For the majority of racers out there a ladder bar will do everything they require. “But a four link is better”, I hear this all the time, and yes in some perspectives a four link is a much better drag racing rear suspension setup. The biggest advantage to a four link is it’s adjust-ability. This can be a great advantage. . . if you know how to adjust it. When I look around the track, I see very few people adjusting their four links. Generally what I see are four links that have been setup, either by someone who knows how to tune them, or after a certain amount of aggravation, or “that’s how I bought the car, and I am not going to touch it ’cause it is working just fine the way it is”., and because the car is going straight, and running consistent, the owner leaves it alone.
The problem with a four link is that very few people have spent the time to work out how to make adjustments. I am not going to get into how to adjust a four link here, but I will tell you that the procedure starts when the car is being built. The chassis builder should have given you a chart of instant centers. If you don’t have one, you need to get one. You can generate one yourself. There are computer programs you can buy that make it easier, or you can do it the old fashioned way on the floor. I have done both. I now have a computer program I find reasonably easy to use although it does not allow for as many holes as we are using in newer four link setups, so I have to run the program twice. Once you have your instant center chart, you then need some sort of preload reference. How much weight, does a quarter turn on the bar add, or subtract, form each corner. Of course this will change with a change in instant center, so if you change bar locations, you then need to know the preload adjustments for that location. The point I am trying to make here is that although a four link is a great rear suspension setup, the adjustment procedure is too involved many drag racers. They generally get it working, then never adjust it.
If that is going to be the case, why not use a ladder bar then? A ladder bar generally has three choices for instant center, and you can see them all on the car, you don’t need a chart. Preload can be set with a ruler if you want, although I do prefer to weigh the car so I have a reference point for preload adjustments. Often, unless it is a very high horsepower car, little or no preload is required with a ladder bar.
Ladder bars do take up a little more space forward in the chassis, if you are building a car with a lot of rearward driver position it can be a challenge to get a long enough bar in the car.
One of the cars I am currently building is a ladder bar setup. It is a very short wheelbase at 100.5 inches, so getting bars in there can be done. This one is interesting in that the owner wanted to use his rear axle housing which was already setup for a four link. It has a range of holes on the axle housing that could be used, and they are not in the usual location for a ladder bar. I had to get creative. One problem the typical ladder bar has is because of the angle of the two bars, as soon as you adjust the rear of the bar for pinion angle, the hole spacing changes, causing a side load on the rod ends, this is not good at all. It can also cause a certain amount of stress and cursing when trying to install the bars after making an adjustment.
For this car, I wanted to take advantage of the range of holes in the housing. I accomplished this by putting a pivot in the top tube, allowing the hole spacing at the rear to be adjustable. This solves two problems, first there is now no side loading on the rear rod ends after adjustment, and second, because I made the top bar adjustable, pinion angle can be changed very easily. Now that I have built these bars, I wouldn’t build them any other way. They are a little more expensive, but worth it. They offer a large range of adjust-ability, without the confusion of a four link. They are very strong, with large diameter thick wall 4130 tubing. And they can be built to suit any set of brackets you may already have.
Ok, everyone seems to want to know how to setup their ladder bars, so I will see if I can shed some light on the subject.
I have given a lot of thought to how to approach this. I have decided that I will try to keep away from the engineering and physics as much as possible, and try to make this a “practical” guide. This is aimed at what I would consider the average bracket racer, with basic understanding, and tools.
First, you need a mostly level surface. Ok, here it comes already, the ‘experts’ out there are going to tell me it has to be perfectly level. Yes, perfectly level would be nice, but tell me this; how level is the startline at your dragstrip? Do you even know? If you want ideal, you will set your car up on the startline, and the lane, of the track you are about to race on. That is most likely not possible, so we will settle with mostly level. Dragstrip startlines are generally pretty good, some much better than others. They usually have a very slight amount of camber to the outside for drainage, which is why you always start your burnout a little to the inside right? I will assume you are using your garage or workshop floor to work on here. To get mostly level, put the car in nose first, and centered in the doorway. The floor will usually have some drainage built into the floor so liquid will drain out. By nosing the car in, it puts the front end on the high side, more like a wheels up launch, and centering the car should have it pretty close left and right. What you want to do now, is make some marks on the floor, or take some measurements, so you can park the car in the same spot in the future to get the same results. You will want a reference of some sort.
