The Stovebolt Site Forums The Shop Area Driveline Pitman arm angle/Drag link length

I'm changing out my stock drag link and tie rod on the '36 and putting modern tie rod ends on them. Attached is a picture of the pitman arm with the stock drag link with the tires pointed straight. You can see it's on an angle backward. Is there a specific reason for this? I'm thinking about making the drag link shorter so the arm is either straight down when the wheels are straight, or perhaps even slightly forward to make the pivot point closer to the pivot point of the rear spring hanger.

The only reasons I could come up with for the pitman arm to be angled back like this would be to minimize bump steer, or maybe to level the drag link, but if bump steer is the reason, I'd think being closer to the rear spring hanger is what would be best. The steering knuckle attachment point is right in the middle of the front axle with the wheels straight, so I would think bump steer would actually be minimized with the pitman arm closer to the rear spring hanger?? One other thought I had was perhaps that since the pitman arm attachment point is lower than the spring hanger, maybe the extra length on the drag link is necessary to make the travel arc of the drag link closer to the suspension arc?

Someone who knows this stuff please let me know what I'm overlooking or what's wrong with my thinking on this. Seems Chevy did this on purpose for a reason, but the reason is escaping me.

I'm interested too!

If one moves the steering wheel to find center, that is a good location, angle wise, for the pitman arm. The wheels should be pointed straight ahead in that position. The drag link should be level with the ground in that position.

Ed

Yes, all of those conditions exist in the picture. The question, though, is "why"? Why is that a good angle? What's the benefit?

The steering gear center high point will dictate the angle of the pitman arm. I don't know how your pitman arm is keyed to the steering gear shaft but you may be able to choose a different angle. When I rebuilt the front axle on my truck, I replaced the drag link due to worn ball/socket ends. I bought an adjustable drag link with the correct tapers and adjustability in the original length. I centered the steering gear with no drag link connected, centered the front wheels, then adjusted the drag link to fit the distance between the pitman arm and the steering arm.

The draglink, pitman arm, and steering arms are where they are for a reason. They match the design of the chassis and work together. As far as the pitman are being back, that is so the steering box will be centered when driving straight ahead and the draglink will be parallel to the frame. To keep the draglink parallel to the springs and frame, you would need to shorten the pitman arm, that inturn will change the steering ratio, and likely not for the better. I will post some information on the '36 and '37 in a little while.

Joe

Here is some reading for you from the 1937 Engineering Features book. It tells why the '37 is a better truck then '36 so you will buy one.

Perhaps I should have mentioned that my pitman arm is splined the entire way around. Moving it one spline at a time moves the end of the arm where the drag link attaches approximately 1". I know some pitman arms/steering box shafts have several wide, flat indexing splines which locates the pitman arm angle to a specific angle, but mine is not like that. I can put it wherever I want, which is why I was considering making the new drag link shorter than stock. But before I do that, I want to figure out if that would make bump steer better or worse.

What if I move the pitman arm to angle forward at approximately the same angle, rather than backward? That would keep the drag link parallel to the frame (or at least the relationship to the frame/springs would be the same) while shortening the drag link and moving the pivot point closer to where the rear spring eye attaches to the hanger, which I would THINK would make the arcs more similar?? Maybe not.

Thanks for posting the information. That was interesting reading. If that drawing of the '36 is to scale, I can blow it up and print it out and use that to draw arcs from different pitman arm locations to see how the arc changes. That is, if it's to scale.

The angles will be way off when making full turns if you change the orientation of the pitman arm and draglink length. Ideal location is to move the steering box forward to make spring and draglink the same length.

My truck has a 1941 steering box that uses worm gear and roller balls, an adjustable draglink, and stock '37 steering arms with stock type tie-rod ends. I have never had bump steer. I can run 65 -70 miles per hour all day long with good radial tires and disk brakes. As for the driving, it's as good as my '03 Tacoma when on good roads, it drives itself with very little input from me to keep it in the lanes.

