2014 - HPI Nitro Bullet ST
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My Bullet is finished.
I have a little 1/16-scale Turnigy Nitro Buggy, which is great fun, but eventually I found myself wanting a nitro-powered vehicle that could actually handle running off-road. I didn't want to go 1/8 scale because I have no space for a vehicle that large, but I knew a large wheel diameter was a requirement for running in grass. The Bullet presented a good compromise -- a 1/10-scale vehicle with 1/8-scale wheels, and 4WD too. I opted for the ST version, which really only differs from the MT version in that the ST body actually fits down onto the chassis properly, instead of being hoisted-up by extra-tall body posts.
The Bullet comes from the factory with a .18 engine, and I used that engine for several months, but eventually I realized I couldn't tune it to produce enough power to run on grass without risking severe overheating on every run. I'm sure it would've been fine if I had a dirt track to run on, but I don't, so the vehicle needed to be able to handle grass on a regular basis. A larger engine was necessary.
Fortunately, a helpful Nitro Nerd on URC made me aware of the Losi 3.4, a .20-sized engine (actually .207, so for all intents and purposes it's a .21) with mounting tabs that fit the .18-sized motor mounts that came with my Bullet. That made replacing the engine easy enough that I decided to go for it, even with cold weather coming. The stock air filter didn't fit anymore, but conveniently, my local hobby shop had a Traxxas air filter that's a perfect fit, and clears all the nearby hardware without any trouble.
In case you're wondering, the little black box with the label "FUEL PRESS." on the side is a pressure-bleed valve, which allows me to run a custom-made bronze-wool silencer on the exhaust to keep the noise down, without screwing up the fuel trims. It works very well, even better now that I've rearranged the tubing so the pressure-outlet is within a couple inches of the feeder nipple on the exhaust pipe.
Both servos were replaced, to improve steering response and braking response. (throttle response was always fine, because it takes almost no force to operate the throttle valve.) The throttle/brake servo was replaced with a Hitec HS-5645MG metal-gear servo with 3x the torque of the stock servo, and the steering servo was replaced with a Hitec HS-7955TG titanium-gear servo with a whopping 5x the torque of the stock servo. The brake servo can lock all four wheels, and the steering servo makes sure the vehicle goes exactly where I want it to.
Also, I wasn't happy with the stock antenna mount, so I drilled a tiny hole up through the lid of the receiver box, just wide enough for the antenna wire to fit snugly, and I used a universal antenna mount to hold the antenna tubing in-place.
All 8 shocks were replaced with HPI 103410 shocks, originally intended for the Blitz SCT, but they work fine on the Bullet as well. I just don't trust plastic shocks to not deform over time, and I also wanted the ability to adjust the ride height without having to dig out a bag of preload clips. I reused the lower eyelets, lower spring retainers, and springs from the old shocks, and I added 5mm travel-limiters below the shock pistons to keep the shock travel the same as stock, and filled the shocks with 45wt oil for slightly stiffer response than stock.
The hub carriers and caster blocks on the Bullet are undersized compared to the size of the wheels, and I decided to replace them before they could break and leave me waiting for replacement parts to arrive in the mail. TopCAD made aluminum hubs in orange, which matched the other accent bits on the chassis, so I went with those. I also upgraded to CVD axles, after a crash that caused the steering to hyperextend and the stock dogbone to pop-out on one side. I like that I can trust CVDs to stay put, and they also produce more predictable wear-patterns on the diff outdrive cups.
Also, the diffs were cleaned-out and refilled with 100,000wt oil in the front, and 30,000wt oil in the rear, to improve the vehicle's ability to maintain its momentum when the wheels are bouncing up and down over tufts of grass. And speaking of the wheels, I replaced the wheels with 3.2" Revolvers from RPM, so there wouldn't be any chintzy chrome-plating to chip off from exposure to nitro exhaust. The tires are stock, though; they work more than well enough for my needs.
For some reason, there was a big hole under the spur gear. Lots of vehicles have that, I guess because it's easier to cut a hole than to stamp an indentation to allow the necessary clearance without having a hole in the bottom of the chassis. Whatever the reason, I don't like it, so I covered the hole. Originally I used a bit of an aluminum can, like I did with my Turnigy Nitro Buggy, but I discovered the aluminum sheet got torn to pieces by scraping over the edges of curbs when I drive the vehicle up on the grass. So, I replaced with with a piece of plastic from an IKEA spare-part, which should last longer before needing replacement. And it will reduce wear on the spur gear, because it won't be constantly bombarded with grit from whatever surface the vehicle is driving on.
