Ackermann Principle

[artie on edge]

I doubt it played a significant role on carriages.

Then I have to ask.... Why did they do it?

[fabr]

I think he meant that slip angle had nothing to do with how ackerman was optimized on a carriage.

[artie on edge]

Reverse Ackermann technically is not a valid description, but is also a common expression which people understand what is being said.

Reverse Ackermann is a "technical" description or label and that is what we are discussing at this point.. Found a pic to highlight. M Schumaker at Monaco.

[artie on edge]

Ok... yeha that I agree with..

[fabr]

Reverse Ackermann is a "technical" description or label and that is what we are discussing at this point.. Found a pic to highlight. M Schumaker at Monaco.

Check out his hand position.There's something about to happen.LOL!

[Admin]

Normal ackerman for cars that turn both directions is when the IS tire in a corner turns more sharply than the OS tire in a turn. This is accomplished by the steering arms being angled to the centerline of the car on a rear steer setup and by the arms being splayed to the outside of the car on a front steer setup. It is rare that you would use reverse ackerman.

I believe that is the prehistoric way of setting up ackerman..

[Admin]

"It's an attempt to have all four tires turn about a single point so that all four tires roll freely without any lateral scrubbing while cornering.  Legend has it that "to avoid disturbing the gravel in the driveways of the well-to-do" may have provided additional early incentive for development of this concept.  Understand that the idea of steering the front wheels by different amounts dates back to the 1870s, with all that implies with respect to road, vehicle, wheel/(tire?) development and cornering speeds/lateral accelerations. 

The method itself appears at least in part to make use of the nonlinearity of the sine function and the angle the tierods make with the steering arms rather than just the angle of the steering arms themselves.  You can get ~parallel steer, >100% Ackermann, or anti-Ackermann just by moving the rack longitudinally in the vehicle and affecting this angle.

When you apply that angle from the kingpin axis to the rear axle centerline to the steering arm, are you using that for the physical angle of the arm itself or as an angle to locate the coordinates of the outer tierod pivot relative to the kingpin axis at the steering arm elevation and then building a steering arm to fit?  The presence of any steering arm lateral offset means that the virtual steering arm doesn't coincide with the physical one.  If you're working with the physical arm orientation I think that any given offset would push the intersection of the construction for a front-steer arrangement back faster than it would pull the intersection for a rear-steer arrangement with the same length steering arms forward, though I don't know which would represent the greater effect on %Ackermann.

Anyway, and as Greg suggests, it's at best an approximation thats most applicable at low levels of lateral acceleration.  Lots of things are being assumed in the basic construction that's typically illustrated, including zero static toe at the front and zero static toe/zero toe change at the rear (none of which is necessarily true and usually none are). 

Understand that the relation between where the steering arms are pointed and the directions in which the tires are actually traveling is not mechanically fixed either.  Once you have much of anything going on in the way of slip angles and lateral load transfer, by definition you don't have pure rolling any more, contact patches move around, and the construction itself will be "off".  Hence the use of computers (or lots of instrumented test-and-tune). 

Unless you're designing primarily for low speed on unpaved roads (golf cart?) and/or really short-radius turns (fork lift?), there isn't anything particularly sacred about 100% Ackermann correction." Norm aka an automotive engineer...

[fabr]

Where'd you copy that from? 

[Admin]

Where'd you copy that from? 

Norm, aka an automotive engineer....

[fabr]

WTF is norm? The guy from Cheers? I have read that stuff in many forms before and it shows how complex steering geometry really is.The best most of us can hope for is a nice handling car.I prefer to try for that by designing ackerman of some degree into the front steering for just the reason that is indicated.Slow speed turning capability.With a spool rear end you need all the help you can get turning at low speeds.Reason enough for me to want ackerman on my side!

[artie on edge]

Finally, something posted today that I completely agree with. Well said Mast. :P

[fabr]

Well since I agree with you most of the time Artie it's about time you agreed with me!

[lee1969GB]

Check out his hand position.There's something about to happen.LOL!

I have herd the commontator on F1 Martin brundell (ex F1 driver ) say the steering set up for monoco is a one off because of some of the very tight turns (the main one is the very tight left hander over the zebra crossing) there are also a few places on the track where one of the front wheels are off the ground whilst turning which must cause some problems when the wheel comes back down and takes you in a different direction because of the difference the wheels are turning.
The corner in the picture looks like the one at the bottom of the hill where  a few years ago schumacher "crashed" a few minutes from the end of qualifing, funnily enough he was leading at the time and the session was ended. The officials saw through it and gave him a 10 place drop on the grid.

[karman1970]

The picture of Schumaker is a perfect example of optical illusion.  Anyone who has sat in an open wheel car will notice that the tires appear extremely toed-in, even pointed straight ahead.  I'd be willing to bet money a Formula 1 car doesn't run anti-Ackerman.

[artie on edge]

I hope your credits good. I competed in F3 for some time and we did run "rev ack" for certain tight tracks. How much do you owe me?