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Guide: 4-TEC Rally Conversion

Preface

Tired of driving on flat, groomed surfaces with your electric 4-TEC? Ready get a little down & dirty? Want to get some WRC rally action going, shades of Colin McRae & Tommi Makkinen? You've come to the right place! Here you will learn the simple & straightforward process of turning a Traxxas electric 4-TEC sedan into a proper 200mm, 1/10th scale rally car.

Granted, a rally car will never have the offroad prowess of a stadium truck or even a buggy. You may be able to scale large jumps, but due to limited ground clearance, the landings may be a bit rough. You also won't be able to tackle terribly rough terrain or go rock crawling. However, once pull off a perfect 4-wheel drift around a sharp corner on loose, dry dirt, throwing up a huge dust cloud behind you and pulling into a 30mph straight dash, you may be hooked on rally racing for life!


What you need:

You could simply slap some rally tires on your sedan and call it a day, but I'm going to assume that you want to do this thing right. A true 1/10th scale rally car, like an HPI RS4 Rally, Associated TC3 Rally conversion, or Losi Rally Weapon, is a 200mm-wide vehicle with enhanced ground clearance. The stock electric 4-TEC is a 190mm car slammed to the ground. That said, let's get to work.

Required:

  • Nitro 4-TEC front a-arms, part #4831
  • Nitro 4-TEC rear a-arms, part #4850
  • Two Rustler/Stampede half shaft packs, part #1951, or one half shaft Pro Pack, part #1953
  • Two 3x22mm or 3x24mm button- or round-head machine screws
  • Two 3mm locknuts
  • 24mm or 26mm sedan rally tires, such as Pro-Line part #1077-00 or HPI part #4470 or #4467, mounted on your choice of wheels
  • Your choice of any 200mm rally or nitro sedan body
  • Dremel or power drill
  • Thin power cutoff wheel or handheld "razor" saw
  • Shock standoffs (as included on your 4-TEC shock parts tree) or medium-walled aluminum tube with ~3mm ID

Strongly recommended:

  • Traxxas slipper clutch set, part #4615
  • Traxxas foam body bumper, part #4835

Optional:

  • Two sets Powerline blue threaded shocks for Nitro 4-Tec, part #N4T1011
  • HPI Super Nitro Rally wheels and tires, two sets each
  • Self-adhesive lexan chassis protector sheet, Associated part #6312

Modifications

Let's start from the rear and work our way forward. The first picture below shows a stock a-arm (left) next to an obviously longer Nitro 4-TEC arm. They're shown back to back; notice that the Nitro arm steps in near the inner mount, whereas the stock arm is straight. We'll refer back to this in a second. You'll need to drill a hole along the flat edge of each arm, about 32mm in from the outer mounting hole. I used a 5/64" Dremel drill bit and enlarged the hole just a tad after going straight through, so that a 3mm screw would be able to thread in.

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Next, you'll want to make some longer slider shafts. You can get away with keeping your stock sliders, but there will be a lot of play after widening the car and they may pop out from eachother from time to time. Take a Rustler outer half-shaft (with the spline on the inside) and cut it down to about 19mm of straight length (from the shoulder of the output yoke to the end). Go ahead and do this to four half-shafts. Below you can see a stock half-shaft next to a longer, custom-cut shaft, along with the piece that was cut off of the latter:

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Now it's time to do a whole bunch of disassembly, adjustment and re-assembly. Refer to the arrows in this photo as you go through the steps:

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First remove your stock a-arms completely and put on your new half-shafts. Mount the new arms to the chassis, with the flat sides (with the holes you drilled) towards the back. Unscrew the camber link at the inner mount and widen it out until there's a 5.5mm gap of exposed threaded rod between the plastic rod ends. When you do this, try to draw an equal amount of threaded rod out of each rod end. Use needle-nose pliers to hold the rod and control how much each end is unscrewed. When re-mounting the camber links, use the lower hole at the inner mount, and make sure you're using the uppermost hole on the bearing carriers. (Speaking of bearing carriers, make sure you're using stock carriers, which give 1.5 degrees of toe-in, versus upgrade carriers that have no toe.) Mount the bearing carriers to the a-arms using the upper holes for more ground clearance.

