Craker

What is the significant difference/relationship between the length of a crank arm and the type of ride one would experience? And, is there a simple way to determine what BB spindle length a particular crankset would require?

I'm trawling ebay for a vintage crank for my singlespeed/fixed gear conversion.

RaleighSport

In my flawed understanding, crank length has more to do with leg length than anything, a second property of concern is pedal strike/toe overlap, so if I had short short short legs, I might want a 160 or 165 crankset, same deal for if I had a very low bottom bracket on a fixed gear and wanted to minimize pedal strike in turns (since you can't coast the pedals in a fixed position on a fixie when turning). My understanding also is 170-175 is fairly average crank lengths. Someone will be along shortly to tell you how wrong I am though and give a better answer I am sure.

Ex Pres 

I'm no expert, but I think not just leg length but femur length more specifically.
I probably wouldn't go over 170 unless I had a high BB height for the pedal strike reason.
Spindle length - no magic formula. There are reference books to be consulted, and you can always ask here. One of the variables will be - if you're using a double crankset for your conversion, are you planning on using the inner or the outer landings? That's a difference of 5mm chainline right there.

gioscinelli 

Also, shorter length more spin, longer length more power to the pedals. Pros usually go with a little longer length for both spin and power cycling.

repechage

There is a more recent favor of longer cranks than what was convention 30-40 years ago for a given rider, in general. But there are many variables as already suggested.

For a fixed gear conversion, a road frame to fixed, I would go 165mm as the bottom bracket will be lower than from a track frame, say, 70 mm of drop being very typical.

For a fixed gear conversion, it will be all about chainline. Probably driven by the rear wheel.

RaleighSport

That gives me an interesting though, a mtb to 700c fixed gear conversion.. assuming you used a smaller frame size with 160 or 165 cranks OEM, you could slap on 700c wheels and possibly get away with 175 cranks and not have much concern about pedal strike..

As for your last point, assuming he uses a true fixed gear wheelset chainline is almost a non issue and will be more dependant on the cranks he chooses than the wheel but you're right, it's all about the chainline.

lostarchitect 

Peter White suggests getting as close to 18.5% of your leg length as you can (see link). I measured and came pretty close to 165mm, which is the length I prefer, so the formula makes some sense to me. Of course, if you have super short or super long legs, it'll be hard to find cranks in the length you want.

cyclotoine

there is no magic bullet. Try different lengths to see what you like. Chances are if you only ever ride 170 or 175 you won't notice much difference. 165 and 185 you will notice something. I have ridden 165 to 185... I like 180 but also road 175 for most of my life without ever thinking about it so I am habituated and like the extra leverage I get with 180 but find 185 is a little too much to turn over on the road bike for long distances. Again that may be just me not being well adapted to that length and if I road 185 all the time I might find that I prefer it to 180... but 185s are rare and expensive and 180s are relatively plentiful by comparison. I would say if you are average height you should go for 165 or 170

Craker

Why is chainline less of an issue if I'm using a true fixed gear wheelset? Is there an proper order to buy components in, out of curiosity? Should I get the crank first and then purchase wheels to fit etc? Thanks for all the info everyone.

Savagewolf

I view the length of the crank as mainly tied in to the size of the bike and the rider. The taller the rider and bigger the frame, then the longer the crank arm needs to be to stay in proportion.

You can also use it to vary your RPM. Using a smaller length for the arm means that there is less distance that you need to pedal to make a full revolution. As mentioned, a longer crank will allow you to put more power into each stroke as it has more leverage.

Another way to think of cranks is to compare an ordinary ceiling fan to a giant wind turbine. The fan seems to go really fast, but it also has a short distance to travel to make a full circle. A wind turbine can seem to be going slow, but it can be actually going much faster (and with more force) than the ceiling fan is. It just looks slower and takes longer to make a full circle.

RaleighSport

Assuming you know the OLD you need for your hub and buy an appropriate wheel, the dishing of the wheel and the axle spacing should be such that the cog will be placed close enough to the drop outs that you will hopefully not need any crazy solutions (IE going SS and getting a chain tensioner or something along those lines). Whereas say if you were to suicide hub a road bike wheelset (freewheel type of course), a 5 speed might have the cog outboard enough without redishing the wheel or respacing the axle, worst case scenario for that would be something like a 7 speed freewheeel compliant wheel, without the spacers being adjusted or the wheel redished.. you could literally wind up with your cog 3 inches from the drop outs, now picture the bend the chain would be making from the crankset to the cog.. you'd be lucky to keep the chain on hitting a light bump like that.

