them modify steel, aluminum,
and magnesium wheels for me. They do a great job. Here is an example, a magnesium Pantera
Campagnolo wheel that they narrowed down to 5" width so I could mount a collapsible spare tire on it:

Jeff

 

[Thugboat]

Cobrajeff

Wow, these guys are what I have been looking for. I wanted a set of track wheels for the GT and I bet these guys can do almost anything. I plan to give them a call today.

Larry

 

[DoctorV8]

Good info Jeff! I knew there had to be someone out there that could modify magnesium wheels!

 

[Fubar]

FYI, I was informed my a few guys at TWS that going from 315 to 345 may not get you additional rubber on the ground. They challenged me to take my car on dirt road with 315 and then again 345... then measure the contact patch. I'd be glad to get the figure on a 345 Bridgestone if someone else will get the 315 Goodyear measurement.

 

[Mullet]

I GUARANTEE my 345 Pilot Cups are wider and have more contact space than the 315 Goodyears or Hoosiers.

 

[Superfly]

With the additional camber from lowering the car, I'm not convinced I've got more rubber on the ground with the 345 Potenza's. But it IS better (softer and stickier) rubber, so I'm good. Plus it is wider rubber (at least visually) on my 12" rear rims, so again, I'm well pleased.

 

[Yukonranger]

The size of the contact patch is solely a function of sidewall stiffness (which is almost negligible) and tire pressure. If you have 35psi and 1400lbs of load, your contact patch will be 40 sq in (very slightly less since the sidewall will take part of the load). BUT...It turns out that the SHAPE of the contact patch is important and you want it to be long in the direction of force (hence wide tires for cornering). There are other related issues like the fact that as the area of tread gets larger the rubber can be made softer with the same wear rate.

See the "Unified Theory of Rubber and Tire Friction" for great bedtime reading. One of the few remaining things I recall from studying engineering many years ago.

I GUARANTEE my 345 Pilot Cups are wider and have more contact space than the 315 Goodyears or Hoosiers.

 

[Mullet]

properly cambered 345's with have a bigger conact patch than 315's that are properly cambered. I am referring to straight line driving (that's what I do mostly compared to track type driving).

now, deformation of tires going around corners and bigger and smaller contact patches.....I don't have an opinion as I don't have knowledge in that.

 

[Yukonranger]

No, the contact patch changes shape with camber change but not size. The only things that can change contact patch size are tire pressure, sidewall stiffness and loading. When you have 35psi in the tire you have one square inch of contact patch for every 35lbs of load on that tire. psi is pounds of force per square inch.

 

[Mullet]

No, the contact patch changes shape with camber change but not size. The only things that can change contact patch size are tire pressure, sidewall stiffness and loading.

makes no sense.

are you telling me a 225 tire has the same contact patch size as a 345?

 

[Mullet]

FYI, I was informed my a few guys at TWS that going from 315 to 345 may not get you additional rubber on the ground. They challenged me to take my car on dirt road with 315 and then again 345... then measure the contact patch. I'd be glad to get the figure on a 345 Bridgestone if someone else will get the 315 Goodyear measurement.

my Pilot Cup rears are 345's and have 12.5 inches of contact space and every bit of the 12.5 inches has touched the road.

 

[Yukonranger]

Yep the contact *area* is the same size. Your 345s indeed have a contact patch that is 12.5" wide and obviously the 225 does not, but the 225 contact patch is longer and the area (with the same load) is the same.

This is a simple physics problem. it is the same as if you sat on different size balls, it doesn't matter how big the ball is, it will compress until the contact patch (in square inches) is the load (in pounds) divided by the pressure in psi.

I know that this may not be intuitive to you but if you contemplate how the force on ground must be equal to the force on the inside of the tire pushing against that contact patch you will see how it cannot be any other way.

BTW I will probably go with the 345s when I replace tires as they are one of the few tires that have close to stock diameter and they certainly look better!

 

[Mullet]

Yep the contact *area* is the same size. Your 345s indeed have a contact patch that is 12.5" wide and obviously the 225 does not, but the 225 contact patch is longer and the area (with the same load) is the same.

so why do they make wide tires?

are you telling me a 19 inch diameter bike tire (road bike tire the real skinny ones) will have the same contact patch as a 19 inch diameter 345 tire with the same amount of weight on them?

