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Old 02-27-2017, 11:59 AM   #1
whitedog76
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Ackerman angle on street suspension

I've been talking with vendors about putting together an IFS system for my 54. I want utilize an MII setup, but reverse the spindles for a rear steer application.

In conversation, it came up that Ackerman angle may be affected.

So, I went and studied Ackerman steering. It's a simple concept, the outside wheel in a turn doesn't need to turn as much as the inside. My conclusion is that on a Rack&Pinion equipped vehicle, Ackerman has little or no relevance.

I want to point out that I understand the concept of Ackerman, Castor, camber, and other elements of steering. I had Trig. and Calc. in college, so the geometry of it is not above my grasp.

Not to get long winded. When you have a rack&pinion installed parallel with an axle beam or parallel to the axle center line, the input on one side reacts equal on the other side.

To further complicate things, we have King Pin Angle (or included angle) in our suspensions, along with castor angle. When a wheel turns in a vehicle with KPA and Castor, the inside wheel rolls in, thus staggering the wheels and transferring weight to the outside of the vehicle. It also creates a scrub or scuff area on the outside edge of the inner wheel. The car has no option but to turn.

With that being said. The only application I see Ackeman working effectively is with a center mount steering, with an offset steering shaft or in a wagon (which Ackerman was originally designed for)

Not to say that Ackerman cannot be used in a street car. Running with toe-out will create some Ackerman affect. Or, offsetting the spindle from axle center-line will have some affect. I've never seen an offset spindle, I've also never seen toe-out recommended on a RWD vehicle other than race application.

If I'm wrong of off-base. PLEASE CORRECT ME! I'm not an engineer, just somebody that over-thinks things.

Chris
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Old 02-27-2017, 05:59 PM   #2
James the III
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Re: Ackerman angle on street suspension

I don't know why you want to run rear steer..
There is a reason the oem's moved away from it..
I'd rethink this..
I don't know how wide your 54 frame is.. but many going from an I beam to ifs..
use the panther front alum. set up.. (ford crown vic and merc grands mark)
as it is a bolt in complete front end.. with crossmember,
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Old 02-27-2017, 06:49 PM   #3
whitedog76
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Re: Ackerman angle on street suspension

I have a very deep front sump pan, which is why I'm looking into rear steer.

The Wilwood Pro spindles claim to be interchangeable from side to side. No mention of Ackerman. This is another reason that leads me to believe it is not an issue.

http://www.wilwood.com/Spindles/SpindleList.aspx

I do like the Panther frontends, but they are a little on the wide side for the AD trucks. The 55 and newer trucks are a different story.
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Old 02-27-2017, 08:34 PM   #4
PGSigns
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Re: Ackerman angle on street suspension

The part you are missing is the steering arm angles. As you put input into the wheel to turn the steering arm on one side moving towards the steering pivot and the other away. This changes the amount the same input turns the spindle giving you the ackerman angle. The placement of the rack in relation to the steering arms will affect the ackerman and the bump steer. The Wilwood spindles have bolt on steering arms so they are the same on both sides.
Jimmy
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Old 02-28-2017, 01:05 AM   #5
whitedog76
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Re: Ackerman angle on street suspension

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Originally Posted by PGSigns View Post
The part you are missing is the steering arm angles. As you put input into the wheel to turn the steering arm on one side moving towards the steering pivot and the other away. This changes the amount the same input turns the spindle giving you the ackerman angle. The placement of the rack in relation to the steering arms will affect the ackerman and the bump steer. The Wilwood spindles have bolt on steering arms so they are the same on both sides.
Jimmy

I'm going to agree with you on this.

I drew out all kinds of scenarios on paper. I came to the same conclusion. It doesn't matter if you have a trapezoid, square, rectangle, you always have 360 degrees.

What I was missing, is that the Rack is static. The tie rod's can't travel their full potential, because the Rack doesn't move.

With that being said, wouldn't a rear steer setup be better for Ackerman steering adjustment?

Also, it appears that I would indeed need the Wilwood spindles, since the steering arms are reversible, in order to move the outer tie rod towards the center of the vehicle.

(I kind of dumbed out on this one. Sometimes when you look at something hard enough, you miss the obvious.)
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Old 02-28-2017, 01:29 AM   #6
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Re: Ackerman angle on street suspension

https://www.youtube.com/watch?v=oYMMdjbmQXc
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Old 02-28-2017, 02:15 AM   #7
James the III
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Re: Ackerman angle on street suspension

Quote:
Originally Posted by whitedog76 View Post
I'm going to agree with you on this.



