This posting is one in a series. If you have not yet read the earlier postings in the series, you’ll need to, as this posting won’t make much sense without having done so! Just do a search using the word “SLURP”.
In my last posting, I described Red’s high tech exhaust manifolds, and promised a steering posting that would be “over the top”.
As it turns out, the steering posting is truly “over the top” in terms of detail, but also has come to include a fellow rodder who is fighting a health battle. His story is simultaneously both sad but inspiring. In addition, the steering story turns out to have great implications for anyone contemplating lowering his or her vehicle – There are BIG potential pitfalls. Finally, I have to warn you: this posting is a bit technical and mathematical at times. I could have purged some of the technical and math detail, but after my own experience, I figured others could benefit from at least a modest level of detail. So, that said, here we go.
Red’s factory steering is deceptively simple in appearance. Take a look at this photo as you read the following text:
The photo unfortunately already has the factory steering box and Pitman arm removed, and the new Flaming River steering box mocked up into place, but the basics are here to see. The driver’s side front wheel has a curved steering arm attached to it. That is partially visible in the bottom of the photo. That steering arm connects to a factory drag link, which goes rearward to the bottom of a vertical factory Pitman arm, and is connected to the Pitman arm via a tie rod end. The Pitman arm’s upper end is splined to fit onto the OUPUT shaft of the factory steering box. The INPUT shaft of the factory steering box is the steering shaft from the steering wheel of the truck.
Here’s how the factory system works:
The driver rotates the steering wheel. The factory steering box uses a recirculating ball mechanism (for low friction and low steering effort), and a 26.24 to 1 mechanical ratio to convert lots of steering wheel rotation (well over 4 360 degree turns of the steering wheel) into a maximum of just 60 degrees of output shaft rotation. The high mechanical ratio makes power steering unnecessary. The output shaft is also at 90 degrees to the steering coluimn (i.e. the output shaft runs “across” the truck).
The upper end of the Pitman arm is splined onto the output shaft of the steering box, and normally sits approximately (NOT precisely) vertical. So, when that output shaft rotates, it rotates the UPPER end of the Pitman arm, and that causes the LOWER end of the rigid Pitman arm to move forward or rearward. This forward or rearward movement of the bottom end of the Pitman arm is what initiates the turning of the front wheels. Remember that later in this posting!!
When the lower end of the Pitman arm moves forward or rearward, it also of course moves the drag link forward or rearward. When it does so, the FRONT end of the draglink moves the steering arm that is fastened to the wheel and tire assembly, and that turns the wheel and tire assembly left or right.
Simple, right? Well, actually, NO.
If you do some thinking and sketching, you will realize that in this collection of steering wheel, steering column, steering box, Pitman arm, drag link, and steering arm, every single piece’s dimensions and angles have a VERY pronounced effect upon what actually happens when the driver turns the steering wheel. It turns out that any change needs to be very carefully analyzed. That INCLUDES not only changes to what we recognize as “steering”, but also changes to ride height. When you lower an AD truck (and this applies to varying extents to almost ANY vehicle, in ways that vary by type of steering and suspension used), you can very adversely affect its steering control.
Randy and I are both very aware of this, and have tried very hard to keep any required changes as modest as we can. We STILL have issues though.
The reason we need to alter the steering at all is simply because the factory steering system was designed for use with a narrow inline six cylinder engine (Chevy had no V8s yet at the time). That allowed the factory engineers to run the steering shaft straight from the steering wheel right into a steering box that was bolted to the driver’s side frame rail – and it was bolted into BOTH the top and outside surfaces of that frame rail. A factory Pitman arm with just a slight offset bend in it then got the bottom of the Pitman arm right where it needed to be to connect to the drag link.
This simple was simple, strong, cheap, and effective. Unfortunately, once you put a V8 into the chassis, the exhaust manifolds, no matter HOW tight to the block, sit where that factory steering box did.
