Professor Morris in the laboratory. Photo: Joe Bonnello
Author: Professor Tommy Morris
This Month's Lesson: Power Steering Basics
Welcome to the 21st class in the Dirt Sports University Series. We have finished our first class on power steering with an introduction and some background history on our new subject. Moving forward we will examine all the various components involved in the steering system in detail and choices that you will need to make to have your system match your vehicle’s needs. In this class we will focus on a basic Saginaw steering gear box system installed in many prerunner trucks and what it takes to get the mechanical portion to work. There have been a couple of other brands through the years, but the Saginaw box is what you want to work with if you are not going to use rack-and-pinion steering. The other components necessary to power and support the steering gear box or rack-and-pinion steering are very similar. We will get into them, as well as rack-and-pinion systems, in future classes.
WHAT YOU SHOULD KNOW: We have all become very spoiled with the nice power steering systems that come in regular everyday street vehicles that you can buy right off the showroom floor. This is from no small amount of work from way more engineers back at the factory than any of us could imagine. These guys will take months, if not years, designing what we use on just one vehicle. Now take into account that a great many of these brilliant engineers are just out of college and are pretty much clueless when it comes to what we off roaders will do to our vehicles. This is where the problems start. You cannot expect your street-based steering system, which can be steered with a fingertip down the highway, to take the punishment of a serious off-road prerunner or race vehicle. Now think of the next jump needed to survive in a true off-road racecar, and it becomes very obvious that a steering system specialist for off-road will need to be contacted. Otherwise you will just be wasting time and money.
EARLY SAGINAW GEAR BOX: The Saginaw steering gear box has been around since the earlier days of off-road, and is still very useful today in the right application. The fitment on the frame can become an issue for you, as the earlier steering boxes that mounted on the outside of the frame have become scarce these days. Many of these versions started life on 1970s and later Chevy and GMC trucks in half-ton to one-ton models. By the mid-1980s these had gone away, replaced by a smaller version mounted inside the frame. However, finding good usable cores to build a quality part from can be an issue in the long run. These outside-the-frame mounting gear boxes used a sturdy four-bolt mounting pad along with a larger internal ram than the later units that were mounted inside the frame. The early servo valve that controls the hydraulic power assist also had some tuning advantages over the later version, but again these are becoming hard to find now. The 1970s Chevy vans had a Saginaw box that used a long sector shaft to package on the frame as well as fit the steering Pitman arm where it needed to be under the unibody van frame. These long sector shafts were used in off-road with a modified truck box to fit the unique Ford Twin I-Beam steering needs.
Three Herbst prerunners that use Saginaw boxes during prep.
LATE SAGINAW GEAR BOX: Starting in the mid-1980s, GM came out with the new body and frame style GMT400 half-ton through one-ton trucks. These were fitted with an inside-the-frame mounted Saginaw steering box, and the steering ram diameter was reduced to three inches. As I remember, some of these new boxes only came with three mounting tabs and were very susceptible to warping the inside of the steering ram cylinder during off-road use. Be sure that you use a box that has four good mounting tabs on it. If you are working with this later inside-the-frame-mounting steering box, or any of the various year versions for that matter, be sure to consult one of the leading off-road steering supply companies on the best path for your application. I would tend to stay away from companies that primarily do pavement racing, as their needs and reliability issues, from high-impact loads and large diameter tires, are not the same as the off-road environment. Tom Lee Manufacturing was the early pioneer in this area. One of the leaders in the field of off-road for Saginaw steering gear boxes now has become Howe Performance. There are several other companies that make parts, pieces and systems that are good also, but Jeff Howe has really dug in and provided solutions to the industry’s needs as vehicle speeds across the desert have become seemingly almost supersonic.
Herbst's Chevy prerunner with Ford's Twin I-Beam suspension.
ODD STEERING COMBINATIONS: To get started with a good example of how to adapt creative ideas, we will look at the steering system on the Terrible Herbst prerunners that were built close to 20 years ago. These were three identical trucks built at the same time, one for each of the three Herbst brothers (Ed, Tim and Troy). They were referred to by some as the Nina, Pinta and Santa Marina (from the Christopher Columbus days) as seeming like three identical ships exploring the desert. By having three the same, one for each brother, there was no arguing who was going to drive and where they were going. Leading-edge technology for that time, they are still great vehicles that even today can run with the latest creations. All three have been very reliable vehicles, considering how many miles are on them. Mike Smith dreamed up the prerunners for the Herbst team, and wanted to mix simple technology with what was state of the art at the time. He used a Chevy cab, bodywork, engine and transmission. Then he mated a Ford Twin I-Beam front suspension that was custom fabricated and made to fit the frame to provide a simple, reliable means of a long-travel suspension. While the Ford Twin I-Beam design does not provide much for accurate wheel paths in camber angle, it makes up for it in raw wheel travel.
