Stiffness!!!

onthefells

I've read loads of posts on here on various bikes (and many magazine/net reviews )describing frames as stiff, seriously stiff, very stiff etc

Genuine question....has anyone on here ever ridden a bike with a floppy or bendy frame?????

More to the point has anyone devised a measure of stiffness? Or is all this talk really just that...talk? Aren't all bike frames stiff? lets face it you can't easily flex a carbon/alumnium/steel/titanium frame can you? Also what about the wheels, aren't they much more likely to be the cause of any 'perceived flex' than the frame?

Just a few thoughts on stiffness this bright Monday morning!!

Evil Laugh

I feel for the poor sod who found out his wheels weren't actually bombproof.

bazbadger

Ha, so true, about time someone stated the chuffin' obvious.

So,so many factors involved, wheels (like you say) being a major one, tyres, tyre pressure, forks, stem, bars - even before we get to the frame.

Ride the bike and enjoy it!

bazbadger

Evil Laugh wrote:

I feel for the poor sod who found out his wheels weren't actually bombproof.

If they aren't then I want my money back please.

JonEdwards

It depends how many bikes you've ridden and how much you chop and change between them. I'm certainly aware of the differences in some of my MTBs.

Road wise, I don't have massive experience, but what I am aware of is that when I'm attempting to corner hard on less than perfect tarmac, I can feel the frame twisting as the load through the BB increases and decreases. Can be quite unsettling, and makes it difficult to hold a line.

huuregeil

Yes, I've had a few frames that have been lacking in stiffness, notably torsional stiffness of the main triangle. You particularly feel it at higher speeds, e.g. going dowhill, where the handling can be unnerving to say the least, and if you sprint hard. This is in addition to speed wobble issues, which is also caused by a lack of stiffness in the head tube. Plus front forks too, they can make a big difference.

Worst frame I had was a cheaper early-generation look carbon frame (the more expensive ones - e.g. KG281 - were lovely!), where you could move the bottom bracket a mile if you stomped on the pedals and was scary to ride at speed. But also a lightweight Columbus steel frame (i.e. too light!) which had similar things going on, and an alu frame which speed wobbled.

As a taller guy, this tends to be more an issue with bigger/heavier riders and larger frames.

PS And all this is quite possibly distinct from magazine reviews harping on about stiffness ;-)

Lillywhite

huuregeil wrote:

As a taller guy, this tends to be more an issue with bigger/heavier riders and larger frames.

I think the taller/heavier riders had a problem with 60cm Reynold 753 tubing built frames in the late 80's/early 90's with whippy frames. :evil:

daveydave43

alright, just to throw this into the mix.
im 63kg, not a tall or heavy rider, and i can tell the difference in stiffness between my 'summer' cannondale Caad4 and my 'winter' dawes giro 300.
its most marked on the climbs; where the caad4 squirts up, offending gravity, the dawes squishes around a lot more. there is 2kg difference in the bikes but the caad4 seems more taught and direct than the soggy sofa dawes.

onthefells

davey, do you have the same wheels on both bikes? I guess 2 completely different bikes may be easier to detect varying degrees of stiffness however what about similar bikes from different manufacturers?

would be interesting to see the manufacturers devise a stiffness index. can't be that difficult as there are all kinds of industrial measuring devices that gauge tensile strength in any number of materials.

an example of what I'm talking about...I buy Procycling which has a 'professional bike review' every month.Pinarello Dogma 60.1.... 'fast and amazingly stiff'....Trek Madone 6.9 ...'its extremely stiff, very light'...Basso Astra...'stiff, light frame' etc etc...they might as well say 'yeah its got handlebars'

edeverett

I think Tour magazine from Germany does proper proper testing of a frame's stiffness. If you google it you should be able to find some results.

What these lab tests tell us about how a bike rides is a different matter. I heard a theory somewhere that a lot of what a stiff bike gives people is psychological rather than mechanical - you sub-consciously push harder against something that's stiffer.

onthefells

edeverett...couldn't help but laugh at that last sentence...I'll ask my wife if its true

wildmoustache

It does matter, though I agree with the general sentiment of the OP. It's laughable how many newbies here and elsewhere - who must put out all of 150W and will struggle to complete the Etape - fret about stiffness.

