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-   -   Undestanding Ackerman/suspension geometry (http://www.hotrodders.com/forum/undestanding-ackerman-suspension-geometry-227762.html)

coryhussey 12-30-2012 07:25 PM

Undestanding Ackerman/suspension geometry
 
I have been trying to wrap my head around the whole understanding of steering ackerman and suspension geometry. I am currently using a MII custom front suspension on my chevy colorado pickup truck. It is a tubular crossmember, with heidts upper and lower control arms. Also im using heidts 2" drop spindles. I am currently trying to use a ford ranger power rack and pinion. My issue i am running into is the bumpsteer. The upper control arm width is 25", the lower width is 18". This setup puts my steering arms on my spindles at 44" apart. My ford ranger rack is roughly 24" pivot points. My question about the whole ackerman issue is the rack pivot points vs. the placement of the rack. If the pivot points do not fall in the ideal location of the LCA pivot points, or the point of intersection of the two control arms, will this affect my bumpsteer. Or can this be fixed with the proper location of the rack. If it is moved up or down relevant to the LCA's, will this eliminate any bumpsteer. If the tie rods line up with the LCA's, will this do anything, or does the pivot points along with the rack location make everything work properly.

I am considering biting the bullet and eliminating my power steering, and going with a unisteer custom manual steering rack. It does cost a bit, but I can make my pivot points work with my current suspension setup, and eliminate most of my bumpsteer.

Also does anyone have any links to MII IFS suspension geometry. I would like to know if my rack is properly placed relative to my control arms.

thanks
cory

timothale 12-31-2012 08:43 AM

Heidts,
 
The first question is why are you not using the rack Heidt's recomends for your application. ? ? If you go to Heidt's web page and down load their catalog, Near the back of the book they have several pages of Engineering info about the Mustang II design. moving the rack up-down fore and Aft will affect bump steer and ackerman angles. Other ford pieces can be used when the width is changed . another source for info is Welder Series. They build weld it yourself hot rod components and their catalog has a lot of good installation information.

MARTINSR 12-31-2012 09:40 AM

You are so right Tim, why redesign what they have provided? Moving the rack or changing it's body length (distance between the inner tie rod end pivots) is moving it out of the arch and pivot points of the control arm and will provide bump steer.
Moving the rack doesn't effect ackermann at all. The only thing that effects it is the location of the tie rod end pivot point in relation to the knuckle or spindle pivot point in relation to the center of the rear axle.

http://scottishcarties.org.uk/files/...ermann_New.jpg

This is a good example of how moving the rack or puttinging the wrong one in effects things.

http://www.pozziracing.com/Media/bump_steer.gif

If the racks tie rods are not matching the control arm they make a different arch. This different arch in effect "lengthens" and "shortens" the rod. Look at the drawing, when the wheel goes up the arch of the end of the tie rod goes in, turning that wheel in! When the wheel goes down, it's pivot is out further, effectively "lengthening" it turning the wheel out!

The control arm pivots and thus arch has to match.

Brian

kso 12-31-2012 09:48 AM

(I see just prior to my posting this, there is a pretty good explanation just put up...but I'll hit "post" anyway in case saying things over again helps.)

I'm not sure if Ackerman geometry is quite what you're dealing with here, that is about steered-wheel angles and generally pictured "from above"...for anything having to do with bump steer you're going to be picturing it from ahead or behind.

Whatever the actual suspension or steering member/pivot-to-pivot lengths may be, you can get close to a bump-steer-free deal by having the steering tie rod be the same length as the lower control arm, and at the same angle as the lower control arm (as seen from directly in front or from the rear and going by the pivot locations), if it's at the same level above the ground as said member....or...if the tie rod were at the level of the upper control arm, then you'd want it at that same length and angle of that upper arm then. If the tie rod height falls somewhere between the two like with a Mustang II spindle, split things up proportionately. This is a bit of an over-simplification and skips a couple considerations but like I said, will have you in the ball park.

Anti-dive geometry where the upper control arm pivot is angled back (as seen from the side) causes the spindle to "rotate" slightly as it goes through it's travel and makes zero bumpsteer theoretically not possible, but that's splitting hairs a bit.

