Introduction: "12 Thumbs" R/C LEGO Dragster

About: I am Electronic Visualization Artist. I look at things through the Looking Glasses.

Impressed by these two LEGO® Dragster sets, a 250 pieces LEGO® TECHNIC Dragster 8847 (released in 1983) and a 391 pieces Top Fuel Racing 5533 (released in 1999). These two Dragsters inspired me to build my own LEGO® dragster, the "12 Thumbs".

The name "12 Thumbs" came after I finished the design. While I was playing with my finished dragster on the cutting board (that I usually use as the background of my instructable photos). I spot that the the distance between the front and rear wheelbases is more or less 12" long (See photo 4, 5)
I thought about the name "12 incher" but I don't like the name. So I googgled around and came across the measurement term used in the old days, thumb, which is equivalent to an inch. (Source:
I like the word! So that's it! My dragster got it name, "12 Thumbs"

My "12 Thumbs" is the easiest of all my custom LEGO® car to build. It only used 129 LEGO® components (not included Arduino and motor controller PCB). It used the same kind of wheels and tires as LEGO® 8847 model, a lot smaller than the LEGO® 5533 "Red Fury" that used the wheel and tires of 81.6 x 34.

The "12 Thumbs" equipped with a small servo to steer the car, same type of servo (9g) that I used with my other cars, R/C LEGO® Car (& Redux), R/C Race Car, or "The Bull" Buggy, etc.
But this time, instead of using the DIY servo supporter that made out of 2x2 plates. I design LEGO compatible, 9g servo mount that is (See next Step.)

For the motor housing, "12 Thumbs" is also using my 3D Printed Motor Housing. (See details in my R/C LEGO "The Bull" Buggy, Step 1)

In the mid section of the car is a battery compartment that is big enough to fit the 9V battery (or small 7.5V rechargeable LiPo battery) into. At the rear section, is where the Arduino and motor controller PCB is located next to the motor housing.

With 7.5V LiPo battery for motor power supply, and 9V, 24mm dia. round DC motor, Arduino, and DRV8833 motor controller IC (Details in Step 8), my "12 Thumbs" just flew out from the start line just like the real Top Fuel Dragster. On the first run "12 Thumbs" I set it speed to 3, it run so fast that I could not steer it straight, so I crashed it to the wall (See the Video in Step 10). Luckily there was no damage, the front wheels just flew off both sides.

"12 Thumbs" Dragster can be controlled wirelessly via XBee, or any other wireless radio, like bluetooth, or WIFI. In this instructable I chose to use XBee module since I have more than two XBee modules with me.

For the XBee Radio Control see R/C LEGO® Car, or Processing Controls R/C Car with XBee modules as examples.

LEGO®, TECHNIC, are property of The LEGO Group of Companies (, which does not sponsor, own, authorize or endorse this creation.

Video is available in Step 10.

Step 1: 3D Printed Servo Mount

3D Printed Servo Mount idea came after the 3D Printed Motor Housing. And with the same reason, since I was not very happy and was tired of hacked or modified LEGO plates to use as the 9g servo mount. Even though, I could used the servo mount from my previously build cars. But that what I do not want to do, I always keep my custom design LEGO car!  

So I am prototyping a LEGO compatible Servo Mount. This 3D Printed Servo Mount is the fifth version. The design and 3D modeling processes were done in 123D Design.

The opening in the 3D Printed on the top of servo mount is for the wiring from the servo. The Servo Mount has LEGO compatible mounting plate at the bottom. 

This 3D Printed Servo Mount fits any 9g servo with the dimension of 23 x 12.2 x 27mm. The servo that I used for this 3D Printed Servo Mount is Tower Pro's micro servo model SG92R (available from Adafruit's ID#169 or Tower Pro website).

Attached is the STL file, you download below. You can view this 3D Model at my 123D project page. The Servo Mount is still in prototyping stage, any comments and suggestion will be really appreciated.

It is a good value for the long run. My Servo Mount for Lego car design can be used in my other custom LEGO car design too.

Gallery of photos below show the 3D model in 123D app. and the actual 3D Printed ordered from a third party fabricator.

Step 2: Parts & Tools

Following is the list of Lego Technic parts that I used for building this "The Bull" Buggy.
If you want to do this project you could use the differenct color for the car.

Note: The number in the bracket is the Lego's Design ID.

