I mentioned the Wheely King toy I got recently, and that I can not leave it as it is; somebody like me simply must make things better and more fun. This is a recap of what I've done so far, with some notes as to what works and what doesn't.

The obvious problem with the wheely king is...wheelies: it does them way too easily, what with battery (and the general center of gravity) located high and to the rear.

My task is to make it crawl up the hill and rocks and stuff behind my house better than in stock shape: I want an rc-almost-crawler that is still fun for a bit of backyard bashing.

I wanted not to destroy any of the stock parts, so all my mods so far are easily reversible. Also I didn't want to spend heaps but rather do "just enough". That's why I aim only for an almost-crawler: I don't intend to change gearings, motor or tires yet.

After reading up a lot on what other people have done, I decided on making the following modifications:

  • mounting the steering servo on the axle
  • flipping the chassis so that the battery is in front
  • extending the wheelbase a lot
  • semi-locking the diffs for limited-slip action

The first item was building a servo mount that sits on the axle, instead of the dreadful lotsa-cams-and-links affair the stock king has. In this monster thread Joshua Thiede describes his nice asymmetric template for mounting the servo (complete with 1:1 pdf), but in the end I decided to build my own: the asymmetric one requires that the axle mount brackets are turned 90° which I hadn't done when the servo mount was made. Thus I aimed for a center mount, which is the only place where there is space for a servo with the stock config.

First I made this mock-up in thin sheet metal, which worked...sort-of: it bent like a wet noodle.

 2007_10_09-servo-mount-thin.jpg thin sheet-metal, bent everywhere

But the concept at least was ok, and thus I proceeded to the next step: same thing, stronger materials. The servo sticks out quite a bit (13mm between the front of the servo mount and the plane of the diff case screws), because there is not enough horizontal space between upper link mount and the front of the diff case (and I wanted this to work with the stock upper link setup). Two strips of steel and some aluminium and quite some grinding and cutting and bending later I had this:

 2007_10_12-servo-mount-mockup.jpg 2007_10_13-servo-mount-finished.jpg 2007_10_13-servo-mount-done.jpg

Note that the first photo has the angles all wrong (90°), but as the King axles face down about 20° I had to change the bend of the bracket somewhat. The metal pieces were joined with four self-threading screws and superglue.

As I didn't have any spare rod ends I couldn't improve the wobbly drag link and tie rod yet, but it ran fine that way for a few days. The white stuff in the photos is 450g of lead sinker balls as ballast to improve the front traction a bit.

A few days later I got the rod ends (Traxxas part 1942: 20 rod ends and balls, $6 ebay). I couldn't find out what threads these have before ordering, and was happy to find out that they are unthreaded, just with a 2.8mm hole and thus work fine with 3mm or 4-40 threads.

A friend donated some carbon arrows with 5mm outer diameter. He said they cost $90 a piece originally, but the set was no longer a full one and thus unusable(?) so he was happy to give me a few. With one of these I built new steering rods: carefully cut off the right amount of carbon tube, then superglued in some sawn-off 4-40 screws as studs and threaded the rod ends onto these. Works great. (I haven't found a source for 4-40 or 3mm threaded rods locally or I would have happily foregone the screw mutilation.)

 2007_10_19-wk-arrow-steering-rods.jpg  2007_10_19-wk-arrow-steering-rods-bottom.jpg

You can see how the servo sticks out a bit, but that is not to be helped: it's either that, mounting the servo a lot higher up or completely changing the upper link mount.

The original owner of my Wheely King had already spent a lot of pocket money on hop-up parts: alloy chassis, alloy steering knuckles and hub carriers, alloy lower and upper links, chassis stays (and also alloy steering pivots). Which is nice, because that took care of pretty much all of the weak plastic parts.

The next big step was working over the chassis: flip, wheelbase change etc. To pull this off cheaply (and lazily), I got a further few Traxxas parts: one set of 4928X and 4951X, long shafts and yokes, $15 and $12. These shafts came out of a TMaxx and I got four (but need only two; good to have spares anyway); the yokes are for 6mm (drilled and flattened) shafts and fit the Wheely King perfectly (and were also cheaper than HPI replacement parts). The original shafts are way too short for any sensible wheelbase.

Originally I had also planned to make my own lower and upper links but in the end I partially chickened out because of a really good bargain: one full new set of Traxxas Revo tie rods and turnbuckles (4x 144mm and 4x 122mm s/steel rods, all rod ends, balls and screws) for $7.50 :-) (That's Traxxas parts 5338, 5348 and 5318, usually worth about $30).

