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Author: Subject: Rack and Pinion Steering
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posted on June 27th, 2004 at 05:22 PM


Quote:
Originally posted by Desert Moose
This cool backyard tool can be used as a bodgie wheel aligner as well. When the car is back together but it at the front of the wheels then roll the car forward till the toe beam is at the same height at the back this will give you your total toe reading e.g. the difference between the to reading. Bodgie but it works. (I use a laser aligner now to setup but it takes longer).


I use a similar method to do my wheel alignments, and I find it works fine. :)

The prob with taking it to a wheel alignment shop is that the next time you go offroad it seems to move slightly and go out of whack anyway. :(

I am currently trying to get accurate measurements of the geometry of the stock beam. It's very hard to do actually!

Once I have that sorted I will know exactly what toe and bump steer probs the stock set-up has. Earlier measurements indicate it has some weird built in bump steer on purpose. I'm not sure why yet, which generally means I am missing something. I'm sure I will figure it out. It's related to the non-symmetric pitman arm.




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posted on June 27th, 2004 at 05:41 PM


Quote:
Originally posted by MikeM
The best place to have the steering pivot would be right up the front at the torsion bars (where the adjusters get welded)


Just flicking through this post again. Thought I'd clarify this one. The best position for the rack is imaginary posts that are offset from the centerline of the top torsion tube, the exactly same amount that the steering arm tie-tod center is offset from the top arm ball joint center.

So that is up and back a certain amount. From rough measurements it's about 60mm up, and 110mm back.

The stock pitman arm tie rod end centerpoints are very close to this offset, so the stock set-up comes very close to eliminating bump steer. The prob is the left and right pitman arms tie rod ends are different offsets from the beam, I haven't quite put my finger on why.

I thought the left and right steering arms on the spindles may have had different offsets, but they don't seem to have. I'll keep thinking. Once I have the 3D model working it will make it fairly obvious why I think.

Here's my rough spreadsheet if anyone is interested. It is a simple first past calc of bump steer vs susp travel. It is ignoring the wheels turning left or right, camber adjustments and any difference in length of the top and bottom arms.

Desert moose, the idea of fine tuning the exact rack placement is a good one. A lot of race cars actually fit the rack, measure the bump steer and then use shims for fine tuning. Sometime they dial in very very small amounts of toe in or toe out bump steer to change handling characteristics.

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posted on June 27th, 2004 at 08:16 PM


Quote:
Originally posted by Baja Wes
Quote:
Originally posted by MikeM
The best place to have the steering pivot would be right up the front at the torsion bars (where the adjusters get welded)


Just flicking through this post again. Thought I'd clarify this one. The best position for the rack is imaginary posts that are offset from the centerline of the top torsion tube, the exactly same amount that the steering arm tie-tod center is offset from the top arm ball joint center.

To maintain ackerman - yes.
To eliminate bump steer - no. The rack would have to be in or very close to the plane of the torsion tubes. But some bump steer may be desirable as you say.


Quote:
The prob is the left and right pitman arms tie rod ends are different offsets from the beam, I haven't quite put my finger on why.


So the ackerman is similar on both sides with different length tie rods.

[Edited on 27-6-2004 by lexm]
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posted on June 27th, 2004 at 08:24 PM


isnt the ackerman affected by the poistioning of the tierod end on the spindle, not by where the steering box is mounted???
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posted on June 27th, 2004 at 08:36 PM


Quote:
Originally posted by dak dak ute
isnt the ackerman affected by the poistioning of the tierod end on the spindle, not by where the steering box is mounted???

In an ideal situation it would be set by the relative position of the outer tie rod end to the king pin position (or in ball joints the pivot position which is a line through the ball joints) as long as the pitman arms moves in an arc the same radius as the distance from the outer tie rod to the King pin axis. The raplacement of a radially moving pitman arm with a linear moving rack end will modify the ackerman calculations, as will the change of length of the tie rods.
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posted on June 28th, 2004 at 08:25 AM


Quote:
Originally posted by lexm
To eliminate bump steer - no. The rack would have to be in or very close to the plane of the torsion tubes. But some bump steer may be desirable as you say.


No, if you mount it in the plane of the torsion tube, you will get bump steer. The arms pivot at the torsion tube, and swing an arc from there. But the arc the tie-rod end on the spindle does is offset from that arc by the distance from the top ball joint to the tie rod end. You want the rack to be at the center of that second arc. So you want the rack offset from the torsion tube centerline the same amount that the tie rod end is offset from the top ball joint. I've ran the calc's and this is definitely correct.

You may be right about the pitman arm having more to do with the ackerman. I haven't ran these calcs yet, but when I do it will show whether that is the case of not. If it is the case then VW have purposely given the car bump steer in order to keep the ackerman correct. The ackerman on the spindles seems to be correct from the factory, with the line from the ball joint to tie rod end to center of rear axle in one plane.