I'm just not going to be convinced on this one.

 

[Yukonranger]

Yes, if the tire pressure is the same, the car tire and the bike tire will have the same contact patch size. Try it! Naturally, if you mount the bike tire on your car, the tire will compress to the rim before the contact patch gets large enough to support the car. If you are talking about very light loads then the tire sidewall stiffness eventually becomes significant.

Think about it this way: air pressure is the only thing keeping the rim from coming down to the ground. the air is exerting a force on the inside of the tire in pounds per square inch. If the load is 1050lb (approx gt rear tire load) and the pressure is 35psi there has to be 30 square inches on the ground. If the tire tread in contact with the ground is 12 inches wide then the length of the contact patch is 2.5 inches which is about what it looks like.

They make wide tires for several reasons:

The wider tire has more total tread area (tread width * tire circumference) so the rubber compound can be made softer and still get reasonable wear. A wide, high performance has about half the treadlife (or less) of a normal tire while having significantly more total tread area. Thus the wide high performance tire compound may be wearing at 4x the rate of the normal tire (twice as much total tread wearing out in half the distance). The compound can be significantly softer and this is where most of the traction advantage is.

The second main advantage is that traction is improved when the contact patch is made longer in the direction of force. A wider tire has a contact patch which is long in the axis perpendicular to the centerline of the car. If you look at the tires on a top fuel dragster the tires are not particularly wide but they are tall providing a contact patch which again is long in the direction of travel. They also use very low pressure to get a large contact patch.

 

[satx]

Yes, if the tire pressure is the same, the car tire and the bike tire will have the same contact patch size. Try it! Naturally, if you mount the bike tire on your car, the tire will compress to the rim before the contact patch gets large enough to support the car. If you are talking about very light loads then the tire sidewall stiffness eventually becomes significant.

Think about it this way: air pressure is the only thing keeping the rim from coming down to the ground. the air is exerting a force on the inside of the tire in pounds per square inch. If the load is 1050lb (approx gt rear tire load) and the pressure is 35psi there has to be 30 square inches on the ground. If the tire tread in contact with the ground is 12 inches wide then the length of the contact patch is 2.5 inches which is about what it looks like.

They make wide tires for several reasons:

The wider tire has more total tread area (tread width * tire circumference) so the rubber compound can be made softer and still get reasonable wear. A wide, high performance has about half the treadlife (or less) of a normal tire while having significantly more total tread area. Thus the wide high performance tire compound may be wearing at 4x the rate of the normal tire (twice as much total tread wearing out in half the distance). The compound can be significantly softer and this is where most of the traction advantage is.

The second main advantage is that traction is improved when the contact patch is made longer in the direction of force. A wider tire has a contact patch which is long in the axis perpendicular to the centerline of the car. If you look at the tires on a top fuel dragster the tires are not particularly wide but they are tall providing a contact patch which again is long in the direction of travel. They also use very low pressure to get a large contact patch.

I have heard this before and your right, it is not very intuitive. Thanks for the great explanation.

 

[Indy GT]

Yukonranger-
Very good post and technically informative. These type of esoteric topics are difficult for most owners to fully understand. It is just so easy to believe wider tires are always better. Then throw into the mix that the tire tractive or friction force is coefficient of friction (Mu) times the Normal force and thus physically independent of contact Area, and people really fall off the wagon.

Thanks for you informative post!

 

[Yukonranger]

Actually tire and rubber adhesion is not simple friction. It is more of an interlocking effect and contact area is important. It is also non-linear, doubling the load does not double the grip.

 

[Thugboat]

OH CRAP!!!!

I hope there isn't going to ba a test at the end of this!!!:eek

Larry

Really thanks, The analogy of the funny car finally put it into perspective for me. Their tires aren't very wide and they have a tad more horsepower than we. Dang it, now I can go home and tell my wife this web site is informative also. (I am on it so much she is accusing me of watching porn. It is car porn)