Ackermann

[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!

[David Pozzi]

Bobby,
Great comments! Can you describe your testing methods? I assume you use a skidpad and vary toe out/in to get the best lap time, then modify the linkage to incorporate that amount of ackerman change? How many different corner radius do you use?
David

[B_Alley]

Thanks David!

Yes, we used test method: bunch-o-circles, aka, skidpad testing. It was for a small formula car, so the radius were quite a bit tighter. We did 25, 50, 75, 100 and 150 ft radius circles (dont see more than that in autocross, at least at nationals) We also had a slalom that we'd test different settings on. We actually kept pulling more and more g's with larger the radius (we run aero) but Gs at 75 and 100 ft were the same, and at 150 ft it was less. This is because the steering geometry we chose was not suited for this big a turn. We had made toe plates that attach to the upright that allowed us to choose different ackermann settings.

One thing we used to do, set the car up for parallel steer, then run different toe settings on different radius' circles to see what works best. Then design a system that achieve the best of both worlds. I'm not sure how much you can alter these systems by shimming things, I'd have to take a look!

We absolutely rely on data acquisition to see what works best, especially for the slaloms, watching what the driver is doing and how the car is reacting.

Does that come close to answering your question?

[mfain]

Not to rekindle an old discussion, but I ran across some interesting data points for the "for what it's worth" column. The Jan. issue of Road and Track had an article on drifting (YUK) that included a chart that showed the inside and outside turn angles for several cars including the '10 Corvette Grand Sport, Mustang, BMW M3, Miata, and others. It was interesting to note that the longer the wheel base/wider the tires/stiffer the roll resistance/higher the horsepower, the less steering angle difference between the inside and outside tires there was (i.e., less Ackermann effect). At about 30 degrees of turn angle, the Corvette had .3 degrees of toe out (30.6 degrees - inside wheel, 30.3 degrees - outside wheel), the Mustang had just under 3 degrees of difference (toe out), and the BMW and Miata had between 6 and 7 degrees. You can draw your own conclusions.

[David Pozzi]

Thanks for that info. I wonder on the Corvette if it's at an ideal Ackerman or is it just that they can't achieve more than that??? I know the outer tie rods are as close to the rotors "outboard" as much as possible and they would have to move the rotor to get more.
David