If the pitman arm is like that due to ackerman, then it means VW should have went with an idler pitman arm set-up to make the two side symmetric and eliminate the bump steer problem, but I guess it was intented to be a cheap car.




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posted on June 28th, 2004 at 08:31 AM


all very interesting really :alien



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posted on June 28th, 2004 at 01:43 PM


Quote:
Originally posted by Buggy Brad
all very interesting really


LOL....yeh, that it is.

Something to think about when you do you calculations Wes is Caster, thatís if you havenít already. The caster effects camber and toe on turns which effects Ackermanís.
Itís all getting to hard and my head hurts :cry




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posted on June 28th, 2004 at 02:12 PM


yeah I got castor covered. Castor and Camber are adjustable with the eccentric adjuster, just to make the calculations more difficult. Just look at the excel sheet that does a simplified calculation and you'll get an idea how complex it gets real quick.

The compound effects of the various angles and adjustments makes everything more complex. I mean you can even set the long tie rod a little longer, short one a little shorter, and end up with the pitman arm slightly turned with the wheels straight ahead. This also has a slight effect on the geometry (but it won't have an effect with a steering rack).

Building it is easier, cos you just fine tune the position from trial and error as you said. But to make it legal we need to prove through calculations that it's equal to or better than the stock set-up :o




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posted on June 28th, 2004 at 02:33 PM


Quote:
Originally posted by Baja Wes
But to make it legal we need to prove through calculations that it's equal to or better than the stock set-up :o


I think even with bump a rack will be better then stock.....lol
I didn't see the excel doc before ! looks like a lot of work to prove your case but I'm sure you will.




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posted on June 28th, 2004 at 03:34 PM


the excel sheet is an attachment to a post about 9 posts up this page. 41kB or so. Just click the link and it will open in Excel. If you have Excel I guess.

It will look kinda like this when you open it...

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posted on June 28th, 2004 at 07:23 PM


lot of work .... lucky he gets paid by the hour :cool:



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posted on June 29th, 2004 at 07:33 PM


Quote:
Originally posted by Baja Wes
Quote:
Originally posted by lexm
To eliminate bump steer - no. The rack would have to be in or very close to the plane of the torsion tubes. But some bump steer may be desirable as you say.


No, if you mount it in the plane of the torsion tube, you will get bump steer. The arms pivot at the torsion tube, and swing an arc from there. But the arc the tie-rod end on the spindle does is offset from that arc by the distance from the top ball joint to the tie rod end. You want the rack to be at the center of that second arc. So you want the rack offset from the torsion tube centerline the same amount that the tie rod end is offset from the top ball joint. I've ran the calc's and this is definitely correct..

Quite correct. I neglected that. When are you going to expand the spreadsheet for different steering angles. Should be interesting as the effective length of the pitman arm will reduce as it rotates, as will the tierodend-balljoint on the inner wheel but increase on the outer wheel, all by different amounts. :):)

Interesting to see how a small change affects the bump steer. I believe the distance between upper and lower ball joints is different to distance between torsion bars, so this would also affect the calculations.



[Edited on 29-6-2004 by lexm]
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posted on June 30th, 2004 at 08:29 AM


quite right with everything there lexm.

I have started measuring everything, as that is the first step.

I have decided that an Excel sheet to calculate it will be incredibly complex, and even if I made it I would have trouble getting someone to believe it without some seriously detailed checking.

I am currently planning to use some advanced 3D motion software. Basically I draw the front end, add spherical constraint for ball joints, pin constraints at the beam, and simulate the motion of the front suspension. The program can output graphs of positions and angle of rotations of various parts through the movement. It will all make sense when I post some pics up here. :D




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posted on June 30th, 2004 at 09:47 AM


here is a pic of my first rough model of the front end made up of cylinders and spheres, it basically picks up the mainly workpoints of the front end which is all you need to animate the movements properly.

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posted on June 30th, 2004 at 07:59 PM


hehe must be a stock front end with those stub axles :>



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posted on July 1st, 2004 at 11:43 AM


Quote:
Originally posted by Baja Wes
here is a pic of my first rough model of the front end made up of cylinders and spheres, it basically picks up the mainly workpoints of the front end which is all you need to animate the movements properly.


It's not moving :thumb:thumb:thumb:thumb
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posted on July 2nd, 2004 at 10:40 AM


gotta finish the non-moving front end before I get the movement sorted.

I'll tell you what, measuring the workpoints of the front end accurately is extremely difficult. It's hard to measure from the center to center distance of a ball joint, without cutting it in half to know where the center is!

From what I can measure, I think both arms might be 153mm long. (very hard to measure). Long in a perpendicular to the beam direction, not total length.

The center to center width of the top ball joints appear to be 1170mm, and the bottom ones are a little wider at 1205mm.

The pitman arm appears to be sweeping up at 59-60 degrees.

My head hurts :(

Anyway, here is a front beam, shock tower not drawn yet (they are trickier to draw, and not need for the calcs).

