tyre width vs HP

[Alan L]

I am currently running 17x255 rear and 235 front - Bridgestone wet/dry track tyre.
They are due for changing. I am considering going to 275 rear, 255 front. I could always use a bit extra grip But there must be a trade off vs the amount of rubber on the track compared to how much HP you run. My engine is stock, except for the mods listed below. I know guys are running up to 315 rear, but probably with a lot more HP than mine. It would seem obvious, if I was only running 200 HP, the car would seem to be carrying too much rubber to utilise it effectively.
Also , does the ideal formula change in the wet, or is it same for both?
So my Q really is, am I likely to be carrying to much rubber for the car by going to 275/255.
Thanks
Alan

[racer]

Which car is this? Stock SC? 930? How much HP do you have? I'll assume the 930 and at least 350hp? How does the car feel now? Are you getting lots of wheelspin? Are you looking to improve cornering ability? I would think a 245/275 would the "smallest" you'd want, esp in the dry.

The wet changes everything... from shock/spring settings, to tire widths/compositions, driving style etc.

[cstreit]

How much does the car weigh?

315 is a LOT of tire and lighter cars will not get it up to effective temps. I admire your thinking as many people miss this fact. The only way to be sure is to take hot temps once you upgrade.

[Alan L]

Ok, it is the 930, and my best guess at HP is around 350+ - with the headers and modified turbo.
Last I weighed it , was 1350kg. Still trying to get some more weight out of it. I run these tyres wet and dry - these are the rules - ie no slicks. I would like more corner speed - I am loosing a bit of time in the corners - due to the turbo lag, and the fact the rear does like to get loose (but coming off a corner with full boost spooling up, this is likely to happen, I think). The front grip seems good - never sledges into a corner. I did a posting recently about tyre temps and on current tyres, and not getting more than 120F at hottest point - but it is winter here, and these tyres are past it I think and just not working/gripping hard enough. So the Q is replace with same 255/235 or take the opportunity to go 20mm wider - my rims will take the extra 20mm. I am sure 315 is waaay to much for me - I need 600HP I would think, to make use of them.
Obviously I will get some tyre data once I get these tyres going, it is just a matter of trying to make a useful decision. Any input into the thinking will help. It is the only 930 on the track regularly here in NZ, and the other 930 that does track from time to time is a waay different beast - EFI, water cooled turbo etc.
Thanks
Alan

[winders]

Alan,

I have a 2375 lbs (with me in it) race car with ~300HP. I use 11.5" front and 13" rear slicks on the car. They get up to temperature just fine.

If I had a ~3000 lbs 930 with 350+ horsepower, I would run a 255/315 or 275/315 setup for sure. I would go so far as to say it is a "no brainer".

Scott

[Alan L]

Whoo, that is a lot of rubber. Actually I think I put you wrong on the weight. it is about 4 yrs since I weighed it and I keep finding ways to pull a bit out. researching lite weight windows at moment. But I think 1150 kg was nearer to the mark 4 yrs ago.
I am going to 275/255 then. This is as wide as I can go as per the rules (2" over original spec)
Thanks
Alan
Edit - according to my latest corner weights I've got 2630lbs with my weight in it. Makes me feel a bit better.

[RichardNew]

Now for the interesting thing. The size of the tire doesn't mean you have more rubber on the ground. The size of the footprint is determined by the weight of the car and the tire pressure.

Changing the size of the tire will alter the shape of the footprint but not it's size.

Richard Newton
Newton's Laws

[Alan L]

OK, I'm all ears. But why do people upsize tyres for more grip? On this basis, are you saying I could run less rubber and have same footprint.
Alan

[Pete R]

Originally Posted by cstreit

How much does the car weigh?

315 is a LOT of tire and lighter cars will not get it up to effective temps. I admire your thinking as many people miss this fact. The only way to be sure is to take hot temps once you upgrade.

Can you explain that a bit. It seems like (to me) if you can drive fast enough no matter the weight of the car you could get them up to temp. If they are below optimum temp you will start to slide which will increase the tire temp. You would keep sliding the car until you are in the sticky range and not sliding as much. Or even with all the extra sliding they still won't heat up enough?