The exhaust header that came with the Losi 3.4 was bent around too large of a radius to fit comfortably in the Bullet, and it caused the back of the exhaust pipe to stick out a lot. After spending a week looking for a better-suited header, I gave up and resorted to using my own BONE-CRUSHING POWERRR to bend the header into a tighter curve. Then I cut off the extra bit of header pipe at the end that connects to the exhaust pipe, sanded the edges smooth, and reassembled everything. In the process, I discovered that HPI had glued the silicone joint onto the exhaust pipe, and to make sure it didn't pop off later, I re-glued it once I got the exhaust back together.
The stock body-cover that came with my Bullet has seen its day, and there are many places where it has lost paint on the inside from the heat and oil of the engine. I recently bought and trimmed a new body-cover, so to extend its lifetime a bit longer, I covered the areas that I expected to be affected by heat and oil with heat-reflecting aluminum tape from the auto parts store. I also cut a section out of the front "windshield" to add extra ventilation for the engine, and this time I did it neatly enough that it actually looks good.
The new engine is a bit taller than the stock engine, and the carburetor adjustments are positioned differently, so I added a couple cutouts to the body cover, one to provide a bit of clearance for the heatsink, and the other to allow easy access to the HSN screw without having to take the body cover off -- a huge help when tuning a nitro engine to run in the wide range of temperatures I get on the Mid-Atlantic coast.
It's a good vehicle; I like it a lot. And I'm still breaking-in the Losi 3.4 engine, but even tuned rich it still handles grass better than the stock engine ever did.
A running video of my Bullet with its upgraded engine, mostly broken-in at this point:
Whoops, I forgot to post the clutch details on RCTech:
There isn't a 3-shoe clutch specifically made for the Bullet, so I did some research on HPI parts that might fit, and then did some experimenting, and I came up with this:
Savage X 4.6 3-pin flywheel: http://www.hpiracing.com/en/part/86271
Trophy Truggy 4.6 carbon clutch shoes: http://www.hpiracing.com/en/part/101199
(aluminum shoes are also available): http://www.hpiracing.com/en/part/101778
Trophy Truggy 4.6 1.0mm clutch springs: http://www.hpiracing.com/en/part/101201
D8S pilot nut, shaved to 9.50mm: http://www.hpiracing.com/en/part/HBC8059
(this may also work, but I never tried it): http://www.hpiracing.com/en/part/86076
Bullet Nitro 12-tooth clutch bell: http://www.hpiracing.com/en/part/101258
(the 13-tooth clutch bell also works, but this truck benefits from as much torque as it can get)
Here's what the pilot nut looked like after I shaved it down. Yes, I really did get it to 9.50mm using a combination of careful Dremeling, emery paper, and a digital caliper. I even beveled the edges and corners to prevent them from digging into the surface of the aluminum flywheel.
And here's what the clutch looks like all-put-together.
Much better than the stock clutch. I suppose the optional aluminum shoes would give more bite off-the-line, but the carbon shoes won't overheat the clutch bell when they have to drag for a few seconds while climbing over stuff, so I think carbon shoes may be the better way to go with this vehicle.
So, I ended up gearing-up to 14t in October, to try to make better use of the Losi 3.4's powerband. It can still pull wheelies while accelerating, but it happens less often, and the engine still isn't overheating, so 14t is a better fit overall.
Also, I got sick of the Losi 3.4 engine sucking the fuel tank dry in a matter of minutes, and there being no options for larger fuel tanks, so I made my own:
The capacity increase is about 125cc vs. 75cc.
Another year, another replacement body shell. Also I cut off the upper bar on the front bumper because it always interfered with the fitment of the body, and a straight-across bumper will still protect the front of the body adequately.
It's just a stock body, but I did a few things to improve it.
I cut a hole in the windshield to improve airflow to the engine, another tiny hole to allow access to the HSN without taking off the body, I sprayed the entire inside with Spaz Stix nitro-proof clearcoat, lined the side near the exhaust and heatsink with reinforced reflective tape, stuck some felt on top of the tape where the exhaust will directly contact it, and I reinforced the body-post holes with Traxxas plastic discs backed with double-sided foam tape.
