Wheel Horse 500 Swept Front Axle Geometry Correction

[Damien Walker 246]

Following on from my recent 518H power steering and swept front axle conversion (see earlier thread), I noticed that the front tyres were leaving very bad marks in the lawn on turns.

 

Looking at the wheels during a turn showed that one was turning in too much and was effectively being pushed sideways...... and was scarring the lawn as a result.

 

When I looked closer at the steering geometry, I found the there was no appreciable Akermann angle in the steering arm.

 

Apologies if you understand Akermann's patent, but if not, here's a quick description:

Clearly, when turning, the outer front wheel has to describe a larger circle than the inner wheel or one or both will skid (thus marking the lawn). See wikipedia for the full details: https://en.wikipedia.org/wiki/Ackermann_steering_geometry

 

I have cloned the next two images from the above article (hope that's ok):

and you can see how the system works,

 

This arrangement is made possible by positioning the track rod ends such that lines drawn through the kingpin and track rod end on each side, intersect on the centre of the rear axle when driving straight, like this:

 

To my surprise, the steering arms on my swept axle are near parallel and so were presumably the cause of the problem. When I look at my C175, I think it is similar (ie also 'wrong') but I haven't done any accurate measurements so the jury is still out on the earlier WH models.

 

I did a quick geometry check to prove my point.The red wire in the picture crosses the centre of the rear axle and should intersect with the kingpin and the track rod end...and it clearly doesn't. I have to say that I am very surprised that Wheel Horse did not apparently incorporate this.

 

Here's the geometry:

 

 

630mm being the distance between the kingpins and 1220mm being the wheel base (sorry for metric units). These are approximate measurements done with an uncalibrated tape measure waved in the general region of interest.....14.5 degrees being the required kingpin to track rod end angle.

 

I didn't want to cut my steering arms and reweld, so designed a triangular bracket (one for each side) to achieve the same effect. 26mm being the required reduction in the track control rod length on both sides.

 

Note the 12mm holes are to fit my power steering modified tractor. If you want to do the same mod, drill smaller holes as they are not as big on the original machime!

 

My brackets ended up being a slightly different shape to better align with the sloping sides of the steering arms, but the positioning of the holes didn't change:

 

 

I have only cut the lawn once since the modification but the difference appears to be dramatic, with almost no scarring (still occurs if I use full lock at speed, but that is hardly fair!).

 

Has anyone else experienced this?

[Damien Walker 246]

Oh, and if anyone is sceptical about my calculations, they were checked by my very loyal friend... she's an expert with tangents...

[Lee1977 6,666]

My 520 is an 89 with a straight axle when I built my new tie rods I made them longer and went to the back side of triangle on the frame.  As there isn't much room between the triangle and the axle.

Didn't do any calculations but I think it improved the steering on it. 

Edited May 25, 2021 by Lee1977

[Pullstart 62,912]

I have checked Ackerman on 60’s models and they seem to be pretty spot on.  Funny how that could have been overlooked later on in the builds!

 

great wright up @Damien Walker!

[Damien Walker 246]

1 hour ago, pullstart said:

I have checked Ackerman on 60’s models and they seem to be pretty spot on.  Funny how that could have been overlooked later on in the builds!

 

great wright up @Damien Walker!

 Thanks for that, that's good to know. I'll check my Commando7, C121 and C175 this weekend and report back. As they all share the same axles and swivels as far as I am aware, I expect them to be the same as each other.

[Damien Walker 246]

10 hours ago, Lee1977 said:

My 520 is an 89 with a straight axle when I built my new tie rods I made them longer and went to the back side of triangle on the frame.  As there isn't much room between the triangle and the axle.

Didn't do any calculations but I think it improved the steering on it. 

Nice track rods!

There's no need to do anything if it doesn't damage your lawn but the error is easy to spot just by looking at the relative positions of the kingpins and rod ends. On my swept axle they were visually obviously wrong.

 

With the mower off, it's easy to check the geometry with a piece of string.

[Lee1977 6,666]

My 520 had the old style fixed tie rods and the tires were toed out quite a few degrees. I guess it came that way as I have seen other early 520's the same way. 

