When is flex in a bike frame a good thing?
 

It seems that all you ever hear about is how dadgummed stiff a bike frame is as if that is all that is important. I don't pretend to know it all but I would imagine that in certain situations, flex would be a good thing. It seems to me that if a bike flexes, that there would be some kind of rebound energy passed on to its rider. Perhaps this would be beneficial in a crit turning corners at high speed. Anybody know more about flex?

Elastic flex is like a spring, it gives back all of the energy that goes into it. There's been studies ad nauseum that proves flame-flex does nothing to affect the efficiency of a bike of it's speed. Check out Chester Kyle's "Bike Tech" series of newsletters or Ed Burke's "Cycling Science". Flex is more a control and comfort issue for the rider.

In my middle years of racing I weighed a slim 145lbs with 4% body-fat and rode the fat-tube Cannondale frames (before they came out with the slimmed down more-flexible 3.0 version). It was by far the stiffest frame ever made with less than a 1/8" lateral defection at the BB with a 200lb sideways load on the pedal (all other bikes I've ever tried had 1/2" of lateral deflection).

Anyway, all my time-trial times were identical, compared to my earlier wet-noodle Vitus 979 and Peugeot Comete frames. Downhills on my favorite roads were actually slower because the tyres would skip under cornering over bumps. Trail-braking was impossible to control as a result, so I reverted back to the amateur-technique of maximum-braking in a straight before entering the corners. Long 4-5 hour 80-110mile endurance rides beat me up like never before, It would take 2 days to recover from one of those instead of just 1 day like with the softer frames.

One area where the super-stiff Cannondale excelled was in smooth crit courses. The response to the steering was crisp and quick, almost telepathic. I had reverted back to downtube shifters because I hated how STI's 1/4-lb of extra weight on the handlebars slowed down the steering response. Sprinting was a joy when I could have a solid base to use to push against people; on the Vitus & Comete, I could feel the frame flex a little when getting slammed or slamming people in sprints. One thing I did notice was that the flexy handlebar and stem was even more noticable with a super-stiff frame. I went to a steel track handebar to take care of that feel issue. I went back to my earlier Peugeot Comete for training, TT and road-races and used the Cannondale only for crits.

I dumped that Cannondale when we got Allez Epics. It was an intermediately stiff frame in between the superstiff Cannondale and the earlier soft Vitus & Comete frames I used earlier; a perfectly balanced bike that I could use for everything. If I had waited another 2 years, I might have picked up the Cannondale 3.0 which would've fit the bill as well. Something that doesn't pound out the fillings in your teeth... :)

That flex is wasted energy that could go to the road.

True, flex is wasted energy but flex is good when the frame flexes and doesn't break first.

Tim

There's a difference between elastic and inelastic flex, look up any 1st-year mechanical-engineering text. When the load on the flexed part is relieved, it flexes back into its original position and returns the engery. The frame basically pushes back on the leg, this is sensed as increased backpressure on the pedal and an opposite push is deliverd to the chain and rear-wheel

Inelastic flex is like the dampers in a shock or a piece of clay. Pushing on those parts causes a permanent change in position that requires further energy to bring back into the original location.

Show me just one double-blind study that has passed peer-review where frame-flex results in less power making it to the rear-wheels and causes slower speeds.

WTF..shock dampners are not inelastic flex, it's hydraulic flow.

Elastic flex does transfer energy, but not 100%, there is always loss in energy transfer.
To stiff can lead to metal fatigue as mentioned, it can also lead to rider fatigue. When considering the design of a bike, the art of it is in realizing that humans are not machines, and sometimes the most theoretically mechanically efficient design is not a good bike to ride, and therefore inefficient.

