Tire Width Differences in Performance

[GT408]

Hello all. I'm currently in the design stage of a 1966 Mustang Fastback that will be built with some modern day Pro-touring style in an effort to be unique in styling, but more importantly to improve handling. I'm building a modest 408W and am shooting for 500hp, nothing too crazy. One day I might install a 4-link coilover in the rear and coilover in the front. But in an effort to get the car up and running while not waiting on the extra $5k difference to invest I'm going to start with leaves in the rear with sub-frame connectors along with some chassis modification to improve rigidity, and improvement of the front end geometry. My question is this: has anybody experienced, or can anyone honestly tell me if they see a valuable difference in running a F275 R315 set of tires versus a F245 R275 set of tires. The first set of numbers is the tire combo I plan to run with a 17" rim, the second is with 16". I might also consider R295 with the 17".

I don't plan on being the fastest person out there, I don't have the time or money to allow a full dedication to becoming competition worthy, but I would like to be able to build something that allows me to have a good time and turn some decent numbers in performance. Second thing is, if I go with 315s then extensive body mods to the rear will be in order to fit that width...mini tubs and stretching the fenders...all which I am willing to do if it's worth it since I will be doing some body modification anyway. I just don't want to add a whole lot more work for a minor difference in performance. I want to get up and running sooner than later! Thanks for the input.

[dontlifttoshift]

compound and construction > width

Good tire choices in 16" are hard to find. 17s are a little better if you don't want to go too wide.

I would wager that 245/255 - 17s RE71s or RivalS would be faster than what ever 200tw tires you found in 315/275-17s. Fits the budget by not requiring a bunch of mods, fits the car without requiring a bunch of mods.

The car can only go faster if _you_ can go faster......have you ever autocrossed before? If not, run the 245/255 on the widest rims you can fit and go have fun!

[Tomswheels]

Donny speaks with much wisdom....

[RobNoLimit]

I'm with Donny. And, as one of the few who has drivin a well prepped 'Big Tire' car (or truck) I will say that a big contact patch makes a difference, but at a price. A really really - really light 18"x 11" wheel weighs 19 lbs, and a 315/30/18 weighs 34 lbs. So thats 53 lbs per corner of un-spung rotating weight. Add 3 more lbs per corner if you totally man up and go 18x12 with 345's. Understand that in real world "best case scenario" you will use 70% of the contact patch at any one moment, but you cary 100% of the weight all of the time. BTW, Un-sprung rotating mass id the worst there is. To control all this requires more brakes (more mass) and better shock valving and tuning. - ++$$$$. My wife Tina's 65 Mustang runs a 245/40/18 on all corners, with the same wheel offset so we can rotate tires. The car is quick, neutral and fun to drive. I would go with a square car, even with a slightly smaller tire to get there, over a big/little car every time.
Here's some more info. There are many wheels on the market in 17x9, 5 x 4 1/2" (BMW and WRX) that weigh less than 16 lbs, and are relatively inexpensive. A 255/40/17 Bridgesone 71R weighs 24 lbs. Now thats 40 lbs per corner. Almost 25% less rotating mass. I am so convinced of this direction that (even though I built one of the first 'Big Tire' cars/trucks) That my current project under the knife is not being built around a 315/335 tire size. Wheels cost $$. plan the built out to the end. I would set the rear axle width and the front suspension width so that the same wheel can be used all around. If that means pulling the trigger on a good IFS first, do it. - Ours is less that 3200 bucks. More fun to drive in the long run.

[Peter Mc Mahon]

This is fantastic info. Rob, using the 17x9 example is the 255/40/17 the most appropriate width tire you would recommend on that rim? Thanks.

[RobNoLimit]

This is fantastic info. Rob, using the 17x9 example is the 255/40/17 the most appropriate width tire you would recommend on that rim? Thanks.

That's the best choice I have come up with. A 255 tire is apx 10.1" wide, Bridgestone specs that tire at 10.2", with a rim spec of 8.5" to 10" wide. A 9" rim is really 9.8" or so wide, (outside lip to outside lip, depending on rim manufacturer). So it's a pretty nice fit. I have been trying to run a wheel that is close to 1/2" or 3/4" (O.D.) narrower than the O.D. tire width. This matching seams to support the sidewall/contact patch the best. For 315 tires we have been using a 10 1/2" wheel lately with good results.
On my current project, I started out focused on a 275 tire, and although that is common in an 18" tire, it's not in a 17". After a lot of tire research I chose the 255/40/17, it has a good choice of tires/manufacturers from 400 TW all season tires to 40TW R-1 compound slicks. The difference layed out like this. The 255/17 combo weighs 40 lbs, tread block is 10.12" wide mounted. The best fit I could find for the 275/18 weighed 47 lbs, with a tread block 10.78" wide. The total difference in the contact patch is 1.8 sq.in. - but, if you follow the rule of only using 70% of the contact patch at any one moment, the usable difference is 1.26 sq,in. At some point there comes a point of diminishing return for a bigger, heavier tire. - This of coarse is assuming that you are maximizing the the tires full potential. So the question becomes, would you add 28 lbs of rotating un-sprung mass to gain 5.04 sq.in. if contact patch?
- BTW, I pruchased a set of Wed's Racing TC-105N wheels and 255/40/17 tires, mounted/balanced and shipped to my door, for a touch under 2800 bucks. - That's how I roll

[dontlifttoshift]

I'm not an engineer but I am going to over simplify this anyways.

