Originally Posted by
Mike Mills
There are many racers who intentionally introduce flex into their frames in order to improve their performance. Part of that "responsive feel" you mention is low hysteresis spring back.
I seem to recall the Paris-Roubaix (?) racers all wanted a more flexible frame because a significant portion of the race was over cobble stones and other poor road surfaces. A super-stiff frame was hard to control and beat the racer to death. Mountain bikers also introduce LOTS of compliance into their frames (front and rear suspension) for the same reason.
I think we all know we want a little compliance in between that front wheel and our hands, too. The fork stiffness, handlebar padding and gel inserts in our gloves is how we get it.
I'm not sure the OP was restricting the question to just racers, either. What about a touring bicycle?
The frames of UCI pro roadies are some of the stiffest bicycle frames made. They are about pure efficiency, getting the produced wattage onto the road, with minimal compromises. Racers absolutely do not want to introduce 'flex' into their frames.
What they do want is bikes that don't compromise efficiency for the sake of comfort.
A steel bike, aside from the nonsense you'll hear from the 'steel is real' cult, is just generally inefficient, and just generally flexy. The benefits of comfort come at the cost of pedaling efficiency and stiffness. If you don't believe this to be true go test ride a Santana steel tandem some day then go hop on a Cannondale tandem and the difference will be beyond evident.
A modern race bike can utilize a carbon front fork, a carbon seatpost, a carbon stem, and carbon handlebars to contribute to racer comfort. The frame itself is all about performance, not comfort.
However, carbon frames can be laid so that there is vertical compliance without any compromise of lateral compliance or drivetrain efficiency. Essentially building into the bike 'micro' suspension for the cyclist isolated from the drivetrain. This is possible, to a lesser degree, with titanium frames as well (ti can be made stiffer than carbon or aluminum, or made merely to be 'better' steel).
As for Mountain bike suspension frames. These are the stiffest and strongest frames of any bicycles. The frames are designed oversized and overbuilt, and are specifically engineered to not flex, but rather, to let the suspension absorb the riding surface irregularities and impacts. The frames on these bikes are off the scale in terms of how small the rear triangles are, how stiff the frames are, and how strong they are.
You can use inferior frame materials like steel, and make them less inferior with compact geometry. You can also take epic frame designs made from Titanium, aluminum, and carbon and make them even more legendary.