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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

RE: Fine tuning ackermann

If you are running on banked ovals, is there significant steering angle at all??

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

(OP)
30 degrees sounds about right for how much I turn the wheel in a turn, 45 max.

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

(OP)
Not sure how to edit.

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

Looks to me that you are trying by "trial" and error to "lock" in into your best Ackermann percentage (which is not a bad thing at all). Ackermann is defined ideally for a "static toe" of 0 and any value different from that value does change your percentage Ackermann quite drastically. So by doing what you have done you have already "filtered out" the sensitivity of your car to this particular parameter. If I might make a suggestion, get yourself a proper suspension kinematics tool that allows you to investigate what you have done in detail. You are already halfway.

Cheers,

dynatune, www.dynatune-xl.com

RE: Fine tuning ackermann

(OP)
I would love to play around with that kind of software, but its not in the budget. I do most of my drawings in photoshop to get me in the ball park. It's worked great for me so far, even with this issue the car is still fast, but I know I have an issue with 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

static toe dominates over the ackerman setting in the range for 0-30 degs SWA

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

(OP)
I can't do the math cause I don't have conventional ackermann built into the chassis. I do see your point tho how regular (for lack of a better word) ackermann would have little effect with such a small steering angle.

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

Quote:

I can't help but think it would be beneficial to front end grip to have my contact patches parallel in a corner, no?
I think that would require the more heavily loaded outside tire to develop peak lateral grip at the same slip angle as the lightly loaded inside tire develops its peak lateral grip. Sounds like wishful thinking.


Norm

RE: Fine tuning ackermann

As an example here's a graph showing lateral force vs slip angle for 4 different vertical loads.

[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

(OP)

Quote:

I think that would require the more heavily loaded outside tire to develop peak lateral grip at the same slip angle as the lightly loaded inside tire develops its peak lateral grip. Sounds like wishful thinking.

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

Quote:

Why do the tires have to be operating at the same slip angle for this to work?

They don't, but that's what parallel contact patches implies.


Norm

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