The size of the contact patch doesn't change unless you change the load on the tire or the pressure in the tire.....255 to 315 will have the same _size_ contact patch.

The _shape_ of the contact patch changes drastically when you change tire width. A 315 will have a wider (side to side) contact patch than a 255. A 255 will have a longer (front to back) contact patch than 315. Both patches will be the same size.

[Peter Mc Mahon]

Donny, is this due to the outside diameter?

[Boxjohn]

Doesn't that ignore sidewall stiffness? It'll limit how long the contact patch gets regardless of width. Even an uninflated tire has lots of resistance to being squashed and lengthening the contact patch.

[Peter Mc Mahon]

So a car weighs XXXX lbs, it will compress a tire by that much so on a narrow tire it will compress more front to rear, and on a wide tire it will compress more side to side? If this is correct then in a straight line, not under load, both tires would have the same grip?

[RobNoLimit]

I'm not an engineer but I am going to over simplify this anyways.

The size of the contact patch doesn't change unless you change the load on the tire or the pressure in the tire.....255 to 315 will have the same _size_ contact patch.

The _shape_ of the contact patch changes drastically when you change tire width. A 315 will have a wider (side to side) contact patch than a 255. A 255 will have a longer (front to back) contact patch than 315. Both patches will be the same size.

Not sure I agree with this, but I am quite willing to hear the explanation. Pressure may be the key. My C10 with 335's needs 21-to-24 lbs in front to get the temps even all the way across the tire. The rolling contact patch is appx 1.75" x 9.75", or 17.05 sq,in. With a 275 tire, it needs 26-to-28 lbs to get the temps even, at that pressure the rolling contact patch is appx 1.65" x 8.4", or 14.12 sq.in. (note that the larger tire is only 17% larger on the contact patch at these pressures). I would agree (haven't tried it, so this is a guess) that if we aired the 315's to 27 lbs, the contact patch would get smaller, maybe equal to that of the 275 tire at the same pressure, but this set up would run hot in the center, and not maximize the tires ability. Likewise (I guess) if we dropped pressure on the 275 tire to 22 lbs, the contact patch would grow - here's where I get lost - but, due to the cupping of the tire under load, I'm not sure the center of the contact patch would actually touch the pavement, as this would show the center temps much cooler than the outsides. - Donny?

[dontlifttoshift]

Donny, is this due to the outside diameter?

No, but that is a factor in the shape of the patch. Taller tire, longer patch than a shorter tire of the same width.

Doesn't that ignore sidewall stiffness?

Yep.

It'll limit how long the contact patch gets regardless of width. Even an uninflated tire has lots of resistance to being squashed and lengthening the contact patch.

Yep. Over simplified, I said :wink:

So a car weighs XXXX lbs, it will compress a tire by that much so on a narrow tire it will compress more front to rear, and on a wide tire it will compress more side to side? If this is correct then in a straight line, not under load, both tires would have the same grip?

Grip is sort of a function of load so I'm not sure I can accurately answer that question.

Not sure I agree with this, but I am quite willing to hear the explanation. Pressure may be the key. My C10 with 335's needs 21-to-24 lbs in front to get the temps even all the way across the tire. The rolling contact patch is appx 1.75" x 9.75", or 17.05 sq,in. With a 275 tire, it needs 26-to-28 lbs to get the temps even, at that pressure the rolling contact patch is appx 1.65" x 8.4", or 14.12 sq.in. (note that the larger tire is only 17% larger on the contact patch at these pressures). I would agree (haven't tried it, so this is a guess) that if we aired the 315's to 27 lbs, the contact patch would get smaller, maybe equal to that of the 275 tire at the same pressure, but this set up would run hot in the center, and not maximize the tires ability. Likewise (I guess) if we dropped pressure on the 275 tire to 22 lbs, the contact patch would grow - here's where I get lost - but, due to the cupping of the tire under load, I'm not sure the center of the contact patch would actually touch the pavement, as this would show the center temps much cooler than the outsides. - Donny?

I don't have to explain it, you just did. Same pressure, same load, same size patch. But the wider tire can operate at a lower pressure. Less pounds per square inch means we need more square inches to support the same load - boom, more contact patch. As far as the tire cupping under load and giving up grip through the center, that could only be determined by measuring tire temps in real time. It's probably not as drastic as either one of us would think. It would probably be extra sloppy on turn in and likely would roll over onto the sidewalls alot.

