Bent vs. Straight Drag link

I say don't. It is like a split wishbone on a tube axle, 10 guys will say no problem and then you see a picture of one failed. I bought an old T bucket with a dog leg in the drag link and it looked like a death trap to me, way to many forces on it that don't belong..

 

Originally Posted by swissmike

You will never see a bend in a factory set-up for good reason.

1929 hudson 4 foot long, bend right in the middle. Been this way for 79 years and still workin(the streets here in chicago beat the shit out of it too)

 

When you move a draglink up or down during suspension movement it changes the length thus unvolentarily steering the car on a nasty bump at speed. so when it is level to the road there is less movement. I found out the hard way years ago when I had my draglink at a severe angle.

 

an easy way to check if you have bump steer, have a couple of big friends bounce the front end of your car up and down while you watch for steering wheel movement, if your wheel moves back and forth as the car moves up and down then you have bump steer.

 

OK should not have said NEVER, but it only goes to show everybody can have a bad day....even ol'Henry haha!

Obviously both of the above examples are stock and surely appropriately dimensioned, whereas the first pic of a severely bent drag link made from a straight piece is a different animal. I would not be surprised to see that drag link take on the shape of a (swiss?) pretzel if you hit a good size pot hole.
Everything is a matter of degree. No matter what, to bend a drag link is bad practice if it can be avoided. Especially if people don't understand the mechanics behind it.

 

>>> A lot of stock cars in racing have bent panhard bars. I always wondered why they would want a bend in them...>>>

Mostly clearance issues and an attempt to get the longest bar possible in a limited space, but those J-bars are built out of heavier stock than a regular straight bar and often braced.

I once used one that went both over and under the driveshaft (kind of a loooooong oval/loop connected to a couple straight sections at the end) so I could get it all the way over to the other side of the car, when I didn't have room for a straight one behind the rear end.

It worked pretty good but was heavy so I went to a shorter one and lived with it.......

Al in CT

 

I just realized I have very few pictures of the drivers side of my car. My drag link goes up hill, I never liked the way it looks, but it drives fine, no bump steer. A Zed drag link might look better, but it would only be for looks. Dean

 

Yours is about as good as it can get. If your drag link were horizontal it would be horrible, twitching all over the road at every lump or bump. Zeeing it would simply make it look odd, to an engineer anyway.

So many people state that the arms all have to be horizontal, or parallel, or whatever, but they have no idea what they are talking about. Bump steer occurs when the various suspension links move around different centre points, changing radii and pulling or pushing the steering when you hit a bump.

 

Bump steer affects all suspensions except for one. When someone claims they don't have any bump steer they are wrong, their geometry is good enough that they've minimized it. The only suspension that has no bump steer happens to be a straight front axle, but not just any. It has to be a straight axle with a rack and pinion steering box mounted DIRECTLY TO THE AXLE! With the rack mounted to the axle all of the links move with the axle, so there are no components traveling in arcs to cause bump steer.

To understand how bump steer works all you need is a piece of paper and a compass. Draw a line say 2" long horizontally on the paper. This line is your drag link. Set your compass so that the pointed end is on the right (your pitman arm) and the pencil end on the left (the steering arm on the spindle). Use the compass to draw an arc that would make the end points of your line an approxamate 30 deg. angle (one hell of a bump). Now draw two vertical lines one at the left end of the horizontal line and one at the left end of the 30 deg. line. The distance between these two vertical lines is how much that bump just pulled your steering arm back. That is an overly simplified description, but it illustrates how suspension arcs work. Off of paper you also have your radius rods, or wish bones traveling in their own arcs all of the arcs affect every other arc the suspension travels in. To truly minimize bump steer all the links on one side all would be the same length, all of the pivot points in line with each other, and all of the angles have to be the same. This way all the arcs will travel in the same path (timing of the arcs). All of this is normally not possible, so we have links with different lengths and different angles, we have bump steer. Say you did manage to get all of the lengths, angles, and mounting points the same while the wheels are pointed straight. Now you need to turn left. Once you turn the wheel the pitman arm moves and changes the location of your pivot points and bingo you have bump steer again. Other than mounting a rack to the axle there is no way to eliminate bump steer only manage it.

To see how much bump steer is actually in your car, unbolt the drag ling from the steering arm on the spindle. Put a jack under only one side of the axle and jack it up a few inches, then try to bolt the drag link back to the spindle. The amount of misalignment is how much bump steer that you have.

As far a a Z'd drag link, any link with a bend in it reacts the same as a straight link except the angle and length of that link is determined by an imaginary straight line between the pivot points. There is a diagram of this towards the top of the post.

