Build a HUGE RC MONSTER TRUCK - Golf Cart Wheels - Moped Motor - Remote Controlled

Here's a DIY to build a HUGE RC Monster Truck. You'll need to have a welder.


I have enjoyed seeing remote controlled trucks come a long way over the past couple decades. I've even owned several of them along the way. It all started with the AA powered 2WD trucks with simple springs as shocks. Then along came 7.2V with oil filled shocks, then 4wd, then NITRO powered and then 2 speed transmissions capable of 50MPH with reverse. The past several years the industry has had no where to go except bigger. From 1/16th scale to 1/10th scale and even 1/8th scale. Well, I'm going to beat everyone to the punch. I'm going to design and build a 1/4 scale remote controlled truck with my own unique twist.

I've made several drawings and I couldn't wait to get it out of my head and start welding! I've had this idea of an independent suspension where all swingarms pivot from a common center pin and all shocks are inline and attched to a parallel upper pin. I useda solid steel rod as the pin and welded matching tubing to the swing arms and honed them for a good fit. I'll do the same for the shock mounts. Here' s the start of it all. The 4 swing arms with the lower pin, front steering knuckles and 4 golf cart tires with ball bearing hubs. I made the swing arms out of 3" electrical conduit. The steering knuckle are simply 3/8" thick steel plates with washers, bolts & bearings to make for a proper knuckle. I had to weld a keyed sleeve into the rear hubs to make a proper fit to the 3/4" keyed axle shaft.

This is after the upper parallel pin is installed using H-links and the pneumatic cylinders (soon to be converted into hydraulic adjustable shocks) mounted. The engine is just sitting on it's mounts for now. But the differential and pillow blocks are in place. I had to design, weld and machine a sprocket that would fit perfectly over the differential and use the existing bolts. Then I designed, welded and machined a sprocket incorporated with the drum brake and wheel hub that fit the moped engine. Of course I crunched some numbers and calculated for a proper gear ratio. The differential sprocket is a 40 tooth and the drive sprocket is 15 tooth. This should give an end result similar to the moped's original ratio and performance which was speeds up to 48mph. Whooahh Can't Wait!!! I better plan on having an EASILY accessable kill switch on the vehicle and the remote.

Here's a picture of the pneumatic cylinders plumbed for a hydraulic shock application (another one of my hair brained ideas). There's a 10-turn valve inline so I'll be able to adjust the rebound rate. Also, the springs and spot welding are temporary (from ACE) until the proper ones I ordered come in. These shocks worked pretty darn well. I can close off the valves and stand on the truck without it moving or I can open them up and push the suspension to the ground with ease. This design has 18 inches of travel!

Here's the motor welded into place. You can get sprockets and chain at mcmaster.com.

Mount a gas tank up high so gravity will feed it down. I found this body at the dump for free.

Can you see the proper springs under there now? I designed and machined spring guides and stops out of aluminum specifically for this project. Boy were those a pain to install! Also notice the muffler's under the rear wheel well and the gas tank under the dash. You can also see the servos on the drivers side swing arm. I coupled 2 super high torque 1/4 scale airplane servos together. They don't seem to have much power and the pull-pull cable system seems to have a little too much play so we'll just have to see how it all work out. This engine is electric start too! Yeah baby! I used two 6 volt batteries so I can split my RC controlls off of one and use them both in series for starting and getting charged by the engine. Oh, yeah, the engine is a 50cc air cooled two stroke with and automatic variable belt transmission out of a honda moped. I just happend to have rebuilt it about a year ago and had it laying around. You can also see the servo horn (small white round) just above the muffler. It's mounted to the engine and controlls the throttle and brake.

After the first test run I noticed the steering wasn't stable enough. The wheels kept wanting to turn in on each other. So, I made a 12V linear actuator controller using the electronics of a servo and 4 solid state relays. I bought a 12V actuator and hooked up solid linkage. It works much better now. Check out these videos. Also, the chain tention was an issue (I knew it would be) so I made an idler tensioner which works great I really got a feel for how dangerous this could be if it got out of control. I've taken every safety precaution I could think of. It has a "fail safe" that lets off the throttle and applies the brakes if it loses radio communication or gets a low battery and it has a kill switch on the vehicle which is also remotely activated.

I've been asked for plans on my steering "servo". Where do I start. First of all my truck is 12V but the radio stuff is 6V so I paralled two 6V batteries so they could be used to start the engine and get charged by it too. Then I just pulled off of one battery to power the receiver and servos. With that said. The solid state relay dirver I made for the actuator is controlled by 6VDC pulsed from the servo board but switches 12V source power to the actuator. You MUST use solid state relays and not the coil type because of the fast pulses. Also, you must use 4 of the. Two for left (+ and -) and two for Right (+ and -). All the relays do is take what the servo board sends to the servo motor and controlls the relays with that signal to give a similar signal only 12V and high amperage.

Here's the schematic. (REVISED 01/02/09 ... Thanks to Alan)

Here's what the circuit looks like put together.

R, C & L are just soldered to the terminals where I removed the pot off the servo board so I could rig a pot on my steering linkage and it would give the proper signal to the servo board. I drove it without the pot in place and just soldered on there and in center position for a while but it doesn't have the "centering" effect. You have to steer left and then steer right to straighten it out.

Yes, messy and crude but it get's the job done and very nice and smoothly too.

Here is the completed unit. Just attach it to the steering linkage on the truck and I guarantee it will steer just about any size project you make.

Here's a compilation Video and attached are some other videos including slow motion suspension movement.

Prototype Results: There were a few small mechanical type issues that were easily solved in the maiden voyage. The vertical movement of the suspension was wonderful but lateral and centrifugal aspects of handeling were a bit "tight". The power wasn't as high as expected (probably a carb/jetting adjustment). Radio communication was good. There were no glitches or interferences with engine noise or other electrical interference. I used a resistive type spark plug and was conscious of radio type issues durring design in anticipation of those types of problems.

Well, it's been fun. Unfortunatley I don't have room for this beast in my garage so it's getting disassembled soon. All in all it worked out pretty good. I got the idea out of my head and all I wanted was to see a working prototype. So, these parts will sit on my shelf until I come up with something new to make. Any ideas?