Technical STEERING, Vintage C9: Bump Steer

Digging through my HAMB tech archive and found this gem

Zeke

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C9 posted 05-17-2001 12:48 PM
Bump Steer

A parallel drag link and tie rod is what you should strive for.
At least when viewed from overhead.

As a practical matter, it can't always be done and a minimal angle between the two won't hurt.

If there's considerable disparity between the two angle-wise, the steering ratio will be - in effect - different on one side than it is on the other.
Get it too far and there's a real danger of getting the pitman arm in such a position that it doesn't steer well and if it's really way out there, you could get into an over-center situation at the pitman arm end of the drag link.
I doubt that would happen, but if you're not paying attention and trying the steering out lock to lock while the car is up on stands it's possible.

Depending on how much of an angle you have and also if you're running an aftermarket pitman arm that's fully splined and will orient anywhere on the pitman shaft you can set it up so the pitman arm is at a 90 degree angle to the tie rod and have equal steering on both sides.
Vega factory pitman arms only install at zero and 180 degrees.
Making things somewhat limited in the adjustment department.

Thing to watch out for then is that you don't have such an extreme drag link angle that you end up pulling the steering arm so far at the extremes of steering travel that there's not a sufficient angle left between drag link and steering arm.
Another place where an over-center situation could occur.

Practically speaking, you can run 2-3 degrees angle disparity between drag link and tie rod - again in plan view, from overhead - and you won't notice any problems or have any real danger of the pitman arm or steering arm going over center with the drag link.
My 32 is set up this way, steers fine and has no problems.

As a matter of interest, note that the more the angular disparity between tie rod and drag link, the less the bump steer is due to less of a horizontal input due to travel.
Even so, you don't want to use a radical angle between the two or you'll run into the over-center problems mentioned above.
Among other things.


Other things you should consider:

The front panhard length should match the length of the drag link.
That's the optimum.
As a practical matter they're usually shorter.
But not by much.

Off the subject in a front end discussion, but make the rear panhard as long as you can.
I'd say that some of the very short commercially made ones I see today don't add as much as they should to the ‘good steering' equation.
The frame bracket for the rear panhard should be on the same side as the steering box.

Make the front panhard parallel to the tie rod as viewed from above and parallel to the drag link when viewed from the front or back.
With the cars weight on the front axle..
This means, with engine, trans and all the expected weight actually on the front end before you set the panhard's drag link angle.
The angle easily set by the length of the drivers side panhard frame mount and the height of the pitman arm.
Get this one as close to zero as you can and it will keep bump steer to a minimum.
At least bump steer induced by the panhard.


Bump steer - for those that don't know - is defined as a steering input from a front suspension deflection.
Swing any panhard, 4 bar, ladder bar or the like and the free end will describe an arc.
Short ones describe more of an arc than do long ones.
The arc in this case being measured for the distance of travel.

To see what I mean, swing a string on the garage floor or workbench.
Draw a straight line on the floor about 3' long and hold the anchored end of the string at an arbitrary anchor point on one end of this line.
Hold a pencil at the other end of the string, at the length of the suspension component in question and swing an arc about 10" tall scribing a line with the pencil.
5" above and 5" below the horizontal line.
Mark two points on the arc that are the same length as the cars suspension travel.
IE: travel = 4", the two points will be 2" above and 2" below the horizontal line.
Draw a straight line between these points.
Said straight line will be at 90 degrees to the long straight line drawn on the floor.
Measure from the center of the straight line to the high point of the arc and see how much deflection you have.

Which means, simply put, the longer a suspension locating component the better.
At least within the bounds of practical use.

There's a lot of talk bandied about, about bump steer, but if you look at how little suspension travel there really is on a solid front axle car, you'll see that the horizontal deflection is not too much at all.
Not as much as some would have you believe.

Even so, strive for the minimum deflection by careful location of components and make them as long as is reasonable.

In a word - short panhard's suck.
Ok, three words, but you see what I mean.


If you can get the right tubing, making your own panhard, drag link and tie rod to the required length is easily done.


Again, as a practical matter you can run a panhard/drag link horizontal* angle mismatch of 2-3 degrees and not create any noticeable problems.
Probably a bit more, but you'll have to be the judge here.
I did this one too, with about a 2 degree mismatch on the 32.
Bump steer is still not noticeable and the car handles quite well.

*Horizontal defined here as viewing from front or back.

What happens here is, the drag link and the panhard are describing a differing arc and when you measure the deflection and compare the difference it's just not very much.


You definitely want a panhard bar on the front end of a solid axle, transversely sprung car.
Without it, centrifugal forces create an unwanted steering input.

What happens here is, without a panhard, the front of the chassis is free to swing left and right on the spring shackles.
Enter a turn, centrifugal force pulls the chassis to the side and a steering input is created.
The driver corrects so as to stay on the same turn radii, the centrifugal forces are altered and the centrifugally inspired steering input is changed.
Requiring the driver to make another steering input and the cycle starts over again.
Most times not ending until the turn is completed.

