Ladder Bar Setup. Part one, center and square.
October 14, 2009 · Print This Article
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. For those of you with four links. . . work it out yourself. No, no, I promise, I will do four links too, but they are a little more involved, and in effect build on the ladder bar principles, so I will start with ladder bars, and do four links at a later date.
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 centre of the front cross-member I am confident is in the exact centre of my front wheel track, I will make a permanent mark, usually a centre 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 centre 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 centre. 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.