BB stiffnness (myths?) some thoughts

neeb

The marketing hype for frames is always going on about lateral bottom bracket stiffness and how it contributes to power transfer. Pros even seem to talk about it quite a lot, as if it is one aspect of bike design they really do care about (or their sponsors like them to talk about?)

One interesting thing I tried today makes me think that it must be largely b*llocks, although I'd be interested to get some input from people with more knowledge. If you put your bike up against a wall with the left side dropout and the handlebars firmly pressed against the wall, and then press down on the drive side pedal when it is in the 6 o'clock position, you will notice some flex. Now turn the bike upside down, again with the left dropout hard against the wall, and try flexing the frame using the pedals. The flex is much, much less, I guess because the vast majority of the flex when the bike is the right way around is actually due to the wheels, and particularly the tyres. The eurus wheels on my bike are pretty stiff, but still this is the case.

With my rather limited technical knowledge I can't see any other interpretation from this other than that lateral frame stiffness is usually going to be a very small percentage of total lateral bike stiffness in real life cycling situations, but maybe someone can enlighten me.

Another interesting thing that seems to come out of this is that lateral tyre stiffness seems subjectively to me to be the biggest factor in total lateral bike stiffness in real life situations. Might this indicate that higher tyre pressures could make you faster, not because of decreased rolling resistance, but because of increased lateral stiffness?

redddraggon

I pretty much agree with what you say.

IMO a certain level of BB stiffness is required, which most decent frames have these days, and the level of stiffness above that, is more of a personal preference than a necessity. Afterall a frame acts pretty much elastically, so if you the flex frame you'll get most of the energy returned on the alternating stroke. Obviously you don't want too much flex because then the bike will start changing gear for you without your consent, but a little flex isn't too bad.

Front end stiffness IMO is more important, as flexiness there will adversely affect the steering and feel of the bike

LJAR

http://www.sheldonbrown.com/rinard/rina ... etest.html
Check out some of the stats on there.

It seems that steel bike have some of the stiffest rear ends out there (stiffer than a carbon Trek 5200), but the front end is more bendy.

Stiffness isn't all its it cracked up to be, nor is light weight, but the marketing departments need to sell bikes. so "LIGHTER AND STIFFER" is what they have gone for. Despite the fact that frame flex is more stored energy than lost energy.

The stiffness of the whole package is the important thing, but also remember holding a bike still and flexing it is not the same as pedaling it while moving. The flex will be less on the road because the bike can move sideways instead of bending. Stiffer wheels and tyres may just make it more uncomfortable on British roads.

I think my next frame will be a custom aluminium, good fit is more important than anything else I reckon.

sandbag

redddraggon wrote:

if you the flex frame you'll get most of the energy returned on the alternating stroke.

What you don't see is, not all of the stored frame energy, returns to the wheels! A proportion of it, is sucked up and absorbed by the legs, body and muscles! By having to counter this energy force, it makes you fatigue more. You lose power.

So it not bollocks.

Keeping the BB stiff, stops the energy spreading through the frame, and that is why you can have the recent modern dampening chainstays for a softer ride, without effecting it's perfomance qualities from the stiffness.

redddraggon

sandbag wrote:

So it not bollocks.

Unfortunately everything you post on here seems to sound like bollocks. You seem to have ignored everything I posted, and just typed out a load of random rubbish, thinking you are contradicting what I said.

neeb

Re: possible OK BB stiffness of steel bikes, but bendier front ends - a German magazine report (I stuck it into Google translate) said something similar about Ti frames, they found that BB stiffness was comparable to carbon but that many of the bikes (not all) had front ends that were a little flexy (but not excessively so). Of course as someone pointed out on a blog somewhere, Germans tend to be a lot bigger and heavier than, say Italians, so probably need stiffer frames!

As the owner of a nice Enigma Ti bike and no carbon bikes, I am always having to fend off the urge to fall for the marketing hype and buy a carbon bike. I am very happy with my Enigma, which is light and certainly doesn't seem flexy in any way, it has great acceleration and tracks very well at the front end (I'm only 63kg anyway). The marketing is trying to tell me I would be faster on carbon, but the facts (and experiments like the one above) suggest that this is probably not the case.

I'm intrigued by the whole argument about energy from frame flex being returned during another part of the pedal stroke. To me it makes a lot of sense to suggest that different levels of BB flex (assuming it is even significant) might return energy to the system at different points in the pedal stroke. So if the amount of flex is just right, it could do something analagous to rotor Q rings in absorbing some energy during the most powerful part of the stroke and returning it during the "dead zone". If the degree of flex is too much or too little though, perhaps the energy is returned too late or too soon in the pedal stroke, possibly making the power distribution more uneven rather than more even.

sandbag

redddraggon wrote:

sandbag wrote:

So it not bollocks.

Unfortunately everything you post on here seems to sound like bollocks. You seem to have ignored everything I posted, and just typed out a load of random rubbish, thinking you are contradicting what I said.

Ignored? I just knocked hole straight through your opinion. Not all of the dispersed energy from the frame, goes to the wheels.

redddraggon wrote:

if you the flex frame

Try improving the grammar.

STEFANOS4784

Second time lucky eh Sandbag? Don't worry i don't think anyone saw

sandbag

You pay more for a performance stiff frame that offers comfort as well, for longer rides.

Barteos

Most of flex in a frame and components will rob you of some energy but it's largely IRRELEVANT for 99% of cyclists in 99% situations.
At typical cruising speed/power at the "proper" cadence, the load applied to the cranks is too low to cause any significant flex.
The cyclists that may be concerned about the "issue" of flex are either the pros or those who spend more time on forums than on the bike

rake

it feel nice to ride a bike with very positive power transfere and it got to be more efficient. its definitely noticeable pummeling up hills. its not the be all and end all but nothing is. its the way forward for design though if thery striving to make the best they can. ive contradicted what redragon said

White Line

sandbag wrote:

Ignored? I just knocked hole straight through your opinion. Not all of the dispersed energy from the frame, goes to the wheels.

redddraggon wrote:

if you the flex frame

Try improving the grammar.

Honestly? :roll:

redddraggon

sandbag wrote:

Ignored? I just knocked hole straight through your opinion. Not all of the dispersed energy from the frame, goes to the wheels.

Obviously you either cannot read or are just stupid, because this is what I said:

redddraggon wrote:

Afterall a frame acts pretty much elastically, so if you the flex frame you'll get most of the energy returned on the alternating stroke.

No where did I say:

sandbag wrote:

all of the dispersed energy from the frame, goes to the wheels.