Crank arm lengths for tri popular
Written by: Dan Empfield
Date: Mon Mar 16 2009
At first blush, the answer seems to be it doesn't much matter. But let's look a little further, because I think it does matter. Further, the answer is counterintuitive, as is often the case in this milieu.
Jim Martin is a cycliing enthusiast, a bicycle racer, holds a PhD in exercise science and an undergrad degree in mechanical engineering. He's a professor at the University of Utah and is as august an academic on the subject of crank length as anyone. The conclusion of his investigations is that it's hard to quantify a difference in efficiency between cranks lengths of 145mm and 195mm, regardless of a rider's heights. So why sweat it?
Crank lengths have been a topic of investigation for Velo News' tech editor Lennard Zinn for 15 years, and he's written, rewritten, recast and self-edited many times over the years, probably most notably in a long reply to a Velo News letter to the editor in 2003.
These are two of several such formulae. Others are Bernard Hinault's 28.4% of knee to trunk, and then just 10% of your height.
You'll notice a large variance in crankarm lengths if you use formulae built strictly on the notion of proportionality. Crankarms from 150mm to 195mm appear appropriate for riders ranging between 28" and 36" inseams. A formula like this intuitively flows from the notion of proportionality, and the fact that these cranks are not spec'd on bikes is probably more a result of fear of SKUs-run-amok by crank and bike manufacturers than by anything more rational. Anyone who's spectated bike makers trying to explain why they don't use 650c wheels on their smaller-sized bikes understand the mindset.
But there are detractors, notably Edward Zimmerman, who moderates proportionality based on factors such as the knee stress at the top of the stoke just when power is applied.
What I have failed to see, from the analysis of those listed above, is anything above a cursory sentence on the importance of the hip angle at top dead center, and how this question impacts the choice of crank length. More and more TT/Tri fitters—whose jobs are finding the appropriate bike positions of their charges—are focusing on this particular ergonomic metric: acute hip angle.
Kirby Palm may be onto something with his crank length formula, but he falls prey to two urban myths in the same sentence: that time trialing with a longer crank, and with a lower cadence, are each desirable things. Zimmerman notes, without refutation or commentary, that longer cranks are chosen in both mountain and time trial events, as if these specialties have a kinship requiring parallel technologies or techniques.
Lengthening a crankarm creates problems for a timed racer. If you add 2.5mm to a crank's length, you lower your saddle that same 2.5mm to preserve your knee angle at bottom dead center, roughly 145° of included angle. At top dead center, then, that pedal axle is now 2.5mm closer to the torso, plus another 2.5mm, because that crank now sits 2.5mm higher above the BB axle. So, for every increment you lengthen the crank, your knee is double that increment closer to your torso as you start to apply power to the pedals at top dead center.
Your "closed hip angle," that is, your hip angle at top dead center, is probably between 46° and 52°, depending on your chosen landmarks, but this range is what you'll get using the trochanter as the angle's fulcrum, extending to the center of the knee, and to the acromion (shoulder bone). Our F.I.S.T. protocol measures open hip angle, or more precisely a proxy of it, and centers around 100°. In either case, in order to perserve these hip angles, the armrest drop—that vertical measure between the top of the saddle and the top of the armrest—must be increased by 5mm if the crank is lengthened by 2.5mm. This works against the athlete if he's trying to choose the most aerodynamic position available to him.
But this would be just a starting point for what might be a further analysis into your perfect cadence, taking into consideration your morphology, your height, conditioning, fiber type, and so forth. My hope is not to segue into a discussion of perfect cadence, rather my points are two: that cadences in the timed events, at the highest levels of bike racing (where efforts rarely last more than an hour), are rarely observed below 90rpm and are often above 100rpm; and that cadences trend higher as the event grows shorter or, to put it another way, cadences go up as effort goes up.
You might add a third element that tracks up or down with cadence and effort: seat angle. If you consider just flat terrain, whether in a mass start event or in a timed event, as the effort goes up, the trunk moves forward—when Phil and Paul are describing the rider as "on the rivet" they're not talking about the rear rivet.
This is not to suggest longer crank arms are not desirable, rather to say that longer crank arms do not dovetail nicely with the notion of flat backs; small frontal profiles; and closed hip angles that allow for appropriate leverage at the beginning of the power phase of the pedal stroke—especially if riding a faster cadence is a required element.