The long rods I used as the lower links, which gives me a nice 34cm wheelbase (way up from the stock 23cm), and the shorter links are spare for now. I didn't have to cut the drive shafts (as other people reportedly had to when aiming for a 12.5" wheelbase).


Getting the length of the upper link right was a fair bit of work and quite important, or the steering setup is shot: on one hand the axle shaft angles must be bearable, but on the other hand the caster angle should also make sense. As this is not a racer I went for zero caster (also because I forgot to measure the original before ripping it apart...). You can also see the angle of the servo bracket quite distinctly.

 2007_10_22-wk-lang-hinterachse.jpg 2007_10_22-wk-lang-vorderachse.jpg  2007_10_22-wk-chassis-lang-fertig.jpg

The stock upper links are wobbly plastic, but mine came with alloy wishbone links which I simply extended by 54.5mm: cut more carbon arrow tubes, beheaded two 3mm screws, superglued them in, done. The wishbone was already threaded for 3mm, and simply screwing in the studded carbon tube worked perfectly (without the need for a backing nut). On the axle-side the extended upper link was fastened with a 3mm nut.

 2007_10_27-rear-axle-mounts-detail.jpg  2007_10_27-upper-link-servo-detail.jpg 2007_10_27-wk-servo-mount-axle-mounts.jpg  2007_10_27-wk-shock-lower-mounts-servo.jpg  2007_10_27-wk-shock-lower-mounts.jpg

The axle mounting brackets were turned 90° to lie flat, and I mounted both lower links and the shocks with a single 3mm screw through the lower hole. The mounting brackets don't fit perfectly when turned, so I used a strip of 1.5mm (doublesided) foam tape to improve the fit.

 2007_10_27-wk-top-shock-mounts-esc.jpg  2007_10_27-shock-mount-detail.jpg

With the drastically extended wheelbase mounting the shocks to the shock towers doesn't work: the ball mounts can't move enough. I simply used some metal offsets/stays (from a DB9 or DB25 connector shell) and some convenient pre-existing holes in the chassis. This meant, however, that the shocks are mounted lower than originally, raising the center of gravity. So far that turns out to be no problem and the car rolls over less easily than before, with added clearance as a benefit (10cm at the gear case).

Flipping the chassis around also meant that I had to rewire everything. The speed controller gets quite hot, and I had already added some cooling fins and ventilation slots in the back of the lexan chassis: thus the ESC had to move to the "new" rear. I quickly made a chassis tub out of aluminium flashing and put ESC and receiver there (with double-sided tape). The motor wires needed to be switched (because I had not exchanged the axles...lazy me) and I soldered an extension to the battery cable. The steering servo needed an extension cable as well, with the receiver being the rear-most component now.

 2007_10_27-rear-rods.jpg  2007_10_27-servo-mount-axle-mounts-detail.jpg

I also got rid of the fixed drag link on the rear axle; quickly made two rods from beheaded 3mm screws and rod ends which link the knuckles to the axle shock mounts (fixed with the original 3mm flanged shafts and e-clips).

 2007_10_22-wk-lang-mit-elektrik.jpg 2007_10_27-wk-artic-front.jpg  2007_10_27-wk-artic-side.jpg

And of course I kept the provisions for driving lights ;-)

 2007_10_22-wk-lang-vorne.jpg 2007_10_22-wk-lang-seite.jpg  2007_10_22-wk-lang-von-unten.jpg  2007_10_22-wk-lang-licht2.jpg

Initially I had filled the diffs with heavy grease to limit the differential action, but that didn't last at all past reassembly; now I've put a mix of 20% grease and 80% blu-tac in the diffs and that seems to do the job better. (I don't want to lock them fully because then other things start breaking, the turning radius worsens and so on.) I may try silly putty and grease next (I used too little blu-tac in the front diff and will have to redo that).

Overall cost:

  • one carbon fibre arrow (original price $90, zip for me)
  • one pack Traxxas 1942 rod ends $6
  • Traxxas 4951X and 4928X long shafts and yokes, $15 and $12
  • a bunch of 3mm screws, four 4-40 screws and other small parts $5
  • Traxxas 5538/5348, 5318 lots-a-rods $8
  • a few hours of fiddling.

Net result: lots of fun while doing this, and also more fun driving the car around. It climbs obstacles a lot better now, just as hoped. Success! :-)

[ published on Mon 29.10.2007 23:04 | filed in interests | ]
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© Alexander Zangerl