[TR] high engine rpm

[pethier at comcast.net]

From: DLylis at aol.com
> 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 differentials.

> You give it some gas in  gear and 
> go nowhere.  Are we saying that the ratio to the spinning wheel is  not 3.7:1?  

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 shaft.

100 right-wheel revolutions divided by 13.5 prop-shaft revolutions is 7.407407407.

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/
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I decry the textmessagization of the American-English language.