[B][U]Vail Valley National Tour update - the battle of scrub:[/U][/B] Amy and I went to the Vail Valley National Tour with the EVO last weekend. We got back on Tuesday but have been playing catch up all week, so I'm just now adding our event report. It was a tow from hell getting there... 16 hours straight thru, after working a full day. Towing over several mountains including an 11,000 foot high pass wasn't fun in our truck, either. We got there Friday morning at 7:30 am, and Amy was entered in the EVO school at 8:30 am. So we grabbed an hour of sleep, our only rest for 2 days, and spent the next 3 days testing and racing.
Weather couldn't have been better - it sure beat the 100°F heat we had in Texas! Beautiful mountain views surrounded the Eagle Airport site as well. We had been battling some serious understeer the past several weeks, which I will touch on in a bit. Amy got a lot of test runs during the Evo school, but I think she could be teaching at these things by now (she has instructed at several driving schools in the past and has more wins at Nats than many of her former Evo teachers), but she says "it never hurts to get an outside perspective" and she got a good number of laps in before a freak downpour at 11 am. It dried quickly and (co-driver) John Scheier and I started taking laps at around 2 pm. The car was pushing as badly as ever, so we quickly headed for the pits and made some big changes between the many test n tune runs we made for the next several hours.
We swapped rear swaybars (from a Cobb hollow to a Works solid), adjusted all of the shocks, changed the front camber by a full degree, tweaked the front toe, and made laps in between changes. Nothing helped. We could get the turn-in right but as soon as the front wheels turned more than a few degrees the front end would wash out. The tires were howling, and we had no front end grip. Front tires were SO hot, rears were dead cold. Frustrating! Slaloms felt OK but the many 180 degree turn-arounds we had to drive through all weekend on the narrow airstrip courses were murder. Losing so much time in the big turns!
We've been playing with all sorts of adjustments on the car of late to try to re-discover some seriously missing front end grip. Ever since we switched from the 265mm AD07s we've been fighting a serious push, but the lack of performance wasn't as apparent at higher speed events (time trials/HPDE) nor at local autocrosses, where we still seemed to be doing well overall (but slowly slipping back in PAX). Too many spring rate changes to count, ending up with 700# fronts and 1000# rears, which seem a bit extreme because they are. On top of this push, the formerly flawless SST S-Sport automatic shifting mode was acting funny at this event for the first time ever, probably due to the 7000 foot elevation of the site? It needed coaxing to hold a gear and not upshift at odd times. Never done this to us before, so this was a mystery. We went to full manual shifting on some runs.
[I]285 Yokohamas and 18x10s on an E36 M3!? Mayfield was FAST[/I]
At this Tour event we had a lot more competition (19 cars in STU!) and it was fairly obvious that the car was off the pace with all 3 of it's drivers. It wasn't until we got back that one of the obvious deficiencies was found - tire pressure. [B]Look at this picture:[/B]
That outside front tire looks like it is about to come off the rim. We were running the sub 30 psi pressures that had worked so well on the wider 265mm AD07s. We'd noticed odd tire wear on the 245s lately though - it as wearing the insides more then the outsides up front. WTF? There was something else going on that was masking the lack of tire pressure that is so obvious from these pictures. We're running 40 psi up front on future testing, starting today. 3700 pounds with fuel and driver is too much for 29 psi. It may have worked on the 265s, but not on these 245s. We also noticed a lack of dynamic camber in higher angle turns, not readily apparent until you study multi-sequence pictures from the same turn. It needed more caster.
Anyway, we fought the car all weekend, barely maintaining our mid-pack finishes in this field of STU heavy hitters (driving our butts off to get 10th an 13th places in Open). Amy still won STU-L somehow, but she was struggling more than normal to stay in front. She had a "mystery cone" assigned to her fastest Saturday run that knocked a cool second off her next best time (I watched that run from start to finish - it was bogus) which made it too close for comfort. We tried all manner of changes all weekend, even adding as much as 1/2" of toe out in the rear to try to get this thing to rotate. Nothing made a difference, which just didn't make sense. Something else was fundamentally wrong. We finished our runs, taking our licks, then packed it up for an 18 hour tow back to Dallas (record traffic jam at the Eisenhower tunnel made for an extra 2 hours getting from Vail to Denver - yay!)
