Ackermann

[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?

[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