2017 - HPI Nitro MT2 #2 Electric Conversion
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(first of all, a random thought: The HPI MT2 really should've been called the ST2, because it's a stadium truck, not a monster truck, and the same goes for its predecessor, the RS4 MT.)
My continuous stream of RC projects has slowed way down in 2017 -- adult life and having to pay bills, or something like that. By this time last year I'd managed to acquire 15 new vehicles, most of which were assembled from kits; this year the number is down to 7, 4 of which were bought RTR and 2 were returned unused because I didn't like them once I had the chance to examine them. That doesn't mean I haven't been working on anything, though; I've been catching up on a lot of boring/tedious/irritating back-burner stuff that I'd been postponing since I moved last October, possibly longer, and the vast majority of my *creative* energy has gone into 3 long-simmering projects. The first was my HPI RS4 2 build, which involved cannibalizing no less than 5 donor cars to get enough parts to build a runner and a shelf-queen to keep it company. The second was my Tamiya DF-03Ra 2-speed electric touring car. This is the third.
I like the MT2 I built for running in cold weather so much, I decided to build a second one for warm weather. That one has a blue body shell, this one has an orange body shell. Notice a theme? ;)
I finished the last few details a couple days ago, and then I decided to scroll through my email to see when I started this project. The first email I have about it was sent on March 20. It took me five months to finish this truck. Why did it take me five months to finish building a HPI MT2? Well, in addition to the normal trials of hunting down upgrade parts for a vehicle that was discontinued 5 years ago if not longer, I did a few things...differently...on this one.
You see, my collection is getting so large that I can no longer justify adding more vehicles unless there's something unique about them. I have multiple vehicles from every major category except competition-grade rock crawlers and drift cars. So this MT2 is not just a second MT2 for my collection, it's also the result of several ideas I've had for a long time
Now that I've talked around it for a while, I'll get to the point. This one is electric instead of nitro:
But wait, it's not just electric, it's also a 2-speed:
But wait, it's not just an electric 2-speed, it also has disc brakes:
But wait, it's not just an electric 2-speed with disc brakes, it also has a custom-wound 11-turn Holmes TorqueMaster Pro 550 motor with a Team Orion V2 endbell:
But wait, it's...okay, actually that is all of the really interesting stuff.
Why do all these things differently than conventional wisdom suggests? Well, why not do all these things differently? This is a hobby, and I had a bunch of ideas I wanted to try. People had been telling me for years that I needed to try driving an electric RC with disc brakes, because it has such a different feel from an electric RC that uses the motor to brake, and I'd kept it in the back of my mind for a long time.
When I was building my RS4 2 this past winter, one of the donor cars I bought came as a pair with an Electric RS4, and that started a whole new round of research. I found out that HPI had made a 2-speed transmission specifically for electric RCs; I was confounded as to how it could provide stopping power without separate disc brakes, so I bought one on eBay to examine it. I suspected it wouldn't be useful for my still-hypothetical electric RC with disc brakes, but I was curious how it worked anyway. After examining it I was impressed with its ingenuity and I decided to use it on some kind of RC, but it made some compromises that limited its applicability. The electric 2-speed transmission can't even roll backwards, because both spur gears are mounted on one-way bearings oriented in opposite directions, and both of them lock-up at once when the transmission is turned backwards. Also, it uses a centrifugal ratchet to engage 2nd gear, which engages very harshly, so its usability was limited to lightweight low-power vehicles that wouldn't break from repeated driveline shock. (more on that topic will be forthcoming in my DF-03Ra 2-speed thread.) So I decided to buy a second Tamiya DF-03Ra kit, which I tweaked for maximum steering angle, and I installed the electric 2-speed transmission on that car instead.
So really, that project was an outgrowth of this one; I started it after this project, and finished it sooner. Funny how things work-out sometimes.
But not being able to use that 2-speed electric transmission in this truck wasn't a big problem. Honestly I hadn't fully decided what vehicle to use for this project at the time I was building my RS4 2. However, after I finished that build I started overhauling several other vehicles, specifically my T-Maxx (which got a new engine and a HPI MT2 fuel tank installed with a homemade adapter plate) and my cold-weather MT2, which had gone through two winters without major service and was ready for a tune-up. To begin with, I bought a HPI Firestorm 3.0 with thoughts of converting it to electric, but I ended up selling it without ever running it, because I was unimpressed by the design and didn't feel like fighting with the combination of RWD and small stadium-truck tires on grass. I already had a ECX Ruckus and a Traxxas Slash, both of which are RWD, and they are hard enough to drive on grass even with their larger tires.
