Ron Olds wrote:
> Would somebody who was Dunlop SP4 165x15 tires on a TD of TF please
measure the distance from the ground up to the center of the rear axle.
.... The car needs to be complete so the correct weight is on the tires.
I'm getting my speedometer rebuilt and calibrated by Mo-Ma ....
What counts for speedometer and odometer accuracy is the effective rolling
radius of the tire which doesn't change much with the load on the tire or
with the compressed ride height of the sidewall, but does change some with
tire pressure variation. So measuring the height of the centerline of the
axle is not as accurate or effective as you would like. You really want to
know turns per mile, and I can help here. I run Dunlop SP20 165x15 tires
on my MGA. Used to have SP4 before they went out of production, but
according to my rally computer they have the same rolling radius as the SP4.
Bottom line is that the nominally correct overall rolling diameter for a
tire for my MGA is 24.92" to make the odometer read correctly. With the
proper inflation pressure these tires can be made to roll at exactly the
correct diameter to make the odometer accurate (at least when the tires are
new). I use a rally computer which is accurate to +/- 1 foot per mile to
measure rolling distance. I find that at 25psi these tires (when new) will
register 100.4 miles on the MGA odometer in 100 miles of travel. Increasing
the tire pressure by 3 or 4 PSI can get the MGA odometer to register 100.0
miles in 100 miles of travel. After I wear 1/8" of rubber from the radius
of the tire tread, the odometer will register about 1% greater distance,
and the speedometer reading will also increase by 1% (along with engine
speed).
The 24.92" rolling diameter of the tire translates into 809.30 turns per
mile of travel. Multiplying by the final drive ratio of 43/10 (4.300:1)
gives the drive shaft 3480.0 turns per mile. Further multiplying by the
5/12 ratio of the speedometer take off gear in the tail of the transmission
gives the speedometer cable 1450.0 turns per mile. I have the number 1450
on the face of my speedometer, indicating the number of turns required to
make the odometer register exactly one mile.
When my odometer is registering 100.4 miles in 100 miles of travel it
calculates to an actual 812.5 turns per mile for the tires. As the tires
wear down this will increase by about one percent, making it close to 821
turns per mile. Starting with these numbers your instrument guru meeds to
know the reduction ration of the final drive gears in your car, and the
reduction ratio of the speedometer drive gears in the tail end of the
transmission in order to get the required turns per mile for the odometer.
Barney Gaylord
1958 MGA with anattitude
http://www.ntsource.com/~barneymg
|