Kingpin Torque

[gpommer]

I am working on the design of a steering system for a construction vehicle and I am trying to determine the steering force I will need to rotate the tires. Parker Hannifin has a design guide for their steering units that has an equation for determining kingpin torque based on tire section width, kingpin offset, and axle load. According to Parker, the kingpin torque decreases as the tire offset increases, which, in my mind, makes sense if the wheel is not braked. With the tire offset, the wheel will roll as it is turned. In this situation, with 0 tire offset, the tire contact patch will have to scrub about itself. The thing I can't quite wrap my head around is what happens when the wheel is braked. It would seem that the kingpin torque would increase but I can't find anything to verify this. I have included the link to Parker's design guide for reference and would appreciate any insights anyone would have.

Thanks.

http://www.parker.com/literature/HY13-1560-002 HGF Cat.pdf

 

[patprimmer]

I can only give anecdotal evidence, but I can testify from steering wheel reaction that application of the brake with a positive scrub radius causes toe out.

This was very evident in a car I had which had a large scrub radius and sticky brake calipers so the right always hit a fraction before the left (RHD car so left was the dirty side). The steering initially kicked right so hard, I got RSI or tennis elbow from driving it. Well that is from the heavy steering in general and the kick from potholes and the initial kick from brakes.

Regards
Pat
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[bradleyelwood]

Charlynn has systems for steering and some good information on the forces involved.

When all the wheels are symmetrically placed about the front axle, and the braking forces are equal right to left, the braking force causes a moment about the king pin causing a compression or tension in the tie rod, but this does not cause forces in the steering cylinder or steering mechanism.

However, hitting a bump or dropping one tire in a hole will cause a-symmetrical forces which must be absorbed by the steering system.

Is the vehicle all wheel drive? Rough terrain and all wheel drive loads can present shock loads to the steering system.

For a practical introduction to design, go to your local equipment dealer such as Caterpillar or John Deere and look at their equipment. Ask the parts manager for hydraulic operating pressures. You should be able to get an idea of steering requirements on similar types of vehicles.