# ACHERMAN ANGLE

Discussion in 'OffRoad Design' started by RUSS84K5, Jan 9, 2001.

1. ### RUSS84K51/2 ton status

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I'v had this question for awhile. If the Acherman angle is correct on the top nuckle arms you sell (and I feel they are because by truck drives very well with the x-over steering), then why do other shops have passenger side nuckle arms that are longer and have two holes in the front one for the drag link and one for the tie rod. These would allow tie rod over and the longest drag link for less bump-steer. I'v seen these for D44 and D60 nuckles. dose the acherman angle calculation use wheel base as a component. if so then one for a blazer,pick up ,and sub. would all have to be different. what is the calculation for the Acherman angle?

2. ### TX Mudder1/2 ton status

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I'm pretty sure it's Ackerman.
Sorry, I don't know the answer.
-- Mike

3. ### RUSS84K51/2 ton status

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I think I found some info on this. it sounds to me like the the axtra degree of turn for the inside wheel is set by the axle yolk dimenstions. so if your ball/king pin joints are good then the ackerman angle should be fine. Dose this make sence to you guy. I found this info at
http://www.auto-ware.com/setup/ack_rac.htm

Ackerman Steering and Racing Oval Tracks
Many racers are becoming aware of Ackerman Steering geometry and are concerned with how it influences their race cars. Conventional Ackerman Steering was developed around 1800 AD. Yes, 1800, not 1900. The Ackerman concept is to have all four wheels rolling around a common point during a turn.

The example above shows that the inside front tire must turn a larger number of degrees than the outside front tire for this principle to work.

Most short tracks (oval tracks) have a corner radius of 150'+/-. For a race car that has a 108" wheel base and 60" width, the inside tire would need to turn 3.4 degrees and the outside tire would need to turn 3.3 degrees to have Ackerman Steering. In other words, the inside tire needs to turn 1/10th of a degree more than the outside tire in order to fulfill the Ackerman requirement for this car and corner.

However, at racing speeds, tires develop what is known as slip angle. Despite the name "slip angle" it has nothing to do with slipping or sliding. Instead, it describes the flexing or twisting of the tire's contact patch. It's not unusual for racing tires to develop 6 +/- degrees of slip angle before they loose traction. DOT (street) tires can develop as much as 10 degrees of slip angle before they loose traction.

Due to the magnitude of flex (slip angle) in the tires, that tenth of a degree steering difference mentioned earlier is negligible. So, when it comes to tuning your race car, take conventional Ackerman off the list of concerns. For more information related to this subject see Slip Angles and Handling.

Alignment shops and some Dealerships will refer to Ackerman as "toe-out." This is because Ackerman steering geometry causes the wheels to toe-out during a sharp turn. Be careful not to confuse this terminology with static toe-out. For more information on toe-out see Toe-out and Handling.

4. ### tRustyK5Big meanieStaff MemberSuper ModeratorGMOTM WinnerAuthor

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Some good info there, I was always a little concerned because my front Dana 44 was out of a 3/4 ton pick-up. With its longer wheelbase I thought it might make a difference.

Rene

5. ### Stephen1/2 ton statusModeratorVendor

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I don't think ackerman angle is that big a deal for what we do since it is wheel base dependent and you can take a D60 from the front a crewcab and put it in a jeep or blazer and it works just fine. And all the front axles interchange parts between 1/2 and 3/4T no matter what the wheelbase is.

Ackerman is set by the location of the tie rod holes, so running xover steering to the knuckle doesn't mess with it. You just have to worry about the steering ratios.

The big problem with running a right arm with two holes is finding a pitman arm long enough to make the steering ratio work out and then fitting it under the frame and on top of the spring.

SW-ORD

6. ### Triaged1/2 ton status

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Don't worry about it. I think that our trucks run reverse acherman (toe-in while steering) because of the front steer setup. To run classic acherman (toe-out in steering) an imaginary line drawn from the tie rod end through the line between the ball joints should meet at the center of the rear axle. Next time I have some spare time (never) I'll measure it to be sure.

7. ### Stephen1/2 ton statusModeratorVendor

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One thing to watch out for is a lot of cars are rear steer, meaning the steering linkage is behind the front wheel centerline. Course rear steer means something different to us, but that's what they call it.
Anyway, that messes with stuff a little.
Basically, Ackerman angle is a guess and appoximation for most vehicles, especially in the era we deal with. Anytime one axle was used in wheelbases from 155 to 106, you know it's not a big deal.
Ackerman angle is important in that if you don't have any Ackerman effect at all, you can get some funky feel to the steering, like decreasing force to turn. At some point, you could even have wheels that want to turn to the lock instead of straightening out. That could be strange to drive.
That's about all I know that practically applies to the 4x4 world.
Fundamentals of vehicle dynamics (Gillespie)
Steve Smith's race car engineering books

SAE's bookstore is a cool place....

Making the world better, one truck at a time.

SW-ORD