Ted Bennett or Lisa Kaskan wrote: Because the left and right wheels turn on different radii in a turn, the inner wheel, to reduce scrubbing, must be turned more sharply than the right. This known as Ackerman steering compensation, and an approximation is achieved by arranging the steering arms so that they point at the centerline of the rear axle.
It is widely believed so, but I
calculated and found it to be wrong. The intersection
should be between the front and rear axles. Then in page
294 of John C. Dixon, "Tires, Suspension and
Handling," SAE, 1996, I found the following.
It is widely believed that aligning the steering arms so that their lines intersect at the rear axle will give the true Ackermann steering (the Jeantaud diagram). However, this is far from true; the actual Ackermann factor varies in a complex way with the arm angle, rack length, rack offset forward or rearward of the arm ends, whether rack is forward or rearward of the kingpins, and with the actual mean steer angle. Moving the rack forward or backward to change the tie-rod angles can be a useful way to adjust the Ackermann factor, the most important single variable being the angle between the tie-rod and the steering arm in plan view, the Ackermann factor being proportional to the deviation of this angle from 90°. With straight tie-rods, to obtain an Ackermann factor close to 1.0 may require the projected steering arm intersection point to be at about 60% of the distance to the rear axle.
This 60% matches my analysis. If you want, I can send you the derivation in a Word document and the calculation in a Mathcad file.
I have not considered the arms pointing forward yet.
This certainly does NOT apply to our "centre point" or "zero scrub radius" steering!
MY definition of this type of steering is that the projected axis of the kingpin will go through the centre of the contact patch of the tyre in FRONT view.
This means that when the front wheel hits a bump, there is no torque reaction on the handle bars, and likewise when one wheel brakes, again there is no torque reaction in the steering system.
However, when viewed from the SIDE, the projected kingpin axis is some distance in front of the contact patch, in much the same way that the projected fork axis is some distance ahead of the centre of the tyre contact patch on most bikes.
This distance, commonly know as trail with bikes, or as caster in cars, which do not normally have an offset, is what gives the feeling of "weight" to the steering while cornering at *some speed*.
As I designed my own car, and a number of road race side car outfits before I started on trikes, I was acutely aware of the relationships between steering "feel" and steering geometry.
Thus, because the trikes are a fraction of the weight of cars, the trike trail or caster was carefully designed to give a good "feel" on the road, so one can sense, for instance, exactly HOW much grip the tyres have on the road, so you can adjust your speed accordingly.
IF you were to have ZERO caster and trail, then you would have "power steering" as Bob suggests, and there would be no feel to the steering.
It's easy to measure toe-in, with the seat on and the rider aboard as well, if you measure from the outside. I made up a wooden jig in a wide U-shape, using 3 pieces of 1/2" x 1" (anything similar will do) plus a couple of corner braces to keep the sides of the U rigid. The jig is wide enough to slip over the outside of the wheels, and deep enough to clear the machine, yet reach the mid-point of the wheels. On the inside of one leg, near the end, there is a nail, which stands proud of the wood. On the opposite leg, there is a bolt, with nuts to either side, so that the length which protudes is adjustable. It is set so that when the jig is slipped over the rear portion of the wheels, and the nail and bolt make light contact, the toe-in is correct. As a cross-check, the jig can be placed over the front portion of the wheels, and a small ruler used to measure the gap between rim and bolt or nail. Alternatively, a cap of the appropriate size can be placed over the bolt or nail. It's all very ad hoc, but is quite precise and works quickly. I made up the gadget because in addition to a seat, I've got to get over a full fairing.
IMO it is NOT centre point steering or zero scrub radius which takes the feel out of the steering but the lack of *CASTOR*!
Generally speaking with cars, it was normal NOT to have zero scub radius, simply because it was fairly difficult to achieve with wide wheels, and not having it reduced some of the suspension loadings under cornering. Plus with hydrulic braking there was no steering reaction under braking.
However tyre blowouts did produce a fairly severe pull on the steering to one side, and after a number of deaths, car manufactures started introducing centre point steering, or at least reduced the scrub radius in their designs.
Most cars run between zero and six degrees of caster, a common passenger car figure being 2 degrees.
The more caster you run, the greater force you need to turn a corner at the steering wheel, and the more "feel" you have. E.g. if you suddenly come apon and icy patch while you are corning, the wheel will go light in your hands, indicating that the front wheels have reduced grip on the road.
This quite different from the "feel" that you might get from NOT having center point steering, which will simply give you "kick back" from the bumps on the road, which IMO is NOT helpful or useful.
Thus I use centre point steering with 11 degrees of caster, so that on such a light machine as a trike, there is a good amount of feel in the steering WITHOUT kickback from road shock, or steering bar reaction from uneven braking.
Thursday, 29 January 2009