I toed mine in an 1/8". If I can get the front tires to run straight and it's not hard to steer I'm not going to do anything else with it.

[Ed Kennell 38,227]

19 hours ago, Damien Walker said:

      for for this explanation that even a "cave man" can understand.     Not sure why WH failed to dot it right.

 

I just checked my '88 520, my 417A, my 312H, and an 855.    All steering arms are parallel ... no Ackerman angle at all.

[Lee1977 6,666]

46 minutes ago, Ed Kennell said:

      for for this explanation that even a "cave man" can understand.     Not sure why WH failed to dot it right.

 

I just checked my '88 520, my 417A, my 312H, and an 855.    All steering arms are parallel ... no Ackerman angle at all.

True Akermann  steering on a Wheel Horse as short as they are will probley make the turning circle larger. At 5 to 7 MPH it's not really needed. 

If your going to make it fast, you need it plus correct caster and camber.

Stigian did a great job on the build with the two cylinder motor cycle engine,

Edited May 25, 2021 by Lee1977

[Handy Don 12,237]

11 minutes ago, Lee1977 said:

True Akermann  steering on a Wheel Horse as short as they are will probley make the turning circle larger. At 5 to 7 MPH it's not really needed. 

If your going to make it fast, you need it plus correct caster and camber.

I've no doubt the steering arms were engineered (and tested) as well as any of the other components and were not overlooked at all. That the angles vary across models tells you these were neither accident nor mistake.

On the non-paved surfaces these machines were designed for (grass, dirt, gravel, etc.) using the tires with which they were normally delivered, the increased angle of the inside wheel makes for tighter turning.

[Damien Walker 246]

1 hour ago, Ed Kennell said:

      for for this explanation that even a "cave man" can understand.     Not sure why WH failed to dot it right.

 

I just checked my '88 520, my 417A, my 312H, and an 855.    All steering arms are parallel ... no Ackerman angle at all.

 

Good to hear I'm not alone but sorry to hear that really. Other similar tractors of the era got it right...very odd.

[Damien Walker 246]

57 minutes ago, Lee1977 said:

True Akermann  steering on a Wheel Horse as short as they are will probley make the turning circle larger. At 5 to 7 MPH it's not really needed. 

If your going to make it fast, you need it plus correct caster and camber.

Speed isn't an issue, it's the way the wheels run that does. No Akermann angle means at least one of the front wheels will be skidding in any turn....as evidenced by the damage to my lawn. That is quite a serious failing for a lawn mower, especially when the tyre manufacturers go to a lot of trouble to stop the tyres marking the grass.

[lynnmor 7,308]

21 hours ago, Damien Walker said:

This arrangement is made possible by positioning the track rod ends such that lines drawn through the kingpin and track rod end on each side, intersect on the centre of the rear axle when driving straight, like this:

 

 

 

What you are missing is that your yellow tie rod is actually two pieces connected to the triangle in the center.  As the triangle turns the effective length of the two tie rods changes.  I did a quick check on a 520H and found that the inner tire turns 43 degrees while the outer turns only 31 degrees.   I didn't do the math, but Ackermann is alive and well.

[Damien Walker 246]

1 minute ago, lynnmor said:

 

What you are missing is that your yellow tie rod is actually two pieces connected to the triangle in the center.  As the triangle turns the effective length of the two tie rods changes.  I did a quick check on a 520H and found that the inner tire turns 43 degrees while the outer turns only 31 degrees.   I didn't do the math, but Ackermann is alive and well.

Ah, now that is an interesting take on things. My power steering conversion loses most of the original steering gear and has a single track control rod between the two wheels...looks like it might be me that is the dunce! Apologies for casting doubt on Wheel Horse engineering.

 

This smacks of the ingenuity that Wheel Horse applied to other issues...not the least of which is the chain drive path on the single stage snow thrower...the outside of the chain drives the rotor and hence it rotates backwards (because it has to wrap around the sprocket in the opposite direction) - clever!

 

Well spotted and thanks for pointing it out. (I'll need to update my power steering conversion guide - again)

[Damien Walker 246]

1 hour ago, lynnmor said:

 

What you are missing is that your yellow tie rod is actually two pieces connected to the triangle in the center.  As the triangle turns the effective length of the two tie rods changes.  I did a quick check on a 520H and found that the inner tire turns 43 degrees while the outer turns only 31 degrees.   I didn't do the math, but Ackermann is alive and well.

 

Ok....here's the proof:

 

I have added to my original Akermann geometry diagram by drawing the triangular plate that drives the track rods....but I have scaled it up by a factor of x10 to show it alongside my original tractor sized geometry.....it's the shape of the triangle that matters  not the size.

 

So:

Tractor dimensions on left, steering shaft plate x10 on the right.

 

 

And look at that, they are the same shape! (well nearly), and it looks like the triangular plate will provide an equivalent Akermann angle of 12 degrees whereas my measurements reckoned 14.5....I reckon that's good enough. Interestingly, certainly on the swept axle, the length of the triangular plate is exactly the same as the length of the steering arm between Kingpin and track rod end....giving a 1:1 ratio.

 

I would therefore argue that this provides the correct Akermann effect in an indirect way. A 'direct' Akermann set up would be as my power steering conversion with solid track control rod requires, with the Akermann angle built into the steering arms but the use of two track control rods with the inner ends spaced at the correct distance, clearly works very well. I suppose they could have just as easily built 'direct' Akermann steering arms and mounted the inner ends of the track control rods on the same pivot point on the triangular plate (it wouldn't need to be triangular then!) and whilst this indirect approach has confused a slightly dopey Englishman, it is a very neat solution. Thanks to Lynnmor for pointing this out!

 

In conclusion then:

No standard Wheel Horses need the suggested steering arm mod but if you do my power steering conversion with a solid track control rod, you must modify the steering arms to include the Akermann angle or your lawn will suffer!

[c-series don 8,723]

Okay, I’m thinking you should have been an engineer for Wheel Horse! 

[Damien Walker 246]

8 hours ago, c-series don said:

Okay, I’m thinking you should have been an engineer for Wheel Horse! 

 

What a dream job that would be! Thank you, but it is quite plain, they didn't need me!

[rmaynard 15,514]

Now if someone could find a way to decrease the turning radius of a 701 you would have my eternal gratitude.

[Damien Walker 246]

1 hour ago, rmaynard said:

Now if someone could find a way to decrease the turning radius of a 701 you would have my eternal gratitude.

 

Interesting! (I must get out more)

 

 

It seems the 701 must have had 'direct' (to use my nomenclature) Akermann steering with the angle built into the steering arms (because it has a fixed length track control rod).

 

You'd reduce your turning radius if you could increase the length of item 222 in the drawing...a longer arm would give greater lateral throw on the track control rod (and would require more effort to turn it) but I imagine you are hitting those hefty looking steering stops cast onto the axle on full lock, so increasing the length of the steering lever would have no effect. 

 

If so, it would be interesting to know how much further the steering gear would turn if those stops weren't there. If more travel is available then you could 'adjust' the end stops with an angle grinder (though that would be fairly barbaric!).

 

 

 

 

 

 

[ebinmaine 67,619]

Interesting thread. 

 

@Lee1977 I did move my replacement tie rods when I installed them. I agree that it makes a positive difference.  

 

 

@Damien Walker would you mind posting a link to your power steering conversion? 

[Damien Walker 246]

2 hours ago, ebinmaine said:

Interesting thread. 

 

@Lee1977 I did move my replacement tie rods when I installed them. I agree that it makes a positive difference.  

 

 

@Damien Walker would you mind posting a link to your power steering conversion? 

 

Here it is:

 

I haven't updated the step by step to incorporate the  necessary Akermann mod yet....I'll try to do that in the next couple of days....there are a couple of editions of it on this thread too so you need to download the last upload not the first!

 

[Ed Kennell 38,227]

Interesting pic from the tractor show.    Yes, projecting the steering levers to the rear, they do intersect at the center of the rear axle.

[Damien Walker 246]

24 minutes ago, Ed Kennell said:

Interesting pic from the tractor show.    Yes, projecting the steering levers to the rear, they do intersect at the center of the rear axle.

 

How bizarre, that appears to have both aligned steering arms and the twin track rods and triangular plate arrangement. It must work out though.