.

[zout]

Not mention screwing up your PPM (pulses per mile input) used for speedometer calibration.

 

[Csm Davis]

Okay I hate to eat crow but sometimes we do. The Sprague will let the rear turn a very small amount more than the front but only until the Sprague locks and then it is one to one. This next part for the sprague transfercase is wrong(And the ratio is one to one with it unlocked so the only time the rear is a minute faster is actually a slip not gearing. The rear tires slip and move just a little but more than the front and it locks as a solid straight they shaft. There is no gearing difference.) They built in a small difference in ratio on the sprague to keep the front trying to unlock the sprague as you go down the road. This is not the case on most Transfercases and unless someone can show me that in a tm that is NOT for a sprague transfercase which the OP is talking about a 939 series not a 809 series then I will admit to being wrong about my post to scarecrow which is still correct.

 

[supermechanic]

http://www.steelsoldiers.com/showthread.php?21343-T-138-T-case-gear-ratios

 

[73m819]

The sprag gets the ratio difference NOT by gearing, the front and rear drive shafts turn the same speed going down the road because the F/R have the same gearing (if F/R tires are the same), The overrun (sprage) gets the the F/R speed difference RATIO from the difference in diameter of the INNER COLLAR and OUTER COLLAR, the CIRCUMFERENCE of each collar is different there by TRAVLE at a different speed to cover the same distance. O verses O, I don't know the actual dia. of the collars, but look at it this way. usung CIRCUMFERENCE = PI (3.14 x D (diameter) so a 3" collar would have a C=9.42", a 6" collar would have C=18.84, so 1 revolution of a 3" would cover 9.42", it would take the 6" 1/2 the time to cover the same distance, hence the speed difference, as long as the outer collar is turning faster then the inner collar it cannot catch the outer collar and engage the clutch dogs (outer collar is overrunning the clutch dogs, hence the nane "overrunning clutch"), as soon as the outer collar stops (rears start to spin), the inner collar is no longer overrun and the dogs engage. In this example our outer collar to inner collar speed ratio would be 2 to 1

 

[supermechanic]

THANK YOU, 73m819, finally a reasonable explanation for the inner workings of the T-case clutches.That was so clear, even Helen Keller could see it.

 

[Bolkbich]

At the end of the day you still need to run tires with the same diameter on the front and the rear..especially the 809 trucks.

 

[Csm Davis]

Okay to break it down, because it is clear as mud, on a operator engaged transfercase you can run different sizes in front or rear just don't engage on solid ground, on sprague transfercase you need same sizes front and rear or smaller front tires so that the tc is not engaging going down the road.

 

[quickfarms]

A small difference in gear ratio from the front to rear axle is very common on a lot of trucks. My truck runs 4.88 in the front and 4.90 in the rear.

I would not run different tires due to fact you could not rotate them if the fronts start cupping,

 

[Csm Davis]

Most OTR big trucks run different tires up front anyway not always a different size but a steer tire in front and a traction tire in back. Think I would like a 916 with floats up front and the road supersingles in the rear. Would looks mean and should work well.