Front Steering Geometry of Tractor

[rmalik30]

Hi all!

I am making a tractor's front wheel steering arrangment with driving front wheels driven through hydraulic motors. Because of space constraint I was wondering if steering axis (kingpin) can be shifted towards front of wheel axis ie positive mechanical trail but without any castor angle.
Following are related specs :-
1. castor angle = 0
2. kingpin offset (or scrub radius) = 150-200mm
3. Mechanical Trail = 100-150mm
4. Tire Rolling radius = 510 mm (11.2-28" Traction tire)
5. Max tractor speed = 30kmph

The doubt is whether shifting of steering axis towards front would increase tire wear/scrubbing ?

Any comments on above are welcome.

Thanks

 

[mender]

Offsetting the pivot point can introduce some unwanted steering torques because of the driving and braking loads. You'll have to beef up your steering mechanism to compensate, especially on a tractor that will be expected to pull pretty hard. Can you mount the motors directly on the hubs instead?

Tire wear shouldn't be affected if you keep the steering geometry the same, i.e. Ackerman.

 

[rmalik30]

Thanks for the reply.

Your point is right that the load on linkage would increase due to offset and some strengthening of linkage would be necessary.

Motor can't be mounted directly on to hub because some reduction is taken through a spur gearbox to reduce motor size. With a little offset of steering axis I am able to extract more clearance of motor with the front beam-axle, which carries the king-pin hub.

I was able to do some simulation on ADAMS with a simple model having a single tire pivoted on a straight arm, other end of which acted as steering axis. On turning the tire in a circle about the steering axis one could observe the difference between with and without offset of steering axis. A high lateral force on wheel got introduced increasing linearly with the offset.
This lateral force may lead to increased tire wear.

I couldn't really understand the introduction of force. However, I feel that with steering axis at an offset the tire centre-plane is no longer tangent to the circle while turning. This circle being drawn with centre at steering axis and radius equal to direct distance between steering axis and the tire contact point/patch on ground.

I hope above theory is right. But still not clear how much offset to consider or not at all.

 

[mender]

You're right, the tire doesn't know the difference in steering offset.

Draw a circular path with the appropriate arcs for the front and rear tires, then add in different steering pivot points. The additional force to steer the tire is on the steering linkage, not the tire. Something wrong with the sim.

 

[rmalik30]

Thanks for the inputs.

I would re-verify the same on sim.

Besides that I have one more issue with the steering system design. I am not able to assess the effect of scrub radius on the force required to move the linkage. The configuration in my case is hydrostatic steering. Hence, I would like to calculate amount of pressure generated in the balanced cylinder.

To calculate above I used the manual attached herewith. Similar ones are also available, so I don't doubt the correctness. Now the questions are ?

1 Why should coefficient of friction depend on L/B ratio (scrub / tire width) ?

2 At other places I have read that in automobiles the trend is to keep the scrub radius to a bare minimum so as to minimize steering effort and to eliminate kickback from bumps or while braking. This makes sense and can be proved easily by taking moments about the king pin.

However, from this manual it comes out that as scrub approaches to zero the coefficient of friction becomes maximum and pressure inside cylinder also reaches max value, which corresponds to increase in steering effort (although the driver wouldn't feel it, only cylinder would be bigger in size).
The above two outcomes are contradictory and utterly confusing !

3 I also wonder that what would happen to a tractor traction tire with almost zero scrub ? Will the tire wear increase ?

It would be great if you could throw some light on above.

Thanks again