Steering theory: Shouldn't angles be different, outside vs inside radius of turn?

Maintenance/Repairs
21

From a Danish site:

http://gsp9700.dk/align.html

Look at the bottom.

Translation from google:

TOE OUT ON TURNS or SPREADING IN SWING
Dispersion of swing rotation difference between the left and right wheels by a given number of degrees.
Feature: Ensures that the tires do not wear crooked when turning.
Effect and error: Is the angle difference is too large or too small, this will cause uneven tire wear on the outside of both front wheels. Here there may be bent steering components.

And that’s also what I learned during my apprenticeship back in the seventies.
You may have another word for it in USA.

This site is good reading with regards to bumpsteer:

22

Ackerman (spelling variable!) and tire slip angle are very closely related. 100% akerman is needed when there is no slip angle - as when we are talking about wagon wheels and very slowly moving cars.

Most street cars need less than 100% akerman, but more than zero.

My Ford vans (twin I beam) reportedly had 0% akerman - also known as parallel steer - and it sure seemed like it dragged a tire around a corner.

But vehicles that use a lot of slip angle need less akerman - like race cars.

My race car - a converted early 70’s Mercury Capri (It was a Ford, in Europe) needed to have some of the akerman dialed out. The car seemed to bind up in a corner - fighting itself. When I took out as much akerman as I could, I got a car that was free and went where I pointed it.

2 Likes
23

I happened to walk by one of those huge 4x4 pickup trucks the other day, not a new one, but a classic-era old truck somebody has modified so it looks like an entry to the next monster truck show. Since the body was so high up in the air it was easy to see the steering system. And on this truck the steering arms on the front hubs pointed forward, rather than backward as shown in @Barkydog 's diagram above. Otherwise similar, a fixed length tracking rod between the two front facing hub arms.

What’s puzzling me is the statement above from the Wikipedia Ackerman steering link, that the tracking rod for front facing arms should be longer than the axle. How could the tracking arm be longer than the axle? There isn’t enough room for that, right? On this monster truck the tracking rod wasn’t longer than the axle, as far as I could tell anyway. So what’s up with that?

24

The “longer than the axle” comment is only slightly off. It has to be longer than the kingpin axis line, or the imaginary line through which the knuckles turn. Really not that hard to do from a design standpoint but you rarely get 100% Ackerman with any design.