2016 - HPI Nitro RS4 3 SS Electric Conversion
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Last summer I built my first road car, a HPI RS4 3 Evo+ which I converted into a rally car so I could drive it on rough pavement. Then I dove into electric road cars, building four Tamiya XV-01s, a TB-04R, and a DF-03Ra, not counting the off-road trucks and buggies I also worked on during that time. A couple of the XV-01s were built as rally cars, but the other two (as well as the TB-04R) were built for on-road use only, and I learned a lot about tuning road cars for better performance than I originally achieved with my RS4 rally car without giving up the ability to handle a bit of uneven pavement. So, a year later, I circled back around and built another RS4 -- this time the older "Type SS" version.
One of the biggest differences between this build and my RS4 rally car is the carbon-fiber upper deck; the upper deck normally consists of plastic pieces at either end of the chassis, braced together by the quick-release electronics box, but the carbon-fiber upper deck ties everything together more solidly. It's not as easy to clean around, so it's not as good for rally use, but it's much stronger. Rather than painstakingly sealing all around the edges of the upper deck, I just sprayed the whole thing with with nitro-proof clearcoat.
Like my RS4 rally car, this car has shocks made from Tamiya TRF internals, Max Speed Technology caps and collars, and HPI lower eyelets, spring carriers, and springs. Unlike the RS4 rally car, this car's shocks are the stock length and use the stock dual-rate springs. It also has adjustable toe and camber links using HPI purple-anodized titanium turnbuckles and Tamiya ball cups and ball studs. The rally car only has adjustable toe links, the camber links being the stock fixed-length plastic bits for better durability; since this car doesn't get bashed around as much, the risk of stripping or breaking a turnbuckle is diminished and thus a reasonable tradeoff for being able to adjust the wheel camber.
The electronics package is the same, with a Spektrum receiver and two Hitec servos, though the carbon-fiber upper deck required the use of a rechargeable battery pack, the first one in my entire nitro collection. (Energizer Lithium AA batteries have worked great in all my other vehicles, usually lasting most of the summer before needing replacement.)
The Spektrum receiver is small enough that, with a little trimming of the carbon-fiber upper deck, I was able to tuck it into a slot directly on top of the steering servo.
The wiring is all run along the underside of the upper deck, for a super-clean look with almost no wiring visible from above.
The upper deck was made by Hot Bodies, and has a very thoughtfully designed removable plate that the battery pack attaches to. I used black Tamiya fiberglass-reinforced tape to hold the battery pack in-place, as well as the wire bundles.
The brakes use the same GPM steel disc that I used on my RS4 rally car and my MT2 winter truck, with the same sintered composite pads borrowed from the Hot Bodies R10. The brake linkage uses a gear-diff adjustment spring because it's the right size and I had several handy, instead of a piece of fuel tubing, for more progressive brake engagement. The brake lever is topped with a purple countersunk washer because it looks pretty.
The engine is a vintage HPI .12R SS, which uses a cylinder sleeve coated with nickel instead of chrome. This is old tech nowadays, because the HPI .12R SS was designed and built by Toki, which used to make RC airplane engines before they went out of business. Nickel is generally considered inferior to chrome, at least in car engines, because it's more prone to damage if the oil content of the fuel is too low. (airplane fuel has a high percentage of oil to compensate for the engines being run very lean compared to car engines). But thus far I've had no problems with my Byron's Gen 2 fuel (30% nitro, 11% oil) spiked with an extra 50mL of castor oil per gallon. The engine actually runs so cool I've accidentally tuned it too lean several times. I might have to switch from a medium plug to a hot plug just to improve the idle when the engine is cold. The engine is fitted with the carburetor from the Nitro Star T3.0 engine; the carburetor has a wider throat than stock, so it can supply more air and fuel to the engine at redline; the transmission gearing is lower to compensate for potentially reduced performance at lower RPMs with the larger carburetor.
The drivetrain uses the same GPM center driveshafts and MIP CVDs as the RS4 rally car, but the gearing is different in several ways. First, the front diff has had the internals replaced with a diff-locker from the HPI Wheely King; the outdrives slot right into the diff-locker, though they can't be retained with the normal C-clips, so instead they're held in-place with foam dots stuffed into the drive cups, to provide compression between the outdrives and the axle ends.
