Project: TRX-4 - The 4WD Buggy Traxxas Never Made

Project Log

4/22/06

I realize that my last update over a year ago stated that this project was complete, but sometimes I just can't help it. I never was really satisfied with the way the front hubs came together (see the 10/29/03 update), even after all of the trouble I went through to come up with a solution. Today I finally put my mind at ease with a new setup.

On each side I pressed a 5x10 flanged bearing into a 10x15 to make a 5x15 that would fit snugly inside the HPI hub carrier and accomodate the stock Traxxas axle. I still had to use one of my original trimmed-down Teflon washers to space the back of the stub axle in from the bearing just a hair. Finally, I bevelled the hub carrier to the outside of the bearing seat to remove any chance of rubbing against the wheel.

This new setup a little more expensive, but otherwise superior in every way. The axles ride more true and have far less play.

And finally, I have a new shot of the chassis, now complete with an antenna mount I made from Delrin -- one small detail I had neglected in the past. Additionally, I've rebuilt the front shocks with 25wt oil (down from 35wt before) and switched to softer (Losi red) springs. The installed battery is a GP3300 sport pack from Ballistic. It's actually charged & everything. I'm just waiting for the weather to stay clear for more than a couple days so that the ground can dry and I can get some video footage for your enjoyment.

1/16/05

Correct me if I'm wrong, but I do believe this project is complete. Last night I got the body painted and also trimmed the front shock tower for better aerodynamics. Now all I need to do is hook up the electronics and test it out.

It's rare that you'll ever catch me patting myself on the back, but I'm very happy with how this vehicle turned out! I was able to keep it true to its Traxxas & 4-TEC roots, while stretching it off in a clear design direction that makes it truly unique. Plus, it simply looks hot!

1/6/05

I was getting ready to mount a XXX-4 wing (#A-8109) to the back of the buggy with Losi mounts (#A-4222), but decided to go the extra mile and cut out my own custom rear shock tower first. The new piece is cut out of G-10 fiberglass for some thickness and gives me a handful of new shock mounting positions.

I also got pictures of how the body is mounted, and took a final poser shot with the wing on.

1/5/05

A new year and new hope. Today I got the final nagging bugs in the steering worked out (put in a couple of hard stop pegs on the knuckles to keep them from turning too far & binding the front outer u-joints), and finally got the body mounted. Next up, rear graphite shock tower, and wing mount. Then I think I can actually go to the paint shop, and start some suspension tuning.

The body is from a Kyosho TR-15 1/10th scale 4WD buggy, part #KYO39091.

5/31/04

Still going, like the Energizer bunny! In today's precious time in the workshop, I got my upper deck trimmed down and mounted. This piece is cut from G-10 fiberglass sheet. Sure I could go for carbon fiber, but I had some of the G-10 to spare and it's much cheaper, while still being quite stiff. Weight is a little more than I'd want, but I'll try milling it out a bit. I used the front and rearmost mounting points and added two of the HPI chassis standoffs formerly used on the Ultimate Bandit for support. The steering posts are mounted only to the top deck, and are cut from a Rustler upper chassis deck.

In the second paragraph, you can see how the extra area on the lower chassis gives me more than ample room to mount the electronics down low, even leaving some breathing rom in between.

All in all the car is really coming together now and I'm itching to try it out on the track. I need to tune the front suspension (mostly the shock angle and downtravel limiters) and then re-dye the a-arms (had died them green/teal, but they're turning red... what the?) and paint & mount the body. The final step will be to add a proper wing. The end is almost in sight!

3/7/04

Today I found myself with an hour to spare and an itch to do some project work, and here's the result:

Why the a-arms are turning from green to purple beats me, but I will be re-dying them to black. The new lower chassis is fashioned of 0.084" laminated graphite sheet and is mostly a copy of the stock 4-TEC chassis, with the exception of the extension on the left side of the midsection (the lower part in the picture above). This extension will allow me to mount all electronics on the lower deck. The holes need to be countersunk yet, of course. I will not be milling out any holes as I want the chassis to keep dirt out, as much as possible.

Once I get the new lower deck installed, I'll pull off the upper and fashion a replacement for it. I'll likely do this piece in G10 fiberglass, as it's cheaper than graphite or carbon fiber, I have several large pieces to spare, and most importantly, it will help to keep glitching problems to a minimum.

