TECHNICAL TOPICS
  
     
 

The BLACK Art

  
 
or how to get the best from your race tyres
  
 
Paul Smith (BMTR)
  
 
 
  Apologies first to all of you who have been racing for years and know all about tyres already for maybe wasting your time... then again you never know "you're never too old to learn something new".  
  For a lot of competitors, particularly those new to racing, tyres can be something of a mystery; in some instances other competitors perpetuate this by being less than helpful with information, and sometimes drivers don't seem to want to ask for help. We also seem to be involved in a sport where hearsay quickly becomes fact and no one thinks to ask the specialists whether something is true or not.
 		 
  To get the best from your tyres the following points are the most relevant.  
 
  1. Fitting and balancing
  2. Correct pressures
  3. Bedding in
  4. Tyre condition
  5. Car set-up
  
  1. Fitting...
should always be carried out by someone familiar with racing tyres and wheels. It is also important to make sure the wheel you are using is designed with fitting race tyres to it in mind (there are certain "minilite" replica wheels to which is it is impossible to fit a race tyre without damaging the beads). It never ceases to amaze me the appalling condition some competitors allow their wheels to deteriorate to. We are regularly presented with wheels that have bent and buckled rims making achieving any sort of seal difficult and, in my opinion an even worse crime, wheels that are just plain filthy and obviously treated with no respect at all. How can you be expected to spot cracks on such a safety related item if competitors can't even be bothered to clean them? Have you tried sticking a self-adhesive balance weight to a surface that obviously hasn't been cleaned since the last time you tried? No matter how small your budget there is no excuse for having a dirty car. Excuse me while I step down from my hobbyhorse...		  
  Tyres should always be fitted with the correct direction of rotation in mind. This is important, as when a tyre is manufactured there is always a loin in the tread and by running the tyre in the correct direction you are pushing this join together rather than pulling it apart. This is why fronts and rears rotate in different directions as the maximum torque applied to the tyre on the front is under braking, on the rear, under acceleration. A small amount of lubrication is advisable to help the tyre slide on to the rim and to help the tyre pass over the humps on the wheel when inflating.  
  Cross ply tyres are susceptible to stretching of the carcass, beware of over-inflation. If you have to put 40 to 50psi in a tyre to seat it on a wheel and only 20psi to seat the other tyre on the same axle, you will end up with the one tyre being bigger than the other, this will effect your comer weights. To avoid this it is important to sit the tyre as squarely as possible on the rim before inflation begins so one part of the tyre doesn't seat well before the other, and always inflate pairs of tyres to the same pressure - if one seats at 20psi and one at 30psi, blow them both up to 30psi.  
  Always inflate your tyres with clean, dry air. This is vitally important as pressure maintenance when in use is much more difficult if the air in the tyre is full of moisture. Things to avoid include compressors that are not drained regularly, compressors without water traps and adequate filters, foot pumps etc. Never be tempted to use a source of air you are not familiar with i.e. a garage forecourt airline or you could end up putting significant volumes of water into your tyres. We all know what happens to water when it is heated, yes it turns to steam and increases your tyre pressures! Balancing should be straightforward most race tyre/wheel combinations are fairly light and a static balance should be adequate. We always try and do a dynamic balance - this involves putting weights on the outer edges of both inner and outer rims and explains why we sometimes look quizzically at competitors who do not want weights on the out side of their rims, it surely is better to have the best balance rather than it looking pretty? Adhesive weights need a clean surface to adhere to and it also takes a few hours for the adhesive to fully "go off" and give the best "stick".  
 

2. Correct tyre pressures
are vital to realise any race tyres potential. Assuming you have used clean air referred to above you should not experience any more that 4psi rise in pressure from cold to normal racing temperature. On a K-Sport car we would expect the average hot running tyre pressure to be about 20psi front and rear. A cold starting pressure of 16psi should result in this hot pressure being achieved on a normal UK racing day. If it is abnormally cold you still need to achieve the same hot running pressure that you have found is the best for your car and so you will need to start with a higher cold pressure to get up to say 20psi. It is wrong to think that starting with a lower pressure will generate heat more quickly - all you will end up doing is running your tyres under-inflated, the sidewall stability will be less resulting in more deformation of the contact patch and hence less grip not more. Any race tyre is like having another undamped spring in your suspension, so the stability of the sidewall is important to maximise the mechanical grip of the car.

  
  For the same reasons of stability and also to keep the tread opened up to clear water, wet tyre pressures should normally be set at the same pressure cold that you would run slicks at hot. You should only experience a very small rise in pressure in a wet tyre on a wet track - if your pressures are, rising a lot then maybe you should have changed back to slicks a few laps before?  
  A couple of other points to do with tyre pressures-have you noticed the thread that valve manufacturers very kindly provide on the end of the valve? This is for a valve cap to screw on to. I have seen many tyres destroyed and also cars crashed because competitors have not fitted valve caps. Centrifugal force when a race tyre/wheel combination is rotating at speed is quite high and if your valve is fitted at the wrong angle or you have a weak spring on your valve core, the rotating force will lift the core off its seat and start to bleed air out of your tyres if a cap is not fitted. A good quality metal cap with a rubber seal is the best fitment; we are always delighted to provide these at race I meetings. Should scrutineers not be looking for valve caps to be fitted at all times?
 		 
