Differential Failures

[urq]

... perhaps vehicles with very high torque output are different, I can't 
comment on that other than to say that I remain skeptical.  Any chance there 
are reports that can be viewed on line?

It was pretty hard to figure out where the thread started with the original 
subject ... but it seemed to be something to do with risk of damage with the 
diffs locked.  There's no doubt that if you had a sticky enough road surface 
and sticky enough tires ... if you locked a differential and started driving 
around in circles (or had tires of different rolling diameters) eventually 
you would build up enough torque in the diff/shafts that something would 
fail.  Practically speaking, and IME the place that gives first is the 
interface between the tire and the road.

Open differentials ... it always seems there are some strong opinions on 
that one ... and I happen to be one of the ones that has a difficult time 
seeing how there would be much difference in stress on an open diff with 
identical rolling diameters on its outputs as compared to one with different 
diameters.  I think of the situation where the vehicle goes perfectly 
straight and the rolling diameters are identical.  Of course if this were 
indeed the case you would not need a differential ... but assuming one is 
there, all of the drive torque is transferred through the same gear faces in 
the differential.  Eventually you would have to expect wear on those 
surfaces.  In normal operation those drive forces are distributed between 
all the faces of the bevel gears, and the wear should be pretty even.  Under 
most driving conditions a differential is feeding two different rates more 
often that it is driving the same rotating rates ... and I'd argue that many 
of the situations where the differential is "differentiating" are the times 
where the most torque is passing through the differential.

The one case I might actually buy into an argument for long term damage 
resulting from differences in rolling diameters is in a Torsen.  That said, 
so many Audis have Torsen center diffs, and the center diff is the one that 
is most likely to deal with differentiation between its two output shafts 
...

Steve B
San Jose, CA (USA)

... did a little websearching myself:

http://www.saturnfans.com/forums/showthread.php?t=45182 - looks like folks 
with Saturns report diff failures running a compact spare up front (!?!)
http://forums.vwvortex.com/zerothread?id=1885810 - looks like Audi calls for 
a small rolling circumference difference (saw similar for ScoobyDoo) due to 
Torsens?
... wish I could find something technical from SAE or one of the 
manufacturers!!!  Wouldn't you know I'd be asking this just after I let my 
SAE membership lapse!

>
> The theory is this, and proven on Semi trucks and other large trucks such 
> a
> Dumps, Haulers, etc.,  different diameter tires, such as one worn and one 
> new
> tire of the same size, cause the axles to rotate at different speeds. 
> This
> puts undo pressure on both sides of the diff.  This will  eventually cause 
> the
> diff to fail.  Not urban legend.  The same will  happen in a car.
>
> The report on the trucks was in a maintanence magazine for fleet 
> companies.
> Our Utility followed the advice, and the diff failure rate went  down 52%.
> Savings in the first year of replacing pairs, or in some cases,  quad 
> tires,
> was close to 1M.  The rear tires not worn out enough were  simply used on 
> the
> fronts as replacements.
>
> Greg W.
>
> In a message dated 10/9/2006 9:31:01 AM Pacific Standard Time,
> audi at humanspeakers.com writes:
>
> The  locked diff probably wouldn't get hurt, it's the tires that get
> eaten if  they can't slip a bit.  Running different tires on an unlocked
> diff  can supposedly "wind up" the diff and kill it, though, according
> to, as  you say, the legend of the street.  This goes for 4wd trucks
> (etc.)  as well as quattros.
>
> Anyone who actually knows what they are talking  about, feel free to add
> accurate details...