Safety Officer’s Report:                     Steve Coomer, Safety Officer

Suspension & Steering Basics, Part II

Last issue I presented information on rake and trail as the lead-in for a multi-part series on suspension and steering. After providing rake and trail data for the various current model Harleys, I talked about how rake, trail, and offset are inter-related. I concluded that discussion with the idea that the ultimate steering geometry design is nothing more than a compromise for how the bike is intended to handle and ‘feel’ in the rider’s hands. The physics of the front end geometry affect the bike’s overall road stability and agility. This installment will be the first of a two-part discussion on front fork configurations and how they work. Rear suspension will be dealt with in the final installment in this series.

When Harley introduced the front end that became known as the ‘Hydra Glide” in 1949, it strongly resembled the telescopic front fork that BMW had been using since the mid-30’s. Thanks to the end of WWII, the allies were privy to many technological developments from Germany. Although telescopic forks had been used since the mid-30’s by the Germans on the 500cc BMWs, the post-war refinements made by Harley were the first to make that front end suitable for heavy road bikes. The war also gave Harley the S-125’s and 165’s (formerly DKW’s) and allowed the Motor Company to have the world’s widest range of sizes and models available to the public.

When you think about how your ‘glide’ telescopic front forks work, the slider moves along the main tube at the rake angle. This is accomplished by close tolerance sleeves, bushings, and seals working together with the internal springs and dampers in a bath of fork oil. The resulting motion of the front axle is the reaction of the damping effects of the metering valves and springs within the fork as the sliders overcome the friction of the bushings against the upper tubes. The fork seals and boots perform a twofold task: They prevent debris and grit from attacking the slider bushings as well as preventing the fork oil from escaping the lower legs or sliders.

Understanding how the sliders work against the fork tubes is critical to designing a functional ‘raked-out’ bike. Remember in the last article that I have a 46 degree rake on my bike? As you read on you will understand why…

The ideal rake for most efficient shock absorbency is around 15 degrees. Unfortunately, a rake that small would create some very tricky riding at highway speeds unless the fork offset was extremely negative to increase the trail length. A large negative offset would require designing suitable clearance in the fuel tank area due to triple tree clearance requirements for lock-to-lock motion.

Rake angles between 25-34 degrees provide the best overall compromise for suspension travel in telescopic front ends, whether they are a standard or inverted configuration, such as the Ceriani and Storz fork assemblies that are available. As the rake angle increases, the torsional and lateral loads on the bushings, sleeves, and seals within the sliders become increasingly severe. This same ‘twisting’ effect places additional wear on the steering bearings in the same manner: the lower bearing wears heaviest toward the front of the bike while the upper bearing wears toward the rear of the bearing race.

When the rake angle exceeds 46 degrees, the loading of the bushing and sleeve inside the slider creates too much friction for the fork oil to overcome. This is amplified by the fact that the directional travel of the axle wants to be vertical in response to road surface motion. The result is a flexing and bowing of the fork tubes to absorb road shock as the sliders become incapable of telescoping to absorb those shocks. This can lead to abnormal or uneven front tire wear, excessive wheel hop, poor or erratic handling, and accidents caused by steering neck breakage due to micro fractures of the triple tree assemblies.

The fork seals, wheel bearings, tire balance, and over/under-inflation of the front tire are the biggest causes for poor tire wear and shortened tread life. While these items can contribute to poor handling, improper maintenance of the neck bearings and wheel bearings are the largest contributors to the bike developing poor handling qualities. The best prevention for these types of problems is proper maintenance. Check your tire air pressures before riding and correct them as quickly as possible. Follow the maintenance intervals for servicing the wheel bearings and neck bearings, as well as the fork oil draining and filling. The presence of water in the fork oil or excessive oil on the fork tubes is an indication that the forks should be torn down and new seals installed. Once back together, range of motion should be smooth and even for the sliders and steering.

While the forks are down, the lower triple tree should be inspected along with the neck bearings. As the neck bearings loosen from wear, they transmit impacts to the neck post. We usually fail to notice this progression since it develops slowly and we adjust to it as it occurs. The severity and frequency of these impacts can lead to neck post failure. Needless to say, attempting to handle a 700 lb. unicycle if the neck separates at highway speeds can be extremely difficult. Correct any problems found before riding off into the sunset.

Now, what about those ‘dry’ forks that I haven’t talked about yet??? In the next issue we will discuss the various springer designs as well as girder front ends.