Ackerman question

[jrh3]

I have two Van Diemen FF’s. An ‘81 and ‘82. Front suspensions are nearly identical rocker arms except for the upright steering arm angles.
’81 has rear conventional A arms, outboard coil overs.
’82 has rear rockers with inboard coil overs.
’81 Ackerman is basically neutral.
’82 stock Ackerman is distinctly negative, anti-Ackerman. The 82 car steering arm has been modified to allow positive, neutral and negative settings. The car has been auto crossed and came to me with positive Ackerman. I assume to allow better low speed turn in.
Question: why is a road race car set up with negative, anti-Ackerman?

[BorkRacing]

I have two Van Diemen FF’s. An ‘81 and ‘82. Front suspensions are nearly identical rocker arms except for the upright steering arm angles.
’81 has rear conventional A arms, outboard coil overs.
’82 has rear rockers with inboard coil overs.
’81 Ackerman is basically neutral.
’82 stock Ackerman is distinctly negative, anti-Ackerman. The 82 car steering arm has been modified to allow positive, neutral and negative settings. The car has been auto crossed and came to me with positive Ackerman. I assume to allow better low speed turn in.
Question: why is a road race car set up with negative, anti-Ackerman?

Read this: https://www.me.ua.edu/me364/PDF/Steering_Ackerman.pdf

[jrh3]

Good article, thank you. Lead me to additional reading.

[pooch776]

I owned a Tecno F2 car a number of years ago. It had a very short wheel base yet it handled as neutral as any race car I had ever driven. I could not understand why it did not have a tendency for more oversteer. Then I discovered the car had anti-ackerman which induces understeer. My conclusion was that the anti-ackerman was used to counteract the cars natural tendency to oversteer.

My conclusion was confirmed when another Tecno owner in Japan finding the anti-ackerman decided to change the steering arms to achieve ackerman. He said the car was not drivable with ackerman. It wanted to swap ends in every corner. Those Italian Kart makers apparently knew what they were doing.

Cheers, Joe

[problemchild]

Because the effects of ackerman are so dependant on slip angles of the tires, the optimum ackerman configuration is totally dependent on the tire itself, AND the speed in the corner AND the radius of the corner AND the duration of the corner. It is also extremely time-consuming to test and tune. The same car at Road Atlanta may like a completely different configuration than Mid-Ohio than at St Pete than at a Gymkana. As a result, most of us are setting our ackerman configuration based on what theory we believe. Do you want to follow the theory of brilliant or non-brilliant writers of 40 year old books based on input from 50 year old tire technology? Similarly, do we want to follow the theory of club racers or solo racers that once modified our cars for their applications (tires and their tracks). I don't have the answers but I am very reluctant to follow "generalizations". I have done some messing with different ackerman theories and have found no verifiable conclusions. I essentially cannot tell differences in front toe settings, nevermind ackerman. On the other hand, I have found rear toe to be a critical tuning tool. YRMV.

[jrh3]

I owned a Tecno F2 car a number of years ago. It had a very short wheel base yet it handled as neutral as any race car I had ever driven. I could not understand why it did not have a tendency for more oversteer. Then I discovered the car had anti-ackerman which induces understeer. My conclusion was that the anti-ackerman was used to counteract the cars natural tendency to oversteer.

My conclusion was confirmed when another Tecno owner in Japan finding the anti-ackerman decided to change the steering arms to achieve ackerman. He said the car was not drivable with ackerman. It wanted to swap ends in every corner. Those Italian Kart makers apparently knew what they were doing.

Cheers, Joe

Thanks Joe. That was the direction my thoughts were going.

[Brian Forster]

Funny how this thread came right after the one about Tigas...

I have a 77 Tiga and the steering plates can be reversed to either positive Ackerman or none. My car is set up for none, and would require shorter toe links to use the positive Ackerman set-up. Howden Ganley used to live in the area and has seen and helped me with my car. I asked him about Ackerman and his response was (and I could be misquoting him here), 'As Colin Chapman said, it's the biggest fraud in race car design...'

However, I do have some shorter tie-rods and I want to try it for myself....

[S Lathrop]

One of the side advantages of the steering system that Ed Zink designed for the Zink Z12 FV, and I have used that design ever since for Citation FVs is that I can easily change Ackerman. We started with zero or anti Ackerman and progressed to more than 100% As we increased the Ackerman we got positive feed back from our drivers. Zink started with zero Ackerman on all his cars. Over the years, I have come to build and modify my cars with more than 100% Ackerman.

