Ackerman angle

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Postby FRANK BASILE on Mon Dec 29, 2008 2:15 pm

Thanks Tony and Peter. Many of us just go about transplanting front ends,swapping bits because they fit , or to get around some clearance issue created by using an unmolested front end .And as long as the wheels turn left and right we are content. Doing proper research and having some more knowledge and understanding of what goes on,can only help build up a safer ride,and certainly one comfortable to drive under suburban or highway conditions. Besides steering, how many of us actually consider weight differences between donor and recipient vehicles for braking and actual suspension ride?.....Frank.
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Postby HYPORX on Mon Dec 29, 2008 10:00 pm

Here's a great diagram showing the tie-rod end/kingpin relationship and how they'd be placed in a "perfect" Ackerman situation.

In practice, if the two lines join either 1/3 the wheelbase length infront or behind the axle C/L, it is considered acceptable.

I have studied many different theories and experimented myself on radio controlled cars. The difference between having perfect Ackerman, C/L rear axle, and "almost" perfect is not a great deal.
Image

To figure out the steering arm offset:
Measure the length of the steering arm (kingpin to TRE)
Mulitply that by the front track width.
Divide the answer in half
Divide the wheelbase dimension.

Eg: (inches are easier 'cos they's bigger....)
Steering arm: 7"
Track: 60"
Wheelbase: 100"


7x60=420
420/2=210
210/100=2.1

Therfore your steering arm offset should be set at 2.1"

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Postby Dave on Mon Dec 29, 2008 10:18 pm

Brett.C wrote:Peter, doesn't it become a bit more complex due to the need to find where (or how far) the outer tie rod pivot sits in relation to the steering axis (kingpin inclination)?
I believe it does need to be correct in both planes. The diagrams so far refer to Ackerman but there is also a proper relationship of where the steering arms should be in relation to upper and lower suspension pivot points in the other plane. Perhaps Peter can explain more with diagrams? Steering is a fairly complex issue, which is why using a front end like a Jag or a Rod Tech or similar (or using factory steering for the fitted front end like in my coupe!) where all this has been done for you is so much simpler!
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Postby Warpspeed on Tue Dec 30, 2008 6:06 am

Good point Dave.

I had not thought about how caster, chassis rake, or kingpin inclination effects Ackerman, but it obviously does. In the end it is just the actual measured toe out on turns measurement (at the ground) that matters.

If you know your track and wheelbase dimensions, and that typical steering full lock usually maxes out at around twenty degrees in either direction, you can then work out the "theoretical" required toe out on turns geometrically.

Or you can look up some figures for similar sized production cars, or ask a professional wheel alignment guru for advice.
Cheers, Tony
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Postby Brett.C on Tue Dec 30, 2008 7:15 am

Warpspeed wrote:If you know your track and wheelbase dimensions, and that typical steering full lock usually maxes out at around twenty degrees in either direction, you can then work out the "theoretical" required toe out on turns geometrically.

This why I suggest the 2 to 3deg toe out at 20deg turn, because its toe out that really matters not ackerman. Ackerman is just a means of achieving this and is by no means the only means.

You could set what you think in theory is the correct ackerman angle only to find that in practice it is wrong. This is why it is prudent to check everything before final assembly. And this goes for bump steer as well.
With the springs removed, put the suspension system through its paces and see if it does what you designed it to do.

My 2-3deg comes from checking various car specs and a wonderful book I have called Auto Mechanics Fundamentals, by Stockel

You could also come up with a simple table showing wheelbase, track width and desired toe out.
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Postby Warpspeed on Tue Dec 30, 2008 4:17 pm

Brett.C wrote:

My 2-3deg comes from checking various car specs and a wonderful book I have called Auto Mechanics Fundamentals, by Stockel



Yup, spot on observation there Brett.

The Ackerman angle theory for judging steering arms is a pretty first good bet though, at the initial "selecting and mixing possible parts for my rod from the wrecker" stage.
But as you say, the proof is what the front wheels actually do at full lock.
I had that same 2-3 degree toe out figure buzzing around in my head, but cannot remember the exact source. It may possibly have Been buried in one of Carrol Smith's (several) racing car design, and suspension tuning books ??
Cheers, Tony
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