Axle Bushing Alignment 101

By Steve Sawtelle

(click on images for larger view)

Introduction

Our club (Shoreline Model Raceways) races several classes which are based on a variety of 1/32 RTR cars with plastic chassis. The rules vary from class-to-class but most allow replacing the stock nylon bushings (when present) with aftermarket bronze bushings and gluing the bushings to the chassis. The stock nylon bushings tend to have quite a bit of "slop" to allow the stock knurled axles to pass through. By replacing the stock bushings and knurled axles, the slop can be reduced substantially which usually helps overall car performance. Gluing the bushings to the chassis prevents unwanted movement and prevents them from popping out of the chassis bushing carrier while racing the car. Virtually every 1/32 car I race gets the stock knurled axle and nylon bushings replaced with aftermarket parts.

Initially, I never gave actual installation of the bushings too much thought - after all, it's pretty straight forward. Slip a couple of bronze bushings over the axle, line up the bushings over the chassis bushing carriers, press in place, make sure the axle spins freely, glue in place and lubricate the bushings. However, after performing this upgrade on numerous cars, it became apparent that some rear axles seemed to spin a little more "freely" than others. Why? There are several variables to consider including, but not limited to, axle straightness, axle roundness, the outer diameter of the axle, the inner diameter of the bushing, the surface of the axle (rough or polished), the inner surface finish of the bushing, the chassis (i.e. is it perfectly flat or has it been twisted out of shape?) and bushing alignment with the axle. This article will explore one of these variables, proper bushing alignment, in more detail and provide simple, step-by-step instructions that virtually guarantee near-perfect bushing alignment every time when installing aftermarket bushings.

Note: The same basic technique can be applied to other vendor's plastic chassis which use a similar bushing mounting setup and will accept a Slot.it ™ bronze bushing. If you plan to use different bronze bushings you will need to adjust the size of the tools described in the “Tools and Materials” section below.

Understanding The Problem

Let's start by comparing a diagram of a rear axle and bushings which are properly aligned with a diagram of an axle and bushings which are not. As you view these diagrams, keep in mind they are two-dimensional representations of three-dimensional objects. Properly alignment must take into account all three dimensions. Likewise, improper bushing alignment must be considered from a three dimensional perspective as well.

First, let's examine a diagram showing proper bushing alignment. Figure 1 (click on thumbnail) shows the relationship of properly aligned bushings with an axle. Figure 2 highlights the areas (red circles) which are critical to proper bushing alignment. The bushings are properly aligned when the longitudinal axis of the axle and the longitudinal axis of both bushings are concentric. In other words, proper bushing alignment is achieved when:

a) the entire outer surface of the axle which is "inside" each bushing is parallel to the entire inner surface (race) of the bushings.

b) the "gap" between the outer axle surface (entire 360o) and inner bushing race (entire 360o) is equidistant along the entire length of axle which is "inside" each of the bushings.

Figure 3 (click on thumbnail) shows two examples of misaligned bushings. Bushings are misaligned when the longitudinal axis of the axle and the longitudinal axis of one/both bushing(s) are NOT concentric.

Before proceeding, let's look at bushing alignment and misalignment from a slightly different perspective. There is exactly one way bushings are properly aligned as shown in Figure 1 above. Conversely, there are an infinite number of ways they can be misaligned so the odds are really stacked against us. Will the alignment procedure which follows align the bushings "perfectly"? The short answer is "No."; however, it will get them very, very close and just as importantly, it will ensure consistent, repeatable results.

Tools and Materials Needed

Basic components needed to install and align bronze bushings in a Scalextric 1/32 RTR plastic chassis are described below.

A 3/16" cylindrical burr bit designed for a rotary tool. Dremel and others offer these to sharpen chainsaws; however, they tend to get "fouled" easily when used for the task described later in this article. The burr bit shown is diamond coated - it is far less susceptible to fouling, cuts very quickly and leaves a smooth finish.

A pair of high-quality bronze bushings. The accompanying picture shows a close-up of the bushings Slot Car Corner offers - if you looks closely, you will notice the bushings are counter-bored to reduce the bushing's inner surface area which the axle rests on. The surface area of the inner bore is roughly 1/3 the original bushing. This is a trick that has been around for quite some time - the smaller surface area helps reduce friction.

A Pin Gage (or drill blank) whose outer diameter is as close as possible to the inner diameter of the bronze bushings you plan to use. I prefer pin gages over drill blanks because the tolerances are generally smaller. Pin gages are commonplace in machine shops - they are typically used to ensure various machining operations are within tolerances defined for a given item. So just how close is "...as close as possible..."? Based on my experience, a pin gage with a diameter within .0005" (5/10,000th of an inch) of the bushings inner bore (diameter) works well. For the Racing Bushings Slot Car Corner offers, a .0940" GO pin gage works well. If you use Slot.it ™ bronze bushings, you will need a slightly larger (.0950" GO) pin gage.

