Hi, I am building a 31 Plymouth coupe, and plan to use a mid 60's Mopar manual steering box mounted under the cowl, with the pitman shaft (or an extension of the pitman shaft) protruding throught the cowl. The car will have a straight axle and parallel leafs in front. It will soon be time to build mounts for the steering box. I need to determine the length of the pitman arm so I can determine the height of the box. Ideally, I would like to mount the box up high to provide room for the pedals and my big feet. But it seems that the the pitman arm would be excessively long or the drag link would not be level. If anyone who has experience or knowledge could please advise, I would be very grateful. Some of the questions I have: Should the drag link be fairly level, thus requiring that the bottom end of the pitmman arm be even with the steering arm on the axle? What about a 2 peice drag link with a link, or a bent drag link if I mount the box up high? Should the pitman arm be the same length as the steering arm on the spindle, to give a 1:1 ratio, or is there a formula to calculate the ideal length of the pitman arm? Would there be any better way to do it, like mounting it under the floor? Any other help or advise? Thanks, Joel
Start by mocking up the box with the arm on it. You can do this on a table. Count how many turns from lock to lock, but also make marks on the table to show where the draglink hole is, on both locks. Then measure how much distance the imaginary draglink has moved from lock to lock. When I do this, I use a yardstick, and mark further away from the box to make errors less likely. Just keep the box stable on the table while you turn it. Now do the same marks on the side of the frame or cowl, with a yardstick, measuring from steering arm hole on LF spindle, to see how much distance the draglink must move, to go to full turns, both directions. Some guys on here like fast ratio around 2 to 2.5 turns of the steering wheel, and one hamber recently said his Mustang setup is 5.5 turns, and he said "you get busy making a tight turn in parking". The quicker the ratio, the more effort it takes to turn. Heavy caster makes it worse. Also, let's say your box does 3.0 to locks, and measuring your front end with it on a jack with no load, you find that it needs exactly 3.0...then when the load of the car/engine is on the ground with any amount of caster, you actually need a bit more that 3 at the box to get to the locks. That is due to flexing in the entire steering system. Draglink "working angle" is figured by the actual centerline of each pivot at the ends of the draglink. So, if the rear C/L is way too high. compared to the front at the steering arm, a bent draglink will not help the bumpsteer you will get. In a perfect world with your twin leaf, is to have those centerlines level, at true ride height. That way, a slight move up or down during driving, will not "steer" the car. "Level" meaning, you are going by the way the axle moves up and down at the pivots of the springs. Those are likely pretty much level to the ground in your case. Your longer-than stock" draglink should also minimize any bump steer if you get the angle correct. A very short link would add to a bumpsteer.
I forgot to answer your question about a much longer arm. Do the mock ups like suggested with the stock pitman. Then figure out how many turns of that box/arm will go lock to lock, if it was hooked to the spindle arm. It's just math. Now if you want to know how many turns it would take with a longer pitman, clamp a paintstick to the pitman, and mark the "new" draglink hole on that stick. Now run through the lock to lock on the box, to see how many turns are needed on the steering wheel, to move your steering arm from lock to lock (going by how many inches the imaginary draglink has travelled). If it ends up under 2, I doubt you will like the feel. It will be heavy, and dart like a go-cart with a slight movement of the steering wheel.
Some pics to help make sense of the above "imaginary" stuff; Pic of how the yardstick is used to make marks on framerail. Also the F100 box on the table with Visegrips with a tape mark showing me where a stock F100 pitman hole should be. I did not have an arm yet, so Dreddybear gave me that measurement from his arm. I ended up with 3.5 turns when all the parts were finally installed, just like mock ups suggested it would be. If I wanted to go quicker, I'd have to make the pitman longer. But I could not do that, as the draglink angle would be worse. And I could not make it quicker by shortening the spindle artm, because then the draglink will hit the tire on a full left turn. So 3.5 it is, and feels good to me.
