Car is soooo sluggish after BBS install...

[S--K]

Sh!t, my car feels like a slug after puttin the BBS RCs
(18 x 8.5 with 235/40s ) on. Is it the taller tire?

Techno you there? How did your car feel with the 235/40s again?

[gerry_miranda]

just a thought....
235/40 x 18s is a whole inch in diameter larger than 235/40 x 17s...which is about 4% larger diameter. This will effectively give you a higher rear end ratio.

Also with wheels and tires, most of the tires weight is in the thread area , and most of the wheels weight is in the rim area. Although your BBS RC might be lighter than the standrd M3 wheel, you have effectively pushed most of the mass on the outside diameter of the wheel / tire combo.

Acceleration is most affected by the wheels rotational mass and more so by it's moment of inertia. A wheel's moment of inertia is dependent on how far off the center the average mass is located. With bigger (larger diameter) rims and tires, the average mass is located farther away from the center.

So your sluggishness is coming from an effectively higher rear end ratio and larger moment of inertia to be overcomed by the engine...

Has anyone evernoticed that a 16" winter tire/rim makes your car peppier.....

[S--K]

Gerry, the stock rears are 245/40s. How did you fit
16" wheels on your M?

[gerry_miranda]

There are certain wheels that are 16" that will fit the M3...tire rack has em......used for winter tires and driving....

As far as differences...with 245/40/17 the diff is now only .7 inches or 3.7% --still over tire racks max recommendation of 3%...

[S--K]

OK, tell me you secret on how to calculate the difference
between the different tire sizes.

Thanks.

[John@BPG]

Who sold you that tire wheel package? You should be running 235/35/18s I think.

As good as 18s look. I've never have or ever will put them on my car cause of the performace degration.

[eurospeed]

Originally posted by S--K
OK, tell me you secret on how to calculate the difference
between the different tire sizes.

Thanks.

Sure thing.

Let's use 235/40-18 as an example.

The first number is a unit of measure in millimeters of the tire's width. The second number is the aspect ratio, i.e., a percentage of the tire's wdith used to calculate sidewall height. The third number is another unit of measure, this time oddly enough it's in inches (who the heck ever decided to mix units like that!) of the wheel diameter. To calculate the overall diameter here's what you do:

First convert 235 mm to inches to get all your units the same. ( 235 / 25.4 = 9.25 inches wide)

Next determine sidewall height. ( 9.25 * .40 = 3.70 inches)

Multiply the sidewall height times two and add that to your diameter. ( 3.70 * 2 + 18 = 25.40 inches diameter)

This can all be done in one painless step with this:

((width / 25.4) * (aspect ratio / 100) * 2) + rim diameter = total diameter

[S--K]

Dang, that's cool Euro!!

Any idea how much of a diff .7 inches should make though?

It's definitely noticeable to me!!

[KraZy007]

That's really weird, i can swear to u that my car felt if not the same then peppier with my 18 rcj's. Odd.

[TechnoViolent]

Originally posted by S--K
Sh!t, my car feels like a slug after puttin the BBS RCs
(18 x 8.5 with 235/40s ) on. Is it the taller tire?

Techno you there? How did your car feel with the 235/40s again?

Right here, bro.
I hate to say it, but I told you so. ;) The 235/40/18 is tall enough to make a noticeable difference in acceleration. I've never tried a 235/40/18, but my experience w/ other sizes ranging in height from the relatively short 235/40/17 to the slightly tall 225/40/18 made me stay away from the very tall 235/40/18's and go ahead and shell-out the bucks for perfect-sized 245/35/18's. 235/35/18 is also a nice size. The 35 series rubber is rare and costly, though. :mad: But worth it in my opinion:D

Sell those damn truck tires and get the real deal!!;)

[S--K]

Techno,

Yeah, you were right about the 235/40s. I'm thinking 235/35 is a nice size. What size do you currently run?

I need a tire hookup, ya think you can hook me up with your buddy?

Thanks!

[TechnoViolent]

235/35 is cool. it is shorter than a 245/40/17 and creates a tiny bit more wheel gap as a result. And it is not as wide, of course. I run 245/35/18's. I might be able to get a decent deal, but S&H charges would prolly kill any benefit of it. My guy is located in Silver Spring, MD. :(

[S--K]

Thanks Techno. I only need 1 pair of 235/35 for the
rears. I'm in Seattle, WA.

Let me know!