Professor Morris in the laboratory. Photo: Joe Bonnello?>
This Month's Lesson: From Saginaw to Rack and Pinion Units
Welcome to the 23nd class in the Dirt Sports University Series and the fourth on power steering. We finished up our third class on power steering with a look at the insides of the Saginaw steering gear box and the detail items that you need to think about to have it match your vehicle’s steering needs. Keep in mind that there are several other key components in the steering system such as the pump, reservoir, cooler and plumbing, along with a few others. All of the parts in the system need to be carefully considered for each vehicle application to have a properly matched steering system. We will cover these in detail now, and in upcoming classes.
In this class we will cover a few finishing details for the Saginaw power steering gear box and then move into rack and pinion steering gear systems. I realize that some of this subject matter can get pretty boring, but please bear with me in this class as many of the details we will be covering can save you from some poor choices of your own or those of others. Just think of how you can help your buddies with their projects, and get them pointed in the right direction. Having the steering system work to its optimum potential will, in fact, allow a vehicle to be pointed in the right direction. This assumes that the driver is capable to begin with, and that is a whole other class session by itself.
READING BETWEEN THE TIRES: Most of what we will be discussing with the Saginaw steering box is based on a front-engine vehicle. That being said, it will also work well for a mid-engine vehicle with precise steering needs. Most rear-engine buggy-type vehicles work well with a rack and pinion unit, as steering needs are not as precise due to the usual rear-engine weight bias of the vehicle that tends to make it like throwing an arrow backward. The rear of a buggy is normally trying to step out of line, and this gets amplified with poor trailing arm geometry that tends to rear steer the buggy. So, at least take the time to investigate what your particular vehicle’s steering is at the front tires, and do not take what the guy who sold it to you says as the final answer. Educate yourself and make use of your newfound knowledge.
Steering racks come in all sizes to fit a wide variety of vehicles and needs.?>
CHOICES IN STEERING: There are many off-road prerunners and race vehicles still out there that use a Saginaw steering box and work very effectively. Unless it is really necessary from all of the parts being shot or the setup was junk to begin with, you don’t need to rip out a Saginaw steering system and replace it with a rack and pinion unit. The Saginaw system still has a little advantage here and there over rack and pinion units, even the latest rack models. We discussed the internals of the Saginaw box in depth, along with suppliers, in the last class.
The last ten years have brought an onslaught of companies building a wide assortment of rack and pinion steering gear systems. You can get real quality-made units in sizes to fit everything from a small play car all the way up to Trophy-Trucks, but you can also end up with a less than desirable piece. At this point in time the big players in the upper end of the off-road racing performance steering market start with Howe Performance. Howe has been doing Saginaw boxes and a multitude of custom power assist rack and pinion units -- each available as a complete, totally matched steering component system -- longer than just about anyone else.
Next up is ProAm with rack and pinion units, and they have been developing other components to go with them. There are a few other suppliers, but most do steering as an add-on product to their main business and do not completely specialize in it. Stick to the companies that make their main business supplying proven solutions to all of your power steering system needs.
PACKAGING THE STEERING BOX: The advantage of using the Saginaw box over a rack and pinion system is packaging. Sometimes there is an advantage in packaging the steering box with the Pitman arm on the left side of the vehicle frame where the steering column can get to the steering box input shaft fairly easily. You then fit the right side of the frame with an idler arm and connect them together with a relay rod linkage system, also referred to as a swing set. This allows the engine to sit right on top of the steering linkage without having to have access to the center of the vehicle with the steering column like you would need to if you were using a rack and pinion unit. The relay rod assembly then has the tie rods connected from it out to the steering arms at the front spindles.
Another packaging advantage of the Saginaw box is that the hydraulic servo valve is built into the steering box and does not need to be a separate component, as is the normal case when using a rack and pinion gear system. This simplifies the steering column shaft and usually eliminates a couple of steering U-joints and connection points that can work loose.
