2
$\begingroup$

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

enter image description here

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.

enter image description here

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

$\endgroup$
5
  • $\begingroup$ 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. $\endgroup$
    – Phil Sweet
    Jun 2 '19 at 15:15
  • $\begingroup$ 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. $\endgroup$
    – Phil Sweet
    Jun 2 '19 at 16:17
  • $\begingroup$ @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. $\endgroup$
    – php_nub_qq
    Jun 2 '19 at 18:21
  • $\begingroup$ 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. $\endgroup$
    – Phil Sweet
    Jun 3 '19 at 2:07
  • $\begingroup$ 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. $\endgroup$
    – TimWescott
    Jun 3 '19 at 23:29
2
$\begingroup$

If you just first turn the king pin on the interior wheel by say 25 degrees and draw a line perpendicular to the same wheel going up till it intersect the rear axel continuation, then from that point draw a line to the center of the other wheel and make that wheel perpendicular to the line, there is only one straight tie-bar that will connect the ends of wheels' bars so that they mach.

Now try for 15 degrees, 30 degrees and compare your length of rods and tie-bar. This gives you a pretty good idea of the geometry. Here is a link with more info. Steering geometry

just so you know the car manufacturers take advantage of three dimensional toe, caster, camber, drop arc, and lateral expansion of suspension and the lateral fricshion in road puddles as to give the car its road handling and comfort characteristics and sharing the handling of bumps with the shocks. basically the suspension twists and deform in a way that breaks the sudden shakes or jerks and vertical moves. Also they make the car surefooted in sudden jerks and even they design them to self destroy the whole suspension and wheels as sacrifice mass in an accident to break the crash and absorb a lot of energy.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.