Try a cheap hire bike with crap cranks, frame, wheels etc. get out the saddle and stomp and you'll find that it does matter.

mroli

The only time I've really noticed it is on our old steel peugeot tandem. When we're full laden and I'm on the front, I go one way and about a second later the rest of the back end follows. V disconcerting!

onthefells

mroli...sounds like it was designed with the bendy bus in mind!!

tache agree re the old bike scenario but how does anyone tell whether the dogma is stiffer than the madone? unless they are being paid to ride them!!

wildmoustache

onthefells - I totally agree. Take any new mid to high-end frame with oversized tubes, BB, and HT and fork crown and they are all stiff enough for everyone here.

It's a fair point to say that front ends have got a bit stiffer over the past few years with the introduction of oversized tubing in that area. It does make a bit of difference - wildly exaggerated by the industry of course!

Bhima

I've heard you can lose about 1-5% power through flex during a hard effort.

If you're riding around at 300W, that's not much of a loss, but if you're a world-class sprinter putting out close to 2kW, that's a lot of energy you're wasting...

Put on the brakes, lean against a wall and put as much force as you can on the pedals. If you've got an average bike, you should see the chainring move sideways as the whole bottom bracket area bends in the opposite direction to the force.

MrChuck

I have no doubt that if you put enough power in some frames can flex. Just because it doesn't really matter to most people doesn't mean that magazines/manufacturers are talking bo11ocks. It's just up to you to decide how much you care.

Mark Cavendish could probably make my frame flex, but it's stiff enough for me.

EDIT When I was at school a mate had a cheap catalogue racer and whenever you stood on the pedals the back tire would rub on the stays!

rake

it makes the bike much nicer to ride. i think riding style has a big impact ie pulling one side of the bars while cranking hard.

Hibbs

huuregeil wrote:

Yes, I've had a few frames that have been lacking in stiffness, notably torsional stiffness of the main triangle. You particularly feel it at higher speeds, e.g. going dowhill, where the handling can be unnerving to say the least, and if you sprint hard. This is in addition to speed wobble issues, which is also caused by a lack of stiffness in the head tube. Plus front forks too, they can make a big difference.

Worst frame I had was a cheaper early-generation look carbon frame (the more expensive ones - e.g. KG281 - were lovely!), where you could move the bottom bracket a mile if you stomped on the pedals and was scary to ride at speed. But also a lightweight Columbus steel frame (i.e. too light!) which had similar things going on, and an alu frame which speed wobbled.

As a taller guy, this tends to be more an issue with bigger/heavier riders and larger frames.

PS And all this is quite possibly distinct from magazine reviews harping on about stiffness ;-)

+1 on torsional stiffness on larger frames. My winter hack's alu frame is shockingly flexy when I go over a ripple in the road when having a drink.

navier_stokes

Everything flexes, it is just a matter of how much.

You can't really give a stifness to something like a frame. Engineering materials have a stated stiffness measured in N/m but this is a very specific property for a laboratory test piece.

Measuring the stiffness of a frame is very different because the material properties of the frame change due to welds/joins, tube thickness, shape, length etc. Carbon fibre is even harder to analyse because its properties depend also on the exact composition of the carbon fibre and the layup.

Stiffness can be calculated for a whole frame using FEA but it's not much use to the average consumer and I doubt that many companies have the resource to do this - more likely it is based on past experience and trial and error.

You may give a frame an average stiffness, but then it may be stiff in all the wrong places, so to use it as a comparative tool is useless.

Either way, I think you can definitely feel stiffness, but it's hard to quantify.

Also, it is not just stiffness we are looking for - there's no good having the stiffest bike in the world if it doesn't give decent ride quality or is too brittle. Imagine a bike frame made out of granite or ceramics for example!

As with all engineering, there is always compromises. As a general rule however, the more you pay for a frame, the less compromised it is. I.e. you get greater stiffness, without sacrificing comfort and less weight. Obviously the law of diminishing returns also applies

DaSy

Science seems to show that energy is not lost when the bottom bracket area flexes, but that the energy is delivered back into the drive-train as the BB flexes back. Stiffness really isn't all we are led to believe in a bike frame.

