firebird854

You could have the same rider use different bikes with the powertap hub + pedal setup on rollers, no need to do "perfect rides" around a track and such.

Campag4life

I am going to throw in my 2 cents on this problem with no solution and just share my recent anecdotal experience.

Per the Di2 thread...my first foray, I bought a new bike, the 2018 Cervelo R3...redesigned for 2018 and why I was interested. Cervelo touts as the 'stiffest bike they have ever made'. That is quite a statement if you know the lineage of Cervelos going back to their Aluminum Soloist and S-series which were and are very stiff bikes.

Of course I was intrigued on many levels including the purported ride quality which I will confirm is stiff but relatively friendly for a race bike.

So, this is the stiffest bike I have ever owned. A pure race bike but not overly aggressive geometry and a bike with remarkable handling because not in the least twitchy but handles like a slot car...no understeer, goes where you point it. LIke Warren from Bikeradar said and now having ridden it for a few rides have to agree, frameset of the year contender. Certainly the best I have ridden without being bone jarring or overly nervous or too aggressive.

Why I bring it up, I just want to share my observations. The bike just wants to go. It beckons me to ride it fast. And will admit, its a bit intoxicating on two levels.
One, the frame is so stiff, that every ounce of force I put into the pedals seems to create a linear response in road speed. At lower speeds, the bike I would say isn't out of sorts but not where it excels. The bike to me is faster at higher speeds. Why is that? I have a theory. Bike frame flex is undaunted by higher frequency assault of the road on the frame at higher speed. At higher speed of course, the force due to uneven road surface is greater on the wheels of the bike. This frame is unflappable by this more intense assault. Second issue is, at cadence of say 100 RPM with 300-400watts which for me is pouring the coal to it, my pedal stroke seems purer. It seems I can spin the pedals faster on this bike at high speed compared to other less stiff bikes I own. I attribute this to how rigid the frame and BB are. I have utter control over the bike at high cadence and higher watts and of course there are stronger riders on this forum than me. This is just my observation. In summary, this is the fastest bike I have ever owned.
It happens to be extremely stiff. Perhaps a coincidence it is so fast but I don't think so. I believe the bike's design allows me to ride it faster. It is part geometry but mostly stiffness of the core frame.

Above is a picture to illustrate. The downtube is just massive. The BB is huge. The chainstays are also massive. I can spin the cranks faster with more watts on this bike than other bikes is what I believe because the frame is a rock.

A last note you guys may snicker at a bit. I mentioned the bike beckons me to ride it faster. It isn't just about the uncanny placidity of the frame when I am putting all the power this old man has into the bike. It is about the sound of the bike. Some may not think about sound or their bike doesn't emit a sound when you ride it. But this frame is so stiff that when the wheels spool up, there seems to be a resonance between the modest Al wheels on this bike and the frame. The bike literally sings above 20 mph or so...the wheels connected to such a stiff frame I believe..and perhaps the thin-ness of the carbon that makes it more like a speaker membrane that telegraphs the song of the wheels spinning on the road. Its an intoxicating bike to ride fast.

HarborBandS

Saying a "stiff frame" is a "faster frame" greatly oversimplifies the complexity of the dynamics involved. Consider all of the factors that transfer power from the musculoskeletal system of the rider to the roadway.

First there is bike fit, including the position of the rider and the contact points with the bike. Is the bike fitted properly for the rider to get the most out of each pedal stroke? Is the frame positioning the rider's center of gravity correctly to transfer power?

Then you have the pedals and the moment arm of the crankset transferring power to the chain and the frame through the bottom bracket. Some of the rider's power is transferred directly to the chain, and some is absorbed by the crankset, bottom bracket, and frame. At this point, a frame that is stiff between the bottom bracket and rear dropout will absorb less energy from the rider and better transfer power to the rear cassette. Additionally, stiffness in any part of the frame supporting the rider results in less energy getting absorbed by the frame. If your fork or tires are "squishy", that can absorb some of your pedal stroke. It's not just the rear of the bike.