So again, per the above responses, except for the one that said that only stiff translates into power to the road, it seems that flex in some cases can be beneficial and that stiffness can be detrimental. Based on the responses, it would seem that ti would be the optimal material; a proper blend of stiffness and flexibility. Still, it seems that most are only concerned with the stiffness of a bike and that nothing but the most stiff will suffice. Cannondale abandoned that philosophy long ago. Many would agree that at one time, they produced the stiffest bike on the market. Great bikes for sure but they tended to beat your spleen out. Their bikes now offer a good balance (at least for aluminum) and are light at the same time. Any idea where this idea of stiff is the best property came from? Im still a bit perplexed about it.

That flex is also absorbed vibration that DIDN'T go to your hands and seat! Flexy frames are comfortable frames. Not racing? GO FLEX!

I agree with you whitemax. This is not a technical response, but in road cycling, as in LOTS of other sports, there can be a frenzy, many times brought about by marketing hype, that convinces the customer they need to have the "stiffest, lightest, dampest, you-fill-in-the-blank____est", where in reality, you get to a point of diminishing returns. I have 2 road bikes, a full carbon race bike at 7.0 kg that's very stiff, and a 10 kg chromo century bike that's got some flex to it. The carbon bike, even though very stiff, could probably be made even stiffer, and even though it now has excellent power transfer efficiency, could possibly be made with slightly better efficiency. But the difference would be so miniscule that it might translate to maybe a 5 second faster time in a 20km hilly time trial. In the mean time, with that extra stiffness, you get jarred around, and if the course has any rough road sections, you might just lose those 5 seconds negotiating the rough roads. For me, I'd gladly take the comfortably stiff bike, give up the bragging rights to having the stiffest bike, and probably have an extra $500 in my pocket. BTW, in just a casual test a few months back, on the same 13km hilly time trial course, my time on my "flexy" chromo century bike was only 10 sec. slower than my time on my full carbon "stiff" race bike. Just my 2 cents.

Cheers! - RJ

It doesn't matter how elastic the bike frame is. The 20lb elastic bike is attached to 165lbs (in my case) of floppy inelastic "meat." A bike that flops up and down dissipates energy in the body of the *rider*. So having a perfectly elastic frame is almost completely irrelevant.

You want vertical compliance to soften bumps but rotational stiffness to minimize bb deflection. Very ridged front triangle; combined with flexy chainstays and forks.

there is no such thing as perfectly elastic flex. every time the frame strains it will absorb some energy that is converted into heating/vibrating the frame slightly. but the flip side of this is that EVERY frame will strain to some extent. it might not be noticeable on some, but its still there. so no matter what, every frame will be absorbing some minute amount of energy.

while i do agree that a more flexible frame may not neccissarily be slower, i dont think you time trial exaple validates that. in a tt you use a very fluid, low torque, in the sadle stroke which isnt going to flex any frame very much. i doubt the same evidence could be derived from sprinting or intense climbing.

"WTF..shock dampners are not inelastic flex, it's hydraulic flow."

Dampers absorb the energy the spring would've returned to the system pushing the handlebars back up. It's the hydraulic flow that's inelastic and if properly calibrated, it will absorb exactly as much energy returned by the spring as was put into it in order to compress it. This energy is then converted into heat. Take the spring out of the shock and you'll find that any position you push the shock to, will require a certain amount of energy that's never returned.

The thing is that the amount of energy wasted in elastic flex is so miniscule that the amount wasted between a super-stiff frame and a wet-noodle is irrelevant. Take a front-shock with no dampers. Hit a bump and it bounces for a long time. Hang a spring from a ceiling and attach a 10kg weight to it. Pull on the weight and release, the spring will bounce for a long time. The energy is transfered back and forth very efficiently.

Also the frame is just one part of the entire bike+rider system. Tyre-friction losses are way higher than frame-flex loses. Wobbling knees and bouncing shoulders waste a lot more energy than frame-flex. Skin-friction of air ruffling over edges of sleeve and rear-pockets wastes much more energy than frame-flex. All of these factors have quantifiable measurements that have been determined through experimentation. Not a single piece of data has EVER been obtained to show that frame-flex results in any change in speeds or energy loss. All the studies have shown the exact opposite, that energy-transmission is identical and speeds are identical. Check out all of Chester Kyle's "Bike Tech" publications and Ed Burke's "Cycling Science".

