I have been getting requests to discuss tires. I can understand the curiosity as I was indeed also very interested in this particular subject when I drove competitively. These rubbery things are, after all, the link between your car and the road.
This read is going to be a bit heavy for I have included lots of details and anecdotes and I might even get carried away... I apologize in advance.
For the sake of this post we’ll concentrate on Michelins which I have experience with. When I was active in the WRC, there were many types of Michelin tires rolling around the service place. First of all, one has to be aware that in those days there were two main categories of tires:
1. Customer tires.
These were tires available for sale to privateers. A limited variation of different compounds and constructions were offered and they were products of the past years of development. For example, a dry weather tarmac tire would be marked with a big “N22” on it. This was typically what heavy 4 wheel drive cars used. It was a “number 2” compound which was rather “hard” and resistant to heat and abrasive surfaces. "N" meant it was a “slick” tire. Another example was the "T04" which was a “0” (zero) compound intermediate for wet tarmac. The "B00" was a "zero" compound full rain tire for tarmac which was absolutely amazing. Stephane Sarrazin used those to set some awesome times on the wet with his private Subaru.
Ok let’s go back a bit and answer some of your thoughts. The Michelin compound scale for tarmac went from 0- (zero minus) to 2+ (two plus); 0- being the softest and 2+ being the hardest. The gravel scale went from 7 (softest) to 9+ (hardest). As I remember, a typical gravel tire was marked, for example, “ZR8”. This was an asymmetrical design tire “Z” and R meant it was for the right side wheels; 8 was the compound category. I mentioned tire construction, which is extremely important for performance and confidence. The construction of the sidewall of the tire is flexible to allow distortion. Basically more flexibility will help in slippery conditions but will move a lot when lots of grip. I’ll get back to this later…
2. Development tires.
These were reserved for factory teams, who tested them and selected the ones they wanted for competitions. Michelin continuously came out with newly developed or improved compounds, there were many tires to choose from and they ranged from greatness to, well, I’ll let you guess… Once or twice, I heard some privateers saying they were happy to have received some development tires from Michelin… I thought to myself: "well I hope you know what you are getting because if you don’t it’s Russian roulette." Except in this roulette there’s one empty chamber out of 6! That’s because the good (and fresh out of mold) development tires were reserved for factory teams and for some privileged privateers who had the right connections. These tires had no particular markings on them but if you’d look carefully you’d find a small engraving in the rubber (on the sidewall). This was a long code made of various digits and, as far as I remember, one letter. In any case, the interesting part was the letter and the two or three digits before it. After every tire fitting, all the factory team’s tire guys would use a grinder to, you’ve guessed it again, grind the code off the rubber. Secrecy was imperative and nobody except you and your tire guy was aware of what you took. Asking another driver what he had and getting an answer like “n°2 compound” meant nothing to me because as far as I was concerned there could be at least seven different “n°2 compounds” which had varying constructions and compounds. And these seven were just the ones I knew of; I had no certainty that another competitor, for example Citroën, had chosen exactly the same tires as Ford had.
Here’s an anecdote to illustrate my point:
Back in Catalunya 2005 we had just taken 20 seconds off Alexandre Bengue in a stage. The stage was bone dry and the next was full wet. I was running on what I thought was the best solution in my list of choices: “183D”. This was an old type of “0+” compound slick usable on slightly wet roads or very cold dry tarmac. I had run the stage smoothly, caring for my tires, avoiding overheating. They had worked well. Bengue was surprised at the time and came up to me. He asked what I had. I said “D…and you?” He replied “J”. I replied “ok” but thought to myself “wtf”. I thought “is he sh*ting me?” The only “J” tire I knew was the “380J” which is a hard slick. We ran the next stage, in full wet conditions, and we lost 20 seconds to him. Later I understood that Skoda was running with something called the “183J”. This “J” had a “softer” construction which made a big difference on cornering.
Accelerating and braking on wet roads with the “183D” was ok. You could feel the grip well and the threshold of grip-loss was progressive. Confidence was there. When cornering, however, it was different. The soft compound alone wasn’t enough and the rather rigid structure made the loss of lateral grip rather sudden. Apart from the lower overall cornering performance it was difficult to be confident about the grip level and therefore the times suffered.
