Author Topic: Pitman arm types  (Read 286 times)

StefanN

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Pitman arm types
« on: 22 Oct 22, 01:27 pm »
There are two types of pitman arm that most people use.   One has the links next to each other and the other has them one in front of the other.  Has anyone got any experience of the relative merits of the two styles?

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RhysN

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Re: Pitman arm types
« Reply #1 on: 22 Oct 22, 05:18 pm »
The side by side version introduces Ackermann effect.
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jim

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Re: Pitman arm types
« Reply #2 on: 23 Oct 22, 05:37 am »
The side by side version introduces Ackermann effect.
That's very interesting. Did you put the front wheels on turning plates and measure the angles throughout the entire turning amount. What angular difference did you find between the side by side and them bolted on top of each other? Do you have a graph of your results or something written down?
If you can drive round corners, you're not going fast enough.

ChrisS

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Re: Pitman arm types
« Reply #3 on: 23 Oct 22, 08:16 am »
As Rhys says, in most cases, i.e. where the pitman arm hangs below the steering column, side by side will increase the rotation of the inner wheel relative to the outer, it will also increase vertical movement on the inner end of the steering link which might cause clearance issues.

StefanN

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Re: Pitman arm types
« Reply #4 on: 23 Oct 22, 03:52 pm »
I was interested to know how much of a difference there would be and so did some rough modelling based on some dimensions that hopefully are representative of cyclekart dimensions (I've not gone and measured mine).   If you're not interested, please move along ;).


Pitman arm radius 55mm
steering arm radius 100mm at 15deg
toe in about 0.5 deg

The charts show the input steering angle and the left and right wheel angles when turning left.
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My take-away from this is that the concentric mounting keeps the steering input to output relationship linear for longer, and then the differential is less pronounced.

Concentric mount



Side by side mounting




Chris Brown

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Re: Pitman arm types
« Reply #5 on: 25 Oct 22, 08:00 pm »
Interesting, may explain some of the steering problems on the Morgan. The steering arms are set up for Ackerman, and I've got a side by side pitman. Changes needed perhaps, so I've got one or the other, or maybe none.

StefanN

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Re: Pitman arm types
« Reply #6 on: 28 Oct 22, 04:27 pm »
I thought it would be interesting to isolate the steering input from how the left and right wheels steer.   The chart below shows the relationship between the left and right wheel angles in a left hand turn.

The black line is for a solid tie rod between the left and right steering arms.  It is fairly linear and shows the right (outside) wheel steering less than the inside wheel.
The green line is for the links on the Pitman arm being next to each other, but no Ackerman angle at the steering arm.  It matches the black line pretty well, but with  the outer wheel getting progressively less steering angle in sharper turns - after about 20 deg.
The blue line is for the steering links on the Pitman arm being concentric and 15 deg of Ackerman on the steering arms.  Again fairly linear for most useful steering angles, just with less differentiation between the left and right wheel angles.
The red line is for 15deg Ackerman at the steering arms and the steering links side by side at the Pitman arm.   This starts to flatten off quite quickly with the outer wheel steering a lot less as the left hand wheel angle increases.

My conclusion? I'll probably avoid the last option, even though it looks neater!
« Last Edit: 28 Oct 22, 04:30 pm by StefanN »

Seven racer

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Re: Pitman arm types
« Reply #7 on: 02 Nov 22, 08:00 pm »
As an aside, I have noticed a couple of cyclekarts using a steering box, or Pitman arm operating one steering arm on the front axle, and a solid bar across to the opposite steering arm.

Is there any advantage, or disadvantage of this arrangement? The only thing I can think is that it might help with bump steer, but I'm really not sure, or why it's used as an alternative.

Peter

Mark Underdown

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Re: Pitman arm types
« Reply #8 on: 02 Nov 22, 09:39 pm »
Peter

My Kart uses a steering box, I did it just because it was in keeping with the original car I was copying.  Not sure of any advantage to be honest although reducing bump steer would be a bonus.  My kart just seems to slide around every corner, I might have to look at the steering geometry before next year having wiped a set of tyres out in 1 1/2 events. :)

StefanN

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Re: Pitman arm types
« Reply #9 on: 02 Nov 22, 10:14 pm »
As an aside, I have noticed a couple of cyclekarts using a steering box, or Pitman arm operating one steering arm on the front axle, and a solid bar across to the opposite steering arm.

Is there any advantage, or disadvantage of this arrangement? The only thing I can think is that it might help with bump steer, but I'm really not sure, or why it's used as an alternative.

Peter

The pitman arm linking to just one side and then a track rod between the two steering arms is shown as the black line in the chart.   It gives you a more consistent Ackerman effect between the wheels when steering.

A rack and pinion “steering box” gives you a reduction gear making steering lighter, and also a more linear relationship between steering wheel input and wheel angle.

Other steering boxes can give the benefit of lighter steering because of the gearing.

How much bump steer you get depends on so many different things - eg type of suspension and geometry, is the steering box operating through a drag link (front to back on the vehicle) or left to right,   etc etc.

Seven racer

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Re: Pitman arm types
« Reply #10 on: 02 Nov 22, 10:30 pm »
Thanks Stefan,
so it's Ackerman that improves, useful to know, and think about before I finally put the front end together. :)
Peter