Drifting RC Car Inspired by Razor’s Crazy Cart

Hey all! Today I’m excited to share my latest project: an RC car that drifts like the Crazy Cart sold by Razor. The cart looks like a ton of fun to drive around in, so I thought, “Why not make a miniature version?”

To complete this project over winter break, I teamed up with some fellow members of my school’s American Society of Mechanical Engineers (ASME) club - Phil Le and Joshua Ng. We’re all engineering students at San Jose State University (SJSU) and this was our first time diving into an RC project.

In this Instructable, we’ll explain how we made our RC Drifter, so that you can make one too!

When choosing parts for our build, we wanted to spend as little money as possible - but since our club has a ton of random stuff lying around from old projects, the parts we ended up using weren’t necessarily the cheapest, they were just the ones available - which made them free! So if the mishmash of parts seems odd, that’s why. If you’re trying to choose parts for your own project, there are better and cheaper options.

Here’s what we used:

Electrical Components:

1. Transmitter - FlySky FS-i6
2. Receiver - FlySky FS-iA6B
3. Battery - Tattu 850mAh 14.8V 95C 4S1P
4. 2 Servos
5. One for steering - Just ‘Cuz 32KG
6. One for activating drift mode - HiTec HS-311
7. Motor - FLASH HOBBY D3548EVO 1150KV 760KV
8. ESC - FlyColor 60A 3-6s 5A BEC FlyDragon Lite

Other:

1. 2 Small Caster Wheels
2. Banebots 3” Wheel with 6mm Hub
3. 6mm Steel Shaft
4. R4-2RS Bearings (x2)
5. Fingertech 42T Pulley
6. 246mm, 82T S3M Timing Belt

Hardware and Fasteners:

1. M3 and M4 Socket Head Screws
2. M4 Washers
3. M3 and M4 Heat Set Inserts
4. Brass Rod (Used for connecting parts)

Tools:

1. 3D Printer
2. Allen/Hex Keys
3. Superglue
4. Soldering Iron (For Heat Set Inserts)
5. Calipers
6. Hacksaw (For cutting brass rods and steel shaft to size)
7. Files
8. Screwdriver (with assorted bits)

This is the rear of the RC vehicle, which includes the caster wheels and “drift bar” mechanism. Designed in Fusion 360 and 3D-printed by Joshua Ng, it allows the back wheels to switch from default to “Crazy” mode, enabling omnidirectional driving.

It's composed of 3 parts: the Drift Bar, the Hinge Plate, and the 4-bar linkage that connects the two

The purpose of the hinge plate is to hold the HiTec HS-311 servo in place and provide the drift bar with a stationary axis of rotation. The drift bar holds the caster wheels, and it should ideally rotate around the same axis as the caster wheels. If it does, less torque will be required from the servo as it switches between default and drift or "Crazy" mode, as the body of the car will remain at the same height.

Joshua designed and printed the hinge plate in tandem with the drift bar, mounting the servo using M3 screws and heat-set inserts. The linkage arm was also mounted to the servo horn using M3 screws.

The magic of a Razor Crazy Cart comes from the Drift Bar Mechanism, and this RC vehicle is no different. The goal is to be able to switch the caster wheels from two different orientations: upright, and ~45 degrees from upright. The upright position is the “Crazy mode” and the 45 degrees from the upright position is the normal, resting position. To achieve this, Joshua used a 4-bar linkage with two toggle positions, connected using brass rods.

Originally he used relatively thin links, but in testing we found that it produced a wobbly or unstable effect on the drift bar. In newer versions, he made the link between the drift bar and the hinge plate much thicker. (It’s almost the full length of the hinge plate!) This solved the issue of instability at the rear.

The small protrusion also prevents the links from becoming colinear, which would cause a jam. This also ensures the links are in the correct place each time the car powers on.

The Drift Bar acts as a mount for the caster wheels. Certain measurements of the caster wheels are important in this design, specifically in this case, the spacing between the holes on the plate of the caster wheels. Joshua used these measurements to place holes in the CAD model to fasten the caster wheels to the 3D-printed part. He used M4 screws and washers with heat-set inserts for this.

This part is the middle of the RC vehicle, which houses the electronics. It was designed by Phillip Le. The design process for this section was relatively simple, as it has no moving parts. Its geometry is dependent on the front and back sections and any electronic components we needed to add. For the middle section, we only needed to house a receiver and a battery, so Phil modeled stacked compartments for both parts. He did this to prevent weight imbalance and to allow the battery to be easily removable. There was also originally a small compartment for the ESC, but it was later decided to move that to the front section for better wire management.

