What is the effect of changing length of crank arms?

Question

There are some length options for the crank arms - like 170mm, 175mm.
What is the difference for the rider? In which way does it effect the commute?

Tried looking at Selecting the right crank length, didn't get a comprehensive answer.

Answer 1

If you keep the rest of the bike the same a shorter crank gives more ground clearance at the bottom of the stroke, and that's going to be the main thing most people notice. You're unlikely to notice a change in power output unless you're shorter than, say, 1.7m, which would put you firmly on the down-slope of the power curve (ie, the part where longer = less power). You may notice a change in comfort or pain levels due to the increased joint articulation from the longer cranks. IMO most people would be better off switching from 170mm cranks to 160mm or 165mm cranks.

At the high performance end the power output varies in interesting ways. Since power = torque x speed, and torque = force x radius, a longer crank means more torque but also more leg movement to produce it. And vice versa for shorter cranks. In practice most athletes have a fairly flat relationship between average power and crank length in the range 150-170mm, but it's also something that is little studied.

A study by Martin (2001) suggests that it doesn't really matter:

Power produced with the 145- and 170-mm cranks was significantly (P < 0.05) greater than that produced with the 120- and 220-mm cranks. The optimal pedaling rate decreased significantly with increasing crank length, from 136 rpm for the 120-mm cranks to 110 rpm for the 220-mm cranks. ... The optimal crank length was 20% of leg length or 41% of tibia length. ... Even though maximum cycling power was significantly affected by crank length, use of the standard 170-mm length cranks should not substantially compromise maximum power in most adults.

I found some thought experiments and this interesting summary/bibliography. (edit to add) Ian Sims at GreenSpeed has long had an interest but I can't access their site right now, so here's an A2B magazine post. It's worth noting that Ian has changed his position completely, based on evidence that he mentions in that article. Previously he was a fan of long cranks, now he's suggesting that short ones work better.

Edit by mattnz replacing Mσᶎ's link: PowerCranks have an interesting sales push with useful links and discussion. They also provide test results showing advantages of short cranks. Apart from being benificial to people with poor flexibilty and knee problems, short cranks provide major racing benefit from improved aerodynamic position. They also discuss why its a myth you need long cranks for climbing.

(moz again) My experience is that shorter than conventional cranks work better than I expected and reduced my knee pain somewhat when riding hard (long tours or fast commuting for 40+ km/day). I'm 1.8m tall and ride 155mm cranks because those are the shortest I could get without paying a price premium. My partner is 1.5m tall and prefers 145mm cranks. Going up to the 165mm cranks on my load bike feels like a significant jump, but I haven't tested them because I don't have access to a power meter.

Answer 2

I have a 39" inseam and put homemade 195mm cranks on my mountain bike and 205mm on my street bike. The increase in performance was exhilarating and the improved comfort indescribable. I had been monitoring performance with bike computer odometers on both bikes before making the change and observed about a 15% increase in all areas. Last week the crank on one bike broke and I returned to the old cranks. My computers returned to the old performance levels and I feel I'm riding a child's trike! I've heard these "crank length makes little difference" arguments many times over the years and have always found those making such claims to be people of normal stature who can't truly conceive of the issues confronted by us outside the norm. It's also possible that manufactures wish to maintain a "crank length makes no difference myth" to preserve minimal parts manufacturing and that race committees want to prevent tall people from having an advantage on the race course.

Answer 3

My experience may be unique but at least be aware. I recently went from 175 - 165 cranks on my mountain bike because I was getting wayyyy too many pedal strikes as the bottom bracket on this bike is low. Although it solved the pedal strike issue, it also created a fit issue I can’t seem to remedy. It feels like a completely different bike. Raising the saddle was of course a must, but everything else fell apart too. Crowded cockpit, sore triceps, poor handling & response. Like when you buy a new bike and have to get it “dialed in”, stem length, handlebar height, reach etc…. That’s my 2 cents. Maybe it’s unique to me but thought you may want to be aware and look out for that possibility.

Answer 4

It affects several factors:

The range of motion during the pedal-stroke

Time per revolution (as a result of the increased radius)

and the maximum torque a rider is able to put on the crank

The majority of riders only pay attention to range of motion. For taller people and those with longer legs, longer crank arms are necessary to generate a larger range of motion in the thigh/hip. This uses more muscles and feels less cramped. However, shorter riders might benefit (if they have the power) from longer crank arms for the sake of increased torque. However, torque is generally ignored since the increase is quite minimal (only heard of a few pros doing this). Most people will be fine in th 172.5mm range. If you are tall then I would say it is an absolute necessity to get 175mm crank arms (I myself am tall and feel cramped when riding anything shorter). Hope that helps.

Answer 5

One size does not fit all when it comes to crank length, and if you are under 5'9" or are on a TT bike, it is important to go shorter. This can increase muscle activation and firing rate.

Imagine 40% more power pulses and a longer duration power pulse for each power stroke and there are momentum aspects and efficiency aspects that need to be understood and addresses and words like torque and horsepower don't cover totally what's happening in the cycling pedal stroke.