Then the cassette transfers energy through a freehub (hopefully a tight fit) to the hub and through the spokes to the rim, and out to the tire. Assuming the tire doesn't slip, this power then pushes the bike forward. So a stiff rear wheel is probably just as vital as a stiff frame. If your frame is stiff but your spokes flex, you are losing energy transfer at the wheel.

So whole-bike stiffness = a faster bike? Well, first of all we should replace the words "a faster bike" with the words "a more efficiently pedaled bike". The forward velocity of the bike is only partially controlled by the power of the rider's pedal strokes. And there are times riding a bike--even in fast-paced crits--where the bike is coasting with forward momentum.


What are we overlooking? What else is robbing the bike and rider of forward momentum and velocity? The answer is resistance. Gravity, rolling resistance, wind resistance, mechanical resistance.


If a bike or rider is heavier, there is more weight to overcome to get it moving forward, and to keep it going... Unless you are going downhill. Then F=ma works to the advantage of heavier objects.

If tires have poor rolling resistance, that must be overcome throughout the ride.

If the bike or rider are not aerodynamic, that wind resistance must be overcome throughout the ride.

If bearings in the hubs or bottom bracket have friction, or the chain is grinding on the gears, that mechanical resistance must be overcome throughout the ride.

So frame stiffness is only one factor in a complex system. And I believe "the industry" is working to offer multiple options that address each of these issues in different ways, some more successfully than others. I've personally found rider comfort and bike fit to be a bigger concern, since I'm in 40's and can't rely on my body to be the "watt crushing racing machine" that it once was if my joints hurt or I'm chronically uncomfortable. So for me, a super stiff, aerodynamic bike may not be the fastest option.

Hermes

@Campag4life Thanks for the review. I have a Cervelo 2014 R5 with the squoval downtime. I have found what you said to be true about stiffness with my bikes. I will have to check out the new bike.

Campag4life

Thank you Hermes. This bike is a bit of a revelation for me. It might not even be the best bike for everybody. Some will go for the S-series. I didn't want the more kiss your sister C-series which is of course more upright and softer and no doubt a great bike for long distance. By contrast, I don't like frame dynamics to completely cave to aerodynamics however and this bike like the new TCR which Bike Radar did give bike of the year honors to over the R3...but mostly based upon cost/benefit and like the TCR, the Tarmac has aero cues for an 'all arounder'. I suppose the new R3/R5 maybe considered a climbing bike. Stiffness and lightweight are the priority. The bike just wants to run the best I can explain it. It makes me a better rider because its easier to control at speed...not handling per se but turning the crank. The frame is simply a rock and I feel I can turn the crank faster at higher wattage compared to other softer bikes I have ridden where I feel like the frame is moving around more when going for bigger speed....what modest speed I can personally generate compared to a good racer. Its fun to push.
Thanks again.

noobcake

After a number of years, its amazing to see how off-the-mark Campag4life's observations about his VeRy StiFF Cervelo R5 is....

Campag4life - "Bike frame flex is undaunted by higher frequency assault of the road on the frame at higher speed....."

Maelochs

I normally do not respond before reading all the posts but .... meh. Ignore me at will.

We had not "all" been debating any of that. People with much sense and particularly engineers know that most of the "debates" are just rants by people who champion their personal preferences. I am not an engineer but common sense is actually pretty easy.

No one is legitimately asking what frame Material is "more efficient" because shapes mater more than materials. This is widely known. if you are talking rigidity versus weight, CF is always going to win, because it can be shaped freely and is lighter for a given load-bearing capacity .... in terms of stength for weight CF is not going to be beat today. But .....

What do you mean by "efficiency"? if you bolt a drive train to a solid steel plate four inches thick, you won't get much flexing a t human-generated power levels.

What do you mean by "configurations"? Tadpole trikes versus two-wheel recumbents?

No one can answer any questions once and for all until you ask the right questions.

If you could clarify Exactly what it is you want to know, experiments can be devised to test those things.

if you just want to know how much a given frame deflects at a given load compared to another frame, ask that. if you want "proof" that there is a heirarchy of frame materials ... don't bother asking please.

If you define your terms and focus your questions people can probably provide answers.

Maelochs

@HarborBandS in post # 53 addresses some sensible issues.

