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MG MGB Technical - Tyre pressures

With all new suspension; bushes, springs and telescopics I thought I had better have the tyre pressures correct also! I am running on 185 by 60 Yokohamas on 15" minilite style wheels( this is giving me the sam rolling radius.) My father used to finely tune his tyre pressures on his 1953 mg td such that at a slight push he could very easily 'drift' on dirt roads out in Venezuela. He said the engine was not powerful, it was just the difference in front to rear tyres pressures that made this possible. To be honest I am not even sure what pressure I have at present, I let the tyre fitters set it. I could look it up I suppose, but perhaps someone out there has the same set up?
J.M. Doust

I was looking for the same answer. Is there a "corect" tire pressure or do you just fiddle with it to find one that feels the best?
Bruce Cunha

Just had a thought; If one found a nice flat piece of road, and laid a square of 'butchers' paper in front of each tyre, then sprayed the tyre with some water soluble poster paint,( on the front surface.) Then pushed the car over the paper. You would have a 'print of the tyre'. If under-inflated it would be 'light' in the middle of the tread. If over-inflated it would be light at the edges. So we might arrive at 'correct' inflation pressures like this to as it were provide full tyre contact for the given weight of the vehicle. But would this necessarily be the correct pressure for the suspension geometry etc? Mike
J.M. Doust

Factory c1972 for 155SR14 or GT 165SR/14,
was 21/24 "normal", 21/26 "max weight". +6 psi for "sustained high speeds"
Base numbers are hopelessly mushy, but give a "good ride" = soft.
The "high speed" are OK everyday for civilians, and especially insensitive civilians, but give too much understeer for my tastes; I usually set customer's cars at 30/32, and if I'm driving I use 32/32 unless heavily loaded, then 32/34 is OK. Can vary somewhat by brand & style & preference.
In theory, larger tires will permit lower pressures - fine as far as the tire goes, but the response will suck.

Mike's idea was previously used for bias tyres (photos through glass floors), but radials have very little tread deformation over anything close to normal inflation range (that's why they work so well), so not much use. In fact, if radials show wear on both shoulders it generally means they are grossly overinflated. They will never show center wear, because the belt does not allow the center to bulge as bias do.

FRM
FR Millmore

Thanks FRM, I'll check my pressures and play with them. Mike
J.M. Doust

Do what suits you best. I use 21/24 plus 4 for 'normal' loads and speeds. For me this gives noticeably lighter steering at parking speeds, but without the harshness of going as far as 30, which is getting on for 50% over. By mistake one day I said '34' instead of 24 to a tyre fitter and he looked at me in horror.
PaulH Solihull

At the risk of sounding pedantic - the first workshop manual said 18 psi for normal speeds up to 9o mph and 24 for sustained speed above 90. The rear tyres should be increased by 2psi when touring with a laden boot.
Yes - these were crossply tyres and the owners manual was updated as FRM writes but normal driving at 30 gives the MGB a very different feeling to 24 - 28 (normal on our cars).
R Walker

this thread reminds me of a little excursion I had almost exactly 30 years ago... (that makes me feel very old!)
A lovely spring early morning start for a long distance, for me anyway (250mls!), jaunt to work. Just to be on the safe side I pull in to a garage, fill up, check the tyre pressures and set to a nice 24psi. 20 mins later heading down the twisty A7 from Edinburgh to Carlise I decide to have a little fun through the bends. Now't wrong with that, MG's are made for old A and B roads aren't they? Well yes, just not with over inflated tyres... 25 mins in and I'm half way around a bend when the back end lets go. Gulp!! A snake, a wiggle and 180 degrees later and I'm traveling backwards for about 20 yards on the wrong side of the road.
Luckily there was nothing coming the other way! I put it all down the exuberance of youth and calmed down for a while.. a week later before the journey back home I checked the pressures again to find an extra 10psi in there. Faith restored in my youthful driving abilities and a lesson never to trust anyone's tools when it comes to my safety..

Never over inflate tyres more than a few psi from the recommended values. I agree some trial is needed for newer products but check with care and don't push it until you are confident in the level of grip available.

Remember "Safety Fast"

MGmike

M McAndrew

Interesting discussion. I would think 21/24 would be pretty low. I think most of my cars run 28 to 30. I thought that is what most say on the sidewall.
Bruce Cunha

Lol Mike that A7 is one of the best roads to have fun on though!!!! :) not that I was able to drive 30 years ago ...!

As for tyres, people keep changing them on me, but I like 28/30 or 30/32 for most things and an increase to about 40 for autotests - it just helps to remember to put them down again after :)
rachmacb

Sidewall marking is max pressure for that particular tyre, as designed by the mfr. Nothing whatever to do with the car. It has been getting higher over time; many passenger car tyres were marked at 32 or 36, but lots are now marked at 52psi, which must mean something in metricals.

FRM
FR Millmore

Glad it was not the Roman A66 Mike !
R Walker

I run 35 PSI on both front and rear tires (Michelin radials). I find that I have no problems with this and, for me, it makes the car handle better. Jacking up the tire pressures is an old racer's trick when running road tires in the race track.

The recommendation to use a lower pressure in the front tires was to give them slightly less grip than the rear tires, creating an understeer condition. It was felt, by the "safety experts", that the "average driver" was safer in a car that understeered rather than oversteered.

Tire pressures should be taken cold. When the tire warms up, driven more than a mile or so, the air inside the tire is also warmed and the tire pressure is greater than when read cold.

Les
Les Bengtson

The general "rule of thumb" for all passenger car tires, is 10lbs of pressure , for every 1000 pounds of total weight (including the driver/passenger, and fuel).
Edward
E.B. Wesson

30/32 all round
Rears a bit higher if you want to drift it
Front and rear the same seems to give a nice slightly under steer balance.
Nice and safe for my fiancee's GT
Onno K

This thread was discussed between 13/11/2011 and 20/11/2011

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