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. :cheers:

[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/chassistech/ctrp_0407_ackermann_steering_system/rack_and_pinion_steering.html

[JRouche]

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/chassistech/ctrp_0407_ackermann_steering_system/rack_and_pinion_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.
http://i117.photobucket.com/albums/o71/davidpozzi/Other/yukontirescrub.jpg

[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?

[David Pozzi]

Thanks, I wish I had access to somewhere to test, it would help so much!

[B_Alley]

Haha, I don't know where your located, but if your in California, Buttonwillow Raceway, near Bakersfield has a good 300' skidpad that I'd love to test some cars on! Rental rates arern't too bad.

When we couldn't get our practice track, we always just snuck out to a nearby church or Homedepot at 1:00 in the morning!

[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

[mfain]

David,

You're welcome. That's a good question about the Corvette. My humble guess is that GM wouldn't let a little redesign of suspension components stop them from achieving the geometry they were after. You can also change Ackermann by moving the rack forward or aft. As you pointed out earlier in this thread, you try to make each tire operate at its optimum slip angle. With a fairly well balanced car like the Corvette, the front tires probably operate at very near the same slip angle during a corner, which means adding a lot of toe out (Ackermann) might cause a lot of drag, especially with wide tires and short sidewalls. A less balanced car (higher center of gravity, more body roll, an "unloaded" inside front tire) or a car with a narrower front tire might not experience the same adverse affects from toe-out induced drag. Like you said, testing is the only sure way to work it out.

Pappy

[B_Alley]

I agree Pappy. I'd be curious to see what the test entails. It was mentioned as a 'drifting' which doesn't necessarily mean steady state skid-pad. You don't want the same ackermann settings for every event whether it be a 100' skidpad, 200' skidpad, or a slalom. Each company probably sets their car up for different situations. From a safety standpoint, if you swerve to avoid something, your going to want the setup designed for a slalom or single lane change.

Each car has a amount of weight on each tire, CG height so a different amount of weight transfered, and different roll gradients and suspension meaning when your rolled over in mid corner, the tire will be at different camber settings which again effects your 'optimum' slip angle, not to mention, the trend between maximum grip vs slip angle per normal load is NOT linear.

But the most important factor is the tire they use! Typically: Vette uses Goodyear, Mustang Pirelli, and, surprise surprise, the m3 and miata use bridgestones, which both operated at the same slip angles.

As I said before, compliance also plays a big issue. I was talking about this last week with Huibert Mees - the chassis and suspension designer of the Ford GT.

How far down the rabbit hole do you want to go? :D

[mfain]

Bobby,

It looks like the turn angle chart in the R&T article was simply a static comparison of the showroom stock cars they tested in their drift article. They turned the wheels as far as they would go and measured the inside and outside turn angles. They commented that big turn angles are important for drifting and, without explanation, they commented on the big difference in toe out between the Corvette and the BMW. It wasn't a very scientific or deliberate Ackermann comparison. I measured the Ackermann on my C6 Z06 a while back and it was virtually zero at 20 degrees turn angle -- not much different than the Grand Sport in the article.

Pappy