OT- 'Akermann' Steering Formula

Deanism · Jul 29, 2006

Does anyone know the forumula for working out the linkage lengths and pivot points to obtain ' Akermann ' steering geometry ?

At least I think its called Akermann steering - that is, when a vehicle turns, its inner wheel follows a circle with a smaller diameter than its outer wheel does. To acheive this the Pittman Arms ( as auto makers call them ) are angled inwards. This causes the inner wheel to move through a greater arc than the outer wheel as the steering wheel is turned ( its easier to understand if you actually see it happen ).

My problem is in how to work out the actual ratios and pivot points when building a steering linkage system. I've looked on the web but came up blank.

Any help will be apperciated so much, it'll make you sick!

DICKEYBIRD · Jul 29, 2006

Dunno so much 'bout no formulas but the "graphical" method worked for me on go-carts when I wuz a kid.

Build up your chassis, spindles, rear axles and steering arms but don't weld the arms on. Mark the center spot on your rear axle, stretch 2 strings from the center of the rear axle to the axes of the spindles and weld your steering arms on aligned with the strings. Couldn't be simpler.

ps: It's Ackerman.

Or you can draw it out to scale as above on graph paper (or use CAD if you can) and measure the nec. angles.

matt_isserstedt · Jul 29, 2006

I recall this being a school project long ago on an incredibly outdated CAD program called AUTO_TROL.

I know there are tradeoffs depending on what kind of geometry you select, but alas, everything about the project but those details has escaped thru the two sidewall vents in my head

CatHead · Jul 29, 2006

I have the formulas to correct the steering linkage angles by way of shimming or slotting the steering gear and idler arm, or are you looking for actual engineeering formulas to create from scratch?

Mlbz420 · Jul 29, 2006

If you have a cad program your better off just laying everything out there. im sure somewhere there is a basic formula, but there are so many componants that effect akerman angle that i doubt that a generic formula will help unless it is tailored to your specific app.

more things than just the pitman arms effect akerman. the angle of the tie rods, where the tie rods attatch to the spindle, ride hieght, suspension stroke, etc.

If you can lay out all your geometry in cad to get you in the ball park then put an adjustable link beetween the pitman arms so you can change the aker angle. you can also give your self a couple of different mounting locations for your inner tie rod to give you more adjustment.

also remember that perfecr akerman isnt always ideal for handling. somtimes advanced or retarded akerman nets a better result.

the length of the wheelbase, toe in or out at the rear wheels, and whether or not you are on the throttle or not when cornering hard will affect your ideal handling and proper akerman setting.

try looking for information withing the performance side of the rc car racing hobby. there are lots of magazines forums and articles you can find that detail the specifics of suspension and steering geometry that appply to real world apps.

TAG · Jul 29, 2006

Take a look at http://home.tiscali.be/be067749/58/c3/index.htm

Mark

Dave P. · Jul 29, 2006

There are several sites on the web that talk
about this, look at sites dealing with racing
karts.
You will have to decide how much ackerman
you want, low speed, low traction off road
situations with lots of tight turning will
be happy with lots of ackerman effect.
In high speed pavement ( racing ) you
may not want any at all or possibly a
negetive amount.
Dave P.

bob308 · Jul 29, 2006

check out the circle track magazine site they have. some articals on steering.

metlmunchr · Jul 29, 2006

Bump steer and roll steer also come into play in determining which way the front wheels are actually headed in a dynamic turn situation. A double A-arm suspension, and particularly one with anti-dive properties designed into it, is a seriously complex linkage to analyze. Doing it graphically or with computer modeling is a major task, but attempting to come up with a mathematical representation of the motion could be a career in itself.

I made my living for a few years doing suspension testing and analysis, and I'd hate to think I had to even derive the formulas to accurately predict what will change at the tire-road interface with something so simple as changing the compliance of the rubber bushings at the mounting points, much less the motion of the entire linkage.

On the funny side, we had a development vehicle on loan from one of the manufacturers one time, and for a particular static test we replaced the front struts with threaded rod and nuts so we could look at some of the properties without the struts affecting the numbers. The rods came a couple inches above the strut towers, and well into the space the hood would want to occupy. When we quit for lunch, one of the guys laid a 2x4 across the engine compartment just so no one would close the hood without looking. While we were gone, our department manager came thru the shop and dutifully removed the 2 by, stood it in a corner, and proceeded to slam the hood. The result looked like a couple bullet holes thru the hood, shot from the inside out

Toms Wheels · Jul 29, 2006

The pitman arm is connected to the ouput shaft of the steering gearbox. The arms connected to the steering knuckle are Ackerman arms or more generally called steering arms. In the simple solid front axles of heavy trucks, the Ackerman arms were available in three angles, this was to cover the wheelbase range from 156 to 300+ inch wheelbase. If you happen to build a vehicle near the top of the any range for a given arm, you were sure of having steer tire wear problem with that vehicle.

my wheels don't slow me down

Deanism · Jul 30, 2006

Thanks for all the replies.

I should have pointed out that this is for a robot I am making ( which will finish up heavier than the U.S.S. Carl Vinson the way things are going ), and so the steering doesn't have to be Formula One grade. But as it will have a fairly tight turning circle the wheels cant use simple steering.

I like metlmunchr's suggestion to incorporate an adjustable link in the tie rod to allow the correct inward angle to be obtained through experimentation. I should have thought of that!

And I like his story about the loan car hood customisation.

JamesM · Jul 30, 2006

I am aquainted with Dickeybird's method.

One great place to research this subject is at the www.eng-tips.com forums. Go to the Automotive Engineering/Steering Systems forum (click "activities" to see all the forums.) and search for this subject. Real-live automotive engineers there talking about all kinds of stuff at length. Very interesting.

-James

Deanism · Jul 30, 2006

One great place to research this subject is at the www.eng-tips.com forums.

Hey thanks JamesM! That looks like a great place to learn heaps.

Forrest Addy · Jul 30, 2006

I don't need no stinkin' Ackerman formula on my Bobcat

aametalmaster · Jul 30, 2006

Heres some more info on suspension...Bob
http://www.heidts.com/heisinfo.htm

77ironhead · Jul 30, 2006

FWIW- per Tex smith in 'how to build hot rod chassis' ackerman is set up by drawing a line from the center of the kingpins thru the center of the tie rod holes and should meet at the center of the rear end

GisMo · Jul 30, 2006

This is an awesome book. Race Car Vehicle Dynamics.

http://www.sae.org/servlets/productDetail?PROD_TYP=BOOK&PROD_CD=R-146

Also, this software is pretty nice to work with

http://www.performancetrends.com/SuspAnzr.htm

the_mechanic · Feb 3, 2010

simplest and best illustration I could find online is here.

I searched all over the place for 'ackerman' (including images) before I adapted (actually, converted) some 'normal' swivel casters to ackerman steering, under my 500 lb welder. this particular dolly took a "criminal amount" of time to make

the dolly -alone- weighs over 200 lbs

good luck with your project :cheers:

Chip Chester · Feb 3, 2010

If you want tight turning, don't overlook 4-wheel steering... Opposing directions will turn quite well at slow speeds. "Pigeon-toed" at both ends will let you spin on axis without resorting to treads.

Chip

Racer Al · Feb 4, 2010

Also worth reading is, well, everything by Carroll Smith. He's the author of the "... To Win" series - Engineer to Win, Drive to Win, etc.

IMHO, if for no other reason than entertainment, these ought to be on your reading list.

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