"i doubt the same evidence could be derived from sprinting or intense climbing."

Again, more energy is wasted moving the body and bike around than's wasted in frame-flex. When you let off the pressure on the pedal after the downstroke, the unflexing frame will actually push back up on your foot.

You seem to be contradicting yourself. A floppy frame is going to set the shoulders and the rest of the body into motion, dissipating energy. So the energy lost in the frame itself is negligible. Who cares? The system of bike+rider is anything but elastic.

You're argument is nonsensical -- akin to saying that an RC electrical circuit does not dissipate energy because capacitors don't dissipate energy.

We all should know that the force put into the frame absolutely cannot be returned 100%, also the energy returned is it the vertical plane, making it wasted energy in propelling the bike on the horizontal plane. "Inertia is a property of matter" -Bill Nye... I dunno, quoting Nye is always good. :)

when is flex in a frame a good thing?

http://softride.com/images/product/large/19-N0004.jpg

You have to remember that human beings are very inefficient, and as a poster above said, 165lbs of jiggling, inefficent bag-o'-bones sitting on top a bike wastes more energy pedaling than the flex encountered in any frame of any material.

Flex is actually a good thing, especially when the design of the bike helps dissipate that energy and not concentrate it into any stress-risers that will lead to failure, or directly translate those forces into other components stressing them unnecessarily.

It's all about dissipation in my book. Whether a frame flexes .3 of a mm or .6 of a mm at the BB under load is pretty much totally irrelevant as far as I'm concerned.

That's what I'm saying. I'm not contradicting myself. Moving your shoulders around due to non-round, non-efficient pedal motion is what causes the bounces. Whichever way you move your shoulders will require an opposite force to move it back in the other direciton. Upper body weighs a lot more than a frame and you don't get back any of that vertical or lateral movement.

I'm not talking about qualitative, all-or-nothing, yes/no blankset statements here. It all comes down to the numbers and how you juggle the numbers. The numbers for frame-flex is so insignificant, it's not even worth discussing. A rubberized skinsuit gives way more benefits over a normal 1-piece skinsuit, which gives way more benefits than any difference in the softest vs. stiffest frame. We should be discussing the benefits of tucking or not tucking in a T-shirt at the back. If you really want to improve your hillclimbs, take a big dump before the ride and toss your bottles at the bottom.

BTW, the frame-flex we're talking about is lateral deflection due to offset location of pedals (Q-factor). Vertical deflection is non-existant and 70% of it is in the fork and the remaining 25% is in the tyres...

heh, heh... it's more like 30-40mm of lateral deflection under a sprint or 20-30mm on a steep hill... Even reducing that to 1mm of flex won't yield any benefits in speed... :)

Danno thanks for the explaination... (Saves me words! :D )

Wouldn't the loss of energy be related to the coefient of resistution of the material (steel, aluminum, etc), and also wouldn't the value be close to 1 anyway (perfect elastic movement? If the value was real low (say .5) the frame would get bent more and more and eventually the material threshold would be comprimized and break?

Frame flex is more of an annoyance than a energy waster. Now if you take say a rear shock it is another story because the movement of the cranks in relation to the vertical component of motion (i.e. up and down) of the legs changes the action the the muscle slghtly, allowing less power draw in certain motions without a change in lever arm on the crank (i.e. leg does not stretch). That difference may be small but the power output must change, expecially because a Hard tail MTB is much more efficient per WATT than a FS MTB>... and we are not even talking forks.

AHHH!

Depends upon how much load you apply and on what material.

1. With steel, if you stay below the fatigue limit, the flex is 100% elastic and the material goes back 100%. Most of the load on steel frames is well below the fatigue limit.
   
2. Above the fatigue-limit and below the yield-limit, the material accumulates microscopic surface fractures. It's still 99.99999% elastic (depending upon load). Over time with enough flexing the cracks propagate and eventually join and the part breaks in two. Fatigue failures are sharp and there's very little deformation of the material adjacent to the break.
   