Example of a good chainline:

The fixed gear cog would be sitting about where the lowest gear on this cassette would be, to give example of the bend.

And please do not try a suicide hub.

Cache

Good crank arm length info here: https://zinncycles.com/Zinn/index.php/custom-cranks

mongol777

If you are going fixed - invest in proper rear wheel first, with threading for cog and lockring.
Measure your OLD, buy appropriate wheel (many companies will re-space wheel for you like velomine or wabi cycles)
Typical chainline on older road bikes would be around 40 to 42 mm
Do you have your current crank? If yes - measure the chainline at the crank (https://www.sheldonbrown.com/chainline.html) and once crank is off - measure how long is your axle. Unless you will be buying track/fg specific crank - use the same BB for new crank. There are several ways to adjust chainline - by moving ring inside or outside, by using spacers between bb cup and bb shell, etc. Make friends with your LBS - good shop will help you with bb length and may even let you try several.

Before you do any of the above - spend 30 min and read this https://www.sheldonbrown.com/fixed-conversion.html You will understand what you need better and will likely save some money by not doing noob mistakes. And of course visit singlespeed subforum here (or on mtbr - great bunch of FGSS people there).

And if you really serious about FG and want the best experience - good chainring, cog and chain should be high on your "to buy" list. Sometimes (quite often actually) it is really long process with no guaranteed results to get even chain tension on regular road c-rings and cheap cogs.
FG is addictive so pick your poison in advance - 1/8 or 3/32 chain, cog and c-ring (will hekp you later to easily swap between bikes :-))

rhm

No, Zinn is all wrong about cranks. He suggests crank arms that are much too long. In my opinion most of us use cranks that are far too long, but he advocates much longer ones.

There are a few factors that matter, and leg length or femur length is really not among them. What's important is:

When riding a fixed gear, make sure your pedal can't hit the ground. If it does, you are leaving the ground and coming down sideways. When in doubt get shorter cranks.

Short cranks are advantageous at high cadences. Short cranks put more strain on the cardiovascular system and less on the legs and in particular less on the knees, except at very low cadence, when you might not have enough leverage to turn them over.

Long cranks are advantageous at low cadences. That's especially relevant on rough trails where you may have to power over rocks and things at walking speed. Long cranks put your knees through a wider range of motion which can be painful or even injurious, especially if your not used to it.

I don't recommend cranks over 170 mm for anyone except Zinn himself. I have 165s on most of my bikes because a vintage version of anything shorter than that is hard to find.

mongol777

Forgot to add - check if the crank you want is JIS or ISO (since you are going vintage I am assuming no ISIS, external bb, etc) and get appropriate BB for it. I have couple of bikes where I use JIS crank on ISO BB and vice versa but preferably match them if you can. Again - 99% of the info you will need is likely already here https://www.sheldonbrown.com

mongol777

Once you get a hand on fg riding - strikes are easy to avoid and shorter track pedals will help too. I use FG offroad as well and as it is much harder to avoid strikes on ice, stones, roots, etc - prefer shorter cranks and slimmer pedals. But then again - I rarely get any strikes now compared to when I just started riding fixed everywhere.
For OP I agree - 165-170 is a safe bet but 175 would work too

Salubrious

Seems to me the French had a rule of thumb that the crank arm should be about 20% of your total inseam. I stand 6 feet and my inseam is about 33 inches. that times 0.2 is 6.6 inches. Converted to mm is 167mm. Round down to the nearest size if you spin, round up if you mash.

Mashing is known to be bad for your knees FWIW.

The important thing here is that the entire rest of the bike is supposedly supposed to fit you- the seat height and angle, the length of the stem, the size of the frame. Why should we not also have the ideal crank arm length? I can make a 175 go around and I have 170s on most of my bikes, but I find that I make more power (spin faster) with a 165.

If you have knee pain you will do better with a shorter crank and a shorter crank means you are less likely to develop knee problems. My girlfriend does quite well with a 140 crank (which was hard to find); she has had knee surgery and often has knee pain. But with the short crank she gets no pain at all even after an all-day ride. She has no trouble keeping up too.

I think there is a sort of macho thing that says you should get longer crank arms. For this reason people tend to discount the shorter crank arm thing out of hand without giving it due consideration (much less giving it a try). Having gone to shorter crank arms after using long ones I'm not likely to go back- I can ride faster.