Move your upper shock mounts to the lower holes on the tower. (In these pictures, I'm using Powerline aluminum shocks -- don't let this faze you, as stock shocks will work just fine.) You'll notice that there's a gap between the lower mount and the a-arm. This is because of the step shape on the back of the a-arm, as mentioned above. Fill this gap with your favorite form of spacer. The one I used in the picture above is one of the pieces that comes on the shock parts tree, which comes with every Traxxas vehicle. If you don't have these, they also come with part kit #2669, $2.39 on TowerHobbies.com. Use a 3x22mm or 3x24mm screw and 3mm nylock nut to secure the lower shock mount as shown. The photo below shows how much more suspension droop you get with the new setup (right) versus stock. Make sure you do both sides identically.

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Now let's tackle the front. Nitro 4-Tec front arms have a tab at the inner mount. Cut this off, as shown in this comparison between an unmodified arm (top) and a modified one:

Click to enlarge

Now the arms will mount right up. Refer to the photo below. Be sure to install your remaining two long half-shafts at this point. Widen your upper camber links until there is a 9mm gap between the eyelet and the inner arm, and again try to evenly thread the ends away from eachother. Move all of your upper arm spacers to the front (see the topmost arrow) to minimize caster and maximize steering effectiveness. Widen your steering links until there's a 7mm between the rod ends. This will give you roughly 0 degrees of front toe-in. You can adjust this later to suit your driving style. Complete the front end re-assembly using stock mounting locations and mount your favorite 24mm or 26mm rally tires all around (or go straight to Super Nitro digs, as described in the Optional Steps below).

Click to enlarge

You'll now have a 200mm rally car platform with about 5/8" of ground clearance. You can go ahead and mount a 200mm body and run it, or you can continue with the advanced buildup...

Optional steps

While off-road driving places far less constant strain on the drive belts, it can introduce sudden shocks to the drivetrain that can punish the diffs and gears. It's always advisable that you let off the throttle a tad after launching in the air. Additionally, you can install a slipper clutch to act as a shock absorber for your entire drivetrain! The Rustler/Bandit/Stampede slipper clutch set, part #4615, is a direct fit for the 4-TEC, and it even includes assembly instructions so that I don't need to repeat them here.

Click to enlarge

To reduce the accumulation of dirt inside the chassis, you can try to track down a Schumacher Cat 2000 4WD buggy underbody, rumored to be a direct fit, or you can make your own basic protector. Clean the bottom of your chassis carefully and lay a sheet of self-adhesive Lexan sheet over it, and trim the edges (but not the holes in the chassis). In the photo below, I left flaps of extra material at the back to help guard the rear diff outdrives a bit. I also tucked both ends of the sheet under the front & rear bulkheads to keep it from peeling up.

Click to enlarge

A foam bumper is always a good thing to have, and you can see my trimmed Nitro 4-TEC piece fitted above.

Finally, if you want to add a bit more ground clearance to your car while giving the appearance of scale 21" rims, use HPI Super Nitro Rally wheels and tires. They are about 1/8" larger in diameter and significantly wider than standard 24mm or 26mm wheels & tires.

Addendum: Shock tuning

In the buildup pictures I was using Powerline's stock springs, with 40wt/50wt oil front/rear as a starting point. That ended up being too stiff and I'm now using cut Losi off-road rear springs all around (just cut them precisely in half and flatten the end coil on each), greys (1.6lb/in) up front and yellows (2.0lb/in) at the rear, with 25wt/30wt oil front/rear on 2-hole pistons.

With stock 4-TEC shocks I would use the same springs, but 30-35wt oil up front and 35-40wt for the rear, with 3-hole pistons.

All done!

Here are some bonus images of my rally-converted 4-TEC.

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