Lego Parts
4 nos. - 1x16 Technic Brick (#3703)
2 nos. - 1x12 Technic Brick (#3895)
2 nos. - 1x10 Technic Bricks (#2730)

2 nos. - 1x8 Technic Brick (#3702)
8 nos. - 1x6 Technic Brick (#3894)
2 nos. - 1x4 Technic Brick (#3701)

5 nos. - Technic Brick 1X2, Ø4.9 (#3700)

1 no. - Roof Tile 2X2/45° (#3039)

2 nos. - Angle Element [1], 0 Degrees (#32013)
2 nos. - Angle Element [4], 135 Degrees (#32192)
2 nos. - Angle Element [5], 112.5 Degrees (#32015)

4 nos. - 1x4 Plate (#3710)
4 nos. - 1x6 Plate (#3666)

6 nos. -  2x4 Technic Plate (#3709)
4 nos. -  2x6 Technic Plate (#32001)

4 nos. - 2x2 Plates (#3022)

2 nos. - 1x2x2 Corner Plate (#2420)

4 nos. - Technic Bush 1/2 Toothed Type II (#4265b)

1 no. - Pin Connector Round (Pin Joiner Round)  (#75535)

19 nos. - Connector Peg with Friction (#2780)

4 nos. - Connector Peg with Cross Axle (#6562)

2 nos. - Perpendicular Axle Connector (#32039)

2 no. - Axle Joiner (#6538b)

2 nos. - Cross Axle 2M with Groove (#32062)
1 no.   - Cross Axle 3M with Knob (#6587)

2 nos. - Cross Axle 3M (#4519)
2 nos. - Cross Axle 4M (#3705)
2 nos. - Cross Axle 5M (#32073)
4 nos. - Cross Axle 6M (#3706)

1 no. - Cross Axle 10M (#3737)

2 nos. -  Bush for Axle (#6590)

4 nos. - Hinge Plate 1 x 4 Swivel Top/Base (#73983)

3 no. - 1x6 Plate with Toothed End (#4262)

1 no. - 1x4 Gear Rack (#3743)

2 nos. - Hose Rigid 3mm D. 5L / 4.0cm (#75c05)

2 nos. - Technic Level 3M (#6632)

Steering Kit
2 nos. - Steering Arm (#4261)
2 nos.   - 8-Tooth Gear (#3647)

Gear Kit
1 no. - 24-Tooth Crown Gear Type III (#3650)

Wheels and Rims
2 nos. - Technic Wedge Belt Wheel (Pulley) (#4185)
2 nos. - Wedge Belt Wheel Tire (#70162)
2 nos. - Technic Wheel 20 x 30
2 nos. - Black Tire 20 x 30 Solid Balloon (4266 / 2857)

Servo Mount or modify LEGO plates to use as servo mount. (Download the stl file below)

9g Micro Servo (T Pro SG90) or similar.

2mm Shaft Adapter for Lego Wheels (Pololu part #1001)
(or Modified Connector Peg/Cross Axle (#6562) to fit the shaft of DC toy motor.)

LEGO compatible Motor Housing for Round 24mm dia. 9V motor. (STL file can be download from my "The Bull" Buggy project.)
or if you do not have 3D Printer (I wish I have one too!) You could order one from Shapeways, here.)

Round DC Motor: 9V, 24mm in diameter.

Two M2.3, 4 mm long round head screws (if not come with the 9V, 24mm. dia. round motor.)

DIY Arduino and Motor Driver PCB (See detail in Step 9)

XBee Module

XBee breakout board (I used XB-Buddy Basic Kit, Jameco Part no. 2163680)

Hook up Wire

Super Glue
X-ACTO Knife
Sand paper
Hand Drill & bits

Step 3:

I started to build the "12 Thumbs" from the front end which is the the front wheelbase.

Following are the photo instruction, which is also included description text box(s) in each step.

Photo 1, 2, 3 Two 1x6 Technic Bricks (#3894) and two 1x4 Technic Brick (#3701), and three Connector Peg (#3673)

Photo 4, 5, 6, 7, 8, 9, 10 Two 1x8 Technic Bricks (#3702), six Connector Peg (#3673), and 9g servo assembled in servo mount from last Step.

Photo 11, 12, 13, 14 Technic Plate 1x6 with Toothed End (#4262), two Steering Arm (#4261), and two Connector Peg with Cross Axle (#6562)

Photo 15, 16 Plate 1 x 6 with Toothed Ends (#4262)

Photo 17, 18, 19, 20 1x4 Gear Rack (#3743), Plate 1x4 (#3710), and Plate 1 x 6 with Toothed Ends (#4262)

Photo 21, 22 Two Connector Peg with Cross Axle (#6562)

Photo 23, 24 Two Technic Wedge Belt Wheel (Pulley) (#4185) and Wedge Belt Wheel Tire (#70162)

Photo 25, 26 2x4 Technic Plate (#3709)

Photo 27, 28 2x4 Technic Plate (#3709)

Photo 29, 30, 31 Technic Brick 1X2, Ø4.9 (#3700), and Cross Axle 3M with Knob (#6587) glued to servo horn.

