Last weekend, I attended the XRS event at the great Cotswold track in the UK. The grip level at this track is medium level. The race was run under BRCA rules, which meant that we were running the Rush 32 (SPC32MR2) tire with the Spider Grip Red additive. Although I do have a lot of previous experience from this track, running the X4 platform for the first time on asphalt in the UK, together with a spec tire that is radically different, it was a steep learning curve, but very productive in the end, with big improvements in performance found. I will share our best findings, as well as my final set-up sheet:

The Rush 32 tire is very soft, and together with a rather soft wheel, it makes the car feel quite "squirmy" going around the track, they make the car move around a lot, as the soft tire aren't supporting the car too well. The balance of the car, compared to when using a harder 36 tire for example, shifts the balance from more rear traction, to having A LOT more front traction, especially when tires are new. This meant that several set-up changes were needed from our traditional set-ups, to improve drivability and consistency.

Basically, everything I did to remove steering, and/or improve rear traction, made the car better and faster to drive, as I was able to get on throttle earlier out of turns, without losing the rear.

The best improvements were:

More front weight bias: 50/50 instead of 49/51. Made the car understeer more, but made it less pointy to drive, and allowed me to drive the car harder. It also made the car "track straight" better down the straightaways.
More rear toe: 3.5 degrees instead of 3.0. Once again, improved rear traction and made the rear more stable, mainly on the corner exit.
Stiffer rear spring: 2.9 instead of 2.8. Improved rear stability, particularly off-power, and meant that I could push the car harder with more confidence. Forward traction was also improved as the rear was squatting less. This was an especially good change for used tires.
Less rear flex: Non-cut rear topdeck, T-brace on chassis. Reduced the over-rotation that was occurring on corner exit.
Lower servo horn height: On the soon to be released adjustable HUDY servo horn, I lowered the height from 19mm to 17mm, which made the steering a lot less sensitive, which helped a lot in the center chicane section, as small steering inputs didn't upset the car as much.
O-ring on rear shock shaft as bump stop: Reduced squatting on-power which improved forward drive out of corners. Less front camber, and more rear: Took away a bit of the oversteer and moved the grip balance from the front more towards the rear.
Softer rear diff: 6K instead of the typical 7 or 8K. Improved steering in the 180-degree corners as initial steering improved and reduced over-rotation coming off the corner.
Topdecks: It's important to not run too much rear flex, hence why – no cuts in the rear deck, and 2 screws installed. We had a good feeling with the 1pc topdeck as well (301070) – this topdeck gave the best drivability in the center chicane as it made the car "follow the track" the best.
Bodyshell: The Speciale was preferable over the Twister as it had a bit more side bite and forgiving handling compared to other shells, combined with a Twister wing (with side wings). Remember to use side stiffeners and rear anti-tucks if you're using an UL version of the body.
Tire preparation: The method that I found the best was:
- For new tires: Heat up tires for 2 min, then clean with brake cleaner. Apply additive. Warm for 5-7 min at 65 degrees. Remove warmers and re-apply the additive and add tissues. Warm again for 5-7 minutes at 65 degrees. Remove warmers as late as possible before going to the track.
- For used tires: Same procedure, except, clean tires with the additive instead of brake cleaner for used tires.

At the end of the day, all the above changes made the car a lot more competitive, where it was both and fast easy to drive. If I had more time, I am sure that I could have improved the car further with more fine tuning.