Errol
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| posted on 7/2/07 at 09:54 AM |
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Steering Rack Position
Hi,
I was wondering what is the best position to install the escort steering rack in a locost chassis.
Regards,
Errol
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02GF74
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| posted on 7/2/07 at 10:11 AM |
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at the front,
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macnab
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| posted on 7/2/07 at 10:15 AM |
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there's always one...
If the cars sitting at ride height and the rack track rods are connected to the uprights, then the rack and its arms should ideally be at the same
level. This may not be possible due to the need for the track rod am joints to be in the same plane as the two pivots for the top and bottom
wishbones. Pic anyone??! Only complication is that the steering column shaft should meet the rack at 90 degs, so make the mounts to that angle and all
should be fine.
[Edited on 7/2/07 by macnab]
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David Jenkins
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| posted on 7/2/07 at 10:19 AM |
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A straight answer - I started off by putting mine in the book position. However, I did strengthen the supports a huge amount, the original design is
very weak and wobbly. I had planned to play around with the position when the front suspension was completed, but in the end I left it as it was.
You could install the mounts lower than designed, with the aim of adding packing pieces later, if you wish.
You will probably get a load of theories regarding bump steer caused by the incorrect length of the rack, or by incorrect mounting height. In my
case, with my standard-length rack, the bump steer is insignificant and doesn't bother me at all... I've driven production cars with worse
steering faults!
David
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macnab
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| posted on 7/2/07 at 10:26 AM |
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I agree totally, its easy to underestimate the shock loads on the rack especially if you clip a curb, being so hidden you want very sturdy mounts. As
for the bump steer it's quite common and not really noticeable unless you go offroad...
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ecosse
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| posted on 7/2/07 at 10:29 AM |
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I have mounted mine high, just below the top bone mounts, and connect the steering rods through the top of the hub arms (use rosejoints), bump steer
is minimal (i.e. none) 
Cheers
Alex
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David Jenkins
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| posted on 7/2/07 at 10:48 AM |
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quote:
Originally posted by macnab
I agree totally, its easy to underestimate the shock loads on the rack especially if you clip a curb, being so hidden you want very sturdy mounts.
And that's from someone who wants zero ground clearance! 
Actually, the main reason I made the rack mounts stronger was because I had been told that the SVA tester would look underneath the car while that
wheel was waggled left and right, looking for unwanted movement - and he did just that! Before my mods I could see the whole rack move if I got a bit
brutal with the wheel.
DJ
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macnab
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| posted on 7/2/07 at 11:19 AM |
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my mate had a fiat 127 and when you moved the steering the whole front of the car could be seen to twist! put the steering on full lock the bonnet
could not even be lifted due to the distortion what a bucket.
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nitram38
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| posted on 7/2/07 at 12:43 PM |
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The correct way on any car is to draw a line between the top and bottom wishbone inboard pivot points i.e. your centres of the bolts.
To prevent bump steer, your ball joints on the rack must center anywhere along this line when the rack is pointing straight ahead and it is
centered.
In otherwords the rack must be raised or lowered into the correct place.
Not locost, but I have a custom titan rack which is level with my top wishbones and the ball joints are in line with the top inboard bolt centres.
I have zero bump steer.
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Phil.J
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| posted on 7/2/07 at 01:48 PM |
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Zero bumpsteer must be the primary consideration, it's at the root of many badly handling cars. The rack mounts also need to be stiff. While
the loads through steering racks are not high, the destructive frequencies can be. (kerbing excepted!)
If you have some freedom on where to mount the rack, I would always temporarily clamp the rack to the frame/bulkhead with 'G'cramps until
you have eliminated the bump steer, then make your mountings to fix it in place.
ATB
Phil
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mark chandler
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| posted on 7/2/07 at 03:28 PM |
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I,ll second that, popped mine on with bookish mounts and the bump steer was really bad, 1" of wheel movement when moving the suspension from
full droop to topping out.
Then spent an hour cutting the book bits out then clamped the rack in various places to improve this, although better still pants !
So next step, cut 4" out of the rack and 3.5" out of the sleeve and start again, now the bump steer is negliable.
So do not attatch only tack until you are sure you have the right location.
Regards mark
[Edited on 7/2/07 by mark chandler]
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Liam
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| posted on 7/2/07 at 07:08 PM |
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As said the most important thing is to minimise bumpsteer. I've driven a westy with bad bumpsteer on a bumpyish road and it was horrible!!
