tyre pressure question

[yellowelan]

Can someone explain the logic behind running 24psi at the front and 28psi in the back? Wouldn't you want higher pressures in the front with the engine and slightly heavy driver carrying a bit too much weight up front?

[ftsoft]

I'm not sure which car you are referring to, but the S2 has a slight rear weight bias. In any case I have run equal tire pressures for the last 45 years, although slightly higher than 24.

Frank

[msd1107]

The Elan has a slight rearward weight balance as standard, accentuated by a full gas tank, two passengers, and a full boot. Thus the recommendation for 18/23 or 23/28. After experimentation, I settled on 28/33.

The lower front pressure also contributes to understeer, a trait Chapman considered desirable, as well as being considered safer for the general motoring public. Look at the history and anticdotal experience of 356 and early 911 Porsches for examples.

David
1968 36/7988

[Quart Meg Miles]

With rear wheel drive the Elan's tyre wear is already twice as high at the rear compared to the front even at 18/23 psi and the car slides nicely when provoked. With equal pressures the wear difference would be worse and the car less stable though some prefer it.

The legality of running non-standard pressures could be a problem and it possibly invalidates the insurance too. Any experts or precedents out there?

[nomad]

The Elan has a slight rearward weight balance as standard, accentuated by a full gas tank, two passengers, and a full boot. Thus the recommendation for 18/23 or 23/28. After experimentation, I settled on 28/33.

The lower front pressure also contributes to understeer, a trait Chapman considered desirable, as well as being considered safer for the general motoring public. Look at the history and anticdotal experience of 356 and early 911 Porsches for examples.

David
1968 36/7988

Well I admit to being completely ignorant on the subject but I would expect lower front tire pressure to contribute to oversteer. Personally I always have preferred a car that tended to be neutral or oversteer slightly.
Old flat tracker here!

Kurt
26/3754

[Quart Meg Miles]

Well I admit to being completely ignorant on the subject but I would expect lower front tire pressure to contribute to oversteer. Personally I always have preferred a car that tended to be neutral or oversteer slightly.
Old flat tracker here!

Kurt

Consider almost flat front tyres, the car wont steer anywhere as the slip angles are so great, i.e. terminal understeer, but if the rear tyres are flat you won't be able to keep the car straight. I've often noted cars steering jerkily and nearly always they have a soft rear tyre causing the driver to keep correcting the car's wandering direction.

[nomad]

Agree Meg but I'm not talking that extreme. Perhaps I'm thinking of something else. If you move the steering wheel with lower front tires the instant that you move the wheel the car would feel as if you had understeer as the tires flexed but if you take it to the point of break-away I would think that the tire with the least pressure would have a larger patch on the ground and thus more traction and less likely to break away.

Just my amateur view on the subject. If I had ever done comparison's on a skid pad or track work I would have more insight. I'm not talking drastic changes in tire pressure and personal experience only amounts to motorcycles on dirt. Too hard a tire and it would tend to wash out.

Kurt
26/3754

[quaybook]

I'm with Kurt. As long as not taken to extremes, lower tyre pressure means more grip, so to increase understeer you would increase front tyre pressure.

Vernon

[msd1107]

Vernon,

Tests show maximum cornering speed increases with higher tyre pressure up to some indeterminate level somewhat dependent on tyre construction and wheel width. The narrower 4.5" wheel width of the Elan will use somewhat lower maximum tyre pressures than the 5.5" wheel or a 6" after market wheel. However, comfort concerns generally result in using a lower pressure than what produces maximum cornering speed.

Often, rainy or snowy conditions dictate reduced pressures.

David
1968 36/7988 using 28/33 with 5.5" +2 wheels.

[Quart Meg Miles]

The Sports Car - Its Design and Performance by Colin Campbell 2nd edition 1959:

Page 104 Table 6.1
Cornering power (lb per degree slip angle) @ Percentage Under/Over-inflation:

56 lb/? @ -20%
63 lb/? @ -10%
70 lb/? @ normal pressure
76 lb/? @ +10%
82 lb/? @ +20%

The increase in grip is almost exactly proportional to pressure, that is: the slip angle required for the same load reduces proportional to tyre pressure increase, supporting the following assertion:

Page 116: Factors Leading to Understeer....