You cannot simply slide the steering box “outward”, for reasons of its large size and its mounting arrangement which needs mount bolts on both the top and side of the surface to which it is mounted. So, we needed a different steering box that was small, mounts only to ONE surface, has mechanical leverage similar to the factory one (so we can stay with manual steering), and has an output shaft that comes out of it in roughly the same position and angle as the factory one.
The Flaming River steering box recommended to me meets these requirements. It can be mounted simply with 3 bolts, one of which will bolt through the factory frame rail, and two of which will bolt through an angle iron extension piece that will be itself bolted to the frame. That angle iron piece is evident in both the above photo and in this “action sht” that shows Randy cutting it (I couldn’t resist taking this twilight photo!):
But, the only Pitman arm that Flaming River makes for it has a center to center hole spacing (splined hole to tie rod taper hole) of only about 5 inches. This is WAY shorter than the factory AD Pitman arm center to center distance of 9.5 inches.
Ok, so why not simply lift the rear end of the drag link to connect to the shorter Flaming River Pitman arm? Because that would change the ANGLE of the drag link relative to the ground, and that change would introduce massive amounts of what is called “bump steer”.
“Bump steer” is defined as unintended steering action that occurs as a result of suspension movement.
It’s exact mechanics vary, as mentioned earlier, by exact type of steering and suspension used on a vehicle, and every vehicle designer works hard to eliminate it or at least minimize it. You can understand why: if a bump in the road turns the wheels left or right without warning or driver intent, the results are going to be at a minimum exciting and at a maximum, both damaging and deadly.
On the AD trucks, they key to understanding how bump steer is minimized (not eliminated) is to understand what happens to that drag link as the suspension cycles upward or downward from normal loaded ride height.
Imagine for a moment that the drag link is horizontal when the vehicle is traveling at normal ride height on the highway. The normal maximum foreseeable suspension compression on a hard bump on these trucks is about 4 inches. When that front axle deflects upward the 4 inches, that creates a problem: the drag link is no longer horizontal - the front end of it is deflected upward 4 inches. Since the drag link is only about 12.25 inches in length, that creates an angle from the horizontal of about 19 degrees!
Now here we need to use that nasty high school and college trigonometry. If you remember and use your cosine and sin functions, you will eventually discover that the 19 degrees of angular movement makes the drag link about 0.67 inch too short when the front axle hits 4 inches of deflection. Since the drag link is steel and cannot change its length, either the front wheel and tire assembly needs to turn abruptly, or the steering wheel needs to turn abruptly. Since the driver is holding the steering wheel, and has a mechanical leverage ratio of 2.624 to 1, it is the front wheels that turn, and the truck suddenly turns when the driver has no intention of turning! This is clearly bad.
But, fortunately, the GM engineers who designed this steering system were pretty smart, and they did NOT place the drag link so that it is horizontal at normal ride height. They placed it so that the rear end of it is about 2 inches higher than its front end. The benefit of this is again related to trigonometry, but to spare the non-mathematicians the gory details, NOW when the front axle deflects upward on the bump, the total bump steer is only 0.16 inch, versus the original 0.67 inch. This is because the cosine of an angle varies much less at small angles than at larger ones, and because the engineers arranged things so the drag link is actually going from a smaller negative angle (-9.4 degrees) through horizontal, and then upward through a smaller positive angle (+9.4 degrees). Very clever.
If you foolishly substitute the Flaming River 5 inch Pitman arm, you would get bump steer effect of 1.3 inches – surely enough to put you into either the weeds or the oncoming taffic.
Soooo, it is vital that the drag link stay at an orientation that puts the rear end of it about 2 inches higher than the front end of it!
This means that not only STEERING changes, but also RIDE HEIGHT changes, can very adversely affect the safety of an AD truck, if not done correctly. “Correctly” means that compensating changes are made to the Pitman arm or steering arm, or both, to keep the drag link at or near the factory angle.
I’m afraid it gets more complicated.