Lifted from Bike Think -

Try this experiment. It will work best if you mount your bike to a trainer and disengage the resistance roller.

Put the cranks in the horizontal position.
Place a rigid block or stool under the forward pedal so that there is a small gap under the pedal.
While holding the rear brake firmly, stand on the pedal so that it is pushed down to the stool.
Keep holding the pedal down and release the brake.
When you pushed the pedal down, the chain did not move since the brake locked the rear wheel. Since the chain did not move, no work energy was delivered through the chain. The crank moved down with the pedal as the frame was strained. When you released the brake, the frame was able to move the center of the crank back up to relieve the strain energy. But the pedal remained in it's lower position, so the crank had to rotate around the pedal as the bottom bracket went up. There was a reaction force in the chain and the chain moved as the crank rotated around the stationary pedal. The strain energy of the frame was converted to rotation kinetic energy in the wheel.

onthefells

DaSy just gone dizzy reading that last paragraph!!

interesting point though, although there has to be some loss of energy going through that process rather than all the force going straight through the chain to the back wheel?

The Hammer

I've used all sorts of frames over the years, and whilst I've never known of any measurement for it, stiffness is really noticable! Cheap winter frames I've had have been sloppy, cheap aluminium frames also, but expensive ones like my Cannondale track frame and Cinelli Unica have been super stiff and responsive. I'm now on a Velo Vie Vitesse 300SE road frame, which is super stiff (and comfortable). I've also just renovated an old steal frame that my bro used to ride 20yrs ago... an old Paul Donoghue frame with Coumbus SLX tubing, which for its age isn't too bad (but nothing like as stiff as the better carbon / alloy frames).

leejdavies

onthefells wrote:

interesting point though, although there has to be some loss of energy going through that process rather than all the force going straight through the chain to the back wheel?

If there was one thing I learnt in Physics at school, it is that energy is never lost. Simply transferred if you transfer energy.

Energy is used to move the frame laterally and then that in turn is transferred to the wheel. So the energy isn't wasted. Kind of delayed pedalling I guess :?: :shock:

huuregeil

Yeah, but the desire for stiff bikes is mostly not about energy loss*, it's about handling and responsiveness. Argue physics all you like, it's very hard to quantify the benefit to the rider of a better frame, hence the endless debates (and meaningless magazine reviews!) :-)

* Unless it's binary, rubbing brake bocks, chain rubbing/dropping etc.

navier_stokes

DaSy wrote:

Science seems to show that energy is not lost when the bottom bracket area flexes, but that the energy is delivered back into the drive-train as the BB flexes back. Stiffness really isn't all we are led to believe in a bike frame.

Lifted from Bike Think -

Try this experiment. It will work best if you mount your bike to a trainer and disengage the resistance roller.

Put the cranks in the horizontal position.
Place a rigid block or stool under the forward pedal so that there is a small gap under the pedal.
While holding the rear brake firmly, stand on the pedal so that it is pushed down to the stool.
Keep holding the pedal down and release the brake.
When you pushed the pedal down, the chain did not move since the brake locked the rear wheel. Since the chain did not move, no work energy was delivered through the chain. The crank moved down with the pedal as the frame was strained. When you released the brake, the frame was able to move the center of the crank back up to relieve the strain energy. But the pedal remained in it's lower position, so the crank had to rotate around the pedal as the bottom bracket went up. There was a reaction force in the chain and the chain moved as the crank rotated around the stationary pedal. The strain energy of the frame was converted to rotation kinetic energy in the wheel.

This is absolutely irrelevant and quite a silly way of "proving" things.

If you are putting energy into moving the bottom bracket (because it is not stiff enough) then the total amount of energy you put through the rear wheel is decreased.

You cannot get useful energy back from a flexing bottom bracket...

Plus the test in no way represents what happens when you are cycling because you have an additional force on the wheel (the brake) which will never be there when you are riding. When you apply the brake in the test you describe, you are essentially making another connection from which force is transferred from the wheel to the frame (or vice versa). Essentially you are increasing the stiffness of the rear triangle.

In addition, in reality the crank can never rotate around the pedal - it is not "fixed" on a stool!!!! In your test there is a reaction force from the stool as you press your pedal against it - this does not exist when you are cycling!

