DMSBD Tech Tips

Construction Tips for a First-Time Car Builder - by Ian Carsten
Article 3: Preparing the Rear Axle and Front Kingpin for Triangulation

In article 1, we inspected the parts and obtained replacements if necessary, prepared sawhorses, and sealed the floorboard. In article 2, we polished the spindles. Now we’re almost ready to begin construction. First we have to layout the location of our center punched dimples in the bottom surface of the rear axle and front kingpin for the triangulation procedure, which is used to set the rear axle 90 degrees to the centerline of the car.

We should think about how the axles will be oriented on the car. The kingpin holes must be vertical and we also want the AASBD logo and date stamp to be visible on top to facilitate inspection. Begin by placing both axles on a tabletop or workbench to orient them in this manner. When that is done, we should also look at the position of the kingpin hole across the width of each axle. It may be obviously offset to one side of the longitudinal centerline. If so, this is actually beneficial, particularly for a front axle. If this is the case, you should position the axle so the kingpin hole is closest to the front edge of the axle, as it will be mounted on the car. You should mark the front of the of the axle between the airfoil mounting holes with a felt marker, indicating this is the front axle, front surface, and draw an arrow pointing towards the top with the label “up”. These notations will be useful and cannot be seen once the airfoils are installed. The benefit of a kingpin offset to the front of the axle is that it creates a straight-tracking caster effect, making the front wheels slightly easier to keep pointed straight ahead. You should also position and identify the rear axle in the same manner.

Now, rotate the rear axle 90 degrees with the bottom surface facing towards you, and place one of the kingpins all the way through its hole from the top side. Sometimes there may be a few burrs left inside the hole from the drilling operation that prevents you from pushing the kingpin through. If that happens, you can remove the burrs by inserting a small round file and removing them with a few gentle strokes of the file. We are going to layout the position of the center punched dimples for the triangulation procedure. This isn’t the only way to do so, but it works well and is quite accurate. You will need the sharp pointed machinist’s scriber, the 12-inch (or 300 mm) blade from the combination square, and a felt tip ink marker.

Lay the square blade with its edge on the table, its flat side against the bottom surface of the axle, and its end against the kingpin. Uncap the ink marker, move the end of the blade nearest the spindle and ink the area of the axle square stock where the outboard end of the blade was.

Next, do the same thing to the other end of the axle. You are inking the bottom surface of the axle so the layout lines you are about to scribe will be easy to see. After the ink is dry, reposition the blade with its end firmly against the kingpin. Hold the blade in position with one hand, or use the two small c-clamps, to hold it to the axle while you scribe a line into the bottom surface of the axle using the end of the blade as a guide. Repeat this for the other end of the axle. Remove the kingpin and rotate the axle 90 degrees so the bottom faces up. Now you have two scribed lines exactly the same distance from the center of the kingpin hole and perpendicular to the centerline of the axle.

Some builders use the end of the square stock to reference the transverse scribed lines we have just made. But this works, if and only if, the kingpin hole is precisely halfway between the opposite ends of the square stock. Due to manufacturing variations, this is not always the case. However, our procedure guarantees our reference lines to be equidistant from the center of the kingpin hole, as they must be, even if the kingpin hole is not perfectly centered lengthwise along the square stock.

Now, put the blade back into the head of the combination square and set it to 3/8-inch (9.5mm), and firmly tighten the binding nut to lock the blade in this position. You may wish to have someone hold the axle down against the table, or c-clamp the axle to the tabletop, with the bottom of the axle facing up. Hold the head of the square firmly against the side of the axle so that the end of the blade crosses one of the transverse lines you scribed previously. Use your other hand to scribe a line into the bottom of the axle using the end of the blade as a guide. Do the same thing to the other end of the axle, making sure you use the same side of the axle to reference the square head against. You now have two pairs of intersecting scribed lines that are precisely the same distance from the center of the kingpin hole, and are essentially on the longitudinal centerline of the axle.

Now we are ready to make our permanent triangulation dimples into the axle. When using any kind of an impact tool driven by a hammer blow, it is absolutely mandatory to wear goggles or safety glasses. This is because the impact of the hammer against the tool sometimes breaks off a small piece of steel, either from the driving surface or the point of the tool. Although the hammer may strike the tool at a modest speed, the fragment can be driven off at a speed of several hundred feet/second. Wear eye protection before punching the layout.

We will need either a center punch or a prick punch and a hammer. If your punch is worn and rounded at the tip, have it sharpened to a fine point before proceeding. The prick punch is more suitable than the center punch since its point is longer, ground to a sharper angle, and usually has a finer point. That will make positioning the point more accurate. Place the point of the punch as precisely over the intersection of the layout lines as you can, and hold it perpendicular to the surface with a light downward pressure. When you are satisfied it is accurately positioned, strike it with a modest blow from the hammer. Repeat this for the crossed layout lines at the other end of the axle. Now the bottom of your axle has both permanent index dimples with each of the two positional requirements we must have: First, they are equidistant from the center of the kingpin hole. And, second, they both lie on a common line parallel to the centerline of the axle.