It turns out that the Flaming River box has four other key differences from the factory box, two of which are negative for us, and one that is an advantage for us:
1. The output shaft of the flaming River box is located about 1 inch lower than the factory box, relative to the frame rail. This means that the center to center length of the factory Pitman arm is 1 inch too long
2. The Flaming River output shaft uses a nominal 13/16” spline diameter versus the nominal 1” spline diameter of the factory box, so the factory Pitman arm cannot possibly be used on the Flaming River output shaft
3. The output shaft of the Flaming River box is not exactly horizontal. It actually tilts downward 9 degrees as it comes out of the box towards the driver’s side fender. This is a plus, because it means a straight Pitman arm with NO bends in it, will “tuck inward” as it goes down to meet the drag link, and miraculously, it aligns wonderfully with the tie rod end on the drag link, making it unnecessary to forge a bend into any replacement Pitman arm – a big plus since forging a RELIABLE (rigid) bend into a Pitman arm would be an epic undertaking.
4. The internal multiplication ratio within the Flaming River box is 20 to 1, versus the 26.24 to 1 of the factory box. This has the effect of making the steering “harder”. However, the Flaming River box is also made with very superior quality features, including needle bearings versus bushings, compared to the factory box. In addition, Red has aftermarket needle bearings in his front suspension/steering, and I also plan to replace the current front tires, which are MUCH wider than factory width, with slightly narrower than current ones, which will reduce the “scrub”, and thus the steering effort.
The combination of above items means that we need a Pitman arm with the following features:
- About 8.5 inches center to center
- Bottom end has a hole sized and tapered to fit the tie rod end on the drag link
- Top end has a 13/16” nominal diameter splined hole, with 32 regular splines and 4 master splines (this also is a TAPERED spline, by the way)
- The master splines must be “clocked” 26 degrees so that the Pitman arm rests at a certain angle (4 degrees) when the steering box is “centered”.
This last point bears some explanation. There are two separate reasons why the “clocking” of the Pitman arm relative to the output shaft of the steering box is critical. One is that the steering box is manufactured in a manner that creates the “tightest” steering at the center of its range of rotation. That tightness is key to highway stability without constant correction. The second reason is that the clocking of the Pitman arm relative to the steering box basically establishes how far forward or rearward the bottom of the Pitman arm is where it connects to the tie rod end on the drag link. If that point is too far forward or too far rearward, it is possible for the Pitman arm to reach too great an angle at either extreme of steering range (too acute or too obtuse), causing VERY high steering effort, or even binding up of the steering. This would clearly be undesirable or even disastrous.
This is why all steering box to Pitman arm interfaces use some form of “registration” (clocking) to ENSURE that the Pitman arm is mounted SAFELY. On the AD trucks, and many other vehicles, the registration (clocking) system used is that the 36 spline interface has 32 regular splines and 4 “master” splines spaced 90 degrees apart, where the master splines are very much wider than the regular splines. This ENSURES that the Pitman arm is mounted at the correct clocking angle.
In addition to the 4 features shown above, Red’s new Pitman arm must be made of a material suitable for the purpose. Pitman arms undergo considerable loadings. The material used must have the right properties to stay rigid, to not strip its splines under load, and to not fracture or bend under the shock loadings that might be encountered in a steering system coupled to a working vehicle suspension.
One approach that I considered, and that was suggested by many, was simply to use the TOP of the available Flaming River Pitman arm, and the bottom of the factory Pitman arm, and TIG weld them together to make a new Pitman arm of the correct length. While this can theoretically work quite well, since TIG welding is a VERY elegant and effective process, it requires a TIG Welder of great skill, and the means to x-ray the resulting weld, since the loads on a Pitman arm are so great. I suspect that the required skill and x-ray capabilities are available somewhere, but they are not obvious, and a better solution came up before I pursued this welding approach anyway.
I had barely begun the process of trying to locate a suitable new Pitman arm for Red, when I happened to respond to an ad for a beautiful vintage Packard car (I am looking for a second vehicle that I can use whenever Red is tied up getting improvements, like he currently is). “Mitch”, the proud owner of that daily driver (!) Packard, happens to be both a rodder AND a very skilled machinist who has worked over 35 years in precision machining and tool and die work. Mitch was also a race car team member for his employer’s race team for a number of years, doing maintenance, repair, and construction of race cars and their components. The combination of skills, experiences, and interests makes Mitch a very unusually capable and valuable resource for any hot rodder making changes to key vehicle mechanical and safety systems. Mitch knows all about materials, machining processes, and vehicle dynamics, and so can provide KNOWLEDGEABLE advice and services to people like us.
