Frame flex and climbing

I recently posted on Facebook and BROL information I gathered comparing the Bacchetta CA 3.0, Cruzbike V20 and Metabike Mystique. One comment read "The rubber hits the road when these bikes are compared on hills, and freeway overpasses don't count. Otherwise stick bikes are completely sufficient on the flats of Florida." This got me thinking. I presume this was a reference to frame flex, Since the "stick bike" has a weight advantage over the Mystique, and all seat angles were the same in my tests. But even if it's not referring to flex, the statement seems like a pretty testable theory. So here are my questions, maybe some of you have answers.

Question 1 - Has anyone quantitatively compared different recumbents climbing ability efficiency? I don't have any hills in my part of Florida, but if I did, I think comparing bikes should be pretty straightforward. Here's what I would do: First, I'd equalize the weight, since we know that weight is a disadvantage when climbing. Then, using the same power meter pedals on each bike, I would climb the same hill trying to keep the speeds slow so as to minimize the effect of aero differences between the bikes. Then, by comparing the total watts used to climb the hill, we should see if one frame, or drivetrain or seat maybe, "wasted" more energy than another. The bike using the least energy to climb the hill should be the most efficient. I have seen a million opinions ..."my (P38,V20,M5 fill in the blank) out-climbs any other bike.." Looking for actual data here.

Question 2 - What about other climbing attributes? Maybe one bike is less efficient, but allows greater power generation because of pedal position or some other factor. I'm not sure how to measure this. Just because I generated more power on bike A than B, doesn't mean A is better. I could have just been more motivated, more rested or in better shape for that ride. How could you test that?

Question 2 - Is a stiff frame actually better for climbing? I have seen that stated so many times, that I assumed it was fact. But in researching the issue, in the upright bike world, it seems like it is considered a myth. I can't post links as yet, but I've seen tests comparing frames with identical geometry, some with stiffer tubes, some with more flexible tubes, covered so as to blind the riders. This test showed that the MORE flexible frames were the better climbers. I also saw an interview with a Cannondale exec, saying that the idea that stiff frames climb better is a myth, and they were designing more flexible frames. I don't know if this applies to the recumbent position, but maybe someone does know.

Question 3 - How would you measure frame stiffness? I know an informal way of measuring an upright bike is to put lateral pressure on the Bottom Bracket and see how much it moves, but is there any way to compare bikes?

Thoughts?

Joe Valenzi

 

This is a big topic. A couple big topics, actually. Perhaps too much for one thread. But we'll solider on...

Yes, I think the comment you received was a reference to frame flex. And my experience has been that frame flex matters much less when pedal forces and torques are low than when they are high (say, on steep hills and during hard accelerations).

1 - I am not aware of any, off hand, that would have the accumulation of contemporaneous data that I think you are looking for. Most people just aren't logging it, even if they are noticing speed differences between bikes on certain climbs that they do often. I don't think your proposed test protocol / method of holding power at the pedals constant is best. This is supposing that the advantage of frame stiffness is entirely in terms of transmission efficiency. I suspect that's not the case. I also suspect it has a supply side effect. That is, the human is more efficient /effective at producing power when pedaling a stiff bike. My hunch is that is the larger effect than energy losses in the actual flexing. That's just a hunch, though.

2 - There is no simple way other than field tests that have the same rider riding the same climb on different bikes many times, in an alternating fashion, over the course of a certain period of time. The many runs and alternation should help mute the effects of confounding variables. Which is basically what you did. I think that can be quite valid, but it's possible another rider doing the same exact tests with the same exact bikes might have different results because, as you suggest, different people favor different positions, postures, etc. So, it doesn't necessarily reveal any universal truths.

3 - Here is what I do. With one leg on the floor, and the other on the pedal in the middle of the power stroke (roughly 12 o'clock position), and both brakes firmly locked, apply modest pressure to the pedal - just to take up all slack in the power side chain. Hold this for a second or two - then apply a large static force to the pedal. Watch to see how much your foot moves. Whatever amount it does is due to flex / wind up in the frame, crank, idler deflection, etc. etc....