Please note, I have not said which one is better but I don't know anyone that has put wider tires on and gone slower. At this point, I'm not sure the gains are as large as I would like to believe. I also don't know anyone that has done any A-B-A testing on a PT car. I would love to see a handful of cars tested that could run 275/275 315/315 and 275/315 all on the same tire brand. Perfect cars for this dream test would be Maiers mustang, NoLimits truck, the ridetech 33, and the DSE 2nd gen but I would like to see all of those cars with 400hp at the wheels because all of those cars will melt 275s on the rear in short order. Those cars are all well sorted. Bring in Mary, Junior, and Strano to drive them. Also test the 275s on 9.5" and 11" wheels.

[Cobra 498]

No, but that is a factor in the shape of the patch. Taller tire, longer patch than a shorter tire of the same width.



Yep.



Yep. Over simplified, I said :wink:



Grip is sort of a function of load so I'm not sure I can accurately answer that question.



I don't have to explain it, you just did. Same pressure, same load, same size patch. But the wider tire can operate at a lower pressure. Less pounds per square inch means we need more square inches to support the same load - boom, more contact patch. As far as the tire cupping under load and giving up grip through the center, that could only be determined by measuring tire temps in real time. It's probably not as drastic as either one of us would think. It would probably be extra sloppy on turn in and likely would roll over onto the sidewalls alot.

Please note, I have not said which one is better but I don't know anyone that has put wider tires on and gone slower. At this point, I'm not sure the gains are as large as I would like to believe. I also don't know anyone that has done any A-B-A testing on a PT car. I would love to see a handful of cars tested that could run 275/275 315/315 and 275/315 all on the same tire brand. Perfect cars for this dream test would be Maiers mustang, NoLimits truck, the ridetech 33, and the DSE 2nd gen but I would like to see all of those cars with 400hp at the wheels because all of those cars will melt 275s on the rear in short order. Those cars are all well sorted. Bring in Mary, Junior, and Strano to drive them. Also test the 275s on 9.5" and 11" wheels.

We tested 275x35x18 Bridgestones vs 315x30x18 Rival S both on 10 1/2" rims on the rears and the 275 Bridgestones were superior. This was an A to B, B to A test. We then tested the Bridgestones vs Rival S 335x30x18 on 12's and the Rival S was superior. Rear wheel power around 700 so anything can be fried at will. Forward acceleration and lateral acceleration were both slightly better but enough to translate to about .5 seconds on a 31 second course. we ran the same air pressures on both the 275, 315 and 335 tires because that yielded the best times on that particular course. I am relatively sure the contact patch of the 335 was larger than the 275 because of rim width and physical tire size. As far as drivers go I would put Scott up against all those mentioned. I believe that the Bridgestone compound and construction are superior to the BFG Rival S but at some point physical size overcame the deficiency.

[dontlifttoshift]

Good info! Thanks for sharing! The only thing that would be cooler is if the 275s would have been RivalS as well. That takes the previously mentioned compound and construction out of the equation.

Did you stretch 275s on a 12? That's awesome.

Do you think the 335s did better due to heat tolerance of a much larger tire?

[Cobra 498]

We didn't try anything over a 10.5 with the 275 although you could probably get away with an 11" rim. The Bridgestones are 10.1" wide at the tread so they are not ordinary 275's.
You might be interested in our front tire test, we ran a 315x30x18 Rival S against the Bridgestone 275x35x18 and they were also very close, the Bridgestone was .2 faster on the same course but more importantly both drivers (myself and Scott) liked the feel better. The Bridgestone construction gave a very precise accurate response. Grip levels were very similar but the Bridgestone was easier to go fast with. The 335 has a little better heat recovery but it just flat has more grip both lateral and tractive. Our set up required some changes to rebalance the car with the 335's vs 315 or 275's which indicates more grip from the 335's. I have heard Bridgestone has a 305x18 coming next year which could change things.
This is a video of a prosolo practice run where I smoked the tires pretty badly but acceleration was still quite good with the Rival S in 335 x 18, showing how much traction is available.
https://www.youtube.com/watch?v=sSNexdP28zU

[dontlifttoshift]

305-30/19 for RE71R next year.

Thanks again! Keep adding info. There aren't many of us that have the time or the resources to test so we rely on those that can to bring useful information.

[GT408]

Thanks for all of the great information everyone! I'll be taking all things into consideration before I even get started on the restoration. At this point my thoughts are leaning towards the 275 just for ease of fitment. I'm not trying to win any events here, and it sounds like that will get the job done.

[72BBSwinger]

I hope they do make a 305/35/18! That would be awesome, otherwise im gonna run the stones on the front and Rivals on the rear. Don't care about contingency, its my money and I want the best for my dollar....