2014 - Losi Mini SCT
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I finally own a short-course truck. Given all the mini build threads I've posted, I'm sure it comes as a shock to no one, it's a mini.
I now own three vehicles based on the Losi Mini-T drivetrain, which I guess is about as glowing a recommendation as I could offer for the platform. In all of my bashing of my three Mini-T-derived vehicles, I've broken two stock parts: a plastic front hub, and a brass ball-stud. Admittedly, that's because I've upgraded everything else that could break in a crash, but still. (also, I broke one titanium turnbuckle and burned-out several motors as well, but those were all upgrade parts.)
Since I have fairly extensive experience with the Mini-T platform at this point, I knew going in what most of my mods were going to be: GPM aluminum hubs, Boca Yellow Seal ceramic bearings, Dynamite metal gears, a MIP ball-diff to control wheelspin in corners, a proper Teflon-lined slipper clutch from Losi (Teflon is the best material for slipper clutches, because it doesn't stick before starting to slip), a GPM metal gearbox casing, Losi aluminum shocks, Traxxas Mini Slash tires on Mini SCT rims (for better tread-life), Losi adjustable turnbuckles, GPM titanium universal-joint axles, a Tacon brushless motor, a Turnigy Trackstar ESC, and a Hitec HS-5055MG metal-gear servo. Piece of cake, right?
I also replaced all of the Losi ball studs and cups with Associated ball studs and cups, because I learned from my experience with my Mini Rival that the Associated parts are made better. The Associated ball cups hold tighter, and they have little holes drilled into them so the ball studs can be unscrewed without needing to unsnap the ball cups, thus preserving their ability to stay securely attached to the ball studs. You can kinda see what I'm talking about below (and you can see the metal hubs and the GPM titanium axles too).
A slightly different gearbox design vetoed the metal gearbox casing upgrade, but everything else was doable, though the slipper clutch did require some shortening to get it to fit. Interestingly, the only reason I was able to shorten the slipper clutch's propshaft is because Losi thoughtfully made the screw-threaded portion removable, though I did have to blowtorch it to burn away the permanent threadlocker used in its assembly.
As you can see from the modified spur-gear cover (the central dome from the dual-disk slipper clutch's chintzy Lexan cover was inserted into a hole drilled in the middle of the stock cover, for a cleaner fit), the modified slipper clutch fits perfectly once shortened to a suitable length. That's a good thing, because the Mini-T's stock slipper clutch has always been a huge pain to adjust, partly because the spring-washer design was way too sensitive to tiny adjustments, and partly because it was nearly impossible to keep the adjuster nut from loosening over time.
So, the not-quite-Mini-T gearbox posed a few unique challenges, but nothing a Dremel and a few minutes of careful measuring couldn't solve. Instead, the really tricky challenges with this build came from the vehicle's handling. Structurally, the Mini SCT differs from most other Mini-T-derived vehicles in two important ways:
1) the motor is mounted in a mid-rear position, instead of a full-rear position, which changes the way weight is redistributed as the vehicle accelerates;
2) the chassis is borrowed from the Mini HighRoller, so it lifts the center of gravity considerably.
Both of these factors significantly affect the Mini SCT's handling, and most of the tinkering I did was related to that. In some respects, it's a good thing I started my Losi mini collection with a different vehicle, because I probably would've given up on this one if I had to deal with its handling problems with no tuning experience.
Unlike my other Losi minis, my Mini SCT handles best with stiffer springs and thicker shock oil in the front, instead of in the rear. This is because the Mini SCT's weight balance is more neutral, meaning the front tends to plow into obstacles (like speedbumps ) instead of bounce over them. Without stiffer springs and thicker shock oil in the front, it would bottom-out the front suspension on every jump, landing, and corner -- and having the front suspension lean too much in corners would make its handling problems significantly worse.
In the front, I cut-down a set of 1600g blue rear springs to fit the slightly shorter front shocks, which increased the stiffness of the front suspension significantly vs. the stock 1200g front springs. However, in the rear I ended up keeping the stock springs, because I really needed the rear suspension to be compliant in corners and keep both rear tires in contact with the ground as much as possible.
I noticed that the stock springs were cut shorter than the actual shock travel, perhaps to lower the ride height, but without travel limiters in the shocks, it just allowed the springs to rattle freely over jumps; that is almost certainly the reason people had so much trouble losing their lower spring retainers with this vehicle. I wanted the full shock travel, so instead I opted to fill the gap with preload spacers. I haven't noticed excessive bottoming-out on the rear suspension with the soft stock springs, but if I do, I'll just increase the shock oil from 30wt to 45wt in the rear.
Tuning the suspension helped somewhat, but after fiddling with the springs and shock oil for a few days, I realized there was a different problem -- weight balance. The rear wheels handle all the acceleration and braking, and they're supposed to keep the rear-end following obediently through corners, but in stock form the Mini SCT has almost a 50:50 weight balance -- not good for rear-axle traction. So, in a departure from what I've done with my other Losi minis, I put weight in the *rear* instead of the front -- and a lot of it, too.
That's 12 little blocks of lead ballast, for a total of 3 ounces of extra weight behind the rear axle. That's 2x the ballast I used in my ECX Smash, and 3x what I used in my Losi Mini Desert Truck. All that weight is necessary to give the rear wheels traction, because the gravitational-force-vector of that weight tilts forward into the rear wheels' contact patch when cornering or slowing down. The motor and gearbox can't help anchor the rear wheels under those conditions, because they're in front of the rear axle already.
Having done all that, my Mini SCT now handled quite respectably, as long as there was lots of traction available. If there was a smear of dirt on the road, though, it would still get really tail-happy and become almost impossible to control. I was nearing my wits' end when I discovered the marvelous black magic called "gyros"; I bought a Futaba GYC-430, added it to the electronics stack on top of the battery, and...
...wow, what a difference it made. I can still spin-out my Mini SCT, but I have to do things that I know the vehicle is mechanically incapable of compensating for. Everything else, the gyro just fixes silently, letting me get back to important job of deciding where the vehicle should go.
I expect this will be my last mini build for a long time; after reviewing the currently-available offerings, I feel like I've got all the good ones that I'm interested in, at least in the 1/18-1/14 range. The Tacon offerings look nice, but there's something to be said for being able to get replacement parts the same week I order them. Plus, they're a little large for my taste. I'll probably get a Mini 8ight-T in a couple months, but from the look of it, that one won't require enough work to justify a build thread.
After a little more fiddling, I got the truck to handle a bit better. I replaced the cut-down blue springs with full-length blue-springs, which basically means they have more built-in preload since they're installed on the slightly-shorter front shocks instead of the rear shocks. I didn't like that setup before, because the front-end was too bouncy and jittery, but I added an ounce of lead to the front to dampen that a bit, and now I have the gyro to help the vehicle hold its line better. The net result is, it lifts its inside front wheel in corners a bit sooner than it used to, giving the rear axle the traction advantage to make the oversteer more manageable. Now I can take more corners at full-throttle without spinning-out, which isn't really how I'd drive on a track, but it's fun for bashing.
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