Think of muscles as engines, the more of the muscle you can use, the less you damage or drain the others. Shorter cranks allow you to change to a broader cadence range from 90 to 125 RPM instead of say 80 to 90 RPM with 175mm cranks.

However, watts generated depend not just on crank length but rideability and position. So at the end of the day only real time ABBA type testing, varied skill sets, different events and time to adapt can tell much. But you have to know why and what happened. A peer reviewed study that doesn't take into account all these variables won't give viable real-time results.

I've found clearly that short cranks are faster. I don't believe the studies to be relevant, including the one by Jim Martin who I knew. I don't believe they are adequately controlled for all the variables like the power phase of the stroke, pedalling style and gearing changes. Other factors may be aerodynamics, frame geometry, standing and sitting rate and shifting rate. Everyone of these things I studied for 15 years for personal interest. I had crank sizes from 150mm to 190mm. I was a pro racer and outputs were high. I performed best with cranks between 150 to 155. I am 5x7".

For maximum efficiency I suggest you to slam your seat back and lower your bars and bicycle if possible. I've tested it on a velodrome and found that your gear ratios need to be 0.8 inches smaller (one front tooth) smaller for every 1mm shorter crankset to keep the same foot speed.

You need to increase foot speed also. Using a significantly small gear allows you to switch to a horizontal pedal stroke where you never push down. Pushing down on the pedals is for beginners and it's ridiculous. Pederson really is off on this. You pedal a Nike swoosh down and through horizontally. This is so important for you to grasp, probably the best advice you could ever have, "Nike swoosh" will engage more motors. Its stomping down but right into a clawing back to you with a flat shoe. This makes short cranks come to life, And moving seat back at least 1.5 inches.

Do not lift up the backside with the heel raised. Many newcomers think this faster. And don't point the heel and toe on the way up. Keep shoe horizontal all the time. Kick up and over and down and through. With shorter cranks, keep a high cadence, put a giant cassette like 13 x 28t on, shift a lot and grab gears and wind it out like a two stroke engine.

The key is far greater gluteal and posterior chain use. Lower position, with your butt back, and shorter cranks are key.

[Edit: Put the bit about knee problems back in because this guy really knows what he's talking about] Also shortening cranks isn't the solution to knee problems. A too wide/narrow Q-factor, cold knees (improper clothing), no warm-up before exercise, previous injuries, pushing only down on your pedal stroke, bad shoe and cleat position, sitting too far forward and curved sole shoes with toes pointing down are common causes to knee problems. If you bounce on your saddle at very high rpm this is evidence to poor pedalling technique. Improve by doing high cadence drills.

Answer 6

Hmm! None of this seems to answer PeteH Jul 2 '14 at 6:32 comment about changinging crank length for two different length legs? I have been experimenting over the past 8 years when I discovered I had a 1cm longer left Femur. I tried increasing the shim stack under my right foot to 10mm. I reduced this to 6mm after a bike fit whereby my seat was lowered to stop cramping that would build up in my right calf. I read up on Steve Hoggs Bike shim system https://www.stevehoggbikefitting.com/bikefit/2011/.../foot-correction-part-3-shimmin... I developed knee pain in my right knee on my training bike with crank lengths of 170mm. I did some further internet research on 'Crank Length' where there were mixed opinions on differing my crank lengths from right to left. I experimented with a left crank of 172.5mm and a right crank of 170mm on my race bike, I also kept the shims under my right foot. I had a 'Retul' Bike Fit and the angels at joints were within the parameters. All seemed well. I Experimented further, after being uncomfortable with an increase in size to my right calf. I changed the left crank on my training bike to 175mm keeping the right at 170mm this was an improvement in many ways at low intensities or on longer hills. I then reduced the shim stack under my right foot to 1x 3mm shim and moved it forward. I also dropped the seat hieght by 3mm. I developed pain in my right Hip so returned the shim stack to 2x 3mm shims which solved the pain. I also recently bought a MTB, now this has two 175mm cranks but I moved the left cleat back and the right shimmed cleat forward aswell as turning my seat slightly so the right seat bone sits me slightly angled and supports that side. I only do 10 miles to and from work on the MTB and that is fine. The result is, I am considering changing the crank length back to 172.5 on the right side of my race bike as I believe that my left leg gets tired with a bigger ark of rotation and my right leg ends up having to produce greater force with a shorter lever hence tiring my right leg and causing tighter right hamstrings (hence the pain). A conflict of changing effort between right and left occurs on tougher longer and more intense training efforts or races leads to bad form or technique. My conclusion is that my body had coped with the adjustment with the build up under the cleat and less force was demanded of a right leg that had already become accustomed to dealing with a greater work load (shorter lever more Torque/Force) and by shortening the crank (Lever) increased the Force required by the shorter lever. There are more variables such as glute strength and calf strength but ultimately the calf was having to deal with stablizing the pedal with more leverage created from the shims and that is enough extra for one leg to have to adapt too.