Regardless of direction, the turn is tightened by centrifugal force steering inputs and the resultant effects are, the driver swings back and forth on the steering wheel all the way through the turn.
Not so true for a very gentle low speed turn, but for most curvy road mountain driving you'll find it to be very true.


Here's another one you have to worry about.

Steering box stops.

First up, don't use the steering box as a limit stop for a solid axle front end.
It creates unnecessary wear and tear on the steering box.

Get the stop nuts that double as the nuts to the king-pin retaining bolt.
King-pin bolts in many cases come with a common nut.
The stop nuts are simply an long acorn nut.

Note too that some of these nuts are overly long.
Which limits the cars steering radius and in some cases the radius is so large that you have to make an appointment for a right hand turn.
So to speak...

Stop nuts you get most places are about the right length to start with.
If you like the shiny stuff, get em in stainless.
That way, if you cut them down they still look good.
The stop nuts I just bought for the 31 were from The Deuce Factory, measured 1 1/4" long and have about a 3/16" thick washer.
They worked out just fine with the Vega cross steering setup on this car.

The ones on the 32 were quite long and when first running, the turning circle for the car was wider than sweeties 5 liter Mustang.
If you've driven one of those, you'll know what I mean.

I cut those down in two stages, .125 each time and trying them each time.
Don't simply cut to match the figures I ended up with, make a cut and try it.
Right now, they're at 1 1/4" long and the car turns about as sharp as it's going to.
Which is to say, I don't feel limited by the 32's turning radius.

Pay attention when you set up the pitman arm angle.
I strongly recommend the use of an aftermarket one.
Moving it just one spline will make or break a good steering system in my estimation.

Get the car up on blocks, car weight on the axle and the spring properly compressed, remove the stop nuts, turn the steering - gently - to the limits, where the internal stops of the steering box come into play and note if the bump stop nuts would stop the travel before the steering box internal stops hit.
If so, you're in business.
Reinstall the stop nuts.
Double check and make sure the stop nuts are what's stopping the steering from traveling too far.

If not, you'll have to balance the drag link length - within reasonable limits, keeping in mind the requirement for good thread engagement of the tie rod ends on the drag link - the pitman arm angle and the bump stop nut length.
Sounds like a big deal.
It's not.
All it takes is some attention to detail and a willingness to work with it until it's in the right place.


Sometimes you can end up with the steering wheel spokes not in the desired horizontal position.
Once the drag link length is set up, the pitman arm properly setup and the stop nuts the correct length you can bring the steering wheel into correct orientation by:
Moving the wheel to the closest position you can by moving it the requisite amount of splines until it aligns where you want it or very close to that.
Then, use the drag link length adjustment to fine tune the steering wheel spoke orientation.
This is, the last thing to do in setting up the steering.
And may take several test drives.

All of which assumes the toe-in was correct before you started.
To boot, if you moved the drag link length adjustment, be sure the stop nuts are what's stopping the travel and you're not hitting the steering box internal stops.
It's a bit of a juggling act, but if you pay attention you won't get lost.


Height-wise installation of the Vega box is important, but there's still a lot of latitude to work with.
Especially so with the use of an aftermarket pitman arm.
These are reamed for the drag links tie rod end so the end can enter from top or bottom.
Making fine tuning for the drag links - viewed from the horizontal, or the back - angle easy.
If a touch more height - or lowness - of the drag links steering box end is required it's easily accomplished by heating the pitman arm and bending it.

Need I warn against heating it while it's connected to the steering box?
Probably not, but there it is anyway.

Use care here not to use too much heat.
Bringing the metal up to a soft orange viewed in a subdued light will do it.
Subdued defined as; in the garage in the daytime with the flourescent's off.

Make sure to clamp one end in a vise so the pitman arm hole or the drag links tie rod end hole is not stretched oval during the bending process.
To that end, use a large Crescent wrench - adjustable spanner for the down-under guys - and heat well away from the hole you're trying to protect.
Protect just another word for ‘keep it round'.

The aftermarket arms and the factory arms bend with heat just fine.
Be sure and let it air cool when done.
No tossing it in the water.
That will make it brittle and definitely lead to later breakage.


I see a lot of hot rods with aftermarket pitman arms that are as flat as the day they were made.
In a lot of cases, the pitman arm points down and is a ripe candidate for damage from a speed bump or similar.

Having the pitman be the low point of the car is something to stay away from.


A lot of guys and gals go for the Vega cross steer as it is a pretty neat setup.
They think too that they're getting away from the bump steer problem which has plagued the fore and aft drag link setups for many years.

The simple truth is, do it wrong and the Vega steering will have all the bump steer you can stand.


As for the fore and aft drag link?
These can be a very good setup.

They're traditional, good looking to my eye and in fact was the first choice of steering for my 31.
Header conflicts are lessened to a degree.

I went for the Vega because I already had a good box sitting on the shelf.

If you pay attention to the length of the hairpins, radius rods, 4 bars or split wishbones and set the fore & aft drag link up with the right length, in the right place with the right pitman arm these will steer just as good as the Vega.

And look twice as cool.....

C9

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