The shortcomings of the Firestorm 3.0 reminded me that I really wanted to build another MT2 that isn't purpose-built and tuned for cold weather, because I consider the MT2 to be a great platform and I missed driving it 9 months out of the year. So I started scouring eBay, and after a few weeks I found a used MT2 that appeared to be not completely run into the ground. Thus began this build.
I had two big problems I needed to solve, though; I'd examined my MT2, as well as my RS4 3's which share most of the same design features, and fitting a motor and battery was going to be difficult. The stock battery-box is nowhere near large enough to hold a full-size LiPo pack, and I wasn't about to buy multiple special-size batteries just for this one vehicle. The other side of the truck, where the engine is normally installed, could fit a battery tray, but then the motor would have to be moved over and it would interfere with the stock battery-box and the servo mounts. I really didn't want this to be a hackjob, so I looked at available upgrade parts and I realized the optional carbon-fiber upper deck would give me space between the upper and lower decks for a battery to fit -- and I already had one that I never used on my first MT2 build! All I needed to do was find a suitable battery tray to wedge in there, but the choice of battery tray was a problem to worry about later, when the truck was closer to completion.
Having settled on a battery location that didn't require chopping the center gearbox to pieces to fit the motor in a weird location, I could move on to the next problem: How to actually mount the motor. I could've gotten a sheet of aluminum or mild steel, bent it into an L-bracket, and cut some holes in it to make a motor mount, but like I said, I didn't want this to be a hackjob -- anyone can do something badly, I wanted to do this well. Enter RC-Monster.com:
I emailed Mike in late March and asked him if he could design and machine an electric motor mount for my MT2 (it also fits on the RS4 3, by the way). He said yes, so he and I spent the next month or so discussing options, and then in late April I sent him the MT2 chassis with the wheels and suspension removed so he could take measurements and make a proper high-quality motor mount for it. It was kind of a back-burner project, because he has a business to run and regular-production items to keep in-stock, so it took him about a month to design the part...and then one of his CNC lathes broke. That put him behind on his regular-production items, so my project had to sit on the back burner for another month and a half while he got his CNC lathe fixed and took care of his backlog of work. I finally received the finished product in mid-July. Unfortunately there was a minor programming error when feeding the CAD design into the CNC lathe and the motor mount I received was defective. No hard feelings though, these things happen when prototyping new parts. The CAD design was correct, so he was able to correct the programming error and send me a revised motor-mount the following week. In late July I was finally able to get to work on a vehicle I'd been planning for the previous 4 months.
In other words, I had lots of time to plan what I was going to do with this build while I was waiting for my custom work to get done. I decided to go with more of an on-road build, because the grass is taller in the summer and it would slow-down a lower-profile truck like the MT2 more. Since I was going to keep the original disc brakes, that meant the truck could only go forward unless I wanted to rig-up some kind of fancy switching mechanism to cut power to the brake servo when the motor was running in reverse, and I didn't feel like putting that much effort in. Another option would've been to find a way to shoehorn a reversing gearbox into the MT2, so I could press a button to shift into reverse, similar to a T-Maxx or a HPI Savage, but there really isn't room for that on this smaller chassis, and it would also mess up my ability to use the fancy custom motor mount I had already ordered. So I decided to stick with forward-only, which works fine during the 99% of the time the truck isn't stuck with its nose directly up against an obstacle.
As long as I was building an on-road forward-only truck, why not go 2-speed? And as long as I was going 2-speed, it would make more sense to use a brushed motor, since they benefit from 2-speed transmissions in much the same way that combustion engines do. I'd wanted to see how well that would work anyway, and I already had experience building 2-speed transmissions for my RS4s using discontinued clutch-type transmission parts made by OFNA. Clutch-type transmissions have the huge benefit of slipping a little bit as they engage, which helps protect the drivetrain from sudden shocks when shifting, unlike the cheaper centrifugal-ratchet design that HPI uses. They're also a lot less likely to get stuck in 2nd gear, which can happen if the centrifugal ratchet jams too hard into the notch that catches it, especially when shifting near max RPM, which is how electric 2-speeds should be tuned.
Of course, if you look at those clutch shoes, you can see they've been modified. One thing I forgot to account for at first is, with the larger tires on the MT2, the gear ratios have to be lower, and that means the centrifugal clutch in the transmission spins more slowly at any given vehicle speed. Getting the centrifugal clutch to open-up and engage 2nd gear was actually pretty frustrating, because even after I replaced the pressure springs with much softer ones, it still wouldn't shift. So what I ended up doing was drilling holes into the clutch shoes and fitting them with weights that I cut from a piece of copper-tungsten rod:
I knew I'd need a lot of weight in a very small space, because most of the interior of each clutch shoe is occupied by holes that already exist for various other bits of hardware to fit into. I was originally going to use brass, then I looked up the specs on various metals and discovered copper was a little denser...and then I found out about copper-tungsten. Copper and tungsten don't alloy together, so copper-tungsten actually consists of tungsten powder glued together with copper; in this case, the rod I used is made of 70% tungsten, so it has a density more than 50% higher than pure copper, and that made making centrifugal weights a lot easier. Those two little chunks of copper-tungsten doubled the weight of the clutch shoe in the picture above.