With that being said, wouldn't a rear steer setup be better for Ackerman steering adjustment?

.)
What math are you using that you think a rack with the same travel be it in front of the cross member or behind it.. and same length steer arms no matter if front steer or rear.. going to change??
6" of travel is 6" of travel.. 5" arms don't get longer bolted on one way (front) or (rear) steer when the arms are straight..
Only thing I see changing is the column intermediate shaft being shorter and the rack mounting not working if moved.
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Old 02-28-2017, 08:27 AM   #8
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Re: Ackerman angle on street suspension

When you flip it front to back you may not be able to get the same rack position due to the design of the lower control arms. They may force the rack back some as many have the ball joint biased to the front. This will affect both bump steer and ackerman. Then then is the issue of mounting the rack as some are set up with a mount pad on the back side to go to the cross member that will now be on the wrong side. Lots of things to consider in the design. There are rear steer crossmembers out there for hodrods like the one from Walton Fabrication or Jim Myer racing that could be adapted to your 54.
Jimmy
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Old 02-28-2017, 08:43 AM   #9
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Re: Ackerman angle on street suspension

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Originally Posted by James the III View Post
I don't know why you want to run rear steer..
There is a reason the oem's moved away from it..
The reason the OEMs went away from rear steer is because it was a super expensive option that didn't sell well; not because of any performance issues.

It was, in fact, a great system on pickups (and Suburbans). Parking lot maneuverability was greatly improved and it was phenomenal for trailering (the wheels turned in the same direction at speed, allowing for very stable lane changes).

But GM chose to package the quad steer option with the composite pickup box, which drove the price up into the $4000 range for the option. Not a lot of appetite for that among consumers.

K
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Old 02-28-2017, 10:08 AM   #10
whitedog76
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Re: Ackerman angle on street suspension

Quote:
Originally Posted by PGSigns View Post
When you flip it front to back you may not be able to get the same rack position due to the design of the lower control arms. They may force the rack back some as many have the ball joint biased to the front. This will affect both bump steer and ackerman. Then then is the issue of mounting the rack as some are set up with a mount pad on the back side to go to the cross member that will now be on the wrong side. Lots of things to consider in the design. There are rear steer crossmembers out there for hodrods like the one from Walton Fabrication or Jim Myer racing that could be adapted to your 54.
Jimmy
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I've thought about this. First off, I realize that a Pinto/MII rack won't work, Unless I made it RH drive. I'll need to find something different. Fortunately, the salvage yard is full of FWD cars and mini-vans.

Mounting the rack to the rear of the crossmmber is an issue. I'll have to come up with this on my own.

As far clearance goes. Moving the Rack isn't that simple either. The lower control arm may pose an issue due to it's shape. I'm thinking the Wilwood spindles will give me a few more options, being the steering arms are removeable.

All this is actually just theory. Something swirling around in my head. It's hard to say what will actually work, without having the parts in my hands.

Chris.
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Old 02-28-2017, 10:16 AM   #11
whitedog76
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Re: Ackerman angle on street suspension

Keith

When I say rear steer, I'm actually talking about the rear of the front axle, not the rearend.

James, the OEM didn't actually move away from it completely. All FWD cars are rear steer. My Trailblazer is actually rear steer, it's a RWD full frame vehicle too.

I think the biggest reason the manufactures went away from it, particularly in trucks, is the 4x4 factor. It's hard to get steering components in through a spot that has a driveshaft going up an down.

With that said, my Trailblazer contradicts that statement. Being it is a 4x4 with rear steer. Of course the front Diff. is fixed and does not move. However, the Trailblazer turns sharper than any vehicle I've EVER owned. Whether it be a compact car or anything else. GM did something right there.
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Old 02-28-2017, 11:27 AM   #12
James the III
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Re: Ackerman angle on street suspension

Quote:
Originally Posted by Keith Seymore View Post
The reason the OEMs went away from rear steer is because it was a super expensive option that didn't sell well; not because of any performance issues.

It was, in fact, a great system on pickups (and Suburbans). Parking lot maneuverability was greatly improved and it was phenomenal for trailering (the wheels turned in the same direction at speed, allowing for very stable lane changes).

But GM chose to package the quad steer option with the composite pickup box, which drove the price up into the $4000 range for the option. Not a lot of appetite for that among consumers.

K
rear steer tierods/etc behind the wheel not in front of it..
not rear axle steering
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Old 02-28-2017, 12:56 PM   #13
Keith Seymore
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Re: Ackerman angle on street suspension

Sorry; my bad.

Carry on -

K
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