Second, the diff gearing is lower, using the 13/38 heavy duty gearing for the Wheely King and MT2, instead of the 14/36 heavy duty gearing that the newer RS4 3 Evo+ came with from the factory. (the stock gearing for the SS version was also 13/38, but the gears were made from pot-metal instead of hardened steel.)
Third, this car has a two-speed transmission, using the same pinion gears that I originally tried to use in my RS4 rally car before I converted it to single-speed. (that's why the gear teeth already have noticeable wear.) Not content to use the stock gear ratios, I replaced the 18/47 1st-gear with a 16t pinion from the HPI R40, and a 49t spur from the MT2's optional 2-speed conversion kit. This gives the car a 1st-gear ratio that's 15% lower than stock, helping to keep the tiny .12 racing engine in high RPMs most of the time.
The stock shift mechanism changes into 2nd-gear very abruptly, which doesn't matter as much on the newer RS4 3 Evo+ with the larger .18 engine, because that setup runs a narrower gear range and the shifter can be set to shift at a lower speed. But the wide gear range and high shift-point on my car caused the stock shift mechanism to change into 2nd-gear with a *click* that could be heard from dozens of feet away over the sound of the engine revving-up at full throttle. (which is to say, it was very loud noise to be coming from such a small component.) It also jerked the car noticeably, and I was concerned that the shift-shock would start to damage drivetrain components after a while.
I went in search of a smoother shift mechanism, and ultimately found the solution I was looking for in the form of some truly ancient OFNA parts intended for use with the RS4 2; some careful modifications detailed »here« made it possible for me to fit it to my RS4 3, giving the car the ability to shift smoothly from 1st-gear into 2nd, and back into 1st when slowing down.
To top it all off, I decided to use a HPI Pontiac Firebird body that came with another RS4 3 SS kit I bought late last year, to build as a shelf-queen, and I'd been saving the body in case I decided to use it for something else in the future. The body is painted Duratrax metallic black, backed with Duratrax base black, and sealed with Spaz Stix nitro-proof clearcoat. (sadly, it turns out the nitro-proof clearcoat can't withstand being completely doused with the entire contents of the fuel tank, as I discovered after a crash today -- but I know from experience that it's good for protecting the paint from *small* amounts of fuel spray, as well as residual oil.) Parts of the body that sit near the engine heatsink and the exhaust are backed with reinforced aluminum tape to reflect heat away, something I learned to do after melting the sides of a couple bodies on other vehicles in the past.
The whole thing rolls on HPI Ray's Gram Light wheels, 0mm offset in the front and 3mm offset in the rear, to fill the wheelwells properly, and HPI V-Groove Pro Compound tires for lots of grip. The rear-right wheel, being exposed to nitro exhaust spray, was coated with the same Spaz Stix nitro-proof clearcoat prior to mounting the tire, so the chrome will last longer.
I don't know if this is the ultimate retro RS4 build, but I think it comes damn close. And it handles like a dream; it can spin donuts no problem, and then drive off into the sunset just by letting the steering re-center, without ever letting off the throttle.
Yes, I have Pirellis on the BMW. In that video I was running S-compounds; I have since switched to D-compounds; at first they didn't give me enough traction, but then I put a wing back on the BMW and suddenly I have all the traction I need.
I speed-tested the RS4 SS with the higher-ratio diff gears this morning, before it got hot outside.
Not a bad increase from the 40mph I saw previously. It was actually higher than I was aiming for. Unfortunately I ran the engine too lean at one point. It didn't lean-out the way I usually see an engine lean-out, where the engine speed suddenly drops when I hit the throttle; instead I noticed after a few passes that the engine would start to slow down as it ran at full-throttle for more than 10 seconds or so, and I checked the engine temperature and I saw 266°F -- much higher than I've previously seen with this engine. I enriched the fuel mixture again and the engine resumed running as expected.
I just pulled the cylinder head to check the condition of the cylinder sleeve; it looks fine. This is why I add a bit of pure castor oil to my fuel, just in case.
I find myself wondering whether I really need the extra shim that I added to the shim-stack when I built this engine. Removing it would probably give me more power, but at the cost of a higher risk of overheating.
