Technical FAQ: Sitting on a carbon top tube at speed, especially a counterfeit one

This article originally appeared on Velo News

Have a question for Lennard? Please email us veloqna@comcast.net to be included in Technical FAQ.

Dear Readers,

I have been thinking more about last week's topic of sitting on the top tube of a carbon bike and have more to say about it. I was addressing static sitting on the top tube, when the bike was stationary, and I think that this is how the manufacturers who weighed in on it were, too. I started thinking about sitting on the top tube when tucking to increase descending speed, and I think that opens an entirely new can of worms.

Descending positions 1, 2, 4, and 7 (Taylor Phinney, Peter Sagan, Matej Mohoric, and Chris Froome), when the rider is not sitting on the saddle, have all been banned in racing by the UCI. However, since they are clearly faster than other descending positions, there are lots of riders still using them recreationally and in training; I certainly use the Sagan position a lot.

Also read: Analysis: What is the fastest descending position?

Think about Chris Froome on the descent of the Col de Val Louron-Azet during stage 8 of the 2016 Tour de France. He was flying down that descent, way off the front of his saddle, often not only sitting on his carbon top tube, but also often pedaling while sitting on it. Hitting bumps and dips in the road in that position would certainly put more pressure on the top tube than static sitting would, and the consequences of a top-tube failure in that instance would be much more catastrophic.

The responses from Alchemy, Pivot, and Specialized indicate that those companies have confidence that their frames would suffer no damage from sitting on the top tube, while carbon-repair shops Broken Carbon and Calfee warn against it, having repaired frames that cracked from it.

A well-made molded carbon frame generally has continuous carbon fibers curving around the top tube/seat tube intersection that would probably prevent the frame from separating, even if it were to develop cracks there due to sitting on the top tube.

Modern, high-end, molded frames often also have a joint in the rear section of the top tube, just forward of the seat tube, since the top tube/head tube/down tube is often molded as a single "monocoque" piece, and the seat tube/bottom bracket shell is correspondingly also molded as a single piece with stubs sticking off of it for joining to the top tube, down tube, seatstays, and chainstays. I think that you can generally be confident of the bonding of that joint on the top tube against separating when sitting on the top tube. Same goes for quality carbon frames built in a tube-to-tube construction method, where the carbon tubes are mitered much like a frame made of metal tubing and are joined to each other with resin-impregnated carbon fibers wrapped around the joints; I can't imagine a well-made one separating at the seat tube/top tube intersection.

However, I started thinking about an unsuspecting rider descending on a counterfeit carbon frame. He or she would have bought that frame thinking that they had gotten a great deal on a brand-name frame and would not understand the risk they are taking every time they ride the bike. Counterfeit carbon frames not only are shaped, painted, and decaled to look like the brand-name frames they are mimicking, but they are often also built to weigh the same as those brand-name frames, since that is something the customer can check with a scale.

Envision the danger. Toray, which manufacturers the fibers used in most carbon bike frames, only sells the super-strong, high-modulus fibers, like Toray T1000, to certain high-end manufacturers. In some cases, the restriction on dissemination of certain fibers has to do with military defense technologies that are only released to defense contractors who have agreed to not export them and to only use them in completed products (which then can be exported).

A brand-name, superlight frame with these exclusive fibers in it would have precision-cut each piece of unidirectional pre-preg (already impregnated with resin) carbon fabric and would have them aligned by properly-trained workers in a carefully-engineered layup pattern with alternating directions in each layer to properly oppose the stresses anticipated at any given section of the frame. They would have then molded it, under heat and pressure, in such a way that there is no excess resin between layers or squeezed out into folds in the inflated membrane inside during molding. A low-end manufacturer pumping out counterfeit frames would neither have the high-modulus fibers nor the layup engineering, nor the precision molding techniques that prevent resin concentrations.

Resin weighs more than carbon fibers, so if there is excess resin solidified within the frame, and it still weighs the same as the superlight, high-end, brand-name frame, that means that there is less carbon in it, and the layers are less densely packed; plus, the individual fibers are not as strong. Now imagine going fast downhill while sitting on the top tube of that counterfeit-carbon frame and then hitting a big bump. In that circumstance it's not hard for me to envision the top tube cracking cleanly through with the rider's butt smacking down on it hard.

If the top tube separates from the seat tube while weight is still providing downward pressure on the frame, especially while it is also subject to lateral and torsional forces due to the motion of the bike, the down tube will have to break somewhere as well. The bike is now in two pieces while it is going fast downhill with a rider on it. I cringe at the thought.

I think that the warnings from Craig Calfee and Brady Kappius alone are enough to give someone second thoughts about sitting on the top tube at all, much less while descending, and especially while descending a bumpy road. And if there is any question whether the bike's frame is actually genuine, made to the standards that the brand and model on it should guarantee, those concerns ought to be multiplied many times over to the point that the rider absolutely ought not do it. In the immortal words of Forrest Gump, "And that's all I have to say about that."

-- Lennard

Lennard Zinn (https://www.velonews.com/byline/lennard-zinn) , our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder and purveyor of non-custom huge bikes , a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”
He holds a bachelor's in physics from Colorado College.

Follow @lennardzinn

For exclusive access to all of our fitness, gear, adventure, and travel stories, plus discounts on trips, events, and gear, sign up for Outside+ today.