Monday, April 6, 2020

Just Trying To Keep Busy.....

Besides baking, house cleaning, garage cleaning, and goofing off with TLG, I've been quietly tinkering away on my Arrma Limitless "Speed Run" car.

The best it's done is 62.6MPH, and that was limited by the amount of room I had to run it in.

Following the advice of the guys on the Arrma Forum who have run these cars well over 120MPH, I trimmed down the front and rear spoilers to cut the drag down.

Arrma provided some molded in "cut lines", so I trimmed the front spoiler down to the lowest cut line.

And trimmed the rear spoiler down a little over half-way. The prevailing wisdom, proven by track testing, is the the rear wing has "way too much" downforce, which will push the rear of the car down. If that happens and the front hits a bump, the front of the car can lift, causing a "Blowover", like I'm sure you've seen with hydroplanes, and Top Fuel dragsters.

I'm going to trim the rear wing (more of a "spoiler" than a true wing) down one more line. I'm not entirely convinced you need to cut ALL of it out, as a bit of downforce makes the car more stable.

But too much is bad.....

I also took the center "spool" apart and shimmed all the slop out of it. It was grossly loose, with .05" clearance on the left side where the big silver washer is sitting, and .025" on the right side, where you can see a thin silver shim between the bearing and the thin red spacer.

And I ordered a set of driveshfts that are about .100" longer than stock, to get rid of most of the fore-and-aft slop as shown by the two pix below. One picture shows the shaft all the way forward, and the other shows it all the way aft in it's drive coupling.

Another trick is to put a small O-Ring into the drive coupling before you install the driveshaft. The O-Ring will fill the remaining space, and yet be soft enough to have some compliance to compensate for chassis flex.

And I installed a motor cooling fan using a 3D printed fan mount that one of my friends sent me.

After just a few runs the motor is getting to the "Ouch Point" when you touch it, indicating it's about 135*F. The little fan should keep that somewhat cooler.

And I installed a new motor mount plate and center spool mount made from billet aluminum.

The motor mount uses a screw thread to move it back and forth so you can set the gear lash precisely, and the other aluminum parts are both lighter and stiffer than the plastic parts they replaced.

Tonight I'll get started on making up the new wiring harness so I can install the upgraded speed controller.


  1. I understand very little of what you are doing but it brought to mind an old memory of my sons and Cub Scout Pinewood Derby contests.

    Living in greater Seattle, many Cub Scouts had engineer fathers. The cars those kids brought to the races were amazing. My sons did all their own work with some help from me. Their entries were crude. Their entries won, consistently. My contribution was some graphite treatment of the axles.

    1. I helped my son with his Pinewood Derby cars. I guided him, but he did all the work.

      We sprayed Liquid Wrench into a bottle so we could drip it into the axles. The solvent carried the graphite into the wheel/axle, and then evaporated, leaving just the graphite behind.

      It was nice because we could re-lube the axles without taking anything apart.

  2. " you can set the gear lash precisely,..."
    That brings back a bunch of memories of being a mechanic in the shipyard.

    1. Your gears were probably quite a bit bigger than these!

  3. Replies
    1. ZOOM-ZOOM!

      I *still* have to get out and reconnoiter the five places I found on Google Maps....

  4. It occurs to me that a wing mounted midway between the tires might be the optimum location. Downforce without front or rear bias.

    1. Having separate front and rear wings makes it easier to adjust the balance between front and rear.


Keep it civil, please....