Wednesday, July 21, 2010

Cap Kart Summer Rebuild 3

In the previous post, we crossed the state of maximum entropy, having taken apart basically the entire rear of the kart and all the wiring. I'm happy to report that things are now being put back together, starting with the shiny new rear axle differential, which is pictured above (some assembly required). By virtue of bad machining good design, the precision shafts press tightly into one side of the case, but are a free fit in the other, so assembly is actually fairly easy. The finished product:

Savor the shiny multi-tone metallic look, because next time you see this it will probably be an indistinguishable cylinder of black grease. We were happy to see that it does in fact function as a differential should, even with all the screws tightened. It won't be the smoothest, straightest, or quietest differential in the world, but guess what, we made it and it works. Props to Charles for the simple concept and Max for the execution.

We had a few options for mounting the differential to the kart. There are two steel frame tubes under the rear axle that at first seemed like a prime target for welding on new bearing mounts. Upon further inspection, though, the rearmost of the two was attached to chassis ground structure with cracked aluminum clamps. So, we chose instead to mount everything to the aluminum shelf above the rear axle using our best friend, 80/20 t-slot extrusion. First, we got some of the nicest waterjet parts I have ever seen from Big Blue Saw's low-taper waterjet:

No joke, that's how they came out of the box.

 The boring part: finish-machining the bearing holes.

Angry brake caliper mount.

A new bag of t-nuts and some finicky alignment later, and we have a mounted rear differential:

All that's left to do, mechanically, is cut the rear axle and reattach the two halves to the outputs of the differential. The proposed method of attachment (34mm ID hollow shaft to 1-1/4" solid keyed shaft) involves thin-walled aluminum sleeves, 8-32 screws and small pattern nuts, and hot glue. I'll leave it at that.

The controller and new wiring have successfully survived power-up, so nothing is wired up backwards. We're taking the approach of writing the new software in its entirety without testing it piece by piece. It will just work on the first try. For my part, I became obsessed with having a flashy telemetry display after I watched the Solar Impulse videos. (Is it sad that after watching video of a solar plane, I decide the thing I want to do is write new telemetry software?) Anyway, I upgraded the existing telemetry system to have a scrolling plot:


Left-clicking on any variable sets it to the blue scope trace. Right-clicking sets it to the yellow trace. The telemetry is also recorded for later analysis and can be synced to video. In other words, we just might have the most over-instrumented go-kart in the world.

And yes, I created a GUI interface using Visual Basic. See if I can track a go-kart.

Next: movement.


  1. I just put ball bearings straight into waterjet cut holes.

  2. well aren't you mr. fancy-waterjet-trousers

  3. Yeah, I'm not that much of a Waterjet Ninja yet.

  4. OK. Well, we need to load the chain, too. And the missing lug nuts.

    I did, however, charge the new batteries, so that when we do load the code we'll be ready to dri -uhhh- troubleshoot the code.