To model the middle section, Phil used TinkerCAD, producing several iterations that were somewhat blocky. Most of these shapes were made through booleans and model subtraction instead of extrusion.

These models were transferred briefly to Blender for specific shapes and then finally, Fusion 360 with my help to iron out all the kinks. The width of the middle baseplate varied throughout the design process as the cart width was discussed and changed.

One of the components we decided on scrapping was a switch. We realized that it would’ve been more work to solder and attach the switch, and we could simply plug and unplug the battery to turn the drifter on and off. The switch was scrapped, which saved us some space and led to the final design, which was incredibly compact and easy to print.

Like the Razor Crazy Cart, our RC car has only a single front wheel. This is part of what allows the car to drift, but it presents an interesting design challenge. The front wheel must simultaneously spin to provide acceleration and turn to allow steering. As a result, the motor and wheel have to turn left and right in unison.

Taking design inspiration from Razor’s version, I aimed to design a housing unit to hold the motor, wheel, and accompanying hardware to all turn in unison. I started by laying out all the components in CAD, basing all dimensions on the center-to-center distance between the motor and wheel axes, calculated with Fingertech’s Pulley Belt Distance Calculator. Then, I could model a structure around the components to be 3D printed and bolted together.

As we didn't have the smaller 26 tooth pulley that the design called for, I ended up 3D printing one in two pieces, gluing them together. This worked great! The aluminum pulleys are designed for combat robotics, but the 3D printed one worked just fine for this application. I wish I did that same for the larger 42 tooth pulley too, since I had to drill out the hole to fit the aluminum pulley on the 6mm shaft (and I lost the set screw the pulley came with!). If you use intend to use pulleys, I highly recommend trying printed versions!

The housing for the motor and wheel has to turn independently from the rest of the car. So, I designed the steering mount, which would provide a place to mount the steering servo and connected the housing to the middle section.

The shown model was later modified slightly to provide a mounting spot for the ESC.

The steering gears required several iterations. This is because the cheaper servo we intended to use for steering, the HS-311, had a relatively low range and torque output. By changing the gear ratio between the housing and steering servo, we could increase the amount the wheel housing could turn, but at the cost of torque. The amount the housing can turn is important because we didn’t have the ability to reverse the motor, so if you ran the car into a wall, it would get stuck. After iterating through several gears, it was decided that the HS-311 did not have enough torque to turn the housing reliably at a satisfactory range, and we switched to a different servo, the Just Cuz 32KG.

This servo was incredibly overkill, but it’s all we had available, so we had no choice. It was also a different height and had different sized servo horns, causing the need for several redesigns and reprints. In the end though, this servo was more than enough to turn the wheel housing reliably and at a satisfactory angle.

While driving the RC Drifter, we found some changes we wanted to make.

First, the car had a tendency to flip over because of the top heavy front and the relatively long and narrow wheelbase. To rectify this, I designed and printed an anti-tilt piece to go between the front and middle sections. It's significantly wider and lower than the base of the car, which stops the car from tilting and causing a flip. It's also curved on the bottom, encouraging it to glide along the carpet instead of getting stuck. This piece is similar to the legs that Razor's Crazy Cart has, which you can see stick out in front of the front wheel, preventing the vehicle from overturning.

We also found the motor to be insanely overkill for this project - In fact, it would spin the wheel so fast that the wheel would expand due to centrifugal force, rubbing against its housing and tearing itself apart. While I expected the motor to be a bit much, this was a surprise to say the least. The solve here was simple: we limited the throttle in the Transmitter settings.

A small improvement was to tape the battery in with masking tape, as it had a tendency to fall out when the car crashed.

Finally, steering. Due to the unplanned change in servo choice, we found the increased range of motion - from 96 to 200 degrees - caused significant oversteering. The wheel could not only turn to the left and right, but all the way to the rear and beyond. This caused the steering to be jerky and hard to control, so we changed the endpoints in the servo settings to limit its range.

After much designing, printing, testing, redesigning, and reprinting, we were finally finished! I'm incredibly proud of how our RC Drifter turned out, and it's a ton of fun to drive (if chaotic, to say the least).

I had a ton of fun working Joshua and Phil, and I hope I get to do another project with them in the future.

If you enjoyed this project, I highly recommend giving it a go! The electronics behind RC projects were intimidating when we started, but this was an incredibly engaging way to learn about them. It was also Phil's and Joshua's first time modeling for 3D printing, so I'm super proud of what they were able to produce. If you're looking to get into 3D modeling, 3D printing, or RC cars, a project like this is a great place to start!