PiWars Suspension System

Alexpl, CC BY-SA 4.0 , via Wikimedia Commons - Christie derived Suspension System from a Polish 10TP tank
Christie-derived suspension system from Polish 10TP tank:
1. Roadwheel
2. Spring
3. Arm

Alexpl, CC BY-SA 4.0, via Wikimedia Commons

Due to PiWars course for “Lava Palava – Escape the volcanic eruption!” containing at least one speed-hump, and the obstacles in “The Temple of Doom – Go on an adventure and tackle the obstacles in your way!” are unlikely to be flat, there should be some compliance in the drive and suspension system to allow the robot to keep the tank tracks in contact with the floor as much as possible.

The East-Devon PiRates have an image with the dimensions for the speed hump on their page. It’s a minor segment of a circle, with a base dimension shown to be 570 mm and a height of 53 mm (The diameter of the circle is not shown).

With the maximum length of the robot being limited to 300 mm, this means that the speed hump is longer than the robot. As such the robot needs to climb one side and descend the other (rather than ride over it like the orange circle in the animation of the Polish 10TP tank’s suspension). The compliance is therefore required to keep the tracks in contact while transferring from the floor to the ramp, and the ramp to the floor, and allowing the tracks to conform to the curvature of the ramp.

I have moved away from using the Vex Robotics Tank Track and am now referencing the track design on the Speed Tank by Janka, this allows for more of the design to be 3D printed and is easier to replace anything that may get damaged in use, or perform redesigns as required. This also allows for the drive sprockets to be sized appropriately, and match the dimensions of the hull.

Suspension System Design

The suspension system we are using is a Christie-Derived system, that, unlike the suspension system in a car, uses either coil-over (dampers) in a vertical arrangement, MacPherson struts or in the case of more commercial vehicles leaf springs. Is based around a design used on full-size tanks, designed by the American engineer J. Walter Christie. The advantage of the Christie design is that it allows considerably longer movement than conventional leaf spring systems then in common use.

To allow for a lower profile hull base, we are using a bell crank similar to the second animation to allow for greater travel without the height required for the spring system.

Since we are using a spring under tension, rather than compression, I will be looking at using elastic bands rather than coil springs. Elastic bands allow for easy adjustments, and replacement if they get damaged.

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