MIXING THE RIGHT COMBINATION: Take a look at the front suspension picture and notice how long the steering arms are that mount to the front spindles, located just behind the brake rotors. The steering arms are nice and long so that the leverage put to the steering arm and its related hardware by the massive 39-inch BFGoodrich race tires pounding through the abusive desert is minimized. This is where many teams have made the mistake of using too short a steering arm when planning out the steering system combination. The forces put into the steering system are huge, but you really don’t feel it at the steering wheel if all of the hydraulic power-assisted steering system is working properly. The greater the mechanical leverage ratio of the steering system versus the tires, the easier it is to use lighter weight parts that will survive. There will be inherent steering travel limitations, such as the tires hitting the frame or other limiting factors, that you cannot fix without major work. Once you know the total steering angle available at the tire, then you work backward through the entire steering linkages, all the way to the steering wheel and the steering input you want to have in feel and the number of turns the steering wheel will make from lock to lock. While this might sound simple, it is a very delicate balance of mechanical components pushed around by lots of hydraulic pressure and power. Proper planning will allow you to get the steering travel that you need to make tight corners, while minimizing the forces put back through the steering system components. Each part of the steering has its strengths and weaknesses that need to be taken into account for the entire system to function properly and reliably. If your current system falls more to the unreliable side, then you need to work backward to see where you missed the proper combination.
Saginaw steering parts combination.
MAKING IT FUNCTION: These well-used and reliable Herbst trucks were planned very smartly for the components used. The early style Saginaw steering box mounted on the outside of the frame uses a long sector shaft to get the Pitman arm under the frame to connect to the relay rod that then connects to the swing set assembly (the big red arm). Note that the Saginaw box is fitted with extra AN ports on the bottom of the servo valve area, and has hoses to power an auxiliary steering ram mounted between the relay rod on the swing set and the vehicle frame. When you look at the steering shaft coming down from the cab you will see a four-inch diameter aluminum round planetary gear box, which is called a Speeder Box. It can be used either to speed up or slow down the steering input from the steering wheel to the steering box. Depending on your needs, this Speeder Box can be mounted anywhere along the steering shaft that allows easy fitment. In this vehicle’s case, the Speeder Box is being used to speed up the steering input from the steering wheel to the Saginaw steering box. By doing this you reduce the number of turns of the steering wheel it takes by the driver to keep the vehicle aimed in the direction that you want. In some cases this will make the vehicle very twitchy to steer, and you will need to work with a steering specialist to tune the steering servo torsion bar diameter and barrel valve assembly.
Pitman arm connecting to the swing set.
PITMAN ARM CONNECTION: The Pitman arm serves the function of connecting the steering box to the relay rod. While this might seem like a simple task, there is more to it. The Pitman arm length also works as a function of the total effective steering system ratio. So you need to figure in the length along with the Speeder Box, steering gear box, swing set and steering arms. This is not a simple task. The Pitman arm locates to the steering box sector shaft on a spline that is tapered, so the installation procedure, along with removal of the Pitman arm from the steering box, does have its own special details to keep from damaging parts while performing correctly. There are also other concerns when designing what your Pitman arm is going to look like. It must have as much of a simple push and pull operation as it can. Avoid any sort of dogleg or offset arm that will load the sector shaft up and down in the steering box. The steering box is designed for rotating loads on the sector shaft, not up and down. Look at the picture of the Pitman arm along with the short drag link that connects the arm to the relay rod portion of the red swing set (left of page). This design locates the Pitman arm loads correctly. You can also see from these pictures how it is important to get the arc of the travel of the Pitman arm correctly matched with the relay rod so that the ratio and travel of the Pitman arm matches what the steering system needs in a matched left to right travel from steering straight ahead. Confused? The Pitman arm moves 90 degrees, so with the steering gear box centered in its travel, you want to have the same front wheel angle steering from left to right from center. If you are not getting the same angle both ways from center, then look to see where your problem is and correct it if possible.
AUXILIARY STEERING RAM: With just about every hardcore off-road vehicle that uses a steering gear box, you will need an auxiliary steering ram in the system to reduce the loads from the steering box’s internal hydraulic ram as well as give the desired steering performance. This ram will need to be securely mounted to the frame on the ram side and then attached to the relay rod steering assembly at the shaft end to do its best job. If you look around the off-road industry you will see many ways that this extra ram has been incorporated into the steering system. Whichever way you go with in layout, be sure to remember that it will see very high loads, so the mounting method needs to be strong and there must be no binding of the mounting ends during the entire steering travel. The high-pressure hydraulic lines need to be treated with respect just like you should with the high-pressure line that comes from the power steering pump to the steering box. They can see upward of 2,000 lbs. of pressure and 10 gallons per minute of flow. This is nothing to fool with, and if one line fails, the steering system goes down.
IN CLOSING: The Saginaw steering box system is a great way to go if this design fits your vehicle and needs. By incorporating the relay rod and swing set system you can do a nice job of minimizing the bump steer while providing positive Ackerman in your steering system. For many, the current trend is to use rack and pinion steering to simplify life while providing great steering. Remember, at the end of the day, you and you alone are the one in control and responsible for your choices. Ideas and thoughts of any kind that we may put in your head are your responsibility to investigate properly how you will apply and use them safely. Be responsible and make your decisions wisely on any project you are going to undertake.
HOMEWORK: Put a list together of where you are with a steering system. What does your off-road vehicle have for steering? What issues are you having, or what can make it work better? If you are planning a new vehicle for yourself, what do you want to end up with? This will help you as we move forward in the next classes. If you have missed some of our past classes, make the effort to get a hold of what you missed. Remember that knowledge transforms itself into safety and speed that you need.
Class Dismissed. Professor Tom Morris Dirt Sports University and School of Hard Knocks