CRANK LENGTH Discussions
DISCUSSION #1 – Damon Rinard, Cervelo Engineer
I’ve heard many triathletes and time trialists are switching to shorter cranks: 170, 165, even 160mm. What do you recommend?
Great question. It is true that many top athletes are switching to shorter cranks for timed racing such as triathlon and TT. This is relatively new, because traditionally longer cranks were thought to be better since they give more leverage.
However, crank length is just one lever in a drive train composed of a system of levers that transmit your foot’s force on the pedal to your tire’s thrust on the ground.
The other levers in this system are the chain ring radius, cog radius and wheel radius. We vary two of these (chain ring and cog) at will whenever we shift gears. So we don’t need a small difference in crank length to change the leverage available to us.
What does Dr. Martin say?
For many athletes, the idea “longer is better” has changed in part because of Dr. Jim Martin’s 2001 study titled “Determinants of maximal cycling power: crank length, pedaling rate and pedal speed” (Eur J Appl Physiol (2001) 84: 413-418).
Jim’s study involved 16 bike racers of various heights doing maximal sprint power tests, typically less than four seconds duration. During the study, they repeated the efforts while systematically testing the following crank lengths: 120, 145, 170, 195, and 220mm.
Believe it or not, the test results showed no statistical difference in maximum power among the three middle crank lengths (145, 170 and 195mm). The saddle height (measured to the pedal) was maintained throughout and the researchers did not adjust fore-aft saddle position or handlebar height despite changes in pedal-to-knee relationship and handlebar drop with the various crank lengths.
For years crank length tests had been inconclusive and the general working knowledge came more from experience and intuition than science. Now athletes can choose the crank length they like without worrying they’re affecting power.
What does the wind tunnel say?
With the leverage-dependency myth debunked to a certain degree, it was the application of these lessons which really drove the value of this study. The figure above graphically shows how the aerodynamic drag area (CdA) changed when four pro athletes tested multiple crank lengths in the wind tunnel. (Keep in mind lower CdA is better.)
Rider1’s CdA increased (from 0.271 to 0.277 m2) when he changed from longer to shorter cranks (from 180 to 175mm), but the other three riders’ CdA stayed the same or decreased slightly when changing from longer to shorter cranks. The crank length and CdA data for each athlete is listed in the table below.
|Wind Tunnel Run||Rider|| |
|LSWT 0908 Run 756||Rider1|| |
|LSWT 0908 Run 757||Rider1|| |
|LSWT 0908 Run 806||Rider2|| |
|LSWT 0908 Run 807||Rider2|| |
|LSWT 0908 Run 805||Rider2|| |
|LSWT 0908 Run 701||Rider3|| |
|LSWT 0908 Run 702||Rider3|| |
|LSWT 0908 Run 707||Rider4|| |
|LSWT 0908 Run 708||Rider4|| |
Table 1 Crank and CdA data used to generate the Figure above. Only CdA pairs with adjacent run numbers are comparable; other position changes were made in between non-adjacent run numbers which make them non-comparable.
As you can see from wind tunnel test data, changing crank length by itself doesn’t always have a predictable effect on aero drag (CdA). But for each of these pros, the change to a shorter crank solved a range of motion issue at the hip that allowed them to comfortably make other changes to reduce their aero drag without decreasing power.
What is the application?
With maximum power essentially unaffected by a wide range of reasonable crank lengths, athletes are now free to choose crank length based on other criteria. Convenience (your might already have a serviceable crank on your bike), comfort, pedal clearance (to the ground), toe overlap; all of these are affected by crank length.
However, what is now understood is that, especially in an aero riding position, shorter cranks can sometimes alleviate a common fit problem: if the hip angle is too tight at the top of the pedal stroke, the athlete can be uncomfortable, or is unable to produce maximum power at the top of the pedal stroke.