I tend to agree that flexy DF frames climb just as well, perhaps better, than stiff ones. But be careful about extending that to recumbents. The stiffest recumbent has more flex than the flexiest DF frame in the vertical direction. This is triangulation at work. I am personally quite convinced that a stiff recumbent frame climbs better. It matters more than 5 or 10 LBS of bike weight difference, in my opinion. I have owned a succession of different bikes, and DFs too for that matter, and I have a very good sense of my climbing times up certain hills / mountains on different bikes. I don't log the numbers.

By the way, here is one theory about why a flexy frame messes up your ability to put power into the pedals during high torque efforts.... The pause between muscle contractions is critical for blood to nourish the muscles. Blood flow is impeded by muscle contractions, and fuel is delivered and wastes removed during the pause. When you have a stiff bike, you can use a strong, concentrated contraction to add power, and you don't need to maintain pressure on the pedals as long to make sure the wind up in the bike gets redirected into the chain once the pedal force is reduced, later in the power stroke. This results in a longer pause that allows the muscles to be better nourished, and get ready for the next contraction. The opposite happens with a flexier frame.

Sorry to be a blog shill, but here are my overall thoughts about climbing on a bent:

https://rothrockcyrcle.wordpress.com/2022/01/29/climbing-on-a-recumbent-part-i/
https://rothrockcyrcle.wordpress.com/2022/02/05/climbing-on-a-recumbent-part-ii/

 

I forgot to mention, for the flex test, for comparison between bikes, even if you could somehow push with equal force each time,, you'd also need to have the same crank arm to chainring ratio each time, and you'd want to use the same leg to push with each time.

Even without the ability to push consistently, you can feel the difference in flexibility between machines.

 

Thanks Steamer for your thoughts. I think I'll need to study the two blog articles before I reply. That's a lot of information!

 

My best climbing is with motor assisted bents! (Just joking... don't own one of those...yet...)

 

I forgot to mention that I would recommend holding perceived exertion constant. That's what most riders use to govern their riding most if the time. And it would capture the effects I was theorizing about (that flex makes it harder to apply power).

 

Steamer,

I read your blog posts part I and II (looking forward to part III). I'll need to read them a few more times. But I can see that my original question about comparing different recumbents and their climbing abilities is too huge and complex a question. But let's set aside issues of frame geometry for the moment, and just focus on flex.

There are a number of bike models that have been made in different materials: Bacchetta stick bikes in steel, aluminum, titanium and carbon, Easy Racers in steel, aluminum and titanium, RANS Stratus in steel, aluminum and titanium etc. If we compared only fram variations within each model, we should get some useful information. This would eliminate many of the issues you mention in your blog post about the direction of force due to the chainline position relative to the frame, frame triangulation or lack thereof etc.

I realize that material selection doesn't equal frame flexibility. A thick walled fat titanium tube can be made stiffer than a thin walled, narrow aluminum tube, even if aluminum frames are "in general" stiffer than titanium. But still, we should at least be able to agree on a ranking of the flexibility within a line. So we might not be able to quantify how much flexibility there is for a given force, applied as you describe above. Still, we should be able to agree that, for example, a TiAero is more flexible than a Carbon Aero (I don't have a TiAero so I don't know if this is the case).

Then, we could compare climbing efficiency . I misspoke about using Watts as the metric. What I meant was energy, not power. So, Joules, which my power meter can easily give me. If a given bike can get up a hill using less energy, (controlling as best we can, total weight, wind, speed, tires, tire pressure, chain lubrication yada yada), wouldn't we have to say it is a more efficient climber? I'm not an engineer or a physicist, so forgive me if I'm not setting this up properly at all.

You talk about the "meatware" part of this problem. As I understand it, power meter and speed chart gives us information about what a bike will do given a certain amount of power applied to the pedals, but as we ride the bike, what matters even more to us is how much energy WE have to produce to move the bike. You have an excellent discussion in your blog about the many factors that go into our perceived and actual exertion expended riding the bike, and how things like COG and steering geometry factor in to that exertion. I wonder if we can get a handle on this by looking at our caloric expenditure while climbing. So let's say we agree on a test method that kept speeds low to minimize aero loss and cadence within a range so that we don't mash on one bike and spin on the next, I think we could start to get good comparisons of both the hardware and meatware parts of the problem at the same time, by looking at calories for meatware, and efficiency for hardware.. I know we are not going to go into a lab and get hooked up to respirometer, but heart rate is a pretty reliable proxy for calories.

