This Upgrade was required as the Rock Crawler is going to be the chassis of my Autonomous (Arduino Controlled) RC Car. Also I wanted better control and build a robot project which uses an Arduino.
Research on the internet gave me some insight to the drawbacks of this modal of RC car. It's main fault was it's lack of turning circle and limited amount of lock between turns, so unable to make sharp turns. It's good points where good off-road performance, 4 wheel drive also it's articulation of each front and rear axle.
I had used the car before I took out the PCB which controlled Steering, direction and the speed and indeed it did have poor steering control; Something this instructable is going to address.
Please note : All the pictures used in instructable, show the modifications carried out on the original modal. By looking at the original model you should be able to workout which bits need to be removed to obtain the picture above.
Step 1: Parts/Tools Needed
Maisto Tech Rock Crawler Jr. RC Car
A dremel type tool, small cross head screwdriver
4 M3 x 20mm countersunk screws + 4 nuts
2 small strips of 2mm fluted rubber mat (50mm x 20mm)
1 HEXTRONIX HXT 900 9g Micro/Mini Servo Size:21x12x22 mm / 0.74x0.42x0.78 in
Bought on Ebay from Pro Flight UK http://stores.ebay.co.uk/ProFlightUK for £3.95
Various servo horns and screws (one from the servo above and another from a normal size servo)
Step 2: Completed Front Axle - Dissembled
Step 3: Removing the Front Axle
First, remove the body by removing the 4 screws, accessed from the underside of the car. Carefully pull the aerial lead thorough
the body as you lift the body off.
Now remove the 2 screws which holds the suspension strut onto the chassis.
Next you need to undo the 4 screws which will expose the PCB which controls the motor and steering.
Remove the screw near the center and desolder the battery terminals (right side, looking down) in order to remove the PCB.
Next desolder the wires which go to front axle. One is for the main motor, the other is the steering motor.
We are going to keep the main motor but, replace the steering motor with the servo.
Now turn the car over and remove the 2 screws which hold the axle to the main body. Open the main body by undoing 4 screws, you should see the underside of the original motor controller PCB, which can be removed by removing the single screw, then just pull gently up to remove it. You will need to desolder the steering and main drive motor connections from the PCB so able to mod the front axle without causing damage to the wiring or connections.
You should now have removed the front axle from the car. Place the rear section out of the way
Step 4: Disassembling the Front Axle - Remove Top Section
To Remove the top section undo the 4 screws in the corners. Picture only shows 3 due to the modification carried out later on.
Step 5: Disassembling the Front Axle - Middle Section - Motor Mounted
This picture shows the main motor in it's position, with the top cover removed, this doesn't change. The 3 wire lead goes to the servo motor mounted on the underside the bottom section. The servo lead comes out in front of the motor.
Step 6: Disassembling the Front Axle - Middle Section - Motor Removed
Picture below shows the motor removed,
You will need to cut a slot in the middle section to pass the servo cable though from the bottom section.
It's position is near the back of axle assembly shown by the servo wire.
Remove the gear wheel before removing the middle section.
Step 7: Disassembling the Front Axle - Bottom Section
This picture shows the middle section removed.
You can remove the small steering motor and the arm plus the mount used.
Parts modified in this picture include the steering arm.
1)Remove the stops on the front edge of steering
2)Remove the spring back, associated knob from the steering arm then enlarge the hole to make way for the servo wire.
3)Completely remove the Front Right stud. Looking from the same view as the photo. It's the one where the screw is.
To reassemble the front axle just do the reverse of the steps you carried out to dismantle it. You won't need the steering motor as it's being replaced with by the servo. Ensure that each part is allowed to go back in the exact same place it came from otherwise the front axle will not function and go back together correctly.
Step 8: Attaching the Steering Servo to Bottom Section
As you will see when holding the servo and looking at the original bottom section from the RC Car. Alot of time was spent with the dremel tool removing various bits of plastic to allow for the servo to fit in such a small space, without hitting any other part which will cause problems with the mounting of the needed components.
Parts which can't be replaced easily are the steering arm, front axle, gear wheels, the mounts upon which the gears are mounted. So stay away from these areas.
On the servo you will need to remove the shoulder mounts down to the body on both sides so it's a lot smaller width ways.
Picture above shows the Steering Servo mounted and ready to be resembled.
When mounting the servo, find the middle of the servo's travel. keep it in the middle. Mount the servo so the servo horn is behind the steering rack. Applying a little pressure helps keep the knob of the steering rack in place. The slot cut in the bottom section will stop the arm moving the steering to full lock both ways.
The way my servo is mounted and the slot is cut. Is as so
When steering is at full left lock (looking from the front) the servo is at 90 deg at full right lock is ~45 deg when dead center is about half-way (45/2 = 22.5). These values will be needed to set-up the servo in the Arduino code so not to overload the servo and will be adjusted as needed. More pictures have been added to show this. Also if you look at the left lock photo you don't need to cut the slot to the right, just extend it to the left to allow the arm move all the way so it hits the steering arm
Step 9: Making the Servo Horn
1) Using a servo horn from the ones supplied with the HEXTRONIX HXT 900 9g Micro/Mini Servo.
2) Cut it down to about 2 holes from the center, then using an normal size servo horn, with a slot cut into it (closed at one end)
3) This horn needs to be cut to length (31mm) so the end of it doesn't touch the middle section. The length is 31mm.
4) Using small screws attach both horns to each other by passing one screw one way and the other the other way as shown. If you have access to a 3D Printer you should be able to print one. This was something I did after this photo was taken. Pictures shown below.
The servo wire is passed around the bolt nearest the wire outlet and laid flat under the servo towards the opposite corner, so not able to move once clamped by the rubber straps shown.
Step 10: Bottom Section - the Underside
This photo shows the 4 (3mm) holes drilled for the screws used to mount the servo to bottom section. Also the slot thorough which the servo horn is passed to engage with the knob on the steering horn.
Using the dremel tool the CE label has been removed and the area around the 4 holes has been made flat, also the area around the steering spring back peg as it's no longer needed. Near the 3rd hole (bottom left hand side) I removed the screw which holds on the front bumper attached it back with the right hand screw which seems to hold it well.
You need to space the servo from the bottom of this section, this helps the servo move the steering arm with the least amount of force. For this I used 2 pieces of the rubber mat. one was cut in the shape of a rectangle and mounted with the flutes horizontally so the top square piece could interlock and lay flat. This pad also provides good grip for holding the servo. It was attached using superglue and placed under pressure by mounting the servo, then allowed to cure for 24hrs.
Step 11: Bottom Section - Inside View
The number of missed drilled holes was because I first thought of mounting the servo on the inside, which later on decided not to, as you can see on the completed picture in step eight. The hole near the 3rd hole is where the servo wire is passed up though the bottom section into then in to the middle section, in front of the motor and out towards the rear and a new hole cut in the top section. The holes started as 3mm ones which where enlarged to help with the mounting of the servo using the rubber straps.
Step 12: Final Thoughts - Conclusion
It still needs to be tested and it's going to interesting how the servo deals with being so low it's going to act as a plough. I'm thinking about attaching some of the rubber mat to cover the front and provide some protection.
Now it's mounted and you can turn the steering from full lock to lock, you can see that the steering has greatly improved, giving greater control. I'm not using the original PCB as made one, which will drive the motors front and rear using a H-bridge IC chip.
Photo of PCB shown above.