Photo 32, 33 Axle Joiner (#6538b)

Photo 34, 35, 36, 37 Two 8-Tooth Gear (#3647), Cross Axle 5M (#32073), and Technic Brick 1X2, Ø4.9 (#3700)

Step 4: Battery Compartment

This Step will be showing the continuation of the middle section and form the battery compartment.

Following are the photo instruction, which is also included description text box(s) in each step.

Photo 1, 2, 3 Two 1x16 Technic Bricks (#3703), and Two Connector Peg (#3673)

Photo 4, 5 Three 2x4 Technic Plate (#3709)

Photo 6, 7, 8 2x4 Technic Plate (#3709)

Photo 9, 10, 11 Four Connector Peg (#3673), and Two 1x16 Technic Bricks (#3703)

Photo 12,13 Three 2x6 Technic Plate (#32001)

Photo 14, 15 Two 1x2x2 Corner Plate (#2420)

Photo 16, 17 Four Hinge Plate 1 x 4 Swivel Top / Base Complete Assembly (#73983)

Photo 18, 19 Two 1x6 Technic Bricks (#3894)

Photo 20, 21 1x6 Plate (#3666)

Photo 22, 23 Two 1x12 Technic Bricks (#3895)

Photo 24, 25 Roof Tile 2X2/45° (#3039)

Photo 26 Middle section or Battery compartment is done.

Step 5: Rear Wheelbase & Motor Housing

This Step will be showing the building of the rear wheelbase and the installation of 3D Printed motor housing for 24 mm diameter motor, the same one that I used in "The Bull" Buggy project

Following are the photo instruction, which is also included description text box(s) in each step.

Photo 1, 2 Cross Axle 10M (#3737), and 24-Tooth Crown Gear Type III (#3650)

Photo 3, 4 Two Bush for Axle (#6590)

Photo 5, 6 Two 1x6 Technic Bricks (#3894)

Photo 7, 8 Two  Technic Wheel 20 x 30, with Black Tire 20 x 30 Solid Balloon (4266 / 2857)

Photo 9, 10 Join the rear Wheel base to the middle section frame.

Photo 11, 12, 13, 14 3D Printed motor housing, 24mm. diameter 9V DC motor and two 4mm long M2.3 screws.

Photo 15, 16, 17, 18 2mm Shaft Adapter for Lego Wheels (Pololu part #1001) (or Modified Connector Peg/Cross Axle (#6562) to fit the shaft of DC toy motor.)

Photo 19, 20 8-Tooth Gear (#3647)

Photo 21, 22, 23 Assembled motor housing

Note: For installation details of the Arduino + Motor Controller PCB see Step 8

Step 6: Rear Wing Flap

This Step will be showing the building of the rear wing flap.
Note: In case, if you want to build this Dragster and you could not find the part as shown below, you could substitute them with different components. Or you could design your own rear wing flap base on the one shown here. This is another spot that you can show off your creativity!

Following are the photo instruction, which is also included description text box(s) in each step.

Photo 1 Two 1x10 Technic Bricks (#2730)

Photo 2, 3 Four Connector Peg with Friction (#2780)

Photo 4, 5 Two Angle Element [1], 0 Degrees (#32013), Two Cross Axle 6M (#3706) and Two  Angle Element [5], 112.5 Degrees (#32015)

Photo 6, 7 Two Cross Axle 3M (#4519)

Photo 8, 9, 10, 11 Two Tan Angle Element [1], 0 Degrees (#32013), Two Cross Axle 6M (#3706) and Two White Angle Element [1], 0 Degrees (#32013).

Photo 12, 13, 14 2x6 Technic Plate (#32001)

Photo 15, 16, 17 Axle Joiner (#6538b), Two Cross Axle 4M (#3705), Two Technic Bush 1/2 Toothed Type II (#4265b) and Two Perpendicular Axle Connector (#32039)

Photo 18, 19 Two Angle Element [4], 135 Degrees (#32192)

Photo 20, 21, 22 Pin Connector Round (Pin Joiner Round)  (#75535), Cross Axle 5M (#32073) and Technic Bush 1/2 Toothed Type II (#4265b)

Photo 23, 24 Technic Bush 1/2 Toothed Type II (#4265b)

Photo 25, 26 Cross Axle 2M with Groove (#32062)

Photo 27, 28 Two Hose Rigid 3mm D. 5L / 4.0cm (#75c05)

Photo 29, 30 Two Technic Level 3M (#6632)

Photo 31, 32 1x6 Plate (#3666)

Photo 33, 34 Assembled the Rear Wing Flap to the Rear Wheel base.

