Re: brakes and carriers

[Howard R Pletcher <n9ads@domain.elided>]

><p>PS - an interesting note - I was talking to a mechanic about front
and
>rear R&amp;P sizes.&nbsp; He was saying that up to the '70's and the
production
>of full time 4WD, manufactures made sure there was a slight difference
>between the front and rear R&amp;P ratios.&nbsp; Since most of the
vehicles
>weight is in the front of the vehicle, the front tires would have a
different
>diameter as it had the weight of the front end, engine, trans, and some
>of the driver.&nbsp; This would affect the front and rear ratios when
locked
>into 4WD.&nbsp; This accounted for a smoother ride going down the road
>in 4WD.&nbsp; Full time 4WD had a center differential that had some
give,
>so companies like Dana began making R&amp;P's with identical 
>ratios.</html>
>
>
Makes an interesting urban legend and it has enough elements that seem
logical to be believeable, but I don't think so.

The biggest reason for slight difference in front and rear ratios was
availability.  Most manufacturers built their own rear axles, but few
including IH built driving front axles.  If their rear happened to be a
41/10 tooth combination, this 4.10 ratio was combined with Dana's 45/11
or 4.09 (I don't have a chart of the actual gear combinations, but am
using these as an example).  This is a difference of only .3% which would
be barely noticable and easily absorbed by tire slip, even on pavement. 
I think I recall seeing some combinations that differed as much as .05 in
the numeric ratio.

Tires are probably a bigger variable to be concerned with.  I have seen
differences in rolling radius of up to 5% within a set of  "identical"
tires, which was enough to cause some T/C jumpout problems.  Even bigger
differences in distance traveled, which has to be absorbed by tire slip,
would occur in a corner.

If all the manufacturers "made sure there was a slight difference", then
it's no wonder the Scout died, as we did it wrong from the start.  :-) 
The ratios have always been identical Dana gearsets since 1961 with no
full-time 4WD.  And someone recently noted here that some pickups had
identical ratios when using the Dana 60 in the rear, but slightly
different ratios when an IH rear was used.

For heavy duty trucks like mixers, unmatched ratios were carefully
selected as often the diameter of the floatation front tires was not the
same as the rear duals and because of the heavy weights involved, tire
slippage even on dirt could not be counted on to accommodate more than a
very slight mismatch without drivetrain damage.

My $.02 on why we see the differing ratios.

Howard Pletcher
Howteron Products Scout Parts

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