Ackermann ???

[Art Smith]

what is the accepted theory for Ackermann for road racing cars with large track to wheelbase ratios??? asked another way, as the track increases with fixed wheelbase OR as the wheelbase get shorter with fixed track, how does the Ackermann need to be changed?? excluding Mini's (front wheel drive), are there any known examples in the neighborhood of 0.9-1.0 track to wheelbase......?

thanks for your thoughts!

Art
artesmith@earthlink.net

[Joe_914]

We believe the ackerman is all screwed up on our 88 VD. How does one change this ona car.

[Art Smith]

Joe-

Ackermann is adjusted by changing the intersection point of the steering arms on the front uprights extended. Race Car Vehicle Dynamics by Milliken & Milliken is best engineering reference I have.

Art
artesmith@erthlink.net

[Diamond Level Motorsports]

IMHO zero Ackerman is preferred. Especially in low powered formula cars.

[Dick R.]

We believe the ackerman is all screwed up on our 88 VD. How does one change this ona car.

For show and tell, here are couple of shots of my 85 VD (rocker arms compared to your pushrods but still "rear steer").

If you project a line from the upright pivot point through the tie rod attachment pivot it can give you a good idea of your current ackerman. If the line is parallel to the centerline of the car you have "0" ackerman as I understand it. If the line intersects the car at the center of the rear axles you have "100 %" ackerman.

My car was a road racer when I bought it but I still run 100 % plus about 3/16 total front toe out with my 93 inch wheelbase for autox. Other guys seem to run more (commonly the "eyeball on grid" lines intersect at the roll hoop when they do). In general, for autox the combination of 100% plus static toe out works very well for me given the fact that we are dealing with tight to very tight corners and a little toe out drag just helps heat the front tires in my opinion.

FYI: Edit to remove rambling. Think carefully about the impact on bump steer if you change ackerman. With my car the rocker and the tie rod are the same length and are parallel so they don't steer when moving up/down.

Dick
CM 85
85VD

[Brian]

The project a line and find the intersect only works if the steering rack is in line with the steering arms and the tie rods make a straight line with the rack when the wheels are straight ahead. If the rack is forward or back of the steering arm pickups, and the tie rods are at an angle to the rack, all bets are off and you have to actually think about ackerman. I initially measure the angle that the arms intersect the tie rods - if that is 90 degrees then I initially say there is zero ackerman. But you do have to actually analyze the geometry.

Ackerman depends on the tire, the problem and what else you can do to fix it. You can have fun testing with it. I favor having some ackerman if only to make the car easier to push around the shop...

Brian

[S Lathrop]

Art;

I think you are going to have to think of akermann as something that has to be tested for each car design. I have run settings from 0 to more than full akermann.

On the steering system for the Z12 and 84 Citation FVs, I could adjust the akermann almost as easily as changing toe settings.

It is important and getting it right makes a big difference. I have run 0 akermann cars but I have had more success with other settings. I would suggest that you start with full akermann and try other settings.

[ghickman]

Ackerman has much to do with the minimum turning radius that you might see on any course you'd be running. The difference in the arc path of the frontend versus the rearend.

In my opinion rear steer cars are far easier to get any amount of Ackerman out of. To get anywhere close to 3 degrees of Ackerman out of a front steer chassis is not easy to do without compromissing bump steer etc.

Back in the early days of Swift DB1 testing we went all the way out to 5 degrees of Ackerman on tight courses. As I recall we ran 2 degrees at Road Atlanta.

I always found formula cars with 2-3 degrees of Ackerman turned in better and took a set faster.

Some of the modern F1 cars run reverse Ackerman.

[Dick R.]

Gary,

Is the number of degrees the difference in "tire angle" between inner and outer at some specific amount of "tire angle" for one of the tires? For example, at zero toe, my car measured 18 degrees outer with 20 degrees inner and at "autox full lock" 28 inner gives 24 outer. With 1/8 inch toe out 20 degrees inner is about 17 outer (i.e. more ackerman I think).

Projecting the lines per my post above they intersect at the rear "axle".

The car has no apparent scrub when being pushed at any steering amount which "I think" is consistent with "100 percent ackerman" but don't really know for sure.

Thanks,

Dick

[Hardingfv32]

How is the measurement made when you make the statement "2-3 degrees of Ackerman"?

