How is this notch under the seat post not a tremendous concentration of stress that is just waiting to snap and render the rider genderless?
by uberbeetle
How is this notch under the seat post not a tremendous concentration of stress that is just waiting to snap and render the rider genderless?
by uberbeetle
5 Comments
I think you underestimate just how strong carbon fibre is.
Would you question this if the exact same frame was made with steel?
Carbon fiber has multiple times the tensile strength of steel.
I think it’s important to keep in mind that for a high end race bike, the forces that the upper seat tube has to contend with are pretty trivial, it’s basically just needs to deal with a fraction of your body weight. Your eyes go that that gap, but remember that the very thin rails on that saddle is also carrying similar loads with zero issues.
There were issues with the Gen 7 Madones. The Gen 8s seem OK.
In other words, you’re correct. It is under a lot of stress. And Trek did in fact bork it up even after lots of design and testing. But carbon is strong enough if laid up and designed the right way to make even a cantilever design like this work.
>How is this notch under the seat post not a tremendous concentration of stress
It is.
>that is just waiting to snap and render the rider genderless?
Because Trek knows it is a significant concentration of stress. They have done all of the engineering and testing work required to build up the carbon in that area strong enough that even the greater concentration of stress does not exceed the carbon’s structural limits – at least, for any rider inside their weight limit under any reasonable riding condition.
There are *lots* of bike designs that are technically a “tremendous concentration of stress” in one specific area. For example, any bike with no seatstays (such as many triathlon bikes) see a ton of bending loads concentrated in the chainstays and bottom bracket area. Any bike with comfort features that rely on intentional frame compliance, such as the old Zertz Roubaix’s, probably see a ton of stress concentrated in the compliant area of the frame; that’s why the frame is able to deflect there in the first place. The Lauf Grit fork is this same idea taken to an extreme.
The reason most of them don’t typically randomly fail is, again, because the engineers know what they’re doing. They *know* that the frame is concentrating stress in those areas. They’re going to build up those areas sufficiently that they can withstand the stress anyway.