Technical Advice to get more steering

I'll just leave this here:

http://www.jalopyjournal.com/forum/threads/tech-submission-the-dreaded-bumpsteer.325015/

 

Metalman got it right...Gimpyshotrods got it right.
Halfdozen mentions the pitman arm not moving...correct.
But the axle effectively moves back as it goes up, the draglink remains the same length, the pitman arm doesn't move....so the resulting length variance between the wishbone and the draglink (when set up parallel and a distance apart) causes the spindle to rotate around the kingpin:
Bumpsteer.
Gary...the spring socket thing doesn't compensate for bumpsteer. It just allows the pivot cups to be set to a particular tightness...and remain tight even as parts wear slightly.

 

Thanks for this. Everyone should have a look at Mart's and Morrisman's drawings, they illustrate the effects of moving pivots and shortening drag links very well. Differing arcs at the spindle equals bump steer.

 

Well this thread got spirited?

Metalman's comments here sum up the entire theme.

People tend to think that the geometry used in a parallel four bar carries over to the wishbone/ drag link relationship, but they are two different things.

Instead of debating, go grab a tape measure and draw this stuff up.
It shouldn't take you long to see with your own two eyes that the center of the front axle really does move closer to the rear of the car as it moves up and down.
At 36" arc, with 2" of travel either up or down, the difference is 1/8" that you'll see on a tape.

Don't forget that straight ahead isn't the only driving position of these suspension components either. In sweeping turns the pitman arm is in a different location as well as the effective length of the axle's arc - the steering arm pivot will be closer or further away from the frame pivot, or off the axles center fore or aft. That doesn't change the arc of the axle, but it does throw in conflicting arcs that are either focused into the king pin ( aka bump steer) or they are focused into the center's of the arcs and minimized.

In many ways bump steer and drag link angles are similar to Ackerman's angle.

Then again, these are hot rods full of compromises for looks and some of them will have bump steer. Form should follow function but when the over all looks are important sometimes the minor details of flawless function get thrown out the window.

 

http://www.gasalleystreetrods.com/Steering.html

 

Thanks for posting that , Gimpy. That is exactly what I was trying to explain with words. Drawings are better. - Jim

 

So, Is the pittman arm vertical when the wheels are straight? Im confused. I have a nearly identical setup to yours and the tires will touch the bones before hitting the steering boxes limit on both sides.

 

His probably will now too, since he fixed the steering box.

 

Sorry you misunderstood me. That is not what I said, but in fact the common way that I was saying is wrong .

 

The drawings that gimpy posted explain my rambling better than I can with a million words.

 

That would be my thoughts as well.

 

Sorry to beat a "dead horse", but the drawings you all are relying on do not appear to me to capture the OP's geometry. In the linked post, 'pic #1' is going to result in bump steer because the attach point of the bone to the frame is depicted well behind the drag link/pitman arm joint. The bone is significantly longer than the drag link, yet they both have the forward ends in approximately the same place...the centerline of the axle.

Whereas, the OP's setup has both the bone and drag link very, very close to the same length. That results in different geometry than depicted in 'pic #1". The other thing that seems inconsistent to me in poster's claims is this......it is mentioned, correctly, that the axle moves reward as suspension deflects up or down. However, the drag link's similar movement seems to be dismissed or overlooked. It seems to be treated as a fixed point in explanations offered. However, when the axle moves rearward, so does the spindle and attached steering arm...the one to which the drag link is attached. It seems to me that with both forward attach points (axle and steering arm) moving in unison...AND...in the OP's case, near equal length of bones and drag link, bump steer would be negated were parallelism established between the drag link and bones.

I will concede the possibility I am in error, but obviously I am not convinced of that so far. I do intend to make a layout with movable and fixed points and prove or disprove the arcs subscribed. I am big enough to admit if I prove I am mistaken in my reasoning and will report back to this thread.

Ray

 

Wow, Ray, I really opened a can of worms. It was not my intention to claim that the OP's setup is perfect, just that it is better than if he moved the pitman arm 180 degrees, and/or made it parallel to the wishbone.
Hopefully the drawings made you see that? Also, I'm definitely not thinking of the drag link to pitman arm point as a fixed point, that is exactly the point that is moved in the parallel setup that you defend, which causes the bumpsteer.
You mention the OP's drag link being longer than the one in the drawing. The way I see it, that length difference is not an issue provided the pitman arm end of the drag link is on that imaginary straight line drawn through the front drag link pivot and the wishbone attachment to the frame.
I'm going to attach a couple of images that are similar to gimpy's, but slightly different, and maybe you will see what I mean. The drag link is still shorter than the wishbone, but maybe you can see why that doesn't matter. Long drag link or short drag link, it is still being moved forward and back the same amount through suspension travel. These drawings are based on a mustang box, but of course it doesn't matter what box is used, the geometry still works the same.
The bad

And the good

 

You can see the line I keep talking about, and a longer drag link would be just fine as long as it falls on that line, as it does in the second picture.
Also, if the drag link in the first picture were longer, it would not fix the problem. If the wishbone in the first picture I posted creates a 36" arc, that moves the pitman arm 1/8" for the first two inches of travel, and it gets worse with more travel,no matter the length of the drag link.If you read the captions under the pictures, maybe they ( Pete&Jake's) explain it in better terms than I did.
In an ideal situation, points D andB would be in exactly the same place.