Now we must prepare the front kingpin. The following is a technique recommended by Detroit Metro racer, Amanda Karr. If you have access to a drill press, you can drill the pivot hole for the point of the trammel. Place a block of scrap metal, either steel or aluminum, across the center of the drill press table. It has to be long enough to c-clamp both ends to the table. If a suitable piece of metal is not available, a piece of wood such as a length of 2 x 4 may suffice. Drill a hole through the block with a size “D” drill, which is .246-inch diameter. Now place the kingpin in the drilled hole. Since the kingpin is .245-inch diameter, it fits the hole without any noticeable side play. Remove the drill and replace it with a small center drill. A 1/8-inch center drill works well since it is short and quite rigid. Further, it has a small tip, usually 1/16-inch diameter or less, to produce a shallow dimple just sufficient to form the pivot point for our trammel. The friction between the block and the head of the kingpin should prevent the pin from rotating while drilling. If not, prevent the kingpin from turning by holding it with a 7/16-in wrench while drilling. Run the center drill into the head of the kingpin just deep enough so that the resulting hole captures the trammel point. This setup ensures that the hole is perfectly centered on the kingpin.


Drilling the .246" dia. hole for the kingpin. 


Drilling trammel pivot  hole with center drill. 


  Double ended center drill.

If a drill press is not available, we need a prick punch or center punch dimple in the center of the head of the kingpin. It has to be large enough in diameter and deep enough to capture the point of the trammel. Start by inking the center area of the head of the kingpin. Set the combination square to 7/32-inch (5.6mm). Gage the head of the combination square against one of the flat edges of the hexagon head of the kingpin so that the blade crosses the center of the head. Now scribe a line across the center of the head using the end of the blade as a guide. Next rotate the kingpin 60 degrees (i.e. rotate it by two flats of the head). Now repeat the scribing step. Rotate the kingpin another 60 degrees in the same direction and scribe the third line.

All three lines should intersect at the center of the kingpin. Even if they don’t meet perfectly, they will form a very small equilateral triangle centered over the kingpin’s center. That should make an easy target for the point of your punch. You may wish to fasten the axle in a vise if one is available. What we are going to do is place the kingpin through the hole in the axle and let its length hang down below the axle. We are using the axle to support the kingpin so we can strike a blow with the hammer to place the dimple into the head. If no vise is available, we will have to improvise.

Some workbenches are equipped with small holes drilled through the top to facilitate driving out press pins and the like. Placing our axle with the kingpin hanging down into one of these holes would work well. If that is not available, we can bridge the axle across two strong, solid objects of the same height and placed just far enough apart to allow the kingpin to hang down between them without touching the bench or tabletop below. Now you can prick punch the dimple into the kingpin head. Be careful here, since the kingpin is quite tough. Use a modest hammer strike so as not to break the point of the punch. If the dimple is not deep and/or large enough, then reposition the punch and strike it again until you are satisfied Use the trammel point to verify it is large enough and sufficiently deep to capture the point. Remove the kingpin from the axle. Now you can remove the ink from the bottom of the axle by wiping it with an alcohol saturated rag or paper towel and quickly wiping it off with a dry one. Leave the markings on the front to aid in reassembly should you ever need to take the axles off for some repair job. AASBD rules allow you to prevent rusting of the axle square stock by the application of either oil or auto wax. Oily axles are very messy to handle and the oil can soak and stain your airfoils. Waxing them and buffing them smooth is the only sensible alternative. You should do so now, since it will be too difficult after the car is assembled. The rear axle and front kingpin are now ready for assembly and triangulation.

Alternate Layout Method
If you have access to a trammel set that accepts a ballpoint holder and you have the holder and set of ballpoints, you have an easy alternative to laying out the position of the reference dimples in the bottom of the axle that does not require a kingpin. Here’s how it works. You first ink the bottom of the axle at some convenient distance from the kingpin hole near the end of the square stock. Fasten the holder into the trammel and attach the smallest ballpoint to the holder, which is usually 1/2-inch diameter. Adjust the trammel so that it will swing an arc from the kingpin hole across the inked area on each end. Place the ball into the kingpin hole and maintain some downward pressure on that end of the trammel so the ball stays in seated in the hole. Use your other hand to scribe an arc across the inked area on both ends of the axle. Any two points on either arc are precisely equidistant from the center of the kingpin hole, as we require. Now use the combination square set to 3/8-inch (9.5mm) as before to scribe lines through the arcs, being careful to gage the square head against the same side of the square stock. If you have no combination square, you may use a steel rule carefully indexed from the edge of the axle to the same dimension given above. Also, some large dividers, such as Starrett’s number 85E will accept a ballpoint holder (number 88B) and swing a sufficiently large arc for our purposes. A trammel or divider equipped with a 1/2-inch ballpoint produces a layout that is just as accurate as the first method, and is slightly easier.

This concludes Article 3.

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