10/29/03

As of my last update, I was moving right along with the front end rework. I had the whole thing mostly worked out and just needed a few more small parts and a bit of Dremelling to finish it off; or at least, that was the plan.

Generally when I attack a conversion project, I figure out how to make the desired changes very early on, and then it's pretty smooth sailing as I follow my own instructions. Not so with TRX-4. My ideas for the front end have fallen through repeatedly! At first I was going to use simple turnbuckles with Traxxas rod ends for the camber links, but then I realized that this would let the hub carriers rotate fore & aft too much.

So then I was going to use stock 4-TEC hub carriers in a partially Dremelled out HPI RS4 Sport 2 caster block, but this didn't leave enough room for plastic slider shafts to fit through; I refuse to buy a $40 set of CVD's for this project. I ended up taking off the entire top halves of the RS4 c-blocks, leaving just the part for the hingepins to go through and the brackets for mounting the lower swivels of the hub carriers.

HPI's hub carriers would have fit the c-blocks well, but they only fit one bearing per side with the Traxxas axles. I figured this setup would be too flimsy for offroading, so I went back to 4-TEC carriers. Well whaddyaknow, these were too bulky inside. After a great deal of Dremelling, I got them hollowed out enough to allow decent suspension travel. Upon assembling everything together, the steering linkage would run into the a-arm when the suspension was compressed. The problem was so fundamentally bad that to Dremel it away, I would have to destroy the structural integrity of the arm. Blast!

Alright, so then I fashioned some carbon fiber extension strips to mount onto the ends of the steering arms. This worked like a charm and led to some very handsome-looking steering geometry. So I turned it full lock right. No good. The servo linkage that connects to the right-side bellcrank got in the way of the right-side steering link (bellcrank to hub carrier). To correct this, I remounted the servo link to the inner hole on the bellcrank and cut off the excess material from the end of the 'crank. This gave the servo too much mechanical disadvantage (the servo would only have to turn 1/2 as much to turn the tires just as much, thus requiring 2x the torque). So, I had to remount the linkage at to a lower hole on the servo saver. This led to binding between the rod end and the servo saver! Does it never end? Some quick work with a X-Acto knife solved this latest issue.

Alright, then I turned the steering full lock left. Something else bound up. Argh! The inner steering link on the left was running into the drive belt! Why didn't this happen on the right? The 4-TEC's belt is offset to the left of center! I needed to get more steering from less bellcrank movement. After trying my hardest to make it work with the 4-TEC hub carriers, I gave up and switched to the HPI units. Making them work would end up being more than a minor pain as well.

The one 5x11mm bearing that the HPI hub carrier holds presses into the outside. On the RS4's, a 10x15mm bearing then fits on the inside, with the large inner diameter fitting neatly around the dogbone hub of the axle stub. Click here to see exactly what I mean, as a picture is worth a thousand words. The Traxxas 4-TEC axles (TRX 4353) don't step up the way the HPI ones do, so the 10x15mm bearings wouldn't help me out at all. I could try to find some 5x15mm bearings, but if I could find such things, they would doubtless be prohibitively thick. I ended up concocting two sets of stand-ins, each made with a modified HPI wheel spacer (HPI 6125), a 5x8mm bearing, and two Traxxas 5x8mm washers (TRX 1685) cut down so that they'd only contact the inner race of the bearing:

I had to slightly gouge out the large aluminum spacers to allow the axle to slide in a fraction of a milimeter further, to allow the pin to fit through the outside. The good news is, after all of this hassle, I finally have something that seems to work! For going through the trouble of reading about all of this mess, you have earned a reward, yourself -- a handful of pictures of the finished product!

