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Author: Subject: Rack and Pinion Steering
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posted on July 6th, 2004 at 02:47 PM


Mike,

We think alike. Yes I thought maybe it moved in a new arc, but yes I also thought it'd probably actually move in an ellipse and therefore not actually have a fixed center.

But instead of attempting calc's I decided to draw it. It's very complicated this steering business, especially considering traditional steering theory doesn't apply to the VW torsion bar front end. I like to thoroughly calculate everything before I comment, but this post is an exception where I am saying what I am thinking while calculating so that forum members can make sure my thoughts are on the right track. Anyway, back to the R&P.

So I drew a simple 2D drawing of the side on view. I then rotated the lower arm in 20 degree increments. I then relinked and rotated the spindle, top arm and tie rod end point for each 20 degree rotation. So what we end up with is 5 different colours, showing the front end in 5 different positions through the travel. Hopefully the drawing makes sense. The two circles are the torsion tubes, a line for top and bottom arms, a line for the spindle axis, a line for the spindle to tie rod end attachment point.

From looking at it something else becomes obvious. The castor angle changes through the suspension travel. Good positive castor when the suspension is raised, and bad negative castor when the suspension is lowered.

This is interested because we all know that when you lower a beetle it loses castor and you need to fit castor shims to correct the problem. But the explanation I always saw was that tilting the whole pan by lowering the front tilted the beam and this caused the problem. Which I though was silly cos you normally lower the front and the back. From the drawing, we actually see we need castor shims because of the arm geometry. Because of the different spacing of the torsion bars and the upper & lower ball joints. Why did VW do that anyway....

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posted on July 6th, 2004 at 02:53 PM


Quote:
Originally posted by Baja Wes
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?


Well, firstly let me say what a wonderful job you have done on this.
I have never set up a ball joint front, all the cars I do are link pin type and there is little if any difference between the link pins and the beam. The other thing is the beams are much longer (1275mm) and set at 14deg. As for bump yes and no, from ride height about 3” up and down I get +/-1mm at the most. But max travel (bump stop) it always + about 2mm but full extension (in the air) it would be –1mm at the most. Please remember that there is nothing on the front end of these cars that’s VW and there run from 15” to 18” of travel.
So the long and short of it NO I don’t get bump steer (I don’t think -/+ 3 over 15” is a problem but it is still bump steer.
:duh




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posted on July 6th, 2004 at 02:55 PM


but back to the rack and pinion again :D

So the question is.... does the tie rod point swing an circular arc or an elliptical arc?

well...


the answer is....


a circular arc!

How convenient. So maybe we won't need a Camira rack. The back and up point is completely different. It's a smaller back distance than the tie-rod to BJ back distance at any point. I know what it is and will stick it in my 3D model tonight, reanimate the suspension and see if it cures the bump steer. It should of course, unless I am missing something, again... :P

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posted on July 6th, 2004 at 02:58 PM


Quote:
Originally posted by Desert Moose
The other thing is the beams are much longer (1275mm) and set at 14deg.


14 degrees, wow, that's some serious castor!

Thanks Moose. The rack model was getting more bump steer than that, like about 11mm per wheel! Looks like I figured out why in the above posts. :D




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posted on July 6th, 2004 at 03:02 PM


But I still have to set each car up different as no to are the same. I will post some photo's of mounts I have made plus I'm about to do one soon and I'll make sure I take pic's for you.:thumb



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posted on July 6th, 2004 at 03:04 PM


Quote:
Originally posted by Baja Wes
So the question is.... does the tie rod point swing an circular arc or an elliptical arc?

well...

the answer is....

a circular arc!



Cool!

:bounce :bounce :thumb

That should make it easier.

[Edited on 6-7-2004 by MikeM]




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posted on July 6th, 2004 at 03:06 PM


Quote:
Originally posted by Baja Wes
14 degrees, wow, that's some serious castor!

:D


Yeh, lol. looks like a grader on lock :P

But it will center on a dirt road at 200kmh

[Edited on 6-7-2004 by Desert Moose]




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posted on July 6th, 2004 at 04:59 PM


Quote:
Originally posted by Baja Wes
... Because of the different spacing of the torsion bars and the upper & lower ball joints. Why did VW do that anyway....


I just thought of another side effect caused by that and the spindle changing angle. The ackerman angle will change through-out the suspension travel too!

Either VW did something really weird for no reason, or we're all really missing something...