Jughed

Didn’t read the entire thread…

I would think the only way to test the frames themselves would be to use all the exact same components on each frame, pedal mounted power meter - hub mounted power meter and compare the delta numbers.

Kontact 

I had a conversation with noted bike engineer Damon Rinard about this subject when he was at Cervelo. The upshot was that there wasn't data that stiffer bikes are faster or more efficient. But stiffer bikes feel more efficient because people cannot feel spring return, but they can feel stiffness.

You talk about the industry like it is data driven - like aviation. It isn't. It's still largely a cottage industry of amateur designers favoring a particular paradigm that sells bikes, and then doing the engineering to meet that paradigm. And the current fashion of enormous tires means that there is less downside to making the frame super stiff. AND, super stiffness is necessary if you want to make the frame as light as possible.

Pro teams don't know better either. It's a sport, and a very underfunded one compared to football. They ride what their sponsors tell them to ride. And racers make choices all the time that are far from scientific.



I think using a crank or pedal powermeter and a hub meter, swapped between frames, is a great idea. But I would purposely choose bikes that have a reputation for different ride qualities so we could see some definite effects.

terrymorse 

Having used both stiff and not stiff frames, I make the claim that the stiff frames are definitely faster. Not because they are more efficient, but because their precise handling instills more confidence.

rsbob 

I would think if a gig was built, with a mechanical device pressing the pedals with constant force using a rolling road measuring torque at the rear wheel, one could get a pretty accurate view if the drivetrains were the same.

Kontact 

I don't think "not stiff" is a very useful description. I had a Vitus 979 that felt inefficient in the way it flexed, and I have a Merlin Extralight that seems to leap up hills in a way I could only characterize as "springy", yet doesn't rub the front derailleur. And then I rode a Cannondale 3.0 with the enormous downtube and short stays for two decades that was very satisfyingly stiff, but definitely scrubbed the rear tire if you weren't careful on climbs. Scrubbing can't be efficient.

Handling stiffness is also a real thing, but if the fork and frame are supposed to act as suspension to keep the tires in contact with the road, there is also an upper limit to how much stiffness is actually helpful for handling. But there is a real question whether drivetrain stiffness contributes to handling nearly as much as a good fork.

79pmooney

The top five race bike manufacturers all have to provide race bikes they are also going to sell to the public. And who wins here? The folk that have say in the bottom line. Bottom line? Stiff sells. (As proof - would our champion here ride anything else?) And certainly, stiff is fast enough to win. So these top 5 make stiff bikes. So does everyone else who wants a piece of the action.

Stiff has another advantage. Stiff bikes take less skill to ride. The attribute of "supplesse" is not needed. And hasn't been for so long that most don't even know what the word means. 45 years ago, on much more flexible bikes, it was fast. And if you had it, you didn't need (a lot of) stiffness. Back then we rode rollers to achieve finesse. Now, everyone rides trainers that don't improve riding styles at all. Many now cannot make a flexible bike go fast because they aren't smooth enough to do it.

Trakhak

Agreed. After 40 years on Reynolds and Columbus steel frames, I made the switch to aluminum bikes because I can't feel any difference in so-called comfort, but I can easily feel the difference in the improvement in torsional rigidity and thus in handling. (My all-time favorite bike has oversize aluminum frame tubes and an oversize straight-blade aluminum fork.)

Maelochs

The sort of "stiffness" the OP seems to be discussing in deflection due to high power ... hence measuring power output at different points.

Thing is, if the BB moves under high power, there will be no way to measure power loss. The power would never reach the pedals. Possibly one could put sensors on different frames and use lasers to measure actual flex under high load, but then how would one translate that flex accurately into power numbers?

My initial questions remain unanswered. What questions is the OP actually asking?

We all know that in a straight line, a stiffer frame is going to transmit more power from the legs through the pedals to the wheels. Stiffer frames also provide a more stable platform for high speed cornering---and nowadays, wider, softer tires provide the compliance needed to avoid hysteresis losses.