The Saginaw box also has less steering kickback to the driver’s hands due to the internal reciprocating ball setup that acts like a worm gear, while also serving as part of the internal gear ratio reduction to the gear teeth on the sector shaft. The Pitman arm attaches to tapered splines on the end of the sector shaft outside on the bottom of the steering box. This is not like the direct-acting rack and pinion system that relies solely on the steering ram to stop feedback to the driver’s hands.
A Howe buggy rack with the servo attached.?>
THE SWING SET: This is where the magic happens, so I guess that it needs to be explained before we go much further. The swing set name is used because the sweeping motion of the inner tie rod ends from left to right in the vehicle, while steering the tires, very closely mimics a child’s swing set in the motion of the seat hanging from the chains as it is propelled by a child. This is very much the same as an old standup grandfather clock where the pendulum arm and weight swing in a similar arc.
When this arc of the steering relay rod or swing set system is engineered and plotted correctly to match the front A-arm pivot geometry, you will be rewarded with a steering system that will have very minimal bump steer through the entire wheel travel range as well as while the wheels turn full left and right. You will also have an added bonus of positive Ackerman, which has the inside wheel turning a tighter radius than the outer wheel, just like it should for proper racecar steering geometry.
The swing set system is only really practical to use with a Saginaw steering gear box system. There have been a few souls who have used a rack and pinion unit to drive a swing set linkage system, but this is very rare due to some of the complications of packaging the two together in the same tight space.
I only know of one race program that ever built the advantages of a swing set into the actual steering rack itself. These were a few of the PPI/Toyota factory race trucks where cubic dollars were often lit fire to for the sake of having the best available system. “Available” only meant that it had to be designed and machined from scratch from nothing but chunks of metal. These one-off steering racks had the pinion input shaft come in from the top for ease of access in the vehicle, and incorporated internal opposing diagonal moving mount points to which the inner tie rods attached. As long as you have lots of time, money and patience, this works okay. Otherwise, stick to what you can buy off the shelf.
Easily replaceable service parts help cut costs and keep a rack fresh for use.?>
PACKAGING THE RACK AND PINION UNIT: For many play or smaller engine race buggy applications, a rack and pinion unit fitted with the smaller servo valve assembly attached directly to the rack can be used. This can simplify fitment into your vehicle and, while I prefer using the larger servo for larger vehicles, a smaller buggy can work very well with the small servo system. Once you move into a larger buggy with a V8 and so on, you will need more hydraulic oil flow to keep up with the vehicle speeds and quick steering needs. This will take you away from the use of the small servo’s oil flow capacity and move you into needing the larger servo valve that is also used in the Saginaw box. We covered the details of the servo spool valve assembly in the last class.
There are many sizes and steering travels of rack and pinion units available to match what your vehicle’s needs are. This is not the time to skimp on dollars by trying to get away with using a smaller rack than you really need. Not only does the steering travel of the rack come into play, but also the physical strength of the parts and the diameter of the power assist ram. Spend the money on the longer-travel rack instead of trying to cheap out and use short steering arms on the spindles to get the high steering angles you need. It makes no sense not to get as much steering travel as you possibly can with your vehicle. A minimum of 35 degrees each way from straight is where you need to start. Any less and you will be making three-point turns in tight corners. Even years ago, Ivan Stewart had 45 degrees each way from center in PPI 015. I am not saying that you need that much yourself, but Ivan never had to take a couple of moves to get around the tight corners such as the ones on Baja’s La Rumerosa switchback-laden mountain.
A custom inner tie rod link for a rack and pinion.?>
RACK AND PINION STEERING LINK: Most off-road race and high-end prerunner buggies and trucks built in recent years have used a commonly available rack and pinion steering system. Much of this is due to the ease of availability and the simplicity of mounting one all-inclusive rack and pinion steering unit with a few mounts into the frame. This all works well as long as you have considered what front suspension geometry and A-arm pivot points will best match the somewhat limited steering geometry of a rack and pinion unit. All that you have available to direct the front tires properly is a simple left to right motion of the inner tie rod ends.
