Ackerman & Steering Effort 3

[shabba]

Hello all,

I am currently looking at a steering system for a heavy duty truck, driven on tarmac (asphalt, bitumen call it what you will) with regard to the load on the actuator and hence ball joints.

My questions are these: I have worked out the Ackerman Error for the axle combination and wheelbase I am using to be ~66% (lines join behind rear axle). I take it from this that the vehicle may experience accelerated front tyre wear but will this affect the force required to steer the wheels at speed? Does it influence the 'auto-centering' of the front wheels?

The truck is built for low speeds (30mph max.) but heavy loads, it has a solid beam rear axle with no suspension as such.

any comments greatly appreciated...

 

[GregLocock]

Ackermann primarily affects the steering load under low speed manouevres such as parking. It has little effect at high speed because the steer angles on the wheels are so small. It is pretty easy to get a ball park estimate of this - just work out the steer angle to generate a turn of (say) 0.4g at 30 mph for the two wheels. You should find the Ackermann is only making a slight difference, and, if your tyres are in the linear range, so are the forces.

It won't have much effect on self centreing, that tends to be controlled by trail, castor angle and perhaps a little bit of toe in.

Tyre life will be affected if you do much low speed stuff, it is a quick way to chew through the shoulders of the tyre, since you are combining ackermann errors with large cambers.



Cheers

Greg Locock

 

[shabba]

Cheers Greg,

The vehicle is a tractor and as such is used for moving trailers. This obviously can mean a lot of low speed manouvering and some of the vehicles in service have experienced the tyre wear you describe. Is it worth modifying the track arms to improve the ackerman or is 66% generally ok? Also on road tests which involve long periods at top speed, the front tyres seem to heat up, I had assumed this to be due to toe-in/out but the axles are supposedly set up parallel.

Is there any hard and fast calculation for the force required to turn the steer wheels? I have been working from one I found in a book which takes king pin offset, tyre contact, load etc into account but is just for static 'dry' steering, is this worst case scenario? Or do I need to consider the forces acting at speed also?

Cheers

(ps. what do you mean by the linear range re. tyres?)

 

[GregLocock]

In reality the Ackermann at the straight ahead position can go all over the place due to cmpliances in hte system. SO we plot Ackermann as a function of steer angle, and try and get that right.

I have no experience with suspensions for tractors, but I can't think of a convincing reason to use anything other than full Ackermann, with the useage you describe. Note that odd camber settings can also cause the shoulders to wear, because the tyre leans over at extreme steer angle.

Working out the steering forces will always be a bit of a guess unless you have a full tyre model, not very easy to come by. If you are an OEM you may be able to get the tyre supplier to tell you.

The linear range of the tyre is the bit where the forces are proportional to the slip angle, typically up to 3 or 4 degrees.





Cheers

Greg Locock

 

[shabba]

Yeah sorry, I thought I'd mentioned that but reading back I didn't.
It is a solid beam axle with the hydraulic steering ram mounted along the beam of the axle. There is not really any justification for a full suspension model in my view. One; because we can't afford the time or expense, and two; ride comfort is low on the design criteria, outweighed by durability and strength. Also this is a low speed vehicle (30mph max) so the handling boils down to purely traction. Will it pull 65tonne, and will it be able to steer under this load. The real question for the thread was whether or not the ackerman has any effect on tyre wear and the steering force required from the ram. And what other things influence this.

Cheers
Shabba