Ackerman

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Ackerman

Postby jphoenix » December 5th, 2014, 10:12 pm

This past season I spent most of my time at the track adjusting my suspension. I read all the threads on this forum, the Schings book and other sources to understand the mods to the front and rear suspension. I replaced numerous worn parts and adjusted things repeatedly because I wasn't sure I got it right the first time, so I kept adjusting toe and bump steer and front/rear alignment and droop, etc. until I was satisfied it was all perfect. At least as close as a novice can get to perfect :mrgreen:

I looked at a number of other FV's I was racing against (to steal their speed secrets!) and I noticed some cars had the original steering arms with original ackerman angles and others - like mine - were modified to zero ackerman. I have since seen a variety, looking at cars for sale, I see zero ackerman modifications on some, but others have the standard steering links.

So, my question is why? Does it make a difference whether ackerman is zero or not? I ask because I don't understand the ackerman effect (if any) on the slip angle in turns.

Thanks in advance for educating me!

I also learned the importance of tire pressures the hard way - one day I had the right rear go down to 13 psi and for some reason missed it - that was interesting - on AR's of course with their self-adjusting pressure feature :oops:

Jim
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Re: Ackerman

Postby billinstuart » December 6th, 2014, 9:56 am

Personal choice for many. If your car "pushes"..understeers..you may want to dial in some ackermann so the inside front tire helps steer the car. However, that can affect bump steer, which, to me, is more important.
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Re: Ackerman

Postby brian » December 6th, 2014, 5:04 pm

If one thinks of ackerman as maintaining a consistent radius on all four tires it will be better to visualize. If parallel steering is used, the outside tire has the same radius as the inside tire. That means that both tires are likely to scrub. The tighter the turn, the higher the scrub. It is this scrubbing the induces push or understeer.

Like most terms we use, zero ackerman is like zero roll, which both are really misnomers. To an alignment specialist or engineer, proper ackerman, which is dependent on wheelbase, results in zero radius variance and scrub between the four corners. If you push your car in a small circle and hear one of the front tires scuffing, the ackerman is not balanced.

On race cars, the amount of ackerman is determined by the nature of the race track being run. If you are running a super speedway oval, chances are you are very close to no ackerman or parallel steering. As the turns tighten up, ackerman should increase as well. Chances are neutral or proper ackerman is probably too high since most turns we run are fairly large radius.

My shop made a very nice bit of change reestablishing ackerman on limos, When the wheel base is stretched out, the ackerman has to change accordingly or you'll be grinding front tires off in no time.

Race cars are a bit more challenging. On sprint cars we designed a huge amount of ackerman into the left front. It gave the car what we called "lead". Meaning, the front left tire would open up quicker and lead the entire car into the turn forcing the car to oversteer or "backin". That's a bit radical for a road race car.

In the past, some builders used a slight bit of toe out to get the "lead" or turn in, better but that's not desired since under braking toe out will make the car very nervous and unpredictable.

To figure out how much you need will take a bit of testing. If the car is reluctant to turn in try more ackerman. For me when I am testing cars, if I see rpm dropping in large radius sweepers, it's likely the ackerman is too high.

I have different steering plates that adjust ackerman depending on the track. My car was designed to minimize bump steer so it's not an issue when I change steering arms. If you relocate the outside tie rod location, be sure to maintain the same height and be cognizant of turning rate to maintain the lock to lock that you currently have.

Hope this helps.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
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Re: Ackerman

Postby jphoenix » December 6th, 2014, 11:36 pm

Thanks for that Brian, it helps me understand.
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Re: Ackerman

Postby enrikerace » June 10th, 2017, 9:13 am

Hello,

Which are the ways to adjust akerman?

Modifiying sprndles? changing steering plates?

Do you have images?

Thanks
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Re: Ackerman

Postby BLS » June 11th, 2017, 4:23 pm

You need to study ackerman first, just search it. Then you will understand the answer to your question better, which is that yes, you must modify something. It is the relationship of the steering arm, the tierods and the outer spindle that determine ackerman.
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