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

[chrisGT4]

Can somebody tell me why an AWD or 4WD must have the same width tyres front and back.

When I had my tyres replaced (ST185) I wanted to have a wider tyres at the back but I was told that it's not safe. I wanted to know why but he didn't have the knowledge or experience to explain it at that time, all he knows was it was not safe. The person who could've explained it to me was not there (smoko). typical.

Why is it not safe? I don't see having wider tyres can have an affect, yeah I can understand different profile but not width.

Confused.

[Ben12a]

Maybe it messes with the torque split? I'm only having a stab in the dark.

Or something to with the electronics controlling the show (heavier rims and tyres might take it outside the design parameters)

Being the type of person I am, I'd fit some and try it. And probably learn the hard (expensive) way.

Take it from me, you dont want staggered fitments. I have two cars with staggered fitment meaning you can't rotate tyres (leads to shorteer tyre life) and you cant get good deals on 'sets'.

[LeaThaL]

i think its only different sizes not width

being 4wd/awd if the front is smaller than the rear the gears are moving at different speeds and there for break , but ive got a mate that ran 285's on the rear and 265's on the front on his GTR, didnt make a difference.......but he did need a new box when he got a pair of 18's off a mate and ran them on the rear just for the trip home.....he only got about 20m before the box shattered

[WDE_BDY]

Its actually very dificult to get two different width tyres with exactly the same diameter, remember the profile is a % of the width. I would even be wary of mixing tyre brands front to rear. For 4WD it simply isn't worth the risk of damaging your gearbox if it isn't designed for different front/rear sizes (some cars are designed for it but most people can't afford them).

Callum

[TRD-12A]

most your GTR's come out with fatter tyres on the back anyway. And like your BMW X5 and stuff like that.So technically there is no problem doing aslong as you do it right.

You have to work it out cause if you go a 235/45 series on the front you would then go a 255/40 series on the back and it would be the same diameter anyways so i dont know what his problem was.

[LeaThaL]

You have to work it out cause if you go a 235/45 series on the front you would then go a 255/40 series on the back and it would be the same diameter anyways so i dont know what his problem was.

if my comment is what your talking about, he was running 17's on the front and 18's on the rear and profiles were way off

[Roundy]

Supposedly the main issue is the centre diff.

With different size tyres it will always be active and trying to lock a little bit.

With a GT4 just run the same all around, you can rotate them then if need be.

[RONA]

if my comment is what your talking about, he was running 17's on the front and 18's on the rear and profiles were way off

Surely you'd pull the attessa fuse and run it rwd only.

[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. . .

[MWP]

Also remember that, for example, a 225/45 tyres does not have the same rolling radius as a 235/45 tyre.

You can get combinations that will have a very close rolling radius, like 225/45/17 and 245/40/17 (1% difference).
GT4 center diff should handle that, but im not going to gamble destroying a gearbox.

[TRD_021]

Lucky i read this, I was due for some tires and was going to put wider tires at the rear. I might just stick to the same profile all round

[Z2TT]

Also remember that, for example, a 225/45 tyres does not have the same rolling radius as a 235/45 tyre.

Why is that? They are the same profile so how could the width affect rolling radius?

I assume rolling radius is the measure from the center of the wheel to the tread of the tyre

http://www.raceandroad.com/acatalog/Rolling_Radius.html

[WDE_BDY]

As I said above, the profile is a % of the width. A 195/50 has a sidewall 50% of the tread width, so approx 97mm. A 295/50 would be approx 147mm. To retain a total rolling diameter you have to drop the profile on a wider tyre. My KE70 for example is running 205/40 17's up front ((17x25.4)+(205x0.4x2)=596mm) and 245/35 17's on the rear ((17x25.4)+(245x0.35x2)=603mm) to keep my rolling diameter fairly even. This applies to road tyres, race tyres often have specfic measurements like 240/625 17 which means 240mm section width with 625mm diameter for a 17" rim.

Callum

[Mr Ed]

most your GTR's come out with fatter tyres on the back anyway.

No they dont.

[boxh34d]

WDE_BDY has it correct.

Quote:
Originally Posted by TRD-12A
most your GTR's come out with fatter tyres on the back anyway.

Sir_2jza70: No they dont.

I haven't seen any GTR's with different sized tyres. That said, keep in mind that they transmit the vast majority of the drive to the back wheels 95% of the time anyway, 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.