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How Does Changing the Frame Geometry of a Bicycle Change the Weight Distribution at the Contact Points?

By Georgena Terry

Q: “How Does Changing the Frame Geometry of a Bicycle Change the Weight Distribution at the Contact Points?”

A: This analysis can become complex very quickly, so I’ve made some simplifying assumptions to get the discussion started.

  • I’ve only used the weight distribution at the hands and saddle. The feet are important contact points, but the forces there can vary quite a bit, depending on how hard you’re pedaling, and whether you’re standing or seated. As those forces change, so do the forces on your hands and seat. So, for this analysis, I’m going to look only at the weight of the upper body, the trunk, and arms and head.
  • I’ve assumed the saddle and the point on the handlebars where the hands rest are at the same level.
  • The arms are straight, not bent at the elbow. This is to simplify the trigonometry of the analysis.
  • I’ve used a 5’4” rider who weighs 120 lbs as my model.

You may remember this diagram from a prior article on women’s anatomy. It’s a “free-body diagram” that’s often used in engineering to calculate forces. Using the research done by Laura Lund, which was the basis for my article, I’ve calculated the forces at the saddle and handlebars for this rider. Varying the geometry of the bike essentially means changing the length of the “rider compartment,” the distance from the center of the saddle to the resting point of the hands on the handlebars.


This diagram shows the effect of the length of the rider compartment on the weight sustained by the hands. Note that this is the total force; each individual hand bears one half this amount. As the distance to the handlebars increases, so does the weight on the hands.





Here’s what happens at the saddle as the rider compartment is lengthened. No surprises here — the weight has been redistributed from the saddle to the hands.

If you look at the total change in weight on the hands and the saddle as the rider compartment changes by 5.5 inches, it’s only about 0.9 lbs. Not a lot, considering that this rider weighs 120 lbs. Since comfort is affected by the reach to the handlebars, it seems that the change in weight distribution is not a significant factor.

Given the discomfort many riders feel when the reach to the handlebars is too long, this may seem counterintuitive. As the rider compartment gets longer, shouldn’t the hands be bearing a lot more weight instead of slightly more weight?

Well, it depends. Your hands won’t bear significantly more weight unless you rise out of the saddle and shift your body forward. So something else is at work other than just the the force of gravity pulling down on the rider and the reaction of the handlebars and saddle pushing up on the rider.

An important factor in comfort is core strength, particularly the strength of the muscles of the lower back. If those muscles are sufficiently strong, they can support the upper body as it leans forward. If your lower back was strong enough, there could theoretically be no weight on your hands. They would rest gently on the ‘bars.

This is why this analysis is difficult. It’s examining a static situation. But when you ride, your core comes into play and adds a dynamic factor. In Lund’s analysis, which I wrote about in my prior blog, she assumed that 5% of the rider’s weight was on the handlebars. She calculated the forces required in rider’s shoulders and back to maintain this state over varying lengths of the rider compartment.

Notice that my rider, who weighs 120 lbs, bears about 13 pounds of force on her hands. Lund’s 120 lb rider would only bear 6 pounds. But Lund’s analysis takes into account the action of the low back. Mine does not. Neither analysis is wrong; they just set out to accomplish different goals.

Georgena will answer your questions about bike-fit, bike-frames, saddles, and bike-maintenance. So don’t be shy. Take advantage of her knowledge and decades of experience to get answers to any questions you have about bicycles. Email me at [email protected].

When Georgena Terry first started designing bicycles, over 30 years ago, there were no women-specific bikes. Many female cyclists were forced to put up with neck, and shoulder pain, and other physical problems that came from riding bicycles that didn’t fit them. Georgena Terry changed that. Looking at anatomical differences between men and women, such as body mass and body strength, she began designing bike frames that optimized comfort and performance for female cyclists, with special attention to women under 5’2”.

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Working out of her basement in Rochester, NY, Georgena first designed bikes for herself, then for her cycling friends. In 1985, she sparked a revolution in the cycling industry by launching Terry Precision Cycling, selling custom-built, high-quality steel frame bikes for women. Larger cycling companies sat up and took notice. Eventually following her lead, they developed their own brand of women-specific bicycles, clothing, saddles and accessories.

Today, Georgena continues her passion, designing and building custom steel bikes for women cyclists of all sizes. Contact her at georgenaterry.com


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