Project Update for June 13, 2011: We had two autocross events on the weekend of June 4-5, where we tested the new engine tuning (throttle mapping) and some new wheels/tires, with the first real promising results for the car in STX class to date. Let's start with the new rear rubber first, then cover the two very different autocross events separately.
17" Rear Wheels and Tires
After the very positive results we had with some very worn and borrowed 18" Dunlops at our
Tire Test in April, we wanted to try some
fresh 265 Dunlops. We thought about just plopping down for another set of 265/35/18s but instead we tried something different...
Dunlop has the only ST legal/competitive tire that is a 265mm max width for STX in a 17" diameter. This might be one of the only STX cars that can physically fit these 265/40/17 Dunlops under OEM fenders, too. (I've seen these used on an STX classed E36 or two, but those cars had massive compromises in ride height to clear these uber-wide tires) I'd love to run 17" diameter wheels and tires on the Mustang at all 4 corners, but the Mustang's (optional) front 14" rotors and Brembo calipers don't allow this.
Up front. But on the rear...
Hell yes! The rear brakes are small enough to allow a 17" wheel to clear, so we bought a pair of wheels and got some 265/40/17 Dunlops mounted to them. The wheels are lighter (TireRack calls them "15.9 lbs") but the mounted 17" wheel and tire (45.0 lbs) is 3 lbs lighter than the
18x9" Enkei FP01 and 265/40/18 Yokohama (48.2 lbs) but is 2 lbs heavier than the
18x9" WedsSport and 265/35/18 Toyos (43.1 lbs). The Toyo R1Rs are way too soft for use on these big cars, so that lighter tire has its drawbacks. Keep in mind the
OEM 19x9" wheel and 255/40/19 tire was 57 lbs, so its still 12+ pounds per corner lighter than that. The 265/40/17 Dunlop is actually .1" taller than the 265/35/18s we had been running, so we don't lose any tire height or alter gearing this way.
But we didn't go to the 17" Dunlop for "lightness" - we did it for
increased sidewall height and lower cost. The dollars and cents are easy to understand: its $100 per tire cheaper to get the same exact Dunlop Star Spec in 265/40/17 vs 265/35/18. That's $400/set, and hard to ignore. But we've wasted $3K testing various tires and wheels on this car this year, so cost is pretty much thrown out.
Corner Exit Acceleration is the biggest aspect we need to improve on this car (and most autocross cars, for that matter), and we had a long way to go on the Mustang. Throttle mapping was part of it but we're looking for other ways to improve this acceleration direction. The thought that a taller sidewall could reduce some of the shock load to the rear tires crossed our minds. Going to the 17" tire allows for more sidewall height and potentially less spring rate in the tire sidewall.
There are "rules of thumb" regarding sidewall aspect ratios and in one book (
ThinkFAST) race engineer Neil Roberts (Costas' close friend, and an old college racing buddy of mine as well) claims that he likes to use nothing shorter than 40 series tires. Sometimes we don't have a choice, but this time, we did. So we tried it. Looks goofy with 19" wheels on the front and 17s on back (above left), but the 18/17 combo (above right) doesn't look quite as jacked up. We've seen other autocrossers mix wheel diameters front to back, and even OEMs do it (but usually the other way around). So I had planned on running the 17's on back during the first 3-4 runs of the NSTC event and switch to the 18s for the last half. The best laid plans...
National Street Tire Challenge, Saturday June 4, 2011
Note: I've re-written this NSTC event summary several times, trying to be less negative of the course layout and the event in general. This is considerably "dialed-back" and as "PC" as I can make it. I do appreciate all of the hard work that went into putting on this event, and my main gripe is really just the uber-tight course. This is my brutally honest assessment from a 24 year veteran of SCCA autocrossing. Remember - I'm also driving in a very wide car with lots of power and limited rear grip - the proverbial bull in a China closet on a course like this - so that paints my views in a certain light. I wasn't alone in my harsh criticism of this course.
70 cars were there for Saturday's NSTC event, which was an autocross strictly for street tire equipped cars (140+ TW).
Great idea for an event, as I'm a big supporter of the various ST classes and feel they are pushing racers out of dated and to some extent
broken classes, like Stock and SP. For this series (4 events in 2011), run by the Milwaukee SCCA Region, they have 4-5 classes (all PAX based - my
least favorite way to create a class), prizes for each class, and a free set of Bridgestone tires to the top PAX time of the event. Well.... except for any of the Street Touring Shootout class cars - they were not eligible for the PAX ranking or tires. That wasn't spelled out very well (none of the classes/rules were very clear to some) and some competitors felt a bit slighted by the way they handled that. Oh well, now we know - don't enter the PAX-combined ST class if you want to win the tires. And don't show up without an AWD car, either.
We noticed some serious issues as soon as we walked the course. The course...
the course was BAD. It was not representative of anything I would call an "autocross", as it was more akin to a gymkhana course, with lots of stopping, crazy tight turns. The course drove over the dirtiest parts of the event site, with 2-3' tall grass over many sections - they needed a lawn mower on this thing.
Not kidding. It "walked" so tight and un-flowing, but it
drove tighter than Hell. It is
hard to describe how bad this course flowed, you just had to drive it. I came to a near stop about 6-7 times on each run, there were never two corners that flowed together, every slalom cone and offset was spaced/timed differently from the one before, and the course was heavily biased to narrow cars as well as AWD cars. For having 40+ acres of asphalt to play with, and for making a 100+ second long course, to never get over 50 mph? That's just wrong. It was dubbed
"The Milwaukee Beast" by the Miata drivers. Yes, the Miata guys thought it was a big, long, tight, nasty mess.
After making my morning runs I couldn't understand how our local course designer JJ could have made such an
abortion of a course. I found out later that JJ
had almost no say over the layout this time; the NSTC event organizers walked the course and made changes to every corner, insisting that it be super tight. A good 90% of the drivers I talked to
hated the course and vowed to never to go to another NSTC event.
The throttle mapping WAS much better, but this course made everything feel bad. A go-kart would have felt big and cumbersome in this mess. It was also very hot outside, and with such a long damned course the rear tires got greasy halfway thru the run if I allowed any wheelspin at all. I was spraying the rear tires 3 times after each run, fighting for rear traction through the many tight corner exits. It felt pretty loose and disconnected on the 17" Dunlops and
almost as bad on the 18" Hankooks (which I ran up front all weekend and out back after my 4th run Saturday). I did a quick pit stop tire change after run #4 of 6 Saturday, and was faster on the 18" Hankooks, but its not a fair test - this course shouldn't be used to judge anything about any car's handling. The results from the rear tire testing really have to be thrown out, this time.
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