Next, you do want some weight on the driver’s seat to simulate the driver. It does make a difference. You know how much you weigh, add that amount. Within twenty pounds is fine. Set your tire pressures to where you would normally run the car. Now you get to get dirty.
The very first step is to square the axle in the car. And see, it is already getting tricky. What will you square it to? Is the chassis square? No sense squaring the axle to the rear cross-member if the cross-member was installed out of square, how will you ever know? Let’s triangulate. No, it is not physics, it’s geometry.
Head up to the front end, at the front cross-member, between the front wheels, measure the distance between the front wheels, this can be inside to inside, or to straightedges held on the outside by your assistant, whatever is easiest, but be accurate, a sixteenth of an inch accurate. If you are measuring from the wheel or the tire spin the wheel an measure from a couple different points to make sure you are getting consistent measurements. Take that measurement, find the middle, the exact middle, and make a lark on your front cross-member at this distance. Sometimes I will use a plumb bob and drop points to the floor, or use a carpenter’s square. Once I have a mark in the center of the front cross-member I am confident is in the exact center of my front wheel track, I will make a permanent mark, usually a center punch mark, then it is always there. When I am building a car I mark it while it is still on the jig. Now you have the center of the front. Perfect.
Next. To use this procedure to square the rear axle, you need the rear axle to be centered in the car. The best bet is to center to the chassis, as the body may be a little offset. From under the car, measure from the inside of the tire or wheel, to the chassis rail where it goes up and over the rear axle. Sometimes it is way too tight to get in there, in that case you will have to pull the wheels off to get this measurement. So now you find your rear axle is half an inch offset one way or the other, not at all unusual. Sometimes it is less, sometimes more, but we can fix it. You will most likely have a panhard bar, or a wishbone setup to center the housing. Loosen the jam nuts and adjust the housing from side to side. A wishbone is a little more involved as you will have to remove the bar from the brackets to make the adjustments. Another note on the wishbone, if you wind one rod end in, wind the other one out the same number of turns to keep the width the same so you can get the bolts back in. (a little pet peeve of mine, the rod ends should be parallel). If any of your rod ends are difficult to turn, remove them from the car, clean the threads, inside and out, spray some good oil up inside the tube, (I use engine store fogging oil) to prevent corrosion, and grease the threads. I don’t use never seize, it is dirty and abrasive, I use good quality grease. Now next time you need to make an adjustment, there will be less cursing.
Back to squaring the axle in the car. Ok, now you have the rear axle centered between the rear chassis rails, and a center mark on the front cross-member. The next step is to choose a point on the ends of the rear axle to measure forward to the front cross-member center mark. You can use the front edge of the brake drum, or disc, the backing plate, or the backing plate flange. The important thing is to use the same spot on both sides. measure from your chosen point to the center mark on your front cross-member, (again you can use a plumb bob, or carpenter’s square to drop marks onto the floor if it makes things easier). The objective here is to have the dimension the same on both sides, right rear brake drum to front cross-member center the same distance as the left rear brake drum to front cross-member center. Again, be as accurate as possible, a sixteenth of an inch or better.
They’re not the same are they. That means one rear wheel is further forward than the other. Now it is time to look at your ladder bars and see if one is adjusted longer or shorter than the other. What we are concerned about here is the bottom of the bar, we will use the top to adjust pinion angle later, but for now we want the axle square. I generally start by removing the ladder bars, remove and inspect all the rod ends, and the bars themselves for any damage, rust, cracks etc. Lubricate everything, and assemble the bars with all adjustments in the middle of their range, (which is about six threads showing on the rod end). Before re-installing the bars I will adjust the bottoms to the same length, if the chassis is all square and true, the housing will be square. Back to your car, if you have adjustable ladder bars, set the adjusters, (which are normally on the bottom and rear of the bar) in the middle of their adjustment range, and adjust the front of the bar, the front rod end, to square the housing in the chassis, to make both dimensions from the rear brake drum to the front cross-member center mark equal. Ideally you want both front rod ends in or out of the tube the same amount, and preferably with about six threads showing, (that is above the jam nut). A couple threads either way is not too bad. If you are finding that you have to have much more difference than that, either your housing or chassis brackets are on crooked, or your chassis is crooked. I am going to assume that everything is pretty close.
