[M1500Z]

June 2024 update

NASA TX | Round 4 Hallett Motor Racing Circuit

NASA TX round 4 was the annual event at —a small club track in Oklahoma. This marks 1 year of me driving with NASA TX since it was last year's event that my friend convinced me to tag along for their DE. The Hallett event is a really great example of how fun this sport can be at the grassroots level. Hallett is somewhat remote, so quite a few people camp out at the track, and in addition to the usual Saturday evening banquet, there's a competition called the Slip and Flip that involves a long plastic sheet slicked down with soap and water, some beer, and a rather unique relay race. There are some other traditions and good times as well.

The track, in my opinion, is fun, flowing, and challenging. It's 10 turns over 1.8 miles with 80 feet of elevation change. At first glance, it's pretty simple and the line is not hard to pickup. Where it gets tricky is many of the turns are blind and require commitment, and the surface is a constant barrage of bumps and cracks which makes grip less predictable. To complete the difficulty—this is a track where I'd rather not put a wheel off with many deep ruts outside the curbs just waiting to damage a wheel or worse. In my limited driving experience, it's the most challenging track for me to actually execute a complete lap.

Last year, I'd traveled with 2 sets of wheels/tires but would end up only using a set of questionable 265 RS4 takeoffs. The plan was to learn the track on the RS4s and then swap to my faster set but some weather and just general coming to grips with the track meant I only ran the RS4s and ended with a best time of 1:26.492.

At the track
I didn’t make any changes to the car from the last time out at COTA aside from taking damping out to account for the bumps and lower speeds—ok I replaced my passenger seat with a Cool Shirt system. I again brought 2 sets of wheels—both RE-71RS— one set with ~20 heat cycles and the other a sticker set. My frugality making sure I got the most out of the old set and running them on Saturday. 2 car field again, so the only person I was competing with was myself.

Saturday would be on the old tire set and the car was struggling with the front end. This was somewhat expected given the nature of the tires but nonetheless frustrating dealing with understeer and trying to work the car to rotate more mid-corner. One somewhat humorous mistake that I’d made was with the new Cool Shirt box—first hot lap of the firs session, coming out of the last corner something catches my eye. It was the Cool Shirt cooler flopping around! I’d set it all up and forgotten to strap it down ��. One thing that’s gotten more complicated is the little checklist of things to do when rolling out to grid… hopefully a lesson learned.

My fastest lap on Saturday was a 1:24.856, so about 2s quicker than I’d gone previously. Even though my tires were pretty much toast, I’ve improved much as a driver over the last year. I’ll go into data in another post, but this is a big improvement—it’s not small things. I put this down to 2 major improvements:

• Getting rid of my habit of getting to throttle early
• Continued improvements in braking

That’s very blunt, but those changes have led to better rotation and higher corner speeds.

I learned another lesson. I should have been testing damper adjustments all day Saturday but I chose not to thinking that it wouldn’t be very useful since I was on trash tires. This was a mistake. I should have adjusted compression to find an optimal setting prior to putting on the good tires. After swapping to the sticker tires Saturday evening, I would be very cautious to make any adjustments.



Sunday I had 3 sessions. It’s a long trek home from Hallett and the last session is always in the opposite direction anyway so we pack up to hit the road as quickly as possible. Because of the limited number of sessions and being on new tires, I added 2 clicks of rebound to settle the car in transitions and that was as experimental as I would get. My original thought of pulling a click or 2 of compression out might have been a good experiment but I went with what I know would work. So new tires (these were pre-heat cycled) and the car now does what I want! Within 2 sessions, I’d taken 2s off my previous time and would end the day with a fastest transponder time of 1:22.725. Oddly enough, AiM says I ran a 1:22.543 but the transponder data for that lap was about .3s off.

The whole weekend was another success and a really fun. Our team came up the bracket in the Slip and Flip competition but lost close to the final, I took 2nd place in TT3 both days (duh) and won the Time Trial Target on Saturday for most consistent driver. I’m left with some things to contemplate for the future.

[M1500Z]

Continuing a conversation about active M differential lockup from the top-times thread.

Originally Posted by E90convert

Cool data, thanks for sharing. Adding in the front wheel speeds might also give an indication of what the diff is doing. Of course, there is response time from when the ECU reads the wheel speed difference under load, to when the diff ball ramp actuator finishes it's position change and the clutch material brakes the spinning wheel.

What do you mean by the front wheels giving an indication of what the diff is doing? For that screenshot, I hid the front wheel speeds but they're usually displayed to see lockup during braking. I read that the diff is able to provide lockup faster than a clutch type diff, I went looking for where I'd seen that but couldn't find it. It's definitely quick though.

Here's an overhead view of the data from COTA. The slip % is the purple trace in the 3rd row—the higher the spike, the higher the slip %. Plotted in the same row is steering angle to see how they relate.



Digging in a little further, this is what I can glean:

• Under decel, diff slip* increases as steering angle increases
• Under accel, the diff locks* up by ~160lbft torque being delivered
• The time it takes to achieve diff lock* is achieved in .3 to .4s.

*All inferred from a math channel to calculate slip as a % using difference in rear wheel speed.



The other thing I looked at that I found interesting is using RS3's scatter plot to look at slip % vs. longitudinal Gs. This image compares 2 laps, my fastest lap from last December (left) to this year's fastest lap (right). In terms of time, there is less slip over the course of this year's lap, but there is more amplitude of slip. I haven't been able to figure out what the major difference is that would cause that.

[hjj2385131896]

hello, nice driving skill and great car build! i have quick question, how you fix the problem that M2CSR rear sway bar conflict the hose? is rear sway bar link fix that contact? i am plan to do the m2csr front and rear sway bar but the problem make me confuse.[IMG][/IMG]

[medphysdave]

Originally Posted by hjj2385131896

hello, nice driving skill and great car build! i have quick question, how you fix the problem that M2CSR rear sway bar conflict the hose? is rear sway bar link fix that contact? i am plan to do the m2csr front and rear sway bar but the problem make me confuse.[IMG][/IMG]

My understanding is that you need to use a different rear sway like the Hotchkis sport. That is my plan. I run the CSR front bar.

[hjj2385131896]

Originally Posted by medphysdave

My understanding is that you need to use a different rear sway like the Hotchkis sport. That is my plan. I run the CSR front bar.

Hello, thanks for the replay. just check Hotchkis sport sway bar on bimmerword and those seems been discontinued by the Manufacturer lol. It would be perfect if the m2csr rear sway bar could be installed directly on the car.

[M1500Z]

Originally Posted by hjj2385131896

hello, nice driving skill and great car build! i have quick question, how you fix the problem that M2CSR rear sway bar conflict the hose? is rear sway bar link fix that contact? i am plan to do the m2csr front and rear sway bar but the problem make me confuse.[IMG]

Hey, appreciate the comments on my driving and the car . In your photo, if you're referring to the fuel filler pipe and not the blue line (not sure what that blue line is), then no, I haven't fully resolved it.

What I did was remove the fender liner to gain access to where that pipe mounts to the chassis. It's a plastic bracket that I slotted with a drill bit to allow me to push the pipe further away from the sway bar. The next thing I'll do is take a piece of PTFE like a piece of a plastic oil container and zip tie it in place as a barrier. I think that'll resolve the issue for the foreseeable future. If the bar rubs through the PTFE, then I can replace it, but that's doubtful given how slick that stuff is.

Eventually, I'd like to figure out the rear bar stiffness and see if there is another aftermarket option that I could swap to that would mirror it, but until then, I just check it when I check the rest of the car between events.