PeteHski

I don't have any incentive and don't think the "flexing" losses on a modern CF frame are worth worrying about. Also I don't happen to have 2 test bikes or a hub power meter. If someone wants to fund those, I'll give it a go.

Kapusta

Bill Murry and the Chemical Brothers both agree...

PeteHski

So there we have it. Stiffness doesn't matter after all.

Maelochs

You think they do?

ofajen

God exists by two falls to a submission. 😊

Otto

livedarklions

No, but they admit that in such pretty words.

Also, acronymania!!!

ChamoisDavisJr

Figures.

I assume they're all on something...and enjoy the show.

AlanO

Ummm, might want to correct that original post. Of course energy is dissipated when the frame flexes. Where do you think that energy comes from?

If what you actually mean to say is it might not matter to some people that’s fine but the laws of physics do indeed apply to bicycles. I, personally, prefer a steel frame because, yes, it flexes more than aluminum. Can I measure it? I doubt it and I don’t care.

melville

Oooh! Stiffness! Perhaps better called Rigidity--Stiffness is a property of a material. What's being discussed here is rigidity of a structure.

I skimmed a bit and saw the Merckx hour record bike mentioned--I have touched that thing and it is crazy light, especially for the era. It was, however, probably a fairly close match for what he was riding on the road as far as rigidity is concerned and that was probably an important consideration.

My own personal experience is as a trackie who favored sprints. A rigid frame is nice to have when you're out of the saddle because you can feel confident in where the wheels are going. I had this in my track bikes and in a crit bike I had, with slightly oversize steel tubes on that crit bike and one of the track bikes.

My main road bike, to be contrary, was a Vitus 979. So flexible that for the first year I had it, I was convinced I had a tire going flat. And at no time going down the road did I ever think the wheels were in the same plane. But for anything but the last 200 meters it was just fine. More than fine, as it wasn't beating me up.

Kapusta

If one thinks of the frame as a spring, a spring does not dissipate energy. It stores energy (most of it, anyway). And in the case of a bike frame, releases that energy. Steel and aluminum are pretty efficient in terms of storing and releasing energy. I'm not sure about CF.

A damper dissipates energy.




A damper dissipates energy.

AlanO

It's not a spring. It's mainly hysteric damping, the frame is its own damper. The same reason a steel beam returns to its original shape after the load is removed. You don't need a viscous damper to experience damping.

peterraymond

Yes I skipped 90% of this. But I do see it may be past time for serious answers and maybe this has been answered as well.

If a whole system is elastic springs, then physics says (claims?) energy has to be conserved. But the rider is not an elastic spring. If the rider puts out effort to deflect the bike, the rider may not get that energy back when the bike relaxes. It's hard to analyze a full bike, so I think it might make sense to try to imagine the simplest system possible.

- Assume a perfectly efficient, perfectly stiff bicycle, except for the handle bars. Start with the bike coasting, rider standing, equal weight on the pedals, zero weight on the handlebars.

- Assume the rider keeps their body stationary put reduces weight on one pedal and increases on the other. This provides torque at the crank, but also tries to lean the bike. The rider then pulls up on one bar and pushes down on the other, so the bike and the riders torso are still stationary, but the bike stays balanced and is propelled forward.

- The forces on the bar will flex one side up and the other down, but elastically - no energy lost. Just a conversion from work energy to potential.

- When the pedals turn the rider will reverse the forces and the bar will flex the other way. The bar gives up it's potential energy in one direction and stores the same amount in the other direction.

- However, when the bike feeds energy back into the rider arms, the arms can't store that energy. Your lungs understand somewhat the energy difference between force as you contract a muscle vs extending it. That's why your cardio system works so much harder hiking uphill vs down. One direction burns sugar and oxygen and the other doesn't, but feeding energy into a muscle doesn't unburn the sugar.

- Conclusion: Flex may not make the bike less efficient, but it can make the rider less efficient. The flexible bike may conserve energy, but the resulting additional motion of the rider can waste it. At least in this simplified case. At a minimum we can say that at least some types of bike flex will waste energy. Stiffness matters.

Wes in Missouri

Just a quick observation from my experience. We have a hill that is a descenders dream. I reached 63 MPH on my 88 Cannondale and it was rock solid. The only thing that slowed me down was that I ran out of gears. Then I descended it the next day with the same weather conditions on my trusty steel Bianchi. At 55MPH it started getting a little squirrelly and at 58 I nearly wet myself. Despite this the Bianchi remains my favorite bike for long rides due to its softer ride. So, in this case, stiffness does matter.

KKBHH

Seems to me that a few years from now, most road bicyclists will be riding Carbondale carbon-fiber frames with the Kingpin feature. The rear of the frame flexes to make a rear suspension while the rider weight in the seat dampens it. The system could only be quirky if the rider was out of the seat while hitting bumps. Well, maybe the seat post could have a hydraulic lever to lock it in place when needed.

Of course we already have flexing forks. The trick is that steel is okay for flexing and carbon-fiber is okay for flexing. Of course steel is very historical while carbon-fiber is relatively new.

Flatforkcrown

We’ve had flexing forks for over 100 years. All of the steel road bikes I’ve ridden had forks that flexed noticeably over rough road.

Kapusta

I am skeptical of the highlighted claim.

Something returning to its original shape has it not an indication of it being damped (internal or otherwise). In fact, if you deflect a steel beam and let it go with no external damping, (not a common situation, as what it is bolted to may be damped) it will keep moving back and forth (vibrating) because steel itself has low hysteric damping (some, but little). And eventually return to its original shape.

The fact that steel has such low hysteric loss is the reason it makes a good spring for uses like a trampoline and pogo stick. Why would it suddenly have higher hysteric losses when you change the shape to a bike frame?

