Ackerman adjustment
Finally got an explanation of Ackerman, from Eric Clements, at the Crows Tour. Turns out my car was still set up for road racing. It only took about 45 min to reverse the brackets on the outboard end of the steering rods. I expect miracles at my next event. Is it possible to have too much Ackerman? If so, what handling problem would I experience as an indication?
Mark
1990 Van Diemen, the Racing Machine, CM AutoX, 2016 Frontier
You can try to make a street car into an autocrosser or you can do a lot less work and make a race car into a great autocrosser
Yes, too much of any setting is always possible.Originally Posted by mwizard
Too much will generally result in overheating and/or graining of the inside front tire. That may not be evident soon enough. The difference in handling may lead to a car being loose from apex to exit.
Ackerman effect will be greater the tighter the turn radius (the more you turn the wheel towards lock). The article link below explains what to expect.
https://www.longacreracing.com/artic...t/Ackerman.pdf
“Racing makes heroin addiction look like a vague wish for something salty.” -Peter Egan
Mark,
For autocross, it's better to have a bit too much than too little. But you can experiment over time by fabricating your own steering arms.
Jim
I wish I understood everything I know.
Check your bump steer when you make steering arm changes.
And, "bump steer" is?
1990 Van Diemen, the Racing Machine, CM AutoX, 2016 Frontier
You can try to make a street car into an autocrosser or you can do a lot less work and make a race car into a great autocrosser
The simple explanation
http://www.longacreracing.com/techni...aspx?item=8162
“Racing makes heroin addiction look like a vague wish for something salty.” -Peter Egan
Simplified: Change in toe through suspension travel
Well played
“Racing makes heroin addiction look like a vague wish for something salty.” -Peter Egan
Have you plotted your steering Ackerman setup as it is to see what you actually have?
It is fairly simple but does require taking some measurements and laying those measurements out. At a minimum you need to plot the top ball joint of the each front corner, the centerline of the chassis and the center of the rear axels..
Now you need a straight line through that ball joint and the steering joint on the upright. All of this is in plan view (looking down from above the car).
Next, project those lines rearward until they intersect. For 100% or full Ackerman the intersection point should be on the rear axel centerline. Less than full Ackerman the intersection will be behind the car. In the 1970's it was thought that zero Ackerman was desirable. Zero Ackerman is where the steering arms are parallel to the center line of the car.
If you want to get a lot more you will need to plot out the inner steering joints. This way you can actually plot the radius of the inside and out side wheels as they are turned. !00% Ackerman will have the inside and outside wheels turned on the exact same center point. This is also the same center point the rear tires are turning about.
My guess is that you will want more than full Ackerman. In that case the intersection point will be forward of the rear axel center line. A good starting point would be a point that is forward of the rear axel center line by 10% to 15% of the wheel base of the car. So, if your car has a 95" wheel base you would have the intersection point 9.5 to 14.25 inches forward of the rear axel.
Because of the crazy steering system on a Citation FV, we were able to play with Ackerman very easily and thus could experiment with it in testing. Most cars you have to build new steering arms so you are always guessing at the exact Ackerman setting.
Now that is likely more information than anyone wants to know on this subject.
Not at all. Some of us love to keep learning.
I've never engineered a car from scratch, so I'm very limited in knowing what good starting points might be, but I have been fairly successful in getting a car balanced.
In my opinion the steering angles are so small (in road racing) that ackerman would have to be pretty darn aggressive to have much noticeable effect. The reason to utilize it would be to attempt to get the inside front tire to work without needing too much static toe out.
IF you find yourself needing more toe-out than you would like to run then tuning with ackerman could prove useful.
Ackerman doesn't cost you any speed when the steering wheel is straight like toe does, but can allow the inside front tire to assume more of the work load in a corner.
You are right about the magnitude of the change. The differences are shuttle but when you are functioning right at the limit of the performance of the tires, the changes can be very noticeable to a driver. The change of moving the intersection points 10 inches forward of the rear axel center line does produce a noticeable change.
Now that change may be like many, in that it makes the car noticeably more pleasant to drive but does not put anything on the watch. The difference between zero Ackerman and full Ackerman is very noticeable and positive.
Especially for Solo, as is the case with the o.p.
Jim
I wish I understood everything I know.
Those kinds of changes produce results. Making passes when you are comfortable putting the car anywhere you want, working traffic is simpler and arriving at the checkered less mentally and physically fatigued are all key.
Yes, the tighter the corners/more steering angle the more important proper ackerman is.
Wouldn't more ackermman also cause more diagonal weight transfer across the chassis based on the KPI and caster?
Scott
And scrub radius.
Chris Livengood, enjoying underpriced ferrous whizzy bits that I hacked out in my tool shed since 1999.