Even in athletes with no existing fit problem, some choose shorter cranks in order to further lower the torso by lowering the arm pads. Perhaps this is not a surprise, but the hours of wind tunnel testing we’ve done with various Cervélo-sponsored pro athletes over the years confirms that for nearly all athletes, a lower bar means lower aero drag.
Keep in mind that hip angle isn’t the only limiter on lowering the torso. Saddle discomfort, digestion and vision are other common limiters. If an athlete is limited in these ways then shorter cranks won’t help get them any lower.
Some athletes keep their long cranks and still perform well. Some try short cranks, aren’t happy with the results and switch back again. Others keep the short cranks and tell us the following:
- They pedal faster. The effort and foot speed is about the same, but the RPM is higher, typically about the same percentage higher as the change in crank length. For example, the difference between 165 and 175 is about 5%; some athletes find themselves in a gear about 5% easier than before, with a matching cadence about 5% higher. Coincidentally, the difference between a “compact” 50 tooth chain ring and a 53 is close to 5%. Likewise 20 and 21 teeth are about 5% different.
- They adapted immediately. The leg muscles operate over a slightly shorter range of motion with shorter cranks, so no “new” muscle training is needed. Also the faster cadence doesn’t need to be learned or trained, because the foot speed (and thus the muscle fiber shortening velocity) is the same as before.
- They feel more similar between aero and road bike positions. The typical idea is to rotate your road position into your aero position, but usually the torso rotates farther than the rest of the body. This closes the hip joint, and shorter cranks on the aero bike can maintain a hip angle more similar to that of their road position.
- They can run better. Triathletes say the initial part of the run feels better coming from shorter cranks.
What does the Race Engineer say?
As Team Garmin-Cervélo’s Race Engineer, I advise athletes to choose whatever crank length they like. Those who are interested may try shorter cranks on the TT bike; in that case I usually recommend a 5mm difference: longer on the road bike than the TT bike.
In all cases, regular training on the TT bike is important to promote adaptation to all aspects of the aero position. The main thing is to realize that the choice of crank length doesn’t significantly affect power, so any length is now free to choose for any other reason. This lets the athletes relax about crank length, knowing it’s not as critical as we used to think.
Dr. Martin’s results are not widely understood yet, so crank length is still controversial, and many athletes have strong preferences on crank length. Let me know what you think in the comments section below.
DISCUSSION #2 – Power Cranks
On this page you should learn
- 1. You should learn that even though most people are comfortable with their current crank length this is probably because it is the only crank length they have ever used. We like what we are used to but, for most, their current crank length is probably too long for them for optimum performance.
- 2. You should learn there is scientific support for the potential superiority of shorter crank arms.
- 3. You should learn what you need to do to determine the crank length best for you.
I am perfectly happy with my crank length, why should anyone think I could riding somethng much better?
Ponder this: Why is it that bicycle riders come in sizes ranging from under 5 ft tall (60 in, 152 cm) to 6 ft 6 in tall (78 in, 198 cm), a variation of over 30%, bicycle frames come in sizes from 48 to 62 mm (a 30% difference) yet most bicycle cranks are only available in lengths from 165 mm to 175 mm, a variation of only 6%? Bicycle frames are readily available in 18 sizes but bicycle cranks on original equipment generally come in 3 sizes, 170, 172.5, and 175. As far as bicycle cranks go the industry may as well be taking a one size fits all approach. It serves the bicycle industry just fine because it is much less expensive to have just a couple of sizes but it doesn’t make sense that limiting this choice would serve the athlete well. Is it possible the bicycle manufacturers care more about their profits than your performance?
Why would crank length be so important? It is simple, each rider only touches the bicycle in three places, the pedals, the seat, and the handlebars. Crank length (and rider flexibility) is really what determines bike fit (seat and handlebar position) because both seat and handlebar position are determined by pedal position which is determined by crank length. The crank length-seat height determines the knee and hip angles when the cranks are at the bottom and top of the stroke. The crank length-handlebar distance determines how close the knee comes to the chest, especially in the aero position.
