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MG MGB GT V8 Factory Originals Technical - Tyre Pressures

Hello all, I wonder if someone can help me with this one.
My GT V8 is fitted with 15x7 Compomotive ML (miniltes)
On the front I have 205/50 15 tyres and on the back 205/55 15 tyres.
Presumably the Tyre pressures I have seen quoted in the manual are not right for these tyres. How can I find the correct pressures?
At the moment there is 28psi in the front and 30psi at the back.
I will be having a go at a track day next week at Brands Hatch and would like to get them ok by then.

Thanks in advance

Mark
Mark Rawlins

Mark,

Most tyre companies refer to the manual. On the Toyo site they recommend in increase if using lower profile tyres. The RV8 use 22F 24R The MGF 26F28R Morgan V8 22 all round (These cars are about same weight and similar tyres to your own). About 6psi should be added for high speed motoring and for a track perhaps up to 10psi. I use a base of 24F 26R and 28F 30R for Motorway use. Certainly a higher pressure for tyre wall stabilty reducing heat build for track use may be a place to start.

Dunlop do provide advice on track tyres and I'm sure other companies may provide this service. What is clear from most of the sites I've looked at is they do not recommend pressures for any car but only refer back to manufacturer. So seat of the pants.

Paul
Paul

I am not sure about tires in UK, but here in the US all our tires have min/max pressures listed on the sidewalls. I would recommend you follow those pressures. I also agree with Pauls reasoning, with low profile tires you have to increase pressure, other wise you risk pinch flatting the sidewals and damaging your wheels..

As Many here in the US know, the car MFG know nothing about tire pressures as evidenced by the rash of Ford Explorer rollovers and the Firestone recall that resulted.
Larry Embrey

The factory recommendations are ideal for the factory wheels and stock tire size. New tire pressures may be desirable if either of these are changed or if you need to achieve performance different than originally intended.

Proper inflation will exert the same force equally across the width of the tire therefore provide the maximum grip and minimize tread wear. Because an under inflated tire will wear more on the edges and an over inflated tire will wear more in the center, you can monitor tread wear to find the ideal pressure. Because itís not practical to run the tread off tires to find the ideal pressure, professionals use a pyrometer to measure the temperature across the tread. If the center of the tread is hotter that the edges after a run then it is known the pressure is so high that the tire is bulging.

I have no idea where a person could get a pyrometer for measuring tire tread temperature, so your best bet will be to use the factory recommendations and monitor the tire pressures to discover if your tires wear evenly, the proof of proper inflation and maximum grip.
George Champion

Thanks for the help.
The rears are wearing slightly more in the centre so Ill experiment. I think Ill leave them as they are for the track day.

Mark
Mark Rawlins

Mark,

If you are in the money Demon Tweeks sell tyre pyrometers.

Paul
Paul

I always thought pressure to be a personal thing, what you feel happy with as with choice of tyre. Just keep bear in mind the max. pressure stated on the tyre. In the 1980's I ran my B with 30F and 32R and I got at least 40k out of them. They were Pirelli P2000 front and Goodyear GradPrix70 rear, both now obsolete. I still run higher pressures than 'the book' on my current car because I feel it stops shoulder wear and I dont drive like mad most of the time.
I went down this road of thinking after reading an article about the MG C which never had a good record for handling. The article was written by one of the people involved with its design/release and it basically said that when they took it for track testing and press day reports just prior to its market debut, the only way they could get it to handle was to run the tyres at around 36 f and 38 R and it transformed its handling. But come its release the maunfactures hand book gave pressures of around 26 F and 28 Rear. The guy who wrote the article recons it changed its history!
Ian Rutherford

Mark - the foregoing reflects the conventional wisdom of more speed/more pressure - if you compare notes down in the pit, you may hear that pressure should be only as high as needed to keep the tyres on the rims.

You may also hear that the B performs best on narrower tyres (with the negative camber front suspension mod )

Sorry if this confuses
RW
Roger

Ian,

If you have consistant wear The tyre pressures appear suitable.

The interesting point about the C is that if they were track testing I presume they need higher pressures for tyre wall stabilty and stopping the tyres overheat.
Intersting the front rear diff stayed the same yet I gather from C Owners that the first improvement to handling is to increase front pressure (although I would need to check that statement not being a C Owner).

Roger,

Can you be more specific on pressures tyre sizes and camber adjustments?

Paul
Paul

As explained in the book How To Make Your Car Handle, higher tier pressures can improve cornering speed because of less deflection and less body roll. This is not true if the tread is not kept flat. Increasing the wheel width up to an inch wider that the tire tread will require increased pressure to keep the tread flat. Putting wider tires on original rims will cause the center of the tread to bulge and require lower air pressure to maximize contact across the tread width. If you picture uninflated tires on extremely wide or narrow wheels it will make sense.
George Champion

To find the right tire pressure on any given vehicle, start with the minimum tire pressure recommended by the tire manufacturer.
Acquired some white shoe polish, marked the outer sidewall between the tread and the wall of the tire, about 2 inches of polish and four places on the tire. Drive the car on the skid pad (an empty parking lot) at max traction. Maximum traction is determined when the tire begins to squeal. Check the white shoe polish. If polish is marked or gone on the sidewall tire pressure is to low. The ideal is when the white polish stays on the sidewall, even with the tread of the tire. Record this tire pressure (Hot) let the tires cool about three hours, recheck tire pressure and this will be your cold tire pressure.

Some tires require low tire pressure to achieve the correct temp.
Some high performance tires preformed best with low air pressure Cold 20 hot 32 it takes low pressure to get them hot.
If your tires are new and low wear number, then it's a good idea to put them thru a heat cycle. Get them hot, removed and store them in plastic trash bags for a week, re installed them on your car.
This allows the resins to bread or spread on the casing.
Bill
Bill Guzman

This thread was discussed between 17/03/2002 and 22/03/2002

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