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


#1

Got to thinking, when a car turns to the left say, the outer (right hand wheel) is turning on a slightly wider radius than the left hand wheel. Isn’t that the case? It seems like it is to me. If so, when turning, shouldn’t the turning angle of the left wheel (w/respect to the long axis of the car body) be a little different than the right hand wheel? I could be all mixed up about this, maybe the theory says the steering angles are indeed the same, left vs right. But it seems like they should be a little different. If so, do car designers take that into account in the design of the steering system, or is it such a small effect they just ignore it?


#2

Nope, you’re not mixed up. You’re exactly right.
And yes, car designers take this into account. The inside wheel does, in fact, turn more than the outside wheel. Take your steering to its limits in a parking lot and get out and look. At the limits, the difference is clearly visible, obvious even.


#3

Of course on a totally unrelated hijack of thread, if you have helium balloons in your car when you turn to the right the balloons go to the right, not related I am sure but interesting.


#4

Yup, and the heavier air goes to the left. The balloons float to the right. It’s buoyancy on a horizontal plane. :nerd:


#5

You’re right. They should be and they are. Do a search for Ackerman steering (spelling?) and read all about it.


#6

For all of those who wish to skip a search, Ackerman steering

"Rather than the preceding “turntable” steering, where both front wheels turned around a common pivot, each wheel gained its own pivot, close to its own hub. While more complex, this arrangement enhances controllability by avoiding large inputs from road surface variations being applied to the end of a long lever arm, as well as greatly reducing the fore-and-aft travel of the steered wheels. A linkage between these hubs pivots the two wheels together, and by careful arrangement of the linkage dimensions the Ackermann geometry could be approximated. This was achieved by making the linkage not a simple parallelogram, but by making the length of the track rod (the moving link between the hubs) shorter than that of the axle, so that the steering arms of the hubs appeared to “toe out”. As the steering moved, the wheels turned according to Ackermann, with the inner wheel turning further.[2] If the track rod is placed ahead of the axle, it should instead be longer in comparison, thus preserving this same “toe out”.

stolen from https://en.wikipedia.org/wiki/Ackermann_steering_geometry


#7

I have to say that paragraph gives me a bit of a headache … not saying it isn’t interesting, just headache producing … lol …


#8

PIc added to avoid brain freeze


#9

It’s also called 'toe-out on turns*, and for most cars, there are specs to go by from the factory.


#10

Where this phenomenon becomes more interesting is on cars that have tie rods that run at an angle from the steering rack down to the hub. Combine that with inadequate sway bars, and the body lean on curves makes the inside wheel turn less and the outside wheel turn more the faster you go around a curve. Hence, the faster you go, the worse the handling in the corners. I had an '81 Toyota Starlett that was scary at any kind of speed.


#11

Yes, early on the Ackerman design of steering geometry was designed to allow for that. Even farm wagons that are puled have that layout.


#12

That is not correct unless the steering arm from the bodymounted steeringsystem is located below the fitting point on the hub in neutral position. I haven’t seen such a configuration, but it could exist.

My manual for my Audi 100 Coupe -71 even states specs for toe-out on turns with deflection of 80 mm under/above neutral at full turn. Values has to be the same as in neutral. If it ain’t, the steeringrack is off center or something is bent.


#13

Good Ackerman geometry is difficult (impossible?) to achieve if the tie rods are ahead of the front axle line. Many Fords, like my early '80s Fairmonts, had the rack mounted in front, so in parking lots, in tight turns, the tires would squeal loudly. My cars with the rack in back of the front axle line have had no tire squealing in tight turns.


#14

I have that tire squealing thing w/my Ford truck. Esp noticeable in parking garages with cement floors. That must be the reason, as the tie rods are front of the axle. At least I know why now, thanks @insightful … .


#15

I have heard that a lot of race cars use parallel or even anti-Ackerman steering. It has something to do with that barely on the ground inside front wheel operating at a lower slip angle.


#16

I wonder if it could have to do w/race track curves being banked at quite severe angles?


#17

You are making a joke aren’t you?


#18

No it is not called “toe-out in turns”. That is another concept completely. It called roll steer in the industry or bump steer in some racing circles although they are not quite the same.

It refers to the change in toe angle from relative wheel and body motion and includes each of the 4 wheels.


#19

That’s an Old school theory about Ackerman on race cars and has basically been debunked. I personally think it was lazy suspension designers. Some Ackerman allows race cars to “turn in” with more crispness (less transitional understeer).


#20

Not entirely true. You can achieve close to perfect Ackerman with the tie rods ahead of the axle centerline. You need a decent inclination angle to achieve it but cars rarely have perfect Ackerman at every steered angle and so they squeal in parking lots.