Wednesday, December 24, 2014

Lets call this the Christmas special

A recent question from Bryan caught my attention: 

"Amazing the variety in subtle spec changes even between teams. In regards to gravel, what tread width and block cuts were more efficient for the different gravel surfaces?"

In 2005 on gravel we always used the same tread pattern called "Z" but we sometimes cut it to half "ZA" or full "ZA". Except in Cyprus where I used something called a "GW" pattern which was designed for rough events. I am not sure about the tread width anymore...Anyway it was the standard width for the 15 x 7 inch gravel wheels, perhaps 185mm tread width. 

A full "ZA" cut tire looked like this:

Full ZA cut (left tire)

Just for reference, the triangular shaped blocks sit on the outside edge, once mounted on the car. In this particular photo the tire on the left is a full cut but not the right one! Compare the second row of blocks from the outside. 

As you can see the transverse grooves that run across the tire have been opened up and the inside edge blocks have been cut in halves. This sort of cut is helpful for muddy conditions; sandy roads where the hard surface is broken up and loose as in for example in the Fafe area in Portugal; you could perhaps use this also on New Zealand stages on the first pass if you are among the first 3 cars on the road and its damp; or also on Finnish style roads if it's very wet. In Wales you should use a "Z" if its dry whether it's loose or not because from my experience a cut tire will overheat quickly due to the hard surfaces and the fast nature of the stages. If it's damp, do a half cut and only a full cut if it's pouring down. A "half" cut would be: to open up the transverse grooves from the outside, going until the fourth row of blocks. Nothing else is touched

Generally speaking, you need to be aware that fast stages generate lots of tire heat. A cut tire will tend to create more unwanted movement on hard-ish surfaces for a given compound. Mainly because 1. there is less rubber in contact with the ground and 2. the blocks move due to overheating. It will cause loss of general performance, precision and confidence. Back in 2005 I drove the "Halfway" stage (18,85 km) in Wales with an 8 compound full cut "ZA", thinking it was going to be wet. It was dry and the result was a 10 second loss. 

 For the second pass the tire choice was correct.

At the end of the day it's all a matter of compromise between performance, endurance and confidence. 

Having said all this it's important to remember that, just like on tarmac, the most important factors are your tire constructions and compounds. Back in Mexico 2005 I was running shakedown and it was our first gravel rally of the season. Michelin had just come out with a new 9 compound development tire called the "L". Toni, Roman and Dani had them in their tire pack but not me. I ran the older "M" type. 

The shakedown stage was rather long, a nice uphill climb followed by a downhill climb. All that on a hard surface covered in sand and lasting some 8 kilometers plus. We did our runs and stabilized the clock around the same stage time each run, literally within tenths of a second. No way to go faster. Toni was within an arm's reach but no way to match him. Then came the moment back in service where the tire guy did me a favor and let me have Dani's set of used "L" for a go. Off I went for one final round. I had a clean run. The time was 8 seconds faster, just like that, easy. Suddenly this put me in front of Toni and even got me a thumbs up from Loeb. What an amazing tire. I was all smiles. Needless to say I was begging for more of these tires! 

Happy Holidays!