APIII

Perhaps reviewers should use 'responsive' rather than 'stiff' when they're talking about bikes. Everyone wants to ride a responsive bike don't they? Most of the bikes I've ridden have a different feel, from a slightly soggy cheap alu frame to a super responsive carbon wonder bike. I agree on the point about the Procycling tests. Once you get to a certain level, they all feel good to ride, with only small differences seperating them.

Escargot

DaSy wrote:

Science seems to show that energy is not lost when the bottom bracket area flexes, but that the energy is delivered back into the drive-train as the BB flexes back. Stiffness really isn't all we are led to believe in a bike frame.

Lifted from Bike Think -

Try this experiment. It will work best if you mount your bike to a trainer and disengage the resistance roller.

Put the cranks in the horizontal position.
Place a rigid block or stool under the forward pedal so that there is a small gap under the pedal.
While holding the rear brake firmly, stand on the pedal so that it is pushed down to the stool.
Keep holding the pedal down and release the brake.
When you pushed the pedal down, the chain did not move since the brake locked the rear wheel. Since the chain did not move, no work energy was delivered through the chain. The crank moved down with the pedal as the frame was strained. When you released the brake, the frame was able to move the center of the crank back up to relieve the strain energy. But the pedal remained in it's lower position, so the crank had to rotate around the pedal as the bottom bracket went up. There was a reaction force in the chain and the chain moved as the crank rotated around the stationary pedal. The strain energy of the frame was converted to rotation kinetic energy in the wheel.

Sorry but that is the biggest load of cack I've read in a long time. How this is a scientific test is beyond me. It merely shows that the chain stores some energy but it is impossible to tell (without using other measurement devices) just how much energy is lost.

When you flex a bottom bracket energy is instantaneously lost in bending the frame. It is possible that some of this energy may come back trough the drive-train but how much of it is actually beneficial to propelling you forward is another matter.

In fact I'm almost certain than none of the energy would come back through the drive-train as flexing the frame will involve forces that are not in line with the motion of the chain/chain rings.

DaSy

As I said, I lifted the text from part of an article on Bike Think, which is here - http://www.bikethink.com/Frameflex.htm

Escargot wrote:

When you flex a bottom bracket energy is instantaneously lost in bending the frame

As someone said above, energy is not lost, but just converted to different forms, so in the case of the bb flexing, the only obvious way the energy would be lost is via heat, and there is no noticeable build up of heat in a flexy frame.

I do agree that a stiff frame feels and handles better and more predictably, but I think the impression that reviewers give, that a flexy frame is consuming more energy for the same speed etc is not really true.

balthazar

DaSy wrote:

As someone said above, energy is not lost, but just converted to different forms, so in the case of the bb flexing, the only obvious way the energy would be lost is via heat, and there is no noticeable build up of heat in a flexy frame.

I do agree that a stiff frame feels and handles better and more predictably, but I think the impression that reviewers give, that a flexy frame is consuming more energy for the same speed etc is not really true.

I agree. I don't believe that a flexible frame absorbs energy (which would have to be radiated away as heat), any more than any other spring.

Whether our bodies are efficient at pedalling flexible frames or not is a distinct matter. As a tall rider, I prefer frames as rigid as possible, for tracking control, particularly downhill.

Escargot

DaSy wrote:

As I said, I lifted the text from part of an article on Bike Think, which is here - http://www.bikethink.com/Frameflex.htm

Escargot wrote:

When you flex a bottom bracket energy is instantaneously lost in bending the frame

As someone said above, energy is not lost, but just converted to different forms, so in the case of the bb flexing, the only obvious way the energy would be lost is via heat, and there is no noticeable build up of heat in a flexy frame.

I do agree that a stiff frame feels and handles better and more predictably, but I think the impression that reviewers give, that a flexy frame is consuming more energy for the same speed etc is not really true.

Don't get me wrong I wasn't shooting the messenger so to speak but just criticising the theory

My comment regarding the loss of energy was the fact that whatever comes back will undoubtely NOT go back into propelling you forwards. As you are not permanently deforming the frame then it will store some spring like energy, some will possibly be lost as heat but I can almost guarantee that none will go back into the drive train. Hence it is lost.

I guess it's all down to terminology but in some respects the reviewers are kind of correct