So with your front suspension sitting level (hopefully the lower a-arms will be near level, with the uppers angled down toward the middle slightly), kneel down and have a look, imagining lines created through the pivot points and not being fooled by some shape of the a-arms. The angle and lenth of the tie rod needs to be working with that of the a-arms.

MARTINSR 12-31-2012 10:21 AM

Quote:

Originally Posted by MARTINSR (Post 1629047)
You are so right Tim, why redesign what they have provided? Moving the rack or changing it's body length (distance between the inner tie rod end pivots) is moving it out of the arch and pivot points of the control arm and will provide bump steer.
Moving the rack doesn't effect ackermann at all. The only thing that effects it is the location of the tie rod end pivot point in relation to the knuckle or spindle pivot point in relation to the center of the rear axle.

http://scottishcarties.org.uk/files/...ermann_New.jpg

This is a good example of how moving the rack or puttinging the wrong one in effects things.

http://www.pozziracing.com/Media/bump_steer.gif

If the racks tie rods are not matching the control arm they make a different arch. This different arch in effect "lengthens" and "shortens" the rod. Look at the drawing, when the wheel goes up the arch of the end of the tie rod goes in, turning that wheel in! When the wheel goes down, it's pivot is out further, effectively "lengthening" it turning the wheel out!

The control arm pivots and thus arch has to match.

Brian

By the way, where I said it is turning the wheel in or out in my description, this would of course depend on where the rack was, ahead or behind of the wheel! So "in" when the wheel goes up, would be "out" if the rack were behind the wheels and "in" if it were in front of the wheels".

Brian

timothale 12-31-2012 10:27 AM

more geometry
 
Moving the rack fore aft will change more angles, arc swing and change the effective akerman angles inducing more toe- in or toe- out during turns than ideal akerman angles. Racing geometry gets more complicated hard cornering suspension squat changes effective camber and that affects where the tire wants to roll. My old friend Bruce Cambern, retired head of Ford SVO designed the last suspension on his 427 Cobra on his home computer, And his theories worked.

coryhussey 12-31-2012 10:29 AM

thanks everyone for the wealth of knowledge. I will look into the heidts catalogue and see if i can find something that will work for me. I wanted to build a tubular front crossmember to fit my whole tube chassis, that is why i went with a custom front IFS. I may have to redo my rack mounting points as it may be out a little from what most MII kits come as. The rack is relatively close to the same "arch" angle as the lower control arms, but the pivot points are not the same. And yes my wheels toe in and out when my air ride suspension goes through its cycle.

timothale 12-31-2012 10:39 AM

suspension design
 
I have some Jag stuff I plan on using on my next roadster build. I am going south next week and the College there has a street rod program, I last time got to Audit For $ 200, the senior discount, Regular students have to pay over $ 1100 per semester, I plan on building my own crosmember for the later jag pieces and will have to sort thru the Rack placement and geometry.

MARTINSR 12-31-2012 11:09 AM

Quote:

Originally Posted by timothale (Post 1629070)
Moving the rack fore aft will change more angles, arc swing and change the effective akerman angles inducing more toe- in or toe- out during turns than ideal akerman angles. Racing geometry gets more complicated hard cornering suspension squat changes effective camber and that affects where the tire wants to roll. My old friend Bruce Cambern, retired head of Ford SVO designed the last suspension on his 427 Cobra on his home computer, And his theories worked.

I never thought about it that way but yeah in reading the little bit on the ackermann principle in racing with suspension rebound and what not they control the toe and that is all I use see it as, but if one toe is more or less than the other THAT is part of the ackermann principle so where I use to think of it as simple "bumpsteer" the experienced engineer is USING this "flaw" to do what he wants, very interesting.

Brian

coryhussey 01-01-2013 11:36 AM

here are some pictures of what i am up against. They are not the best angles to see exactly what i am talking about, but you can get the point
[IMGhttp://i29.photobucket.com/albums/c2...ps02973a5a.jpg[/IMG]
[IMGhttp://i29.photobucket.com/albums/c2...ps28d786fe.jpg[/IMG]
[IMGhttp://i29.photobucket.com/albums/c2...pse40e3198.jpg[/IMG]
[IMGhttp://i29.photobucket.com/albums/c2...ps0e02034b.jpg[/IMG]
[IMGhttp://i29.photobucket.com/albums/c2...psa7c05cc5.jpg[/IMG]

MARTINSR 01-01-2013 11:44 AM

Yep the body of the rack is too long, that is all there is to it.

