Tyre Pressures

PostPost by: hatman » Wed Oct 13, 2004 6:32 pm

Hi all. Recently discovered this site, having owned my S4/Sprint convert for c4 years. I'm intrigued why the advice nowadays should be to run the tyres at higher pressures than the sainted Colin Chapman advised back when the cars were new. Why?
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PostPost by: steveww » Thu Oct 14, 2004 1:33 pm

Modern tyres even in old sizes are a lot different to the tyres that were around in Colin's day.
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PostPost by: hatman » Thu Oct 14, 2004 2:04 pm

I accept that tyre technology has moved on over the years but, where we're still using the same brand/model of tyre (Dunlop SP Sports) that was fitted when new, how can it be that it's better nowadays to run them 5lb harder (23/28 instead of 18/23) than back in 1970(ish)?

The caveats on tyre-pressure maintenance still remain the same: if you pump 'em up too hard you sacrifice grip/ride quality and create premature wear in the middle of the tread; if you run 'em too soft you risk overheating through excessive wall-flex and create excessive wear on the edges of the treads. I'm puzzled.
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PostPost by: type26owner » Thu Oct 14, 2004 3:04 pm

I've run the SP40 on my S2 and at around 20 psi I'd say they had the handling close to that of a flat tire at around 100 mph. Ran them with at least with 40 psi so they would not terrify me while cornering at speed. Wasn't going that fast either because the grip level was very low. Second worst tire I've tested so far. The worst tire by far was the BT70s. Took the BT70s off after two weeks because they made the car unsafe while threshhold braking from freeway speed and finally gave them away. The CHEAP performance tire to use is the RE92. Trust me!

Paraphrasing what Carroll Smith says about tires is that 80% of the envelope is solely from the tire choice and only the remaining 20% is what you've got to tweak on to make it handle any better. Picking the right tire makes the difference between making the Elan perform like a slug or a hare. :D
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PostPost by: rgh0 » Thu Oct 14, 2004 11:39 pm

Personally I believe the original pressures recommended by Lotus were to low and even in the early 70's I ran my tyre pressures for normal road use around 26 front and 28 psi rear. Lotus like most manufacturers tend to pick the low end of the acceptable pressure range to maximise comfort and minimise noise , vibration harshness (NVH). Also the lower pressures selected tend to give a more predictable understeer characteristic that most manufacturers consider safer.

Even if the pressures were right when new in the 60's and early 70's tyres have changed a lot in the last 30 years, even the ones with the same nominal brand name. The construction details and rubber compounds are significantly different and as a result you would expect differences in optimum pressures. The range of optimum pressures between different barands of modern tyres is huge and a similar difference would exist between current and the original tyres.

If ACBC was designing the Elan today he would use a substantially different suspension geometry and stiffer spring rates to work well with modern tyres that would run at different pressures. If he could not change the geometry and springs I am sure he would be playing with the pressures.

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PostPost by: axis77a » Tue Oct 19, 2004 1:47 pm

I run around 26psi front and 32 rear, the car seems more balanced when there is a 5 to 6 psi difference front to back. This still works afor me at lower pressures eg 22F 27R, but I prefer the higher pressures as they keep squeal and breakaway at bay longer. I sometimes go up to 28F 34R for track days, but 26F 32R seems to be best road & track compromise. Agreed about tyre brands - for ultimate grip/laptimes but for me the fun of enjoying the balance and controlled slides is more important..... my current tyres are Dunlop sp4 165/70's R13 ...very average.. but slightly more grippy at the back. And unbranded (Pnuemat) 155R13 (155/80) .... Although non -maqtched front to rear I prefer this balance to having same tyres all round. I'm always interested to hear other's recommendations on tyres/pressures (especially softer UK brands).
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PostPost by: steveww » Wed Oct 20, 2004 8:18 am

I have been told good things about the Uniroyal Rallye 680. Have not tried them myself yet but will give them a go this spring when the car is back on the road. It is in lots of bits on the garage floor at the moment :o
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