[winders]

Originally Posted by RichardNew

Now for the interesting thing. The size of the tire doesn't mean you have more rubber on the ground. The size of the footprint is determined by the weight of the car and the tire pressure.

Changing the size of the tire will alter the shape of the footprint but not it's size.

Richard Newton
Newton's Laws

Please finish off the explanation!

The reason wider tires are used is that you can use softer compounds and carcass designs which allow the tire to grip better.

Scott

[Alan L]

Originally Posted by Pete R

Can you explain that a bit. It seems like (to me) if you can drive fast enough no matter the weight of the car you could get them up to temp. If they are below optimum temp you will start to slide which will increase the tire temp. You would keep sliding the car until you are in the sticky range and not sliding as much. Or even with all the extra sliding they still won't heat up enough?

I'm sure Chris will chime in with the right reason, but I see it like this;
How much sliding can you tolerate before you get to temp - you are loosing track time all the while this goes on,
and, once you stop sliding, can you keep enough heat in the tyres to not start sliding again? This all assumes you are somewhere close to where you need to be tyre-wise. If you imagine an extreme with more tyre than you could possibly use, you may just keep sliding because the heat loss down the straights etc may be too much from the surface area of the large(er) tyre, that even the sliding can't get enough heat in. Like everything else, I see it as a balance/compromise between the sort of driving you do and the type of car and set-up. Which is where my original Q came from. Once you get beyond that balance point, my guess is you end up over- driving the car just trying to work the tyres.
Alan

[Cajundaddy]

Originally Posted by RichardNew

Now for the interesting thing. The size of the tire doesn't mean you have more rubber on the ground. The size of the footprint is determined by the weight of the car and the tire pressure.

Changing the size of the tire will alter the shape of the footprint but not it's size.

I gotta call BS on this one. Looking forward to the physics explaining this. It is well known that adding tire width adds mechanical grip all other variables remaining the same (tire compound, pressure, car weight etc.) There are practical limits though. Not sure how you get more mechanical grip without more contact patch.

[RaceProEngineer]

Originally Posted by Alan L

OK, I'm all ears. But why do people upsize tyres for more grip? On this basis, are you saying I could run less rubber and have same footprint.
Alan

Originally Posted by Cajundaddy

I gotta call BS on this one. Looking forward to the physics explaining this.

While Richard is off doing something else, I have not contributed for awhile . . .

Before calling BS too loudly, Richard is having a little fun with you gentlemen. There are two hidden and critical pieces behind the physics.

Alan, you can achieve the same TOTAL AREA footprint with narrower tires AT THE SAME PRESSURES. The problem will be the SHAPE of that footprint (contact patch). The wider tire offers more lateral grip, where the contact patch is wider from side-to-side -- better for cornering, not as good for drag racing!

The other element is this: with a wider tire, one can generally run LOWER pressures, further adding to the area of the contact patch. Physics: The PSI can be slightly lower because the VOLUME of the air inside the tire is greater.
I know I should have let you gentlemen figure this out on your own, but I couldn't resist.

Ed

[Flieger]

It would be nice if it was only the Ff=mu*Fn principle from basic engineering classes. Unfortunately mu is not constant. Temperature is one variable affecting it and load is another. The coefficient of friction drops off with increased pressure on the rubber, and the dropoff is all dependent on the tire compound and construction. Furthermore, the tire construction has inherent stiffness that means air pressure is not the only thing affecting the size of the contact patch. Then you get into things affecting the shape of the contact patch...

Its not so simple, which is why the best engineers get paid the big bucks.

[Alan L]

The other element is this: with a wider tire, one can generally run LOWER pressures, further adding to the area of the contact patch. I know I should have let you gentlemen fogure this out on your own, but I couldn't resist.

More to come . . .
Ed
OK, I'll bite. How come the lower pressures with wider tyres? And how do you get same contact patch with narrower tyres at same pressures? - same weight on less rubber = more squat?
Sometimes it seems easier just to turn the key and mash the pedal. But getting a primo setup has to be the goal.
Alan

[Cajundaddy]

Originally Posted by RaceProEngineer

While Richard is off doing something else, I have not contributed for awhile . . .