When you layout the steering geometry in plan view, and project the steering arm axis to the center line of the car, 100% would be when the line of the steering arms intersects the rear axle at the center line of the car. Moving the intersection point to something like 10% of the wheel base forward of the rear axle would be a good place to try and see if that is an improvement.

Bottom line do what testing shows works best. It may be different for each car type and driver. Done right, it mcan make cornering a lot nicer.

[DaveW]

One effect of Ackermann greater than theoretical 100% is that during cornering the inside tire has more slip angle than the outside tire. This makes drag force on the inside suspension greater, sort of like an individual wheel brake. The resultant torque on the chassis aids turning, and it's greater during tighter turns where understeer is more likely to be an issue.

The opposite is true for less than theoretically perfect Ackermann (anti-Ackermann) - more drag on the outside tire which can help reduce oversteer.

These effects are in addition to what the different slip angles do to overall grip and cornering force.

As Steve said above, what works best depends on the specific car/driver/tire combo.

[jrh3]

One of the side advantages of the steering system that Ed Zink designed for the Zink Z12 FV, and I have used that design ever since for Citation FVs is that I can easily change Ackerman. We started with zero or anti Ackerman and progressed to more than 100% As we increased the Ackerman we got positive feed back from our drivers. Zink started with zero Ackerman on all his cars. Over the years, I have come to build and modify my cars with more than 100% Ackerman.

When you layout the steering geometry in plan view, and project the steering arm axis to the center line of the car, 100% would be when the line of the steering arms intersects the rear axle at the center line of the car. Moving the intersection point to something like 10% of the wheel base forward of the rear axle would be a good place to try and see if that is an improvement.

Bottom line do what testing shows works best. It may be different for each car type and driver. Done right, it mcan make cornering a lot nicer.

Thanks all for the great input!
I just tried the factory anti setting and still have good low and high speed turn in. Possibly a little loose but the VFF Hoosiers now have a lot of heat cycles. It will be interesting to test with new tires.

[Rick Iverson]

Read this: https://www.me.ua.edu/me364/PDF/Steering_Ackerman.pdf

Good stuff here

[Rick Iverson]

…….On the other hand, I have found rear toe to be a critical tuning tool. YRMV.

Greg;

Would you care to share?

[S Lathrop]

Greg;

Would you care to share?

Rick,

I have use rear toe s a tuning tool for years. I learned the trick when I was doing oval tracks where we would run a lot of toe out on the inside rear and at short tracks like Milwaukee, I would run the outside rear toed out a bit.

Toe our in the rear will eliminate corner exit push on acceleration. Now how much is something that you have to test to find out. But I have used it a tracks like Mid Ohio and Elkhart lake on cars from FF to The Lola T97 Indy Lights.

It is not dictated in the scriptures that you can never toe the rear out on a road course.

[problemchild]

Greg;

Would you care to share?

Nothing specific. I can adjust front toe all over the map without driver commentary or measurable performance change. But 2 flats of rear toe change gets noticed. Sometimes one corner, or sometimes skew the end. I can make a major change with a 20 second stop in pitlane. Its also very quick to undo. It is a useful tool, probably used more by engineers who work on spec cars than old-timers, but try changing springs in pitlane during a 20 minute practice session. I especially like it for tracks like Mid-Ohio where the LH corners are very different from the RH corners, and I want different handling characteristics turning each way.

[jrh3]

Please keep those cards and letters coming in….

[Rick Iverson]

Please keep those cards and letters coming in….

LOL.

Steve and Greg, you both are a wealth of knowledge. Thx.

[t walgamuth]

Nothing specific. I can adjust front toe all over the map without driver commentary or measurable performance change. But 2 flats of rear toe change gets noticed. Sometimes one corner, or sometimes skew the end. I can make a major change with a 20 second stop in pitlane. Its also very quick to undo. It is a useful tool, probably used more by engineers who work on spec cars than old-timers, but try changing springs in pitlane during a 20 minute practice session. I especially like it for tracks like Mid-Ohio where the LH corners are very different from the RH corners, and I want different handling characteristics turning each way.

I like a little rear toe out....a little goes a long way. It will get rid of push and get you neutral, in my experience.