Disco Stu

Assuming frame flex does waste energy, if someone gets off the saddle to sprint, wouldn't there be vertical flex (along with lateral flex) on a bike with "vertical compliance"?

I can't see how vertical compliance is any more preferable than lateral compliance.

Brian Ratliff

"Vertical compliance" is for comfort at the expense of efficiency. "Lateral compliance" has no benefits and just makes for decreased efficiency and squirrelly handling.

waterrockets 

Neither one affects efficiency. It's all what you're comfortable with.

abeyance

sounds like someone hasn't ridden a 1980's Cannondale....

njkayaker

No, something with no "vertical compliance" would transmit the force from a push of the pedal immediately to the tire/road. Something with "vertical compliance" would first bend the frame (taking some energy) and then transmit the remaining force to the tire/road. Thus, less vertical compliance is more efficient and faster.

Yes. People (other than pcad) would tend to choose less vertical compliance for shorter rides (on better roads).

waterrockets 

So you just flex the frame downward with every pedal stroke until your BB hits China?

Remember, it comes back up too, pushing against your powering foot, which translates to the drivetrain.

Bontrager

They're not talking about the BB when talking about vertical compliance.

waterrockets 

I would think they're talking about the dropouts moving up and down in relation to the geometric center of the frame. So, either the BB is moving, or the seat tube is a rubber band.

Yoshi

How efficient is frame flex in transmitting energy?

patentcad

Then there is date compliance, which is enhanced by alcohol.

KidTruth

It exists because.. what else would bike reviewers say to try and sound smart?

mihlbach

You might think "vertical compliance" is over rated, but my ultra-stiff oversize steel-tubed track bike, which I regularly ride on the road, tells me otherwise. Its awesome on smooth pavement, but it bounces and bucks over bumps and waves in the road that my road and cyclocross bikes easily absorb. The difference is very apparent, and its downright scary when cornering fast over wavy pavement.

The_Cretin

Vertical compliance and lateral stiffness are very important. Would marketing lie to us?

Point

Please show your proof of decreased efficiency. On a green engineering pad, state the problem, assumptions, and show your work. Mark your answer clearly at the conclusion of your work. Quantity, if possible any decrease in efficiency by both lateral and verical frame flexing.

I'll wait for the proof

ezee

This makes me wonder.
How much theoretical vertical (or horizontal for that matter) "compliance", deflection, sway, flex etc. could one achieve before the paint cracks?

AEO

paint is elastic enough that you wouldn't see cracks unless you broke the tubing.

I see it like this, since I experience this myself...
Lateral frame flex is bad when you have to control a squirrelly ride, you put more energy into controlling the bike than just putting more power into going forward.
Vertical frame flex is good because your bike doesn't beat the crap out of you, which fatigues you quicker from having to keep the bike under you.

Fat Boy

Good question. The answer is how much heat is produced due to that frame flex. Have you ever rode your bike over rough pavement and noticed it getting warm to the touch? Of course not. Without heat being produced, there are no losses. You could argue delayed response (although we're talking small deflections and very small phase shifts in response). That is different from a power loss.

I'm with WR on this one.

AEO

technical term for it is spring potential energy

there is some loss to heat, but it's negligible enough that you won't notice any heat build up.

human muscles are by far worser than any frame for just absorbing the energy rather than returning it to the pedals. Kangaroo muscles on the other hand are so elastic and springy that they barely lose any energy hopping around like they do at high speeds.

logdrum

By design it is hard to imagine some vertical compliance without a corresponding lateral flex. If it is vertical flexing you betcha it is laterally twisting.

You think you have a VC-LS bike, you are just not strong enough to bend it sideways.

I think most bikes are laterally stiff, it's the vertical compliance I am having a problem about.

Data would be nice for this VC-LS BS.

none

Any efficiency losses that occur due to frame flex result from losses in the system's (the frame's) energy. The largest contributor to this is probably the damping of the frame material, as the damping resists motion, and thus when the seat- and chainstays flex, the material's inherent damping resists the deflecting and returning motion, which should create heat (as already pointed out). No detectable heat is created because the losses are so small, but all materials have some (small) damping properties when they deflect and return, so there will be a small loss in efficiency, although it may well be negligible.

Think of a dual suspension mountain bike - if the rider stands up and hammers on the pedals, there may be some rear suspension bob, which definitely does decrease efficiency. Road bike frame flex is similar, but on a much, much smaller scale.

Disco Stu

gee, good answer

AEO

This is why we have shaped tubes like Rovals and Squovals. Perfectly cylindrical tubes will flex the same in any direction, but the double triangle diamond shape of bikes adds to it's vertical stiffness. So, like you say, it's pretty weak to torsional and lateral forces.
Certain shapes have differing amounts of flex for a given amount of force in X and Y directions. Let's say, an oval will flex more on the thinner side and will flex less on the thicker side.
What that means is that the engineers from the manufacturers will play around with the shaping of the tube to get desired flex in X direction without sacrificing Y and torsional stiffness.

some examples are specialized Roubaix and Tarmac top tube, Cannondale's hourglass seat stays, Cervelo's Squoval tubing.

Now, hearing the engineering is nice, but do you actually feel it? Well, that's subjective to the rider and his setup. I definitely feel the CAAD9 is much stiffer laterally (vertically as well) than Columbus SL is. Does it translate to more power efficiency? Not too sure, I don't have numbers to backup any conclusions I come to.

zzzwillzzz

most of you are missing the point. frame flex is very little about loss or transmission of energy but more about feel and handling. frames that have lateral flex feel sloppy and are harder to handle in a sprint but frames that can control the lateral flex while allowing the vertical feel more comfortable over bumps but still sprint well.

Brian Ratliff

I have a ring of cracked paint around one of the tube joints near the bottom bracket of my Madone.

531Aussie

is the frame busted? That's how I found a crack in one my alu frames: I saw a crack in the paint that didn't look like a scratch, but like a miniature 'squiggly' fault line.
Sure enough, after I sanded off some paint, there was a crack underneath