Best advice I can give is, make sure everything is tight and in good working order, kingpins, wheel bearings, steering box, and tie-rod ends. Steering boxes are pretty crude, simple gear on gear with heavy steering ratios so any movement from the front feels like a lot in the steering wheel. The '41- "46 boxes have a higher ratio 19.8: 1, so it takes more turns lock to lock, but it makes for easier turning and bumps don't feel nearly as bad. I drove my truck for 15 years with the '37 box, the '41 box made it much nicer to drive.

I don't know if you can adapt a newer box or not on the '36 frame, but you want to change something, that's where I would start.

Play with the drawing and see what you come up with, but remember, the pitman arms makes almost an 90* arc lock to lock.

As JoeH says “ arms are where they are for a reason“.
When I did my adjustable drag link it was only to center up the steering wheel at middle of lock to lock, but I considered this question quite,a,bit.
My conclusion is that the location (on my 46) at 4:30-5 o’clock position was chosen to compensate for body roll when cornering, not necessarily bump steer.
Since the steering box is attached to the chassis and the attachment to the left wheel is at a somewhat fixed height…..body roll raises and lowers the steering box/pitman arm.
As you turn right the left side chassis/ springs compress/lower, so the pitman pulling back raising its lower pivot point, when you turn left the left side of chassis rolls up, and the pitman rotating forward lowers. It all keeps the drag link a little closer to level at any given time.
The opinion presented is just that….

Thanks again Joe! What I may do is just make two different length drag links and experiment with it a bit to satisfy my curiosity. Everything will be new and tight, except for the steering box. The axle is fresh from Sid's with new kingpins. Tie rod and drag link ends brand new, and brand new roller bearings. I may very well be talking about a bumpsteer problem that doesn't even exist. I'll find out when I drive it with all the new stuff. All of this just crossed my mind as I was measuring for drag link length and I started pondering this stuff.

Did you adapt the '41 box to your '37 steering column, or replace the entire thing? I do have what I believe is a '38 steering column and box, which is a little longer than my '36 and could be moved forward, but it doesn't sound like that would be worth the effort if you're saying the '41-'46 made your truck much better. Might as well pick up the later box if I'm going to do it.

If one has a pitman arm with 4 skip splines scattered at 90 degree intervals, it is easy to remove them to allow registering to any location. A three cornered file and some time will allow them to be removed to allow any registration.

Ed

I'm certain everyone here knows this but years ago I made a little drawing to explain to a fellow who just couldn't visualize why an angled drag link made a difference to bump steer. Bottom line: the AD trucks were designed to have an angled drag link...I don't know about the 1936 but it might have been the same. And if you replace springs, kingpins, balls/cups, etc all with new, you'll have an angled drag link, too. It will become more parallel to the ground when your springs start to sag. You can see what happens as you move up/down on the arc of the imaginary circle with the attached drawing and how movements (even slight ones) become more and more pronounced the further away from parallel you go. It should help with any year vehicle, hopefully.

Now there's a theory I haven't considered! You certainly did consider this question quite a bit. Thanks for sharing your thoughts.

I have both style pitman arms. One with the 4 skip splines, and one that's splined the whole way around. The one that is splined the whole way around is stock on the '36 (unless someone changed this previous to my owning it), and obviously the pitman arm shaft on the steering box is splined the whole way around as well.

1941 was fist year for the better gear boxes, so no real need to up grade to the '38 other then a little better ratio. I cut both steering shafts, the '41 and '37 so I could keep the '37 wheel and horn assembly. The '41s have a slightly different steering wheel offset which put the wheel to high and to close to the driver, adapting the '37 shaft was a matter of measure, cut, and weld so the interior still looks and feels like a '37. I also have a adjustable drag link, its super handy to center the gear box and make slight adjustments as needed. I changed from 4 degree caster to 6 degrees and the steering wheel moved a little more then 1/8 turn off center, I pulled it back with a turn of the drag link bar.

