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Front Axle design measurement Question

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Rhysn Rhys N
New Zealand   NZL
Ernest, thanks for referring back to my "essay", it does come from close on 50 years of being around the race car scene. Of course "way back then" a 5 1/2 rim was quite wide. All the factors go together, and as you will have seen I don't take too much heed of ackermann. Scrub radius, centre patch steering etc, interchangeable terms.

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Neto Ernest B
Berlin, OH, USA   USA
In reply to # 28454 by Rhysn Ernest, thanks for referring back to my "essay", it does come from close on 50 years of being around the race car scene. Of course "way back then" a 5 1/2 rim was quite wide. All the factors go together, and as you will have seen I don't take too much heed of ackermann. Scrub radius, centre patch steering etc, interchangeable terms.

Well, I have found it quite useful, and it has contributed greatly to my understanding of steering design. I will admit that I am still a bit confused regarding the part application of the Ackermann principle plays in avoiding scrub on a tight turn. I've looked at my riding mower, and the steering arms point straight back, and there is also no KPI at all. And yet when I turn it to lock, the inside wheel is at a greater angle that the outside wheel. (I do realize that this is attributed to the differing angle of the steering arms in relation to the center part where the two tie rods connect to the steering shaft. Can't think of the proper name for it right now. Pitman arm?) So Ackermann is apparently not all there is to avoiding scrub.



Edited 1 time(s). Last edit at 2018-01-06 12:40 PM by Neto.

Rhysn Rhys N
New Zealand   NZL
Ernest, if you take the line of the steering arms alone, it may be that the line passing through where the pivot line of the wheel and then through the steering linkage is not parallel to the centre line of the mower. Remember that I have stated what all the textbooks do, that the line to consider is where the wheel turns around, not just the pin.(Sorry, bad explanation).
Also many mowers do have a system that does achieve the greater turn of the inside wheel without the arms at the spindle doing what you might expect. Ackermann is a theory, achieved in many ways.I have yet to go through an exercise I have been meaning to for 30+ years to establish if the radii of differing turns can be achieved as the theory suggests.

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Neto Ernest B
Berlin, OH, USA   USA
In reply to # 28481 by Rhysn Ernest, if you take the line of the steering arms alone, it may be that the line passing through where the pivot line of the wheel and then through the steering linkage is not parallel to the centre line of the mower. Remember that I have stated what all the textbooks do, that the line to consider is where the wheel turns around, not just the pin.(Sorry, bad explanation).
Also many mowers do have a system that does achieve the greater turn of the inside wheel without the arms at the spindle doing what you might expect. Ackermann is a theory, achieved in many ways.I have yet to go through an exercise I have been meaning to for 30+ years to establish if the radii of differing turns can be achieved as the theory suggests.

Yes. I did understand your explanation regarding where the real pivot point is, and I agree. (I have read a lot of articles on the Ackermann Principle, and I don't think any of the ones I've seen, other than your essay thread, have clearly stated what I had began to think, that the point on the KPI that is pertinent for this discussion is the point at which the KPI intersects with the road surface.) The mower I was looking at has vertical king pins, no inclination at all (I measured it with a rod run through the king pin hole, and a level). The wheels swing around on that axis, like I suppose most go carts do. That is, the pivot point really is directly below the KP. With the small wheels & tires on the mower, and very low speed, I suppose this does not matter, but with a tire size of 22.5" diameter, this type of design will mean that one is even more restricted in the frame width at the front.

But regarding the steering rods themselves, the overall distance between the steering arms increasingly shortens as you turn the wheels sharper, because of the offset created by the angle of the pitman arm where the steering shaft connects in the center. That is how this design gets a tighter turn on the inside wheel w/o any attempt to implement the Ackermann Principle.

As you mention here, I have also been thinking about how a person could build a testing system that would have adjustable KPI (along with being able to adjust the stub axle angle, to keep them level while changing the KPI) and adjustable steering arm angles as well. Then check the effects of different configurations on scrub and turning radius. (I have a design in my head, but trying to think how to do this w/o spending a lot on machining parts that are ultimately only for testing.)

Someone linked to an article on suspension design [https://www.rqriley.com/suspensn.htm] (I think in your thread of the essay you wrote), where the author states that the lighter the vehicle, the more important proper suspension design becomes. I know that the overall objective here is simplicity, but I would think that once properly determined & described, an optimal design would not be much more complicated to build than a relatively bad one. I also suspect that front suspension design becomes more important when the engine is placed ahead of the driver, but I don't know to what degree.

Rhysn Rhys N
New Zealand   NZL
This is one of those many times where I wish I had learned a drawing program when I was physically incapacitated. I am going to draw on paper a fully adjustable system I have seen used, largely on the racing lawnmowers. It gives all sorts of possibilities.The only one it doesn't readily allow is for the angle of the axle relative to the spindle to be changed, so the guys I saw had a few to choose from.

As you rightly say the mower uses the travel disparity of that quadrant to achieve the different angle to the centreline. I'm not too sure whether to say that isn't "ackermann" as it achieves the same principle, just in a different manner to angling the steering arms. It was his suggestion that the inside wheel needs to turn more to achieve the differing radii, and one way to achieve it.. I suspect that any way you can he would say achieved his principle.

Is there a "right answer" to any of this? I don't think so, if there was, why would every race team of any sort keep adjusting for a little more something in the systems? That's everything from straight line dragsters via dirt, to F1, and LMP.



Edited 1 time(s). Last edit at 2018-01-08 12:09 PM by Rhysn.


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Neto Ernest B
Berlin, OH, USA   USA
OK. I tried to draw my idea out here. Two each of the top pieces in the drawing would fabricated. The one on the left would be connected to one another with a piece of flat steel, the part where the bolt is shown passing through. The parts on the right in the drawing would have threaded square stock at top & bottom, or perhaps long nuts. Two bolts would pass through the flat bar at the top & bottom of the outside piece (on the left in the drawing).

The main axle inserts into the space between the two flat pieces, and pivots on the point near its end. The hole farther in on the main axle would be threaded, or a bolt could pass through, to maintain what ever KPI adjustment is chosen for a given test.

The stub axle (spindle shaft) fits between the two pieces as on the left. Using the adjustment points, the stub axle & the main axle would be maintained in plane.

The purpose of this device would be just for testing, to verify how the KPI will affect scrub on tight turns. It would, however, need to be able to safely hold up at speed, in order to make a decision regarding whether one wants a bit of positive scrub or not. (I think that is the correct term. What I mean is to have the KPI meet the roadway at a point a bit inboard of the center of the contact patch. As I understand, this helps maintain a feel for the road, and diminishes shimmy.)

I may try to make a working model of this design of wood, so as to be able to take pictures, which would be much more explanatory than what I am able to draw.


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Test KPI.jpg

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