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Esoteric Tire Question

To: mgs@Autox.Team.Net
Subject: Esoteric Tire Question
From: dmeadow@juno.com
Date: Tue, 25 Mar 1997 18:31:03 EST
Most of you are probably aware of the T-Series (especially TD) "rear
differential problem".  The problem being that the stock TD came with a
5.125 ratio rear end.  This high ratio means that the motor will thrash
itself to death at highway speeds.  Many people have put MGA rear
differentials (4.55, among other options) into their TD's to make them
more drivable at highway speeds.

My question is this:  Does placing radial tires on a TD exacerbate this
problem?  Some time ago a friend of mine measured a 5.60/15 bias ply
(original TD size) and found that the circumference of the tire was a
good inch more than the circumference of the equivalent (155/15, I think)
radial tire.  This makes sense given that radials are supposed to be
"lower profile".  It seems that if you put a radial tire on the TD, it
will make the engine RPM even higher for the same speed than if you use
bias plies.  I looked at the Coker tire web page that someone mentioned
to compare statistics on radials vs. bias ply.  A certain 155/15 radial
had about a 9/10ths of an inch smaller *diameter* than a 5.60/15 bias
ply, which would translate into a 1.41" difference in circumference
(thank Pythagoras for good old pi), which seems to more than confirm my
friend's observation.  What would be the math to figure out the
difference in RPM?  If I use the increase in circumference as a
percentage of total circumference, I get an increase of about 3.5%, or
175 RPM at 5,000 RPM.  Is that a valid calculation?

Now this may be real off the wall, but would this problem be further
exacerbated by the soft sidewalls of a radial?  As you may know, it is
the soft sidewalls that give the radial its cornering capability and
large footprint for roadholding.  But does this reduce the effective
diameter of the tire even further since the rim is closer to the ground
than it would be with a bias ply with its higher, stiffer sides?  My
first thought was that the circumference wasn't changing, but in the
process you are distorting the shape of the tire and you no longer have a
"perfect" circle, so pi may not be relevant.

I'm trying to decide which tire to eventually put on the TD, and, if
possible, avoid the considerable expense of putting in an MGA diff.

Thanks in advance for your help.

David Littlefield
Houston, TX
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