Ackermann

[mfain]

For those still thrashing with Ackermann considerations, there is a new, 6-page article by Bob Bolles in the Mar 2010 Circle Track titled "Ackermann Technology, The History, Application, and Truth About..." It offers a good technique for checking Ackermann along with an Ackermann Toe Chart, and it offers techniques on how to change Ackermann. You just have to "filter out" the techniques for the "left turn-only crowd" such as different length left and right steering arms. They don't have the article uploaded on their website yet, but that should happen soon. There is also a pretty good article on GM's new CT525, a 750 carb'ed LS3 that makes 525 hp/471 ft-lbs of torque for less than $10,000. With a different pan, it might make a good Track Day motor??

Pappy

[speedjohnston]

Thanks Pappy, I'll be checking that out.

[JRouche]

Cool!! Thanks for the tip. Ill be looking for the article when it comes online. Im still trying to wrap my head around Ackerman with my front steer. I get it with rear steer, but for some reason I cant visualize it with the front steer. JR

[Bryce]

with front steer the steering arm has to be outside the lower ball joint. Hard to do with a large offset wheel smaller than 17"

but post up a link to that article when it gets on line.

[mfain]

Guys, try this link. It has the chart for checking Ackermann. The bottom line is that if your car is not "balanced" and the outside front tire is doing most of the work in a turn, then Ackermann may help turn entry and will have little adverse affect mid-turn. If, on the other hand, you have a more modern set-up (mostly a function of moment centers and roll stiffness) where both front tires are contributing to the turn, then Ackermann will introduce mid-turn drag that will hurt performance.

Pappy

http://www.circletrack.com/chassiste..._steering.html

[JRouche]

Originally Posted by mfain

Guys, try this link. It has the chart for checking Ackermann. The bottom line is that if your car is not "balanced" and the outside front tire is doing most of the work in a turn, then Ackermann may help turn entry and will have little adverse affect mid-turn. If, on the other hand, you have a more modern set-up (mostly a function of moment centers and roll stiffness) where both front tires are contributing to the turn, then Ackermann will introduce mid-turn drag that will hurt performance.

Pappy

http://www.circletrack.com/chassiste..._steering.html

Thanks Pappy.. I read the entire article. It tends to jump around and talk about the negative parts of ackerman but doesnt really go into the design and measurements needed to get your head around ackerman. More about the effects, hence ackerman effect. But not a great article to determine how to tune the suspension for an "ideal" ackerman effect.

I have read many papers on the effect. But not alot (some) on the design of the suspension and tuning for ackerman. Maybe because its a set figure with the overall package. And you really dont want to sacrifice one critical suspension aspect just to get a proper ackerman effect because the changes are not worth a better ackerman effect just to loose a stronger suspension quality. Or prolly more to the point of some major structural changes in the overall suspension.

The way I see it is ackerman effect is a concern, but its low on the totem poll as far as other geometry concerns are thought of. Mainly the overall packaging of the suspension, front and rear.

Suspension SYSTEMS are a trade off all along the path of creating a balance. Yup, like you said, balance is the key to dialing the proper suspension... Thanks for the article... JR

[David Pozzi]

Changing Ackerman is an attempt to get each tire working at it's optimum slip angle for a given load. Ideally, you need to know the dynamic loads involved, then look up the optimum slip angle for that loading and adjust Ackerman to achieve the optimum angle for each front tire. Lots easier to do if the turns are the same, over and over like an Oval track. You need tire slip angle to load data or loads of experimentation to get anywhere.
David

[mfain]

JR, you are absolutely right about the minor significance of Ackermann in the big scheme of things. IMO, the big thing is to check to make sure you don't have an unintended Ackermann effect that is off the chart. The referenced article did give a technique for checking Ackermann that is not too bad. When I designed and installed my front steer rack and pinion set-up, I slotted the rack mounting holes (with alignment slugs) so I could move the rack forward and back to adjust the Ackermann -- a lot harder to do with a front steer drag link car where the mounting points are fixed. A suspension that is set up around the factory geometry should be in the ball park anyway. I get concerned about the "less-engineered" suspensions where Ackermann is not considered when designing things such as steering arm length. I recently had to correct a car where, in an attempt to cure his bump steer problem with new steering arms, the owner introduced 2 inches of toe out (Ackermann) at 20 degrees of steering angle. I could hear the tire scuff when he turned into the driveway! I wouldn't get wrapped around the axle about Ackermann, I would just check to make sure it is "reasonable". By the way, congratulations an your Guldstrand picture.

Pappy

[David Pozzi]

Take your daily driver out in a dirt lot and make a sharp U turn at low speed. Then go back and look at your tire tracks. When Ackerman is off, one tire will dig up the dirt much more than the other. It also causes tire squeal in parking lots. You can really hear it squeal in concrete parking structures.

My '04 Tahoe below is bad. After driving forward, I completed the circle and positioned the Yukon back where I started. You can't even see where the LF tire rolled on the dirt, but the LF tire dug up the dirt a LOT. It leaves rubber marks on pavement when I do this test.

[B_Alley]

I'd say the importance of Ackermann varies with what your using your car for. I wouldn't necessarily write it off as unimportant, we've see over a .15 G increase in skidpad grip from adjusting Ackermann, but we were trying some pretty crazy stuff. 100% is usually fine for a street car, helping in parking lots, but for all out road racing (depending on your tire) you'd probably want anti Ackermann if your running cross-plys because the lighter loaded tire achieves max grip at a lower slip angle. For Autocrossing, we'd run about 70% anti with a little toe out, to help turn in, but not enough to give up grip in the big sweepers!

Another thing to consider: When these cars are outfitted with different wheels that may increase scrub radius, you now have a lot more loading going into the steering so your wheel is going to be twisting like crazy! Just be aware! I've seen some scary stuff looking at NASCAR data. That's why we just test at the track!