Randy's - Tech Library Myths regarding diffs
by Randy Lyman, © 1998
Randy's Ring and Pinion Service, Everett, Washington
more technical info on Randy's web page
Over the years I have heard a lot of myths regarding the setup and design of differentials.
One of the biggest myths is; Who is really qualified to setup a ring & pinion? I have heard time and time again that the
following situations make a person a qualified differential expert:
· They are an old timer
· They have a race car
· They are in a four wheel drive club
· They are a machinist
· They built a few rearends for their buddies
If people in these situations really know what to do then maybe we should look for an old machinist, with a race car, in
a four wheel drive club, who has built a few differentials for their friends. Would that make them a complete authority? I
dont think so. I believe that there are only a handful of people who really know how to setup a reared correctly and
they have learned from a combination of collecting facts and experience.
After all you wouldn't go to someone who lets a neighbor borrow their tools or mower and call
them an expert on equipment lease would you?
The biggest myth I continually encounter has to do with contact patterns for setting pinion depth. Although many
disagree, I have found that the method used by General Motors, Dana Spicer, Richmond Gear, the Gleason Gear
Company, and many others works best. The method that they use involves centering the contact pattern from root to
top land and not from heel to toe.
Have you ever heard that the gear ratio in the front of a four wheel drive has to be higher (lower numerically) so that the
front wheels will pull more? Over the years there have been many different ratio combinations used in four wheel drive
vehicles but never so that the front will pull more. Gear manufactures use different ratios for many different reasons.
Some of those reasons are; gears strength, gear life, gear noise (or lack of it), geometric constraints, or simply because
of the tooling that they have available. I have seen Ford use a 3.50 ratio in the rear and a 3.54 ratio in the front, and I
have also seen them use a 4.11 in the rear and a 4.09 in the front. I have found that as long as the front and rear ratios
are within 1% that the vehicle works just fine on the road, and can even be as different as 2% and work just fine
off-road with no side effects.
1 point difference in ratio is equal to 1%. To find the percentage difference in ratios it is necessary to divide, not
subtract. In order to find the difference divide one ratio by the other and look at the numbers to the right of the decimal
point and how far they make the answer different from 1.00. for example 3.54 ÷ 3.50 = 1.01 or 1%, not 4% different.
And likewise 4.11 ÷ 4.09 = 1.005 or only a 1/2% difference. These differences are about the same as a 1/3" variation
in front to rear tire height which probably happens more often than we realize.
A difference in the ratio will damage the transfer case. Any extreme difference in front and rear ratios or front and rear
tire height will put undue force on the drive train. However, any difference will put the same strain on all parts of the
drivetrain. The forces generated from the difference have to travel through the axle assemblies and the driveshafts to get
to the transfer case. These excessive forces can just as easily break a front u-joint or rear spider gear as well as parts in
the transfer case.
Setting the pinion too close to the ring gear or setting the backlash too tight will make the ring & pinion stronger. Ring
& pinions are designed to be strongest and last longest when setup with a centered contact pattern and manufacturers
recommended backlash. I have found that setting the pinion too close only helps in a top fuel car where the aluminum
housing flexes and the pinion moves severely under load. I have never found that setting the backlash too close makes
the gear set stronger or last longer.
Positraction is better than limited slip. I have heard many people call a standard open differential a "limited slip" and I
have been asked for a limited slip instead of a positraction because they did not want something too aggressive. I
believe that positraction and limited slip are just two different names for the same thing. If anyone out there can find a
SAE standard or printed industry accepted definition please let me know and I would be glad to correct myself.
SJ 1962 - 1991
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