"frame flex matters much less when pedal forces and torques are low than when they are high (say, on steep hills and during hard accelerations)." - They would be be less for the low speed, moderate to higher cadence trials, I'm suggesting, but of course that might reduce apparent differences between frames. I have been amazed at the variety of analytical software out there for bike riders, like Veloviewer or MyWindSock. There may be software to look at instantaneous acceleration at each point of the pedal stroke. This could show how much power is converted to forward motion at a very detailed level, given any input power. So, maybe the tools are already out there.

"- There is no simple way other than field tests that have the same rider riding the same climb on different bikes many times, in an alternating fashion, over the course of a certain period of time. The many runs and alternation should help mute the effects of confounding variables. Which is basically what you did. I think that can be quite valid, but it's possible another rider doing the same exact tests with the same exact bikes might have different results because, as you suggest, different people favor different positions, postures, etc. So, it doesn't necessarily reveal any universal truths."
So yes I am proposing field tests, but by getting more data than simply speed up the hill, I think the conclusions would be more generally applicable.

Now all we need is someone who has a heart rate strap, power meter pedals, two or more varieties of a given bike and a hill. I have a Tour Easy and a Gold Rush, but no hills. I recently did a century ride with a total elevation of 128 feet, so really, no hills.

 

Something Steamer mentioned on this topic (probably on BROL) was the flex he witnessed in the chainstay region while riding behind a guy with large legs on an M5. I never thought of this section of frame flex and I'm not conceptually convinced that it would 100% appear on the static test.

Given that I'm a weakling, I suspect frame flex impacts me to a lesser degree than those who actually have muscles (but probably still matters).

On the topic of weight, in my pea brain world, it matters more than in Steamer's big brain world.

 

That doesn't actually change anything vis a vi the point I was making. A watt is just a joule per second. A fixed quantity as opposed to a rate.

I think a significant fraction of the effect a stiff frame has, compared to a flexible one, is how if affects the rider's ability to put power into the pedals. Your approach won't capture that effect. It will only capture losses due to material hysteresis in the frame. It won't even detect losses in the seat.

 

I think I suggested to you in that exchange that you do a test for yourself similar to the stationary flex test I described above with a mirror off to the side (probably should be on the same side as the leg that is on the pedal, so the frame isn't partially obscured by the leg being down on the ground).

 

I realized I probably should explain a little more.

The losses I am talking about already do their dirty work "upstream" of the crank or pedal based power meter. The rider on the flexible bike will just work harder to render the same power, or to perform the same total amount of work in joules on a given hill, as measured by the power meter at the input side of the drivetrain. Hysteresis in the frame would show up in the results, but that very well may be the smallest source of loss. Thinking that's all that's going on will lead to bad conclusions.

One way to use a pedal based power meter to detect what I am talking about is to do maximum power tests. This levels the playing field, in terms of the rider's effort.

I was suggesting holding RPE constant just because it's reflective of real world riding, so the magnitude of the differences measured will be just as real. It's also less unpleasant to conduct. A maximumal power test is probably more objective, however.

 

Tymber,

Thanks for this thread. It made me realize that I had left some of these things out of Part I. I've updated to include part of our discussions here.

This whole topic is a moving target of sorts.

 

The more I think about frame flex and climbing, the more it makes my head hurt. Still, as complex as the subject is, there has to be SOME way to compare bikes in a reliable fashion! It doesn't seem like it should be an unknowable mystery. So let me turn this around, and ask you Steamer: How would you design a test protocol to compare bikes? Let's say the NHTSA has commissioned you to head up the new government hill climbing ratings agency, like we have the NHTSA crash test ratings, or how the EPA has fuel mileage ratings. You have to make HCR (Hill Climbing Ratings). Imagine this isn't a question that only about just 6 people in the world are interested in, no, this is a matter of vital importance. We need a methodology to rank bikes on their hill climbing ability. How would you do it? More realistically, how could interested recumbent fanatics (meaning 2-3 people) produce a "crowd sourced" answer...and I'm using the term "crowd" very liberally.

 

If it makes you feel better, it makes most people's head hurt. I know mine does. If your head doesn't hurt, it means you are either a genius or you just aren't thinking very hard.