3. Higher up is the yield limit. If you stress steel above it's yield limit, it will take a permament bend and not flex back. This requires A LOT of force, like a crash or hitting something.
   
4. The final upper limit on steel is it's tensile-strength or ultimate-limit. This is the point where you apply sufficient force to break the material. Chromoly's tensile-strength is around 90,000-110,000psi. A forced break above the ultimate-limit always shows deformation beyond the break, because the part was stressed above its yield limit (it was bent) just prior to overcoming its tensile-strength.

With aluminium, there is no fatigue-limit, so any loads on aluminium at all will generate micro-fractures that propagate over time. No matter how little you stress aluminium, it will eventualy fail. So you can arm-wrestle your alloy frame and after a couple billion cycles over a million years, you'll be able to snap it with your bare hands! Aluminium also has an upper yield and tensile-strenght limit just like steel, but it's about 1/3rd the level of steel. Combined with 1/3rd lower density and both steel & aluminium end up have similar strength-to-weight ratios; thus fully optimized frames of both materials end up close to the same weight.

On steel and alloy, stressing it above the fatigue-limit and below the yield limit will result in some dissipation of energy in non-elastic flexing. This is converted into heat. Measure the delta-T in a frame between the top of a downhill to the bottom and compare to the change in temperature of a front-shock and you obviously will notice the difference in amounts of energies dissipated.

"Aluminum doesn't have a fatigue limit" again? Hello...? Steel frames are designed for a finite life, just like aluminum frames.



I agree that the energy wasted in frame flex is negligible. But I disagree that, generally speaking, elastic "flex" does not waste energy... for a perfect motor, elastic deformation doesn't waste energy, but the human body is not a perfect machine.

Visualize pedaling your bike with foot-long elastic springs between your shoes and the pedals. You could store energy in the spring over your foot's downstroke, then fight the energy of the spring on the upstroke.

We don't pedal in circles. The upward force on the pedal on the 9-o-clock side of the pedal stroke is small. So flex is bad, but it's apparently not wasteful enough to make a difference in speed, so who cares?

Flex can be annoying when it makes your front derailer rub.

A little knowledge is a dangerous thing. Your pro-steel BS arguments are from a cheap textbook. And websites that push figures on steel vs. carbon vs. aluminum, without telling what steel or aluminum alloy they are using figures from.
1.Every material has a fatigue limit.
2. According to these psuedo-engineering pro steel posts, all Cannondales over 5 years old should have disintergrated by now. ~20 years later after the use on ally in racing, it ain't happening.
3. The arguments you make are all based on pure aluminum metal. This has never been used in cycling.
4. Steel bikes are not entirely steel, they are in fact, mostly aluminum parts, including the rims, the highest strength component of any bicycle. If your REALLY believed your own BS, you'd ride steel seatposts, cranks, stems, and rims -welcome to 50lbs.
5. Steel can fail at braze points due to over heating, which is builder-dependent, toss the coin. At least robots don't have good and bad days.
6. Few (any?) steel frames are welded with the same alloy, and often brass alloys, this becomes the weakest link.

I've only got experience of racing since I was 16 (23 years) on steel italian hand built frames, I've had two fail. I haven't had an aluminum frame fail. If I felt steel was a better frame material, I'd use it. I'm still undecided on carbon, and know I don't like titanium. Magnesium is theoretically the stiffest, but has not been fully developed, scandium is basically aluminum with a different marketing name.

So back to the point: is stiffer better? To a point, yes. Basically, you decide how much stiffness you can handle, but keep in mind pros always choose stiffness over weight, and they are aware that 4-6 hours of racing means a compromise.

"A little knowledge is a dangerous thing. Your pro-steel BS arguments are from a cheap textbook. And websites that push figures on steel vs. carbon vs. aluminum, without telling what steel or aluminum alloy they are using figures from."

