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Color0's Micro RC Blog -- A technical brain dump from the mind of yours truly...
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Box Stock Mini-Z Tuning

Posted 01-31-2012 at 05:42 PM by color0
Box Stock is a Mini-Z racing class getting popular nowadays because of its low entry cost for serious racing action -- anyone can go out and buy an MR-02EX for $180, put in some AAA's and off you go! It also appeals to veteran racers because of the potential for even-performance racing: in theory, no one racer will have the advantage of the part-of-the-day cutting-edge upgrade, everyone will gravitate to the same gear ratio, same motor, same everything.

But of course, this is my technical blog we're talking about. There are things we can do to a "Box Stock" car to make the car more enjoyable and predictable to drive, and therefore better. In keeping with the spirit of most Box Stock rulesets, I'm not talking about tweaking motors to make them faster or making the spec tires stickier via special truing tricks. These tweaks are simply going to turn a decent Box Stock car -- in my case, a FOB (fresh out of box) MR-03W-MM -- into a smooth-driving, beginner-friendly car suitable for battling other FOB Mini-Z's in one of the closest racing classes I've ever partaken in. I apologize for the lack of pictures in this post btw, but at least these tips aren't really requiring of detailed diagrams.

Step 1: Wheels and tires.

Depending on the track, tire tape and truing wheels and tires may be allowed to get the cars operating within a comfortable performance bubble (decent steering and no traction rolling). Almost no track owner I know of complains about taping the tires -- it's a simple matter of helping keep the cars right-side up. When truing is allowed on top of taping, I recommend cutting the tires down to something smaller but reasonable, say like 23.5mm. Truing too small is unnecessary (this isn't Mod racing) but not truing at all sometimes leaves the car too high = traction rolling. My Box Stock car runs at RC Kenon with a spec tire package, so I have trued the front PN Slick 25's to 23mm. My PN Radial 8's in the rear are not trued, just left as is. No traction problems even after several weeks thanks to the weak stock 70t motor.

Step 2: Run in the motor and bushings.

Sometimes Box Stock classes will allow for bearings to replace the stock bushings, and in those cases, that's fine. But sometimes they don't -- and in those cases that's still fine because the stock Kyosho bushings are surprisingly smooth once run in correctly. The procedure isn't rocket science -- pop those bushings into your stock wheels with mounted tires, and spin spin spin! I don't like to use polish with plastic bushings just because I might not be able to wash it all out, leading to eventual wear and slop. Just hone out the bushings a little bit with a natural break-in process. In the rear, I recommend gently running in the rear axle bushings and the motor on the bench: put the car on a stand and hold full throttle for 10-15 minutes or so. This break-in process helps the car run smooth and fast the very first time it hits the track.

Step 3: Check other sources of binding.

With Kyosho's build qualities, typically the only other source of binding is on the older MR-02 chassis, at the front kingpin suspension. Try to work the suspension up and down to free it up, but if that doesn't work then polishing the kingpins in a dremel will be the final solution. I also recommend flipping the spring perches such that they rest against the top of the chassis' "arms" -- a much smarter setup than stock, where the perches slid up and down the kingpins. Luckily, the MR-03 comes with very smooth kingpins and suffers no such issue.

Step 4: Lower the front.

Tuning theory as covered by myself and many others teaches that a little rake towards the front increases steering, and with no rear traction issues to worry about, a little more steering means faster. The MR-03 front suspension comes with one shim above the knuckles and one shim below -- move both shims below the knuckles to drop the front end about 0.3mm. At the same time (not applicable for all bodies), MR-03 chassis kits and readysets should come with an extra pair of shims: rules allowing, I like to lower the front clip with these (good for another ~0.6mm I believe) to add just a little more rake to the setup. For the MR-02 or 02EX chassis, it's possible to lower the front ride height by removing the preload shims on the front springs. You may or may not be able to get away with gluing them to the underside of the knuckle to use as lowering spacers.

Step 5: Add a little damping.

Some tracks will not allow this, but I would push for it as it makes all Box Stock Mini-Z's ten times more enjoyable to drive. With the exception of the unfortunate RML (Rear Mount -- Low) chassis configuration, all MR-02's and 03's come with a top spring in a crude tube damper assembly. Shove a significant quantity of thick diff grease (think Kyosho 30,000wt thick!) into this top shock, and you will have sufficient damping to significantly smooth out the rear end of your Box Stock car, making it a lot more predictable in corner entries as well as high speed corners. This makes the biggest difference for experienced drivers I think as we can see the chatter and intrinsically want it gone -- but for any driver of any skill level, eliminating the chatter makes the car more obedient to commands and IMO generally improves the racing action.

If you want to take this trick to its logical conclusion, then you can achieve roll damping too to complement the bump damping you just implemented. Pop the metal ball out of the end of your top shock, and find the thickest, heaviest industrial grease that you can get your hands on. Slather this grease around the inside of the plastic ball joint before popping the metal ball back in, then roll the ball around a bit to distribute the grease. Attach the shock back to your stock motor pod and notice that there's now a tiny bit of damping action when you roll the pod. Even that tiny bit of damping will reduce the chatter you get from these cars and improve corner speed just a tiny bit.

With these little tweaks, my Box Stock MR-03 GT-R is not the fastest in a straight line, nor does it have the most turn-in steering compared to its competitors, but it is without a doubt the smoothest-driving car in the field and allows its driver to push it hard. At a recent club race at RC Kenon we held a hot lap challenge for the Box Stock class cars, and while most of the hot laps being laid down were in the 10.6xx and 10.7xx range, I was able to push my little GT-R to a single 10.0xx hot lap, turning in early, maintaining corner speed and not being afraid to lay on the throttle VERY early. That's really all it took, just five very slight changes to the car, fully allowed at Kenon, that took off more than half a second from the car's lap potential.

My final point in writing this post is that Box Stock class is a fine class for beginners, in that no tuning is expected, but at the same time it's the best environment to introduce the concept of competitive tuning -- here are the very limited areas that you can modify on your car, go play! -- without being overwhelming. Not only does doing so prime casual hobbyists for more involved and technically challenging racing, but it also educates, teaching basic mechanical sense and (hopefully) inspiring a "tinkering spirit" in a fun package deal. As such, I'm a big proponent of the class and I look forward to helping as many new racers as possible dial in their Box Stock cars and have an enjoyable experience their first time in Mini-Z.
Total Comments 1


I think there is one more thing that can be played with which is screwing or unscrewing the rear shock shaft to it's bottom post mount and changing rear droop. If you can comment on the effect, that'll be great!
Posted 02-07-2012 at 01:29 AM by mugler mugler is offline

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