Yet, most bike fitters ignore crank length as a variable, doing their fit just using the crank length that is on the bike when it rolls through the door. Yet, crank length is just as adjustable as seat position and handlebar height but the problem is it is not normally easily adjustable during a fit so it is ignored.
But, it is clear, one cannot do a really good bike fit unless one also knows the best crank length (or range of acceptable crank lengths) for the kind of riding the cyclist will be doing.
If you ignore the crank length variable you are simply letting your equipment limit your potential rather than finding the equipment that facilitates your reaching your FULL potential. Crank length is a vital piece of equipment in that equation, it should not be ignored. We suggest that you do the work to determine what is the best crank length for you BEFORE you spend all that money to get a professional bike fit.
If you find that you:
1. are having trouble getting into an excellent aerodynamic position (chest parallel to the ground) for racing…
2. find your knee being thrown out when the pedal is at the top of the stroke…
3. have a lot of knee or back discomfort when riding…
4. have trouble getting your cadence up as high as you need…
5. have trouble getting your cadence up when sprinting…
6. are not racing as well as you think you should be for the amount of training you do…
7. are always injured…
then you are probably riding the wrong crank length for you and almost certainly your crank length now is longer than is optimum for you. Be sure to read more to better understand why crank length is now, probably, too long and why it is, probably, holding you back. If you pay attention to this detail and optimize it then I predict it will pay big, big dividends.
But, the real question is can I race well on shorter cranks?
Professional triathlete Courtney Ogden has taken this seriously. Since experimenting with short cranks he has won two major events (Ironman Western Australia and the Metaman Bintan triathlon and its $40,000 first prize) using 145 mm crank length. Drew Peterson improved his placing in the Everst Challenge (28,000 ft of climbing) from 26th to 9th after changing his crank length from 180 to 110! The proper crank length for you (even if it seems very short compared to what you are used to) actually helps you to race better. Shorter cranks are less fatiguing for the hip flexors and it has been shown in several studies (#1, #2, #3) that hip flexor fatigue can adversely affect performance at the end of a race, usually the most important part of any race. If shorter cranks are less fatiguing and do not affect your power (as will be shown below) what is there to lose by experimenting and seeing what happens? Of course, it is possible to go too short (you can’t generate any power at a crank length of zero) but you won’t know what too short for you is until you try different lengths.
How did we get to this position?
The first thing you are probably asking is why, after 150 years of racing bicycles during which crank length has evolved to what we all know of as “best”, am I telling you it evolved wrongly? Haven’t the current crank lengths people use stood the test of time? Well, first, we should look at how current crank length evolved. Bicycle racing has been around for about 150 years and it started on the “ordinary” bicycle, the bicycle with the big front wheel.
The bigger the wheel the faster the rider could go and the longer the cranks the more leverage he had and the faster he could accelerate. Crank length evolved to what it is now by the experience of these racers on these machines in the kind of racing done at the time.
Cranks of about 170 mm in length proved to be about optimum. Later, after the safety bicycle was developed (bicycles with two equally sized wheels one driven by a chain) cranks could be of any length but these were still single speed bicycles and behaved very much like the ordinary bicycle so riders stayed with what they were used to, because people, by and large, do not like change (plus, it seems tradition really counts in cycling).
And, manufacturers made bikes that gave people what they wanted, so every new rider became comfortable with this crank length and for 100 years this crank length is what everyone became used to. And, of course, we all like what we are used to.
In addition, in the early years of road racing, when the ability to shift gears was limited or non-existant, riders gravitated to longer cranks because they thought it would help them to gain extra leverage for climbing. Since the best cyclists used pretty much the same crank length, everyone thought it would be best for them also.
Then, around 1950 the derailleur was developed and riders now could change gearing on the fly to optimize gearing for different conditions. Gearing is nothing more than changing the leverage the cyclist sees between the pedal and the ground.
