Maximum front/rear tyre width difference for 4motion?

[kapowww]

I'm looking at tyres to go on my Bora 4mo and I've bought a set of 18x8.5 and 18x10 wheels. I know it would be ideal to have the same width tyres all round, but this depends on the amount of stretch I have to have after lowering. I'm guessing I'll have to go 235/40/18 on the rears and 225/40/18 on the front. Plus I think 235s might look too fat on an 8.5" wide wheel.

The concern I have is with possible damage to the diff/haldex system.. On the Audi forums there was an argument about this with my old Quattro but I don't think it was resolved. So I'm guessing that manufacturers take into account differing tyre pressures, etc so there's some tolerance for different tyre widths.

So question is would the 235s and 225s be ok, can I go smaller to 215s, or not at all?? Also can I extend this question to different brands/tread patterns?

EDIT: I know the ideal size is 215/40/18 when it comes to rolling diameter (the front's at least) but the tyre place says the loading's not sufficient.

EDIT 2: Also I realise my car's not a RWD so please don't give me crap about wider rear wheels. I admit it's for aesthetics.

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CONCLUSION: In VW's AWD system it is ideal to have identical rolling diameters for the front and rear, otherwise get them as close as possible. The width isn't important but it's the overall circumference of the tyre after it's fitted that matters. Use an online tyre rolling diameter calculator to experiment with sizes, but also keep in mind load rating, etc for the particular wheel widths you have.

NEW INFO (22/08/2012): I ran staggered wheels on my 4mo with slightly different rolling diameters and had no problems whatsoever.

[dylan8]

its more about rolling diameter on a 4mo, width doesnt matter as much id imagine.
if you have 225/40 up front and 235/40 on the rear you will end up with different rolling diamter (although the stretch might fix this!!)
loading wont be right with a 215/40, but then again your car probably wont be legal in a lot of other ways if your putting wheels like that on so id image the load rating will be the least of your problems.

215/40, 225/40, 235/40 all have different rolling dimaters. so youll be wanting to do something like 225/40 on the front and 265/35 on the rear, (dont quote me on those numbers, just a example) as something like this will give you the same side wall height which means same rolling diameter. But then youll run into problems of them not fitting...

But as above, stretching will change the heights of the tyres, need to find a calculator that will accurately work this out.
Think one of the QLD boys is running staggered tyres on there R32 mk4 on porsche wheels, so maybe suss them out.

[dylan8]

oh and fat rears ftw!

[PranK]

Having had many GTR's and other ATTESA based cars I can say that on those vehicles you can break lots if you dont have the same width and rolling diameter. It would surprise me if other systems aren't at least similar.

[balkanac]

I dont think you' have any problems with 225-40. My R32 came from the factory with that size and the AWD system is the same. I'd probably get 245-35 on the back tho

[kapowww]

Yep, fat rears ftw!

I found this from Haldex: http://www.vaglinks.com/Docs/Misc/Ha...ion_System.pdf

Enhanced Vehicle Driving Comfort and Transparency
- No wind-up during tight cornering and parking.
- Optimal traction during acceleration.
- No functional problems with tyres having uneven wear,
- pressure or size (mini spare).
- No functional problems when towing with one axle lifted.
- Transparent actuation.

So it looks like it's saying it should be fine.. They obviously have to engineer the car for mainly FWD cars which will wear more at the front, plus sometimes people have to use a spare tyre..

Also found this:
"An algorithm in the ECU detects differences in the diameters of the tyres and compensates the characteristics accordingly."

[dylan8]

I remember looking at it for another 4mo bora, there is some play in the system. cant remember the percentage.
Check out the other 4x4 and AWD cars that have staggered tyres. BMW X5 for example, not sure how that system compares to the 4mo though...

[Diesel_vert]

So question is would the 235s and 225s be ok, can I go smaller to 215s, or not at all?

Whatever is the closest match for the OEM tyre's rolling circumference (CR) whilst still meeting the required load index (LI), so no 215's - if you want to keep it legit.

Also can I extend this question to different brands/tread patterns?

From a regulation perspective (and it does vary from state to state), I think you are only required to keep the same type of tyre construction (i.e. either cross-ply or radial, never both) on the same axle. But I would recommend that you keep all four tyres the same brand and tread pattern, for reasons of performance consistency and predictability - especially in the wet, since wide tyres are more prone to aquaplaning.

