2020 - HPI Nitro RS4 3 Evo+ #3

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Now that the Nitro RS4 3 has been officially discontinued and removed from the HPI website, I guess now is a good time to post this -- just in time to benefit nobody!

Back in 2016 I built a Nitro RS4 3 that could go 65mph. It looked like this:

It did what it was supposed to do, but the problem is, going fast in a straight line is boring to me. Also the Losi 3.4 engine is loud -- louder than the larger HPI F4.6 in my new Trophy Truggy, in fact -- so driving it anywhere near a residential area was just asking for someone to complain and ruin my day. So after I drove it at top speed a few times ([url=http://i.imgur.com/b1OXpRE.jpg]and crashed it very badly one time[/url]), it just sat on my RC shelf for the next few years. It was too dirty and scratched-up to be a shelf queen, so eventually I decided it was time to rebuild it into a car I could actually drive in my neighborhood. I already had my first Nitro RS4 3 Evo+ rally conversion as a template to work from, and in fact I wondered whether I should retire this car completely in favor of driving my rally-converted RS4, but that car has a single-speed off-road transmission with hardened-steel gears, and extra-long soft suspension. So while it can drive on pavement, it doesn't go very fast and makes a lot of noise in the process. It's really optimized for driving on hardpacked dirt or fine gravel instead:

So, I decided to press ahead with rebuilding my speed-runner into a "daily driver". Even though there wasn't much to figure out about this build, it still took a surprising amount of time to finish because having a toddler keeps me obscenely busy. Here's what I eventually came up with:

At first glance, it looks...mostly normal. And that's because it is mostly normal. The Losi 3.4 engine was downgraded to a stock HPI T3.0 engine, the carbon-fiber upper deck was downgraded to a stock water-resistant plastic electronics box, and most of the upgrades are the same as I installed on my rally-converted RS4: Aluminum shock towers, titanium turnbuckles with Tamiya ball-ends and ball-studs, MIP CVDs, Hot Bodies sintered brake pads and a GPM steel disc, HPI heavy-duty drive cups, long-discontinued HPI "racing" fuel tank and swaybars, clutch-type 2 speed transmission custom-built with OFNA Z10 transmission parts, Tamiya TRF shocks with longer Yeah Racing shock bodies and color-matched MST shock caps, and so on. These upgrades make the car vastly better than the stock configuration, but I wasn't blazing any new trails with this build. In fact, most of those parts simply remained in-place from the original speed-runner build. I even soaked the HPI belted X-Pattern tires in acetone to dismount them so I could reuse them with different wheels. And that's okay, I wasn't trying to build anything new and revolutionary here. I just wanted to make a Nitro RS4 3 that was built to go fast on poorly maintained pavement, similar to my Nitro RS4 2 build from a few years ago:

...but without any consumable belts to worry about. Since the Nitro RS4 2 has been discontinued for so long, I'm always a little afraid I'll smash it to pieces when I drive it, or maybe just break some obscure part that I can't find anymore. Of course, just after I finished this build HPI decided to discontinue the Nitro RS4 3 also, but at least it has fewer potential weak spots to begin with.

Anyway, I did do one interesting thing with this build that I hadn't tried before -- I installed a Centax clutch:

Or the HPI equivalent of a Centax clutch, which HPI called "Center Force", because I guess Centax is a trademarked name. It was originally intended for use with the R40, but I made it fit my Nitro RS4 instead. Why did I install such a complicated clutch on a basher car? I have no idea! Why not? I was bored and I wanted to try it. It works well and the hardened-steel pinion gears that I buy from RC-Monster.com fit the clutch bell with only minor modifications (shaving off unnecessary metal spacers and whatnot), though I had to add extra weight to the flyweights to get the clutch to engage at a lower RPM than it was originally designed for:

In case it isn't obvious, those are 3mm ball-diff bearings that I've glued into convenient ball-shaped sockets in the flyweights. I have no idea if that's what those sockets were intended for (maybe they were supposed to reduce the weight?), but if they weren't put there for people to add weight to them, then it's a remarkable coincidence that each 3mm ball bearing weighs exactly the same as each plastic flyweight. With one ball bearing in each flyweight, the weight is doubled; with two ball bearings in each flyweight, the weight is tripled. This drops the engagement RPM to just above idle, which is much more reasonable for bashing.

I'm sure someone out there is thinking "Bro I can't even with you right now, the whole point of Centax clutches is to engage at high RPM for racing!" Yeah, well...tough. That's not what I wanted to use it for.

Installing the Centax clutch required moving the engine forward from its normal mounting position, because the engine's normal mounting position assumes that the original flywheel will be used. The original flywheel has a ~8mm forward offset, which was necessary to accommodate the position of the brake disc on the Nitro RS4 1 and 2. HPI just carried that design decision on to the Nitro RS4 3 as well, even though the Nitro RS4 3 doesn't have the same clearance issue. Fortunately, HPI also made a one-piece engine mount for the Nitro MT2 (the stadium-truck variant of the Nitro RS4 3) that moves the engine forward the required amount to use a normal flywheel and clutch assembly; UNfortunately, the one-piece engine mount doesn't fit the T3.0 engine (which has its own integrated engine mount in the bottom of the crankcase) without significant modification. I had to cut-off the back and bottom pieces from the single-piece engine mount, turning it into a normal two-piece engine mount instead. That was a pain, but it was a one-time effort, so whatever, it's done now. And now my "daily driver" Nitro RS4 3 has a trick racing clutch for funsies, just like a real wannabe street-racing car. ;)

So what body did I put on top of this touring car with lifted suspension? I decided to go with something vintage, where a taller ride height would look appropriate:

It seems pretty likely this will be my last Nitro RS4 build, since the market has now completely abandoned basher-grade nitro cars. But at least it's going out on a high note.

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