Now no longer was crank length important for “leverage” but this “longer crank length means greater leverage” myth was now ingrained in the psyche of the cyclist. Further, in the early days, not many were thinking of rider aerodynamics and since power doesn’t vary much over a wide range of crank lengths, it wasn’t obvious that crank length needed to be revisited, so racers stayed with crank lengths that were familiar to them, the crank length that came with their bicycle.
And, since everyone was using the same crank length, it wasn’t obvious there were gains to be made by changing. And, of course, manufacturers saw no reason to change either as they prefer to sell what everyone already wants rather than try to convince people there is something better. We like what we are used to, nobody likes unnecessary change.
But, back to the original question, Does crank length really matter. Well, if I didn’t think so, I wouldn’t be writing this. The only real question is how much does it matter and in which situations does it become most important. Even Lance Armstrong (after he started racing triathlon and before he got banned by the world) stated he had moved to shorter cranks for many of the reasons stated in this essay.
Lance, if he was about anything it was about taking any advantage he could if it would help him win. It appears to me that going to shorter cranks (sometimes much shorter cranks) is simply finding free speed, speed that is sitting within you waiting to be discovered.
How did I arrive at this determination? Several years ago I began to think that it would be much easier to get a cyclist into a good aerodynamic position if the cranks were shorter but I didn’t know what would happen to the power as one went shorter (going fast is a trade-off between power and aerodynamics). I asked the question on the internet to see what “science” had been done in this area and was directed to a study done by Jim Martin at the University of Utah (Determinants of maximal cycling power: crank length, pedaling rate and pedal speed). What did this study show?
The Martin Study
The Martin study looked at the effect of crank length on max power in cyclists. Below is the most relevant figure from that study:
This study concluded that while the power was maximum with a crank length of 145 mm there was little lost by most riders when using 170 mm cranks because the difference was only about 1%. Let me repeat, Martin found that in his study POWER WAS MAXIMUM AT A CRANK LENGTH OF 145mm. It is strange to me that even though the difference in power between the 145 and 170 mm cranks did not reach statistical significance Martin would have concluded that crank length didn’t make any difference to the racer.
There was a clear trend to his data and failing to reach statistical significance may only mean the study was not powerful enough to pick up a real but small difference. Further, there is a big difference between racing significance and “statistical significance” for the purposes of a scientific study.
It also seemed obvious that if there had been more participants that the 145 mm cranks superiority would have, probably, eventually reached statistical significance. What would he have “concluded” then? At a minimum the authors should have concluded this potential existed and this area needed additional study.
But, they did not and “everyone” now believes, based upon this one study and its flawed conclusions, that it has been proven that crank length makes “no difference” in bicycle racing. Nothing is further from the truth. If the differences were real but small why would a racer want to give up any additional power? Or, conversely, why would they want to give up any potential aerodynamic and comfort advantages that smaller cranks provide when power is not compromised?
Even if there was no real power improvement, how does one explain the fact the power didn’t drop as cranks went shorter (until 120 mm anyway)? Everyone “knows” that longer cranks offer more leverage so should offer more power, correct? Yes, until one considers crank length affects two confounding factors that also affect power generation. That is knee leverage and pedal speed. Knee leverage? Yes, knee leverage. The more the knee bends the less leverage it has.
The knee bends less as the crank shortens so even though some leverage is lost because the crank shortens leverage is gained because the knee is in a more favorable position to apply force so it is a wash pretty much. Then there is pedal speed. The faster the pedal is moving the harder it is to apply force to the pedal. Longer cranks tend to have higher pedal speeds.
So, while it takes less force to generate power when cranks are longer it is harder to apply that force to the pedal both because the pedal is moving faster (usually) and the knee is bent more. Change one thing to make it better changes other things to make it worse. This explains why power stays pretty much constant over a wide range of crank lengths.