This is a strange little engine. It doesn't sound like it's revving very high, but at 47mph the wheel RPM is 6080, and multiplying that by (36/14)*(43/22) means the engine RPM is 30558. Not quite as high as the engine RPM on my RS4 3.4 (35515rpm @ 61mph), but the .12R SS engine has slightly more than half the displacement so it's working a lot harder. I suspect a lot of it has to do with the pipe I'm running, which is the same pipe that comes with the Nitro Star .18 engine. Maybe I should reinstall the inner cone and see how that affects things.
For comparison's sake, the BMW's wheels (with slightly taller tires, 68mm vs. 66mm) hit 5524rpm at redline, and multiplying that by (36/14)*(46/19) means its engine was pulling 34939rpm in order to go 3mph slower than this car did.
I made a couple updates to this vehicle today. I installed a Novarossi pipe, which I had to order from Austria and took 3 months to arrive, I changed the gearing from 16/49 and 22/43 to 16/47 and 22/41, and I replaced the HPI Crawler King front diff locker with a LockedUpRC FI "Lunchbox" locker. It's much more snug than the Crawler King locker was. I won't know until spring if the Novarossi pipe does anything useful, but it must be at least as good as the stock HPI pipe.
Small update:
A few months ago I swiped someone else's good idea to deal with the button-head motor mount screws being damaged from scraping against the ground, and the result looked like this:
I removed a couple of the locknuts so you can see the solution more clearly; it consisted of set-screws driven into the motor mounts and secured with red Loctite, then locknuts tightened onto the set-screws to hold the motor-mounts in-place. The advantage of this solution is the flat surfaces on the sides of the locknuts were protected from being scraped against the ground, so the locknuts could be easily removed even after several hard hits. The disadvantage, however, is the locknuts stuck down off the underside of the chassis plate even further than the original button-head screws did, causing them to scrape more often; while they weren't damaged to the point of being unusable by all this scraping, it did throw off the handling of the car on rough pavement -- not good when driving at high speeds with curbs on either side of the road.
A couple days ago I was digging through my spare-screws jar and I found a better solution -- pan-head screws from Axial, part number AXA0109:
They're flat so they have very good clearance, and the wide heads mean it will take a LOT of scrapes before the heads are damaged to the point that the hex driver doesn't fit anymore. The wide heads also mean I don't need to use washers with these screws, which improves clearance even more. This is especially important on my speed-run car which has the lowest suspension of any of my cars, and on my RS4 2 which doesn't have insets surrounding the motor-mount holes so the screw-heads stick down further.
This isn't exactly a revolutionary solution -- plenty of nitro vehicles use pan-head screws for the motor mounts, including my two Bullets -- but those screws are much larger and don't even come close to fitting my RS4s. This is the first time I've been able to find pan-head screws that do fit this application.
Small update: I finally got around to cutting some steel gears from RC-Monster to fit the clutch bell on this car, so I could replace the pretty (but fast-wearing) aluminum gears with steel gears that will probably outlast the rest of the car. (the aluminum gears pictured below are spares, not the worn-out ones I replaced.)
The modification wasn't difficult, I just had to order a pair of "1st gears" for the Losi LST, in 16t and 22t, and then cut 5mm shoulders off the backs of the gears so they could thread onto the clutch bell with the correct spacing. The 22t gear also required some beveling on the back of the screw-hole, because the base of the RS4's 2-speed clutch bell bevels outwards. It only took about a half-hour of measuring, cutting, sanding (to get a flat surface) and treating with blackener to seal the exposed steel surfaces. But oh man, there were SO MANY SPARKS. Hardened steel loves to spark when it's cut. I could barely see what I was doing even with a bright light shining directly on my work area.
The weight penalty is about 10g, comparable to the weight saved by using the lightweight flywheel, so I guess I lost whatever benefit the flywheel gave, but hey, I'm not racing this thing, I'm bashing it. If I were racing it, I'd deal with the aluminum gears, but I'd also be making a bunch of other compromises to reduce weight, like getting a cutout chassis plate, so it's not like these steel gears would be the straw that broke the camel's back anyway.