 

http://web.mit.edu/4.441/1_lectures/1_lecture15/1_lecture15.html

Its called a two force member, and the only effect that putting a bend in it has is to make it weak.

 

As long as it turns who cares.................? I did not know anything excep connecting the steering arm to the spinlde, I made mine with 3 positions to change the speed to one that works........if necessary.

 

I hear you all on the bent drag link arguement. It is always best to leave tubing straight for strength. But if it is absolutely needed to clear something I really don't see why it can't be done, provided the proper wall thickness and diameter of tubing is used. Think about all the stuff we use tubing for that is bent, roll cages, bumpers, rub rails, tube chassis race cars, control arms...... And yess there are plenty of factory steering parts that do come bent. Look under you cars and notice the center link and plenty more.
You need to think about how much force is actually going on that drag link. First of all hitting a curb or a brick wall in your average rod is a total anyway. Does it really matter at that point if the drag link bends? There are probably not even seat belt in most of the cars so steering is a non-issue if you're thrown from the car! Getting more realistic, a huge pothole will (should) only send the wheel assembly up on a straight axle car. Yes slightly back, that's why we discuss bumpsteer but if you drag link is getting any hard rearward force you have other major issues to address. Your bones/haipins had better be holding the front axle out there and be taking all that force. The drag link will see hardly any rearward/forward force as it is on either tie-rod ends of heims which allow it to swivel up and down. That it provided that you have gotten the swing/travel/bumpsteer issue right when you built the car. Which by the way has a few different theories. One being that the axle and steering arm angles should be as close as possible swinging in in arc that is even through the entire travel range from full down and full up. There is another that says that a straight line from the front drag-link point should cross the bone pivot and line-up with the pitman point. I use the first one, no bump steer. There is another one that says if your bones and drag link are parallel you will be "close". Take that one for what you will.
As far as the drag link being bent vs. straight and bump steer, there is no difference. Just like old school ford pedals that go through the floor (boomerang lookin' ones) and straight pedal have no difference in force or travel. They are just to clear obstacles. You could also think of this theory pertaining to a throttle linkage. Bend it all you want, 2" of travel is always 2" of travel. Bends just get you around stuff. Bumpsteer is simply a matter of points, point "a" (pitman arm), point "b" (steering arm at spindle) and how they swing with point "c" (axle).
For a bit more justification on bent drag links. I ran one on my car for the last 3 yrs. And I do drive it fast and hard. 1" od .120 wall 4130 moly tubing with 2 bends in it to drop it under the l/s header. I live in central NJ. The roads here have potholes which could swallow a small car. It never bent.

 

Everyone has commented that if you are going to bend a bar, make sure the material is strong enough to handle the bending loads applied. What I haven't seen mentioned is that the joints should be taken into consideration as well. In any tension/compression application where the bar itself is bent, the heims or tierods will also see a bending load instead of a tension/compression load that they are designed to see. I have personally had a tierod fail due to a bent heavy-duty steering link. It's hard to explain without a drawing, but if you really think about it you will see what I'm talking about.

The bump steer conversation is amusing because bump steer is static as well as a dynamic issue. One comment was made that you cannot eliminate all of your bump steer except with one suspension design, that is only partly true. You can eliminate all of your static bumpsteer on just about any suspension design, but dynamic bump steer is a different problem. There are spindle inclination issues as well as too wide of a tire or even compliant joints that can all cause dynamic bump steer. But that is all getting out of the scope of this thread.

 

There is no bump steer on any static suspension. Bump steer is the result of dynamic forces applied to it. AKA a bump

 

Yep, assuming both end are able to swivel (as is the case) the force is along a straight line between the 2 pinned ends. You can make little loops, smiley faces or pics of chinchillas in between and it makes no difference from a geometry perspective. However, a bend in a drag link does make it much more likely to buckle.

 

Haha. True!

Ok then, how would you describe the difference between bump steer that is caused by misaligned suspension parts and bump steer that is induced by too wide of a tire or poor spindle inclination/scrub radius?

 

My brain is getting dizzy.

 

Bump steer is bump steer. It is a culmination of the effects of every suspension component on every other component. Misaligned suspension parts is the main source of excessive bump steer. The main thing is that there is no perfect alignment that will eliminate bump steer. When most people think of alignment they think of caster/camber/toe when in the realm of bump steer it also includes the timing of every other component. An example of this on my race car is that I run extended arms on my rack and pinion in an effort to make the tie rods the same length as the lower control arms. A way of getting both to travel in similar arcs. Another thing is that I run spacers on the stud that mounts the tie rod heim to the spindle and on the studs that mount the tie rod heim to the rack arm. This is an effort to get the tie rod and lowers at the same angle so they are "timed" together. I could arrive at the same caster/camber/toe without either of these changes, but their alignments affect the amount of bump steer.