 

Can you shed some more light on those bent drag links?

 

Metal Man,

I think you missed my point. I did not address the existing setup of the OP's steering linkage....except .....to say the length of the drag link and bones are about equal. The reference that I made to 'pic 1' was not your post content, but the link Gimpy offered to another thread post that had three or four drawings, the first of which claimed to prove/disprove parallel line theory. My comments were intended to point out that drawing used very unequal lengths for the bones vs the drag link, thereby invalidating the claim made.

Further, I did not say the pitman arm was a 'fixed point', though it may have been implied unintentionally. The following sentences should have dispelled that notion though, as I clearly mention the steering arm on the left front spindle, to which the drag link is attached.

With that explanation, I think if you care to reread my post, my comments will be clearer. Also, I wish to add, I don't necessarily disagree with the use of the intersecting angle geometry to deal with bump steer. I just think it is unattractive and personally prefer the parallel drag link bones setup.

That is about all I have to say as I think further attempts to clarify may just cause readers eyes to glaze over.


Ray

 

Ok, this horse still has a little bit of life left in it. I've been out here in the shop going over this deal in my head while welding (mindless work).
Ray, I now understand where you and I are differing . You are looking at the problem from the point of view of the upper left steering arm being pulled back by drag link traveling on a shorter arc than the wishbone.
You are correct that if you take the first drawing I posted and make the drag link the same length as the split bone, the two arcs up front would be very similar and it would not generate noticeable bumpsteer from the drag link "pulling" the upper left steering arm rearward.
The problem that remains is that the axle and everything that travels with it moves rearward slightly as it pivots on the axis at the rear of the wishbone. That still forces the pitman arm to rotate, steering the car, right ?
So , that is why , in my mind ,picture #2 with the rear of the drag link on the aforementioned imaginary line, is the correct way.
Sometimes, as with cowl steering , we have to compromise , and with the small amount of suspension travel most of these cars have, a little compromise isn't usually so bad.
Edit : you were typing your reply at the same time I was,haha. I think we understand each other now, and I agree we've probably said enough on the subject. I'm out...- Jim

 

Just to confuse everything completely....something else has to be considered when looking at these drawings.
The axle location represented is just that, the AXLE.
The wheel centerline is usually about 4" or more above that due to the axle drop most use.
Now...with the wishbone arms being mounted level with the axle position, the pivot points of the arms are actually 4" below the centerline of the wheels.
This will create more of a rearward motion as the suspension works because the arc is already in play...not floating around in the neutral zone like these drawings make it appear.
That would only be the case if the pivots were at wheel centerline height.

 

With a 4" drop and 7* camber the wheel center is roughly 1" behind the spring perch, or in other words the wheel travels in a 1" shorter arc than the radius rod / bones or axle does.

Now I'll confuse it more.
As the wheels move into a turn, the drag link pivots move on both ends.

For example look at the radius rod pivot to steering arm, here that measures 38-1/2 when going straight. In a full right hand turn that measures 34 while a full left turn measures 43".

This one is cowl steering (so it's already compromised) and i just got the pitman arm today and I'm not done playing with and adjusting it yet so try to ignore that, it's irrelevant to the point of this post. But I did manage to pull off a steering arm I like it.
Here you can see both the steering arm and the radius rod pivot. Everything that moves pivots about that point where the bones/radius rods attach to the frame.


Here is straight ahead position. Real nice even pivot points, this is the sweet spot.
Steering arm to rod pivot is 38-1/2"


Here's a left turn. Steering arm to rod pivot measures 43"


And here's the right turn
Steering arm to rod pivot is 34"



Even though the pitman arm moves, it's anchor point doesn't and all force against the pitman arm from any direction is directed to the box's shaft.

So, everything is controlled around the bone/radius rod pivot point and as far as steering these old solid axle cars that really is the center of the "steering universe" for all intensive purposes. As it pivots within a 4" of suspension travel, the arc proves these arc sections get closer to the rear at the top or bottom of their range. The longer the radius, the longer the arc section given the same 4" travel and the the opposite for shorter radius the shorter the arc section.

The arc section VS its radius will determine just how far back this stuff moves.
So within its normal operating limits there are several radii that all need to play nice together and while doing it try not to vary their distance too much from the steering box's pitman arm. The only leg of these triangles that has any give or that is variable or movable is the steering arm to rod pivot, if there's any conflict or bind within the motion the steering arm will be forced to move thus steering the car via bump steer.

Measured From the rod pivot:
37" to center of spring perch
36" to center of wheel's axle
34"min- 42"max to center of steering arm

Now, any of the arguments ever posted about tube axles not twisting but beam axles twisting can't be ignored. well all that twisting they do changes the radius of travel too, That is if the move from the spring perch out

 

Great info here ... just wondering what final outcome was. To dig with no end in sight ... just makes the shovel heavy. Charles, help a reader out. Whatja' do ?