[wakaday]

I’m running M403 19” wheels with standard tyres front 225/45 8J and rear 255/35 8.5J on my 335d F31. I was wandering if anyone had fitted wider tyres to these wheels. The fronts look like they need a bit more rubber to cover the rims properly, maybe 235 is a better combination, but I would like to more rubber on the rears to match say 265, but I think 255 is close to the limit on 8.5J rim. Anyone tried different tyre sizes?

[Alscoob]

Originally Posted by wakaday

I’m running M403 19” wheels with standard tyres front 225/45 8J and rear 255/35 8.5J on my 335d F31. I was wandering if anyone had fitted wider tyres to these wheels. The fronts look like they need a bit more rubber to cover the rims properly, maybe 235 is a better combination, but I would like to more rubber on the rears to match say 265, but I think 255 is close to the limit on 8.5J rim. Anyone tried different tyre sizes?

I asked similar a while back but no reply.
I think that 235's on the front are fine for the wheel width and would also be cheaper than the 225's but as you say I think that the 255's are about optimum for the wheel width on the rear.

On my previous C class Merc it too had a staggered set up 255's rear & 225's front - I change the fronts to 235's without issue. IMO the 235's on the front would give a better rim protection similar to the rears, which is a good thing.

[KM79]

Be very careful when playing with different tyre sizes on an xdrive.I've seen transfer box juddering even when folk put different brands front to rear never mind changing the size of the tyres.

[Alscoob]

Originally Posted by KM79

Be very careful when playing with different tyre sizes on an xdrive.I've seen transfer box juddering even when folk put different brands front to rear never mind changing the size of the tyres.

I'm S drive so OK but didn't realise that this was still a problem on modern 4x4 types. 225 to 235 on a 19" rim = 8mm difference in diameter ...ooops! if you change the rears to a 265 then you are down to 3mm, the existing set up is 2mm so guess it could be done.

[KM79]

Yeah it definately is.Changing the rear also to compensate would be fine as you say.

[markoroogo]

Has anyone fitted 245 40 19s on the front and 275 35 19s on the back? Tire sizes are close enough not to be an issue, I wonder if there is clearance.

[teaston]

The main problem is insurance, officially you should declare different sizes to the factory fit tyres as a modification!

[wakaday]

You should of course declare any factory spec changes, and I did so, with the remap & B3X suspension & front splitter. Since I've up rated the car and enjoy pushing it harder, I notice the tyres getting closer to the limit. I've seen both under and over steer , probably a little more over steer. So a little more quality rubber may help. 245 / 275 probably a little OTT for 4WD, but would look cool though. My main concern, is stretching 265 on an 8.5J rim, shame they weren't 9J. I know many tuners but 265 on 8.5J, on VWT5, 300ZX & 200SX. Just hoping someone's been here before.

[HighlandPete]

Originally Posted by wakaday

You should of course declare any factory spec changes, and I did so, with the remap & B3X suspension & front splitter. Since I've up rated the car and enjoy pushing it harder, I notice the tyres getting closer to the limit. I've seen both under and over steer , probably a little more over steer. So a little more quality rubber may help. 245 / 275 probably a little OTT for 4WD, but would look cool though. My main concern, is stretching 265 on an 8.5J rim, shame they weren't 9J. I know many tuners but 265 on 8.5J, on VWT5, 300ZX & 200SX. Just hoping someone's been here before.

If you are talking of 265/35 R19, the tyre size is outside ETRTO (The European Tyre and Rim Technical Organisation) specification. Rim widths are 9J - 10.5J, with 9.5J being the measuring rim. Being outside the permitted sizes will have insurance implications, both due to the ETRTO standard and being outside BMW type approval.

Even the 255/35 is at the upper limit for the 8.5J rim (8.5J - 10J, with 9J being the measuring rim), so quite a 'bulgy' tyre with less lateral stiffness and softer handling characteristics than a smaller section.

[wakaday]

I agree, like many aftermarket tuner wheel/tyre combinations. I wasn't going to the expense of type approval, just tick the box non standard wheel/tyre fitment and it's down them to ask more detail.

[HighlandPete]

Originally Posted by wakaday

I agree, like many aftermarket tuner wheel/tyre combinations. I wasn't going to the expense of type approval, just tick the box non standard wheel/tyre fitment and it's down them to ask more detail.

What characteristics are you actually after from the wider tyres? Adding wider sections (on the same rims) can make for worse steering response, lower cornering forces and due to the overly curved sidewalls more tyre flex and a softer ride.

[wakaday]

My objectives are more rim protection, particularly on the front and improved understeer. Improved grip in wet, full load rolling start 2nd gear can see the whole car trying to change ends. Running Bridgestone S001 on rear, not class leading in wet, and Contisport 6 on front. I know what you’re thinking, I being asking a lot put down remapped 335d torque in the wet, even with Xd.
Noted that the Alpina D3 runs 245/35R19 8J front and 265/35R19 9J rear , its RWD, but still runs wider fronts, F80 M3 runs 255/35 R19 9J front and 275/35 R19 10J
I know Birds have been running 255 all round on their 435xd demonstrator to reduce understeer
Maybe I should just change the rears to conti 6, standard size and widen the fronts to 245/35 or 255/35?

[HighlandPete]

Originally Posted by wakaday

My objectives are more rim protection, particularly on the front and improved understeer. Improved grip in wet, full load rolling start 2nd gear can see the whole car trying to change ends. Running Bridgestone S001 on rear, not class leading in wet, and Contisport 6 on front. I know what you’re thinking, I being asking a lot put down remapped 335d torque in the wet, even with Xd.
Noted that the Alpina D3 runs 245/35R19 8J front and 265/35R19 9J rear , its RWD, but still runs wider fronts, F80 M3 runs 255/35 R19 9J front and 275/35 R19 10J
I know Birds have been running 255 all round on their 435xd demonstrator to reduce understeer
Maybe I should just change the rears to conti 6, standard size and widen the fronts to 245/35 or 255/35?

Not asking much are you?

To be frank here, IMO unless you start with some sort of optimum bench mark, (even if not giving what you want), it will be hard to get the best changes.

Have you run the car on '4' same spec' tyres? If not, in my view it is hard to see where you are at, on mixed tyres.

Plus you are xDrive, with all the complexities of torque bias, compromised with mixed tyres, front to rear. OK there is tyre tolerance logic involved, but it can still compromise the torque bias, as there is the possibility it can't give you better understeer control.

Same will apply if you get the new tyre sizes wrong, besides the mechanical issues of transfer box bunch up, you could be defeating the control you are after, by wheel speed sensors sending data that results in compromised xDrive performance.

Your examples of tyre widths are much influenced by rim width, not just upping tyre sections. Alpina will have experimented with rim widths and tyre sections, to get the best Alpina ride/handling balance.

Even my standard OE setup (on my 5-series) uses a 245 section on 8" rims, but that is partly to improve ride quality.

Your wet grip is more about tyre selection, than simply tyre width. Remember wider sections can make wet grip worse than decent rubber on a narrower section. Certainly the wider section can lose grip and find it harder to get grip again than the smaller section tyre. Takes my mind back to RS Escort days. Friend of mine had better grip driving his father's Orion on narrow tyres in the wet, than his RS 1600, unless exercising a lot of care and restraint.

Also remember with xDrive, you have to get the front and rear sizes within the RC tolerance. You just can't widen the front and expect the xDrive system to work correctly, you could exaggerate the very issue you are trying to reduce.

[wakaday]

When I started this post I hoped that someone had some practical experience, but now I'm looking forward to my own D&D program although my budget is initially limited to new rear tyres which are required. Obviously need to get quality rubber, so I'm happy to dispatch the S001, and match the Conti 6 fronts, and retain rolling diameters all round.
I noticed earlier that Alpina fit 245 / 265 to their RWD or XDrive F31/31, and that would be my idea choice if changing rims.
But I doubt if I will change rims, and squeezing a 265 on 8.5J makes me a bit nervous , so I guess a will replace the rear with standard size ContiSport 6. Then re-evaluate the handling and possibly look at increasing the fronts to 245/35.