DMSBD Tech Tips

Construction Tips for a First-Time Car Builder
Article 6: Alignment & Crossbind - by Ian Carsten

Since wheel misalignment and crossbind increase a derby carís rolling resistance and slow it, you need to understand and control them to maximize your carís performance.

Crossbind
A source of increased rolling resistance in a derby car is crossbind. If all of the parts of your car were perfect, then both axles would be parallel to the floorboard, pavement, and each other (as viewed from the front or rear). Unfortunately, one or more of the parts responsible for holding the axles parallel may be imperfect, causing one axle to be oriented at an angle to the other vertically. When this happens, two wheels diagonally opposite from each other will bear a greater portion of the weight than the other two. This is what is meant by crossbind. A car with crossbind rolls harder due to the increased load on the bearings and tires.

For example, suppose that the front axle is parallel to the floorboard, but the rear axle is angled down at the left. This causes the left rear and right front wheels to support more of the weight than the left front and right rear wheels. The result is, the car now rolls with significantly more resistance than if the two front and two rear wheels carried equal weight. This is similar to the car being tail or nose heavy, except with crossbind, two diagonally opposite wheels carry more weight than the other two.

This can happen for a variety of reasons. For example, your floorboard may have a slight twist from front to rear. Or, one or more of the washers on a kingpin may not be flat. Usually, the top and bottom surfaces of such washers are not parallel. In either case, when the kingpin nuts are tightened, the two axles are held firmly in position by the kingpin tension at an angle to each other vertically. If the washers are identified as the cause, then by removing those that are not flat and replacing them with good ones, the crossbind may be eliminated.

Whether caused by uneven washers or a twisted board, placing a piece of feeler stock between the washers on the low side of the washer stack to compensate, may remove the crossbind. You will have to experiment to determine what thickness of feeler stock is required. Also, it is important that the feeler strip be placed between the washers. If the feeler strip is placed between a washer and the floorboard, the kingpin tension can easily force the feeler strip uselessly down into the wood of the floorboard. Current AASBD rules allow the feeler stock be held in position by driving a small nail through each free end of the feeler strip and into the floorboard to secure it in place. Also, the rules state you may use only one feeler strip per car. Of course, crossbind caused by a twisted floorboard could also be eliminated by replacing the floorboard with an untwisted one, but that is a rather expensive, and usually, unnecessary remedy. You should check for crossbind and adjust to eliminate it, since a car with crossbind will be slower than one with the crossbind removed.

A quick and easy way to check for crossbind is to place the car on as flat a surface as you can find, such as a driveway or concrete garage floor. Some experienced derby builders use a pool table. If you are really fanatical about it, try to find a tool & die shop or mechanical engineering firm. You may be able to persuade them to allow you to check your car for crossbind on a large surface plate, which has an extremely flat and smooth surface. To check your car for crossbind, raise one end until the wheels are just above the surface, and give each of them a good spin. Now slowly lower the raised end until the wheels just start to touch. If both wheels stop simultaneously, you have no crossbind. However, if one wheel stops before the other, then your car has crossbind. The greater the difference between the wheels, the worse the crossbind is, and the slower the car. You should adjust only one of your axles, as explained above, until the crossbind has been eliminated. Remember, if you use a feeler strip to remove crossbind, the rules state you may use only one per car. Since the front axle has to pivot to steer, it is best to leave the front axle alone and make any necessary adjustments to the rear axle. You have to remove crossbind to allow your car to attain the highest speed it is capable of. The benefit is proportional to how much crossbind was present. Although the results are usually more modest, experiments we have done show that removing crossbind can reduce rolling resistance by as much as 30% in some particularly bad cases.

Spindle Alignment

In order to minimize the rolling resistance of your wheels, all four spindles must be mutually parallel when the car is loaded and running down the track. When supporting the load of a car and driver, the axles of a derby car naturally spring down in the middle. But this makes the spindles point slightly upward. Your wheel bearings are designed to support a load perpendicular to their common axis and this can only happen if the spindles are horizontal. With the spindles angled upward, the bearings are forced to support the load at an angle and generate more rolling resistance in doing so. Some of the most competitive racers pre-bend their spindles downward about .025-inch at the ends so that when the driver is in the car, the weight of the car and driver is just sufficient to level the spindles. Thus, they can roll down the hill with less resistance and are, therefore, a bit faster than if the spindles were not pre-bent.

The next idea is not universally accepted. Some racers believe that as the car accelerates to speed, the rolling resistance of the tires and bearings and the air resistance exerted on the axles and wheels tend to spring the spindles rearward a very small amount. If so, this causes increased rolling resistance at the bearings, and at the tires as they are forced to slightly side scuff the pavement. For this reason, some racers very slightly pre-bend the spindles forward about .003-inch at the ends to compensate.