Me and Geoff minimised bumpsteer on his car very successfully with the following simple method. We supported the front of the chassis and removed a
front shock so we could move the wheel up and down. Then clamped the rack in place on blocks. We taped a cheap laser level to the wheel so that it
projected a line on the floor. We could then visualise bump steer by moving the suspension up and down and measuring how much the laser line moved on
the floor (marked it on a couple of pieces of paper on the floor). The rack was then adjusted up/down using washers as spacers until we found the
position where the laser line 'steered' the least. It's surprising how much difference a mere mm makes, and in the end we got his bump
steer down to nothing. In the final position final mounts were tacked in place. Easy!
Here's a piccy from Geoffs archive showing the 'top engineers' in action, in case any F1 team bosses are reading this...
Image deleted by owner
We used two bits of paper to record laser line movement, one near the front wheel and one near the back (just off the piccy). This is because the
line will move laterally anyway due to camber change. What you're looking for is the point when the line moves the same amount on the front and
back pieces of paper. This is no bump steer. When the line moves more at the back than at the front the wheel has obviously steered.
Might sound complicated but was actually a doddle and really quick too, and geoffs car has no bump steer (and neither does mine cos i used a similar
method).
Liam
[Edited on 7/2/07 by Liam]
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TangoMan
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| posted on 7/2/07 at 11:21 PM |
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I've pondered this one many times.
It is fairly straightforward to set up your rack for zero bumpsteer when driving straight. This is done my ensuring the inboard pivots are all in line
as has been said and that the steering arm follows the plane of the lower suspension arm.
Obviously when you turn the inboard pivots are no longer in line and is is likely that the suspension will be in droop one ride and possibly rebound
the other.
What does this do for bump steer in mid corner??
Adding onto this.. When would you least want to have bumpsteer, on a straight or mid corner when you have the tyres at their limit.
I never really put this one to bed as all the 'experts' say to follow the accepted practice but I was never much of a sheep. I need to
fully understand things before I buy into them.
Summer's here!!!!
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nitram38
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| posted on 8/2/07 at 12:05 AM |
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You don't want bumpsteer anywhere but suspension is a compromise.
What is good for a track is different on the road.
Most people (tintops) shouldn't be going around corners fast so bumpsteer is not an issue, but get bump steer at 70mph on a straight, it is a
different story. On the track, you just have to put up with it in fast corners. That is where driver skill comes into it.
But you do not want continual bump steer on the road. Imagine a 2hr drive with the car trying to take right and left turns without your
permission!
It will soon wear you out, never mind it also being dangerous.
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Liam
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| posted on 8/2/07 at 01:31 AM |
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When you set up your steering geometry to minimise bump steer, what you are actually doing is setting the length and angle of the track rod such that
it's outer pivot follows the same arc during suspension travel as its attachment point on the upright wants to follow (which is determined by
the wishbone geometry). Get these two arcs matched and you have zero bumpsteer.
What about when you steer? In the simplest analysis (parallel equal length wishbones with the track rod also parallel and equal length - no straight
line bumpsteer) moving both track rod pivots sideways (i.e. steering) has no effect at all on the arcs the outer pivots follow during suspension
travel, so you still have no bumpsteer at any steering lock position.
In real unequal length converging wishbone geometry things arent so perfect. The kpi changes the angle of the trackrod slightly during steering and
camber change comes into play during suspension travel too. But these effects aren't huge. In effect, setting up the steering geometry for no
bumpsteer in a straight ahead position does a good (but not perfect) job of minimising bumpsteer for all points in the steering travel.
Liam
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sgraber
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| posted on 8/2/07 at 04:08 AM |
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The laser trick is exactly how I did mine. Except my chassis was up on a table, the wheel was off and the laser line was pointed down. I had a line
extending 2 meters forward and 2 back from the centerline of the wheel. The wheel assy was moved through it's range of motion, and at bump,
droop and level, the laser line was marked on the floor with tape and a marker. As stated by Liam, adjusting the rack up or down a mere millimeter at
a time made a difference in the bump steer. Using this method I was able to induce less than a millimeter of bump steer over the full 8" range
of travel in my suspension.
I would strongly suggest that this is a VERY important step in making your car handle correctly. You do not want the car making steering inputs for
you!
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
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