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posted on July 6th, 2004 at 05:06 PM


in one manual i have, factory was toe in, and as you come through changes the steering to toe out to allow fo the different arcs each wheel has to follow
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posted on July 6th, 2004 at 06:35 PM


alos are you taking in to account that the torsion tubes arent dirrectly above and below each other... bottom one is slightly forward

(this is according to volkswagen technical manual by henry elfrink)
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posted on July 6th, 2004 at 06:38 PM


Quote:
Originally posted by dak dak ute
alos are you taking in to account that the torsion tubes arent dirrectly above and below each other... bottom one is slightly forward

(this is according to volkswagen technical manual by henry elfrink)


I think he is Dak Dak U thats what Castor of the beam is :thumb




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posted on July 6th, 2004 at 06:41 PM


okay cool just checking
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posted on July 6th, 2004 at 09:11 PM


I'm not taking account of the tilt of the beam, mainly because it has no effect on where the beam has bump steer or not. If the beam is on an angle of 0 or 5 degrees tilt it won't matter as the steering box or rack is mounted to the beam. does that make sense?

modelling it vertical is a little easier.

So anyway, I re-ran the model and I have found a position for the rack which give no bump steer at all, and the length of the tie-rods or whether the rack is center-mount or not has absolutely no effect. I will post a pretty pic in a minute...




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posted on July 6th, 2004 at 09:46 PM


here's a pretty pic, looks similar to before but with the rack in a slightly different spot.

The measurements still aren't 100% accurate, but I am convinced that any changes in measurements will simply mean a slight relocation of the rack, and importantly still no bump steer.

Now of course the racer boy's already know this, but I can prove it so I can get it legal. :D

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posted on July 6th, 2004 at 10:03 PM


So let me explain what I've done for those who are interested.

I model the front suspension in a fancy 3D motion program. First I draw the suspension in 3D of course. Once drawn I still local co-ordinates onto the parts (the little red bits). I then join the various parts at those local co-ordinates using various constraints.

Constraints range from spherical constraints (ball joints etc), and pin constraints (the arms in the beam), and sliding constraints (like the steering rack moving rack), etc.

Then I can put controls on various constraints as well. So I have put slider controls on both upper arms, which lets me move the arms by moving the slider. There are 3 sliders in the screenshot. The last one is the rack position, so I can turn the wheels too.

The program uses interation to calculate movements, forces, velocites, accelerations, etc. Very computer intensive.

So I can also graph practically anything. I have graphed the rotation of the left and right wheels (in a steering sense of rotation). I ran the model, moved one arm through it's travel, then the other through it's travel, and the graph recorded the bump steer. It recorded a max bumpsteer of 0.07 degrees, which ends end less than 0.5mm at the edge of the wheel. So within the tolerance of what I've drawn and the accuracy setting I have the program at.

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posted on July 6th, 2004 at 10:10 PM


so as a comparison, here is the stock front end, charted the same.

The rough curve is because I wasn't very steady moving the slider with my mouse :)

You can see the short tie rod gets about 1.7degrees of bump steer (about 10mm at the front of the wheel!), but the long tie-rod is much less effected. These bump steer figures may also be because I haven't measured the exactly location of the pitman arm exact enough. But it's clear at least one tie rod on the stock set-up will always have bump steer due to the unsymmetric pitman arm.

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posted on July 6th, 2004 at 10:33 PM


here's a quick avi

www.offroadvw.net/rack.avi

I think you'll need the DivX codec from somewhere...




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posted on July 7th, 2004 at 08:15 AM


Actually ignore the avi above, I made a much better one. Only 768kB

http://www.offroadvw.net/rack2.avi

I'll find a link to the DivX codec




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posted on July 7th, 2004 at 08:37 AM


Go here...
http://public.planetmirror.com/pub/divx/windows/

and download the DivX505Bundle.exe file.

Install DivX

Open the movies above in the DivX player.

There are DivX codex for windows media player out there, but I can't find it right now.




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posted on July 7th, 2004 at 12:16 PM


Wes, can you make the tie rods longer as in 100mm apart in the centre, :thumb



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posted on July 7th, 2004 at 12:41 PM


I can, but it won't do anything any different than what the short tie-rod rack design is doing. Although it didn't look like it at first, any rack can be set-up for theoretically 0 bump steer.

Long tie-rod versions will just be less sensitive to how accurately it is initially set-up than short tie-rod versions. But they will both be equal is they are both set-up perfectly.




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posted on July 7th, 2004 at 02:57 PM


Quote:
Originally posted by Baja Wes
Although it didn't look like it at first, any rack can be set-up for theoretically 0 bump steer.



True, but there are other things to consider. The centre tie rod rack (Camira) has an advantage in that the angle changes in the up and down plane are less as the suspension travels. I have done a diagram. It's not as cool as Wes's diagrams, but I don't have the cool tools he has.

In the diagram you can see the Two black lines are the torsion bars viewed from the front (or rear) the red objects are the spindles and the blue lines are tie rods. Points A are the inner mounts of the tie rods on a long rack, points B are the inner tie rods on a Camira style rack, and points C are the outer tie rod mounts.