This past spring I was driving my RS4 3.4 when the power cut-off suddenly, and the car drove around in a large circle until it crashed into a curb. I repaired it, but since I had to replace the body, I decided to move the Firebird Trans-Am body from this car onto that one. It didn't really seem to make sense having the Firebird body on a RS4 with a .12 engine when it could be on a RS4 with a .21 engine instead. So I had to find, trim, and paint a new body for this car. Here's the one I settled on:
This Alfa Romeo body is extremely discontinued, but somehow I managed to score 4 of them -- plenty of spares in case of another "incident".
Major update to this vehicle:
Yep, I converted it to electric. Why? Two reasons:
1) After months of tinkering with the ancient HPI 15FE engine in my Nitro RS4 2, I improved its performance so much that the 12R SS engine in this car seemed slow and boring by comparison -- especially since the Nitro RS4 3 chassis is a couple hundred grams heavier than its predecessor.
2) I built an electric-converted Nitro MT2 last year, but after all that work the truck just wasn't that much fun to drive. The 2-speed transmission was neat, but it meant the truck could only be safely run on pavement because there was no slipper clutch to handle drivetrain shock from jumps. Meanwhile, the suspension never handled sharp impacts (especially speedbumps) very well, and a truck that can only be run on pavement and has to slow down for speedbumps is kinda pointless.
So I retired the electric MT2, cannibalized the custom parts I'd used in that car, and used them to convert my Nitro RS4 3 SS to electric instead.
Currently this car is running the same hybrid motor that I have in my Tamiya XV-01 SH-AWD -- a Sagami can, an Orion 12T armature, an Orion SV2 endbell that uses diagonal brushes, and ceramic bearings. It hauls pretty good. I only have these diagonal-brush motors in a couple vehicles, but they do seem to live up to the hype -- the diagonal brushes don't seem to spark nearly as much as normal brushes. The rest of the electronics are the same stuff I use in all my builds -- a Spektrum receiver, Hitec servos, and a Hobbywing WP1080 ESC which will be my go-to ESC for brushed vehicles in the future. It's very helpful that the WP1080 is fully programmable, because I was not only able to switch it to Fwd/Brk mode, but I was also able to turn-down the brake strength to 0% so the motor wouldn't screech to a stop for no good reason every time I tapped on the car's mechanical brakes.
Unlike the Nitro MT2, the Nitro RS4 3's chassis is too short to accommodate a full-size LiPo pack on the left side without moving the steering servo to the right side. Fortunately the Nitro RS4 1 and 2 had their steering servos mounted on the right side, so there are steering bellcranks available for those cars that I could use in the conversion. I also drilled a couple mounting holes into the chassis for the steering servo, rather than sticking it in-place with servo tape and hoping it wouldn't come loose. I had to insert a couple cylindrical shims under the pivot-balls on the steering knuckles to correct a bit of bump-steer that this setup caused, but Team Schumacher makes a variety-pack of purple cylindrical shims so I could preserve the car's color scheme. (these things are important, dammit!)
If you've seen the optional carbon-fiber upper deck for the Nitro RS4 3 before, you'll notice the one I have installed is shaped differently. That's because I cut a bunch of extraneous material off the outboard side of the upper deck, to make accessing the Velcro battery strap easier. I also modified an existing hole near the front of the upper deck, enlarging it and merging it with a nearby screw-hole to make room for the negative battery connector to pass through.
Battery retention is provided by a pair of Team XRAY battery holders sticking out past the left edge of the chassis. I had to drill holes to mount these as well. I had to cut into the center gearbox case a little bit so the battery could sit inboard enough that the Team XRAY battery holders could mount properly, but I cut the center gearbox case carefully so it has a lip on the upper edge of the cut that helps hold the battery in-place. With this setup, the battery actually snaps into place; the Velcro strap is just a formality in case of a major crash.
For the sake of thoroughness, I countersunk all the extra holes I drilled into the chassis. I love my hardware-store-brand countersinking bit; it's one of only two precision-machining tools I have, and it's very satisfying to be able to keep the underside of the chassis perfectly flat for maximum clearance over road debris.
So now my RS4 3 SS has a new lease on life. It used to be kinda slow and uninteresting compared to other nitro touring cars I'd built since then, but now it's unique again.
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