Motorkhana yesterday

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I attended the latest motorkhana yesterday with mixed results. I had a ball as always but the car's performance seemed to take a step backwards. Having now had time to think about it, I think I have the answer.

 

The previous time I used the car was the MG Iron Man and the temperature was mild. The car seemed well balanced.

 

This weekend the car seemed to understeer.

 

I checked the tyre pressures in the early morning before leaving for the track. At that time the tyres were not on the car. The temperature was quite hot yesterday. So I think by the time I was in mid event they may have been well overinflated.

 

Has anyone ever measured the pressure before and again after putting a tyre on a car? If so what pressure difference does the weight of the car make? 

 

Needless to say I will pay more attention to the tyres next time.

 

 

[PZG302]

Don't know about what effect a motorkhana will have on tyre pressures but I do know that in a 5 - 10 lap race the slicks will grow from 22 psi to about 26 psi and if the wheel alignment is good the temperature across the tread of the tyre should be resonably even. Previously I have gon eon the tread wear and where the rubber balls up on the tyre, though now I have access to an infra red thermometer I can start to be a bit more precise.

 

Also I always over inflate the tyres before the day and then at the last moment, or within 10 minutes of heading out drop the pressures to what I need for cold pressures and check the hot pressure as soon as possible when I get off the track.

 

The tyres you are using will also influence what pressures to run and you will need to adjust the pressures slightly depending on the age of the tyre and what tread there is left and also the surface you are running on will have an effect as well.

 

About the most important tool you can take to a race meeting is a good tyre pressure gauge, for example I would hate to have to replace mine as its a Longacre guage, that when I got it 10 years ago was over $100, and is very accurate as well as easy to read and only goes up to 50psi with the ability to accurately adjust to 1/4 psi increments due to the size of the scale on the dial.

 

The weight of a car will make a big difference.

 

For example my cars and the pressures run for different tyres are:

• XR8 Ute - OEM Dunlop Sport SP 3000's 245/45/18's - 40psi all round, rears to 42psi when towing race car
  
• Mazda MX5 - 15x6 wheels with 205/55/15's - 32 psi for road and 34 psi for track
  
• 240Z race car - 18x8 with 245/35/18 radial slicks - 22 psi all round
  
• 240Z race car - 15x7 with 225/50/15 Toyo Proxes RA1- 26 psi all round
  
• Valiant Charger - 15x7 with 235/60/15 Yokohama 352- 38 psi front 40psi rear

 

The weights of the cars plus tyre types make a big difference to what I run in pressures. The slicks and semi slicks have very stiff sidewalls so I can get away with less pressures, however if I was to run the Toyos on my Charger than the pressures would need to be up around the 36-38psi due to having an extra 500 or so kg to deal with over the Z, as it's at 960kg with 40 litres of fuel on board.

 

The road tyres need the extra pressures to help the side wall not flex, especially when loaded as in towing, this also helps with not overheating the tyres as the extra load will cause more flex and therefore heat which will reduce the life of the tyre dramatically.

 

Depending on what I am doing at the track, and which car, I will sometimes chack the tyre pressures at every opportunity, other times it's set and forget a the start of the day.