Joe, keeping my stock wheel is exactly what I would like to do as well, not only for originality, but because mine is in great shape. I'm going to add this gearbox conversion and relocation to my list of things I'm going to do. Now I just need to find a good box.

Since you mentioned caster, I'd like to change the subject a little...My axle is a '50 at factory specs. I'm not sure where the caster was set, but it appears to be right about 0. I didn't measure it. I have 4 degree shims that I was going to try, but I was concerned about the affects of negative caster. Sounds like you aren't experiencing any issues, since you say you can travel 65-70mph. Does your steering wheel return to center coming out of a turn? No problems with darting around/stability/going straight on the highway? Thanks again for your input.

Ron

Mine drives as it should, wheel return is normal. I had 4 degree for a long time, when I changed springs, the front sat lower and I could tell it needed more caster, the steering wheel was light and the truck didn't want to stay going straight. When I changed to 6 degrees, it was right back to tracking straight with out the wondering around. Caster changes as you raise or lower the front, rear, or both, my new springs kept the rear about the same height but lowered the front. I have since raised it slightly with longer front shackle plates. When at a stand still on a slick garage floor, the nose of the truck raises slightly when turning to full lock position. Zero caster will not do this, and to much caster will really raise the nose as you turn, you want just enough to return to center, but not so much you are lifting the nose everytime you turn. My front axle is also a early '50s, at somepoint in the trucks life it was changed out, probably due to the small '37 kingpins being worn out, ( it was plumbing/heating company truck in the Flint Hills of Kansas all it it's life). Start with four and see how it acts. I had to buy wide shims and cut them down, I couldn't find the 1 3/4" wide ones at the time.

FYI on steering boxes, the '41 is the same design as the '42-'46 only the roller balls change size so the cage nut and worn shaft do not interchange. Good news is, 1/2 to 1 1/2 trucks all used the same gear box, the steering shaft is longer on the bigger trucks, but since you are cutting it anyway it doesn't matter. New worm and cage nuts are available if needed. Be sure to check the lower bearing race for water damage, the steering shaft is not sealed, so any water that goes down the steering tube lands right at the lower bearing.

Thanks for the tips on the steering box. As for the shims, just to be sure I'm understanding this clearly, you are using the shims to achieve negative caster? In other words, the thick part of the shims are toward the rear of the truck? Or are you giving the truck more positive caster, with the thick part of the shims toward the front?

My understanding is that negative caster will give a lighter steering wheel, but at the expense of straight tracking and stability. Positive caster will do the opposite. Heavier steering (which doesn't matter with power steering) but better straight line tracking, and better steering wheel return coming out of turns.

Do I have this correct?

Yes to both. Positive caster keeps your wheels straight ahead and re-centers the steering wheel easier.

Just to follow up for anyone who is interested...I took the drawing Joe H posted and printed it out. I drew an arc for the pitman arm travel, and selected two points on that arc to represent places I could move the pitman arm to by moving it one spline at a time. One of these positions is almost straight down (rather than angled back at the 5:00 position which is stock), and another angled forward (7:00) which ends up directly underneath the spring hanger.

My original curiosity was to see if locating it directly below the spring hanger would improve the drag link's travel arc (bump steer) to get it closer to the leaf spring travel arc.

It turns out neither of these pitman arm locations would improve the drag link travel arc. They may even make it slightly worse.

This of course depends on how close to scale the drawing is. I did attempt to check the scale on the drawing by measuring both the length of the actual pitman arm and drag link , as well as the length of each of them on the picture, and comparing the ratios. It's close, but not exact, so I can't say the drawing is perfectly to scale, but I think it's close enough to draw the conclusion that different pitman arm positions and drag link lengths will not improve anything with the steering box in the stock location. So to answer my own original question, I think Chevy chose this particular pitman arm location and drag link length as the best compromise between bump steer and consistency of how quickly the wheels turn left and right.

Ron

Thick part at the rear, axle lays back.