So now, let's tally it up. We have Bandit rear a-arms (well, technically, old white TRX 1950 arms, dyed with RIT) mounted front forward with Bandit suspension pins, shaved down in random areas (the white spots) for the sake of clearance. The inner pins needed to be spaced (with the arms moved forward) due to the narrower mounts of the 4-TEC, and for this I used trimmed-down pieces of 4-TEC shock mounts (TRX 4365) (just the narrow ends), plus one 4-TEC caster spacer per side. At the wheel end, the outer hingepin goes through the arm and an HPI RS4 caster block from parts tree A461 that has been cut down to just the base. To this is secured a hub carrier ("steering knuckle") from the same parts tree, with the help of a 3x8mm socket hed screw, passing through a A838 metal bushing to allow the whole assembly to swivel. You've seen what goes through the hub carrier above. On top of the carrier, I've mounted an Ofna Hyper 7 raised 5.8mm ball end from this set (OFNA 19055), secured with a 3mm roundhead machine thread screw. Over this is snapped an HPI Savage large rod end from this parts tree (HPI 85050). The rod end is threaded onto a long 4mm set screw from the OFNA Hyper 7's servo saver, available in a bag of 2 pieces as OFNA #19061. I cut off about 5mm from the end of each of these set screws to allow more room for adjustment, as proper camber required screwing the rod end all the way down. The other end of the rod goes directly into the stock 4-TEC upper arm, which has been moved fully forward with the caster spacers. This upper arm setup is fairly rigid & supportive, holding the hub carrier in upright position. As mentioned much earlier in this update, a simple 3mm turnbuckle with swivelling rod ends would not have given any fore-aft support.

I've already explained how I modified the right-side steering bellcrank. The steering links are fairly straightforward -- rod ends on ball studs. The only trick here was that I had to space the inner studs downward to reduce bump-steer.

And in the final development of the day, I made a rough cut of my new front shock tower, fashioned from 2mm laminated random-matrix graphite sheet. It has three shock mounting positions which are on perfect arcs centered at the lower shock eyelets, to allow the shock angle to be adjusted without changing downtravel much. I actually could have mounted the shocks to the stock shock mounting locations, laying them down nicely. However, the geometry of this was unfavorable. After a lot of eyeballing & measuring, this is what I came up with:

I will eventually be cutting more material out of this rough for lightness and aesthetics. But wait, is that an HPI logo on that bulkhead cover? It is! It really shows how much influence the early RS4 series of cars had on the design of the 4-TEC -- the part, from A340, is nearly an exact fit. Imagine that! Why did I install it if it's so close to the Traxxas part? It adds an extra shock mounting bracket hole on each side, thus giving me an extra option for how I could attach the front tower.

The shocks are mounted with a long 3mm screw that comes in from behind, backed with a flat nut tightened down to the tower. Over this is slipped a stock 4-TEC shock mount, the shock, and then a 3mm nylock nut. At the a-arms, I'm using the stock middle holes on the Bandit arms. I will need to put some downtravel limiters inside the shocks to keep the arms from over-extending, but I'll get to that later.

WHEW! Time to catch my breath! This has become one of my most challenging projects yet! Enjoy the bonus beauty pics below. By the way, the front wheels are Pro-Line B3 rears with the center holes slightly enlarged to fit over the 4-TEC axles. The inner beads were cut off to narrow them down. The tires are some discontinued Pro-Line treads I got out of the LHS' dusty old clearance bin. I'm running no wheel hexes, just the stock axle pins, which fit right into the wheels. Oh, and yes, the front of the car is about 3/16" wider than the rear. I might eventually try some Losi XXX wheels up front to narrow it up. Also, I've cut off most of the front bumper to improve clearance.

9/24/03

I made a great deal of progress today on the front suspension. I found some parts that should work together to allow the steering to work with plastic sliders & stock axles, and I also made a very shocking discovery of an almost exact-fit front upper bulkhead replacement that'll allow the easy installation of a custom front shock tower. I'll post specifics once I get the assembly done, with pictures.

9/10/03

Although a few months have passed since I said anything about the TRX-4 project, I have been steadily collecting parts, brainstorming ideas, and making small changes on the chassis. Today I completed the rear suspension:

The arms are dyed rear units from an LSII. Bandit rear arms would have worked just as well, actually adding a tiny bit of width, to boot. The shocks are Rustler/Bandit/Stampede fronts, with Losi springs. The half-shafts are trimmed Rustler parts. So far, so good. I'll eventually replace the stock tower with a custom-cut carbon fiber piece with more mounting positions. I hope to make a carbon fiber upper chassis as well.

6/15/03

Nearly four months after announcing the project, I have something to show to the public. No details, no questions asked or answered, just one simple, candid photo: