Introduction: K'nex RC Trike: VIDEO UPDATE
Hello. Since the K'nex contest has come around again, I figured I'd finally get this thing posted. I've had it built for about 5 years now, and just haven't had the time to post or finalize it. At the moment it doesn't run due to a very old, and very dead, battery but I will be posting a video of it working from power off the battery charger for proof-of-concept. This is just a rough draft to get the thing entered in the contest, and I will be editing and posting a better video when I get a new battery. I will also be adding pictures as I tear it down to get the dust off it and check pieces for wear. Other than the electronics that make it move and a touch of wire to help secure these parts, the entire trike is made from K'nex branded pieces.
Step 1: Drivetrain
I chose to do a trike for several reasons. The first was that I really love them and want to eventually scale this toy to a gokart, then an actual vehicle. The second, is that I didn't want a fixed drive shaft in the rear shared by two wheels without a differential gear. The car is already under enough stress, and having to worry about turn was not something I wanted. Another reason is that it keeps the most dense part of the trike, the motor and gear set, as small and compact as possible.
The trike is powered by a pretty standard 380 motor that I probably pulled out of another one of my RC cars at some point. While I'd like to use the much larger and stronger 540 motor I have, the pieces just can't take the stress, and parts that house higher rotating shafts are already showing signs of melting and wear. The motor links to the back tire through a set of three small to medium gears resulting in a 15:1 gear ratio to the wheels (15 turns on the motor for one turn on the wheel for those who have trouble with ratios). The shaft drive system was my second design, preceded by a chain drive that skipped too much and couldn't handle the torque of the system when stopping. Unfortunately for some the pieces involved, the shaft drive method (at least with the pieces I have) requires cutting a few pieces so that shafts don't run into each other. At one point I had re-purposed the wheels for another RC buggy and damaged two of these large wheels, so I had to help support the rear wheel with two other medium wheels. Overall, the motor mount and housing are very overbuilt, but I would rather have the car little heavier than have to worry about extra friction from parts torquing or flexing out of place.
Step 2: Steering
The most challenging part of the car was the front end. The steering utilizes several designs that took many iterations to perfect. The first is the hub that allows each tire to spin, turn, and support the weight of the trike. The largest issue was that the large tires have a mounting point that's almost dead center in the rim. This means that the shaft that holds the tire will be subject to a very large amount of torque since its almost two inches from the swivel point. To fix the this issue, I built a hub that supports the shaft much closer to the center of the wheel, and also provides a swivel and mounting point for steering. As a hidden bonus, the steering mount is just strong enough for normal operation, but will break free from the servo arm if it runs into anything too hard, thus saving the servo gears from the impact
Another issue was pointing the wheels straight. Since K'nex parts don't come in many sizes, and I didn't want to worry about this issue, I made the steering arms adjustable. They allow for toe-in/toe-out adjustment, and aid in pointing the wheels straight.
Step 3: Suspension
To help reduce the impact on the body, I put a flex point on the middle of the body and use the flexi-rods as shocks. It works well enough, and allowed me to find out that each type of colored flexi-rod has a different level of flex strength. The light purple are the strongest, followed by green, then dark purple from the old sets.
Step 4: Electronics
All the electronics involved came off my old Duratrax Evader BX(pic above), aside from the battery and motor. If you guys want any more details on these, let me know in the comments. Most info is better looked up online, and most of these components are 5-10 years old anyway.
Step 5: Videos: UPDATED!!!
I finally got a round to uploading a video of the trike working. It's only plugged in to the slow charger at the moment, but I will try to have a battery for it soon.