EDIT: I know the ideal size is 215/40/18 when it comes to rolling diameter (the front's at least) but the tyre place says the loading's not sufficient.

That is correct. 215/40 R18 has a LI of 89 - which is less than 91, so therefore not acceptable.

I found this from Haldex: http://www.vaglinks.com/Docs/Misc/Ha...ion_System.pdf

Enhanced Vehicle Driving Comfort and Transparency
- No wind-up during tight cornering and parking.
- Optimal traction during acceleration.
- No functional problems with tyres having uneven wear,
- pressure or size (mini spare).
- No functional problems when towing with one axle lifted.
- Transparent actuation.

So it looks like it's saying it should be fine.. They obviously have to engineer the car for mainly FWD cars which will wear more at the front, plus sometimes people have to use a spare tyre..

Also found this:
"An algorithm in the ECU detects differences in the diameters of the tyres and compensates the characteristics accordingly."

The computer will do what it can to protect the system, but I would guess that there are limits on how much the system can compensate for unexpected values. Just because the system can cope with small variances like different pressures, uneven wear or space savers, doesn't mean you should assume it can deal with large changes in rolling circumference and such.

I remember looking at it for another 4mo bora, there is some play in the system. cant remember the percentage.
Check out the other 4x4 and AWD cars that have staggered tyres. BMW X5 for example, not sure how that system compares to the 4mo though...

I would think the vehicle manufacturer would have programmed in the expected values for rolling circumference, whatever they may be - though I suspect BMW try and keep the CR values of their front and rear tyres as closely matched as possible.

So in the same vein, I would also try to minimize the CR difference between the front and rear tyres. For example...

1919 mm : 215/40 R18 89Y XL (!) - note LI
1919 mm : 245/35 R18 92Y XL

1943 mm : 225/40 R18 92Y XL
1937 mm : 255/35 R18 94Y XL

1967 mm : 235/40 R18 91Y
1961 mm : 265/35 R18 93Y

1967 mm : 235/40 R18 95Y XL
1961 mm : 265/35 R18 97Y XL

The second example ( F: 225/40 R18 / R: 255/35 R18 ) have CR values closest to OEM spec whilst meeting the required LI of 91.

I realise my car's not a RWD so please don't give me crap about wider rear wheels. I admit it's for aesthetics.

I was tempted, but that was before I realised that you've already bought the wheels - so I didn't.

[kapowww]

=]

Thanks for that wealth of info. I would ideally go for 225/40 and 255/35 IF the rears fit under the guards without too much work.

I will though make the assumption that if the computer can compensate for a mini spare, it would have quite a tolerance. I trust their engineers would've predicted that some noob would put staggered wheels on.

Another thought.. by putting a bigger rolling circumference on the rear would make the Haldex think the front is spinning faster and transfer more torque to the rears, which an upgraded Haldex controller would do right? More rear wheel biased? So therefore I can save money by not upgrading my Haldex. Wishful thinking or utter nonsense? Most likely.

[Diesel_vert]

Thanks for that wealth of info. I would ideally go for 225/40 and 255/35 IF the rears fit under the guards without too much work.

Your wheels are much wider than the OEM spec, so if the second example is too difficult and costly to make it work, you may have to consider going for the first example ( F: 215/40 R18, R: 245/35 R18 ) which entails contravening a couple of regulations - depending on how comfortable you are with that.

I will though make the assumption that if the computer can compensate for a mini spare, it would have quite a tolerance.

The space saver may be narrow, lightweight and not grip too well, but it's CR would be very closely matched to the full size tyres on the vehicle. Also, the space saver is designed to be used as a temporary measure to get by, and most have a limited maximum speed (80 km/h IIRC). These factors all combine to make sure one cannot (or shouldn't) use the space saver for extended periods <- assuming that last condition is met, this is probably something the system can cope with.

I trust their engineers would've predicted that some noob would put staggered wheels on.

There are very little (if any) performance gains from a staggered setup on a FWD or 4WD passenger vehicle, hence being a very uncommon setup. I really wouldn't expect engineers to waste time and resources trying to build in (and/or program in) a safety margin for such an eventuality.

In any case, the system wouldn't need a such a large safety margin - In the event that it were to break due to a non-OEM staggered wheel/tyre setup, VW would be entitled to deny any warranty claim, which in the end saves them research, development and warranty costs.