dmeadow@juno.com wrote:
>
> 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
> --------- End forwarded message ----------
Dave,
Might suggest that you refer your questions to the guru of
differentials, Carl Cederstrand of Brea, California. He wrote the book
on changing the ratios. Carl is at <cederstrand@earthlink.net>. BTW, I'm
in the act of installing a set of 4.30 gears into my '52TD.
Bud Krueger
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