2015 - HPI Nitro Bullet MT
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(originally posted on UltimateRC.com on 2015-02-20)
This is my "spare" Bullet, this one with the MT body instead of the ST body, and it exists so I can have a nitro truck to loan out to friends and family when they want to have a turn. I didn't do as much to this truck as I did to my Bullet ST (especially not retrofitting a larger engine), but for a budget build it came out pretty nice.
Note the carefully-trimmed ventilation hole in the right side of the windshield, cut to leave the "rollcage" decals intact. The engine really needs extra ventilation when it's working hard to move the truck off-road, and this should do the trick. The tires are Pro-Line Trencher 2.8's, pre-mounted on F-11 wheels. The wheel offset is intended for the Traxxas Stampede 4x4, but it works on the Bullet too.
I lowered the body cover as much as possible, to minimize the "peekaboo" effect under the side-skirts.
Conveniently, the lowest position that the body would fit in also allowed the bottom edge of the body to line up perfectly with the top of the back bumper, which is the easiest place to pick up this truck. I cut off a couple overhangs on the rear corners of the body, because I took one look at them and I knew they would get mangled the first time the truck dragged the back edge of the body against the ground.
I trimmed around the right edge of the engine access hole, to maintain a safe distance between the edge of the body and the boiling-hot engine heatsink.
The engine is a GO .18 bought from HobbyKing. It's not a particularly remarkable engine; slightly narrower bore and slightly longer stroke than the stock engine (which was removed by the chop-shop I bought this truck from), but the key thing is the engine costs $45 to replace, and new pistons and sleeves cost $25 on eBay. So I can let kids bash it around without worrying about them damaging an expensive engine. Both of the servos are Tactic TSX45's, one of which came from my Wraith and the other was bought for $12 on eBay. I added a steering gyro to help keep the truck stable, after watching my girlfriend's nephews struggle to drive my Bullet ST last summer. I opted to not install my custom exhaust silencer on this truck, unlike my other nitro vehicles, because this one will only be run on a farm in the middle of nowhere, as opposed to the apartment complex where I live.
The radio antenna is a custom-fit job, because my Spektrum SR201 radios don't have long-enough antennas to reach to the stock antenna mounts. I like my setup better anyway. The receiver is bound to a Spektrum DX2L, so I can hand it to kids and still drive my nicer vehicles using my Spektrum DX4S. Notably, the DX2L has a Throttle Limit switch (L/M/H), so if a really little kid wants to play with it, I can just flip the switch down a notch so it won't go too fast. Braking is unaffected by the Throttle Limit switch.
The engine is fitted with a 3-shoe clutch built from a combination of Savage X and Trophy Buggy parts, with a 12-tooth clutch bell fitted to reduce strain on the motor a little. The stock dogbones were replaced with HPI CVD axles because I learned the hard way that the stock dogbones can pop loose and jam the steering, and the steel brake disc was sanded flat for better performance. The rest of the drivetrain is stock.
The Bullet trucks use body clips *below* the body to set the height of the body, and I hate that design with a passion. (it's just so lazy and cheap!) On my Bullet ST, I actually threaded hex-nuts down onto the body posts to act as body supports, but they cut-up the surface of the body posts really badly, so this time I tried something different: I ordered a set of Tekno clamping wheel hexes designed for 6mm axles, and I clamped those in-place instead. Time will tell how well they hold their position, but I can always brace them with cross-pins through the holes in the body posts if they can't stay-put on their own.
Another view of the Tekno wheel hexes serving as body supports. Doesn't that look better than some crappy body clips?
The body cover itself was trimmed as necessary, as I mentioned before, including a hole to allow access to the HSN without removing the body, because I learned that the Bullet is VERY sensitive to changes in engine temperature caused by the body shrouding the heatsink -- it can't be tuned with the body cover off, because when you put it back on, the engine will overheat fairly quickly. Also, learning from previous experience with my Bullet ST body, I covered the area near the engine and exhaust with sticky aluminum tape, to try to forestall the inevitable paint dissolving off the inside of the body due to fuel exposure. I also glued strips of felt where the exhaust touches the body, both as insulation and to prevent buzzing when the engine is running, and a couple more strips of felt at the front and back to prevent the bumpers from scraping the paint off the inside of the body.
Rawr.
About a year ago I got fed-up with the awful blue slipper pads that don't actually slip, because they get stuck to the slipper plates instead, and I cut a set of Teflon slipper pads for my fully-modded Bullet ST.
After running my MT2 this winter, I discovered that the grey ceramic-coated slipper pads used on the Savage are just as good as the Teflon pads I made, slipping smoothly and not getting stuck to the slipper plates even after months of continuous pressure from the adjustment spring.
Then I realized the grey ceramic pads for the Savage are almost the same size as the blue pads on the Bullet, so I decided to install them on my second, less-modded, loaner Bullet as well. They need a little trimming around the inner circumference but other than that they fit right into place. I'm quite pleased with the result.
I doubt I'll go back and pull the Teflon pads off my Bullet ST, since they're working well and I already went through the trouble to make them, but if I had it to do over I'd use the Savage slipper pads instead. Same result, much less effort.
Wow, I made it more than a year without doing anything significant to this truck.
The latest updates are:
- Ceramic bearings in the diffs and the center gearbox, because the stock bearings were just about to disintegrate;
- Traxxas captured-ball turnbuckles for the camber and steering links;
- HD hardened steel drive cups for the diffs, because the stock drive cups were so worn-out they were starting to break the dogbone pins;
- Molybdenum di-sulfide powder as a dry lubricant for the drive cup that carries the brake disc;
- Sintered brake pads from the Trophy Buggy 3.5;
- Last but not least, a bigger heatsink for the HPI T3.0 engine, which REALLY woke up the engine by letting me run it leaner without overheating.
I also briefly installed a THS tuned exhaust for the old T-Maxx, but it turned out the cheap stock exhaust gave better performance when driving in grass. The THS tuned exhaust sacrificed too much low-end power in exchange for balls-out power at high RPM; that just doesn't work well for an off-road truck that has to fight the constant drag from grass while accelerating.
Definitely still my favorite nitro truck platform. It's incredibly simple, which means there's almost nothing to go wrong. That goes a long way towards compensating for the inherently fiddly nature of nitro, because the engine is the only thing on the truck I ever have to adjust.
After 3 years, I decided this truck had earned a new set of shocks:
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