Another way to look at this is to look at what goes on around the entire pedaling circle. Power generation is more than pushing hard but also involves getting the foot out of the way on the backstroke. The best way to maximize the average power around the pedal stroke is to do what is called “pedal in circles”, where the work performed by the muscles remains pretty much constant. This concept is more fully explained here. Below is an example of a real world pedaling pattern that can further explain why longer cranks rob the rider of power or why power doesn’t drop (or increases) when cranks are shortened.
So, while one can argue that one can get more pushing leverage with longer cranks we can see that it is likely that you lose more at the top that you gain in pushing advantage. There may be more leverage pushing but what happens around the rest of the stroke is just as important.
Here is some actual data from someone who tested this for himself. This coach for many elites put himself on an Excalibur ergometer, which measures pedal forces around the entire circle, and compared what happened between 170mm and 150 mm crank length, close to what Martin did. Here are his results (the 150 crank length results are on the left, the 170 crank length results on the right).
Note, the total average power for this 90 second test was slightly greater (313 vs 308) on the 150mm cranks despite the fact that the average maximum power was greater (633, 646 vs 610, 620) on the 170 mm cranks.
This is hard to explain until one looks at what happens on the upstroke where the negative forces are much less (-73, -145 vs -96, -166) on the 150 mm cranks. It is the ability to gather this kind of data that makes having a 2nd generation power meter worth the cost to the serious athlete. If you would like one of these for your own training you need to read more about the iCranks and all the wonderful things it will be able to do when it becomes available.
If all we had to worry about was power in bike racing then crank length would hardly matter at all, which is what most people now believe (because all most people think about is power, especially in this age of being able to measure power). But racing also involves aerodynamics and efficiency. Once I saw the Martin data it got me to thinking.
Why would any serious racer want to give up any power, even if it were small, just so they could ride the cranks they are used to that came with their bike? And, even if there were no power advantage, the Martin study only looked at power production and ignored the effect of crank length on bike fit and aerodynamics, a huge factor in determining how fast you can make your bike go.
This bike fit issue was really the original problem that got me thinking about this. This got us to doing some of our own experimenting and our early data shows that for ordinary sized men, when in the aero position, that power does not start to drop until crank length is below 130mm crank length. After experimenting with this Pro triathlete Courtney Ogden won Ironman Western Australia racing on 145 mm cranks. Most of our customers who have experimented with this are settling on crank lengths between 130-150mm.
I have experimented with cranks as short as 85 mm but have found a crank length between 130 and 145 is probably optimal for me. I am 6’2″ tall with a 34″ inseam and as a result of going to very short cranks have been able to drop the front end of my bike more than 5 inches without any comfort issues (the main reason aerodynamics will improve with this change).
But, what about YOU
Here is the real problem faced by yourself and almost everyone else. How does one even find short cranks to experiment with let alone go though the bother of affording and changing out all these different cranks.
It is just too much trouble. That is why, as soon as we discovered this we immediately redesigned all of our cranks (except for our cheapest entry model) to allow the user to experiment with crank length. All of these adjustable models will go as short as 145 and some will even go as short as 90 mm. Now, with PowerCranks, you can experiment with crank length while, at the same time, learn better pedaling technique.
What we believe you will find when you do this for yourself
Based upon my own extensive testing and the reports of many users I believe the following axioms will be true:
1. Everything else being equal, a rider putting out 150 watts will test ” shorter” than a rider putting out 400 watts. (This means beginners will test shorter than pros, endurance athletes will test shorter than sprinters). I believe this to be true even though the numbers for any particular person will be somewhat different than shown – why you need to experiment for yourself.
2. Everything else being equal, a rider with less flexibility will gain more aerodynamic benefit going short and will need to go shorter than those with above average flexibility to attain the same aerodynamic position.
3. Everything else being equal, a rider trained to pedal in the PowerCranks fashion will be able to go shorter than another rider who gets all their power from “pushing” alone. (more about this below).
4. Those with significant knee issues should do better with short cranks than long cranks – even though short cranks may require the rider to push a little harder, the knee is bent less so overall stress should be lowered on the knee because it is easier to push hard if the knee is not bent so much (can you lift more weight doing a partial squat or full squat?).
