Why AWD/4WD can't have different width tyres from front / back

**Mr Ed** · 27-01-2010, 05:25 PM

If you want more traction on a 4WD, just put wider tyres all the way round. I fail to see why having wider tyres on the rear is better for any reason other than the narrow fronts costing less money.

**Duk** · 27-01-2010, 05:38 PM

Originally Posted by boxh34d

so the centre diff (whatever set up that is, not sure) would be far less likely to have dramas compared to conventional transfer case 4WD/AWD. I still wouldn't like to be trying it though, and find out the hard way it didn't like it.

They use a multiple plate wet clutch that is engaged electro-hydraulicly.

**Talasas** · 28-01-2010, 12:09 AM

As mentioned above it's all about the rolling diameter, sidewall height is an aspect ratio.

I've come across this once, an AWD car in the family was purchased cheap due to a suspected "broken centre diff", indeed the car would exhibit strange rear end locking behaviour in certain situations. Turned out to be different sized tires on the front and rear (40 vs 45 sidewall for the same width in this case). After a new pair of tires it was all fine again. Bargain of the century.

What was creepy was seeing the drivetrain stress unwind as we jacked up the car the first time, this was what gave us the clue.

**Billzilla** · 28-01-2010, 12:23 AM

We've got a part-time 4WD tractor a bit like this one.

The different diameter tyres isn't a problem due to the centre diff, as mentioned above, but you need to work out the front & rear diff ratios very carefully. From memory the WRX's have different front/rear ratios. They're full-time 4WD so the centre diff gets a bit of a workout of course anyway.

**o_man_ra23** · 30-01-2010, 11:03 PM

Originally Posted by boxh34d

Above post are correct. Different rolling diameter between front and back mean that the transfer case (or centre diff etc) tension up (often called transfer wind up) due to uneven shaft rotations on either side. Eventual result is it goes bang, often in a rather spectacular fashion.
It is exactly the same theory to running different size tyres left to right on cars with LSD or locked diffs.

Getting different profiles/widths or even different brands of tyres to the same rolling diameter can be very hit and miss, even if brand x and y are both listed as 235.45.17.

2 easy solutions:
1: run the same size tyres and never have to worry about it, and have the option to rotate as need be.
2: just bag em up all the time! The spinning will allow the wheels to travel at the same shaft speed, so no risk of wind up. This does work out a touch more expensive than option 1 though. . .

That's the best and most accurate description that I have come across. Option 1 is the safe bet, but if you REALLY want fatter tyres at the back, then you are most welcome to test the longevity of the centre diff on your own vehicle (just check to see if it doesn't void your insurance)

**MWP** · 31-01-2010, 03:11 AM

Being a viscous LSD, it could probably take a small (like 5%) mismatch ok around the city.
Itll get warmer than normal, but shouldnt shorten its life at all.
Wouldnt like to do that for long duration high speed driving tho.

**bobjane** · 31-01-2010, 11:10 AM

What if you had to use the spacesaver spare so there are 3 corners with one size and 1 corner with a quite different size?

I don't know the tyre size of my spacesaver off the top of my head but I'd image the diameter/circumference to be off some. I'm running tyres 1.16% larger in circumference than stock.

**o_man_ra23** · 31-01-2010, 10:46 PM

Originally Posted by MWP

Itll get warmer than normal, but shouldnt shorten its life at all.

Heat degrades the oil and expands the internal components. Those two things combined will cause the helical gears to start touching metal to metal (remember, normally there is a thin layer of oil between the metal contact points so they don't actually touch). That will shorten the life.

Seriously, who gets a turbo AWD sports car, goes out of their way to put fats on the back, then putts around town like a granny??

And Bobjane, if you look at the spare, it says to travel at a speed of no more than 80kph, and will give you a distance limit too. If you baby it for a short period to get to out of trouble, it won't kill your diff a significant amount.

**MWP** · 01-02-2010, 02:22 PM

Originally Posted by o_man_ra23

Heat degrades the oil and expands the internal components. Those two things combined will cause the helical gears to start touching metal to metal (remember, normally there is a thin layer of oil between the metal contact points so they don't actually touch). That will shorten the life.

Helical gears?
We are talking about the center viscous LSD here, there aren't any.
The special non-replaceable fluid used in viscous LSD lasts the life of the gearbox.

Look up how a viscous LSD works...
http://auto.howstuffworks.com/differential5.htm

**o_man_ra23** · 01-02-2010, 10:51 PM

Righto, IC now. Was under the impression it used a Torsen type of viscous LSD (A torsen's degree of coupling is altered by changing the viscosity of the fluid).

Still, constant slipping will cause the fluid to heat, which will degrade it, leading to a reduction of viscosity, and the torque transfer will tend toward becoming a 2WD. End result is the same.