# Meeting point of turning wheels with rear wheels

I was watching this video about how steering on a vehicle. It states:

If you track the meeting point of the left and right wheels, you can see that the meeting point always lies on the rear wheel line.

I have a few questions about this:

1. If the car is longer or shorter or wider or thinner, the meeting point of the left and right wheels would not be on the rear wheel line. How then is the steering setup such that the meeting point is always in line with the rear wheels? Is there a process for determining it? Below is a photo of the meeting point not aligning with the rear wheels:
2. Why does the meeting point need to be in line with the rear wheels? Would the wheels skip or slip along otherwise?

## 2 Answers

Wikipedia explains it fairly well.

Figure 1. Image source: Ackermann steering geometry.

A simple approximation to perfect Ackermann steering geometry may be generated by moving the steering pivot points inward so as to lie on a line drawn between the steering kingpins and the centre of the rear axle. The steering pivot points are joined by a rigid bar called the tie rod which can also be part of the steering mechanism, in the form of a rack and pinion for instance. With perfect Ackermann, at any angle of steering, the centre point of all of the circles traced by all wheels will lie at a common point. Note that this may be difficult to arrange in practice with simple linkages, and designers are advised to draw or analyse their steering systems over the full range of steering angles.

If the vehicle is stretched then the steering pivot points will move out and the angle of the solid arms (dark brown in the image) will change to suit.

You might enjoy the Project Binky steering setup. This is episode 11 of (currently) 31 half-hour episodes and is a fantastic series.

The meeting point of the angled front wheels will always intersect the line from the rear wheels.

All that happens as the vehicle changes dimension is that the turning circle becomes larger, or smaller. The Volvo series (140, 240 etc) of cars were known for being easy to manoeuvre due to the angle the front wheels would turn to.

There will always be a meeting point and it defines how the car turns and also helps set up things like the Ackermann steering angle: that the inner wheel of the turn is at a greater angle than the outer to reduce scrubbing.

One point to consider is that the meeting point should be the same distance on either side of the car, one thing that is checked (by good body shops) when dealing with chassis damage after an accident. Some have driven cars or vans that feel like they are "crabbing" down the road...

There are some good books about this, one to start you off is Fundamentals of Vehicle Technology by Hillier & Pittuck.

Edit: One of our questions at college during the apprenticeship was to calculate the angle of the inner wheel, given the wheelbase, track and turning angle of the outer wheel. A scale drawing was all that was needed. Mark the points of the wheelbase and track, extend the centreline of the rear wheels, locate to point where the outer wheel intersects and then draw the line for the inner wheel and measure the angle. Plus or minus 1 degree was allowed for the result so you had to be careful with the scale and quality of drawing.

• Thanks for the answer @solar-mike. I'm struggling to understand how the meeting point angled front wheels would always intersect with the rear wheels. For example, if the length of the vehicle shown in the video was extended a couple of metres, the meeting point would not be in line with the rear wheels. Could you please clarify how it would always meet? I may be missing something. May 24, 2020 at 4:33
• The centre line for the rear wheels is extended as far as is necessary, defining the turning circle - why some cars are poor at parking etc. May 24, 2020 at 4:35
• I've added an image to clarify what I mean. If the body of the vehicle is extended, the meeting point of the left and right wheels does not align with the rear wheels. Would this mean that you would have to adjust the turning angle of the outer wheel right, as you have mentioned in your edit or something else as well? May 24, 2020 at 5:25
• If you have to adjust the turning angle of the outer wheel, how is this achieved? Do you have to extend the length of the tie rod? May 24, 2020 at 5:34
• That diagram shows what I have explained, if the wheelbase is increased, then the lines from the front wheels will still intersect... Changing the angle of the front wheels means turning the steering wheel, changing the angles between the two front wheels means adjusting the linkage between them. May 24, 2020 at 5:36