HPI RS4 Electric Conversions

[OldSchoolRC1 983]

It came to my attention that a former roller skating rink in the area has abandoned skating and the owner is planning an RC megaplex... the inside of the rink will be a mudboss oval and they started doing drag racing on the weekends. Hmmm... interesting. Indoors, warm, 10 minutes away, and cheap no prep racing. And a nice concession stand with cheese fries. 

Not having a drag car (at the time, I've since bought one) and having since sold my Slash and all the parts I would have needed, I decided to build a touring car. My buddy runs a Traxxas 4Tec and it's pretty fast - maybe not as quick as the drag cars, but it's quick. I've got plenty of touring cars laying around, so I decide to convert one over on the cheap and give him a run for the money. 

I flailed around a but trying to decide what to run. At first, I thought a mini would be cool...tiny, big motor, and some larger 1.9 wheels...should haul the mail.  So I dug out my MF01 bug and did some mods... 

But I thought a 4600kv motor would be a handful in such a tiny car, so I abandoned that one. Next was a TL01... I had some huge F1 slicks here, and again - AWD, lots of tire, and lots of power should be a winning ticket. I had to adapt an M05 suspension to the car to get the tires somewhat tucked... all was going well...

Until I got to the front and the upper control arms would have to be so short, I couldn't squeeze anything on there. Plus, both cars were kind of iffy...they were never meant for tons of power and I had some serious concerns about the drivetrain holding up. Back to the drawing board. 

I had most of an old HPI Nitro3 laying around I bought years ago for SCX10 IFS conversion parts... after scrounging around, I managed to dig out most of the parts. Found an Associated buggy trans in my stash and did some mockup... this could work! Steel gears, all metal driveline... should handle the power much better than a 20 year old Tamiya.  

and heaven knows I've got a ton of spares and assorted HPI junk.... Smells like a plan!  

Found all kinds of stuff in my stash... HD outdrives, HD gears, and limited slip diffs... interesting. Plus I can always rob some stuff off my Super RS4's should I ever be so inclined. 

[OldSchoolRC1 983]

First thing to do was strip, clean and assemble the diffs. No idea on the best drag setup for an AWD car, so I took a stab at it. Front diff is locked with an Axial locker, rear is limited slip for now. Front has the HD gearset and the rear has an Axial HD underdrive set. I'm thinking a bit of front OD will help keep the car in a straight line. Hopefully. Maybe... Found some universals too - bonus! 

Drilled some holes in the nitro chassis and got the Associated trans mounted... it does have a center diff which is not ideal. I put the thickest oil in I have, but a locked setup would be nice. I think Associated makes a slipper set-up for this transmission, if so I'll be looking into it.  Found some random dogbones in my stash, a full set of turnbuckles, and an aluminum steering set and it's coming together nicely! 

(upside-down! Sorry!) I cut up and re-glued a TRX4 battery tray for my super shorty Capra pack...it's a 90C pack so it should do OK with instant throttle (I hope so anyway....) and it's nice and compact... 

I knew the chassis was a bit tweaked, and I tried to bend it back straight and thought it was ok...while building it up, I noticed it rocked all over my workbench. I was tweaked diagonally too, not to mention swiss cheesed from all the extra holes. I like swiss cheese, but not on my cars. So I broke out some 3MM aluminum, clamped them together and started making a new chassis... 

Couple of hours later, and I have this along with aluminum shavings in my hair. 

Much better! Not only looks better, but straight as an arrow and much stronger. It's a bit on the heavy side, I may shave it down here and there where I can. Designed it so I can flip the trans so the motor can sit front or rear., and nice and narrow since it's going 30 feet in a straight line and anything hanging off won't matter so much. Going to start with the motor up front for now...Hoping for a 50/50 balance front to rear.  

[OldSchoolRC1 983]

Chassis is complete! Took a bit more fabrication to get it where I wanted it, but it's all ready to go.

Had to make a brace for the steering, not sure why the stock piece didn't fit... didn't fit on the stock chassis either. I started with measurements from the HPI "drift" chassis, maybe it's different from the standard RS43 slightly, IDK but nothing a bit of fabwork can't fix.  

Since I'll be swapping electronics around, I just zip tied everything to the upper braces for now. I made standoffs for the front and rear diffs and the transmission and used 2 turnbuckle links to brace the top of the chassis. It was pretty stiff with the 3mm lower plate, but now it's solid as a rock. 

Battery tray tucks in out back - it just squeezes in there. As close to the centerline as I could get it... sliced some openings in the sides so I could use Velcro straps to hold in the battery.  

Electronics mount on the opposite side of the battery. I've had this ESC in a box for a long time...finally had a chance to use it. 120a sensored, should do the trick.. (or will it??)

View from underneath. I'd like to mill out a bit more, it's a bit of a porker! 

Wheels and tires...stickiest 1.9's I had laying around mounted on old, old, old 0mm offset HPI wheels. Keeping it all in the family I guess! Slicks would be better, but I don't have any handy. 

Body painted up and dropped on... I've had this 200mm Skyline NIB for almost 20 years! Can't believe it's been that long. Bought it for my old belt drive HPI Nitro RS4 car back in the day, but wrecked the car on a bad crash and never used it. Simple paint, I love the lines on these bodies, they do all the talking. No need to gum it up with flashy paint. I lost the wing somehow, so found another one in my stash and honestly, I think it looks better than stock! All sitting low and lean... 