2. Increasing the rear tyre pressure.

[nomad]

Well, thanks Meg, apparently my logic isn't logical!


Kurt
26/3754

[quaybook]

David and Meg

Sorry but I still find it difficult to accept that higher tyre pressures tend to increase grip. Logic tells me that increasing tyre pressure reduces contact patch and lowers tyre temperature, both factors that, within the proper working range of the tyre, will reduce grip. When I've messed about with tyre pressures on my Seven at trackdays I've always been advised to set tyre pressures at the lowest level that will avoid either excessive carcase distortion or tread overheating and that advice has certainly worked for me (running 70% aspect ratio tyres). To quote from Paul van Valkenburgh's 'Race Car Engineering and Mechanics (1976 edition) -
"the proper pressure is the one at which there is an even rubber pressure in contact with the road, all across the tyre patch. Pressure must also be at a minimum, to get the maximum rubber area on the ground and provide some bump absorption, and yet it must be high enough to minimize heat-generating tire deformations."
This seems very logical to me.

Meg. I have the 1978 edition of Colin Campbell's 'The Sports Car: Its design and performance' and both the passages you quote are absent from the later edition. Instead he says-
"When tyre profiles were all at 100% the contact patch was very narrow and more heavily loaded. The actual tread profile presented to the road was very rounded and an increase in cornering power could usually be obtained by running at 10 to 15% above maker's recommended pressure. All this has changed with 70 or 60 ...series tyres...."

The standard road tyre for an Elan is around 80% aspect ratio and, these days, inevitably a radial with more advanced construction than when the car was designed.

Going back to the original question, my guess is that the 5lb difference in recommended front to back pressure on the Elan is primarily about loading, the car fuelled and with two occupants has a significant rearward weight bias. Again, my own experience is that, if one up with an empty boot, my Elan oversteers less with a smaller difference between front and rear pressures than recommended.

Vernon

[Quart Meg Miles]

Vernon,

I'll buy that and thanks for the update! I had already been thinking about the bulging of a modern tyre and change of contact patch but thought it might have been minimal from designed pressure upwards and compensated by better retention of carcase shape. And running at 145 mm and 80 section I thought I might have been below that (modern) threshold, especially with the stiff tread radial. Time to change a few pressures and find out.

Racing presents the different problem of terminal grip which is not the same as handling balance. Ignoring that, what is David and others, running at 28/33 psi, actually achieving apart from central-tread wear and lighter steering; if they are on fatter rims and/or tyres then the pressures should be lower shouldn't they?

[richardcox_lotus]

As I'm not an expert, I assume that Lotus knew what they were doing with their specifications. One key factor that needs to be considered is the ride comfort. Going for maximum grip without considering the ride quality is a step too far for a road vehicle, as opposed to a race set-up

However, I find my ?lan just a bit more precise with slightly higher values front & back - I think I'm running 25/21 instead of 23/18.

Regards
Richard

[msd1107]

Everybody is correct in some way.

A road tyre has a tread pattern and profile that distorts under cornering forces. Higher pressures stiffen the tread, increasing cornering forces. The higher pressures also round the tread profile, decreasing tread area on the road. There is a trade-off point that produces maximum cornering force. A wider wheel flattens the tread, allowing higher pressures to be used before the tread rounds too much. Thus the maximum usable pressure for the standard 4.5" wheels is probably somewhat close to Lotus's high speed recommendation. Using the +2 5.5" wheel would allow for higher pressure before reaching the optimum cornering force, like I found.

Racing tyres are a different matter. They have a flat tread and no pattern, so do not squirm as much under cornering loads. In addition, the Elan is lighter than a normal vehicle, which would normally call for less pressure. The upshot is that racing tyres may need less pressure for optimum cornering force

So, it is a complex question, and your optimum pressure depends on many factors. Test your setup, and the pressure that produces maximum cornering force is the best pressure for you. Someone else with a different setup probably will come up with a different result, but they are both correct.

David
1968 36/7988