5. The major racing benefits to come from going to shorter cranks are to be found in the aerodynamic improvements that are possible and improved bike comfort.
While power might be optimized also, the power variation seen with crank length is small in most (10-20 watts), as shown by Martin and our own data. But, the aero benefits seem to be huge such that this change should provide huge dividends to the serious cyclist. The picture shows a super imposed photo showing how pro triathlete Courtney Ogden was able to improve his aerodynamic position when moving from 172.5mm cranks to 115 mm cranks.
Not only did his power improve (slightly) as he went shorter but look at the improved aerodynamics. He would be faster even if his power dropped a bit with this change. But, what is best for Courtney may not be best for you. Another rider willing to push this envelope reported this:
“I went for a ride yesterday for 75 min @ 120mm lengths then did a 15 min run. It felt really good – I averaged 90 rpm and did a 5 min time trial in the middle @ 300W @ 29.7 mph avg. Compare to a TT in (a few weeks earlier)w/170mm cranks @ 280W @ 27.7 mph avg., first time on the tri bike since July it was 7% faster @ 7% more watts so aerodynamics seem to be improved. (ed: a 7% increase in speed would normally require about a 20% increase in power) I’ll continue to play around with cadence and power but it felt pretty good yesterday.
It feels a lot more aero with an extra 10cm drop to the bars and more comfortable than the old setting too. I need to measure the hip angle to check position. Sitting forward on the seat felt more powerful so I will test moving the seat forward. I’m at about 23cm drop from the seat to the pads. I think I’ll stick with 120mm cranks for now because I don’t think my neck could take any more drop and I’m concerned about bike handling if I go to 100mm crank and move the seat even more forward.”
What is optimum for each person can only be determined by trial and error experimentation.
But, what about climbing?
Everyone “knows” that to climb well you need the leverage that a longer crank offers. So, what happens when you try to climb with short cranks. First, as I have shown, it is simply a myth that you need long cranks for the leverage to generate power. The leverage that is important is not the leverage between the pedal and the bottom bracket axle but the leverage between the pedal and the rear wheel. Since there are several links in the leverage chain it is possible to adjust the leverage using gearing to make up for “loss” of crank arm leverage.
And, there is one more lever in the chain that people generally ignore, the knee. The more you bend the knee the more “leverage” you lose (can you press more weight with a full squat or half squat?). Increasing crank length reduces knee leverage such that there is no net leverage benefit using longer cranks within a large range. Climbing is not about leverage it is all about power so you should have the crank length that optimizes power, not a crank that “optimizes” one aspect of the overall leverage equation.
To illustrate this here is a real world example. Drew Peterson is an experienced ultra-cyclist. He had done the Everest Challenge for many years. He believed he needed long cranks to perform optimally in a race that involved 28,000 ft of climbing in 208 miles.
His times were typically just over 12 hours for this effort on 180+ cranks. In 2010 he decided to do the race on PowerCranks, using the longest length available to him, 182.5mm and he cut about 30 minutes off his usual time, doing 11’47”.
This effort convinced him that racing on PowerCranks, using his typical long length, was the way to go for him. I then convinced him to experiment with shorter cranks. He gave it a try and kept pushing the limits and decided to race the 2011 edition on 110mm PowerCranks (he also used arch cleats because he felt they were more energy efficient, stressing the calves less, and allowed him to fit on the bike better).
In 2011, using 110 mm PowerCranks, he improved his placing from 26th to 9th, beating most of the pro/1/2 riders in the race. The course was slightly shortened this year but we think his time improved about 30 minutes compared to most others who did both races. So, yes you can climb just fine on very short cranks if you are geared properly and train yourself to do so.
More benefits – Moutainbikers
So, using shorter cranks will usually give the rider: 1. More (or, the same) power. 2. More ground clearance. 3. Better aerodynamics. What is the mountainbiker not to like?
