# Ackerman steering

I'm designing a steering mechanism following the Ackerman principle. I've met the condition that my steering arms' normal projections must cross in the middle of the rear axle however when I do a turn my wheels' axles do not cross in a single point on the rear axle. I'm applying some visuals

where the bottom most line is the x axis of my drawing, so the wheel base is 1210mm. I've set the length of my steering arms at 150mm for demonstration purposes, where 756.82mm seems to be the distance between their joint points. The white lines that go off to the right are the projections of my wheels' axles.

Here's a visual of when turning at 15 degrees.

The lines should cross, I can't figure out what I'm doing wrong.

• Simple Ackerman fourbar linkage via a rigid tierod is only approximate. You also don't have a practical geometry for the front end. On real front ends, the tire track centers are offset from the pivot (offset and drag), the pivots have caster and camber angles, and the wheels have an initial toe and camber setting when centered. These are all factors used to tweak the tracking, along with not actually using the precise ackerman angle you are starting with. But it doesn't have to be perfect in practice, Chevy used to use the same ackerman angle for five different wheelbases on their pickups. Jun 2, 2019 at 15:15
• I'm more than a bit disappointed that I couldn't find a single interactive steering model with adjustable kingpin angles (or realistic ones, for that matter). Not even for solid front axle, solid tierod systems. Can someone with access to ADAMS/CAR post a few representative practical steering linkages - MacPherson strut, double A arm, solid axle, Pitman + trackrod, rack and pinion. Jun 2, 2019 at 16:17
• @PhilSweet I see, I completely agree that there is a lot more to it in reality, but for this simple example considering there are no camber, toe or caster angles I don't understand why the lines don't meet. Jun 2, 2019 at 18:21
• Because the Ackerman angle you chose is only exactly correct at zero degrees turning. If you use 1 degree of kingpin rotation, the angular error in the tires will be very, very tiny. If you want to limit the maximum tire error over a kinpin range of, say, 0 to 35 degrees for the interior wheel, then you have to experiment to with different Ackerman so the worst case scenario is minimized. And it is normal to have the outside tire a bit tighter at full lock. You don't drive around at full lock very much. Jun 3, 2019 at 2:07
• Use a stiff anti-roll bar in the back, no Akerman, and drive fast. Then you'll always be in a two-wheel drift, and parallel front wheels will be appropriate. Jun 3, 2019 at 23:29