The night before the Circuit's first race I was still struggling with the rear suspension. I suspect that it is indeed the AKA tire inserts that are too firm and they are causing a bounce before the suspension has a chance to properly work.
In this photo, I'm using Losi orange (2.9 rate) springs. I switched to yellow (2.0 rate) and this reduced the "hop/bounce" of the rear when the car was dropped onto a bare concrete floor. Since it was getting late I decided that would have to do since I'd really need to spend time at the track to properly tune the suspension... this was all "theory and no practice." Literally.
Then I double-checked the toe and camber using some old wheels.
2-deg negative camber front and rear with 2-deg toe in. A relatively neutral starting point.
Final prep and assembly. (Temporarily switched to Losi red springs in the front and orange in the back.) All components are in place and have checked out.
Now can I say that I have the beginnings of a "fleet" of race cars? The TLR 22 3.0 (an excellent buggy) and the ECX Circuit, ready as they'll ever be for race day. (The TLR 22 is such a boring car. Easy to drive, handles like a dream, nigh indestructable.)
Unfortunately I didn't arrive at the track early enough to really experiment with suspension setup and really only had time to get a feel for how the truck would drive on the astro track. Initial impression? Yep- It's a rear-motor ECX car. Jumps just like my Boost does; nose up.
However, the Circuit handles MUCH better than the Boost, primarily due to it's much wider track and slightly longer wheelbase.
The truck liked to push and was taking right turns quite a bit wider than left. After my 2nd heat I was able to tweak the endpoints and get that taken care of, but the truck was still pushing (understeering) a little in the turns. By the third qualifying heat I was getting a good feel for how the truck handled so was crashing into walls less. It didn't help that the track layout had changed since my last race two weeks ago and featured a couple of challenging sections: a single jump just before the entry of a hairpin turn to back straight and a super-tricky double-jump to tabletop to single-jump that gave even the Pro-Stock racers a tough time. And lining up the jump wrong would put you right off the track. We had to have extra marshals by this section and they stayed busy putting cars back on the track!
Racing two classes kept me busy making sure that I switched profiles on my Spektrum DX4C and move the transponder from car to car.
I discovered* that the Hobbywing 3650 motor has a relatively short shaft (My Reedy M2 did not have this issue when it was mounted in the Boost). Sliding the Robinson pinion onto the motor shaft as far as it could go before running into the motor mount plate only let the set screw engage with the very end of the motor shaft. This caused the pinion to sit askew on the end of the shaft. Not a race-stopper, but not something I want to continue running for too long. Looking for a quick fix in the track shop, I was recommended
175RC's "PolyPro" pinions. They look just like slot-car spur gears and feature an aluminum hub with plastic (polypropylene) teeth. These are VERY low profile and solved my problem while cutting pinion weight by 60% over the steel Robinson pinion. Durability is a question though, but asking around and I've only heard good things and they seem to be quite strong (they are recommended for 17.5 and 13.5 only, but some Mod racers running 10.5 and lower were using them with good results). I'll run the pinion (with a slightly looser slipper) until it self-destructs. I plan on taking the Robinson pinion to a bench grinder to remove some material from the tooth side of the gear.
*In Stadium Truck heat two the Circuit just stopped after landing on the tabletop. Wheels responded to the radio, just no driving. The marshal pointed out that the pinion had fallen off! (Since it was indoor I was racing with the gear cover off.) At the end of the race I retrieved the pinion from its resting place at the side of the tabletop. 
After racing was done for the day I took third in 2WD Stock Buggy with the 22 and... last place with the Circuit. That's okay. The truck had just undergone a HUGE transformation and it handled significantly different compared to my TLR 22 3.0. Interestingly, the winner in the truck class was driving a Traxxas Slash with a beat up VW Beetle body! The other trucks were AE T5s. He was also a Mod Buggy racer and winning that class which just goes to prove that it's not the car, it's the driver!
I spent some time after the track opened up and got the Circuit's front suspension dialed in. Then I was able to take turns much faster and stay centered on the track. I still have to exercise more care when jumping as the truck likes to point its nose to the air (and the jumps all had very short transitions which had a tendency to kick the cars straight up).
Racing the Circuit was a blast and I look forward to the next race on March 4.
