Fine tuning ackermann
Fine tuning ackermann
(OP)
I have a fwd race car on multiple banked paved ovals. I have been fine tuning the suspension geometry for some time now. I am having trouble determining the correct ackermann geometry. Everything I've read for some time has said to set it up so the inside wheel steers more than the outside wheel (pro ackermann), the exact amount determined by the radius of the turn. I have done the math and set it to my calculations and at times I have a howl (audible loss of traction) at the inside tire.
It has recently been brought to my attention the need for race cars to run anti ackermann due to the slip angles generated by the tires when taking a corner at speed. This new information has introduced an unknown variable into my equation. I can not find any information on the slip angles my tire generates but have been able to determine tires of similar construction range around 5-6 degrees.
Which brings me to my question;
Is it safe to assume the inside tire will also operate at the same slip angle as the outside 5-6 degrees? My guess is no and im thinking the slip angle at the inside tire is a lot more load sensitive. I'm looking for a method or advice on how to measure or at least get an educated guess as to what slip angle the inside tire may be operating at.
Any other input you think would be helpful is also appreciated
It has recently been brought to my attention the need for race cars to run anti ackermann due to the slip angles generated by the tires when taking a corner at speed. This new information has introduced an unknown variable into my equation. I can not find any information on the slip angles my tire generates but have been able to determine tires of similar construction range around 5-6 degrees.
Which brings me to my question;
Is it safe to assume the inside tire will also operate at the same slip angle as the outside 5-6 degrees? My guess is no and im thinking the slip angle at the inside tire is a lot more load sensitive. I'm looking for a method or advice on how to measure or at least get an educated guess as to what slip angle the inside tire may be operating at.
Any other input you think would be helpful is also appreciated





RE: Fine tuning ackermann
If you are turning the steering wheel (say) 30 degrees to go around the corner, the wheels are only being steered (say) 2 degrees more or less depending on your steering ratio. When running near straight ahead, the Ackermann effect - whether positive or negative - is near nil compared to the static toe-in setting.
Does the car turn any differently if you set it with more static toe-in?? It'll have more effect than Ackermann.
Autocross or very tight road courses can be a different story.
RE: Fine tuning ackermann
I have played around with the toe. I usually like to run around 1/8th out but when I was getting that howl from the inside tire I set the toe an 1/8th in and noticed a huge improvement. I then went another 1/8th in (1/4" in over all) and it was again better. The howling went away and it took some of the twitchy-ness out of the car (probly from the left front dragging before)
I didn't want to try any more toe in than that. I know I need less ackermann, I'm just trying to figure out exactly how much less. Try to get as close as possible to perfect as I can.
RE: Fine tuning ackermann
Even if the steering angle was minimal wouldn't the slip angles from the cornering forces cause toe out making anti ackermann beneficial?
RE: Fine tuning ackermann
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Fine tuning ackermann
Cheers,
dynatune, www.dynatune-xl.com
RE: Fine tuning ackermann
Greglocock, could you explain how static toe could "dominate" in this scenario? I'm just looking at it seeing the introduction of slip angle as dynamic toe out pointing my contact patches on either side away from each other causing a fair amount of drag.
I did some quick trig and determined that if the right front tire is working at a slip angle of 6 degrees, that is equivelant to 61mm toe out. That's huge! I can play with toe all I want but I can't get anywhere close to that and have it go down the straits without massive drag. 61mm may be an exajuration because the left front will be operating at some degree of slip angle, say 3 or 4 degrees (which is a conplete guess) which would almost half that, leaving me with 30 something mm toe out. That's still a lot.
Is there no way to measure dynamic slip angles without software? Or am I stuck with the old trial and error?
Adjusting this isn't easy on my car, it requires a lot of cutting and welding on my part so trial and error is rather time consuming. Only want to do it once more and get it right.
Or am I completely off base here? Help me out, always looking to learn.
RE: Fine tuning ackermann
Let's run some numbers. Assuming steering ratio is 15, so we are talking about 2 degs of steer. 100% ackerman for a car at 1.5m track, 2.5 m wheelbase, at 2 deg front axle average steer is, well tell you what, you work it out and show your working. Then you can refine it later on.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Fine tuning ackermann
Which helped me realize my problem. I cheated when I set my ackermann and shortened the left side steering arm until I got my target (for the radius of the turn) 6.4mm toe out with 35 degrees steering input. So basically I just introduced a ridiculous amount of toe out into the steering thinking it was needed and it clearly isn't. I'll just make the steering arms the same length again and forget about it.
I still think the car would benefit from anti ackermann due to the toe out caused by the slip angles. I can make it happen even at small steering angles, like I've already done. I'm curious to see if its worth trying the opposite of what I have now. I can't help but think it would be beneficial to front end grip to have my contact patches parallel in a corner, no?
RE: Fine tuning ackermann
Norm
RE: Fine tuning ackermann
[img http://www.google.com/imgres?safe=active&sa=X&...]
Link
As the vertical load drops the slip angle for maximum lateral force drops from about 5.5 to 3.7 degrees.
So, if you happened to have vertical loads of 1000 and 400 N on your tires and you wanted to use maximum lateral force, then you'd want (5.5-3.7) difference in steer angle between the two tires, which will be the sum of the steering arm/tie rod geometry induced ackerman angle, the static toe, and any compliance effects.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Fine tuning ackermann
Why do the tires have to be operating at the same slip angle for this to work?
I just need to know the slip angles of each tire so I can factor it into my ackermann in an attempt to get me as close as possible to parallel to get the most out of the front end grip available. It is a bit of wishful thinking but the closer I am to perfection the closer i am compared to my competition. I don't need perfection, I just need it to be closer to it than they are.
After looking at what Greg has said I guess that's the only way to go about this with any acuracy. I could calculate the dynamic corner weights and use it against a chart like he posted, but one that more closely matches my tire in construction and get a rough idea.
I have corner scales, and I've always wanted to calculate my center of gravity height, which I see is needed for the calculation.
Getting it right on is almost irrelevant anyways because every track varies. I'm sure even running high or low on the track would make it different. So aiming for the middle and just dealing with it is the best I can do really.
RE: Fine tuning ackermann
They don't, but that's what parallel contact patches implies.
Norm