General Cycling Discussion - How does stiffness benefit a bicycle?

I read all the time people saying "oh these cranks are really stiff it feels great", just commenting on a components stiffness. Usually the fork, frame, cranks and wheels. What i dont understand is how does this benefit the rider? I want a somewhere scientific reply with facts not juyst "ride a stiff bike and a flexy bike". With the cranks for instance, your still turning the cranks whether they are the stiffest thing or not,, and that still turns the chain and propels you forward. Can somebody englighten me?

Would you rather ride a tyre with 40 psi or one with 80 psi if your aim is efficiency?

stiffness is nice in the frame and cranks since it reduces flex, which means more of the force you exerted to turn those cranks is transferred to the rear wheel, thus more efficiency.

Some people like flex since they think it's "more comfy" or "works like a spring" and transfers power back...IMO flex is inefficient since any flexing does result in heat production, and heat production is pure wasted energy. Given the heat production by a flexy frame is too low to be of any real notice, but to racerboys who claim 10g is a huge difference, that frame flex would be an even bigger difference.

Yeah we sure care about the 0.001% more efficiency tradeoff with a super stiff ass frame.

No thanks.

In a touring bike stiffness is important especially around the bottom bracket area. The amount of torque generated when pedalling a heavy load can cause the frame to flex sideways losing energy which should go to the forward motion of the bike. My custom Mercian had a specially long b/b lug which was designed to counter this. The same problem with flex can be seen with track bikes--there is a famous photo of old-time track star Reg Harris where the frame is seen to have this huge flex as he pours on the torque.

In a touring bike stiffness is important especially around the bottom bracket area. The amount of torque generated when pedalling a heavy load can cause the frame to flex sideways losing energy which should go to the forward motion of the bike. My custom Mercian had a specially long b/b lug which was designed to counter this. The same problem with flex can be seen with track bikes--there is a famous photo of old-time track star Reg Harris where the frame is seen to have this huge flex as he pours on the torque.
So the reason that flex makes you less efficient is because the energy that would propel you forward is being used to flex?

So the reason that flex makes you less efficient is because the energy that would propel you forward is being used to flex?
Correct. It wouldn't matter so much if you had a contant-power motor. It would twist the frame once when you accelerated and the frame would return to normal when you slowed down. But with a bike, you twist the frame on every pedal stroke, and it untwists every time when you get to the bottom of the stroke. That's 3 times a second at 90 rpm, and it adds up. No power is transferred when it 'springs back.' Of course, you have to put out enough power to twist a given frame before it becomes a problem. What's stiff for me might feel like a noodle to a Cat 1/2 racer in a sprint.

So the reason that flex makes you less efficient is because the energy that would propel you forward is being used to flex?

When you mash on the pedals to sprint for example, you want all of said mashing to be transferred to the pedal and bike, not wasted in flexing of materials. It is analgous to hitting someone that does not move with a punch or hitting someone that takes some of the effectiveness by moving with the punch (the other guy is going to move with the punch allright, but find himself on his keister!).

Interesting question.

Either I'm way less sensitive than average or there are a whole lot of guys who are kidding themselves. Who can really feel flex while riding a bicycle and determine accurately where it's coming from?

First of all, you use a pneumatic tire that probably flexes more than everything else combined. Then you have the now standard carbon fiber fork. Your headset is adjusted by feel. Your stem and handlebars are aluminum lever arms with varying amounts of give. What you feel through your butt is also dampened by various suspensions.

If it does flex, what happens to it? Some energy is actually lost in the form of heat. Most of it, however, is just returned when whatever part flexed springs back.

There are different reasons for stiffness or lack of it.

It's easy for me to tell the difference when I put loaded panniers on my touring bike or an old road bike. The old road bike actually flexes. It's easy to feel. It's much easier to control the touring bike with a big load. I like the stiffness.

A flexible bike can flex so much that the bottom bracket area bends during a sprint and the chain hits the derailleur. This is something powerful sprinters don't want. My track bike is so stiff it makes my joints ache compared to my flexible steel road bike converted to a fix. And I can actually feel the stiffness in the cranks, as a track racer would like. I wish it were a little less stiff.

My Litespeed Ghisalo is so flexible that I can grab the pedal and bend the frame by hand. I can squeeze the top tube by hand and it deforms slightly. It's super comfy for century rides. I can actually feel the thing flex over a bump. I can feel it rebound from the flex. In this case I like the flex.

Racers want stiffness to propel the bike faster in a sprint in case the finish is close and decided by a fraction of a second. In this case the stiffness helps, there is no delay in the forward travel. Or in case they need to accelerate fast for some reason.

I was never able to feel, understand, or really believe in the difference in flex until I had all these different types of bike in my garage and I could ride one, and then ride another right after. Without having them available to test ride one after another it is very hard to tell sometimes. Especially if it's a few days later.
Also some people notice these things more than others. High mileage riders tend to notice more.

By looking at the track bike I would not have believed how stiff it is. There really is a difference in a bike properly designed for it's job.