On our way home we were scratching our heads, looking for answers. The video and data showed what we already knew - there was no front end grip. We had plenty of static camber and tested there with the the extremes. Hanchey called another engineer/racer friend of ours (Matteucci) and they keyed in on the many changes we've made to the car this year. We went from 4200s to 5200s (to test both AST strut designs), changed spring rates many times, played with various camber and toe settings, multiple swaybars, and more importantly used 3 different sets of wheels and 4 sets of tires in 2 widths. We also went back and forth from track to autocross setups, sometimes week after week. The problems seemed to follow the 245mm tires, and more specifically the 18x10" Enkei front wheels we've settled on lately.
[I]Kazera 18x9.5" wheels are 3 pounds heavier, but reduce front track width by a full 2", reducing scrub![/I]
Remember from earlier posts in this thread that the Enkei RPF1's we have on the front (6.7" backspace) had even less offset than the 18x10.5" NT03's we've been running for track events, and these RPF1s also needed 15mm of spacer to clear the calipers. The black CCWs on the back had more backspace as well (7") but couldn't clear the 14" front brakes with any amount of spacer. Well this weird offset issue with the front wheels was causing a terrible scrub condition, moving the centerline of the tire further away from the Kingpin angle defined scrub radius. I rechecked the toe at the event and noticed the front track width was alarming wide... nearly 74 inches! By contrast our old STU car, an E36 M3 with 18x10" wheels, has only a 67" front track width. Even our V8 powered E36 race car with 315mm tires has only a 72" track width! WTF?
There are ways to improve weird kingpin angle/scrub issues, namely adding more positive caster. We already had a much improved over stock +5.6° of caster, but our three previous BMW racecars have always been able to achieve +7 to +9 degrees of caster, with the steering feel and dynamic camber that were much better than what you typically see on the AWD cars. They come with much more caster to start with, and we improve on that with our plates. So the first thing I did when we got back from Vail was to make a new High Caster version of our camber-caster plates. We had also just made a short production run of new HC1 plates for the GD Subaru and EVO 5-9, some of which are already on testers' cars. This EVO X version made this week was hand built and adds .400" of more caster offset, which should add another degree and a half of caster, getting us closer to +7° (we will verify with a laser alignment soon).
We also dumped the RPF1s and CCW wheels this week, as they just are not the right fit for this car at all (CCWs are sold, the two RPF1s are for sale). The 10.5" wide NT03s are too wide for the little 245s we're restricted to for STU class, so we went looking for an 18x9.5" ET30 offset wheel... and found them from one of our employees! Stuart here at Vorshlag had a (discontinued) set of Kazera 18x9.5" ET30 wheels from an S197 chassis Mustang and they fit the Evo nearly perfectly. We're using a 5mm spacer in front (to clear the tender spring we need to run for use with a test sensor - a shock pot) and no spacer in back. The width change alone knocks out a full inch of track width plus losing a total of 20mm of spacer width is another inch less track/scrub... so track width in front is a full 2" narrower now. This is a HUGE change, and a GOOD thing when you are trying to slalom around cones. Every inch removed from track width can make significant time improvements in autocrossing.
These wheels aren't the lightest thing we've ever run (21 pounds) but they clear the brakes better than anything other than the NT03s, and due to the proper backspacing they fix much of our scrub problems. The added caster from the new plates was obvious on our first test drive, with improved steering feel and the most important improvement - the car rotates now! I could saw the wheel and break the rear tires loose at speed, which is so much better than before. No more howling front tires, as the scrub problems in front seem to be fixed with the new wheels, and the increased tire pressures feel better as well.
So now we're sort of back to the drawing board on the rest of our Autocross setup... we've started by resetting the adjustable swaybars on both ends, changed camber again (from -4 to -3°), and have to settle in on the spring rates now that we have the other fundamental issues improved (but it feels pretty damn good with 700/1000 - corners flat!). This scrub issue has probably been holding back performance for a couple of months. Good grief - we've only got a few weeks until Nationals, too. Today Brian also swapped back to the RacingBrake pads after he opened up the struts and put them back on with the new plates. This moved the car away from the ultra-hard race compound HTC-70 Hawk pads in front, as they take too much time to get up to working temperature. Great for the track but not so good on your first autocross run (we can't just "drag he brakes coming to the start line" due to the SST trans, either).
We've already scheduled two more test events in the next 3 weeks, so we've got some work ahead of us. If we can regain some of the performance we had earlier this year we think the STU competitiveness will return. To top off the week we have also had to pull the trans from our V8 powered BMW, as the [URL="http://www.vorshlag.com/forums/showthread.php?t=5479&page=9"]input shaft decided to leave the bellhousing at 155 mph at TWS last weekend[/URL]. Brian was racing it at a NASA Time Trial... good times.
Tune in for more test results soon...