Step 7: Motor Controller & Palm Arduino Housing

Continue from previous step. This Step will be showing the building of the rear wing flap.

Following are the photo instruction, which is also included description text box(s) in each step.

Photo 1, 2 Two Technic Brick 1X2, Ø4.9 (#3700)

Photo 3,  4 Two 1x6 Technic Bricks (#3894)

Photo 5, 6 Two 1x6 Plate (#3666)

Photo 7, 8 Two 1x4 Plate (#3710)

Photo 9, 10 1x6 Plate (#3666)

Photo 11 "12 Thumbs" build up to this point. Ready to mount the Arduino and motor controller PCB.

Step 8: Arduino & Motor PCB Installation

In this project I chose the DRV8833 (datasheet) , as motor controller, because I learned that I can connect the two H-Bridges in the DRV8833 together in parallel, so the single H-Bridge can have the current twice as much. (See the schematic below.)

Photo 1 Shows Motor Controller Breakout Board with DRV8833 IC and 5V regulator for 9g servo.

Photo 2, 3, 4 DIY Arduino and motor controller PCB, XBee module, and XBee Breakout board(Jameco Part no. 2163680)

Photo 5, 6  installation of 3.7V LiPo Rechargeable battery as the power supply for Arduino and Xbee Module.

Photo 7, 8,  9 Installation of Arduino and motor controller PCB.

See how to mount the PCB to be used with LEGO components in Step 10 of R/C LEGO Car Redux.

Photo 10, 11 Shows the installation of the 7.5V LiPo Battery.

Photo 12, 13  Shows an alternative installation of 9V Battery.

Step 9: Processing UI and Arduino Sketch

This Step shows you how I conceptualized the UI (User Interface). 

Since the "12 Thumbs" Dragster is quite different from other type of car, and it's nature is for the straight course race (1000 feet, actual race course). So I designed the UI to have only (See picture below)
  • Speed buttons (from 1 to 6), the low number means lower speed. The highest speed is six.
  • Left and right turn controls are for trimming the steering the car not to run off the straight course,
  • Current Speed: display box, so user knows what is the speed of the car at all time.
  • and Stop button.
The background of the UI shows the steering wheel grips for user to create the feel of driving the car.

The original design of the UI was intended for handheld type devices such as Smartphone, iPhone, iPod, or Android, similar to the UI that I used in Wireless Lego Race Car.

Included are the Processing Sketch (thumbDragsterControl.pde + console images) and Arduino Sketch (thumbDragster.ino) that I used.

Step 10: Summary

The "12 Thumbs" Dragster was designed to be used as a toy race car. The LEGO components used in the project are only 129 pieces and there are no hard to find components.
The race track could be anywhere, in your living room, in the corridor, hallway, but it is too small in these mentioned area.
The most suitable place are at school's gym, or basketball court,  in the park, etc.
The length of the track is up to you. But if you want to make it just like the real Top Fuel Dragster Race. I would recommended the track should be 40 feet.
Since the "12 Thumbs" is already 1:25 (wheelbase) scale to the real Top Fuel Dragsters.
The real Top Fuel Dragsters track is 1000 feet, so 1:25 of 1000 is equal to 40 feet.

Why I am still using XBee as wireless control?
Because, it is very suitable choice for this project. If you are a dad, or a mom, or a kid, who want to build two "12 Thumbs" to race with each in the family. Or if you and your friend want to build more than two "12 Thumbs" and race among each others. 
Xbee module would be a good choice! they could be setup to be in the same network and will not interfering one another. One good example of this kind of project is at Robert Faludi's webpage: Horsie Race XBee Example. You can see the details of how they did it there.

I hope this "12 Thumbs" Dragster would inspire you to build this toy, so you could enjoy yourself with a family member(s) or among friends!

Top Fuel Dragster (
Wikipedia -

Gallery of Photos

Photo 1 to 7
Previous photos of 12 Thumbs with th Palm Arduino V3. These photos were taken after the final design only.
Photo 8 to 12 Latest photos of the "12 Thumbs" after the test run.

- Test run a few times.

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