*** Important *** Always wear safety glasses and follow all manufacturer's instructions and safety guidelines when using tools.

Note: The components shown above are available individually or in kit form in the Slot Car Corner Online Store. The kits and components will work with any 1/32 plastic RTR chassis which will accept Slot.it ™ bronze bushings (Part SIPA02).

Step-by-Step Instructions

Note: Before proceeding, you should install your motor and glue it to the chassis (if desired). For clarity, the following pictures were taken without the motor in place. The chassis used for illustration purposes in this article is a 1/32 Scalextric Monte Carlo.

Use the 3/16" burr bit to enlarge the chassis bushing carriers. Enlarging the chassis bushing carriers slightly has a couple of benefits. First, it will help prevent stress failures which sometimes occur when using aftermarket bronze bushings (whose outer race diameter is sometimes slightly larger than that found on stock nylon bushings) with a plastic chassis. Second, it will make it easier to position/align the bushings in a later step. This burr bit is designed for use in a rotary tool; however, use it by hand as shown instead. You only need to remove a very small amount of material - using the bit in a rotary tool will remove too much. When you insert the burr into the bushing carrier, it will be a very snug fit. Don't force it - instead, twist the burr between your fingers and let the burr remove a small amount of material (you will see a "ring" of black plastic around the circumference of the burr). Make sure to keep the burr bit perpendicular to the chassis bushing carrier as shown.

Once you have removed a small amount of material from the chassis bushing carrier, the burr will "spin" freely and slide through the bushing carrier with minimal resistance as shown. Do not remove any additional material. Carefully remove the burr bit and repeat for the other chassis bushing carrier. Make sure to blow away any plastic dust/debris from the chassis bushing carriers before proceeding.

Be sure the pin gage is clean and free of any foreign material (including excess lubricant) before proceeding. Carefully slide 2 bushings onto the pin gage - because this is such a close fit, be sure to align the bushing properly. DO NOT force the bushing onto the pin gage.

Place the pin gage and bushings over the chassis bushing carriers. Adjust the position of the bushings so they are directly above the chassis bushing carriers as shown.

Place your thumbs above the bushings - press both bushings firmly at the same time to seat them into the chassis bushing carriers. DO NOT press on the pin gage - doing so could bend/damage the pin gage. Once the bushings are in place, press firmly a couple of times with your thumbs to ensure they are fully seated.

Check the bushing alignment by using your fingers to turn the pin gage. If the pin gage doesn't turn or takes considerable effort to turn, use your thumbs to press firmly on the bushings to reseat them slightly - you may have to repeat this step several times. Keep in mind the pin gage will not spin as freely as an axle (because the tolerances are much tighter); however, when the bushings are properly aligned, you should be able to turn the axle with minimal effort.

Caution: If you need to remove the bushings from the chassis for some reason, DO NOT use the pin gage! Doing so could bend/damage the pin gage. Use an axle instead.

Glue the bushings in place using your favorite glue for this task. I use a small dab of superglue - the capillary action will draw the superglue between the bushing and chassis bushing carrier. Be careful not to get any glue on the pin gage! Let the glue dry completely before proceeding.

Carefully remove the pin gage from the bushings. Keep in mind the pin gage is like a lever - be very careful not to inadvertently move the bushings during this step. Because of the close clearances between the pin gage and bushings, a slight back-and-forth twisting action may make it easier to remove. Take your time removing the pin gage!

Take a new 3/32 axle and wipe it with a clean, lint free cloth. Turn the chassis on its side and insert the axle a short way into the upper bushing as shown. Be sure there is a surface under the chassis which will not cause any damage to the axle.

Release the axle. If you've aligned the bushings properly, the axle will drop freely through both bushings and land on the surface below. You may have to experiment with the position of the chassis a bit to allow the axle to fall freely.

Turn the chassis back upright and insert the axle through both bushings. Spin the axle with your fingers - it should feel silky smooth. Make sure to lubricate the bushings before actually running the car. That's all there is to it!!

Summary

Proper bushing alignment is an example of a "little thing" which when combined with other "little things" can make a big difference in how well your car performs. The bushing alignment procedure shown in this article can be performed by virtually all 1/32 slot car racers - no special skills are required. The whole process takes just a few minutes to perform and there is minimal one-time investment required to purchase a burr bit and pin gage like those shown in the article.

• Note: For your convenience, the high-quality components shown in this how-to article are available in kit form from the Slot Car Corner Online Store.
  
• Special thanks to Don “Bubba” Willis, Dick “nhdungeonracer” and Mike “Smokeio” Chiocchio for their input and feedback.
  

Copyright © 2006 by Slot Car Corner L.L.C. - All Rights Reserved.

Copyright © 2006 by Steve Sawtelle – All Rights Reserved.

Copyright © 2006 by Slot Car Illustrated. Used With Permission.