What the rack and pinion steering geometry lacks is the ability to limit toe change with the front wheels turned as they move through all of the wheel travel. You can pretty much forget about proper steering Ackerman also. This is the price you pay for using a simple rack and pinion unit.
Let’s assume that you have had a proper job done of positioning the rack and pinion unit in the frame, and by doing such along with carefully positioning the inner tie rod ends onto the steering rack, you should have less than an inch of bump steer throughout the entire wheel travel with the tires aiming straight ahead. If you are not sure about this, then check it out yourself or, better yet, find someone who can help you do this correctly. Chances are that you have two to five inches of total bump steer through the majority of the front wheel travel. Yes, this is too much, and you will be chasing the steering wheel all around in an effort to keep going where you are trying to point your vehicle. You are just wearing yourself out for no real reason. In many cases, moving the entire rack is not needed and you can redo the inner tie rod link to move the inner tie rod mounting point a bit to correct the issue.
These rack parts lacked proper greasing, greatly increasing wear and damage.?>
COST-EFECTIVE REPLACEMENT PARTS: As in any components that you are going to purchase for your vehicle, the steering system is what keeps you aimed where you point the vehicle when driving it, so do your homework first and be thorough about it. All the horsepower and big wheel travel in the world are nothing if you cannot steer your vehicle in a safe and reliable manner. Think about function, reliability and rebuilding costs. There are some real sexy looking rack and pinion units out there, but look a little deeper as to what component does what, and if a normal wear item is simple to replace or is part of a much larger component of the system that is very costly to replace during a rebuild.
RACK AND PINION SERVICE: You are going to need to service the steering system on a regular basis, and a rack and pinion unit is going to be something that is taken apart, cleaned and serviced after every race. That’s if you want to be a winner; or would you rather settle for just being a contestant? If used in a prerunner or play car, you should grease the rack whenever it even starts to look dry on the outside. This could mean complete greasing every day or so of use. If you wash the vehicle, immediately afterward grab a grease gun and pump away at the rack Zerk fittings until fresh grease is drooling out and flushing any water and dirt out of the insides of the rack and pinion unit.
Major teardown servicing is important to rack and pinion unit life because it is just a big, greasy dirt-ingesting hog. Think of greasy aluminum and steel parts sliding against each other, with dirt acting as a big grinder trying to wear all of the parts out before their time. You need to keep the rack well greased at all times for lubrication, and that nice slimy grease collects dirt that gets sucked into the unit each time you turn the steering wheel. Do not ignore this reality, and the importance of regular service.
I have seen a few clever people adapt rubber boots or leather wraps to the rack to help keep dirt and water out. Anything you can do in this area, as well as any shielding you can think up, will pay off big in the end with reduced maintenance costs.
IN CLOSING: Other than brakes to try to slow you down from an otherwise totally out of control situation, the steering system is what ultimately keeps you in control of your vehicle. How good or bad your suspension works or how fast you can shift gears to propel your vehicle with all that horsepower means virtually nothing if you cannot steer away from the edge of the road that you are about to slide off.
Now is the time to get your steering geometry figured out. Off-road vehicles are not getting any slower and, in fact, are constantly in development to make them go even faster through increasingly nastier terrain. If you haven’t been smoked and left behind yet, you soon will be. Don’t look at your vehicle and say that this class is not about you. Dig in and do your homework to be sure that your hard-earned dollars are steering you in the right direction, and if not, take the time and effort to correct it.
Remember that 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 that you are going to undertake.
HOMEWORK: See what your off-road vehicle steering system looks like and if corrective actions are needed. If you are planning a new vehicle build or a complete redo of your existing system, then do your homework as we have described during class. If you do not have a subscription to Dirt Sports yet, then step up and get one so that you don’t get left behind in class. You can also get back issues as well as electronic subscriptions.
Class Dismissed. Professor Tom Morris University and School of Hard Knocks