A similar story happened to Loeb in Catalunya 2003 where Markko Märtin used an intermediate called “74F” and set amazing times in the wet. Loeb was on an intermediate tire called “74V” which had a more rigid structure and he said afterwards that he wasn’t aware of the "74F"! Needless to say they had it for the next event.
In 2001, before starting the Rallye du Var in France, I visited François Delecour and asked him about tires; we had two types of slick tires: “N22” and “N04”. He said “do you have some “248D”?” I replied “huh?” He had left Peugeot a little while ago and back then “248D” was the thing to have on dry tarmac. By the time 2005 rolled along, all the factory drivers were running on something called “380J” for dry tarmac. So what does all this mean in practice?
|Delecour, San Remo 2001|
In a comparison test between a “N22” and a “380J”, you’d find on the first corner that the latter rendered your steering extremely precise. The front wheel’s reaction to steering input was twitchy and immediate. Why? Because of a more rigid structure. You would have also noticed a slower rise in temperature than the “N22”; meaning it took more work to get it ready for max attack. The big advantage was its longevity. Once it started working, you could drive and drive endlessly without performance loss, so to speak. This tire had been developed for the evolving rally format of having mostly longer stages. In 2005 it had become rare when a tarmac stage was less than 20km. Needless to say that this tire was useless and dangerous on wet roads. Nowadays with the new “all purpose” tarmac tires, Michelin has had to compromise heavily therefore it’s normal to hear drivers say things like “...yeah my tires are moving a lot” on dry tarmac.
In some cases it was difficult to choose the right tire. We had occasions when we knew we’d have dampness in the stage but not enough to justify going softer by one whole step. In this particular case the solution was to cut up your tires and make thin grooves with a knife. A 3 cm cut every 5 cm, perpendicular to the tire, on the outside edge. These thin cuts helped the rubber move and it kicked the rubber’s temperature up a bit faster than normal.
By 2005 Michelin had developed many different tires. For up and around 70% of dry in a stage I’d use a “050C”; this was a "2-" compound which worked well in mixed conditions. It was also a good choice on a fully dry stage made of very smooth tarmac but the “050E” was better for that. If the next stages were very long, dry and twisty with abrasive tarmac, I could use a "2+" compound called “050B”. If you really needed a very soft "2-" compound the “050L” was a choice. We had a bunch of cold weather dry and wet condition slicks which we could use. The softest were the “248F” and the “465T” (0-) of which the latter worked well on low grip types of tarmac. The “130K” was a “0” compound, versatile. The "0+" compounds were “183D” which was useful in dry and the “132C” which was useful for wet conditions. Bear in mind these were just the slicks I knew about!
You have probably noticed that I am constantly speaking of using slicks on wet roads and you may find this awkward. I did as well when I began driving. I quickly noticed that all top drivers always pushed for using slicks on wet roads simply because they performed better. In fact as long as there is no standing water on the road, no need to break out the intermediates or full wets.
Finally I’d like to quickly discuss tire wear. In 2003 and 2004 when we had to make do with our own diff maps, I had big differences of tire wear on gravel between front and rear wheels. Also, in general, my tires seemed to wear very fast. The problem laid in our diff mappings. It turns out our diffs were loose and this meant lots of uncontrolled wheel spin. The result was excessive tire wear, especially on the front. For more on diffs see Differential extravaganza
Apart from that and generally speaking, the geometry was always spot on and the wear was even across the tire. My 2004 Focus ran -2.0° of camber on the front with -1.2° on rear and the tracking was 0 on front with 2mm in on rear (all this for gravel). The car was evenly balanced as far as weight went and this meant the front was wearing only slightly more than the rear.
You may ask why Ford ran with a 2mm in track on the rear. From my experience this helped keep the back steady on straights under high speed and perhaps also helped counteract over-steer in corners. Alternatively if your car suffers a bit from understeer you may opt to change the front track and open it up a bit to 1 or 2mm. this will help with turn in, but be careful because it will also cause your car to “snake drive” a bit on straights. Another way to help with turning in is to do a “Solberg Subaru”. Remember Peter’s Subaru, with the low nose and high arse? Lowering the nose is in fact something you can try to fix inherent under-steer.
This post was about tires and here I am getting carried away about geometry… Let’s continue this subject on another day!