Brian

[Art Smith]

my question must not have been clear enough; I'm not fishing for a number for Ackermann!! I'm trying to understand the shape of the required Ackermann curve as the track-to-wheelbase ratio approaches 1.0.

different approach: how does the Ackermann on a typical FFord compare to the Ackermann on a shifter cart?? ditto typical FC cars. I'm not interested in any of the Ackermann number, only the ratio of the two...............

Art
artesmith@earthlink.net

[ghickman]

Ackerman would be measured at whatever amount of steering input it would take to achieve a specified turning radius.

Ackerman is not as simple as most would think. To really properly analyse it I use a master sketch in Solidworks that I can drive steering angle with.

When I say 2-3 degrees that is given at say 180 degrees of steering wheel which is typically about as much steering input as most use in a Formula car under normal conditions. It would be the diffirence in steering angle from inside wheel to outside wheel.

For those that do Autox with a Formula car you would most likely want Ackerman exceeding 5 degrees....just a guess.

Ackerman and Scrub are not the same. Scrub radius is the angle of the Kingpin and contact patch of the tire. I like as little Scrub as possible...zero being ideal for me but not always the best setup.

I've never seen a published ratio of track vs wheelbase and how it relates to Ackerman.

[Brian]

I think the point may be that the shape of the required Ackerman curve (I don't think it's a curve, I think it's a static setting that produces a curve as the front wheels are moved from straight ahead) is relevant to the car, the track, the suspension settings, the tires, the driver, and not really relevant to the ratio of track to wheelbase except as that influences the others. The traditional measure of Ackerman as a percentage of wheelbase removes both track and wheelbase from the equation.

I personally think of Ackerman as a way of changing the slip angle of the inside front tire in a turn, and very close to static toe. If you run a lot of Ackerman you might not run much toe-out, if you run zero Ackerman you might need more toe-out to get the turn-in characteristic you want. One way to look at Ackerman is variable toe-out.

Brian

[ewcmr2]

different approach: how does the Ackermann on a typical FFord compare to the Ackermann on a shifter cart??

I don't have any numbers but shifter karts have much more ackermann than any FF I've seen. With a casual glance at a shifter at full lock you would think something was broken.

I'll have kart out tomorrow if you want rough numbers... Mondiale is staying in trailer, waiting for muffler.

Eric

[Steve Demeter]

Sifter karts need to turn around much tighter radius corners (at least on a kart track) than a FF. And with a solid rear axle, nothing that has to be done to get the best performance would surprise.

The little buggers really haul the mail.

[Daryl DeArman]

With a shifter kart (when did they quit calling them "gearbox"?) the wheelbase is so short that the difference in radii will look quite severe...while it may or may not be running anything much different in terms of ackerman than your average FF.

Taken to extreme if the wheel base was 12" imagine the angle of the steering arms to achieve 100% ackerman, now take a TFD with a 300" wheelbase and see how little would be required.

My guess is the best ackerman is determined by the tightness of any given corner and the slip angle where the given tires are happiest.

[ghickman]

My guess is the best ackerman is determined by the tightness of any given corner and the slip angle where the given tires are happiest.

You got it....this is exactly how it is done in the real world.

[chrisbillings]

As a suspension design judge in last weeks Formula SAE competition at Fontana I asked each team I judged 'how did you arrive at the Ackerman setting in your spec sheet?' Only the best team in the world, currently Oregon State University, understood that tire slip angle performance can play a very important role in determining ackerman. There are pictures around showing F1 cars with anti-ackerman for exactly this reason. FSAE teams have Tire Testing Consortium data to work with. Club racers don't! So, how does a club racer determine ideal Ackerman? Lots of testing.

Want to learn more? Take Claude Rouelle's class:

http://www.optimumg.com/OptimumGWebS...aySeminar.html

Chris Billings
Shock Shop

[Brian]

I find it quite amazing that only one of the teams had that understanding - I used to follow FSAE web forum regularly and I recall that they had a big deal going where they tested the slip angle performance of the tires they used and had a consortium where you could buy the data to contribute to the cost of the testing. So they not only understood the tire slip angle issue well at the time, they actually paid the tire company to test and determine the exact slip angle curve data. Kind of interesting.