You can the the difference in angle in the tie rods for both scenarios. The center pivot rack has much less angle, especially on the right side where the suspension is dropped further.

Less angle is a good thing as it puts less upward or downward pressure on the tie rod ends, and is less likely to bend tie rods. It will also mean the tie rods ends have more serviceable life as they are not moving as much.

I think the Ackerman angle is effected by the angle to the inner tie rod (front to rear) when viewed from the top. But I don't see this as a huge concern.

[Edited on 7-7-2004 by MikeM]

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posted on July 7th, 2004 at 03:16 PM


yes longer tie-rods will not swivel as great an angle.

However longer tie-rods need to be made stronger to resist buckling, which isn't such a prob with shorter tie-rods. It's easy to fix by just making beefier tie-rods though.

I'm going to try to update my spreadsheet that I started. I want it to calculate everything the 3D model does. That way you can easily move the rack around in excel and see what the effect is. At the moment the rack at the start of this post is looking pretty damn bad, and I would like to calc it just out of interest.

Calculating all the angles in Excel is a very intense process :O

Just to relate the angle that the bottom arm is compared to the top arm is complicated and looks something like this "=90-(DEGREES(ATAN(D30/(150+C30)))+DEGREES(ACOS((L_armlower^2+((C30+150)^2+D30^2)-H_balls^2)/(2*SQRT((C30+150)^2+D30^2)*L_armlower))))"

I knew trigonometry would come in handy one day :D




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posted on July 7th, 2004 at 05:14 PM


I finished work and decided to run some quick numbers on the rack set-up shown in Brad's very first post.

I reckon the rack position looks like it is dead in between the tie rod ends, and maybe an inch above the centerline of the beam. Running the calc's on that set-up is giving me about 50mm per wheel of toe change! (25mm at best)

I didn't realise it was that bad. It would certainly suck to drive. :D

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




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posted on July 7th, 2004 at 05:21 PM


50mm :duh nasty ....



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posted on July 7th, 2004 at 06:30 PM


Quote:
Originally posted by Baja Wes
yes longer tie-rods will not swivel as great an angle.

However longer tie-rods need to be made stronger to resist buckling, which isn't such a prob with shorter tie-rods. It's easy to fix by just making beefier tie-rods though.

:D

Wes,For tie rods I use heim's with adapters and C/molly tube, or 3/4" alloy bar, gun drilled to 5/8". Much stronger than standard and I'm not sure but you could take the material spec sheet to the governing body when you take everything else.

I know this started as brad's thread but I think the world is looking in on what you are doing Wes........lol (I have had 8 u2u's about it, god know how many your gettin').
I really like the way you are putting in such an effort to make this work.


This photo I have put on as a URL so I don’t take up much room on the thread but I have had a lot of people ask about it so here is a pic.
http://img78.photobucket.com/albums/v252/Moose906/7.jpg 




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posted on July 7th, 2004 at 07:42 PM


So I modelled the center rack, at the rear (same as first pic).

I played with the Excel sheet to get it in the back spot, but still near the firewall like the pic shows. The Excel sheet was giving around 29mm of toe change per wheel.

Then once modelled the model gave 5 degree change per wheel, which ends up equalling about 29mm toe change per wheel. So that's a nice check of both methods, it's good they agree.

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posted on July 7th, 2004 at 07:46 PM


and a prettier pic...

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posted on July 7th, 2004 at 09:10 PM


I decided to run a sensitivity analysis on the rack position, for a normal rack and a camira center mount rack.

So basically what I done is calculated the bump steer with the rack in the perfect spot (or close enough to get less than 1mm of bump steer). I then moved the rack up 5mm and recorded the bump steer, down 5mm and done the same, then back 5mm and forward 5mm. I put it in the table below.

I did it twice, only with a normal rack, about 600mm wide, and once with the camira rack, with what I guessed was about 100mm between tie rod connections.

You can clearly see the longer tie rods are less sensitive to rack positioning errors which is a good thing, but not a factor if you set it up right.

More importantly you see the rack is sensitive to up and down height position, but not so much so to the forward and back position. This is good, because the camber adjusters change the forward and back position of the tie rod ends. Also good because the vertical height position is easier to get in the right spot than the forward and back position.

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posted on July 7th, 2004 at 09:29 PM


I have attached my draft bump steer calculating spreadsheet to this post.

I have set it up so the drivers side is showing the correct rack position, and the passengers side is showing the rack 5mm too low and the effect on toe change. Thought you guys might have fun playing with it.

*File deleted - there is a new version further along this post*

[Edited on 13-7-2004 by Baja Wes]




Wes - www.offroadvw.net - 200HP Quad Cam V6 in a VW Baja - with climate control... :)
www.taylorcycles.com.au - My DH MTB racing brothers shop.
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