How much faster are racing tires?

So a few weeks ago, I unknowingly got into a contretemps with Paul Hembery on Twitter.  Someone posted the question “how much slower would road tires be on an F1 car?”; Paul didn’t mention a time difference, so I commented on it and suggested what might happen to the tire from all the downforce and loading.

Sometimes I comment on people’s questions and answers to various people on Twitter.  After all, if they are replying in public, clearly they are leaving it open to comments from the public. I replied a different answer to what he replied, commenting on the propensity of road tires to chunk their shoulders when loaded excessively.  His reply?

@malcolm33 it seems you know nothing about what you are talking about with the last comment. Intelligent opinions valid and welcome.

I was blocked after, so seeking clarification on his opinion was suddenly not possible.

After a minute, I started to think about why he might think have said that.  Perhaps it was because he viewed it as an attack on his beloved P-Zeroes, or maybe it was because we had a different set of assumptions governing our replies.  I believe he mentioned something about never getting heat into them and them lasting an incredibly long distance (he said “forever”, but clearly meant considerably longer than an F1 race distance).  I disagree with that.

I think I do know a bit of what I speak, since I also have a degree in mechanical engineering, have worked as a race engineer more than a few times and have 18 years racing experience as a driver (six years in karts, twelve years in cars – mostly GT, with a tiny bit of sports-racer and open wheel testing thrown in).  None of that has been in F1, or working directly with tire companies engineering their tires, but I definitely know how tires behave on the track, and how they can wear and degrade.

Firstly, how would a road tire react to an F1 car?  Assuming it was a Y-rated tire that could withstand the top-speeds, it shouldn’t explode at the end of a long straight.  I also doubt that tire-failure would occur in fast corners or heavy braking due to extremely high loads, but the tread pattern would definitely suffer from tread squirm.  Because of the tread squirm, the tire would heat up very quickly, likely overheating before the end of a single lap – it’s hard to compare to a road car that, while heavier, does not have near the downforce of an F1 car.  Another assumption of mine was that a brand new, unmodified tire was being compared; a trick often used in racing series that must use road tires is to shave off most of the tread, making the tread blocks considerably more rigid and less prone to squirm – this keeps heat down and improves performance.

If the road tire was shaved, I think it would still begin to overheat by the end of the lap, merely due to the intense energy being put through the tire from the downforce, the high g-forces and even the heat from the carbon brakes.  If it was not shaved, I would bet that by the end of the lap, most of the shoulders of the tires would have been chunked off, as the tire isn’t designed for 3+ lateral g-forces, considerable downforce and incredible braking from 300+ km/h.  The rubber used for a road tire is not designed for the tire temperatures than an F1 car would normally see; it would overheat at a lower temperature than a slick and drastically lose performance.

Secondly, how would the tire perform?  Clearly, a road tire has probably not been fitted to an F1 car for decades, and even then, it was likely only to move the car around.  One of very few possible comparisons would have to be the SCCA World Challenge; they have used street tires, semi-slicks (DOT-R, or “R-compound” tires), and currently full racing slicks.

2002 was the last year that they ran the standard “performance” road tires, Toyo’s T1-S model.  It was not a semi-slick or other racing-intended design; it was just a road tire.  After speaking to the drivers in the paddock, they mentioned that the tire would overheat in every session, causing the oils to come out of the tire, leaving a bluish colour on the surface of the tread and leaving the tire compound rock hard and virtually unusable.  Now I am not sure this would happen 10 years later with a P-Zero on an F1 car, but it might give a few clues.  David Farmer turned a fastest lap of 1:28.375 in the race at Mosport that year.

The next year, 2003, SCCA switched to the T1-R, which was a semi-slick, or “R-compound” tire.  This means it had a softer compound designed for higher operating temperatures, and it also had larger tread blocks for greater stability.  The tires worked much better, and allowed the cars to perform as they were engineered to, without frying the tires.  The fastest time at Mosport was by Bill Auberlen, at a 1:25.319.  That is a full three-second drop over an 88 second lap, which means if you were to fit a shaved street tire to Auberlen’s BMW, it would see a laptime increase of 3.6% (assuming the cars and driving are roughly equivalent from year to year).  David Farmer turned a 1:26.835, but finished much lower in the order, and therefore likely never got a clean lap during the race.

These tires were used from 2003 to 2010, after which Pirelli became the sponsor and wanted to use their slick tires.  From Racer.com, it was noted that there was a 3-4 second improvement from the semi-slick to the full slick by Pirelli: http://www.racer.com/world-challenge-teams-begin-pirelli-tire-tests/article/180898/ …  This test was done at High Plains Raceway, in Colorado, which would provide a roughly 2-minute laptime for GT cars.  At Mosport, this should equate to a 2-3 second laptime difference, or a 3.2% increase if you were to switch from slicks to semi-slicks.

Combining the two jumps yields a difference of roughly 6.8%.  This is comparing spec slicks from 2011 to road tires from 2002, and I assume that tire technology has improved since then.  For argument’s sake, lets assume that a road tire from today would be a little better, so the jump may only be 6%.  Shaving a street tire usually results in a 1-second advantage over a 90 second lap, which would be a 1.1% difference.  If the tire was as-delivered from the factory, that would bring our estimate up to 7.1%

Applying that to an F1 laptime, say around Silverstone (it was a British fan that asked, so I think it’s appropriate), Alonso’s pole time of 1:51.746 would have jumped up to a 1:59.780, a difference of 8.034 seconds.

My off-the-top-of-my-head assumption on Twitter was 10-12 seconds, and this calculation shows 8 seconds, assuming the tire does not degrade heavily or chunk during that single lap.  If the tire did degrade and chunk, the last sector of the lap could definitely see a decrease in performance, where a few tenths per corner could be lost.  A little over three tenths in six corners could definitely bump that up to a 10 second difference.

There are a lot of assumptions flying around here, relating different cars during different years, at a race that was moved from May to August, and then using those differences to apply to an F1 car with probably ten times the downforce, 20% more power, and easily half the weight.

So, what is the difference?  Probably 8-10 seconds at Silverstone, but until someone bolts a set of road tires on an F1 car, we won’t know for sure.

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