Bicycle Mechanics - Crank arm length/ proper size

I am a 5ft 11 in, middle of the pack, age group triathlete who is getting frustrated with the deterioration of my speed as I age ( 48 ). My local bike shop suggested 180mm crank arms instead of the 175 I have now. He claims it will greatly increase hill climbing with no extra effort.

Because I don't have good spin technique, he said it will increase speed without bothering my pedal stroke. Anyone ever try this? Are there any bad effects to be careful of?

Thanks
tryhed

I recently switched to a 5mm longer crankset, I found that it does feel like I climb/accelerate faster, but I cant back that up with proof, just seat of the pants observation. The longer crank arms mean more leverage, so it should help. It didnt seem to affect my pedal stroke either. If anything it should make it smoother, I would think. :beer:

What a load.You still have to be able to turn them,and 180 is likely too long for you under ny circumstances. I'm about 6' and 175 is the longest I would use

175 is considered a long crank for someone of your size. 180 is extra long.
If you think of crank length as part of the total gearing from foot to tyre, then longer cranks give lower gears. By turning your foot through a larger circumference you will be able to turn a higher (cog-to-cog)gear with a lower cadence for the same power. You still use the same power, longer cranks dont magick power out of thin air.

Have you tried just running a lower gear with your current cranks. Many race bikes are quite highly geared, and if you just dont have the power to turn your gears, just get a larger cog or smaller chainwheel. Try the cheap solution before going for the expensive one.

Long cranks will cause you to flex your knee and hip joint through a greater range, which may be biomechanically inefficient . (Think of it as increasing your stride length in running).

Long cranks may cause you to ground your pedals on corners. Big size frames should be designed for long cranks, with a higher bottom bracket to prevent grounding.

At 5'8" with a 30" trouser inseam, I am very content with standard 170mm cranks. For someone of your height, 175mm cranks should be long enough. If you have any knee problems whatsoever, remember that longer cranks will take your knees through a greater range of loaded motion. I would prefer using shorter cranks and a proportionately lower gear, but I admittedly tend to be a spinner. You may also want to read Sheldon Brown's article on crank length.

This is a very interesting subject. :(

Of course, a longer crank has a longer advantage, if you can push it. BUT does that advantage actually dissappear into the modern multigear ratios. So If you have a longer crank, you naturally shift to a lower gear to compensate. etc. etc.

And doesn't the crank length have a closer relationship to thigh length than to leg length or body height? It seems that if you look at the thigh as a "power" lever, the shin/foot as a connecting rod, then the crank is the "powered lever".

So, doesn't crank length actually belong only in the "setup" of body mechanics and not into the mechanical advantages on the bike?

The IHPVA Trike list recently went through this discussion. Still getting a spattering of
short crank entries. That group is looking at 145mm cranks!! The comment about knees
came up often.....shorter cranks are better for knees. You just need the gearing to support
the short cranks.

There should be nothing unusual about 145mm cranks, if your legs (or more correctly as oldroadie says, your thighs) are smaller (about 85%) than the average 5'10 male . Thorn brand cranks from SJScycles are available down to 140mm.
It seems only sensible to set your crank size (pedalling circle) to match your leg size, then set your gearing to match that crank size. This would also overcome toe-clip overlap and excessive bb height on smaller size bikes.
If its so sensible, why do no big manufacturers do it?

Even much quoted and regarded fitting guides like Colorado Cyclist suggest 170 as a good size for everyone of average or smaller size.

I believe Rivendell has many different sized cranks
if you're looking for 145's.
Tryhed, What I'd do is get rollers and work on leg speed
and spin in the off season. Easier on the knees.

Marty

Is crank length measured centre-to-centre from bb to pedal? Or the length of the arm which we know to be longer. I've never honestly bothered to measure one, and I'd like to know.

I'm a born-again spinner, so I'd advise practicing it. It seems like a lot of people (myself in particular) instinctively push, rather than spin; but I forced myself to learn, and my knees thank me for it. BTW, I'm 6'5" and the longest cranks on any of my bikes is 175mm.

well i'm 6'1" and have 170s on my road bike, 175 on my hard tail and 180s on my new Full Suspension --- i didn't really notice this when i bought each one -- just kind of ended up that way.

i do really like the 180s on my newest bike and feel really good with them. i've also been working on spinning more and had good luck doing this, even on big 15-25% climbs (with the low gears on my 9sp X 3 MTB with a 34 as my largest rear cog)-- my only problem has been a little pedal scrape in corners...

i think from a force perspective, from your current setup either using a lower gear and longer cranks should increase your force potential but increase the distance through which you have to move it (more reps with the existing cranks, or more foot motion from longer cranks)...

i've never ridden longer cranks on a road bike or in a long term constant spinning high-speed scenario (i do mostly big climbs on my MTB).

since i like the longer cranks so much on my MTB i've been thinking about getting longer cranks for the road bike (i was thinking upgrading the 170s to 175s), but i've been too lazy and i've spent very little time on the road bike this year anyway, almost always riding off-road.

seems like to me it would most depend on body measurements (leg, etc) and just personal preferences. i would say that since you don't seem too happy with your current setup, why not try a change? buying new cranks shouldn't cost you too much...