Uh, I never said anywhere that "steel was better than aluminium". I just laid out their general properties. I've owned 5 aluminium bikes, a Vitus 979, a Peugeot Comete, a Nishiki Altron, the original fat-tube Cannondale, a Trek 1200. I liked them all and only broke two. The one steel bike I originally had, was the lightest at 1.8lbs for the frame (also the most flexible). It got smashed up pretty good in a wreck, leading me down the path of using aluminium bikes for 8 years.

Let's stop talking in qualitative all-or-nothing, yes/no statements. I'm trying to pointing out things with quantitative numbers, but people just want to be at the extremes with black & white statements. The actual numbers would then determine one way or the other which way you go for any material. Most of the time, you have to be in the middle in order to look at the numbers. The engineering role is to design the parts so that they meet a certain specification, again, numbers. And one must remove subjective judgements from looking at numbers.


"1.Every material has a fatigue limit."

I just said that steel has a higher fatigue-limit than aluminium, which has a fatigue-limit of zero. Sure it has one, just infinitely small. Here's an S-N diagram:

http://oregonstate.edu/instruct/engr...atiguePlot.gif

We can argue about the actual numbers later if you want, how high up or down the axes you want to scale things for differnet alloys. But the shape of the curve will be similar. Note that for steel, if you keep loads below stress-level-C, it can take an infinite number of cycles and never fail. In perfectly-smooth riding, a steel frame will never fail. However, hitting pot-holes, speed-bumps, crashing, etc. will put the stress somewhere between the fatigue-limit and the yield-limit (C-B) and microfractures will accumulate.

Looking at the aluminium plot, there is no minimum load-level where the material will take infinite numbers of cycles. You can lower stress-level-E as low as you want, it will still eventually fail within a finite number of cycles (H). Built into the engineered design (with numbers) is the anticipated load-levels a frame will experience, and the numbers of cycles. Most alloy frames are designed to last 150-200 years under normal load cycles. The few that fails, are from stresses beyond the yield limit due to impacts and crashes.

Failures of both steel and aluminum frames are never from fatigue. People just don't ride their bikes 24/7 over rough-terrain for decades on end with 200kgs of cargo. Most failures are from sudden impacts and stress levels above the yield & ultimate-limits of both materials. It's not practical from a performance standpoint to design a bike-frame that never fails under any kind of load.

Now you may be thinking of another figure, the "fatigue strength", which is how much load a sample material can take for a standardized number of cycles, 5000mcycles is often used. In this regard, yes, both steel and aluminium also have numbers attached to them, but this gets tricky because AR of the surface makes a difference here as well.


"2. According to these psuedo-engineering pro steel posts, all Cannondales over 5 years old should have disintergrated by now. ~20 years later after the use on ally in racing, it ain't happening."

Where did you get this data? I certainly never said anything about 5-year lifespan for a Cannondale. If you want to discuss things, let's stick with the data and facts that have been presented. If you want to bring in your down data, let's see the numbers as well, but stop jumping to conclusions like "all Cannondales over 5 years old should have disintergrated by now". I worked in a shop for 10-years and have never seen an alloy frame fail from fatigue, it's always been overstressed beyond it's yield or ultimate-limits. I interned at Steelman and the lifespan of a Excel frame has always been in the specifications. And it's always some super-high number like 100years+. The guys showing me how to weld beer-cans at Cannondale even tested their welds and they hold up infintely longer than the can itself. It comes down to numbers and you can design parts to give you the performance numbers you want regardless of material.



"3. The arguments you make are all based on pure aluminum metal. This has never been used in cycling."