Ok, I'm gonna try to answer this. Some of what follows is as much thinking out loud and wondering myself what is the best way, as it is an actual "answer". I'll preface it also by saying there are scientific (mostly physiological) methods for doing such a thing (of which I have little expertise), and there are some practical but non-rigorous ways of doing it that may not meet other people's standards but satisfy me perfectly well.

The first thing I started to realize about this subject is that approaching this purely like a mechanical engineer (which I am) is not going to actually cut it. We'll come back around to that point in a little bit. I need to first explain the framework for thinking through this:

Speed on a bike can be distilled to supply vs. demand. Supply is how much power you can put into the pedals (net power) at a given moment. Demand is how much power is consumed by various "losses" to go a particular speed on a particular stretch of road. (The demand part is pretty easy to study like a mechanical engineer. The supply side - no so much.)

The demand side includes things like aero drag, overcoming gravity (when slope is positive), rolling resistance, frictional losses in chain drives and bearings, and losses due to elastic hysteresis (click the link) of materials (like frame or crank flex, for example). Of all of these, I am willing to put $100 down that the last one is the smallest effect on real world routes (i.e. those that include some climbing, however little; that is to say, NOT on a track). That aside, demand is easy to evaluate - you can use power meters.

The total sum demand of everything I just listed can be measured with crank or pedal based power meters. A hub meter is further down the chain of energy transfer and takes frictional losses in chain drives and bearings, and losses due to elastic hysteresis of materials, out of the measurement, and only really measures the sum of the "external" losses - which are aero drag, gravity, and rolling resistance. (That is, incidentally, why hub meters are perfect for Virtual Elevation type methods of measuring CdA and CRR, as they eliminate any issue with estimating those couple "internal" types of losses, and the estimates are bound to be off by at least a little, and the VE calculation can restrict itself to only solving for two unknowns - Crr and CdA.)

Assuming no acceleration / steady speed, these total demand side losses (both the internal and external ones) are balanced out exactly by the net supply of power. Net power is the power to the input side of the drivetrain - the power that you could measure with crank or pedal based power meters.

But the term "net" power has no meaning without considering what "gross" power is. In my non-physio mind, gross power is measurable, in a relative sense, by looking at various physiological variables - things like oxygen consumption, blood lactate concentration, heart rate, respiratory rate, or even rating of perceived exertion. There are probably others I don't know about. For different types of physical effort, or levels of effort, I suspect the best metric varies.

So, I am implying here that there is some difference between gross and net power, with gross power being the larger. This then begs the question - where does that lost power go? Well, it's lost within the rider due to something ineffective / imperfect about their technique, body position, coordination, muscle recruitment, etc. etc. I am not knowledgeable enough to list all of the possibilities there. Sometimes stuff like this is obvious - go look at people running a community 5K or 10K. You can literally see the runners who are efficiently converting their gross power into net power/forward movement, and the flat footed flailers who don't. I am going to invent a term that recognizes this difference between gross and net power - let's call it "cycling economy". (Ok, confession time. I didn't invent it. It's an actual thing that physiologists have defined and measure).

Ok, let's talk about climbing on a bike. We'll tackle the demand side first. External demand side gets a bit simpler: Aero drag diminishes - on a steep hill it's essentially ignorable unless its super windy. Rolling resistance is present, but it's percentage of the total also decreases. It all pales to the power needed to overcome gravity. Internal demand side is there, but again is small. To the extent that elastic hysteresis wastes energy, it probably increases a bit due to the higher pedaling forces and torques usually involved (even if pedaling force weren't to change, you may be in a smaller chainring than before). More force and torque - more flex - more hysteresis. But as I said before, the internal losses are probably pretty small. So, to simplify, the demand side is all about dragging you and your bike and everything on it uphill against gravity. No new news there. And comparing the weight of one bike to another is trivial - it's done all the time. That's why poeple focus on it so much. Sometimes things that are easy to understand or measure are elevated in importance in people's minds only because they understand it or it's easy to measure, not because it's actually all that important.

But remember, speed is net power vs. demand. Power at the pedals vs. total weight. Power to weight ratio. Captain Obvious makes his triumphant return.

But here's where it gets interesting.... How can we get as much of our gross power - the total athletic potential of our body and mind - into the pedals? How do we maximize our cycling economy?

This where I think things like stiff frames and seats, optimal gearing, body angle, torso angle, confident handling, leg extension, seat fit, glute utilization, and pretty much everything I mentioned in my two blog articles comes into play.

