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Old 12-16-2003, 08:16 AM
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ackerman geometry

Ok, i understand the Ackerman Principal, however the engineer in me had me doing some research. Everybody agrees that setting up my T-bucket so that imaginary lines from the king pins through the steering arms and intersecting at the centerline of the rear end is CLOSE, and some will admit that is is not perfect. I know that the slip angle of the tires usually makes up the difference, but the exact proper angle must be able to be derived from a mathematical formula. I know that mechanical engineers are taught this (i'm a Civil Engineer).

I'm running narrow wire wheels on the front and the motor cycle tires probably won't allow near the slip angle that car tires do. Again, maybe it's anal, but the engineer in me wants to at least be able to know what the proper steering arm angles are. I know how to calculate the angular difference of the tires at a given central turning radius, but am having trouble calculating the steering arm angle at 0 degree (straight forward) turn.

Sorry for the lengthy post, but it is keeping me up nights.

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Old 12-16-2003, 08:19 AM
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Why not just streach a string from the king pin to the pumpkin center and heat and bend the steering arms 'til they line up?
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Old 12-16-2003, 08:22 AM
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that's a good rule of thumb setting, but does not perfectly achieve 0 Ackerman. I know it's anal, but i looking to calculate the angle so that the front wheels don't hop in a tight corner.
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Old 12-16-2003, 08:47 AM
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Here is a lengthy discussion about the pro's and cons of Ackerman. It may give you some ideas...
http://www.eng-tips.com/gviewthread....d/60/qid/12793
Here is another with more technical data...
http://www.auto-ware.com/setup/ack_rac.htm
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Old 12-16-2003, 09:57 PM
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Mathmatically do this.....
measure the width of the kingpins at the center of the kingpin boss. Let's say it's 48"

Now measure the wheelbase. let's say it's 100"

Now measure the offset distance between the kingpin center and the center of the tierod. let's say that's 5 1/2"

Now with that info. you can calculate the optimal center to center distance of your tierod ends at 1/8" toe-in.

Kingpin centers Divided by the wheelbase times the offset distance between the kinpins and the tierod.

Subtract that from your kingpin centers and you have your optimal tierod length center to center of the tierod ends.

Divide that number by 2 and you have the distance per side.

This will give you the base and height of a right triangle that you can either trig. the angle or lay it out on paper.

48"/ 100"=.48"x 5.5"=2.64

48"- 2.64=45.36"c-c of tierod ends

45.36/ 2 =22.68" per side

or you can use a string
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Old 12-17-2003, 06:50 AM
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That would only give the measurement for aligning the steering arms up with the centerline of the rear axle. That doesn't produce correct ackerman necessarily. It anal, i know, but why not get it as correct as my measuring instruments will allow.

Thanks
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Old 12-17-2003, 08:45 AM
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Here is a diagram of the angles you need to develop,



Here is a link to a free downloadable spreadsheet that will do the caclulations for you. You want the last one titled "For Direct Type Geometry",

Acerman steering design spreadsheet

Last edited by willys36@aol.com; 12-17-2003 at 08:51 AM.
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Old 12-17-2003, 09:11 AM
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there you go...i knew it had to be out there..

Thanks!!!!!!!!
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