I am 5' 11" and i just got a track crank set that is 165mm. From reading this discussion i understand the effects of shorter cranks, but i was wondering if there would be any problems with this size for me?

They are really nice cranks, so i would like to keep them, they are for a new build i am about to start. This wouldn't effect the size of the new frame i plan on purchasing would it?

thanks

There are no absolutes with when choosing cranks. The number one myth I hear is how long cranks will damage your knees. This is a fallacy.

It may hurt the knees of those it hurts, but for most people it doesn't. We usually hear from those it hurts, not from the many who use them without issue.

Going from 170 or 175 to 185 actually helped my knees. It should be noted, I don't push big gears, and spin anywhere from 60-120 rpm, depending if I'm going up or downhill. I have a 35.5" inseam, 40" femur and size 15 feet.

The size of ones foot and position on the pedal also needs to be considered when choosing cranks. There is no "formula" that adds that into the equation..... but it's a part of your human lever and cannot be overlooked.

The bottom line is you have to know your own body and what works for you..... it doesn't matter what anyone else uses.

Usherenko ....... the only way to find out is to ride them cranks. Just remember to adjust your seat accordingly compared to what you have used. From a 170 down to 165, raise the seat 5mm... etc. This will also effect your bar height, so adjust according there too. If it's a new build, leave plenty of room on your fork until you figure it out. Even then, always leave yourself a few cm's if you can. If it's a threaded fork, do the same, and add a cm or two of spacers into the headset.

Since you said you race triathlons, I recommend reading this... http://www.slowtwitch.com/Tech/Crank_arm_lengths_for_tri_727.html

Recent/current thread in Road Cycling:
http://www.bikeforums.net/showthread.php?t=547683

I'm a fan of longer arms (I ride 175s, sub-30" inseam, 5'7", strength=sprinting, could never climb/tt) but I'm pondering if this preference works for me because of my riding style.

cdr

I have a 35.5" inseam, 40" femur...

Typo much? Curious to know your actual stats...this is an interesting discussion.

Typo much? Curious to know your actual stats...this is an interesting discussion.

Yes .... a typo .... 40" referring to floor-to-top of femur !

You might find this link interesting:
http://www.nettally.com/palmk/crankset.html
It says crank length should be proportional to inseam, specifically CrankLength(mm) = 5.48 x Inseam(Inches).
If your inseam is in the same proportion to your height as mine you should have a 179mm crank.
I'm not sure I subscribe to this idea, although I have just ordered 177.5mm cranks on my new Sram group as a step towards the 184mm the formula suggests. Have to be careful not to ground the inside pedal on corners I guess.

Longer crank arms will give slightly higher mechanical advantage, but they also increase the likelihood of pedal strike when cornering and are harder on your knees when spinning.

It's your call whether the benefits are justifiable. Can you borrow a set of 180s to try before committing to the purchase?

Like Garthr, I use long cranks. At 6'4" with a 36' inseam, the 'formula' for crank length shows I need a 196mm crank. I chose to 'err on the side of caution' and got a set of 190mm Zinn cranks. They worked great, except for the cornering clearance! After almost going down after grounding a pedal I decided to put them on my MTB with its higher BB, and got some 180mm XTR cranks for my road bike. They have worked out great, and give me more versatility in choosing chainring sizes. The 190's are good for the MTB as well. I do not know where the "longer cranks will hurt your knees" myth started (I suspect some dealer with an overstock of short cranks, but have no proof), but in my experience, the opposite is true. I am able to maintain the same speed/effort with longer cranks and LESS knee strain. I think it is an individual thing. If it works for you, then use it. If it does not, don't. But please do NOT try to convince other people that you know how it will work for THEM! I have a riding buddy who is 6'8" and got rid of 180's in favor of 175's. Now that I think about it though, he has been complaining of knee pain.

I have recently switched to 180mm cranks on my mtb and I like them... but I have not done any ITTs with different cranks as control subjects. My theory was that longer cranks will help with the abrupt surges of torque necessary for clearing obstacles on the trail... plus I am 6'4" with ~36.5" inseam, so it makes a lot of sense.

However, I am still perfectly comfortable on the 170mm cranks on my road bike and touring bike and the 175mm cranks on my hybrid.

My conclusion: use whatever makes you comfortable. In general, use longer cranks if you are tall and/or you like to push big gears, use short cranks if you are shorter and/or you like to spin. This is what I used to tell customers when I worked at a shop that sold custom assembled bikes... people were given a choice of crankarm length and had no idea how to choose.

I have had 175mm on all but my current Sirrus, it has 170mm. I feel a slight difference in that, but I dont think it does much worse, IMO.

Now, if you were riding a single speed, then yes you want a bit longer, for the mechanical advantage. Geared bike? probably will not notice that much :)

It would sure be nice if there was a larger selection of crankset manufacturers who made them in a wide range, like TA does with their Carmina and Vega.(from 155mm to 185mm).
TA's quality control is lacking though, for the headache I've had getting a straight spider on a Carmina, I'd say they're much overpriced.

DaVinci Designs makes nice cranks in 150mm to 180mm though if you can use them.