Guide To Motorcycle Handlebars.

We as riders place an enormous amount of faith in the proper function of our motorcycle handlebars. Most riders don't even want to think about an important question like: "What connects You (the rider) to your machine, and how reliable is that connection?" Pretty sobering question, considering the trust we place in our machines when jumping distances of 100+ feet!!

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The following is a discussion on importance of motorcycle handlebar design, and the choices you can make as a consumer to increase your safety.

From an engineering perspective the task of motorcycle handlebar design is much more challenging than 1st meets the eye, as motorcycle handlebars are much more complex than their appearance lets on. It's not too difficult to build a handlebar strong enough to exceed any loads that a human body can place on it (12G's far exceeds our limits). What concerns handlebar designers the most is crashing, as an incredible amount of force (easily 20+ G's) can be applied to the bars from unpredictable directions and levels of intensity as your bike cartwheels down the track. To make matters much worse not only must the bars reliably withstand this kind of abuse, the bars must offer resilience/comfort to the rider and be cost-effective to produce.

Below is a technical guide and some key considerations to help in the selection of motorcycle bars to match your particular riding style, and personal preferences:

Challenges In Designing Motorcycle Handlebars:

Dirt bike engineers must review all the pluses and minuses of various materials and dimensions when designing motorcycle handlebars. Tube wall thickness, cross-bracing, tapered walls, butted tube, surface finishes, weight, price, tubing stiffness, barrel stiffness, resiliency, hysteresis, fatigue life, grain structure, and elongation are all important factors. Calculations are performed to determine how the material will respond in various shapes & sizes (such as 7/8" or 1-1/8" diameters), and considerable testing is performed.

What Are Important Considerations For Comfort:

Besides simple sizing, one of the main considerations of motorcycle handlebar comfort is the "Hysteresis" in which it provides. Hysteresis refers to the internal friction of the material or the ability to dampen energy. This is extremely important as a handlebar design with poor hysteresis will directly transmit energy to the rider which wears out the hands and forearms (arm-pump) during racing. Aluminum has a high degree of hysteresis, and thus a great ability to dampen shock loads. Steel has the least amount of hysteresis, and Titanium is somewhat in between. Resiliency is also important as this relates to the quality or degree of flexing in which the design can provide.

What Is The Best Motorcycle Handlebar Material:

At the moment there are basically three main materials used in production handlebars: Chromoly Steel, Aluminum, and Titanium.

Chromoly steel is relatively economical, tough and will bend in a crash. However the drawbacks of this material is the harshness it provides to the rider, and overall weight.

Aluminum is one-third the weight of chromoly steel, and is readily available for production. It's light weight and ease of forming give it many advantages during the design process. A properly designed aluminum bar can be built lighter, but with the strength, fatigue life and ride quality that is unmatched by chromoly steel.

Titanium is almost as strong as steel, and weight is in between steel and aluminum. It's fatigue life is also excellent. Unfortunately due to availability Titanium is nearly five times as expensive as other materials. The material is also very difficult to form, bend and cut making labor costs extremely expensive.

Many discussions have revolved around Carbon Fiber handlebars. However Carbon has little or no elongation during a crash, and may crack or splinter creating a very dangerous situation. Labor cost is also extremely high as this is very labor intensive and would require great efforts of quality control to ensure consistency.

With All The Flexing During Use, Do Handlebars Ever Wear Out:

A motocross handlebar is designed to last a certain number of flexes or cycles. However before you start to panic, rest assured that the engineers have designed the bars to have a fatigue life far beyond what you could ever experience in a lifetime.

However there is cause for concern during a crash event as this may cause the bar material to stop flexing and begin to elongate. Elongation is extremely important as this is a measure of how far the material can distort before failing catastrophically. Some materials will continue to bend under increasing loads, however others will suddenly break. Too much flex with aluminum bars is of extreme concern due to it's relatively large grain structure.

What Is the Importance Of Grain Structure:

Grain structure becomes an issue where a weak or hot spot has developed do to too much flexing. Excessive flexing (particularly at the clamp edge) can score or create a stress riser, and just like denting additional loading will be focused in this area. Materials with larger grain structure will be more susceptible to failure under these conditions.

Microscopically, steel and aluminum have very different grain structures. With steel all the molecular components or grains fit together with smaller and more perfectly shaped elements. This allows applied stress to distribute or spread more evenly across the structure. Aluminum however has larger, more irregular-shaped grains which distributes stress less evenly and is more prone to crack propagation.

Is There Anything Which Can Be Done To Make The Grains Stronger:

By alloying the aluminum with added ingredients the material can be made stronger and more durable. Different types of aluminum have different grain structures which offer improvements in ultimate strength, resiliency, and fatigue life.

Tempering can also be performed to increase the hardness of the material after forming. Aluminum alloy can be tempered to different hardnesses which can make it stiffer, stronger or more brittle.

Lastly a shot-peening process can be performed to increase the fatigue strength of the aluminum. This is normally accomplished by a two step peening process which hits the bar's aluminum surface with small steel balls of decreasing diameter. This process alters the grain structure on the surface and places it into a "compressive state". The performance difference using a shot-peening process can be substantial, up to a 15% increase in fatigue resistance. However this process does not improve upon the bar's ultimate strength characteristics.

What Are The Signs Of Handlebar Wear:

The most dangerous thing you can do as a rider is attempt to straighten bent bars to ride the second moto. Once a bar is bent in a crash you have created a large stress riser in one direction. Attempting to straighten the bars creates stresses in the opposite direction, which compounds the problem and is dangerous. Same thing with surface scores or denting...replace the bars as this is a large stress riser.

If the the cross-brace clamp bolts are loose, replace the bars. While it is alright to double-check the torque on the cross-brace bolts, never tighten them. A loose bolt means a loose clamp and a compromised glue bond. If the bike has been ridden in this condition, the clamp has twisted and scraped against the handlebars during use. Again this will create scoring and large stress risers.

However do periodically double check the bar clamp bolts and tighten if necessary using a diagonal pattern. Reference the manufacturers instructions or motorcycle manual, or use the following as a guide. First snug the four bolts evenly by finger, then with a torque wrench follow the pattern again and tighten to 8 ft-lbs torque. Proceed following same pattern using a tightening sequence of 12 ft-lbs torque and then 16 ft-lbs torque. Do not over-tighten the bar clamp bolts as this can damage the barrel stiffness of the bar and cause a stress riser.

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