[V8] Different Tire Sizes

[dsaad@icehouse.net]

... but but ... :-)

While you can theoretically do this, you would be running the differential in a
mode where it is not designed to be - long term. If you were to take a classic
open diff on say a pickup, and put a big tire on one side and a small one on the
other, with a difference of many inches, the truck will still go down the road
just fine - but the spider gears will be taking a beating. They simply are not
designed for constant use like this. I suspect you would heat up and drastically
shorten the life of the unit.

I really don't know how fluid or viscous couplings work - and most modern full
time systems I know of use these, but a mechanical system like a classic diff
(locking or open) or even a torsen would have to eat itself up in short order if
the differenses were very big at all. As many people have pointed out, the
differences in rotational speeds simply have to go somewhere.


Then again - I only play a mechanic on TV. In real life I am a piano player down
at the local cat house...

Dave



Quoting urq <urq at pacbell.net>:

> ... but ... what if the differentials *DO* give?  ... that *is* what they're
> there for after all!
>
> You *could* put 13" wheels with skinny, low profile tires up front and big
> meats on 20" wheels in the back (assuming an open center diff ... which all
> of you automatic pilots have) and drive down the road without worry about
> drive line stress or tire scrubbing.  There's no doubt that the prop shaft
> to the front wheels would be spinning faster than the prop shaft to the rear
> wheels ... but an open diff really could care less ... (here again,
> *assuming* that the tranny CU doesn't decide to try to lock the center diff
> ... but, here again, I'm exaggerating to try to prove a point).
>
> It seems to me we've lost track of what I thought we were trying to discuss
> here ...
>
> Michael, try those same calculations with:
>
>     235/55-15 (circumference = 79.1")
>     215/60-15 (circumference = 79.0")
>     195/65-15 (circumference = 78.5")
>
> ... by varying profiles you can very easily put together a set of
> considerably different tires that have close enough to the same
> circumference for practical purposes ... I would argue that you could mix
> and match these tires on your car and no matter what kind of
> open/Torsen/locked differentials you had there'd be very little risk of
> damaging the drive train.  Obviously you could come up with some
> combinations that would very likely create a very real risk in terms of
> dynamic stability.  But what if someone wanted to try to put skinnier tires
> in front for some reason to try to improve the overall handling of the car?
> Perhaps on a quattro it would be better to put the wider tires up front ...
> whatever.  My point is that if someone wanted to do that sort of experiment
> they should not feel that they are risking damage to the drive line of the
> car ... given that they went into the project keeping the overall
> circumferences of the tires the same.
>
> Steve B
> San Jose, CA (USA)
>
>
> > Without getting into what the differentials do, if
> > different tire sizes are used at the same time then
> > something has to give. When someone says that the
> > difference between 65% and 60% of a sidewall is tiny,
> > they haven't done the math.
> >
> > 215 215
> > 60 65
> > 15 15
> >
> > 25.16 26.00    diameter in inches
> > 79.03 81.69    circumference in inches
> > 801 775      revolutions per mile
> >
> > If the driveline/differentials do not slip then the
> > slippage must be between the tires and the road. Even
> > if not perceptable to the driver, there is a lot of
> > tire scuffing going on.
>
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