RACECAR/J Chassis Preparation


This is the first step of building a RACECAR/J. Here we prepare the chassis by removing parts we do not use, and upgrading the springs and bumper. Looky here:


RACECAR/J is derived from the open source MIT RACECAR, an “open-source powerful platform for robotics research and education”. The first version of RACECAR/J is based on MIT RACECAR build 2.5, the current build as of November, 2017.

The RACECAR/J Chassis is based on the TRAXXAS Slash 4×4 Platinum Truck, an upgraded version of the normal Slash 4×4 which adds aluminum bits and pieces such as C-hubs, steering blocks, rear hub carriers and axle nuts. An anti-roll bar is also added, which helps better handle the weight that we will add with the autonomy sled.


The tools provided with the Traxxas Slash can be used to complete the procedures we are discussing. However, tiny allen wrenches may not be your cup of tea. I have found the following tools very useful, especially if you’re building a lot of robotic projects:

Chassis Preparation

As shown in the accompanying video, there are several steps in preparing the TRAXXAS Slash. Preparation takes 30-45 minutes. Most of the preparation involves removing parts of the RC Car which which are not used. Here are the major steps:

The video gives detailed instructions on the modifications.

Remove the Body and Body Mounts

The first step is to remove the Traxxas Slash 4×4 Platinum from the package and place it on a work space.
Remove the body retaining clips, remove the body, and then remove the two body mounts. There are two screws holding each body mount in place.
Here are some pictures:
Remove the 4 retaining clips holding on the bodyRemove the 4 screws holding the body mounts, then remove the body mounts

Remove Receiver Box

The Traxxas Slash does not have a receiver, but has a box for one attached to the chassis. In this step, remove the receiver box.
Remove the four screws holding the top of the receiver box in place. Remove the top cover, and thread the ESC and Steering Servo wires from the Receiver box. Next, remove the two screws holding the lower receiver box to the chassis, and remove the lower receiver box.
Here are some pictures:
Remove these screws to remove Receiver BoxRemove the top cover of the receiver box, and unthread Servo and ESC wires from box.Remove the two screws to remove the Receiver BoxLower Receiver Box has been removed.

Remove the Stock ESC

The next step is to remove the stock Traxxas ESC. Detach the wires going to the car motor, and then remove the two screws that are holding the ESC in place. Then remove the ESC.
Here’s some pictures:
Detach the motor wires, and then remove the screws attaching the ESC to the chassis.Disconnect the Motor WiresThe Stock ESC has been removed.

Now remove the antenna holder. A pair of pliers may be useful for this operation.

Remove the Front Bumper

There are five screws holding the front bumper to the chassis. Remove the two screws holding the top the bumper to the chassis. Next, remove the three screws holding the bottom of the bumper to the chassis. Then remove the bumper.
Remove the two screws holding the top of the front bumper to the chassis.Remove the three screws holding the bottom of the front bumper to the chassis. Remove the bumper.

Replacing the Springs

Because of the weight being added to the RACECAR, new springs are added to the Traxxas Chassis. The springs are available on the Racecar/J web store. The entire RACECAR/J can be be bought as a kit (minus cameras and lidar), take a look around. Note: RACECAR/J Kits and parts are currently only shipping to the United States. New regions soon!

Replacing the springs is difficult to describe, watch the video for a walk through. Remove the screw holding the lower shock assembly to the suspension arm. Afterwards, you can flip the shock assembly up for easier access. For the front shocks, access is easier if the front bumper is removed. For the rear shocks, access to the attachment screw is easier if the car is placed upside down.

Once the shock assembly is free, compress the spring. Remove the spring retainer, then remove the old spring. Install the new spring. For the front shock, the short 2″ springs are used. For the rear springs, the longer 2.75″ springs are used. Compress the new spring after placing it on the collar, and reinstall the retaining clip. Reattach the shock assembly to the suspension arm.
Here’s a couple of pictures, but the video may be more useful here:
Remove the two screws holding the shock assembly to the suspension arm.The front shocks are installed.

Bumper Installation

The front bumper is replaced with a foam bumper, for better impact resistance. This is particularly useful for indoor use.

The bumper included in the RACECAR/J kit is a JConcepts Scalpel Bumper kit. For installation, see the directions included with that kit. People also use the Traxxas foam bumper, which uses Traxxas part numbers 7436, 7415x, 7437. Note that these are Amazon links, you can also order through There have been reports of clearance issues with the new springs with the Traxxas foam bumper parts, you may have to modify them to get a proper fit. I have not worked with the Traxxas foam bumper parts, so I have no experience to share.


Preparation of the chassis is straightforward. Basically remove the bits and pieces that are not needed, upgrade the springs, and add a new front bumper.

There are many ways to modify this build to suite any given application. In this build, we remove the stock ESC so that it can be replaced with a VESC. The VESC is an open source brushless DC motor controller. This provides better control at slow speeds than the stock ESC, as well as the ability to monitor engine speed. The engine speed can be used to calculate crude odometery, since there are no encoders built into the car drivetrain.

There are two mounting points for cameras, and a place for a lidar. In the next article, we will cover installing the platform decks and basic electronics that control the chassis. Off to Part II – RACECAR/J Platform Preparation.