THSCC Forums

Thanks Dan. I agree that a front bar would help. We would also gain some rear traction on this car on corner exits with a stiffer front bar since it has an open differential and loves to spin its inside rear tire! Since this is stock class, I'm concerned with going to a stiffer front roll bar for the rest of the time (99.9%) the car is driven since it would shift the overall chassis balance to even more understeer, especially on turn in (and in stock class I can't change the rear bar of course).

For the time being, I'm going to setup the alignment better and also go back to the wear-forever Dunlops.

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2

Chuck,

I have no idea what line you tend to drive and/or how much steering angle you use when the front tires have reached their limit. Long constant radius lines can really stress front tires, especially if steering angle is increased to deal with understeer. So can too high a corner entry speed and late braking.

However, one way to reduce the load/wear on front tires of rwd cars is to avoid long constant radius lines. It can help to take a line where most of the turning is done very quickly and then you "open up" the steering angle. This is commonly called either a late apex line or in some corners a "diamond" line (early apex entry, late apex exit). A bonus is that when you turn the steering wheel quickly for the tightest part of the corner the rear tires can tend towards oversteer, especially if you do the turn-in while still on the brakes which has the rear tires at a more positive camber as well as being unloaded. The resulting oversteer tendency will easily be caught by the combination of opening the steering angle and applying power.

In any case try to take a line where you are not tempted to crank in more steering angle when the front tires start sliding, either under braking, while at neutral throttle, or when applying throttle. Remember that when you apply throttle with a rwd car you unload the front tires which can result in power UNDERsteer unless you also unwind the steering.

One of the real tricks to fast rwd autocrossing is to spend as little time as possible at your maximum steering angle, even if it means that the maximum steering angle is greater (but the time at that angle will be very brief).

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Dick Rasmussen

FS 50 2018 Mustang GT

Yeah, I'm opening up steering angle it I'm past the peak of the tractive force/slip angle curve and the front tires are starting to slide. I rarely get caught out wasting time srcubing off energy; however, my daughter is not up to that level quite yet. However, I will say that this is the first car I've autocrossed since 1978 without a setup suspension (i.e. springs, bars, camber plates, etc..) and no limited slip diff, and NCAC was just my 4th time autocrossing it, so I'm still climbing the learning curve with it.

Thanks for your thoughts Dick. I think the alignment changes will help a lot on the understeer nature of this car's setup since the tail is stuck like glue right now no matter what you do (it's even worse with the street 225 front/255 rear stagger tires too).

Cheers,
Chuck

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2

I have an aftermarket rear bar and i run it full soft.

I know it defies 'classic' sway bar theory, but i really think a bigger/stiffer front bar all by itself will correct the front contact patch and actually reduce understeer.

What he said!

Thanks, Dick, for logically explaining something I have been doing while driving the (understeering POS stock) M3. I have been telling people that to make the car go fast I need to do some really stupid things. Your explanation is far more eloquent

Chuck, FWIW, I corded my rear tires before my front -- on a stock BMW. Trick is always to get the back tires slightly spinning and the front tires out of scrub-mode. Got 70 runs out of my 710 and I don't exactly drive easy. Hoosiers -- no thanks...

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

I bet this guy would have corded A6s in a hurry.


Holden? V8 drifter

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George Bright Member # 141
One of the youngest of the old farts.
Yes I voted against you joining the club.

Mike,

You may remember that I was initially going to with the V710s. I bought them and ended up sending them back in exchange for the A6. The 245/35-18 710 just would not air up on my front 8" rim. Grover McNair tried every trick he knew from ages of mounting every type of race tire, etc. We got help from a Kumho engineer who I think actually learned of a few new potential methods from Grover. If the wheels weren't essentially brand new and pristine, and I didn't want them damaged in any way, perhaps we would have been able to have success with some additional methods...dunno. Anyway, the issue resulted in buying the A6's.

The Kumho engineer said that the tire was "not really" designed for use on an 8" rim (that is just the DOT req'd standard rim width spec, 8-9.5", for a 245/35-18 they have to quote). He said they have successfully aired up this size on an 8" rim, but on some rim types it may not be possible -- he said they have had trouble getting them to air up at Kumho also on 8" rims. He did not recommend using the tire on an 8" rim.