Now you have a rear axle that is centered, and square in your car.
A brief overview:
Car mostly level.
Find the center of the car at the front.
Center the rear housing in the chassis.
Measure from either end of the rear housing to the front center mark to square the housing in the car.
See, this is easy.
Next we get to talk about pinion angle.
Magic is an interesting concept. If we look back through the ages at things which were not understood at the time, they were often referred to as magic. Pinion angle is a term I hear tossed about as some sort of magic incantation quite often. Let’s dispel the myth.
Pinion angle in and of itself does not affect the way a car launches. A change in pinion angle does change the angle of the four link or ladder bar brackets on the rear axle housing. So a change in pinion angle will change the angle of push and pull on your rear suspension components, but, and this is the important part, we do not want to use pinion angle as a rear suspension tuning aid.
What we do want to do with pinion angle is have the drive-line in the right relationship so the universal joints work correctly.
Universal joints; this is really cool. Picture a simple shaft, now add a single universal joint in the middle of it and bend it slightly. You can now rotate one end of the shaft, and the other end of the shaft can also rotate, even though it is, in effect, going around a corner. (that’s not the cool part) If you could rotate the input end of the shaft at a very accurate constant velocity, and if you could accurately measure the velocity of the output shaft as it rotates, you would find that the output shaft does not rotate at a constant velocity. In fact through one revolution it actually speeds up and slows down several times as each cross of the universal joint changes angle, even though it still completes one revolution in the same amount of time as the input shaft.
The amount of this acceleration and deceleration changes with the angle of the universal joint. (you can see where I am going with this already can’t you?) From there it is pretty simple to visualize what we might want from a driveshaft with two universal joints in it, we would want both joints to accelerate and decelerate the same amount, at the same time, if we do not, we end up with vibration. We will not always feel the vibration in a drag car that is accelerating, but it is tearing up universal joints as they fight each other. So, pretty simple, we want both universal joint angles to be the same. Mark Williams has a great drawing here to show what I mean.
Now comes the bit that seems to be magic, we want the universal joints to operate at the same angle. . . when the shaft is turning.
Generally if the car has universal joints, it has suspension, suspension allows movement, which will change the angle of the u-joints, so we need some sort of compromise. Logic says we would want our universal joints to be operating at the same angles when they are under the greatest load. Which in our case is when we launch the car. If you remember back to a previous installment, when we launch the car, the rotation of the pinion gear on the ring gear causes the rear axle housing to rotate around the axles in the opposite direction of the tires. Put simply the front of the axle housing wants to go up, the front where the pinion goes in. . . the pinion that is on the end of the driveshaft. . . after the universal joint. So we will want to take this movement into account when we are setting our pinion angle.
So now, when you are setting up your pinion angle on the floor of your shop, you want to add a little to the angle to compensate for the rotation of the rear housing when you accelerate, here it comes that number you have heard, two degrees down. Ok, two degrees is not always the “exactly” correct angle, but it is a good starting point. but this does not mean that your pinion is set to two degrees, what you want is two degrees more to compensate for rear axle housing rotation. Let’s refer to the Mark Williams drawing here. If Angle “B” is ten degrees, you would want angle “A”, at the pinion, to be pointed down a little to compensate for torque reaction during launch, (the rotation of the front of the axle housing upward), so angle “A” should be about eight degrees. Of course the number itself is dependent on where you measure the angle, but let’s put it simple, you want the pinion pointed down about two degrees more than the back of the transmission. Of course this is all at ride height with fuel load and driver weight in place.
I think one place where many go wrong with the whole pinion angle deal is they do not correct pinion angle after they adjust the rear suspension. Particularly with ladder bars, if you change the front mount location, you must correct your pinion angle.