But .... that has nothing to do with frame material, and there is no specific design which is stiffer .... there are any number of frame designs coupled with almost infinite layup choices (for a CF frame) ...as for metal frames, stiffness means more weight than CF, but weight is really only an issue if you are racing ... otherwise you just ride more slowly uphill.

But what is the question? If the question is, are stiffer frames faster in race conditions? Yes ... and pros are willing to sacrifice comfort to achieve speed. Are stiffer frames therefore "better"? And here we have another unanswered question ... what is "better" to the OP?

What is thread about?

Kontact 

I don't understand this. If the rider puts X watts into the pedals and one bike measures X-4 at the hub and the other bike measures X-6, the second bike has a loss that is 2 more than the first.

The only thing you have to correct for is time, because a flexy bike is going to deliver power to the hub over a longer period than the stiff bike. But the total power in vs power out is what's important.

This isn't really a comfort question, but a drivetrain efficiency question - is stiff actually more efficient?

Maelochs

okay now what about this ....

It is not Drive Train efficiency but Frame Efficiency the OP was discussing .... and power losses after the pedals could mean the chain stays are flexing. If the whole bottom bracket is flexing (twisting) under pedaling forces it will distort the entire frame ... so it is not drive train losses but frame flex which is costing power.

As for common definitions of "efficiency" then stiffer equals more efficient because less power is lost to heat (distortion produces heat) and more power is transmitted to the wheel.. that is by definition "more efficient."

If the OP wants to compare efficiency between specific frames, then that is the experiment to design .... the question I ask is , would a street-level experiment yield real data?

If we are talking about percentage points of difference, then a dirty chain, different stiffness of shoe soles, different gearing, and most of all difference in riders would make the data wildly imprecise. Also, you would have to use a wide range of power levels and eventually truly massive power (UCI World Tour sprinter levels) to make sure you reached the frame's limit ... if I were a tester with my barely triple-digit wattage, I might not bend even the flexiest frame.

Best would be to put the bike in some test jig (was mentioned above by someone) and apply power mechanically. otherwise you have anecdotes, not data, nothing much more useful than ad copy.

But ... if people want to do whatever, that is fine.

Kontact 

It is easy enough to move a crank, BB, chain and wheels between several bikes and have the same rider put them through the same course.

Call it whatever you want, but the frame is a part of the drivetrain, and if the only part of the drivetrain that you change is the frame, then there your are.

Frame flex has heat loss - but not much for any efficient spring. Flexless frames have less transmission loss, but may have other losses due to the asymmetric nature of pedaling and the way muscles don't like halting motion. A compound bow takes the same arm energy as a traditional bow, yet one of them is much more efficient for a human arm to use. Frames may reveal themselves to put similar restrictions on the user.

But that might mean that input losses won't be seen as anything other than lower watts at the crank, so the delta between crank and hub won't tell the whole story.

Some very efficiently stored and return flex may prove to be superior to no flex. Other types of flex will be wasteful - like I would guess a skinny tube aluminum bike. But it is such an easy experiment to perform that it is dumb not to. GCN 'tests' certainly involve more rigmarole than moving a crank and wheels between a couple bikes and riding the same course. Many riders have enough bikes and power meters to do this themselves.

waters60

Too many variables to account for. Pretty simple, really.

zacster

I started this thread? It came up just now in the subscribed section and I don't remember this at all, but it's been 6 years! I don't usually even get involved in these types of discussions. And ironically enough in the last few years I've built up bikes for my daughters based on older Cannondales, one old time aluminum, and one Super Six Evo and they are both stiff. I tested them both out and they felt like they'd climb better, but the rider certainly does NOT climb better. It doesn't matter when you've just turned 69 and are happy enough to be able to still ride, never mind fast.

Carry on...

tomato coupe

Another zombie thread brought back to life by a first-time poster -- why does this happen so often?

Maelochs

Actually a thread revived by a recently revived zombie poster .....

choddo

Google probably

PeteHski

Given that bicycle frames are so light (under 1kg for a modern carbon road frame) it makes sense to maximise lateral and torsional stiffness as much as possible at that minimal weight. Vintage lightweight race bikes were noodles in comparison to modern carbon bikes and I don’t think anyone would argue they were better for handling or power transfer.