All a steel spring is is a long piece of steel that gets deflected when you compress the spring. The coil shape just makes it practical to work with. How is a steel bike frame fundamentally different?

livedarklions

Is that when you cover your mouth so no one can see you're laughing?

tomato coupe

Because the bike frame is now strongly coupled to very lossy blob of dough, aka rider.

Kapusta

That does not change the fact that the frame itself has low hysteric losses. And that is what is being debated in the discussion thread you are commenting on.

tomato coupe

The question is whether a stiffer bike frame offers any advantage over a less stiff bike frame. That question is only interesting if you consider the bike frame with a rider on it; no one cares how efficient an isolated bike frame is.

Kapusta

Look, I know branching conversations are hard to follow when they are all displayed in linear mode, but please try.

There IS in fact discussion and disagreement about how efficient of a spring a bike frame is, as that DOES in fact make a difference in how efficient it is overall is one buys into the idea of planing.

Siu Blue Wind

tomato coupe

Again, the efficiency of a frame "spring" is only meaningful if you include the rider as part of the system, because introducing a rider adds (a lot of) damping. Conclusions based on the behavior of an isolated frame are worthless.

tyrion

Hysteresis loss in the frame can be compensated with low hysteresis chain lube.

greenspark

It is always interesting to look at the Location when reading threads. This is predominantly an American thread. If it were a European thread there would be a nuanced distinction based on use.

In American terms, in speaking about stiffness, think the difference between a Cadillac and an F-150 Ford pickup truck. Stiffness in a truck allows carrying capacity. Ditto in a bicycle. If you buy an Italian city bike with a Dedacciai steel tubing Zero Uno frame it will ride like the proverbial Cadillac (think 1950's squishiness) but offer nimble handling that is called sporty as opposed to racing. You will be unaware your regular ride has constant small bumps and irregularities because the flexible steel springs and frame absorb them. Only when you do the same route in a stiff bike do you start cursing the highway department for using the lowest-bid contractor.

However, if you then put panniers on your Dedacciai frame bike and load them up with 30kg of gold bricks, it will sway to the point of being unsafe. I had this experience once in Berlin when we were relocating to Dresden and had to pack everything (not gold bricks, unfortunately) on the luggage racks to ride to the train station. It took all my concentration to not crash. When we got to Dresden with cobblestones, I gave up and pushed the bike to the hotel. After that I organised a van to carry everything on to Prague and carried a credit card with me instead.

For a freight bike you need stiffness. The ride will not be as comfortable, but the weight will be manageable.

The American approach is different because the DNA of cycling is different. Bicycles in Europe evolved over a century as a mode of transport. They were tools. They went out of fashion for a while until the 1973 OPEC Embargo when Europe - especially the flat lands - began to build bike paths, roads and lanes. Riders were equally men and women, mostly dressed in street clothes going about their business. The bike was a way to get around and was designed as a tool not a toy. In contrast, bicycles in America were forms of mobility for children until they got their drivers license. A small minority would road race, and this gave rise to emulators - 10-speed, drop handlebar bikes great for racing. Think the Peugeot PX-10 versus the Schwinn Continental... often both sold in the same bike store to uninformed customers who would buy what the sales person pushed - both were silly for the average flatland city rider who really needed the Raleigh Sport or DL-1 that was sitting in the back room of the bike store gathering dust as old-fashioned.

Then a group of Peter Pans (adults who don't want to grow up - I'm one of them BTW) began to take the bikes of their childhood and use them to crash down steep hills in Marin County, California. These became mountain bikes, and the American buyer could now go into the bike store and choose between a 10-speed racer and a MTB like a Gary Fisher hardtail. The active shock absorbers necessary when striking stumps and boulders at high speed going down mountains moved into street user bikes (euphemistically called hybrids) to address the same issue spring steel addressed in Europe. From an industry perspective, this is good because shocks don't last. European bikes, like the Raleigh DL-1 can last 50 years or more. The industry needs planned obsolesce to sell more new product. It is not billed for the cost of disposal, so it designs its bikes for a specific life cycle, after which the vast majority end up in the town dump.

A small population of Americans, mostly in places like Boston and Portland, educate themselves on European bikes and small bike shops eke out a living importing and selling them. But for the most part, Americans only know what the bike stores sell, and they sell trends not tools.

Bottom line: yes: flexibility is important, stiffness is important, but not for some obscure blather that marketing types put in their brochures so bicycle sales people can quote it authoritatively to prospective customers as an important reason why to buy their brand over Walmart bikes.

Relative stiffness is all about use.

• If you need a lightweight bike because you live in a walk-up apartment that is different than weight considerations for the Tour de France.
• If you want to go bike camping for a month, you need a different bike than a month bike touring with a van that carries the group's luggage from hotel to hotel.
• If you use a bike to commute to work the same route every day, relative stiffness and riding position will be based on the road conditions, but that also opens up different requirements...

What angle the back and head? In racing, head down reduces wind resistance but you can't see the bus trying to squash you. Head up allows eye contact with others, riding becomes more friendly. Fenders means you can wear business attire and not have a brown skunk line up your back when you arrive.

And of course, electric motors are the big game changer. Suddenly bikes become an option in Switzerland. Their extra speed means better brakes, better tyres and rims and eventually more safety equipment like rear view mirrors, horns instead of bells, always-on lights and in some cases stronger frames that have a different stiffness requirement for different reasons.

And as usual, I try to include a photograph so readers keep interested in my posts.

This was taken on EU bicycle Rt 2 somewhere in Germany. Unfortunately, I did not want to be obvious about it, so look carefully. One bike frame has been welded onto the other and the load is incredible, including juggling pins and leeks for dinner.



On the same route, at what used to be an armed border before the EU, we have bikes packed for credit card touring. Note the umbrella: