[HighlandPete]

Originally Posted by g21

with very similar weight distribution, and RWD

M340i 225/255
M440i 245/255
M3/M4/M3T 275/285
all these cars above have similar weight around 1800kg and same weight distribution

based on that I'd say 245/255 is the sweet spot for OP
i was going to suggest 235/245 but OP already has 255s

From my data, BMW have 'optimised' the M340i and M440i on 225/255 for UK specification. Yes you can add performance upgrades to many models, but that isn't the optimised sizing, after BMW's R&D and testing for the model.

My F11 535i was optimised on 18" wheels, but BMW offer a performance upgrade, with 19" wheels. BMW themselves speak of compromises with upgrading wheel sizing.

What the optimised setup is and what other options a user has are different topics, IMO. The question is, does the user want an optimised setup or an upgrade with the change in characteristics the upgrade gives? An upgrade to one driver could be a downgrade to another driver.

For example, a wider set of tires will change wet weather driving, aqua-planing could be a nightmare for some driving conditions. Even seen as a safety concern. Therefore what are we optimising for? May mean we don't even change dimensions, simply select a tire more suited to our conditions.

I've had a similar discussion with a BMW Technical Manager in the past. I prefer a wet weather biased UHP tire. He agreed user conditions do influence choice. Would be foolish to say otherwise.

[G30M]

M440i is 245/255 F/R

225 F is definitely unsafe for my car at the limits when I need front tyre. Like braking. Don't bother braking over 100mph in the dry.

While it may not aqua plane, moderate braking at 60mph in the damp (not heavy water) abs is going wild. Needs more meat.

My car is 100% stock from factory, under 10,000miles.

That's why I'm guessing they gave new newer 440 245 F

[tranquility]

Ok, am I reading this correctly: the wider the tire, the more danger of aquaplaning?

[RM7]

Originally Posted by tranquility

Ok, am I reading this correctly: the wider the tire, the more danger of aquaplaning?

Well, yeah, but I doubt you are going to have to worry about the level of hydroplaning my 305s experience.

Down at the 255 size (my other car), it's not really on the radar as a concern. Maybe if you go for an almost-slick racing tire, but most tires people are going to put on are fine.

[RM7]

And I'll add this, auto makers are meeting a pricepoint, they aren't trying to make the best possible car they can. They are trying to meet a bunch of benchmarks and they make compromises all over. Just like my BMW, my Stinger came with 225s that were just not appropriate for the weight of the vehicle. Now, with 255s (and sways) it really comes alive...like it should have from the factory. But wider wheels and tires cost money. Especially when they are balancing out with other stuff like the trend for BIG wheel diameter, which generally doesn't help anything, but they design the car around the oversize cartoon wheels and compromises must be made. I had a sporty little Elantra GT N-line recently and THAT was good with 225s, just the right size for the weight and dimensions of the car. That's the thing I like about my 1LE too, it comes with the right sized rubber for the power and size, vs. some of the V8 cars handicapped with 255 and so on. GM recently relented and released the LT1 model, which is a V8 basically on the V6 chassis, to meet the cheapest price point of the Mustang V8. If you are going to build it from the ground up as a drag-racing car, that has some merit, otherwise, it's just not a good starting point for that kind of power, does more harm to the brand names than good.

[G30M]

Originally Posted by tranquility

Ok, am I reading this correctly: the wider the tire, the more danger of aquaplaning?

yes. the wider the tire, the lower the profile, the contact patch is wide and short. skinny tire & high profile the contact patch is narrow and long.

wide and short skims water like surfboard. narrow and long cuts water like knife.

[jmg]

Originally Posted by gabriel.psd

PS PS: The guy telling you to go 285s in the front... that's why you don't ask a forum. The guy probably didn't even read you have a Z4. Maybe you have M4 now?

If you are speaking of my comment, I said “no wider than 285” because you are right, I didn’t read OP has a Z4 because I’m posting from the all which doesn’t show signatures by default nor the garage list under the username. As far as I remember, he had yet to add the edit that he had a Z4 at the time I replied.

[HighlandPete]

Originally Posted by tranquility

Ok, am I reading this correctly: the wider the tire, the more danger of aquaplaning?

Plus it is not just the width, it is also the shape of the contact patch and its affect on the frontal wave.

A good brochure explaining how important the tire is for optimising wheels for any vehicle is Michelin's The Tyre Grip.

Here's one link to the brochure. Contains a good section explaining aquaplaning.

http://docenti.ing.unipi.it/guiggian..._Tire_Grip.pdf

Keep in mind all motor manufacturers are working with the tire companies to get the best set of compromises for road/street use.

Also keep in mind forum member's experience and recommendations are not necessarily covering the same tire performance requirements as a manufacturer's tire/wheel brief.

Optimising, as I've stated before, you really need to know what performance characteristics you are after, and where you are willing to compromise. This is particularly so, when we leave the relevant homologation specification.

[x622]

Originally Posted by g21

Excellent video. I really enjoyed this and even learned something. Didn't know Tire rack made quality content like this.