> OK. let's imagine for a moment that we have a car with a 3.7:1 differential
> and it is sitting with one wheel on dry pavement, and the other wheel is on
> ice. You Northern guys ought to get this!
Minnesota, born and bred. Been there, done that, known people who wrecked
> You give it some gas in gear and
> go nowhere. Are we saying that the ratio to the spinning wheel is not
Yes, we are saying that the ratio to the spinning wheel is not 3.7/1. If the
left wheel is on the dry asphalt and not moving, and the right wheel is on the
ice by the curb and is spinning, the ratio between the propeller shaft and the
spinning wheel is 7.4/1
Let's further stipulate that the speedometer is attached to the transmission
and is reading an accurate 20 MPH. The spinning wheel is turning at the same
speed that it would turn if the car was going 40 MPH on drive pavement.
(Let's not do this for too long. Our hypothetical spider gears are going
crazy. and we are eventually going to toast the differential.)
On your hypothetical car, whether on the straight, going round a curve, or with
one wheel spinning; the AVERAGE of the ratios of each wheel to the propeller
shaft must always be 3.7/1.
To get back to the original question, how do you count the turns on a car with
an open diff to determine the final drive ratio? Jack up only the right wheel
so the left wheel does not turn. Mark a spot on the tire. Turn the tire a
hundred times, while your buddy counts the number of times one of the four
prop-shaft bolts goes by. He counts 54.
54 bolts divided by 4 bolts-per-revolution is 13.5 revolutions of the prop
100 right-wheel revolutions divided by 13.5 prop-shaft revolutions is
The diff ratio must always be the average of the two wheels. Left wheel is 0.
Right wheel is 7.407407407. Average of these two is 3.703703704.
(Looking at another way, you could use 50 instead of 100 to calculate the
ratio, since when you turned the right wheel 100 times, the average of the two
wheels would be 50. 50 / 13.5 = 3.703703704.)
You look in the factory data and see that there is a factory ratio of 3.7/1.
This is most likely the diff gears you have in this car.
If the same car had a welded diff, (or a good stiff clutch-pack limited-slip)
you would need to get both wheels off the ground, or you could not turn the
right wheel. If you turned the wheels 100 times, your buddy would count 108
bolts, which would be 27 prop-shaft revolutions. The average of both wheels
would clearly be 100, since the wheels are turning together.
108 bolts / 4 = 27 prop-shaft revolutions. 100 wheel revolutions / 27
prop-shaft revolutions = 3.703703704.
I hope I have not made any typos!
Phil Ethier West Side Saint Paul Minnesota USA
1962 Triumph TR4 CT2846L, 1992 Saturn SL2, 1993 Suburban,
1994 Miata C package
pethier [at] comcast [dot] net http://forum.mnautox.com/forums/
I decry the textmessagization of the American-English language.
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