More benefits – Good for Bad Knees
And, did I say that short cranks should be good for bad knees? For any given muscle force, the stress on the knee goes up as the knee bends more. The stress increases substantially as the knee bends beyond 60º. Therefore, if you have bad knees one way of minimizing the stress on them is to shorten your cranks.
Here is a link to a web site that lets you explore exactly what happens to the different joint motions as you change crank length and saddle height. Another part of this site has a very interesting discussion of bicycle power production. Despite it being primarily associated with recumbent pedaling, the general principles apply to all pedaling.
The science revisited
How can this short crank length benefit be explained scientifically? There are two major scientific papers here. First is Determinants of maximal cycling power: crank length, pedaling rate and pedal speed by Martin and Spirduso of the University of Utah. This paper concluded that while the power was maximum with a crank length of 145 mm there was little lost by most riders when using 170 mm cranks because the difference was only about 1% and the difference was not statistically significant. Further, the paper concluded that the most important variable that affected power was pedal speed, not crank length.
The most relevant figure from that paper as regards this discussion is again reproduced below.
The trend is clear (in both the average power for each crank length and the power range for each crank) but the differences didn’t reach statistical significance (a mathematical analysis by which the research can say with certainty that the results have less than a 1 in 20 chance of being due to chance).
As a result of this failure to reach statistical significance most observers have concluded that crank length is of no consequence. But, let’s look at this “failure” more carefully. There are only two possible reasons this data did not show statistical significant differences. 1. There are no differences or 2. The sample was too small to show real differences to the degree required by scientific papers.
The reader must look at the data and decide for himself, which is most likely? To my reading, there is a clear trend and, to my reading, the only reason statisical significance was not reached was because the sample size was too small.
From this data alone it is reasonable to infer that if there had been enough subjects in this study the 145 mm crank length would have been “proven” to be more powerful than the 170 mm crank. Even if there were not a “power advantage” to shorter cranks (at least to 145 mm) this study “proves” that 145 and 170 mm crank arms are equally powerful.
But, there are a couple of more issues with the Martin paper. First, it was done in cyclists riding in the more upright “road racing” position and not in the more aerodynamic time-trial position. We theorize that this effect will be even more dramatic when in the time-trial position. Our initial testing supports this theory as a pro-triathlete saw continued small power increases for the same effort as he shortened the cranks until he got shorter than 115 mm.
Second, as cranks become very short it takes a bit of time to adapt to this new length to learn how to ride them with good power. It requires learning the proper cadence and gearing to optimize the crank length. Martin tested without any training time at each length.
We have yet to have an athlete who has tried going shorter report a drop in power with the change, even with very short crank lengths, down to 130 mm. (One issue that prevents most from going much shorter than 130-145 when testing is how high the athlete can raise their seat with their current seat tube.)
The second paper by McDaniel, et. al. entitled Determinants of metabolic cost during submaximal cycling looked at the effects of crank length and pedaling rate (cadence) on the metabolic cost of cycling. In this study, crank length had almost no effect on metabolic cost.
There are some other reasons to think that crank length might affect power. That has to do with leverage. The average cyclist thinks shorter crank length affects leverage to the wheel, but this is not so if one adjusts the gearing to keep the total leverage the same. But, where crank length does affect leverage is in the leg joints.
Leg muscles are most powerful and efficient in a relatively small range of motion. (Stresses on the knee start to increase greatly if the knee bends more than 60º. Standard size cranks typically bend the knee more than 90º at TDC.)
If a crank is so long that it put the muscles and joints (knee and hip) in a less favorable position then power and efficiency can be lost. Everyone “worries” about what crank length does to the “leverage” to the crank axle (which is easily compensated for) but don’t think about what it does to the leverage of the knee and hip joints and muscles.
An “easy” test to look at what range of motion is optimum for you is to put yourself on a Stairmaster (a climbing machine that lets you choose how high each step is) and see what step height works best for you when trying to optimize your climbing rate for various periods.
