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Autocross Test - NTAXS Event #3
With the Moton Club Sports and new camber-caster plates finally on the car, it was time to test them out before switching to Hoosiers the following week. With the Moton install completed over Thursday and Friday (May 10th and 11th), the car was loaded onto a trailer and Amy and I autocrossed it on the 12th. This NTAXS event was a non-SCCA club that is run by the SCCA regulars, with the same course designers we see at Texas Region SCCA events. They used the same sealed asphalt "TMS Bus Lot" site we test at often. They do have twice as many runs in a given day (4 morning + 4 afternoon runs), so it's great event for testing.
The day started off with a freak rain storm that wasn't on any forecast or radar and caught everyone off guard. Nobody was set-up for wet conditions, including the event organizers, and a small percentage of people left the event after getting soaked during the course walk-through. They delayed the start by 15 minutes and got all of the equipment dried out, while attendees dried off and made adjustments for the rain. The Mustang was still running on the 18x11/18x12 Forgestars mounted with tread-less 315/35/18 Kumho V710s, so we knew we'd be slipping and sliding until it dried out - if it dried out.
Luckily, we worked the first heat when it was totally soaked and continued to sprinkle a bit more, but when we made our four morning runs in the second heat it was
beginning to dry. It was so wet somebody still managed to take out a timer at the finish. This club is a bit more informal than SCCA and you can grid up in paddock on your own, with the folks more experienced with this club wisely waiting until the very end of heat 2 to make their runs. Then they take 7-8 cars to the starting area at a time to make all four runs back to back. I wasn't so smart and was one of the first cars to the line, driving in slopping wet conditions. And yes, we left the wing on the rear since this wasn't an SCCA event.
It was wet as can be when I ran and my 4 runs and they were all terribly slow, including a spin on my first run. I have video from these runs, but they aren't worth watching. Amy was smarter and ran a bit later in this heat when it was quite a bit drier - and she was 5 seconds faster than me in our morning runs. The people running at the end of heat 2 were getting an even drier course and were that much faster than her.
Thankfully it was completely dry in the afternoon heats 3 and 4, and we all got some better runs in. Unfortunately Amy left the vidcam on for 2 hours after her final morning run and killed the battery, so all we have are videos from the morning session. In the video below I was riding with Amy on her 4th morning run, and continually admonishing her to PUSH the car harder. She wasn't being aggressive enough with the throttle, but during my morning ride-along she managed to drop another 1.5 seconds.
In-car video on Amy's morning run #4 (she was 7 seconds quicker in the afternoon!)
The afternoon runs were a bit better, but I didn't learn from the morning sessions and ran in the first sub-group
again. The course was dry, but now it was dirty from all of the mud that washed onto the course from the nearby construction. There was about one third of the course that was covered in dirt when I ran, which had markedly less grip. Amy wised up and waited until the end of heat 4 to make her runs and was the last car on course at the end of the day. She made her fastest run on her last, beating the entire 3R class, and set the 2nd fastest time of the day. Nice! The only car quicker was a BSP-prepped E46 M3 driven by KenO, who is one AST/Vorshlag's super-testers.
Terry (at left) was sliding around but Amy (right) was doing nothin' but winning.
In the dry afternoon runs Amy punished me by 1.3 seconds, so I was mired back in 4th place behind some SCCA regulars in SS Corvettes. But my placing didn't matter much to me - I was excited that the car was doing this well on a completely untested, new set of Motons. Getting skirted happens when your wife is this fast, and I'm used to it.
Observations from this Event
What good is going to an individual event if you don't learn something? At this NTAXS event we learned a lot about the new parts we had installed (Motons, spring rates), re-learned to never come to an event without proper rain tires (even the 275 Bridgestone RE11s would have been better on my first 4 runs than the bald R compounds), and I learned that if the club has "optional gridding" that you wait for the optimum time to make your runs! I also found out that if I over-pushed the Kumhos on 4 back-to-back runs that the tires can overheat and get very greasy. Again, watching more experienced folks that run with this club, they were pulling out of line to spray their tires with water to cool them off, something I needed to do. Oh well - live and learn.
One thing we could see from the spring rate and shock change was the front brake dive was less than before, as was front bodyroll. The extra 100 #/in front spring rate and increased compression and rebound settings we used (in the afternoon) helped there (ran the compression full soft int he wet session). Also, the new camber setting of -4.0° up front seems to be edging in to the proper amount, after looking at high resolution pictures of the outside front tire under cornering. As you know, a radial tire works best with a little bit of negative camber when fully loaded, so this picture is almost perfect. A tick more and I think we're there. I will keep an eye out at the next event (a week later at the same event site), when we've switched to fresh Hoosiers and see if it ever gets into to positive camber. If it does, we will add more static negative.
We've always run a lot of negative camber on this car, as learned from testing. Even with street tires it responded best with over -3.3° of front camber. Some folks like to argue that S197 Mustang's don't need this much camber, but I disagree. Again - as a camber plate designer, this is one of the only things I really know well.
Pyrometer data, pictures showing the tire loaded, and tire wear trump theory and internet wisdom, in my book. The S197 is not magical - it is just another strut car to me. And like many other McStrut cars we've raced and designed suspension parts for, this one needs more static camber the higher the grip levels go. With 315 R compounds up front we're seeing more grip than a GRAND AM or World Challenge race car, and we keep adding more negative camber than those race teams run to keep this big tire as close to vertical when loaded.
Another of the tests we did at this event was to look for any indicator of downforce at speed. I manged to snap the picture above when Amy was hitting over 60 mph on course and as you can see, the trunk was deflecting a good bit even at a low AOA on the rear wing (we can always make a LOT more rear downforce with a wing than a splitter can hope to match up front). We knew the culprit - the factory "trunk bumpers". These little rubber bushings are what set the height of the trunk at the rear when it is latched, and we knew from the TWS event they were deforming at speed.
We had a solution in mind and the day after this NTAXS event Ryan made up a replacement set of bushings from some round Nylon stock I had purchased the week before. Chucked them up in the lathe and made a drilled and tapped hole on the underside to mount them to the mounting plate. After tweaking the length to get the trunk height set properly, they were bolted down and now we have no more deflection there. The wing uprights sit right near the edge of the trunk lid (which has structure underneath) and this should now be a more firm mount, and still stay within the SCCA Street Prepared ruleset (normally you build bracing from the trunk floor to the wing mounts, but that's a no-no for ESP class).
My next update will go over the next week's prep, which included a change to Hoosiers and a gamble on rear tire size choice... using the largest A6 tire that Hoosier makes. I rolled the dice, so check back to see if that choice paid off!
Thanks,