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I have a Specialized Roubaix with a 7r carbon frame; the Tarmac has 11r. Will I notice a speed change if I ride "exactly" under the same conditions? Does the additional 4r = more speed? I am thinking in upgrading but I dont want to make the investment without some good feedback.

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It needs to be noted that the largest "damper" on the bike is the cyclist. The body vibrating up and down absorbs more energy than the frame ever could. Minimizing body vibration (while also minimizing tire losses and losses in any suspension components) is the way to avoid "lost" energy. –  Daniel R Hicks Jul 13 at 20:16

4 Answers 4

General rule of thumb, a stiffer frame will absorb less of the input energy and transfer more energy - hence more power from you legs means more power to the wheel.

But....

The bikes have different designs, so the aerodynamics of the bikes and the rider on them will be different which will result in different speeds.

And then...

A stiff frame will transmit the road imperfections into the rider causing fatigue and causing the rider to expend more energy because the bike/rider is being lifted up by road imperfections where as the more flexible bike will absorb them meaning the bulk of the mass is moving forwards.

An ideal bike is stiff where it needs to transmit peddling force, but has shock absorbent qualities to absorb bumps and road imperfections while being aerodynamic for both bike and rider.

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The two somewhat opposing things (stiffness versus comfort) is partially why carbon can be really good in a bike; with the right weaves in the right directions in the right places (and hard to get all this just right) you can have stiff in one direction and comfort in another. To see a real good combination of both, try a bamboo bike like the Boo. –  Ken Hiatt Aug 27 '12 at 23:17
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It should be noted that, to be perfectly accurate, a stiffer frame does not "absorb less energy". Energy is absorbed by damping, not flex. A stiffer frame may (or may not) help "direct" the cyclist's energy better -- it depends on a large combination of factors. –  Daniel R Hicks Aug 28 '12 at 1:08
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"General rule of thumb, a stiffer frame will absorb less of the input energy ..." No, and this is the biggest myth regarding bikes. As noted by Daniel, it's damping that causes power loss, not stiffness or lack thereof. Second, added stiffness (in the vertical plane) can adversely affect comfort. Third, ultra-stiff bikes may not suit everyone. A very small study showed that added frame stiffness resulted in decreased power output. It was attributed to the fact that the frame actually acts like a spring and adds to your pedal strokes. –  Dissenter Jun 7 at 7:50
    
Also, stiffer carbon fiber generally means more brittle carbon. Finally, although the Tarmac 11r might use stiffer carbon, it almost definitely isn't using stiffer carbon exclusively. It's probably a few layers here and there as using stiffer carbon exclusively would likely result in a brittle and very impact-failure prone frame. Conclusion: 11r vs 7r, 8r, 9r, etc. is mostly marketing. Added stiffness may help you, although no one has really shown this to be true. It may hurt you, and a small study has shown this to be true. It may do nothing too. –  Dissenter Jun 7 at 7:53
    
janheine.wordpress.com/2011/10/03/… (yes, it's a blog, but it's a summary of some article published). Also, I need to revise my first statement way way above - strain energy is one way flex can sap power (fairwheelbikes.com/c/reviews-and-testing/…). The strain energy is returned though as bike frame are like springs. Whether this returned energy is productive or not is another issue. –  Dissenter Jun 7 at 7:54

The stiffer a frame is, the more it resists flexing under load.

For the sake of demonstration, say you have a carbon frame that under a load of 400 watts of pedaling power has 2" of flexion. The actual power going to the wheels is 400 - Cost to flex.

Now you take a frame that only flexes 1". Less of your power goes into flexing the frame, so more power gets translated to the wheels, which results in a faster bike.

The stiffer a frame, the harsher the ride in general, so it may not be quite as comfortable.

If you are familiar with cars at all, it's the same concept behind horsepower at the crank, versus horsepower at the wheels.

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Ditto what I said above -- flexing the frame, per se, does not absorb power. –  Daniel R Hicks Aug 28 '12 at 1:08
    
Noted. I was trying to keep it simple. :p –  JohnP Aug 28 '12 at 2:34

As with motoring, you can use stiffer engine mounts so as to help transfer more power from the engine to the wheels. However, I'm told that it can make for a hideous ride quality in the vehicle. Now Imagine that on an ultra stiff road bike. Great on silky, smooth, new tarmac - but in the real world, wincing on every little bump in the road will affect your speed negatively. I know I stop pedalling and wince as I roll over rough bits of road. Mind you- great for powering up the hills I expect.

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I would question whether stiffer engine mounts are effective in motor vehicles, even more than I question a stiff bike frame. As with bikes, they may be "fashionable" for folks with insecurity issues, which would be the attraction. –  Daniel R Hicks Jul 10 at 11:39

The notion that stiffness equals greater performance is more true in the lab than on the road. Sean Kelly won hundreds of races - often in sprints - throughout the 80s and he won most of those on a Vitus 979 aluminum bike that was probably the most flexible bike used in professional racing in the past 50 years.

It's pretty unlikely you'll notice much difference between the two frames. Also keep in mind that for the comparison to be accurate every single other part on the bike would have to be identical. Bars, stems, wheels, cranks all contribute to stiffness.

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