[Edited on 6-7-2004 by Baja Wes]

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posted on July 2nd, 2004 at 11:15 AM


Cool...... very cool.....

I have a couple of balls from stuffed balljoints that are out of their sockets. These would easy to measure with some callipers. I'll post my measurements on here tonight (or Saturday) so you have something to compare against if you like. I will measure the diameter of the ball, the overall length of the ball and mounting bolt, and the length of the bolt sticking out of the ball.

I'd like to do this for a Link Pin beam as well. For Completeness (nothing to do with the fact that both my cars are link pin :) ) If I can do the measurements can you model it for me? The Link Pin front end should model the same as the ball joint if we change the measurements.

The fact that there is 4 pivot points for a link pin assembly won't matter as we can still model these as two if we place the center of the link pin pivot in the same X plane as the Kinpin. That will model just like the Ball joint but still be accurate for link pin.




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posted on July 2nd, 2004 at 11:37 AM


the problem isn't drawing the ball joints themselves, as I don't need to draw them. It's getting a ruler or calipers, sticking it near a beam and trying to actuall measure things like the height from the top ball joint center to the bottom ball joint center. It is tricky to figure out where the hell the center accurately is. Try it and you'll see what I mean :)

The link pin front would be easier. you can see where the pins are, and where their axes are. The program will model the link and king pins are pin restraints quite well.




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posted on July 2nd, 2004 at 11:38 AM


Wes will this help

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posted on July 2nd, 2004 at 11:40 AM


That rack in the ealier picture is also similar of some SAAB racs, though being a GM subsidary I would expect it.



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posted on July 2nd, 2004 at 12:06 PM


Bloody hell Wes, I think you've got too much time on your hands!!! Awesome job though, as usual! :thumb



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posted on July 2nd, 2004 at 01:08 PM


Quote:
Originally posted by Desert Moose
Wes will this help


no, but it does look cool :D




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posted on July 2nd, 2004 at 06:15 PM


I AGREE WITH MOST OF WHATS BEEN SAID!!!!

Except the bit after.....

Quote:Buggy Brad
"Rack and Pinion Steering

Seems to me if thsi works then I have wasted many hours of my life .... "

After that you lost me.




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posted on July 2nd, 2004 at 09:30 PM


It will work... Wes isn't paid to fail ... :kiss



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posted on July 5th, 2004 at 09:24 PM


just to let people know, I am still at it...

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posted on July 5th, 2004 at 09:27 PM


the ackerman angle is showing the desired effect, and the stock front is showing bumpsteer. I still think I need more accurate measurements though...

Oh, and most parts are simply diagramatic with the correct workpoints though.

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posted on July 6th, 2004 at 01:05 PM


for those following the story...

assuming the front arms were equal length, and always at the same angle, then putting the rack at the same offset as the tie rod to top ball joint center would be perfect and give no bump steer.

Unfortunately reality is more complicated. Even if the arms are the same length (not 100% sure yet, but seem the same), the problem is that the spacing of the top and bottom ball joint is bigger than the spacing of the torsion tubes. So what it means is the top and bottom arm are on different angles.

This is a problem because as the arms swing through their travel, they change the forward to back tilt of the wheel spindle. So the effective back and up distance of the tie rod end to the top ball joint actually changes the whole way through the travel. So it is impossible to design a rack with no bump steer. the stock set-up will also suffer from this.

So basically there is an avoidable amount of bump steer (unless you cut the beam and space the torsion tubes another 20mm apart to match the ball joints).

The question then becomes, how do we reduce the bump steer so that it is less then stock. Well I guess we are back to a center mount rack like on the camira. :)

Moose, when you setting your racks up, can you ever completely eliminate the bump steer, or do you always have a little bit?




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posted on July 6th, 2004 at 01:46 PM


Hmmmm. Interesting. Nice work.

As the suspension travels from total down to total up the outer tie rod end travels through an arc. If this arc was constant it would form a circle and the center of that circle is where the inner tie rod end should be (if we are thinking front to rear and up and down).

I'm guessing the issue is that the arc travelled by the outer tie rod, if continued, would form an oval. This give us no point that is a constant distance from the outer tie rod at all positions of travel (the center of a circle).

Therefore there is no perfect spot for the inner tie rod end, it must be a compromise.

My gut feeling then is to aim for a point at the mid-point of suspension travel and set the tie rod end at that point. I'm thinking this can be found easiest with our earlier theory of 60 up 110 back, but scaled. In an exaggerated example lets assume a few things. The Torsion bars are 50mm apart and the ball joints are 100mm apart. That would mean the scale from ball joints to torsion bars is 50/100. If the outer tie rod is 60mm above the upper ball joint the inner tie rod should be 0.5 * 60 or 30mm above the upper torsion bar. The back distance (110mm) stays the same.

This is just a theory in my head but it seems sound. You'd have to model it to be sure. :duh




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