How Spindles are Checked and Aligned
A spindle gage is a measuring tool that determines how much the loaded spindles deviate from their unloaded position. Also, it is quite useful to determine whether a spindle is slightly bent horizontally, such as when a car has struck something with one of its wheels. It has to be used with a pair of spindle stands and a pair of bending wrenches. The spindle bending wrenches are a two-piece set. The first one has a cast bronze head with a square hole that fits over the 3/4-inch square stock of the axle. The second is similar, but has a 1/2-inch diameter drilled and smoothly reamed hole that fits over the spindle. Both wrenches are equipped with a 19-inch handle made of 1-inch galvanized pipe. There are at least three suppliers of spindle gages and related tools. However, the very best and most reasonably priced are the cast aluminum tools made and sold by:

Dennis Wilt  3260 Bull Road   York, PA 17404   Ph:(717)-764-5632

When the spindle gage is unpacked, you have to bolt the 1-inch travel dial indicator to the mounting clamp. The Dennis Wilt spindle gage is supplied with one indicator. However, the gage has mounting clamps for two indicators, one at each end. The second indicator isnít needed, but having two installed can speed up the process. You will need a reference axle to calibrate the indicator. This is simply a straight derby axle that does not cause any noticeable movement of the indicatorís pointer as the axle is rotated in the upside-down spindle gage. Here is how to calibrate the indicator to the reference axle. With the axle in the upside-down gage, loosen the knurled lock screw and rotate the bezel (outer ring on the indicator) until the pointer is on zero. Now tighten the lock screw to hold the bezel in position. Most new derby axles are straight enough for this purpose without any adjustment. If the axle you want to use as a reference is not straight enough, then you can use the two bending wrenches to adjust it until it is.

 Hereís how you align your spindles vertically, which is the only direction most racers use. First we will inspect the axles with the car up on sawhorses. This is not necessary but it is informative and quickly verifies that you are starting with a straight axle. Remove all four wheels. Holding the gage vertically, carefully place the cutout ends over the spindles. Now look at the pointer on the indicator. It should read zero. Now, keeping the gage in contact with the spindles, swing the gage slowly forward until it is horizontal. You should have an assistant watch the pointer on the indicator as the gage is being swung through the 90-degree arc. The pointer should move very little. If so, this indicates that the spindle is straight both horizontally and vertically. This is just what we expect of a new axle in good condition. If you only have one indicator attached to the gage, then youíll have to remove the gage and turn it around to check the spindle on the other end of the axle. Then do the same to the other axle.

Next, we are ready to determine how much the spindles deflect under load. With the wheels installed and the car ballasted to race weight (minus the driver), place the car on a flat surface. Raise the front and have an assistant remove the front wheels, and place a wheel stand under the center of each spindle. Lower the car so the front spindles are supported by the wheel stands. Now, have the driver get in and into racing position. Keeping the gage vertical, carefully place it over the spindles. Look at the pointer on the indicator. The weight of the car and driver should cause the ends of the spindles to deflect upward and the pointer to read between +. 022-inch and +. 026-inch. As you slowly swing the gage forward to the horizontal position, you should see the pointer move smoothly back to zero. Write down the amount of vertical deflection for each front spindle. Put the front wheels back on. Now repeat the process for the rear spindles.

Next, put the car back up on the horses and remove the wheels. You will have to remove the airfoils to get the bending wrench over the square stock. Place the wrench over the square part of the axle so the end of the wrench head is about 1/16-inch from the spindle, with the handle pointing up. Then put the other wrench over the spindle with its handle up too. Hold the wrench on the square stock with one hand while pulling outward on the one over the spindle. You need not be concerned about bending the square stock. The bending force is applied between the two wrenches. Since the spindle has only 35% of the cross sectional area of the square stock, only the spindle will bend; the square part of the axle remains straight. Youíll have to remove the wrenches and put the gage on to see if the spindle has moved down enough. It is better to bend too little and repeat until you have the required amount of downward bend than to use too much force and overshoot. If you do, then youíll have to bend back up. It may take several tries to get it right. Donít expect to get the exact amount. This is a cut-and-try process, and you should be satisfied when the indicator is within +/- .003-inch of your target. The bearings on derby wheels are usually sloppy enough that trying for greater precision than this is meaningless. Then, repeat this process until all four spindles have been aligned. Like learning any new skill, the more you do it, the faster and more accurate you become. Next, you should put the wheels on, place the car back down on the pavement, remove the front wheels, set the spindles in the stands, get the driver in position, and re-gage the spindles to verify that the deflection has been removed with the car under racing load. They should read zero within +/- .003 inch. Now check the rear axle in the same manner.

Will Aligned Spindles Make a Difference?
You may want to know how much of a difference aligning your spindles can make. The answer is, probably not very much. We performed some spring pull-gage tests with a balanced 230-pound superstock car with polished spindles on a freshly swept, approximately level, concrete garage floor. We recorded the threshold force necessary to start and keep the car rolling both before and after aligning all four spindles. We determined that the rolling resistance of our car was reduced from 58 ounces to 57 ounces, or about 1.7 percent by aligning the spindles. It isnít much and the alignment tools cost $325. For such a modest reduction in rolling resistance, you probably shouldnít spend the money unless you are really committed to doing lots of rally racing. Instead, you should try to use the tools available at your local organizationís clinics if possible. Like most speed enhancements in derby racing, spindle alignment will not make a very large difference by itself. However, it is one of many seemingly small things that can contribute to slightly greater speed, and most of your competition is already doing so. 

This Concludes Article 6.

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