Re: Wider tires on the front!

[cheryl_ford@bc.sympatico.ca]

Jeffrey J. Goggin wrote:
> 
> >Yes, having different sized tires will cause extra wear.   I don't think
> >it is a huge amount, hence my decision to run with the different sized
> >tires fro quite a while this past spring.   It is a significant amount
> >extra though.  If the tires have 5% more circumference, then you'll end
> >up with roughly 5% more friction in the centre diff.   The problem I
> >didn't like was that when I locked the centre diff (for maximum traction
> >on a drag race style launch) the dogs in the locker would not unlock
> >without alot of screwing around in the parking lot.
> 
> The 235/40-17s on the front of my Ur-Q have a calculated diameter of 619.8mm
> and the 215/45-17s on the rear work out to 625.3mm ... in reality, because
> the rears are half worn, the overall diameter is just about the same.  In
> any event, the worst case difference is only 5.5mm, which is less than 1%
> ... personally, I don't think this is going to cause much additional wear
> above-and-beyond that caused by normal driving as Brendan pointed out.  (It
> *definitely* helped tame some of the car's understeer, too!)
> ______________________________________________________________________
>     _                _
>    / |      _| o    | \       _| o   Jeffrey Goggin
>   /__| | | / | | __ |  | | | / | |   audidudi@delphi.com
>  /   | |_| \_| |    |_/  |_| \_| |   http://people.delphi.com/audidudi
> ______________________________________________________________________

Hey:

Lets think this thru a bit more....

1. Radial tires tend to deflect (bulge) alot under weight.  Actual 
rolling diameter of front and rear tires will be different when all 
tires are at the same pressure, as there is more weight on the fronts.  
(measure the height of the wheel center to the ground) I think there 
will be more than 1 % difference right there.

2. Different tire models have different actual dimensions, lots vary 
by more than 1 % (model vs. model).  Many people will run snow tires 
on the front in winter, for example

I think the centre diff is designed to handle moderate imbalances of 
front/rear rolling diameters, I wouldn't worry about wider tires up 
front, as long as the rolling diameters are close, like Mr. Goggin's 
car.

related point:
Another issue people tend to overlook is the incredible torques that 
build up in the drive when driving on dry pavement with locked diffs.  
The torque builds up with no wheel slip (static friction coef. is 
almost always higher than dynamic friction coef. for surface 
interactions), and is hard to notice.  If you are building enough bind 
to not allow the diffs to unlock on their own, you will severely lower 
the life of all those expensive components (cv joints, ring/pinion, 
etc.)

exmaple: during one project I was involved with, a semi got stuck in a 
ditch.  One inexperienced Unimog driver tried to pull it out.  He was 
on dry pavement, hooked onto the truck with a cable, and started to 
pull in a lower gear (I think the lowest gear has a speed of 30 
meters/hour @ redline).  He tightened up the cable, kept pulling until 
a loud bang occured, it turned out all four axles in the unimog had 
broken.  The unimog was brand new too! 

I don't run my Q with the diffs locked unless I am driving on a 
surface that has some "give", ie gravel.