Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) Tyre pressures

A question which I am sure those with knowledge can answer.
If I car is involved in a serious accident tyre condition and pressure are taken.
Is it an offence to run a tyre under or over inflated as recommended by the car manufacturer ? I know if you are carrying different loads it is ok to increase pressure.?

If you have access to a flir/infrared camera, after a run look at the tyres to see how the heat is distributed across the tyre it should be even, over or under inflation will show up as hot spots and indicate where the tyre will prematurely wear.

There are 2 issues with running tyres at low pressures:

The first concerns tyre distortion at higher cornering speeds, the tread will move (according to grip available) more than usual leading to a lot of sidewall flexing, (and the ultimate is rolling the tyre off the rim , but I have never seen this happen even at racing speeds).
At "normal" road speeds and enthusiastic corning you might suffer from increased tyre shoulder wear.

The second is resistance to "compression type" punctures. If you drive into lots of deep potholes at speed the ability of the tyre to absorb the impact is reduced and it is possible (although very unlikely) that a particularly deep and sharp hole might just lead to the "Snake Bite" puncture that cyclists often report (if running tubes) where there are 2 almost identical punctures where the tube has been trapped between the rim and the road.

Personally my +8 runs at 24psi on Avon ZV5 205/60 x 15 all round, and I am happy with that. If you are happy at 18 psi then use that , but do be aware that the greater tyre distortion can lead to minor air escapes with tubeless tyres , so check the pressures MORE regularly.

Using 18 at the rear will make it more tail happy, especially in low grip situations, but as long as you are confident of catching it (and enjoying that) I wouldn't be overly concerned.

That's an easier one to answer.

Did the pressure drop during impact?
Can the post impact pressure accurately represent the pre-impact?
What do the manufacturer state as being the pressure?
What do the tyre manufacturers state as being the pressure?
Did the difference in tyre pressure from the 'norm' pressure affect the behaviour of the car pre-impact?

It's the last question that is most important. If there is a debate about the causation, a reconstruction would take place to replicate as best as possible the circumstances, usually at gradually incremental speeds with the results taken.

It's accepted that manufacturers test their vehicles in various states and so the manufacturers recommendations are usually taken as being the best. Morgan of course, spend many thousands of hours testing their products..ahem..

My daily BMW has a pressure difference of 0.3bar between the handbook and the tyre manufacturers recommendation figures. I checked the other day and the service dept follow the tyre manufacturers pressures.

I'm also in the 24psi zone.

The other thing to throw into this is, how accurate is your tyre gauge?
When I had a day job I had the means to check pressures up to 20 psi very accurately at home. One day I checked a brand new tyre gauge and it was over 3 psi out at 20 psi. Most of the few I checked were at least 1.5 psi out. Therefore I would suggest that if you set them to 20 psi on your gauge they cold be anywhere from 18.5 to 21.5 psi anyway and could be even further out than that.
I only tested gauge or plunger type gauges but digital gauges are only as good as the sensor and on a £5 gauge that is not going to be very good.
So I would say set them at what you are happy with, look out for uneven wear across the tread and adjust as required.
YMMV.
Cheers
John

Good point, John..

I used to religiously keep my pressure to the 22psi stated in the handbook. But after being advised by a number of people involved in racing morgans that the handling is better with slightly higher pressures suggesting 24psi I have compromised and am trying 23psi all-round.

I think this one of mine might need a little more air.

Just needs a squirt of this: