Sponsored by The Tire Rack

[consolidated]

The 911 needs big rear rubber so it doesn't try to kill you in oversteer, enormous stagger and a wide rear track are two of the ways to keep the front wheels indeed in front of the rear wheels.

Doesn't big front rubber increase rolling resistance, aero drag, unsprung weight and reduce nimbleness?

[x838nwy]

A few things just off the top pf my head:

1.) Driving characteristics. Changing tyre dimensions and so on will change the way a car drives. It's the same as changing any other component in the suspension system. BMW has a certain concept of the trade off they aim for in the M3 in terms of grip/comfort/cost/noise/asthetics and the oem sizes are what they feel fit the bill best according to their goals. As you can see, it's a compromise between a number of factors. So, there is probably a combination that offers better grip or better looks or whatever. But that will be a particular combination of compromises that best suits that particular owner.

2.) Wet weather handling. In my experience, a narrower tyre width tends to drive better in the wet. Standing water and horribly wet stuff tends not to work so well with the wider tyres - I think this is mainly to do with the smaller contact patch being better able to clear the water (for the same given load). BMW obviously has to design the m3 to be able to drive in all weather and hence the OEM tyres.

3.) The whole "reinforcing the suspension" thing. I agree that if the tyre grips more, more load will go through the components. Greater loads through components will reduce their life spans. BUT unless you drive on R compounds and track the car all or nearly all the time, I don't see how this will be the actual cause of your failure. Bigger tyres have the capacity to load the components more, but it doesn't do that _all the time_.

If we assume that you go from 285->305 then you're increasing the size by about 7% so if we just assume that you gain... okay 10% more available grip. Now, 10% more lateral force capacity means an ability to round the same corner at srqt(1.10) = 1.0488 or ~5% faster. I doubt that cornering 5% faster would significantly reduce component life. Secondly, if you're applying 10% more load in addition to the "design" load, as long as it is an insignificant period in comparison to the design life of the component, there should really be no effect in component life. I truly hope BMW engineers have done at least as good a job as that.

I'm all for reinforcing the suspension joints if you're tracking your car a lot or if it's a purpose-built race car. But if yours is a dd and you occasionally track it then a (relatively) small increase in tyre widths shouldn't cause any problem.

Just my 0.02...

[vm]

I believe most of you are misunderstanding my posts on reinforcing the suspension. The original post was "Why BMW didn't do it" and this is where my point applies. The factory has a lifetime for everything that they need to meet and this is where my point is valid. If you add the larger tires yourself (if you know what you are doing) and only occasionally go to very high loads then this may not have a big effect on suspension lifetime.

[lucid]

Originally Posted by x838nwy

Greater loads through components will reduce their life spans.

I believe Swamp responded to this opinion earlier, but I'd like to re-iterate a relevant point. The above statement does not have to always hold true. In other words, increased loads do not have to certainly mean reduced life span. That depends on the specifics of the new loading cycle. How often and at what magnititude. Two key failure modes are plastic deformation and fatigue fracture. It could be that the increased magnititude and frequency of the loading might not be severe enough to make a difference. It is not clear that increasing tire width slightly would make enough of a difference to say component life is reduced.

[lucid]

Originally Posted by vm

I believe most of you are misunderstanding my posts on reinforcing the suspension. The original post was "Why BMW didn't do it" and this is where my point applies. The factory has a lifetime for everything that they need to meet and this is where my point is valid. If you add the larger tires yourself (if you know what you are doing) and only occasionally go to very high loads then this may not have a big effect on suspension lifetime.

I see what you mean. If the same factors of safety are applied in the analysis during the design process with the stock vs. modified (slightly increased) loading conditions/assumptions, one should arrive at different component dimensions.

[x838nwy]

Originally Posted by lucid

I believe Swamp responded to this opinion earlier, but I'd like to re-iterate a relevant point. The above statement does not have to always hold true. In other words, increased loads do not have to certainly mean reduced life span. That depends on the specifics of the new loading cycle. How often and at what magnititude. Two key failure modes are plastic deformation and fatigue fracture. It could be that the increased magnititude and frequency of the loading might not be severe enough to make a difference. It is not clear that increasing tire width slightly would make enough of a difference to say component life is reduced.

Very true. I was saying it in a general sense. The life we're talking about here is a type of fatigue loading and whether or not the change in component life is significant or not depends on a large number of factors. If one takes things very literally and statically, increasing loads will in general numerically reduce the number of load cycles before a part fails. However, if the increase is slight or the number of applications relatively small then the reduction may be next to negligible. But yes, sensibly speaking, more loads doesn't always mean shorter life.

Regarding the final component dimensions, while that may be true if they were designing to a specific load/stiffness, I think the number they had when they were doing the analysis for the part will cover a range of tyre sizes. The development team were probably the last prople to determine the exact tyre dimensions. May be they re-check and re-design the components again after the final tyre decision, may be not.

[lucid]

Originally Posted by x838nwy

Regarding the final component dimensions, while that may be true if they were designing to a specific load/stiffness, I think the number they had when they were doing the analysis for the part will cover a range of tyre sizes. The development team were probably the last prople to determine the exact tyre dimensions. May be they re-check and re-design the components again after the final tyre decision, may be not.

What you state above is most likely true as these analysis scenarios are not as deterministic as the theory might suggest/allow for.

[vm]

Originally Posted by lucid

I see what you mean. If the same factors of safety are applied in the analysis during the design process with the stock vs. modified (slightly increased) loading conditions/assumptions, one should arrive at different component dimensions.

You got it.

[lucid]

Originally Posted by vm

You got it.

However, if the primary concern is fatigue failure, there might potentially be an exception as even with slightly increased loading it might be possible for the part to remain in the ~infinite lifetime domain--if the part was designed to perform in that region to begin with--and there might not be a need to change the part geometry based on slightly higher loading assumptions. Plastic deformation and yielding is a different type of consideration obviously.

[TRZ06]

I went the 255 front and 295 rear route with HRE monoblocks, and I will tell you that it is far too much rubber and weight for this car.

Yes this car has 414HP, but ONLY 295 lb. ft. TQ. I lost acceleration when I went with my HRE's You also mess up the balance of the car, I had more understeer when I went this route.

I gained just a little in lateral acceleration from the 255/295, but I gained that back when I went back OEM 19's with RE-11's.

I had 295's on my Z06 with 405HP and 405TQ with a weight of about 3150 and 295 was perfect for that, just enough wheel spin to get max acceleration.

295's are way overkill on the M3 though, that has 100TQ less and weighs 300 - 400lbs more.

BMW, knows what they are doing.

[stylinexpat]

Originally Posted by TRZ06

I went the 255 front and 295 rear route with HRE monoblocks, and I will tell you that it is far too much rubber and weight for this car.

Yes this car has 414HP, but ONLY 295 lb. ft. TQ. I lost acceleration when I went with my HRE's You also mess up the balance of the car, I had more understeer when I went this route.

I gained just a little in lateral acceleration from the 255/295, but I gained that back when I went back OEM 19's with RE-11's.

I had 295's on my Z06 with 405HP and 405TQ with a weight of about 3150 and 295 was perfect for that, just enough wheel spin to get max acceleration.

295's are way overkill on the M3 though, that has 100TQ less and weighs 300 - 400lbs more.

BMW, knows what they are doing.

+1,

I would maybe go 1 size wider in the rear but think the stock size is pretty good as is. I did try launch control yesterday (and today again ) and did manage to get quite a bit of wheel spin off the line but you pretty much get wheel spin in any car when you rev the engine and dump the clutch.