Before calling BS too loudly, Richard is having a little fun with you gentlemen. There are two hidden and critical pieces behind the physics.

Alan, you can achieve the same TOTAL AREA footprint with narrower tires AT THE SAME PRESSURES. The problem will be the SHAPE of that footprint (contact patch). The wider tire offers more lateral grip, where the contact patch is wider from side-to-side -- better for cornering, not as good for drag racing!

The other element is this: with a wider tire, one can generally run LOWER pressures, further adding to the area of the contact patch. Physics: The PSI can be slightly lower because the VOLUME of the air inside the tire is greater.
I know I should have let you gentlemen figure this out on your own, but I couldn't resist.

Ed

Still not with you Ed. Willing to listen to your reasoning though. When I run wider tires on the front of my car (same compound and hot pressures) I get noticeably more front grip laterally AND longitudinally (under braking). If a narrower tire/contact patch was better for drag racing, maybe someone should tell this guy:
1996 Dragster Rear Engine Top Fuel Dragster Pictures, Mods, Upgrades, Wallpaper - DragTimes.com
He clearly doesn't get it.

[Flieger]

Originally Posted by RaceProEngineer

While Richard is off doing something else, I have not contributed for awhile . . .

Before calling BS too loudly, Richard is having a little fun with you gentlemen. There are two hidden and critical pieces behind the physics.

Alan, you can achieve the same TOTAL AREA footprint with narrower tires AT THE SAME PRESSURES. The problem will be the SHAPE of that footprint (contact patch). The wider tire offers more lateral grip, where the contact patch is wider from side-to-side -- better for cornering, not as good for drag racing!

The other element is this: with a wider tire, one can generally run LOWER pressures, further adding to the area of the contact patch. Physics: The PSI can be slightly lower because the VOLUME of the air inside the tire is greater.
I know I should have let you gentlemen figure this out on your own, but I couldn't resist.

Ed

I don't see how volume of air in the tire matters one bit unless you are looking at temperature.

[Flieger]

The other thing to consider is the mechanism by which the tires generate grip. It is a combination of mechanical interlocking with the road surface and chemical/van der Waals attraction. Higher pressure on the rubber can increase the mechanical interlocking if the tire is not as soft. The dispersion forces I am not sure of but I think those are maximized with a greater area in close contact with the road, irrespective of load. But I am not sure which mechanism contributes more to load sensitivty.

Then this all changes in the wet, snow, mud, etc.

[RaceProEngineer]

Originally Posted by Alan L

OK, I'll bite. How come the lower pressures with wider tyres? And how do you get same contact patch with narrower tyres at same pressures? - same weight on less rubber = more squat?
Sometimes it seems easier just to turn the key and mash the pedal.

Well, Alan, we are all going to "mash the pedal" regardless, so that part is a given.

First of all, to state the obvious, the weight on a particular corner is the weight on that corner - let's say 800 pounds. So let's think of the tire problem in terms of 2 extremes: a 205mm wide street tire, and a 315mm wide track tire. Inflation pressure is, say, 25 psi.
[1] The 205mm tire will spread its contact patch as wide as possible, limited by it's tread width and sidewall, which will bulge too (to which I shall return shortly). That 800 pounds has to be supported, so once the maximum width of the narrower tire is reached, the contact patch starts to e-l-o-n-g-a-t-e front-to-rear, and eventually enough rubber is on the ground to support the weight. Not having to do with the AREA of the contact patch, per se, but that bulging sidewall now plays havoc with keeping that contact patch in a stable position.

[2] By contrast, the 315mm wide tire will likewise spread its contact patch as wide as possible, limited by it's tread width and sidewall, which now offer approximately 50% more "spread". The sidewall is much happier, being better able to remain more "upright" between the wheel bead and contact patch. Will the contact patch elongate front-to-rear also? Of course - SOME.

In short, the 800 pounds squashes the wider tire down on the pavement in a different way. And, yes, we have yet to talk about HEAT in the two extreme sizes of tires.

Ed

[Alan L]

Now you got us where you want. I'm not doing any work now, waiting for the next episode of the serial.... :-)
BTW, I just ordered more rubber - 255/275.
I hope this serial turns out OK.