Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Correct Tyre Pressure

I was at MWS today to collect my re-sealed and repaired tyre. The technician said he’d inflated the tyre to 30psi. I told him Morgan say 22psi all round and his comment was that he wouldn’t trust their calculations: to him that is too low, especially if the car is loaded.

So I did some web research and from the Caravan Club of all places got the information that resulted in these calculation

First you need you tyre. Note the data on the side: maximum load and maximum pressure.

For the 205 x 55 R16 on the roadster this is 615kg and 51psi.

Now calculate the typical loaded weight of your Mog.

For my Roadster solo I estimate this at 1100kg, based on Morgan’s stated figure of 1000kg with tools and fuel and 100kg for me and my junk.

Now divide the loaded weight by 4 to get the load per tyre. Morgan give, for a max permissible weight, a front/rear bias of 40%/60%.
Given all additional load added is mostly over the rear I’m estimating the Front/rear of a car with 1 person and no proper luggage as 45%/55% so this means:

Normal Load: (45% / 55%)
Front axle this is 495kg or 247.5kg per wheel
Rear axle this is 605kg or 302.5kg per wheel
Actually if you want to be really pedantic for solo use you can bias the weight distribution left/right as well as front/back but do you want to have 4 pressures to remember?

Maximum Load: (40% / 60%)
Front axle this is 560kg or 280kg per wheel
Rear axle this is 840kg or 420kg per wheel

Notice that the maximum permitted load is well below the 615Kg the tyre allows. Actually this means a lower load capacity tyre could be used, normally it is set at 110% of the maximum permitted so this is 462kg. This is an “81” code tyre, but no one makes such a tyre in the size needed.

Finally work out the ideal tyre pressure take the figure for the maximum pressure for the tyre and divide it by the maximum weight for the tyre.
Multiply this figure by the actual load you are going to put on the tyre and you have the correct pressure.

So:

Normal Load: (45% / 55%)
51 psi / 615kg = 0.082927
Front axle is 247.5kg per wheel so 20.5psi
Rear axle 302.5kg per wheel so 25 psi

Maximum Load: (40% / 60%)
Front axle is 280kg per wheel so 23.2 psi
Rear axle is 420kg per wheel so 34.8 psi.

Morgan state 22psi all round. For a normal load it is probably OK, but for a heavily loaded car it is worryingly low at the back.

I’ve set Aldermog at 22 front and 27.5 rear as 2 up with stuff we run at about 1250kg. It wil be interesting to see if there is any difference…..!!

Wow - good research!! I must say I thought 22 at the rear a bit low - but then the 4/4 seems a v well balanced car.

But thanks Peter for that authoritative info!

Interesting Peter. Looking forward to your empirical observations
Nick

Numbers for the 4/4 will be different as it is lighter and has different tyres.

Someone with a love of excel spreadsheets could produce a table for all the Mog models and tyre sizes, covering solo, light load and holiday touring!!

Peter.
Very interesting.
After trial and error over nearly 10 years of ownership of my 4/4 I run at 20 front and rear, with 22 at the back when loaded. The tyre size is 165 . 80 . 15. My handbook recommends (18 psi front and rear).

Peter, nice bit of research.
I also measure tyre wear across the tread and alter psi accordingly.

Interesting stuff, Peter. Since fitting the spare wheel to the rack on my Roadster Sport I have run the rear tyres at +2psi over the fronts. Tyre wear patterns will also indicate whether the pressures are correct for the load. Too soft and both shoulders will wear more than the centre, too hard and the centre of the tread will wear first. I have run all of my previous Morgans at the spec. pressure (22 psi) for road use without any problems or abnormal tyre wear patterns developing, but I increase them for dry competition.
Don.

I think you might find 25psi could be a little bouncy, it maybe ok if you have 4 leaf springs, be aware the break away on the rear will change, especially in the wet. The rears will warm up at a different weight to the fronts, the front getting hot quicker. If you drive at a normal pace you will probably not notice much difference, but spirited driving and you will feel it.

I don't know where Morgan get their tire pressures from. The recommended pressures for both my 4/4s not only resulted in awful tyre squeal but you could feel the tyres squirming as they tried to roll off the rims. (Apologies for the less than technical description) I run mine at 26 psi front and rear and whilst this is definitely firm I at least feel in control. As ever it depends how you drive but for me a Morgan is a sports car not something to amble along and worry about how to make it more comfortable. Not that there is anything wrong if that's what you want from your car but that's not for me.

If you wish to go uber techy apply thermal imaging to your tyres and adjust accordingly until you have even heat pattern for each wheel under each circumstance.

Probably a bit much on a Mog?