Change in Tyre diameter after prolonged running.

[callmekart]

Hi,

I was just wondering that as a lot of rubber gets laid on the track by the F1 cars, it means that the effective radius of the car tyres will get reduced.

So as the radius of the tyre decreases, the circumference also decreases. This will lead to the decrease in the top speed of the car but will increase the acceleration.

If my thesis is correct, then will there be a visible decrease in the car lap times after about 20 laps of running in a low downforce circuit?

[Jersey Tom]

Wouldn't be a big enough change to notice.

[RH1300S]

I'm sure you could work out the differences if you knew how deep the grooves are (it will be in the rules). I suppose you would also need to know exactly how the cars are geared in every gear and which track you were calculating for, also the exact power/torque curve of the engine and grip of the tyres, mass of the car against changing fuel load etc.

But you would be assuming that the tyre performance remains constant.

I expect the change in tyre performance during a stint will have a greater effect than the effect of the slight gearing change on lap-time.

If I were a race engineer (maybe this is why I ain't ) - I would only be interested in what I know (from testing) to be the performance change in a particular tyre over a stint - i.e. that knowledge would include gearing changes, grip levels without worrying about calculating it all. Just give me that hard numbers. Keep it simple.

[Carlos]

Welcome to the forum callmekart. A lot of F1 drivers have raced karts and we have several kart pilots are members. Welcome

[Ciro Pabón]

I've never thought about it.

Quick back of the envelope calculation (actually, I'm not even using an envelope... ): rear F1 tyres have a 67 cm diameter. So, for each cm of wear you will get a delta V of 100*(67/66 - 1) percent, or 1.5 percent decrease in speed. If top speed is, let's say 320 kph, that is 5 kph. That would be true no matter the gears, the mass or whatever, so I disagree (slightly) with RH and Jersey Tom.

Certainly it would be noticeable, because that's comparable with the difference in top speeds among teams (perhaps half the difference).

Maybe, perhaps, it's possible, you could theorize, there is the chance that the thinner tyre would stretch a little more (I imagine it would be very little, the walls wouldn't change much, unless really battered on kerbs), compensating something of the estimation I made, but I don't think THAT would be noticeable.

Am I wrong? I was too lazy to use a calculator or "draw the problem", as I usually do, so I could have made a mistake. Thanks if someone have the time to check it.

Warning, crazy Ciro idea: if the difference is noticeable (and that's a big if, even if the quick estimation is true, I don't know if the tyres wear 1 full cm), the regulations allow you to have a bead with a 4 mm tolerance ("between 328 and 332 mm"). So,

Can any of you imagine a design for a wheel whose bead diameter change in that amount to compensate for the wear while racing, thus keeping some of the top speed? Yes, yes, I know it sounds like Rube Goldberg, the "magical expanding wheel" and I don't even know if regulations allow for it... besides I guess it would be forbidden in ten minutes. There is no practical application for that and if any team "develops" that kind of wheel the others would follow suit in a minute, if it's not forbidden immediately. Anyway, teams invest a lot more in aero to get smaller differences in top speed, so...

[RH1300S]

That would be true no matter the gears, the mass or whatever, so I disagree (slightly) with RH and Jersey Tom.

Ah ha!! [-X

Yes, top speed changes - but the smaller diameter would also slightly change the gearing therefore thrust available for acceleration. So trade off less top speed with getting there quicker - hence my reference to mass

[donskar]

While we're doing a little "blue-skying" . . . Rear tires used by dragsters are designed to grow in diameter, effectively giving them a high speed gear as they approach the end of the quarter mile. This effect is clearly seen any time the wheels sre spinning at high speed, as when they warm up the tires before the start. (The tires are literally screwed to the rims.)

But if bridgestone and Ferrari weren't able to work this out, the same effect might not be feasible in F1/roadracing.

[Jersey Tom]

For every 1cm of tire wear?? I guarantee you the total tread gauge of those tires will not be more than 0.35cm. Even a street tire shaved for racing is on the order of 0.45cm.

Beyond that...

If you really could drop that effective radius.. for the same torque applied you'd get more thrust in theory.. if you're power limited. Contact patch can only generate so much force. If you're already traction limited like hell (as you are in F1 in the bottom gears) you gain nothing.

And most importantly the compound is going to give up way way way more grip than you would ever gain by shortening the tire radius by a tenth of a cm.

Also.. you wouldn't want a wheel that changes diameter. Or.. well it wouldn't make a difference. The bead is in the tire, not the wheel. You'd just increase the amount of squeeze on that area under the bead.

[Ciro Pabón]

Yes, top speed changes - but the smaller diameter would also slightly change the gearing therefore thrust available for acceleration.

That's totally true, but notice I did not mention lap times... that's beyond the space I have in my envelope.

You would have a better overtaking opportunity, as the cars keep the top speed for a while, and frankly, the only place to overtake is at the end of the straight. Of course, you're right, if you calculate lap times, then the thing gets heavier to estimate and there is a compensation, as you mentioned betwee gear ratio and top speed.

For every 1cm of tire wear?? I guarantee you the total tread gauge of those tires will not be more than 0.35cm. Even a street tire shaved for racing is on the order of 0.45cm.

...

Also.. you wouldn't want a wheel that changes diameter. Or.. well it wouldn't make a difference. The bead is in the tire, not the wheel. You'd just increase the amount of squeeze on that area under the bead.

Thanks for the figures, but even that is too much: the grooves are only 0.25 cm deep (I just checked). Anyway, the change in the diameter is twice the thread wear, so it would be around 0.5 cm (twice 0.25): you can change the estimate in top speed to be around 3 kph, which continues to be significative: it's half the difference between a McLaren and a Red Bull.

Sure the bead is in the tire, but as the diameter of the wheel increases, the entire tire will be taller. That, or I did not understood you (which has a large chance).

To incorporate Jersey Tom and RH1300 apportations to my post, IF you could have a "magical expandable wheel" you must design it to have a larger diameter at the end of the straights and decrease it after the curves.

It's attractive to think that the gadget would give you a change in lap times comparable to the one you get from the lack of traction through wear, but I'm unable to estimate the value.

[Jersey Tom]

Expanding the wheel would not make the tire taller. The plies and all in that tire are all tied down by the bead. Bead is a pretty damn stiff steel cable ring. Expanding the wheel would just compress the rubber between the bead and the wheel, it wouldnt force the whole tire to a different diameter.

Plus, another oversight here is you're assuming that top speed is limited by tire size. It isn't. It is drag limited. If you made the tire a little taller down the straight it wouldn't make you any faster.

[DaveKillens]

Welcome to the forum callmekart. I'm glad to meet you. I believe your original theory is correct, but it's almost impossible to determine because of so many factors. The diameter may decrease, and the gearing is altered. But as that happens, the tire degrades, and the car is doing each lap lower and lower on fuel.
I doubt that in F1 the Bridgestone engineers would allow the tire to "grow" under higher speeds (aka drag racing), because it would lead to decreased contact patch. Not a good thing on those fast sweeping turns. But most cars run camber, which determines contact patch area under a straight line. But when under lateral load, the suspension geometry rolls the outside tire to give maximum contact patch. Hmm, now if you made the very inner part of the tire of a harder material, hmm, the rolling diameter would suffer little change.

[Ciro Pabón]

Thanks, Tom, you're right. Now I can "see" your point about the bead. The only feasible way would be to "pull" the steel wire, but, hey... to pull 0.5% a steel wire you need like 50.000 to 100.000 pounds per square inch (almost the yielding point of steel, actually in my steel structures book the yield point is defined exactly by a 0.7% elongation).

Another question: do the cars reach the top RPM at the end of the straight? If I understand Jersey Tom correctly, they don't, so they aren't slower at all.

If the tire is slightly less taller, anyway the car will push some RPM more and finally, balance the drag against the power, as with a larger tire, with less wear, so in the end the top speed is the same.

So, the final answer to callmekart question is: no.

Good point, Dave. Second "another" question: do drag tyres use nylon cords? I mean, do they elongate or do they just deform?

Thank you, guys, interesting.

[DaveKillens]

I don't know of the internal construction of drag tires. Insanely, it's almost as if the tread was fabricated of a stretchable nylon or something. At maximum wheel RPM, the inner part of the tire definitely deforms, and adds at least six inches to the rolling diameter.
http://www.youtube.com/watch?v=gZY7UGMngvI

[Jersey Tom]

The chassis engineer will set the gearing such that you hit top RPM at the end of the straights (or longest straight). It should just be topping out before the driver gets on the brakes.

The whole argument here has basically been to use the wearing tire to make the car more geared down as you go. But the chassis engineer is already going to be doing that.

My point though is, on any given track, with the speeds and amount of DF (and drag), the top speed is going to really limited by the amount of drag the car is seeing. Pretty serious amount of power required to overcome that. While aero drag is a function of velocity squared, the power required to overcome that force at that speed adds another velocity term is, so power is a function of velocity cubed. Engine can only put down so much power. Don't let anyone fool you, power and torque are both equally important in racing. As is gearing.

And the biggest point is the amount of wear is going to be so small it will be insignificant to really notice given that other things are having a much bigger effect (fuel burn and compound giveup). Hell, if you look at the tire changes, usually you can still see some of those grooves left (though also.. one tire will usually be fairly worn down or grained). You could technically drive on those tires a good bit more, down to the cords. Wear isn't much. The compound giveup is huge.

[PNSD]

http://www.youtube.com/watch?v=wL_6dDCCHlg