Maybe. If the increased Ackerman makes the front work better then you may actually use less steering lock and thus less tendency to tilt the front end of the car as more steering lock is input. Maybe reducing caster 1 degree might negate the change in jacking effect.
Now that we are using radial tires, we are going to have to rethink the caster and KPI numbers with the increased camber. I have setup cars with reversed caster for ovals to reduce the jacking effect.
How about negative Ackerman on ovals? Ever tried that?
Chris Livengood, enjoying underpriced ferrous whizzy bits that I hacked out in my tool shed since 1999.
No. I think that Ackerman is desirable on an oval, unless I am trying to dampen the turn in phase of the corner.
Anti-Ackerman appears to be commonplace in F1. Who knows what kind of witch-craft is going on within their differentials and brake by wire technology that certainly plays a part in the handling.
I was under the impression that anti Ackerman in F1 is for aero. If the inside tire is turned in a lot due to positive Ackerman it would be like a wind brake. But that's just a guess on my part.
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Jim
I wish I understood everything I know.
Sent from my SM-G930V using Tapatalk
Jim
I wish I understood everything I know.
I was looking at a recent F1 car and it had a bit of negative Ackerman? Wassup with that.
Thanks ... Jay Novak
313-445-4047
On my 54th year as an SCCA member
with a special thanks to every SCCA worker (NONE OF US WOULD RACE WITHOUT THE WORKERS)
I noticed that also
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Jim
I wish I understood everything I know.
If you can understand the rest of the F1 geometry, you are way better than I. It may be that collectively, the anti Ackerman is a solution to some other characteristic of the suspension geometry. Also think about the tires. They look more like Tundra tires on a bush plane than racing tires.
My understanding was that (at least for steady state cornering), the optimum slip angle for most tires increases with additional tire loading. Perhaps this is a way to get all the tires operating closer to their optimum angle?
Ciao,
Joel
Piper DF-5 F1000
I've also seen this explanation. The local FSAE team experimented with it, but at the high steering angles and low downforce typical of FSAE events, it didn't work well.
More than one way to skin a cat.
That was the theory in the '70's and the Z10 was setup with zero Ackerman. Testing however proved that the car worked better with full Ackerman and even more than full Ackerman. What may be going on is that the side walls of the tires do not allow the contact patch to load sufficiently to actually provide a cornering force. The tire is simply rotating and the slip angle is insufficient to provide any cornering force.
Just a guess.
Haven't done any looking at those cars lately, but one thing to remember when looking at a cars ackerman is that the intersection point at the diff center for 100% is not true if the steering links sweep forwards or backwards.
Ackerman on high speed ovals like Indy or Michigan is set to minimise left side tire scrub - they want the tire to drag as little as possible to help keep speeds up. The left tires are generally not contributing much to cornering grip, so minimising the slip angle the tire is running at can be a huge help. Of course, since the tires are still generating some cornering force, there is a point in the compromise where you can go backwards in your quest for minimal drag and lose too much cornering capability.
I once owned a Tecno F2 car which had Anti-Ackerman. It was used to offset the very short wheel base and helped prevent the car from wanting to swap ends. Best handling car I ever owned.
Cheers, Joe
Ok, I had trouble finding this thread again. Last Sun I drove w/ the adjusted Ackerman. The effect was subtle, but in runs 2 and 3 I took out a slalom cone, something I seldom do. I corrected on run 4 for a clean run. Not going to change back!
Mark
1990 Van Diemen, the Racing Machine, CM AutoX, 2016 Frontier
You can try to make a street car into an autocrosser or you can do a lot less work and make a race car into a great autocrosser
Jim's idea about inside tire excessive toe out angle (from Ackermann) being an aero impediment in F1 is the most interesting thing I've read in a while. Once you have a car that operates in a totally different realm than our own, one really needs to think about what one can copy!
BTW, pos. vs Zero vs opposite Ackermann has been a hotly debated topic for years and years in FSAE. I wouldn't copy what I see on any particular FSAE car either without trying to understand the "why it is what it is on that car" of it first.
My own opinion is that Ackermann effect ought to change with steering lock and that's what I've always designed into my cars. Picking the crossover point is the part I'm never sure of until I test a lot.
BTW, Mark, what you experienced with driving into cones after ur change to more Ackermann is expected.
The more positive Ackermann, the more responsive the car will be to steering input. That also correlates with the post from the fellow with the Techo which had negative Ackermann- to take the twitchiness out. As usual, with all mechanical devices (and cats), there is more than one way to skin a cat.
Slowing down or speeding up or slowing down the R&P ratio is another way to go. With Ackermann angle, one should consider separating best grip from "responsiveness". Also, if you shortened up your steering arms the same time you added more positive Ackermann, you sped up the effective steering ratio, but I think you said you simply turned the arms around on your car, so yours may be a fairly pure cause and effect.
-Chuck
Last edited by B17overhead; 07.02.17 at 12:17 PM.
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