News

Dec 23, 2023

MXA TECH SPEC: WHAT YOU DON’T KNOW ABOUT YOUR HANDLEBARS

Stiffness increases to the third power; strength to the second power of the percentage of diameter increase. What concerns handlebar designers most is crashing. The handlebar is a long lever mounted

Stiffness increases to the third power; strength to the second power of the percentage of diameter increase.

What concerns handlebar designers most is crashing. The handlebar is a long lever mounted to the very top of the motorcycle. An incredible amount of force can be applied to the bar as a motorcycle cartwheels down the track. We’re talking 20 Gs easily. Making matters worse is that the force is placed on the bar from unpredictable directions and varying levels of intensity. It’s easy to build a handlebar strong enough to exceed any loads that a human body could put into it (12 Gs is beyond our limits). The real challenge is to build a bar that can take abuse but still offer resilience and comfort to the rider.

Where the load is. When landing hard off a jump or smashing through whoops, rider energy is transmitted into the bar through an axis that is in line with the angle of the fork tubes. It’s very simple for the handlebar maker to duplicate a similar load in a lab test; however, in a real-world crash, the ends of the bar are being loaded from every possible direction. It’s impossible for a lab to duplicate the level and direction of these impacts.

The benefits of tube diameter. If you take the same material with the same wall thickness and manufacture a larger-diameter tube out of it, its strength and stiffness will grow exponentially with the increase in diameter size. What does that mean? It means that if you take the same material used in a 7/8-inch bar and make a 1-1/8-inch bar, the larger bar will be 2.1 times stiffer and 1.7 times stronger. (Stiffness increases to the third power; strength to the second power of the percentage of diameter increase.)

Round is good. A tube with a thicker wall will be stronger, stiffer, heavier and tougher. Tougher is good because it can take a harder hit without deforming the round profile of the handlebar’s tube. As long as the bar retains its round shape, it can withstand anything up to its original level of yield strength. Think about a soda straw. Take a soda straw and try to bend it. Feel the level of resistance? Now, put a little nick or dent in the middle. Now, see how little force it takes to bend or fold the straw at the site of the nick? Any dent that alters the round profile of the tube creates a weak spot on a straw—and a handlebar.

You gotta give to get. Resiliency refers to the degree of tube flex. It’s different from stiffness because it describes the quality of the handlebar’s give. That’s why aluminum is such a great handlebar material. Not only does it give more than steel or titanium, it also has a superior degree of hysteresis. “Hysteresis” refers to the internal friction of the metal. Bar flex acts like a spring. Aluminum has a high degree of hysteresis and thus a greater ability to damp bending shock. On the other end of the scale is steel. Steel has the least hysteresis and the most spring. The rebounding “spring” wears out the rider’s hands and forearms. Titanium’s spring is between steel and aluminum.

Wear and tear. If you take a paper clip and bend it back and forth, it will eventually break. Every time a tube of metal is flexed, it is weakened. The more it’s flexed, the weaker it gets. Handlebar designers fight this by making the bar stiffer so it won’t flex too much. Too much flex from an aluminum bar is of extreme concern because of its large-grain structure. You can understand the grain structure of a metal by picturing the metal as a brick wall. Smaller bricks help spread stress more evenly across the structure. If a small crack forms, it has a harder time propagating because it has to zig-zag around each brick. All the molecular components in steel fit together like a wall made with small, perfectly shaped bricks. Aluminum is like a brick wall made with larger, irregular-shaped bricks. The bricks don’t fit as tightly together, and there are larger gaps between them. Larger grains distribute stress less evenly and invite cracks to propagate quickly around the larger “blocks.”

Hot spots. Grain structure becomes an issue in areas where hot spots develop. If the bar is weak and flexes too much, it can score at the bar-mount clamp edge. The score creates a stress riser, and just like with a soda straw, the load on the bar is focused on the weak spot. Steel’s smaller, tighter-fitting “bricks” keep the score from developing into a crack. With aluminum, the score focuses stress on the larger spaces between the larger-sized “bricks” where cracks can easily form. When aluminum cracks, it cracks big and fails. The most dangerous thing you can do as a rider is try to straighten bent bars so you can ride the second moto. Once a bar is bent in a crash, you’ve created a large stress zone in one direction. Straightening it stresses it in the opposite direction, which compounds the problem. Never straighten your bars!