In my opinion the car in the pic is a real bad candidate for cowl steering because the piman arm is 6 or 7" hole centers and the drag link from pitman arm to spindle steering arm needs to be level. So measure from the ground up to where the steering arm end is located then add say 7" and measure on your cowl - that is where the center of the cowl steering should be. I'd guess it'll be only halfway up the cowl and that'll put the steering box right into the pedals. Again, the tube running from the pitman arm to the steering arm needs to be level so that it raises/falls at the same 'rate' as the front suspension. If it runs downhill then when the car hits a bump the axle rises against the drag link and the tires will toe-out. Bump steer.
There are race car pitman arms with several holes in the bottom end to adjust length. They come in several different splines also. You will have to research to see if there is something there you can use. You might consider a race car power steering also. They can be had fairly cheap used. We use 1/2 to 1/3 of a turn lock to lock on dirt cars and you can steer them with one finger.
I'd have to say that F&J just gave the definitive explanation on setting up the steering linkage for cowl steering or just about any other drag link steering setup. That's very well done and anyone should be able to follow it.
Thanks for the replies and a great explanation. The more I look at it, the more I'm thinking that maybe, like OJ Says, this isn't a good car for cowl steering. Adding to the problem that he points out is the fact that the Plymouths steering arm is mounted below the kingpin. Of course that could always be changed, the steering arm could be moved up above the kingpin.
The steering rod MUST be equal lenth to the front axle control arm. If the steering rod is shorter you will get bump steer because it travels a smaller arc than the axle control rods. The same lenth the arcs are equal
The car has semi-elliptic springs. (they shackle at the rear...) A drag link starting at the cowling panel will be roughly twice the length of the trailing half of the spring. A long drag link is more 'forgiving' than a short one: Look at F-1s thru F-100 pickups. Pitman are is less than the half length of the spring pack. (and it's shackled at the front) If you were looking for bump steer, this is where you would find it.
Great advice F&J. Have you also addressed the question about the height differential between the pitman arm end and the height of the steering arm end? In a cowl setup, it seems that with the pitman arm length you determined, the drag link will indeed point uphill unless the box is mounted very low in the cowl. Is a raised (bent up and leveled) steering arm the answer?
You can raise it, but there must be an engineering limit as to how far, with whatever style of arm it is. The 32 in the above pics is not mine. The owner wanted traditional F100 side steer. The angle of that link is not the best, but it is better than most 32s with the same box, because this axle is not dropped. Also, that spindle arm was dropped lower than stock 32 arms are. And, the steering box mounting holes were drilled as close to the top of frameail as possible. We live in an imperfect world with the old style side steer. You won't find many that are textbook perfect on length, or angles.
Forgot something on traditional sidesteer. This problem rarely happens but is worth mentioning: On a side steer car with spindle stops like Fords on each spindle... and if the Ackerman is correct...the actual distance travelled by the draglink, from center-to-full Left stop, is greater than the distance travelled from center-to-full Right stop. This is only due to the Ackerman angles of the wheels at full locks. On a full Left turn, the Left front wheel is turned at a sharper angle than the reversed angle of that Left wheel on a full Right turn. You likely know that all boxes have a true mechanical "dead-center". If you had an odd set of parts with a quick ratio gearbox with very few turns, you may end up with barely enough turns to get to both stops, but the dead-center of the box won't be at center if the wheels are straight. It is very rare to come up with that situation, but worth mentioning IMO
The pitman arm (also called a sector arm) works in conjunction with the steering arm at the spindle. Generally, the longer the pitman arm the longer the steering arm. Pictures show how I used a mock up arm to figure out the length and height for an arm that I fabricated.
one more little thing to check. the steering stops on the axle must contact before the box hits the end of its stroke or you can do damage in the box. This is especially true with power boxes.
Quite a bit more work, but check out the set-up in Jim Farley's Deuce in the latest TRJ: Dave Simard ran a linkage inside the cowl side panel, between the output shaft and the pitmam arm. Gets the box as high as you want, and you can run a shorter (and therefore much more practical AND aesthetically pleasing pitman. By varying the length of the links inside the cowl I guess you could 'adjust' the rate without having to have an overly long steering arm on the spindle too. Hopefully someone can post a pic of it.