  How is your poor old tyre pressure gauge? Tyre pressure gauges need to be looked after - most suffer a lot of abuse and are expected to survive dropping, being thrown into toolboxes etc sometimes for year after year. It pays to invest in a decent 0-30 sweep type gauge. Most tyre pressure gauges are not very accurate at low pressures, a 0-60 for example works best in its middle range, and so you could be setting you tyre pressures 1 or 2psi high or low without realising it. When you do buy a decent gauge it is worth while spending £30 or what ever every year having the calibration checked by your local Council weights & measures department. You maybe surprised what you find.  
 

3. Bedding in
is very important if you are to achieve the best performance from any race tyre. In a perfect world you should put the tyre through a full heat cycle before you race on it - this ensures that you will not get any nasty surprises during the race. You wouldn't start a race with new brake pads for example.

  
  Put your normal cold starting pressure in the new tyres, and then go out and do 4-5 laps bringing the speed, and therefore the tyre temperature, up gradually. Be aware that when the pressures are low and the available grip may be high, that tyres could roll off the rim or even turn on the rim knocking the balance out. Pit and set the tyres at your normal hot pressures and go and do another 4-5 laps at racing speed. Be aware that the pressures may still continue to rise at this stage, so try not to get the car out of shape or brake so late that you may risk flat-spotting a tyre. Pit and adjust the pressures again to your normal hot running pressure, and if you are doing this during a timed practice session, this will be the ideal time to go for a qualifying time as the tyre will now be at its best. Allow the tyres to cool down to the ambient temperature of the day and they will now be conditioned for race use and will cycle many times with little loss of performance.  
  Wet tyres do not need to be scrubbed in - the compound is soft enough to bed in quickly even on a streaming wet circuit.  
  4. Tyre condition
a difficult one this as everyone assumes we are trying to sell you tyres you maybe don't need. Lets look a few facts that influence when you should be thinking about changing your tyres.		  
  "My engines' oil pressure is dropping and I know the compressions are down on 2 cylinders but I'm going to try and make it do another couple of races". If you heard a fellow competitor say that you would think he was mad, however how many times do you hear drivers say a similar thing about their tyres? We hear it all the time!  
  Motor racing in any category is expensive and we realise all competitors spend as much as they can afford, however it always seems daft to us to incur all the costs of attending a race knowing that you will not be as competitive as you could be because of something as simple to rectify as poor tyres.  
  In layman's terms, it is the depth of the tread rubber "wobbling" around on the carcass of the tyre that generates the temperature that creates the grip. This is why a new tyre will always run hotter than a worn one - the deeper tread generates more heat, retains that heat, and there is also a longer path for that heat to soak away into the carcass and the air in the tyre. This is more pronounced on a harder compound control type tyre. To run any race tyre much past its half worn stage is really a false economy as the temperature it runs at and therefore the grip available will be lower than that of a newer tyre.  
  At Avon we try to develop compounds that have good durability over many heat cycles and provide good grip even when worn so the clubman gets good value for his money. The compounds used by some other manufactures whilst still appearing to have lots of tread left, loose their performance before being anywhere near worn out. In a professional racing environment this is not a problem as the tyre will probably only be used once or twice before being discarded, but it is of little use to the clubman.  
  The construction of the tyre loses its stiffness after repeated use, you will have noticed how much easier it is to fit a worn tyre compared with a new one. The stiffer, newer tyre, will feel more responsive and will support the tread better, hence loading the compound more and generating more heat, so as you can see it is not just the grip from the tread that deteriorates over time.  
  We sometimes get the comment that a tyre doesn't seem to be working - this is usually because the driver expects to see the surface of the tyre "distressed" by use. It is a fact, however, that if the tyre is working correctly, in its correct temperature range, and that the car is set up neutrally, that little wear or "distress" will be evident. It is wheel spin and sliding about that accelerates wear and causes the "graining" pattern familiar to most competitors, and usually indicates either poor car set-up, or bad driving habits.  
  It's an old cliché but there really is nothing else you can spend £500 on that will find you the lap time improvement a new set of tyres will...  
  5. Car set-up
There are obviously a huge amount of variables involved in this subject and I will restrict my comments to those general areas that affect tyre performance.		  
  Cross-ply tyres as used in K-Sports do not need large amounts of negative camber, half to three quarters of a degree on the front and vertical to half a degree negative on the rear is normally enough (just as well if you have a live axle!). 3-5 degrees negative front castor is normal some drivers prefer heavier steering so you have something to "lean" against in long fast corners like Gerrards.  
  A small amount of toe-in to take up any play in racks/ball joints etc can be useful, but in a low powered car I always think it is best to reduce the rolling resistance as much as possible - introducing geometry changes that cause the tyres to "scrub" will only slow the car down in a straight line.  
  As discussed earlier, cross-ply tyres are not always exactly the same diameters and for this reason it is a good idea to set the comer weights of the car with the tyres you intend to race on fitted and blown up to the hot pressures you normally use.  
  With regard to spring rates, roll bar settings etc, that is up to you, however bear in mind that what you are trying to do is load the tyre as much as possible and a "soft" set up will not work the tyres as hard as a "stiff' one will.