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      09-23-2008, 06:13 PM   #20
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swamp2's Avatar

Drives: E92 M3
Join Date: Sep 2006
Location: San Diego, CA USA

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Originally Posted by watrob View Post
There are couple of points I would like to make, first about your chart:-

I see a problem in your chart and that is when you go up in rim size to 20" the series size of the tyre drops by 5. 19" wheels with 265/30/19 rears change to 285/25/20. You seem to have used the wrong wheel sizes, 265/30/19 are the right size on a M3 not 265/35/19 as in your chart and the fronts are 245/35/19. In most cases the overall wheel diameter of the tyre does not change and in fact if you went to 275/25/20 the wheel diameter would be smaller than the 265/30/19 but the 20" wheel would be wider. Taking into fact that you could also attain a weight reduction with a 20" combination your chart needs to be recalculated. Your chart simply widens the wheel size & width but not the series size and your start point is wrong. The other fact is that a Michellin tyre is actually 5mm wider than its actual size and your will find that with quite a few tyre companies tyres. I could not see anyone running a 295/30/19, you would run a 295/25/19 to keep the rolling diameter the same. The difference between a 265/30/19 (79.5 side wall) and a 295/25/20 (73.75 side wall) is only around 5.5mm and with tyres not being exact widths as stated that maybe only 2-3mm side wall difference.

Further, is straight line drag from 0-100mph the 20" M3 because of more rubber on the ground would be quicker with LC off or on because it could lay more of its power down because there is more power in the M3 than the 18" tyres can handle.

As far as I can see by your chart your are telling me there is a difference between 19" & 20" wheels when in fact you could be presented with a 20" wheel that maybe only 5mm wider, have smaller rolling diameter and be lighter.


I do take onboard your statement that the M3 was (1) setup for 18" & (2) test results were with 18" but thats now the car was tested. I do not see any evidence that was how the car is sold to us. I do not see BMW tunning the car to 18" and then selling an option of 19", nearly all the M3 coming to Australia have 19". I believe the M3 is setup to run both on 18" & 19", the tyre choice of 18" is more to do with the condition of roads in some countries and the 18" being softer tyre to ride on & helps with rim damage. Oh yes don't forget snow chains? hey can you put snow chains on a M3?

I do agree that there is a fair bit of difference between a 18" & 20" wheels, but I believe the HP of the car plays a great part. Here we have a V8 with a e-diff running the same tyres as the e46 M3, the other point is that the DTC 7 speed is quicker than the 6MT.

Now if the M3 has power to spare then 19" would be a benefit. The DTC M3 is faster than a 6MT M3, 3rd gear is less of a problem with the 7 speed DTC, I bet it runs better on 19" than 18".

Your point rests with the fact that you state the M3 was designed around 18" wheels and tested around 18" wheels thats fine, but I say its not sold to us in that format, its been compromised, its been sold to us to run both 18" & 19", its also de-turned for different countries, due to both fuel quality & sub-tropical climates. I don'e see anywhere in the manual that the car performs better on 18" then 19' and I do not see any warnings that the handling will be compromised if you put 19" on. I also don't see anywhere where it says for best track times use 18". If anything the suspension has been beefed up to handle 19" not vice versa.

My principal is give me more power then give me more rubber, I don't drive on a track I drive on a road and I probably apply the brakes more than the accelerator anyway.

Anyway bring it on, I'm waiting?

PS. On M3 test track days I asked why they were running on 18", I was told because the car was more forgiving for the average driver, the tyres were promotional from the tyre company and if they used more than they had they were cheaper than the 19" to buy, they also had less damaged rims.
It is pretty simple. Column A is the OEM 19" rear. Column B is a 20 inch rear option that maintains the same overall OD, which is typically what is done when replacing wheels and tires. The other columns are just the specific sizes mentioned by the OP. I did realize that those were not always an "apples to apples" comparison not do they maintain a fixed overall OD.

Go ahead and download the zip file with the spreadsheet inside - play around with some numbers. My point of this exercise was not to model EXACTLY any specific wheel and tire. If I wanted precise I's I would simple make a 3D CAD model and get the moments of inetria that way. The point of the spreadsheet and going through the calculation was simply to demonstrate that larger wheel diameters will almost universally hurt you when it comes to a much larger I. For rotating mass I is more important than mass and is typically overlooked. The other point the calculation makes is that even a lighter wheel and tire combination can have a larger I than a lighter combination.

These conclusions are quite solid.

As far as dragging I would not bet on the 20s over the 18s. Sidewall flex is as critical in draging as in a road course. Of course tire width is well understood, the wider the tire typically the better. The sidewall size and tire width and completely independent effects.

As far as the way some countries option their cars: That will happen. The difference between an 18 and a 19 is going to be smaller than the difference between and medicore and good driver. It is not an enormous effect, but it is there. I'm sure some track nuts in your neck of the woods specifically requested the 18s.