Tires: calculate torque on tires, metallic surfaces, vertical surfaces

Question
Hi, I am currently making a small battery powered surface vehicle for a project. And I am at the stage of selection tyres for the model. I just have some questions in regards to calculating the following:

I need to know what is the size of the tyres (in terms of the diameter, width, gripping capability on various metallic surfaces) and what formulas do I need in order to obtain the above information and how much materials and contact size so it will be necessary to produce a adhesion of approximately10N/cm2.

The tyre material will be made of Gecko adhesive materials in a normal vehicle tyre profile, so it can apply a “Peel off Action” of the Gecko material for climbing up different smooth and vertical surfaces.  The Gecko materials can apply a contact force of 10N/cm2, and the vehicle could weigh 500-1000grams. The dimension of the vehicle will be approximately 150mm x 70mm x 80mm (L x W x H). The speed of the vehicle is unimportant, but climbing and sticking on vertical smooth surfaces is the most important factor. Although I am using Gecko material instead of rubber, but if I know something about normal tyre calculation, it will help me tremendously.

Thank you for your time, your help will be greatly appreciated.

Regards

Peter Harb  

Answer
Peter,

This isn't a "tire" question - it's a physics question.

For an object to adhere to a vertical surface, you need to know the weight of the vehicle, the height of the center of gravity, the surface area in contact with the surface, and the adhesion level of the tire surface (that you already know).

1)  The height of the center of gravity and the weight of the vehicle are needed to calculate the actual loads on the vertical surface.  BTW the front tires will have a negative force.

2)  You'll need a graph of surface area in contact with the road surface vs vertical load - and the graph must include negative loads.  This will vary according to the actual contruction of the tire (and normally this type of graph would not include negative loads, so you're in uncharted territory - meaning you'll have to develop the graph based on your own experiments).

But from a practical point of view, anything that has ahesion will pick up debris from the surface it is in contact with.  This means eventually the tire surface will pick up dust, dirt, etc. and then the adhesion value will be that of whatever is picked up.  I'm guessing dirt and dust have an adhesion value less than zero - they wouldn't adhere.

Hope this helps.