Fat Bert said:
Dropping the tyre pressures by 25%
and you will overheat the tyre carcass and run the serious risk of the entire boot delaminating
I've seem what happens then cos it happened to Cyclops in Holland and it aint funny!!
Lets ask the Tyre Guru 1200 Pete to add his bit but IMHO 30/31 is too dangerous
Tyre pressures are dropped which in turn makes the carcass flex and bend/twist in greater ammounts. This is where heat comes from in tyres, from the molecules in the rubber moving about in sort of a kinetic energy way (thermokinetic i think) not from the abrasion in contact with the track.
Fitting the higher pressures means the tyre does not flex as much, therefore decreasing the ammount of temp.
There is also the argument of the increase in surface connection patch with lower temperatures, as the tyre is more 'squashed' to the track, therefore the actual contact patch on the track is higher...
Both are the reasons for running lower pressures.
The lower the tyre pressure the more the tyre deforms. The more the tyre deforms, the more friction there is between the tyre and the road surface. The more friction, the more heat. The more heat, the greater the opportunity the tyre has to regenerate itself by shedding the 'used' layers of rubber (to a point). This deformation of the tyre also creates a bigger contact patch at the cost of a little stability.
This is desirable at a racetrack. You want grip. Tyres that are used purely on the track wear the edges first because on a racetrack you generate much more force on the edge of the tyre. You want the tyre to deform, heat and literally 'shed' the used layers of rubber. On the road you have other considerations. The stability or handling of the tyre is extremely important, as you don't have the controlled conditions of a racetrack. There are other road users and emergency situations that you just don't have on the track. You spend more time with the bike upright on the road, no matter how good a rider you are! This means that tyre deformation actually works against you. Now it's deforming when the bike is upright. This means it's wearing quicker in the centre and will 'square off'. This changes the profile and the handling and grip of the tyre. The higher the pressure, the less it will deform giving you the best possible tyre life and handling.
Tyres of 20 years ago aren't a patch on today. A tyre that is designed to be run in a pressure range has a compound that will grip if set in that range.
This means that not only would you run a higher pressure on the road, if you are carrying bigger loads than normal, you would probably increase the pressure.
Now - If your clued up on a track day, what you would do is run a Blackbird at 31 & 30, ride a session and as soon as you get into the pit garage, take a pressure reading. You want to see what your tyres are running when they're hot. If they are still below the optimum pressure, then you top up with a tad of air or let some out if your optimum pressure is exeeded.
Hot air expands, cold air is denser. Tyres get put through the stresses harder on a track than they do on the road simply because you are accelerating harder, braking harder, carrying more lean etc. As explained above the flex in the tyre generates heat and therefore the air inside the tyre expands creating too much pressure. At road pressures the extra heat generated on the track will cause the tyre to go too far over the optimum pressyre and you're handling will suffer.
Some symptoms of running your tyre too hard - reduced stability under braking (tyre no longer 'squashes' into the tarmac widening your contact patch). Less grip, as you are reducing your tyres ability to flex on the tarmac providing the mechanical grip. You will lose feel from the front and, depending on your enthusiasm with the twist grip, you'll spin the tyre on acceleration etc. All ingredients which could end up chucking you and your machine down the road.
Remember bert - if you start with 31 & 30 when they're cold- they'll have a lot more pressure once you're half way into a good session.
Now Pete tests tyres for us and there's nowt wrong with that but I happen to have talked to Sarah J about tis and she races in 'Bemsee Minitwins'
Also have spoken to Steve Weeks who races in the 'Derby Phoenix 750/1000 Twins series' and they both will spend ages going on about why you drop your pressures on the track