Pure metal or alloys, they all have general properties and the specific failure points are what can be quantified with numbers. Here's some numbers to look at:

STEEL
grey-iron: modulus=10mpsi, yield=6.8kpsi, tensile=22kpsi
1020-LoCarbon: modulus=30mpsi, yield=51kpsi, tensile=61kpsi
4130-alloy: modulus=30mpsi, yield=63kpsi, tensile=97kpsi

ALUMINIUM
1199(99.99%): modulus=9mpsi, yield=16kpsi, tensile=17kpsi
6061-T6: modulus=10mpsi, yield=40kpsi, tensile=45kpsi
7005-T6: modulus=10mpsi, yield=42kpsi, tensile=51kpsi

Notice that regardless of alloying content and heat-treatment the stiffness (modulus) of the material does not change much. Although rigidity of a 3D structure like a bike-frame also has geometry such as tubing diameters and wall-thicknesses that has more to do with the stiffness of the frame than the actual material itself. Using the stronger alloys will allow a part to be made with less material (less weight) for the same strength compared to the weaker ones or the pure metals.


"4. Steel bikes are not entirely steel, they are in fact, mostly aluminum parts, including the rims, the highest strength component of any bicycle. If your REALLY believed your own BS, you'd ride steel seatposts, cranks, stems, and rims -welcome to 50lbs."

Again, I have made no interpretation of steel vs. alloy here, you're the one that jumped to that conclusion. Both steel and alloy have their pros & cons and are used in different places in a bike. Steel has a higher strength-to-size ratio than alloy and are used where small-parts require the highest-strength, such as nuts, bolts and small brackets. However, steel has a crumpling limit of 1:50 where thin wall-thicknesses leads to unpredictable failures. On larger parts, aluminium is used to restore that thickness and avert the crumpling failure mode; bending is much more predictable. You can build large-diameter tubing bikes out of aluminium, not easily done with steel. Although Serottas, Eisentraut and Steelman frames are probably the ultimate evolution of the steel frame, it's not going to get any better than that.


"5. Steel can fail at braze points due to over heating, which is builder-dependent, toss the coin. At least robots don't have good and bad days."

Huh? The same production-line is typically used for both steel & alloy frames. Take a look at the Giant bikes. Cannondale, Klein, Serotta high-end frames are hand-welded. Quality-control is a process from beginning to end, the actual welding is just a small step in that process. Many manufacturers have to meet ISO-9000/9001 requirements in order to import/export their bikes. Regardless of whether a frame is hand-welded or robot-welded, the minimum quality-requirements are met.


"6. Few (any?) steel frames are welded with the same alloy, and often brass alloys, this becomes the weakest link."

Welding AISI-4130 chromoly typically uses a ER80S-D2 filler rod. This material has a native ultimate tensile-strength of 70kpsi. When diluted with the base chromoly, its strength will rise to 90-97kpsi, or very similar to the native chromoly's tensile-strength. Chromoly was originally designed for oxy-acetylene gas-welding of aircraft frames. With the advent of modern welding technologies, it's possible to obtain much higher quality welds than with hand-held gas torches.

Previous to that, brass brazing was necessary to prevent overheating the chromoly tubes. Brass has a tensile-strength of 30-40kpsi for joints of less than 0.010", about 1/3rd that of the chromoly tubes it's joining.You can reinforce the joint using lugs, or you can lay down 3x as thick of a brass layer in the joint as the steel-tubing. That's what fillet-brazing is about and the mechanical shape also gives a smoother load-path with less-sharp of a stress-riser as lugs.


Yeah, that's what I've been talking about all along. When you examine the numbers, flex becomes irrelevant really. But no one wants to look at actual numbers, just sweeping blanket statements. Let's talk about reinforced-concrete bike-frames with zero flex... :)

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sources
http://www.matweb.com
Metals Handbook, Vol.2,ASM International 10th Ed. 1990.
Engineering Materials 2, Ashby & Jones, Pergamon Press, 1986
Mechanical Metallurgy, Dieter, G. E., 1988
Statistical design of fatigue experiments Little, R. E. & Jebe, E. H. 1975

I don't know about that one, but there was a double blind study done with identical bikes except for stiffer chainstays on one of three bikes. Cornell University IIRC, (from an old Bridgestone or Rivendell Reader) The riders couldn't tell the difference.