Performance Recumbent Bicycle Frame Design: Designing For a Better Ride

“In my humble opinion, the soul of a bike is embedded in its frame. Everything else is secondary. Herein lies the Giro’s strength and character. The Giro’s frame consists 4130 Chromoly steel teardrop shaped mono-tube that not only looks classy and sophisticated, but also achieves that very hard-to-reach balance between frame rigidity, flexibility and strength. It flexes enough to ensure an extremely comfy ride but it is rigid enough to direct all your energy to the rear wheel. If any power is lost to the built-in frame flex, I sure as hell didn’t notice it. The bike’s performance is excellent!” – Jose A. Hernandez,   for BentRiderOnline.

Needless to say we will not argue with the comments above but we would just like to add that all of our steel and aluminum recumbent bikes share the same custom tube set and display the same riding characteristics as the GIRO series of recumbents. Our many years of experience, both building and riding all types of recumbents, have taught us nothing if not this; the ride is the most important thing. We also felt that if you could make the recumbent design more pleasing to the eye, and still deliver a superior ride above other recumbent bicycles, than we would have achieved something very special. While we know that beauty is in the eye of the beholder, we believe that the Bacchetta line of recumbent bikes delivers just that, a superior ride and great looks, all made possible by a carefully designed custom tube set. Achieving this balance with a mono-tube design also delivers the additional benefit of superior aerodynamics in comparison to triangulated or space frame recumbent designs by eliminating the clutter of tubes below the rider. Less tubing means less frontal area and that makes for a faster bent. Getting this level of ride quality and aerodynamics out of a triangulated frame is extremely hard to do, even with custom tubing, and next to impossible when using off the shelf, straight gauge, tubing. Some may argue that nothing beats a triangulated frame for drive train efficiency but there are trade-offs with these designs as well. Keeping the wheels of a bike in contact with the road is key to overall handling and requires a certain amount of frame compliance. Recumbent designs that ignore this fact in the pursuit of efficiency tend to be skittish on rough roads and transfer more road shock to the rider. Transversely, our early prototypes showed that a mono-tube recumbent frame built with straight gauge round tubing did not provide enough structure for the ride characteristics we were looking for either. This left us at an impasse. Not wanting to give up on what we saw as a superior design, we started to investigate the possibility of producing a custom main tube for our recumbents. It soon became clear that doing so would require a significant capital investment to start with and would probably make our frames more costly in the long run, but we saw no other alternative. Luckily our investor shared our vision and was willing to go the extra mile on the tubing to help us deliver a truly new recumbent design. A recumbent bicycle design with a completely custom tube set. This tube set includes not only the main tube but the chain stays and drop outs as well. It has also been suggested that mono-tube recumbent designs offer little advantage other than they are cheaper to produce and that they suffer from having no depth of structure. This is simply not the case with our recumbent bikes and the custom tube set they are made from. We can assure that pulling custom tubing in short runs is a very expensive exercise (much more expensive than buying off the shelf). We will also assert that a good mono-tube design, using the correct tube, does a great job of dealing with the complex forces at work in a recumbent bicycle. Is this the only way to build a frame? Certainly not. Is it the best way to build a frame? Maybe. But rather than trying to sell you on one style over an other we would rather invite you to try them all, with an open mind, and decide for yourself. OK, enough with the words about our recumbent bicycles. Here is the straight scoop on our tubing… by the numbers. Using a computer program to do a “section moment of inertia” (SMOI) for custom tube shapes (or more simply stated… the stiffness potential of a tube shape) we calculated our tubing at 2.5” x 2” at .039” thickness, cubed, to be 1.95. (Tubing stiffness varies with the cube of its depth and because our tube is not symmetrical we needed to calculate the SMOI in both directions first.) The SMOI of a 2” round tube at .035” thickness is 1.04. Our tube is about twice as stiff vertically, and also substantially stiffer laterally and in torsion than our competitors 2″ round tubing. But we want to stress the vertical direction here because this is where most of the boom wagging forces would come into play with unsupported boom tube designs. Anyway, this shows that our custom main tube is 2 times more vertically stiff than a 2″ round tube of the same length.

Originally posted on bacchettabikes.com Nov. 18, 2008,