Mitch has had a run of pretty bad luck lately. He went through a bypass operation some time ago, and for the past 4 years has also been dealing with a condition known as “CIDP”, a “lighter” form of muscular dystrophy. This is a condition that attacks the peripherals of the body versus the core, and required Mitch to undergo a form of chemo therapy for quite a while. His medical problems forced his early retirement from his work about a year ago, and necessitated many other changes in his life.
In addition, Mitch’s wife was the victim of a car accident that left her with significant spinal injuries, so both Mitch and his wife are unable to work, and are fighting a very brave and vigorous battle that obviously extends far beyond the medical issues now due to the inability to work in the careers they had each been pursuing.
Mitch has a very nicely equipped personal machine shop right behind his home, and despite his medical condition he takes on free lance work from former professional clients and from new customers. He is only able to actually work limited hours and days because of his condition, but I can see he is putting up one TOUGH and admirable fight to win what is clearly a very difficult battle. And I could hear the pride in his voice as he described to me, and then to me and Randy both on a subsequent visit, the capabilities that his expertise and experience, coupled with his carefully chosen shop equipment, enable him to bring to bear on machining projects like mine.
What really impressed me is that as I began to describe what I need, Mitch was able to finish the sentences for me, and to educate me on applicable DOT regulations and material strength issues. I KNEW I had found the right man to build a custom Pitman arm that would be effective and safe.
Mitch specified 4140 steel for the application, and he will actually cut the arm from a billet, and make both holes manually, including cutting the splines individually. He has all the equipment to do it RIGHT, including an indexing table that will enable exact angular clocking. He has my Flaming River box and my drag link, to take actual measurements off, and for trial fitting. I am thrilled that I have someone who is that good working on it, and am equally thrilled that I might be able to help Mitch build that part-time machining work volume by introducing his capabilities to other rodders. I should also mention that Mitch’s hourly rates are very reasonable. I’d be happy to provide contact info for anyone who thinks they might want to hire Mitch to do some work for them.
Mitch received the material we ordered late last week, and will build the Pitman arm within the next few days (he is limited in the amount of time he can work, remember). I am going to San Diego on business on Tuesday, and will return midday Friday, so hopefully Randy and I can install the steering box and Pitman arm Friday or Saturday, right after installing the composite fiberglass leafsprings, and check to ensure we have all the right geometry. If this all works as we hope it does, Mitch may find himself producing Pitman arms for other AD truck owners who want a V8 but want to keep the factory beam axle, and get a nicer steering box in the process.
This whole steering episode has delayed our work significantly, and has taught me a lot so far:
- There is nothing “simple” about apparently simple steering and suspension systems
- There is nothing simple about an apparently simple engine swap, despite what the suppliers may have told you
- Changing EITHER steering components or ride height can have profound effects on the safety of a vehicle, and anyone contemplating doing either, should ensure that they have sound and experienced advice and assistance
- Rodding is at its core a people relationship-based hobby, where the assistance, advice, and cautions of other rodders with experience different than your own can save you a lot of time, work, and heartache. We can try to pretend we are each fiercely independent and can “do it all, and do it all my way”, but if we do, we shortchange ourselves in so MANY ways. My life has been enriched by meeting Mitch. He, his skills and experiences, his current life circumstances battle, his really neat Packard (straight 8 engine, 4.10 gearing, overdrive, etc!), and his daily use of the Packard (it’s his ONLY car!) have inspired me. I responded to an ad for a car for sale, but have ended up with so much more than only the possibility of buying that car: an education on the aerospace machining industry and its pioneers, an important custom part being hand made for my truck by someone who knows what he is doing, and a glimpse into another rodder’s life.
Jim G
Linear Mode