Paramount1973 

Interesting study here indicates that crank length is not that big a deal.
https://www.plan2peak.com/files/32_ar...gTechnique.pdf

DIMcyclist

Although it's been covered quite a bit more technically, both on this forum & elsewhere on the web, I've always felt a good way to think about crank arm length might be to go back to a common childhood experience, at least a common experience for those of us who grew up with snowy winters...

When you were a kid, did you ever try to literally walk in your dad's (or mom's) footsteps, ahead of you in the snow? Their gait was much longer than yours, I'll warrant; and you likely had to almost jump to put one of your feet into one of their footprints. If you can remember such an experience, can you remember how your body had to shift in order to do this? How far you had to stretch your legs & hips to make those steps?

That's essentially what you're doing when you use cranks that are too long for your leg length.

Some people might get quite butch & venture the equation: longer cranks = more power; in fact, too long a crank = hip displacement + eventual knee damage.

The truth is there's no substitute for training and it's best done with an optimal fit.

rhm

DIM, that's a good analogy but, I think, the wrong one. Pedaling a bike is more akin to climbing stairs. Both tall and short people, and even very small children, climb the same stairs. They are not standard, exactly, but pretty close. The important thing is what your legs are accustomed to.

noglider 

I'm amazed that we can feel a difference of 5mm or less. I was doubtful, but I found that I can.


I think rhm might be onto something. I have a pair of TA 155mm cranks I'm going to try soon. They were made for junior racers.


rhm, my question is, I kinda understand how short cranks help us spin, but given their lower leverage, wouldn't they also increase the amount of force going through he knee? Well, yes, of course they would, but the question is, what's the important factor? They change the angle that the knee is bent at when applying maximum force, and maybe that is more important than the amount of force.


I strongly suspect this has not been studied extensively enough. The trend of increasing crank lengths might prove to be a bad one. I've been happy with 167.5mm and 170mm cranks. I recently borrowed my sister in law's bike, a 2003 Specialized Sequoia. She and I are the same size, a 56cm frame. After riding taking a long ride on very big hills, I wondered if it had long cranks. They felt funny. Sure enough, they were 172.5. 2.5 seems tiny. That's a tenth of an inch. Yet I felt it. It wasn't uncomfortable, but is that really appropriate for someone with legs as short as mine?


I want cranks with adjustable lengths so I can do experiments. Rudi, any ideas for building those?


I never got comfortable on my mountain bike (which is now gone, stolen). I realized later that it might be because of the 175mm cranks, standard on MTB's. I was just riding it on roads, fwiw.

JReade

And I know they make crankset shortening devices, espeically for tandems and children. Might be too radical of a change though.

rhm

Yes, the lack of leverage with short cranks is terrible at low cadence. It is hard on your knees and it's hard on the bike and it's just no fun at all. I had 6" cranks on my folding bike a couple years ago and got caught riding home from the station in about 4" of heavy snow. The small wheels (20") didn't help, but my tires were fat enough that most of the time I could ride on the packed slush left by the cars. But I could not go fast enough to keep up a reasonable cadence when I hit the soft stuff. I had to hammer through it, and the short cranks were murder. That was essentially an off-road situation, though. This winter I've been riding 700 x 35c tires with 160mm cranks and it hasn't been a problem.

The important thing, I repeat, is cadence. Short cranks let you spin, and spinning is good. But to spin you have to be able to ride, and if your cranks are too short to ride reliably, then that's no good either.

If you want a rule of thumb, I'd look at it like this: the optimal crank length is the shortest one that will let you ride comfortably through and over the worst obstacles you're likely to encounter. Anything longer than that will impede your spinning at the top end, and therefore show you down when you reach your maximum cadence. On a single speed with a fixed gear, where your cadence may go well over 200rpm on a descent, this will make a difference.

Salubrious

There are problems with this study, causing it to lead to a logical fallacy known as a 'Hasty Generalization'. The lengths of crank arms are not precise enough!

The most important lever in the crank arm mechanism is your knee. That is where the leverage comes from, not so much the arm length. Try some squats and you will see what I mean. If you squat all the way to the floor, its a lot harder than it is if you only squat a few inches. This is because of the leverage in the knee.

dweenk

I think you are onto something with the stairway analogy. Way back in the 1970's, I was wiring high-rise buildings in a resort town. We couldn't afford to wait for the construction elevator, so we used the stairs to get to our work floors. None of us walked up the stairs - it was too tiring. We ran. It seemed to be much easier, even if we were carrying stuff. I think the motion of hopping off your plant foot made the reach to the next stair a bit shorter. Of course I could be totally wrong, but it worked for us.