Finally the transmission shifted properly, but the motor was getting WAY too hot. Aside from having borrowed a motor from my Traxxas Slash and not bothering to hook up the cooling fan just for quick test-drives, the culprit was the gear ratio. Even with the smallest pinion gears I could get from RC-Monster.com (16t for 1st gear and 21t for 2nd gear) and the largest spur gears that would fit inside the center transmission, the gear ratio was still too high. I asked for suggestions on a Facebook group dedicated to RS4s, and someone reminded me that the 13/43 "underdrive" diff gears for the Axial Wraith could fit with slight modifications to the differential. As a bonus, they're helical-cut so they're a little quieter -- not that you'd notice with those big Mod 1 pinion and spur gears grinding away in the center transmission, but whatever.
That dropped the final-drive ratio by 12%, but the top speed of the truck only dropped 2mph. It's one of those hard-to-appreciate truths about electric motors, especially brushed electric motors, that you *can* get them to use more electricity to generate more horsepower by overgearing them, but the increase in horsepower will be incremental while the increase in heat buildup will be exponential. This truck was already beyond the point of diminishing returns, so dropping the final-drive ratio reduced the motor's operating temperature much more than it reduced the truck's top speed. 40mph is plenty for a truck like this anyway.
As for the battery tray, I ended up choosing the battery tray used in the Axial SCX10 II, because it allows the battery to stand on-end which keeps it almost entirely on the chassis plate instead of hanging off to the side. As you can see below, it's held in-place mostly with 3M foam tape, but there are also a couple screws driven into it in places where it overlaps un-used screw holes in the MT2's chassis. The battery strap also passes through a slit in the carbon-fiber upper deck to help anchor the battery directly to the chassis in case the tray ever breaks loose.
That's it for the mechanical ingenuity. The electronics, by comparison, are nothing special.
Like most of my builds, this truck has a Spektrum receiver and Hitec servos -- HS-7955TG for steering and HS-5645MG for braking. The ESC and braking servo are connected to the same port on the receiver using a homemade splitter harness, and that helps explain why I chose the ESC I did; the Axial AE-2 is just a dumbed-down Castle Sidewinder 3, which means I could disable not only reverse, but motor-braking as well. That's important, because when the 2-speed transmission is in 2nd gear, the motor can apply drag as well as power, and I didn't want the motor to interfering with the job of the disc brake. Currently the ESC is connected in normal mode, but eventually I'll probably rewire it to take advantage of the special forward-only "Brushed High Power" mode that Castle offers.
As a finishing touch, I added some foam bumpers for the rear shock tower and plastic reinforcing washers mounted with foam tape to the inside of the body shell, to reinforce it against rollovers.
So that's my latest project, and my latest batch of cool ideas all mashed together into a functioning vehicle. And if for some reason I ever get tired of its current configuration, I can always install a brushless motor with a single-speed transmission -- or just convert it back to nitro. ;)
Quick update. (but I just finished it yesterday! How can there be an update already?) I had a cold-solder joint on one of the motor leads, which broke loose today. As long as I had to take the motor out and scrub the contacts with a steel brush before re-soldering the motor lead, I decided to go ahead and rewire the ESC to use its "Brushed High Power" mode. This hard-disables all reverse functionality, but that's okay because this truck can't use reverse anyway.
Basically, the ESC has two sets of MOSFETs to control power flow, one for forward and the other for reverse. "Brushed High Power" mode uses both sets of MOSFETs together to increase the ESC's amperage limit, at the cost of losing reverse functionality. Since the ESC doesn't need to be able to reverse the polarity of power flowing to the motor when it's operating in this mode, both of the motor leads are joined together and connected to the negative terminal of the motor, and the positive battery lead is split and connected directly to the positive terminal of the motor.
My Castle Link was misbehaving today (probably because I was cheating a bit and didn't bother to disconnect the braking servo from the Y-harness that feeds signal to both the servo and the ESC), so I had repeated connectivity problems and had to change the settings on the ESC multiple times before it finally stuck. So I was a little nervous that it might not have registered the mode-change correctly, and it would fry itself from having the motor leads shorted together the instant I pulled the throttle trigger. Fortunately, that didn't happen and all is well.
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