Tire wisdom, please

[Henry Loh]

Hello, Folks.

I’ve been looking into getting better tires for my autocrosser. I’m driving the all stock White ’91 318is. According to Tire Rack, the hands-down best M+S tire I can get are:

Bridgestone Potenza RE970AS Pole Position
Continental ExtremeContact DWS

And yes, it has to be M+S or else it’ll shoot me into the next class….

2 questions:

1) 225 or 205 width? With what aspect ratio for which wheel?
2) Tire recommendations/opinions?

Additional details: available rims to mount them on are
15×7 MiM 1900
16×7 Alipinas (most choices)
14×6.5 Stock Basketweaves (not a lot of choices in this size)

Will be adding e30 M3 stock springs and Bilstein Sports.

My thoughts – the 225s will add mor grip, but I wonder if the added weight will kill what little power I have and/or will add more tire flex (and less response). So, cost-benefit arguments welcome, please!

Thank you!

Henry

[Hal Dorton]

Overall diameter is more to me right now. The smaller the diameter the quicker the acceleration. Too wide a tire will move around on a narrow rim so just don’t mount a tire two inches wider than the rim. I would use the survey results to determine which tire had the best dry grip and go with that one. Consider the treadwear as well. Lower is better for autocross but the manufacturer pulls this number out of thin air.

[Jeff Roberts]

To add to what Hal said…

DWS example, there are 4 16″ sizes available with a measuring rim width of 7″, which is: [i]the industry standardized rim width upon which the tire must be mounted in order to confirm it meets its dimensional targets[/i]. This is what Hal means by “too wide will move around on the rim.” Stick with a tire with a measuring rim width that matches your rim width.

Of those four tires, the 205/45/16 is the smallest diameter = more revs per mile. This effectively raises your rear diff ratio without taking the point hit.

http://www.mysporttrac.com/mysporttrac/projects/TireSizeChangesAndGearing.pdf

This is good for autocross. If this is the way you want to go, looks like the Conti is your only choice in that size.

On weight, it is a good thing to consider as well. I believe the general understanding is, one pound of wheel/tire (unsprung weight) is about the same as 8-10 pounds of sprung weight. Our classification system does not consider wheel weight = free weight savings. My stock wheel/tire combination weighs approximately 53# per corner, but what I run for a/x is approx 42# per corner. Do the math. 🙂

Treadwear ratings are only comparable to tires by the same manufacturer. A 540 rating on a Conti does not equal a 540 on a Bridgestone or other make. This number is pretty useless other than to compare a 500ish to a 100ish, obviously the 100ish is a stickier tire. But, a 140 on a BStone compared to a 180 on a Conti doesn’t necessarily mean the Conti isn’t as soft or will last longer – too close to compare like that.

Tire Rack has a boatload of tech information on their site: http://www.tirerack.com/about/techcenter.jsp

[Hal Dorton]

225/50-16 seem to fit on a 7in rim and your selection is better. Rotation is 836/mile. We ran the Kumho ASX when we drove the 98 Z3 2.8 in C Class and did fairly well. You’ve got lots of options…

Here’s some links to the TireRack tests for AS tires in this size: http://www.tirerackwholesale.com/tires/tests/chartDisplay.jsp?ttid=150 http://www.tirerackwholesale.com/tires/tests/chartDisplay.jsp?ttid=119

[Michael Kolesar]

I drove the other white E30 325i. I have had a smattering of different wheel sizes from 14,15,16,17 in wheels.

One of the best fitments for the e30 is the 16×7 wheel with a 205/50/16. This maintains the stock 24″ overall diameter of the tire as specified by BMW. As Jeff pointed out the 205/45/16 will give you a smaller diameter effectively increasing your acceleration.

For autocross I have used both 15×7 225/45/15 (Toyo R1R 140TW) and a 15×8 225/45/15 (Hankook R-S3). If you keep with the 7″ wide wheel the wheel offsets SHOULD be okay to where you won’t have to roll your fenders. With my 8″ wheels, I had to roll my rear fenders to fit the 225/45/15 tire (which is almost an inch smaller than the stock tire diameter giving me more acceleration.)

in your case finding a 225 or 215 M+S tire for a 15″ or 16″ wheel will be rather difficult. All of the ones that I saw are summer performance tires and r-comps. I would stick with the 205, a much better selection of tires, they fit better in the wheel wells than 215 and 225s.

Now I run two different wheels sets, my track set, 15×8 225/45/15 with sticky tires that comes in at 35lbs per corner.

My street setup varies with a 17×7.5 currently running 225/35/17 and also in the past and in the near future 215/40/17.

As regarding which M+S tire I can’t help you because I never put less than a Ultra high performance summer tire (as per tirerack’s description) on my car. Living in the Bay Area with a car that will never see snow or off pavement duty, it is something I don’t worry about.

Sorry if that was a little scattered feeling.

[Charlie Davis]

I like the Kumho ASX 225/50-16 the best out of everything I see… There are SCCA Street Touring people who were using this model a few years ago and doing fairly well with it (before the ST tire race sort of ess-ploded…) The Bridgestone RE970AS Pole Position and Michelin Pilot Sport AS Plus would be the others I would look at, based on tread pattern… That Conti looks like lots of very small tread blocks to squirm around to me…

Charlie

[Henry Loh]

Ooooh. Lot’s of good info, here! I like the idea of smaller diam/aspect ratio (45) with wider (225) tire… if I can find it. I’d get the grip of the extra contact patch and in theory the loss in power and increased tire flex would be compensated for by the smaller aspect ratio. I’ll have to do a frresh search in this size. (Unfortunately, I don’t have tire rack wholesale access, so I couldn’t follow the provided links.)

[Hal Dorton]

You can search for tires and click on the test link and find the same info. Sorry, can’t give out the wholesale info but can get the tires for you at cost plus shipping.

[Henry Loh]

Oh wow. That would be awesome! Thank you, Hal.

Well, it seems that the only all-season performance tire I can find in 225/45/16 is a Maxxis MA-V1 (apparently popular in Australia). Idea busted!

So now, I guess I have to decide between

Conti DWS OR General G-Max AS-03 (also in Ecsta ASX and BFG G-Force Super Sport A/S) 205/45/16 (892 revs/mi)

and

Potenza RE970AS Pole Position 225/50/16 (837 revs/mi compared to stock ~870 revs/mi)

It’s still the same question –
Grip (dry grip rating 9.4 vs 8.9 & bigger contact patch)
vs.
Acceleration (weight & diameter) & response.

Oh man, how does one decide?!?! I know what I’m leaning towards. Either way I’ll be better off than my current package, but I am curious what the greater minds would choose. Votes??

[Peter Mottaz]

Pick one and drive lot’s of autocrosses. If you don’t like the setup, don’t worry ‘cuz your tires will be worn out in short order and then you can go the other route. 😉

Pete

[Henry Loh]

True that! Too bad I cannot afford 2 sets and do back-to-back comparisons. I might end up literally flipping a coin to decide.

Also, I unfortunately won’t be able to join in on the fun this weekend… complications arose when swapping suspension this weekend. Control arm thread and nut were all jacked up on passenger side and the metal top spring retainer (front) broke on both sides…. SUCKS!!! I was in 3rd place and ready to make a move in my class, too!!!

See y’all next month (hopefully)

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