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dratkinson

Ackermann Steering Geometry

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dratkinson
All, Several months ago I modified my B-206 (lawn mower) to accept a snowthrower. To do this I had to move the steering drag link from low and inside to high and outside to get some additional clearance for the snowthrower tongue. Every since then I have had problems with the tires scuffing in turns. This week I decided to solve the problem and searched the Internet for information on how to design the steering geometry. What I discovered was that I needed to correctly set the Ackermann angle. Anyway, long story short, I just finished tonight and baby Alice now turns without scuffing the tires. Nice! If any of you change your tractor wheel base or track width, you might want to check this site...it tells how to determine the steering angle you will need for the changes you have made. http://encyclopedia.thefreedictionary.com/Ackermann%20steering%20geometry I laid out my spindle top on a tabletop and made a triangular jig that was the same dimensions as my wheelbase (long leg) and 1/2 the distance between my spindles (short leg). Then a string run from short leg end to my spindle top at the long leg end, showed the line along which the hole for the draglink connection had to be placed. I had previously marked the center of the spindle top and scribed a line across the spindle top parallel to the spindle. Simple. I didn't even need trig to compute the arctangent that gave my 11.3-degree steering angle. :D /r David

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Agricola
Now that is intersting. Thanks for the site. I have always wondered how it was all done and those graphics help me get a better grasp on the concept. I guess the straight axle I used on my soapbox derby car was not the best way to go.

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HubbardRA
If you extend a line along each of the two solid axles, the center of the turn will be where these two extension lines intersect. This is only true at slow speeds. At high speeds the tires develop slip angles when going through a turn because of the high sideways loading on the tires from centrifugal force. At these high speeds a pure Ackerman alignment doesn't work effectively because of the need to compensate for the tire slip angles. Automobiles are not set up with pure Ackerman steering angles, while tractors, mowers, and other low speed vehicles usually are. My Chevy Surburban 4x4 is a prime example. It drives great, corners well, gets great tire mileage, but skids the front tires very noticeably in a very sharp low speed turn where the pure Ackerman steering comes into play. Back when I designed my first motorcycle engine powered pulling tractor, I went through the calculations and developed a curve for Ackerman steering. I plotted aspect ratio (wheelbase divided by distance between steering pivots) versus steering arm angle that would give true Ackerman steering. If I still have that chart around I will scan and post it.

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patrician12
Unlike tractors cars turn on the angle of inclination besides the ackermann radius.Did you ever notice on a sharp turn the top of one front wheel pertrudes from the fender and at the same time the opposite side the bottom of the tire pertrudes past the body.It's had to see on some cars but performance cars with very wide tires it is quite noticeable.The camber actually changes on turns this doesn't happen on tractors.

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HubbardRA
Patrician, There are many aspects of automotive sispension that are different from tractors. The older cars 1920s, 30s, 40s with straight axles worked same as tractors. Newer independent suspensions are quite different. I have also seen tractors with an angled steering pivot (positive caster) that tilt the wheels when turned. The caster, camber, and toe-in are totally different features than Ackerman angles.

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