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Ratio mix up


j hancock
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I was talking to a fellow the other day that has owned a R tandem for a number of years. It is used once in a while but can get loaded heavy and is going highway speed for a couple of hours. After power washing he found the front drive is stamped 4.17 but the rear drive is stamped 4.64. It has an automatic power divider.

Not sure when he is going to open it up but I wonder what he will find with bearings, gears and condition of the power divider? Is the power divider trying to engage all the time or does it need to see more slip between the front and back?

He bought it off a used lot and just never thought to look to make sure both rears were the same ratio.

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Jim

It doesn't cost anything to pay attention.

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Quite funny...

It just might be the housing stamped with some figures has different content in it.

If not I think the divider has to warm up pretty hot and is worn off to the moment.

Would be tooooo interesting to know what he will find out observing the gears.

Никогда не бывает слишком много грузовиков! leversole 11.2012

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It could be someone rebuilt either the front or rear and never bothered to restamp the case. In a similar situation,the local trash hauler has a '98 CH613 which they bought used. The truck was a twin to one my Dad bought new in '98,he wasn't happy with the fuel mileage he was getting and got the dealer to swap the rears from a 4:17 to a 3:86. This helped,so the guy that bought the twin heard about this and had the same done to his. Fast forward to last year,the trash hauler,now the truck's 2nd owner had a rear end go out. Dealer replaces the rear with a rebuilt 4:17 because that's what is suppose to be in the truck according to the VIN,first time he kicks in the power divider,guess what happened. Nobody bothered to check,just assumed because the computer told them the truck had 4:17's,that was gospel.

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When rebuilding or replacing whos responsible for this screw up. The customer usually doesnt know what ratio it is. The shop needs to check the gearing. Here is a formula to make sure. Count the teeth on the ring gear and pinnion. Also count the teeth on the bull gear and heli-cross shaft gear. E.G. ring gear has 17 teeth, pinnion has 16 teeth. The bull gear has 55 teeth and heli-cross shaft has 16 teeth. Divide the ring gear into the pinnion. 17 divided by 16=1.0625. Next divide bullgear into the heli-cross. 55 divided by 16=3.4375. The last step is to mulitply the two numbers. 1.0625 x 3.4375=3.65. Thats the final drive ratio of the carrier.

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Now im not as technical as the rest of you but I do believe theres is a much easier way to check what ratio you have

step 1 park the truck some were level and chock the front wheels leave maxi brakes off ie as no braking applied

step 2 tape a piece of string to the front drive shaft and jack up only one front wheel

step 3 rotate that wheel one complete turn and count how many turns of string are on the drive shaft and divide by two

step 4 return wheel to ground and remove string from front drive shaft and tape to rear drive shaft

step 5 jack a rear wheel and rotate and divide by two

what this will all show is what ratio's are in the front and back diff's with out pulling anything apart and it is easy to do on your own so no need to wait for a mate to show up :twothumbsup:

hope this is of some help

Paul

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If he has had the truck for years and drives it for several miles loaded heavy and does not hear any loud popping then I wouldn't worry about it. If the ratios were different the power divider would have already wore out. We bought a used truck 25 years ago and the power divider started slipping, thinking it was just worn from missed match tires we just replaced it. It ran about 1 month and it wore out again. We checked the ratio stamped and they both said 4.17. We pulled the big side plate and counted ring & pinion teeth and one was a 4.17 & the other was a 4.50 ratio. So if I wasn't hearing a loud pop while loaded heavy the ratio should be the same reguardless of what it is stamped.

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Use your tach, speedometer and GPS to determine which axle appears to have the correct ratio. Them look a the other axle to see if there is any evidence that it has been apart or open it up and check.

Probably one was replaced at some point in its life and the case is not stamped correctly.

We always match parts and do not assume that the VIN or tag is correct.

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