Hopefully the T-Bone wheelie bar I installed will keep this body intact for longer.
MAJOR UPDATE: I finally managed to fit a center differential into this truck *without* having to use a much smaller spur gear and accept a much higher gear ratio. This isn't so much of a problem for the electric version, because you can just install a lower-KV motor to compensate for the higher gear ratio, but that's not possible with the nitro version. This is something I've wanted to do for a long time, but there seemed to be no good way to do it...until I found a guy who could machine some parts for me.
The project started with this:
It's the spur-gear hub for an Integy steel spur gear, intended for use with the WR8 rally car, which shares the same drivetrain with the Bullet. I did some measuring with my trusty calipers and figured out exactly how much the face would need to be cut back, and the center would need to be turned-down, to allow a Bullet 3.0 spur gear to fit.
Once the machinist modified the spur gear hub for me, it was time to modify the spur gear itself. The spur gear has a lip around the inner edge that needed to be cut off, and it rides on a 10x15x4 plastic bushing which needed to be thinned-down to the same thickness as the spur gear itself, about 2.96mm. Then I needed to glue the bushing into the spur gear, drill and countersink the mounting holes, mount the gear to the hub and seal the seams with beads of superglue, and sand-down the face of the spur gear so the differential's gasket would have a smooth surface to seal against.
No leaks...so far, anyway. The differential is filled with 200k oil, which will take a while to leak out, though when I first assembled it I started to see tiny leaks within a few hours, so I disassembled it and tried again. 200k oil is thick enough to provide almost-4WD handling while still allowing just enough slip to compensate for the slightly different rotational speeds of the front and rear axles during normal driving. I may drop it to 100k once I see how it actually drives; right now the weather is too unpredictable.
A view from the opposite side shows the problem the Integy spur-gear hub actually solves; it provides places for the bearing and the outdrive to fit, which the spur gear couldn't provide on its own.
Here it is, installed in the car along with dual brakes from the WR8. Braking power was good before with the sintered pads from the Hot Bodies D8, but with two discs and four pads braking power is now immense. I will probably have to dial it down to actually make it useful.
It took a few years to get around to making this work, but I finally did it. Based on my experience with center diffs in other vehicles, this should improve the handling of my Bullet ST noticeably.
The Losi 3.4 engine in my Bullet ST is a few years old now (actually the first Losi 3.4 I ever bought), and because that engine is well and truly discontinued now, I wanted to make sure it would last as long as possible.
One weak spot on any RTR engine is the pullstarter, because the recoil pulley isn't supported by anything except the one-way bearing. That means it puts a slight sideways load on the pullstart shaft at all times; this isn't a problem for HPI Nitro Star engines and Dynamite engines that have ratcheting pins built into the crankshaft, so the pullstart shaft isn't forced to spin continuously, but all other brands I've seen don't have that feature. As a result, the spinning pullstart shaft ovalizes the bushing in the backplate over time, which allows the pullstart shaft to tilt, which breaks the airtight seal between the shaft and the backplate, and also makes it more likely that the one-way bearing will slip.
My really old HPI engines have press-fit bushings in their backplates, which I've been able to press out, cut shorter, and reinstall along with a ball-bearing to support the pullstart shaft on the recoil side. That wasn't possible with the Losi 3.4, because the bushing is integrated into the backplate, so I sent all of my Losi 3.4 backplates to a machinist to have bearing sockets milled into them.
The backplates should last a lot longer now before the bushings ovalize enough to start leaking air.
Oh, also, after several years I finally managed to track down 4 complete sets of HPI Blitz rear shocks, in orange.
This is extremely important, because as we all know, matching colors go faster. ;) Seriously though, these shocks have rubber O-rings in the preload adjusters, so they retain their preload adjustments without relying on two separate adjuster rings tightened against each other. I always hated having to do that. The orange color is a nice bonus, though. The upper eyelets are fitted with Axial Wraith rubber shock bushings, so the shocks can pivot at any (reasonable) angle and still hang onto the mounting screws securely. I seem to be using those rubber shock bushings on more and more vehicles as time goes on.
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