Brian

timothale 01-01-2013 02:29 PM

Wiki time
 
There is an article here on hotrodders on upgrading 37-57 pontiac, buick and olds suspensions and they used a center pivot rack. (We have a 37 pontiac in the collection) , Chevy caviliar and other same sized cars used it, rear steer and made a new center bar to relocate pivot points. I saw a similar systems on a road race 65 mustang , the pivot point holes could be taper drilled to put the pivots in the right position. I don't know if there is a front steer center pivot that will work , making a new center bar like on the Wiki to get yours right.

gow589 01-05-2013 09:35 AM

The amount of Ackerman is very different for a street car or a track car. The track itself can dictate what is the best Aackerman. Ackerman originated form horse and buggies. When buggy made a turn they made the inside wheel turn more which actually matched the center of the turn. If they didn't, the horse would pull the wheels sideways.

In my 1976 Cutlass, it is a boat. They were making these cars both sporty and so Grandma could drive. If you look at any large mid 70's boat like these the inside wheel turns more making it easy to get around in a grocery parking lot. It does however make it interesting if you jerk the wheel at speed (ask me how I know).

In a race car you want the toe to be correct for the straight away and correct for the turn which depends on speed and the arc of the turn. Often racers find themselves wanting more toe out in a turn which means they have too much toe out in the straight away making the straight always dicey. To counter this ackerman is changed by moving the rack forward or aft so you get the desired toe in for the straight away.

For most street driven cars, we deal with what ever we have, how ever it is for what ever reason. To change the Ackerman more often is done in the design of the car.

Bump Steer

I spent a lot of time getting bump steer out of a Pantera. The stock rack is wrong and is an inherent problem in the Pantera. There is a bump steer fix which changes the height of the rack but ultimatly does not fix bump steer...it does however change the impact location. It makes bump not too much of a problem but it makes droop 2x as bad. The arc is just shifted.

I first needed to measure everything. I cut tubing and bolted them in each suspension point. Then I built and welded a frame to tie them all together. I removed the frame and used it to locate all suspension points. Put this info in a program; suspensionprog.

http://www.rc-tech.net/pantera1/frontsus/jig.jpg


From the program I determined the appropriate height and width of the rack. Reality is often different then the accumulated errors from this kind of excessive; which meant the need to fine tune.

I converted to power steering which an entirely different rack. My a-arms are built from scratch and I modified the distance between the pivot points on the rack to match the information form the software program. I cut one end to not only to adjust the length but I made a threaded shaft so I could adjust it. When the apropriat spot was found, I welded it in place.

http://www.rc-tech.net/pantera1/frontsus/sr2.jpg

http://www.rc-tech.net/pantera1/frontsus/sr6.jpg

Using a HF laser mounted to the rim (not the tire whcih is not always uniform) I was able to achieve ZERO bump steer:

http://www.rc-tech.net/pantera1/bump/bump5.jpg

Now this is really the short version of everything done; the very short version. In fine tuning the rack I made a chart and was able to decipher the problem for tuning. The problem is not always as simple as this chart but this chart generally can lead you in the right direction. This chart is for the right wheel. If your bump does what is shown, it shows you what needs to be changed in your ride height or your pivot distance to fine tune the problem:

http://www.rc-tech.net/cars2/Bump.jpg

NOW, if your only problem is rack width, you can measure how much you are off by turning your rack to change the pivot location and finding the zero spot, doing the same with the other side then measuring how far you had to turn your rack to get the result. I can re-explain that if it did not make sense. (words are hard to understand some times).

gow589 01-05-2013 09:36 AM

BTW, this is also relative to the upright and location of the attachment of the ball on the upright. No one will be able to diagnose what your problem is from photos.

MARTINSR 01-05-2013 09:43 AM

Very interesting stuff, thanks for posting it.

Brian


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