BTW, the relaxed width from bead-to-bead of the Kumho 245/35 was 9 5/8", and the sidewalls are extremely stiff and short. I tried to compress them as much as possible and was able to measure bead-to-bead of just under 9", but they got REALLY stiff at that point, so I had concerns about the mounting before I took them to Grover. The Hoosier A6 mounted with zero trouble on the 8" rim and immediately took air and seated the beads -- expected since the Hoosier sidewalls are MUCH more flexible than the V710.

The old Dunlop SSRs are back on the car now, so we can drive to events with them on, never have to worry about spraying them, and they will never wear out. I won't be nearly as competitive with these rocks, but it's all for fun anyway, right?

Chuck

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2

Chuck, we found the same issue with 245/35/18 710's on an 8" rim. We got 2 to mount, but 2 wouldn't take air. The guys at Discount Tire in Greensboro worked on them for a while and got them to take air. This was after about 6 hours of work.

Donnie found that the warmer the ambient temp is, the easier Kumhos are to mount.

About a week ago, we mounted 285/30/18 A6's on the same set of 8" rims. We had them mounted and balanced in 1 hour and 20 minutes I couldn't believe how easy it was compared to the struggle we had with the 245 Kumhos. - AB

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'14 Toyota Sequoia Platinum 4WD
Super Westerfield Bros - '93 Integra - LeChump Du Jour
STX 93 - Scion FR-S

Sounds familiar. Grover left them in the full sun (it was mid-90's that day) for a couple of hours, but it wasn't enough. Grover spent over two hours of effort on that one tire trying to get it to take air (not counting the sun time). He didn't charge me anything for all that effort -- great guy who is dedicated to doing it right. As an aside, he and I used to battle each other in D-Prepared back in the old days of THSCC auto-x -- circa 1979-1980. He was in an older 1960's Volvo with racing slicks, and I had my '76 Capri. We had some awesome battles for 1st often separated by a tenth of two.

You know, from a tire engineering point of view, it is really interesting how these tires can be SO different (V710 vs Hoosier) in design, yet perform very similarly. That makes me think there is still a good ways to go with performance tire engineering. I sure wish the 710 would have worked since I believe it would perform better on a stock class suspension 330i (i.e. should be within margin of error performance with the A6 while lasting a lot longer).

Chuck

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2

Bringing this story back to life a bit...

We're finally getting around to installing a bigger front sway bar on the car this weekend, and I'm getting the front aligned for 0 toe next week. I'm also going to make sure the very slight front camber adjustment this car has is full maxed out negative (it was at last alignment).

Just to make it interesting, we're making more than one change at a time. Laurinburg finished off the 225/40 Dunlops as we limped home with the LF corded, and in interests of the upcoming Evo school I didn't want to go back to the A6 just yet. Hence I had another set of these SSR Dunlops hanging around the garage (these things wear forever!) that I had run on the M5 -- 265/35-18's, slightly worn from a couple of HPDEs and two autocrosses. The M5 normally runs 275/35's, but the SSR was only available in 265 size, and they are just as wide as a 275 street tire. I wasn’t sure if they would fit on the 330i at all.

Amazingly, they fit on the car with no clearance issues. The tire to strut clearance up front is fairly tight, but it is fine. I had Discount Tire up by Wakefield do the mounting, and I was very impressed with the guys in the bay doing the work...they greatly exceeded my expectations. It took a bit of work to get the fronts (8" rim) to seat the beads, but the rears (8.5" rim) went on easy.

In any event, these will give us some long wearing tires for the Evo school (I'm going to Extreme on Sat and Jackie is doing Phase 1 on Sunday). The A6s are still bagged and waiting to see daylight again perhaps later this year. At the moment, we get to extend the fun part of being able to drive to events on the tires.

It is going to be interesting to see how the 265's work on these narrow rims (they've been on 9.5" rims on the M5). When you think about how the outer edge of the tire is (not) supported by the narrow rim, and how the tire sidewall is pinched inward at the bead, it makes you wonder what the heck the tire contact patch is actually going to end up doing.

I'm at a total loss on air pressures with these. I have been targeting 38 hot on the 225's. I guess I’ll shoot for 36-38 range hot again. Greenville is going to be an interesting event to say the least. If anyone has gone up by 40mm in section width on the same rims, chime in with your experiences please (tire feel, air pressures, wear, everything).

Chuck

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2