PSI
Author
Discussion

kbj149

Original Poster:

131 posts

195 months

Sunday 22nd August 2021
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Currently running 17" aftermarket alloys on my 95 4.0 Chim

Front: 215/40Z/R17/87W/XL
Rear: 225/50Z/R17/98W/XL

Set at 29psi - does this seem a bit high and does it matter that the car has also been lowered?

Apologies if these are daft questions

bobfather

11,194 posts

272 months

Sunday 22nd August 2021
quotequote all
That does sound high, there isn't a definitive answer, from a very basic view it makes sense to load the tyre contact patch as evenly as possible, over-inflation causes excessive load towards the centre of the contact patch, under-inflation loads the outer areas. You can use chalk to draw a wide stripe across the tread, then drive the car. You can examine how the chalk wears off to determine how the load is spread across the contact patch. That only gives you a good (ideal) starting point, cornering and sidewall rigidity often require pressures to deviate from this ideal pressure. If it oversteers easily then reducing rear pressure can help, if it understeers then reduce front pressure.

Classic Chim

12,424 posts

166 months

Sunday 22nd August 2021
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I used 22F 24R on those wheel and tyre sizes. Which is standard pressures on all sizes really.
29 is too high on these cars unless your constantly at high speed over 100 mph.

Light car so as most tyres you buy are designed for heavier vehicles you need to have slightly lower pressures for normal usage to compensate the fact very little vehicle weight will be being transferred onto the tyre on our cars. Lowered and I presume stiffened suspension only makes this worse so reduce them to those pressures and see how it brakes handles bumps etc and grips in corners.

Sir Paolo

244 posts

85 months

Monday 23rd August 2021
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The book upper pressure limits for the standard wheel/tyre combination are 25f and 28r.

This is recommended for fully loaded high speed cruising, but the extra couple of PSI in the front may also benefit drivers of non-PAS cars.

bobfather

11,194 posts

272 months

Monday 23rd August 2021
quotequote all
Sir Paolo said:
The book upper pressure limits for the standard wheel/tyre combination are 25f and 28r.

This is recommended for fully loaded high speed cruising, but the extra couple of PSI in the front may also benefit drivers of non-PAS cars.
The book 'cold' pressures were determined using Bridgestone SO2pp's. These tyres have an abnormally hard sidewall and are not transferable to tyres currently used by owners. It is common practice to add a couple of psi to 'book' pressures to compensate for the softer wall construction of modern tyres

PhilF329

240 posts

255 months

Monday 23rd August 2021
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I generally use 24F 26R as cold pressures on those wheel and tyre sizes, with power steering, 22F 24R in winter.

Classic Chim

12,424 posts

166 months

Monday 23rd August 2021
quotequote all
Modern tyres are still weight rated in the same way as far as I can tell.
22/24 is still near as damn it the most compliant pressure or the correct starting point imho as the car still weighs the same.

kbj149

Original Poster:

131 posts

195 months

Tuesday 24th August 2021
quotequote all
Ok thanks all