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MG MGB Technical - Tire Pressure?

I've generally always tried to keep the tire pressure on all my B's at around 28 lbs. front and 30 lbs. rear, and have never had complaints or concerns about the ride or handling. To be honest, I've done this more out of habit than science.

I just found a couple of different references (Haynes and Moss) that tell me I should be using 20 lbs. front and 24 lbs. rear. (I'm running on radials, of course.) This will certainly soften the ride, but what other benefits will I see from lowering the pressures?

Rick
R. Stevens

Rick,
None that I can think of.
Are your tyres wearing evenly across the tread surface?
I would be inclined to go by that.

Lowering the pressures will increase you fuel consumption too as an aside.

Cheers , Pete.
Peter Thomas

I tend to lean towards your line of thought Peter. I run 30/32 with no adverse affects on tire wear or handling. I understand that auto manufacturers tend to give lower pressures so that the ride is a little softer/comfortable.

Just the way I do it.

Mark
M Whitt

Rick,
You may want to contact the tire company to see what they recommend. The information in the Haynes Repair Manual is the same as what is in the MGB Driver's Handbook which is over 40 years old. The information I recieved from Michelin was to run at least 30 lbs of pressure not the lower amount. They said running on lower tire pressure could cause excessive heat build up which could lead to tire failure. Low tire pressures were supposed to be one the causes of Firestone tire failures several years ago. Just a thought.
Tony
Tony Shoviak

Thanks, all. I've generally figured sacrificing a little ride for more safety was the way to go. Tire wear is generally "normal" with the higher pressures I've been using so I'll go with the flow here and stick with what works. Cheers!

R.
R. Stevens

Tire pressures have changed dramatically over the years. In the 60's the goal was a good ride. (low pressure). Later on the goal was reduced rolling resistance for better fuel mileage. (hi pressure).

Now with low profile wide performance tires you can adjust you traction and the handling balance of the car by putting a little more air in at one end of the car or the other.

One pragmatic way to approach it is to keep adding pressure until you notice handling/traction is being adversly affected.

With the MGB being a very light car with obsolete tire sizes a pragmatic / experimental approach is almost mandatory.

Barry
Barry Parkinson

The original pressures for bias 5.60-14 were rdst 18/18, 24/24>90mph; GT20/24, 26/30>90; Radial 155 rdst 21/24, 27/31>90, GT 165 21/24, 28/31>90; all +2 at the rear with "laden boot". The higher pressure for high speed is primarily a precaution against heating of the tire, but who wants to go that fast on Jello anyway? They stayed the same til the end, despite the increasing corpulence of the cars. Wear across the tread is not a good indicator on radial tires; they wear flat until they are severely overinflated, when they wear more on the outside. Radials give a reasonable ride at higher pressures, with advantages in low heat build up and better fuel efficiency, and they are usually designed for higher pressures, as Tony said. Tire manufacturer trumps car maker! The same pressures with bias ply gave really bumpy results, poor tread to road contact, and broken casings on impact with potholes.
You could always "tune" handling with tire pressures. Factories spec higher rear pressures to give fundamental understeer, as it is easier to control, especially for inexperienced drivers. If you are confident, and we hope qualified, they handle better if the rear pressure bias is reduced. If you can't tell the difference, you are not qualified; and in any event, you may prefer the bias. I generally run the B at even pressure, 30/30 or 32/32, +2 in the rear with "laden boot". I tend to always wind up at 75 or ahem, and hate to slow for corners, and I figure 75 in hot weather here is as good as 90 in merry olde. I set customer cars at 30/32, unless I think they are hopeless, then 28/32.
In theory, a larger tire should safely allow lower pressures, but they tend to feel like Jello. I have met a few brands of tire that were really squirrrley with pressures too low, even though they were at or above manufacturer's spec; and some that acted that way if they were too high. Once a customer brought me some magic tires for his newly rebuilt midget; I started at the factory figure, something like 19, and could barely keep it on the road at 25mph, I was truly frightened, and checked the whole car over to see if I had forgotten to bolt the rear axle in or somesuch. All OK, so going up 2psi at a time, it was barely driveable at 26, but at 30 good and 32 really good, 34 and 36 handled the same but rode rough. On another Midget rebuild at the same time, with different tires, 26 or 28 was excellent all around, 30 handled well but rode rough, and 32 was over the top skittish.
FRM
FR Millmore

With Firestone I understand that the pressure spec from Ford was 26 psi. Firestone's argument was that at 26 psi the load capacity of the tyre was exactly equal to the maximum vehicle design load which is poor engneering practice.

The inflation pressure has a direct relationship with load capacity and overloaded tyre tend to fail, however with a B this should not be an issue.

Inflation pressure also has a direct relationship with the speed capability of a tyre and the manuals recommend a 6psi increase to at least 28F30R, and operating temperature of a tyre.

Lower inflation pressure is not good for fuel economy, wet traction, tyre wear, and steering response which may be as critical as tyre failure.

However, lower pressures give a larger contact patch and potential grip so the old manuals with the 6psi increase are still in right ballpark.
Paul

The early cars also weighed a lot less, the manual recomends higher pressures at higher cruising speeds. This is to prevent heat build up due to flexing leading to tyre failure. With modern speeds and distances 28/30 is a good number. I run this normally, or 26/28 for a better ride if it's just local use.
Stan Best

This thread was discussed between 06/08/2006 and 08/08/2006

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