I have recently purchased an 03 Turbo which will be a dedicated winter car in northern BC Canada. Other than the obvious loss of rear end traction in the dry,(perhaps making the car a little tail happy!) is there any reason why I can't run OEM front rims on the rear then put the same size snows all around? There seems to be a lack of available snows in recommended sizes for the rears and the narrower option would also give me a selection of studded tires available.

In rough terms, I think this would reduce the rear width by about 20%.

A narrower rear would give a slightly increased contact pressure and
cut down into the snow better.

It would also make diameters of front and rear identical.

Has anyone actually done this?

Thank you!

 

A problem is the rear wheels/tires want to be a smaller diameter than the fronts so the rear wheels/tires make more revs per mile than the front wheels/tires (but not to many more revs/mile). This is so the viscous coupling fluid is heated up due to the differences in speed between the discs driven by the rear axle vs. the discs attached to the front axle.

The fluid gets hot (approx. 350F) and the faster turning discs attached to the rear axle transfer some torque to the discs attached to the front axle.

If the front/rear wheels/tires have the same rolling diameter there will be no generation of heat and thus no tranfer of torque.

Actually this may not be that bad as the amount of torque normally transferred is between 5% to 40%. That 40% number occurs at around 150mph.

But in marginal traction conditions where the rear tires are spinning the torque transferred is zilch. All that happens is the viscous coupling gets abused which can only shorten its life.

(There are a number of YouTube vidoes of 996 Turbos in the snow and in every one the rear tires are spinning like crazy while the front tires are eithere turning very slowly or not at all. It is clear there is no torque being transferred from the rear tires to the front tires.)

Thus with snow tires fitted the car will be just RWD which ain't bad. With nearly 60% of the car's weight over the rear wheels/tires and with proper snow tires at all 4 corners the car will get around just fine. Snow depth will be the limiting factor.

(Consider adding say 100lbs or more of ballast to the front trunk. A bit more weight on the front end will help the front tires bite better. They won't be driven by the viscous coupling but of course they have the very important role of guiding the car and helping to stop the car.)

Be sure the actual rolling diameters are the same. Tirerack has a fill in the blanks caculator that will calcuate the various dimensions like diameter and revs/mile so you can know the diameters are the same.

What you want to avoid is having the front and rear diameters either with the rear wheels/tires too small and thus making too many revs per mile compared to the front wheels/tires (which can overheat the viscous coupling) or the rear wheels/tires too big which then can work to brake the front wheels/tires. Even in ideal traction conditions this ain't good. In marginal traction conditions it is to be avoided.

 

Thank you for your reply but I found it kind of confusing. Your first paragraph says the rear wheels must be smaller, then the 9th paragraph says they must be the same size?


If the VC really does nothing then front and rear tires of the identical size shouldn't negatively affect anything, should they?


If no torque is transferred, and spinning rear tires abuses the VC, then by these two facts it would indicate that by design this car should not be driven in the winter?


Again, I'm not trying to be difficult, just trying to understand my car.

 

Sorry. In the 9th paragraph I was speaking in the context of if you had to settle for having the same size, the same diameter, tires on both axles to make sure they truly are the same diameter. You want to be sure so you don't end up thinking they are the same diameter when in fact they are different diameters.

If they are different diameters then you are faced with the problem of if the rears are not smaller than the fronts the torque transfer is in the wrong direction and that ain't good and if the difference in diameters is too much possibly causing damage to the viscous coupling.

The car can be driven in the winter in marginal traction conditions one just has to be aware of the possible risks.

Since I very very seldom drive my Turbo in snowy conditions and already am aware of the issues regarding doing this -- the techs took the time to go over this subject at some point some years ago -- I have not bothered to check but this -- at some level -- may be covered in the owners manual.