I don't see where too wide of tire affects bump steer. I run 12" wide tires across the front without any problem. If tire width is causing bump steer it's because something else is flexing and is either wore out, or just the wrong part for the job.

I'm not sure what scrub radius is? I am just assuming that you are talking about the Ackerman effect where one tire will turn at a greater angle than the other as you steer the car. Ackerman does not affect bump steer, assuming nothing is flexing from being wore out, as the suspension components are limited mechanically to their own arcs. Instead bump steer affects Ackerman. In my previous post I mentioned that if you had everything the same length, same angle, and same pivot points that once you steered the car you have bump steer again because of the pivot points moving. This is how bump steer effects Ackerman and also why you can't completely eliminate bump steer. When the steering wheel is turned and the pitman arm moves the pivot points just changed as well as the timing of the components.

 

Lets put bump-steer into perspective in the real world..........my 2000 Ford ranger has some pretty bad bump-steer at low speeds and steep approaches. Should you worry about it? Yes, but even the sliderule guys can't get it perfect. As for the draglink? What Crease and Kevin Lee said.

 

I think the bump steerissue is turning into a tomatoe tomotoe issue. For the basic dummy keeping everything parelell and traveling though the same arc is going to keep it basically safe. Yes some factory steering components have bends but consider the side loads the are on the different parts of a bent drag link. Those can also transfer into torsional loads in certain conditions. The old T bucket I had there was a bout an 8-10 inch dogleg in the drag link and it mad about a half a twist (to the limit of the tierod ends) which also translated into somethiong over 1/8th of a turn of the steering wheel. I know it was extreme but it also revealed the forces involved in a bent drag link.

 

He is correct. I have two panhard bar set ups one I can use a straight bar, which is just a swedged tube radius rod. The other I need a J-bar to clear the pinion. The j-bar is a massive beast compared to the swedged tube. A drag link with a bend will have to be beefier than the straight one, but will be more than adequate made out of the right material.

 

We're obviously taking this thread a bit out of context...

HanibleH20- I understand what you are saying about the bumpsteer not being eliminated over the full sweep of a suspension's travel, but I don't agree that it can only be eliminated on a straight axle front suspension. I have sucessfully eliminated any measureable (with a tape in my shop) bumpsteer from this suspension. I will agree that beyond the suspension's working limit that it WILL begin to toe in or out. I will also admit that if measured with an acurate enough alignment machine that some minor bumpsteer will exist, but this suspension is adjustable to be able to eliminate what's left to a point that is negligible.


Gotta love the kitty-litter on the ground huh? Haha!


What scrub-radius is, is the name given to the moment arm created by the relationship between the point where the steering axis meets the ground (set by the spindle inclination) and the center of the contact patch of the tire. If the center of the contact patch is more than about 2" from the steering axis, then you will experience steering feedback that resembles bump steer, the longer the moment arm the worse the steering feedback. This is why some stock vehicles would need a steering dampener, but a TrophyTruck would not. A well built TrophyTruck will have an almost zero scrub radius as well as front-wheel drive cars, that is why early Eldorados and such had really weird offset wheels.

 

Man all this drag line bend no bend has me wondering if my is ok it drives ok. It has a little bump steer an old guy told me I need to put a pan hard bar up front and it would steer better is this trus?

 

A pannard bar is only going to help in certain situations. If the steering link from the box to the front end is mounted front to rear in the car a pannard bar isn't going to do much, but if the steering link runs from side to side then it is possible that a pannard bar MAY help.

 

What about four bar suspension, where the drag link is identical in length and parallel to the four bars? All radii are the same, nothing pushes or pulls any further than anything else.

 

You should have negligible bump steer in that setup. I'm curious why you have the kink in the drag link though, as it doesn't look like it has to clear anything?

A panhard rod might make a difference to the handling, but not a lot. They are used more for cross-steer setups.

 

The pitman is higher than the front attachment point so by putting the bend in the DL the rod end lay flater.

 

You should have negligible bump steer in that setup. I'm curious why you have the kink in the drag link though, as it doesn't look like it has to clear anything?

A panhard rod might make a difference to the handling, but not a lot. They are used more for cross-steer setups.

 

Didn't read all replies, but ford used bent drag links on 40-48 fords. But the bend is slight, and is near one end. The drag links are sturdier than the straight ones on 32's for example.
A z'd drag link will not cure bump steer, the relative positions of the pitman arm and the steering arm will cause or cure that. Putting slight bends near the ends will perhaps make the rod ends operate a slightly preferable angle.
If you have to bend the drag link, make it sturdier than if it were straight, put the bends as near the ends as possible, and make the bends as shallow as possible.
Mart.