Crank arm length

7 messages
11/11/2011 at 13:26
Has anybody ridden with cranks of different lengths? I'm on 172.5 and wondering what difference a move to something shorter would make.
Since shortening cranks shortens my levers, I'm guessing that shorter cranks require more power to turn the crank arm by the same distance. On that basis, why aren't we all riding 190s?

Pain is just weakness leaving the body. http://www.hdcc.co.uk

11/11/2011 at 13:27
...apart from ground clearance and having sensible proportions on the frame...

Pain is just weakness leaving the body. http://www.hdcc.co.uk

11/11/2011 at 17:41
I have a range of crank lengths on my bikes from 175 on my full suspension MTB, and an old road bike which spends most of the time on my trainer, 172.5 on a hardtail and on my main road bike. I also have had 170 on previous road bikes.

On your question of power from the science I remeber from school force = mass x acceleration. The point here is there is more to it than big muscles and long levers, neither of which are any good if you can only turn the cranks at a cadence of 10rpm. At a personal level I know that my FTP (functional threshold power), what I can maintain for an hour or so is around 305 - 310 watts, nothing fantastic but not to bad for my age (46) and where I am at in my training. This was achieved at a cadence average of 95 rpm, if I try to muscle a harder gear at a lower cadence I can not reach this figure, likewise I spin out too much on an easier gear and lack the muscle co-ordination to keep a steady force on the pedals if I go the other way. People will come on and be either side of this position.

When it comes to crank arm length my preference is for 172.5 though I am 181cm tall, a lot of bikes my size would come with 175 as standard. I have thought about going lower as well. I personally feel shorter cranks enable you to spin easier and accelerate faster. This makes you far more consistent in dealing with varied terrain. When I used to time trial in my youth, sub hour 25 mile TT to my name, I could handle 175 on a dead pan flat course but would have difficulties keeping it turning freely if the course was undulating.

I think you need to ask the question what is the riding style which is going to bring the most success in a triathlon, not a road racing environment. We don't need to sprint to close gaps or put big force down into a loose surface such as on a gravel track on an MTB to keep momentum going, what we do need to do is keep the pressure off our legs to save them for the run and to be consistently paced throughout the bike discipline. Some people claim that they are better off churning a big gear at a slower cadence to maintain speed, personally I don't get this, please feel free to shoot me down all of you out there, we are an endurance sport and as such efficiency is key and fighting against a machine seems somewhat counterproductive. Look at track bikes and you will see the big stuff soon disapears once you get up to pursuit distances. Turning the cranks smoothly and quickly is a skill and like all skills this can be learnt through practice, it simply takes time for the muscles and nervous system to adapt to pace changes.

To conclude if you can get up to a decent average cadence and put out your best power over a variety of terrains on longer cranks then by all means go that way. If you can't do one of these things then the answer may well be to shorten your crank arm length, not go the other way.
11/11/2011 at 18:57
Gavin, my first bike had a Standard 53/42 with 175 crank, am now on a 50/39 with 170 and quite happy with the set up; pleased that after a bike fit with Russell this met with seal of approval.

This may be of interest:
http://bikedynamics.co.uk/FitGuidecranks.htm

It seemed like a good idea at the time!!

Go Team Timmy
11/11/2011 at 21:21
Zacnici that was a really interesting article to post, thanks.

I think it highlights a few points. Firstly I have come across a lot of formulas on crank arm length and often they result in very different results, this article supports this.

It also supports the notion of a bike fit to come to a good position. I would however question having turned up with your expensive bike how many fitters would have the bottle to say you need to change your chainset to the cost of a few hundred quid, I guess not many but I may be wrong, move saddle position yes, change stem length possibly, change cranks?

What I think is good are the indicators towards the end of the article. As triathletes many of us are not very good at learning from the individual sports which make up our challenge. I know the vast majority of my knowledge about cycling was all learnt at around the age of 16 when I became involved in a highly competitive club environment. The older riders at the time who ran things were pretty strict about things such as how to train, pedal technique, gearing, bike set up etc... Also riding with individuals who were that much better than you was a brilliant learning environment even if at times brutal. I am not saying things have not come on a bit in the last 30 years but I strongly believe the basics remain as true as ever.

Whilst I can see the value of bike fits etc.. Personaly I know within a few minutes of riding behind someone on a bike as to how good they are especially when they are not pushing hard, there is something about good 'form' which is obvious to the well versed naked eye. This is all about how they hold themselves and move between seated and standing positions. For a lot of things like crank length simply riding along with good riders and understanding how you go about the task compared to them would be a real eye opener for many and I would encorage individuals new to cycling to seek out such individuals.

There are a lot of very different set ups in the professional peleton which demonstrates one approach does not fit all and many would not match a full bike fit result but you can see the riders who have form a mile off, the best I think I have seen being Stephen Roche though I might be biased as he was from my era, David Millar being one of the best in the current crop in my opinion.

Thanks for the link a good resource.
12/11/2011 at 00:42
Thanks all. I've run 53/39 with 172.5mm cranks forever, via dabbling in a rotor Q ring (had to go when I changed frame, didn't have it long enough to decide whether there was a discernible advantage). I'm not tall at 172cm but my cadence is high - I'll average over 90 on most terrain while racing.

It would be lovely to be able to experiment with different length cranks, but I guess I will just have to hope I adapt if I change.

Pain is just weakness leaving the body. http://www.hdcc.co.uk

12/11/2011 at 01:47
I did a lot of research on this recently, and it pretty much boiled down to it doesn't matter what length you use.

Longer length = more leverage, but slower cadence as your foot has further to travel
Shorter length = less leverage, but higher cadence as your foot travels less

Overall power remains the same.

The factors that do matter are:
- your prefered riding style, ie do you spin, or do you mash?
- your body position - a shorter crank lets you position your seat higher, so more aero

Fastest loser 2010
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