[OldSchoolRC1 983]

And now for the gut punch... all that work, and after tearing my basement apart looking for it, I realized I tossed my programming card in with a sale last fall being a nice guy....and now have no way to program this ESC (or 3 others in my touring cars) - whoops!!! And they are out of stock everywhere. I need to reverse the motor rotation and have no way to do it! So it flies... in reverse. 

Making a long story short, I tracked down a programming card, only to find the ESC doesn't support reverse rotation anyway! Geez! So an old Castle Sidewinder and 3800kv SC motor got installed, and off we go!  

Anyway, off to the track. I didn't get any pics per se, so here's a gratuitous shot of my pit area. The blue Mustang in the pic is my DR10 drag car....another story for another thread. 

Drag racing an AWD touring car on a polished wood floor is a challenge to say the least. I ended up setting the punch to level 2 so it would get off the line clean, and then pour on the power as it headed down the track. Our track is only 66ft long since it is indoors, and my best pass to date is 1.79 seconds. I experimented quite a bit with motors, tires, tire additives, and diffs during the season. I can say for certain, locking the front diff and leaving the rear open is much easier to drive. I tried with both the front and rear locked and it was absolutely uncontrollable. I'm not sure why that made such a difference, but it sure did. Also, 2 pole motors worked far better for me than 4 pole - too much torque just instantly overwhelms the traction. 

After experimenting with the locked rear, I've now gone back and re-built the rear diff and re-installed, it, and locked the center diff with some putty. The overdrive effect never really came into play with the center diff, so now we'll see if front OD really makes a difference or not. That's where it stands today! 

Sadly, drag racing season is coming to a close. I'm a scale/trail runner for the most part, and with the temperatures warming up finally, I'm itching to get out and explore again. This has been a fun project for sure, and a VERY fun car to drive. The metal gears make a nice "Zing" sound as it whips down the track - doesn't sound like any of my other touring cars. In fact, I like it so much, I've decided to expand the family! 

In future installments, how about a Super Nitro Rally? I tried a poorly conceived electric conversion years ago. Based on what I've learned with this project, I can do much better.   

AND, to round out the family, how about a Nitro MT2? I converted this over years ago....it's good enough overall as is, but was set up for offroading. Needs a few tweaks for go fast duty. I've already started playing with the shocks and tires to get a feel for if it's a worth conversion.... oh, it is!! 

So now I've got a 1/10 (done), 1/9 and 1/8 (in progress) family of electrics, all sharing pretty much the same driveline parts so I can mix and match as I go. Good stuff!!  I've got all summer to finish them up for next season. 

[OldSchoolRC1 983]

Well, drag racing season has come to a close. They are still racing on weekends, but with temps warming up, it's trail time for me. I did get my HPI RS4MT2 conversion completed, and got a shakedown run in on the last day of drag racing... 

On 3S, this thing is a wicked! It even sounds mean with all the metal gears. I never checked the center diff though, and turns out all my fluid had leaked out over the years...On launch,  I was diffing out the entire way down the drag strip - all the power was going to the front. Front tires ballooned so hard, they took off some paint from the inner fenders. A quick trip to the local dollar store and I had a huge tub of putty for $2. 

Packed the center diff and now it's solid as a rock. Car is set up with some old Proline 2.2's and a DeltaPlastiks body. With the diff closed, I suspect this car is going to be quite fast! Won't know until next season though. 

[OldSchoolRC1 983]

The Super Nitro Rally E-conversion is also coming along well. Using a few old HPI chassis as templates, I made a new chassis to get started. Added the bumper mount from the 1/10 chassis and cut a bit longer to accodate something other than very rare and expensive Super size bodies.  I goofed a little on the longer wheelbase holes, my template shifted slightly I guess and they are off to the side by about a mm. Got too much time invested in the chassis, so I'll live with it for now. I don't mind rough edges so much, but I can't stand things being out of square. Thankfully the 280/300mm WB holes are perfect so it's not a total loss. It's a large plate for now, but once I figure out where everything goes I'll trim it down a bit and lighten it up. 

For the center trans I found a complete replacement Axial Exo bulkhead and motor mount. Works out great, I already had a few spare spurs, and it uses standard AX10 stuff of which I have a ton laying around, so it gives me the option of 3 center gear sizes, and a center diff or locked with a diff locker. Works so well, I ordered the only other one I could find on Ebay in the US. Genius, wish I had thought of this earlier. The green color....hmmmm....

Mounted up the bulkheads and the center trans. Took a bit of fiddling, I wanted to make sure I had the right length dogbones for all the different wheelbase options. Which I do! 

Narrowed and milled out the chassis...milled, drilled, and countersunk. Maybe not my finest work, but not terrible. 

Tried to do something different with the battery tray this time. Instead of bolting a huge, heavy tray to the chassis, I bent up some aluminum and made some brackets to hold the battery pack. Works like a charm and seems solid enough, I drilled out a few sets of holes so I can run my 90C packs on drag day, and larger, full size packs on parking lot days. 

Whoa...pulled apart the diff assemblies to check and re-grease and found a 1-way in the front! Nice! Won't need it for drag racing though, I just put in an open diff for now front and rear. Tempted to lock them up, but with drag season winding down, decided to build it for parking lot fun (Though it'll get a few passes I'm sure)

Assembling the chassis ... shocks have arrived (50mm) and decided to embrace the green and purple... it kinda grew on me. Dropped in a Castle 3800 because A) it matched the color and it's all I have.  

Need to finish up the chassis details and figure out an upper brace. Bumper mount is en-route as well.