I predict you will find you take bigger steps if you are trying to optimize your climb rate for 30 seconds than if you are trying to optimize your climb rate for an hour. And, I predict your 1 hour step height will be on the order of about 6 inches, subtantially less than the 14 inches the typical bicycle cranks force you to take.
So, there are plenty of reasons to think that crank length is important to optimizing power in some people but let’s assume, for the sake of argument, that crank length has zero effect on power – is there another reason to worry about crank length? Why not just ride what came with your bike if power isn’t affected? Of course, this ignores the affect of crank length on aerodynamic position and the affect of aerodynamic position (and crank length) on power and speed.
Most experts agree that the lower one brings the front in trying to get into a good aerodynamic position the more the power is going to drop because of the difficulty in getting the thigh close to the chest. We would expect that this “crank length does not affect power” finding would not be so clear if position on the bicycle were added to the mix. Every rider knows they can only go so low before they begin to lose power and/or stop being comfortable.
So, an even more important consideration is the effect of crank length on aerodynamics. Simply shortening the crank and moving the seat up the same amount (and doing nothing else to the bike) does three things in this regard.
1. It moves the butt up in relationship to the shoulders so it flattens the back, generally regarded to be a better aerodynamic shape.
2. A shorter crank and reduces the frontal area, important to good aerodynamics, and,
3. It opens the distance between the knee and the chest at TDC, reducing “cramping and improving comfort.
4. #3 also leads to the possibility of lowering the handlebars even more, reducing the frontal area further. This is where the big aerodynamic improvements can be found.
Several anecdotal reports to illustrate the effects of this. Pro-triathlete Courtney Ogden has reported that he has been able to drop his front end about 10 cm between a crank length of 170 and 115-130. He believes this is saving him about 15 minutes on the Ironman bike split. And, an article in Triathlete Magazine in 2008 reported that John Cobb reported reductions in drag, determined in a wind tunnel, of 30% from simply shortening the crank length and lowering the front appropriately.
At PowerCranks we believe experimenting with crank length will become very important to most serious riders in the future. Therefore, we have recently modified our line to allow the user to experiment with crank length down to 90 mm without changing Q factor.
Our early work suggests that optimum crank length for the average male for time-trial events will be somewhere in the 125-150mm range. Every PowerCranker will now be able to easily experiment with this to see what works best for them should they choose to do so. The key here is for each serious athlete to not be afraid to experiment with this variable to see what is really best for you. Here is what happened to one who did just that: “I read the write up about shorter cranks and it intrigued me. I didn’t believe that shorter cranks would be faster, but nevertheless I couldn’t stop myself from trying them out.
So, I went out and bought a set of 145mm juniors bmx racing cranks. and did some training rides all within a month of the 200mile Seattle to Portland classic. I found in training that speed, endurance, and climbing strength all went way up on the shorter cranks. And in the Seattle to Portland, I finished 4th, 6 min behind a recumbent rider and two guys who were working together the whole way even though my last 100mi were almost all solo miles.
All on cranks and a saddle position adopted less than a month earlier (imagine how fast I will be when my legs fully adapt to this length). Thank you Dr. Day for raising this issue. it has impacted my athletic performance beyond my wildest dreams.”
YOU TOO SHOULD TRY THIS CHANGE. IT IS IMPOSSIBLE TO KNOW THE POTENTIAL FOR YOU UNTIL YOU TRY. WE FEEL THIS TO BE SO IMPORTANT THAT MOST MODELS OF POWERCRANKS ALLOW FOR THE USER TO EXPERIMENT WITH CRANK LENGTH WHILE LEARNING HOW TO PEDAL MORE EFFICIENTLY AND POWERFULLY THE POWERCRANKS WAY.
I have always enjoyed bicycling and, through a series of coincidences, became a Bicycle Industry Consultant and Product Tester. I test prototype products for companies and have published only off the shelf production products on biketestreviews.com.