What both Jordan Rapp (my F.I.S.T. workshop "lieutenant" and one of triathlon's top cyclists) and I, in our n=2 experiment, have discovered is that we pair of 6'2" riders have found that 172.5mm cranks seem most appropriate for our own timed racing efforts. What I don't know is what my best crank arm length is for road riding. I rather think it depends on the specialty. For example, criterium riding is a lot more like timed racing than like, say, long ascents. My flat-back, on-the-rivet, hands-in-the-drops try like hell to keep the elastic from snapping seated sprint out of a corner is pretty much identical to my TT position, just that I don't have amrests under my elbows. Indeed, my closed hip angle in this position is 48°, identical to this angle in my TT position, as determined by a Retul motion capture analysis.
So I cannot give you, as of this writing, an amount triathletes and time trialists should shorten their cranks versus what they'd use on their road race bikes because, even though I've been riding and occasionally racing road race bikes for 30 years, I've only just begun to think about whether my best crank length while climbing is really 175mm.
But what I can at least tell you is, regardless of what crank you ride on your road race bikes, and irrespective of the nature of your typical road riding session, your best time trial or triathlon crank length is certainly not longer—rather it's shorter—than what you'll use on your road bike.
Were I a product manager spec'ing cranks on my size run of TT/Tri bikes, I'd certainly start with cranks no longer than 165mm, and these would go on everything 50cm and below. I'd then add 2.5mm, which means finding the elusive 167.5mm lengths, every 3cm or 4cm worth of bike size increase, ending with 172.5mm on sizes 60cm down to 57cm, and 175mm arms on bikes only above 60cm in height.
This would dovetail with the smaller gears on these bikes required to ride the faster cadences, which advocates for 110mm bolt pattern cranks spec'd OE with 34x50 and 36x50 chain rings. Sadly, none of this sort of drive train spec is very often found on OE tri bikes these days.
In case you haven't noticed, short cranks are all the rage. But what's out there? Will it fit my current bike? How short can you go? We address these questions and more. 6.20.12
We examine bicycle crank arm length and the way it influences gear ratio. Should your chainring and cassette sizes change along with your crank length? Let’s find out. 12.20.13
There have been real studies performed by real scientists on the subject of seat angles. 6.20.02
Hillary Biscay made a bit of news last week with her blog comments. She broke the story, unintentionally, that she, Chrissie Wellington, and Belinda Granger are leaving Team TBB. We asked her to amplify on her comments. 10.31.08
This article exists as part of a series of eleven on tri bike fit, and describes the proper armrest drop for a triathlete; how to measure it; how to determine it. 9.13.07
Crank arm lenght
Reviewed by: Nic, Feb 24 2011 6:32AM
my seat 5mm, but it would not need much force to push the pedals instead of shorten the cranks or I'm wrong?
I asking myself(172cm) whether I had benefit when I would change from 172.5mm to 165mm crank arms.
Confusion after reading this article
Reviewed by: Mark, Jun 11 2009 8:35AM
Crank Arm Lenght
Reviewed by: OldBMXTri Guy, Apr 10 2009 10:14AM
I progressed with those crank arms until the day before a national race I broke them jumping through a rhythm section. When I purchased the proper length based on my height(175mm) which immediately I found my level of power and speed were gone. There is truth to arm length Vs power. But it is down to the individual. For a 40min TT I would go with power. For a 112mi ride I would go with consistency RPM rate.
Crank arm length
Reviewed by: Jack Finucan, Mar 31 2009 8:56AM
Reviewed by: Walt Axthelm, Mar 30 2009 3:03PM
This is some input from a old guy (76 yrs young) 5'10" but with a bad ankle so I trend to pedal with my foot pointing down. This gives a higher seat height for my inseam length.
I have used long cranks in mt bike, road, TT and cyclocross and as a result by average cadance is around 78-80. This works great for climbing and TT and have found that the longer dead spot at top and bottom because of the longer arms for over 2 years I have been using eliptical chain rings, not Biopace, which speed you stroke thru the dead spot. I have timed myself with 175 and 180s and the 180s work the best for me.
I hold a USCF 75-79 age group 20k TT record and several Senior Games Records.
This setup works for me with a old set of legs that do not like to spin and give me a little more mechanical advantage.