Tire pressure, my favorite topic. When I had an 84 Cor**tte, I spent a lot
of time learning about tires. This was one of the first mass produced cars
to use really wide tires so there was a lot of discussion about how tire
design and pressure effected handling. The 84's had 50 series tires and
more flex in the chassis than the springs so there was also a lot of
discussion about the compromise between handling and ride comfort.
Back in the good old days, the typical tire was of non-belted bias ply
construction. These tires were quite sensitive to air pressure and behaved
in the classic manner of too much air wears the middle (from ballooning)
and too little air wears the outsides (due to sidewall stiffness/molded
shape of tire). All modern tires incorporate a very stiff belt under the
tire tread that hold the tread pretty much flat to the road regardless of
tire pressure. In fact, many Vette tires actually act opposite of older
biased tires in that too much pressure stretches the belt more at the edges
and causes them to wear first. MG's don't use that wide of tires so,
unless you are using original type rubber, the air pressure will make
virtually no difference in tire wear for all practical purposes.
For proof of this, just look at the tire pressures recommended on today's
cars. The Vette had huge tires, weighed 3000 lbs and ran 35 psi
recommended because handling (and CAFE) was the name of the game. I had
two H*nda Acc**ds, an 87 and a 94. Both had about the same cross section
vs weight ratio yet the 87 recommended 28 PSI because it had a fairly stiff
limited travel suspension and needed softer tires to maintain a ride
acceptable to us Americans. The 94 recommends 34 psi because it can get
away with it on a much stiffer structure and softer/ longer travel
suspension. I also has to deal with CAFE and higher tire pressures reduce
rolling resistance hence better mileage. So now days tire pressure has
more to do with the chosen suspension/ride/handling/efficiency compromise
than with the weight of the car.
In the past, I have advocated running the tire pressure as high as you can
and still tolerate the ride within reasonable limits and recommended
maximums. Lately I have reconsidered this because the MGA, when running 35
psi in the tires, had a funny harmonic cowl shake understeer sideways jump
when it hit certain types of bumps in corners. I have since cut back to 28
psi and that behavior has disappeared completely. So now I recommend
running the pressure that works best with your car and don't worry about
tread wear. It will almost certainly be higher than the pressure in the
owner's manual but as high as possible may not be the answer either.
When inserting studs into an engine block, DO NOT tighten them in. It is
OK to go past finger tight if the threads are gunked up but you risk
splitting the cast iron if you torque them too tightly. The resistance is
caused be the stud screwing past the ind of the cut threads. These act
like little splitting wedges and try to pull the cast iron apart. Cast
iron is not very strong in tension. I have ruined a casting or two-
luckily on 20.00 auction bought farm machinery- learning this lesson. I
believe that someone on this list lost a newly machined block by torquing
in the main bearing studs.
Regards,
Bill Eastman
61 MGA dodging raindrops
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