alphanumeric

I am wondering how much of a estamated 1\4 time would improve by just having wider rims and tires..

the car I am looking to upgrade is an SLK55.. I am not sure how wide the rims in teh rear are but the tire size is PRETTY dang small in my experiance.. they are 245\35\18s.. and my guess is that they are 8inch wide rims.. 8.5 at the most!

this car is already quick in the 1\4 at 12.8... some MB test have provin 0-60 in less then 5sec.. BUT I also hear that the most recent car and driver.. or some other hot car mag claims LOW 4's 0-60..

now I am not an expert on tires.. but a guy I ran into with a Z06.. who claimed his car ran 10s (LOL) had 295's on the rear on 18inch rims..

now I am wondering if I had some 10inch wide rims on the SLK55 how much quicker would it do the 1\4 in? do any of you guys have experiance in this feild?

mdp c230k

If the tire rim combo is heavier the times will be slower than stock. You get more grip but it is harder to turn the wheels due to the added weight.

vraa

Is it really wider rims you want? Is it traction loss? Remember, wider rims = more friction at higher speeds.

430

+1 will not give you a larger contact patch, but it will change the shape.

vraa

+1 in diamter? Or width.

If width, how does it not change the contact patch?

alphanumeric

wow.. this was pretty amazing.. but I messed around with the slk55 a little last night.. turning the ESP off and on practicing launches.. (all were in Sport mode) and it did not spin the tires in either mode (ESP on\off).. so I guess stock rim and tire combo get enough traction already.

430

Too keep the same ratio in the rear you will change your sidewall height. The reduction of sidewall height will reduce the angles resulting in a contact patch that is wider but shorter with a 0 net change in size. For straight line acceleration you want a long contact patch.

Now if you are going to change your spedo gearing to compensate you will be able to increase your contact patch without a negative impact.

RLS

430,
Could you please explain the above "The reduction of sidewall height will reduce the angles resulting in a contact patch that is wider but shorter with a 0 net change in size"
I was under the impression that what you are doing is decreasing the sidewall height to make up for the ratio of going wider to get the same outside diameter. Isn't the contact patch a combination of outside diameter and width?
Thank you, Bob

430

You are correct the decreased sidewall height is needed to keep the ratio the same. The problem is the shorter sidewall reduces the amount of tire that comes in contact with the ground. The shorter sidewalls are stiffer and will deform less than a taller sidewall.

RLS

Ah!! did not know the sidewall got stiffer as it goes down in size. In that case, it makes sense. Never to old to learn!!

Thanks, Bob

RLS

430, Please excuse my ignorance, but since the wheel diameter is the same, and the outside tire diameter is the same due to the increased width and ratio, doesn't that mean that the wall height is the same? I can understand what you are saying if you use the same size tire but change the ratio from a 35 to 30 only. This has been bugging me.lol

mdp c230k

They are referring to a +1 setup. You are of course right if it is not a different size rim. 205/55 = 225/50 in sidewall height and rolling diameter if the rims are the same.

RLS

Thanks mdp c230k. So on a C55, stock is: front 225/40/18 on a 7.5" wheel. Rear is 245/35/18 on a 8.5 wheel. Tire diameter in front is 25.1" and the rear is 24.8".

Now if I keep the front stock and put 265/35/18 on 9.5" wheels that would make the rear diameter 25.3". Does this sound like a good set-up if you are trying to get more traction for off the line starts? Will it cause any adverse trouble with the esp since it would now be reverse diameters with the front being smaller compared to stock fronts being larger in diameter.

Thanks for the help on this, Bob

mdp c230k

Try to keep the rolling diameter as close as possible from front to back for the esp to work the best. The width of the wheel is only important in the fact that a wider rim is heavier and as a result it is harder for the engine to spin it. That means slower. The other problem can be unsprung weight, more is bad for handling. A wider tire will have more side-to-side grip on a smooth surface but might suffer over bumps due to the unsprung weight. Tires with higher aspect ratios (like 60s or 65s) will have a longer, front to back, contact patch making them better for straight line acceleration but will suffer around corners due to flexxing sidewalls. There is a happy medium that must be found between the two. The best money spent on rims is not on the largest diameter but on the lightest weight on a rim that is 18" or less. I like go more than show.