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#Post#: 7791--------------------------------------------------
Pitman arm types
By: StefanN Date: October 22, 2022, 7:27 am
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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?
#Post#: 7792--------------------------------------------------
Re: Pitman arm types
By: RhysN Date: October 22, 2022, 11:18 am
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The side by side version introduces Ackermann effect.
#Post#: 7794--------------------------------------------------
Re: Pitman arm types
By: jim Date: October 22, 2022, 11:37 pm
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[quote author=RhysN link=topic=994.msg7792#msg7792
date=1666455513]
The side by side version introduces Ackermann effect.
[/quote]
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?
#Post#: 7795--------------------------------------------------
Re: Pitman arm types
By: ChrisS Date: October 23, 2022, 2:16 am
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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.
#Post#: 7797--------------------------------------------------
Re: Pitman arm types
By: StefanN Date: October 23, 2022, 9:52 am
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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.
[attach=1]
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
HTML https://youtu.be/og_xQakxwJs
Side by side mounting
HTML https://youtu.be/yvSe7jErfw0
#Post#: 7817--------------------------------------------------
Re: Pitman arm types
By: chrisenamels Date: October 25, 2022, 2:00 pm
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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.
#Post#: 7838--------------------------------------------------
Re: Pitman arm types
By: StefanN Date: October 28, 2022, 10:27 am
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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!
#Post#: 7859--------------------------------------------------
Re: Pitman arm types
By: peterL Date: November 2, 2022, 3:00 pm
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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
#Post#: 7860--------------------------------------------------
Re: Pitman arm types
By: Mark Under Date: November 2, 2022, 4:39 pm
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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. :)
#Post#: 7861--------------------------------------------------
Re: Pitman arm types
By: StefanN Date: November 2, 2022, 5:14 pm
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[quote author=Seven racer link=topic=994.msg7859#msg7859
date=1667419252]
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
[/quote]
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.
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