These guys' advice seems to be right-on. However, have you considered cross-steering ? Henry Ford used it on his Model T Fords, then chose it for his 35-48 Fords. Works really great, and everything is under the hood. Just keep the draglink parallel to the ground when attaching to the right steering arm. Any comments ?
I stopped putting the steering on the side , cowl and have gone to bottom steering. box is easier to mount. Steering runs up the side to the steering arm nice and parrallel. Worked so well I now wonder why I ever did it the other way. Pitman arm comes out the side of the car just under the frame rail and moves forward and aft in EXACTLY the same plane as steering arm. (Instead of one in a vertical plane and one in a horizontal plane.) Looks better too because it is essentially all out of sight. I like a 1 to 1 ratio or close with the shorter if necesary on the pitman arm. Excellant description by the way. Very interesting. Don
Mopar boxes are a particularly slow box, so you will be looking at a fairly long arm. The longer arm with a slow box will still steer with out much effort. I only used an 8" arm on my modified using a BMW 2002 box . Jeff Eishen from Ohio used about a 12" arm on his modified using a Mopar box. Personally I like the looks of a longer arm. More space to add some detail.
Saw your post on the pitman arm issue so here is my take on this. First with the vehicle you have now I would drop the idea of a side steer box. This is based upon first the issue with the drag link alignment and then in the difficulty of mounting the box in a position comfortable to both the steering shaft and the pedals. Second the position of the box to obtain some degree of bump steer control with the drag link would be difficult. I think in this case I might be going to a cross steer style box to free up the foot room and pedal area. The issue with making a pitman arm requires a little more math than most people want to do. Since the turning radius is a function of the wheel base it is necessary to match the steering to that distance. Example would be that if the ride had a 109" wheel base and a Vega had a 110" wheel base they are very close together so thye would be compatible. To do this you need to know the turning radius of the donor vehicle. You then have to search for a box from a vehicle with a similar turning radius or compute the degrees of rotation of a donor box to determine if it would produce a turn with a similar radius. You are doing this to prevent getting a box that is too fast or too slow a turn steer. After you have slected your box the next step is to make the steering arm connectors match up. If you are using the stock steering arm on the Plymouth then you would want the pitman arm to be the same lenght or close to retain a 1:1 relationship with the steernig arm. This can be altered slightly for correction purposes. Its not that you cant do a side steer but it is mutchy more difficult.
Curious about the terms being used here. The OP asked about "cowl steer", and most responses refer to "side steer". Now I guess that technically cowl steer is a form of side steer, but isn't side steer when the box is mounted down on the frame? Side steer is done on these cars all the time, right? F1 box, etc.
This thread is a 'keeper' tons of information! let me not steal but post another question for the OP, what if he varied the attachment point height at the spindle, he said that the original spindle attached at the bottom. Could he do like another posted and attach at the top, can he bend the attachment tubes upward so that it is higher yet but still in the same plane - if you dropped a rod thru the higher attachment point it would drop thru the lower attachment point, technically. Does the height of the attachment point affect anything?
F&J did a great job explaining, I would like to add some more food for thought to this thread. If one wants to do a cowl steering and the car sits fairly high it stands to reason that the box would have to be closer to the pedals to keep geometry in line. Raising the box up and adding length to the pitman arm to keep the geometry in line, lengthening of the spindle steering arm the same amount will bring the ratio back to what it was. Another option is to make the spindle steering arm taller or arc upwards more, this can look busy but can help to get the box higher in the cowl. This will also help with drag link tire clearance in full turn as the tire link interference is less the higher it is. Also as far as steering effort, or turns lock to lock, what is perfect or acceptable depends on ones preferance. Steering wheel size also makes a big differance and is directly related to steering effort/ratio as well.
I taped a piece of paper to the tube that became my pitman arm and mocked up a few different lengths and measured how far it would move and figured it out. It's 6" center to center with a mopar box and has seems to have plenty of steering. I'll know when I drive it.
Thanks for all the helpful reply's! If I do decide to mount the box dow low, sticking out under the frame, or through it, should I stick with the Mopar box, or is there a better choice?
Maybe I'm not understanding. Do you have a pic? This might be a solution to something I'm working on. Thanks in advance.
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