# How to design a robot chassis so as to avoid bending issues?

I'm good on the programming, control systems and electronics aspect of robotics, but need help with a problem with mechanical design.

How do I avoid bending of my chassis due to weight? It is a 2 wheeled differential drive robot with a castor wheel in front. Since these are the only three contact points, the chassis is bent along Xand Y axis. What changes can I make to the design to avoid this effect by the bending stress?

• What changes can I make to the design? ... we do not know what your design is ... you provided no information about your robot, other than the number of wheels – jsotola Jul 19 '20 at 19:40

## 1 Answer

It is difficult to provide a specific answer due to not knowing details of your design, but consider these factors for the deflection of a beam. They should give you ideas.

For a beam supported at the endpoints, with a load in the center, the deflection increases if:

1. The load increases.
2. The distance between endpoints increases.

The deflection decreases if the material supporting the load:

1. Has a higher Young’s modulus.
2. Is thicker.

Interestingly, the distance between endpoints very much affects the deflection - all of the other parameters are linear for increasing or decreasing deflection except for the distance. For that parameter, the deflection increases relative to the cube of the distance between endpoints.

So what can you do?

• lighten the load
• move the attachment points closer together
• use a stronger base to hold the load
• use a thicker base to hold the load

Now it gets interesting. You don’t need to increase the strength or thickness of the whole platform, just that of the material between the connection points. Consider if you added a thin, flat bar of metal to brace the platform. This bar is much more easily deflected if it lies flat relative to the floor than it is if it is mounted with the longer dimension vertical (like a wall instead of a floor). But that is harder to mount. So here is my favorite idea besides moving the mount points closer together: use angle braces under the platform between your attachment points. The 90 degree angle allows you to have a horizontal surface for mounting to the platform, and the vertical section provides the strength and thickness to counter the deflection force.

• Thickness is part of it; it's the area moment of inertia (second moment of area, not mass moment of inertia) that matters. You can increase the stiffness with a relatively small amount of material if it's far from the axis of bending - this is the concept behind how I-beams work :) – Chuck Dec 16 '20 at 21:37
• Well stated, @Chuck. That was the concept I was trying to convey with the angled brace approach. I never thought to relate it to the second moment. – SteveO Dec 16 '20 at 22:16