Most of these make intuitive sense, but the ones that might seem out of place in the list above is a stiff frame and seat.

Seats first - a fairly large percentage of energy absorbed by a material that has an inherently high elastic hysteresis is not going to be returned. That's the kinds of stuff that seats and seat pads tend to be made of. Any losses in the seat represent a loss in cycling economy - a loss in potential power that never made it to the crank/pedal to be measured as net power.

Now frames - how would a flexible frame reduce your cycling economy and throw potential out the window before it could make it to the drivetrain? I tried to explain that in an earlier post. It's just a theory, but here it is again: The way a flexy frame messes up your ability to put power into the pedals during high torque efforts has to do with how our muscles work when cycling, as opposed to during isometric exercise. When cycling, running, rowing, and similar activities with cyclical muscle use, the pause between muscle contractions is critical for blood to nourish the muscles. Blood flow is impeded by muscle contractions (click link again), so much of the fuel is delivered to, and wastes removed from, the muscles during the pause between contractions. When you have a stiff bike, you can use a strong, concentrated (e.g. brief) contraction to add power, and you don’t need to maintain pressure on the pedals as long past the point of effective torque (when the lever arm gets short) to make sure the "wind up" in the frame gets redirected into the chain once the pedal force/torque is inevitable reduced, later in the power stroke. This results in a longer pause that allows the muscles to be better nourished, and be more ready for the next contraction. Hence, power is higher for a given level of perceived exertion, and endurance likely improves too. The opposite happens with a flexier frame. I theorize that riders can feel this flex, even if it’s not entirely conscious, and instinctively will adjust their pedaling to try to account for it. Basically, to overcome a flexy frame, you have to finish every pedal stroke with what is mostly an isometric exercise that is there to convert flex into drivetrain power but adds no power in and of itself (no distance = no power, no matter what the force is, because power is force x Velocity, and velocity means distance over time), and cuts down on the time interval your circulatory system has available to effectively load and unload stuff in and out of the muscle cells. This malnourishment of muscle cells is certainly going to reduce your ability to produce power.

How to test the loss in cycling economy due to flexy frames? In a lab - same rider, same everything, but two frames that are different in only one way - one is flexy, one is not. Pedal or crank power meter to measure net power, and whatever bag of tricks physiologists use to measure the human animal. O2 meters, blood tests, you name it - to measure gross power. Do the tests at different inertial loads to simulate hills of varying steepness. Do the tests at different percentages of FTP. Do maximal power tests (essentially sprints). See where it matters and where it doesn't matter, and by how much. And if there is an improvement in economy or in net power with the stiffer frame, is it enough to overcome the added weight that it took to make the frame stiffer? That last question is pretty easy - another one the mechanical engineers can answer with confidence. At that point, we have sort of done the study that you are asking for.

Trouble is, we left out a bunch of other things I listed that can affect cycling economy. We've still got stiff seats and seat pads, optimal gearing, body angle, torso angle, confident handling, leg extension, seat fit, glute utilization, and maybe some other stuff I am not thinking about to study next. And what if, when studying something like torso angle, we run into some physiological effects that vary from person to person as much as height or hair color. How to make sense of individualistic results. What if the answer is "well, it depends...." That's not going to be very useful when it comes to calculating a bikes HCR (hill climbing rating).

Short of beating our heads against the wall and wasting years of our lives, I propose something way more fun. Buy a bunch of bikes and ride them all a bunch, and keep track of how fast you ride on them. Which is exactly what you did, and when you posted your findings, you got a comment that poo pooed them by basically saying, hey - all you actually figured out was that these three bikes have slightly different CdA's. And that's essentially correct. You live in FLA, what else can you do? You simply can't evaluate relative climbing performance without hills to test them on. From my experience, different bikes can have very similar CdAs and not be very similar in climbing performance, even if they were all the same weight (or if you handicapped them accordingly). And that was the point the comment you received was making. I think the person making the comment had ulterior sales-related motives, but I can't say he was wrong in principle either. I don't know if he is correct in reality, however, because I haven't ridden them all contemporaneously. Maybe the bike with the highest CdA is also the slowest climber? Or maybe the opposite is True? Those two performance metrics have almost nothing to do with each other. I can make some guesses on which would be the fastest climber based on the basic design, but that's all it would be - a guess.