Steering geometry problem ? or not ?

[stormbird]

Hi all

So this worries me :-

So front is to the left so you can see the front chassis cross member , left chassis side and the rod [ nearest the pedal ] that connects both kingpins together across the front of the car.

sorry picture and drawing in different orientations

If I wanted the move the ' joining link ' forward to be under the front chassis cross member I would have to extend the kingpins forward ?
Would this have either :-
a) no effect on the steering mechanism i.e no change to turning radius/lock/sensitivity etc
b) may change something for the worst ?
c) may change something for the better

Yes there is no Ackerman implementation in this set up , that discussion is a whole new can of lumbricus which I don't want to open at this point

Paul

 

[undertoad]

I'm not sure if I understand - Isn't what you're calling the "pivot" the kingpin?
Also, I think the "joining link" would be what I would call the tie rod.
Isn't the tie rod distance from the "pivot" what creates the Akerman effect ?

For sure, I don't understand that well enough to be able to say what it would do to the steering - But I do think it would do something.
I find that I don't understand a lot of things, more so, the older I get .............

 

[stormbird]

I'm not sure if I understand - Isn't what you're calling the "pivot" the kingpin?
Also, I think the "joining link" would be what I would call the tie rod.

Well it could be , or semantics or being the other side of the pond
I think the kingpin includes the wheel axle and any control arms , the pivot is well just the kingpin pivot.
Well yes not remembering it's name when I did the drawing I called it the joining link , tie rod also works

Isn't the tie rod distance from the "pivot" what creates the Akerman effect ?

No Ackerman is an offset in this case to the outside of the current tie rod mounting point on the kingpin

For sure, I don't understand that well enough to be able to say what it would do to the steering - But I do think it would do something.
I find that I don't understand a lot of things, more so, the older I get .............

You me both , which is why I am asking on here

 

[DannyC]

As long as the extension is equal on either side, I would think there would be no disturbance in the force.
It is just a link moving in side-to-side in & out. There is no Ackerman.

 

[stormbird]

It increases the distance from the pivot and hence the leverage/easier to move ?

Is this covered in Elands spreadsheets ? just a thought ?

Paul

 

[undertoad]

I'm lost ......

 

[DannyC]

It increases the distance from the pivot and hence the leverage/easier to move ?

Is this covered in Elands spreadsheets ? just a thought ?

Paul

I don't really think so.
The primary "movers" are the things your "push-pull" linkages attach to. It is they that cause a motion in the kingpin.
The link between the kingpins just makes sure you cannot point the wheels in completely opposite directions with the two separate push/pull linkages.

 

[DannyC]

I'm lost ......

Eland refers to Peter Eland who very kindly created a spreadsheet for the design of steering systems to ensure near perfect Ackerman performance.
I have used it many times and it is really good.

 

[undertoad]

I'm a fair mechanic and welder - Not so hot as an engineer and designer, to say the least.
I'm building a tadpole trike, from Brad's plan.
When you make the steering arms (which are directly in line with the "pivot", as Paul calls it), the plan has you locate the "joining link" an exact distance on the arm, from the pivot, in order to create the correct Akerman steering ratio.
I can see this working on my own trike - The inside wheel turns a tighter radius, as it should.

Now, on the trike, the steering arms project towards the rear, and the link connects them, at the calculated point, to create the Akerman effect.
So you are saying, that having similar, in-line arms, only projecting towards the front, and connected by a link, will NOT do that?
This is where I get lost.

 

[stormbird]

If you are following Brads plans then there is no need to worry , he has taken care of it.

This is the simple way of working out Ackerman [ taken from Wiki page Wiki ] as you can see 2 control levers @ 90' to each other cannot provide Ackerman. However don't sweat it as many people don't think it is needed in these applications , I suppose due to short wheel base , small steering angles , lack of weight & speed etc etc

Sorry for the confusion Paul

 

[undertoad]

I do understand that - It's only confusing, in that Danny was saying it doesn't work like that, if the steering arms and link are in FRONT of the pivot?
And the plans worked it out fine - The wheels look just right, when turned, so I'm not worried at all, about my trike.
It's the basic concepts I'm having trouble with. OTOH, I've always had trouble with concepts .......

 

[undertoad]

I looked at the Wiki explanation, and it says that the steering arms should NOT be at right angles to the axle, to achieve the Akerman effect.
And that it doesn't work if it's a simple parallelogram arrangement, with the arms at right angles to the axles.
On the trike, however, the steering arms ARE at right angles to the axle, and yet when I turn the wheels, the inner wheel visibly looks like it's turning a tighter radius than the outer - Maybe I'm just imagining this - But it LOOKS like it.

As I said, lost.

 

[stormbird]

Which trike are you building ?

There are other ways of introducing Ackerman , as you say an understand is useful should the results not be as predicted.

Paul

 

[Popshot]

If I wanted the move the ' joining link ' forward to be under the front chassis cross member I would have to extend the kingpins forward ?
Would this have either :-
a) no effect on the steering mechanism i.e no change to turning radius/lock/sensitivity etc
b) may change something for the worst ?
c) may change something for the better

Yes there is no Ackerman implementation in this set up , that discussion is a whole new can of lumbricus which I don't want to open at this point

Paul

If the vehicle has been built without Ackerman then moving the rod is the time to introduce it. If you simply move it forward then you have to extend the arms anyway so extending them to the correct point is no extra work EXCEPT that putting them to the correct angle will then require a longer tie rod. You do not have to move the kingpins forward. All you need to do is extend the arms. The problem with a tie rod forward of the axles is that to generate Ackerman the arms move towards the wheel and depending on the width of the hub and/or how the upright is built may want the same space. Even if you can't get perfect Ackerman because of this it's better to have it close than miles off, particularly on a racer where any lost speed through scrub is very undesirable.

To specifically answer the question then having no Ackerman and just extending the arms to move the tie rod will have no effect on the steering, with the proviso that longer arms will need to be slightly stronger to keep flex down. The increased lateral movement due to the longer arms may mean that bodywork gets hit though.

 

[DannyC]

I do understand that - It's only confusing, in that Danny was saying it doesn't work like that, if the steering arms and link are in FRONT of the pivot?
And the plans worked it out fine - The wheels look just right, when turned, so I'm not worried at all, about my trike.
It's the basic concepts I'm having trouble with. OTOH, I've always had trouble with concepts .......

Sorry If I have confused things. I didn't actually say it does not work like that.
What I said (I think/hope) was that the front tie-bar is merely providing the function of linking the two wheels so they operate in unison.
The Ackerman (or not in this case) comes from the actuating arms on the kingpins.
In many designs this "link" exists aft of the wheels wayyyy back at the steering bar, but this implementation has 2 independent "unlinked" tank-steering arms. The ONLY linking between L wheel and R Wheel occurs with that tie-rod.

http://www.eland.org.uk/steer_intro.html

 

[Popshot]

the front tie-bar is merely providing the function of linking the two wheels so they operate in unison.
The Ackerman (or not in this case) comes from the actuating arms on the kingpins.


http://www.eland.org.uk/steer_intro.html

The front tie bar does exactly the same job as a rear tie bar and does provide the Ackerman. The actuating arms merely provide the steering input.

 

[stormbird]

Popshot/DannyC

There was an attempt to ' add ' Ackerman by adding some extensions to the kingpins and using a longer tie rod :-

This is the nose of the right hand kingpin and it was very poorly implemented. When I asked about the mod I was told
it did not work due to lack of angles on the kingpin.

Curious as the only angles a kingpin pivot would have is :-
Centre point - used to reduce brake & bump steer
rake [ inclination ] - used to provide some self centering

Could either be involved in Ackerman ?

Interestingly a piece of string under the screw and going to the centre of the rear axle passed through the kingpin pivot !

So I have just re-drilled the original holes [ seen on the right ] and build a shorter tie rod.
pending it being driven I actually have not a Scooby Doo as to how it behaves ?

I think in a racing pedal car you are actually doing things you would not consider normal [ honest ]
So while the radius of a bend maybe the same on the street and race track the speeds [ should ] be widely different ?
We don't want to roll the pedal car when riding for pleasure ?
In a race at the higher speeds the car can roll a lot , so much so that the inside wheel goes very light or lifts.

It is possible to argue Ackerman many not be needed in this turn

Paul

 

[Popshot]

Interestingly a piece of string under the screw and going to the centre of the rear axle passed through the kingpin pivot !

Then it already has perfect Ackerman. You can extend the arms to move the rod forward. Just keep the same angle for the tie rod pivot to kingpin and lengthen the rod to suit.

 

[stormbird]

Then it already has perfect Ackerman.

However they said :-

When I asked about the mod I was told it did not work due to lack of angles on the kingpin.

I wondered if they blamed heavy steering on lack of Ackerman ? that could explain why ' adding it ' did not work and the mention of angles ?

None of their cars appear to have Ackerman !

You can extend the arms to move the rod forward. Just keep the same angle for the tie rod pivot to kingpin and lengthen the rod to suit.

I wondered whether it would change the leverage and make the steering either lighter or heavy ?
I rode one car with no rake/trail and if you looked over your shoulder it changed lanes !

Obviously there are sweet spots where all is peaches and cream.

Paul

 

[Popshot]

If you move the tie rod forward it will not make the steering heavier or lighter. The things that determine that are the length of the steering arms that connect to the longitudinal steering rods and the ratio of the leverage above and below the handlebars pivots.