170 vs 175 mm Cranks
 

What is the difference between 170 and 175mm cranks? My son (6'4") is the captain when we ride. The stock 170s seem OK for me, but I'm wondering if the extra 1/2" will benefit him.

What is your thoughts on the effect of that extra 5mm?

5mm is actually only .19 inches. I personally don't think it will make much if any difference. With the larger crank you will (may) feel a slightly faster foot speed. That's the only difference.

If you plan on some serious racing, certainly you would consider all of the bikes geometry and not just the length of the crank arms. I say forget about it.

He might well appreciate the difference, small as it seems to be, in part depending on how much of a
spinner he is. If cadences in the 85-105 range are typical then the 170 is easier to spin but will feel twiddly
to longer legs.
I am 10" shorter than your son, and took a flyer when I was his age with 175 cranks. Felt really
really weird and the experiment was ended about 2mo later. The 0.4" larger diameter circle was quite
noticeable. With his longer legs he might well have the opposite opinion, the guy I ride with is that
height and would not think of riding 170s. Our cadence is in the 70-100 range, usually above 80 a bit.

Maybe height/leg length is the issue here(I'm 6'3"). I have several bikes with both 170's and 175's on them and I can hardly tell the difference switching between the two. I have to actually look at the crank arms if I want to know which ones I'm on. I think if we were talking about going from 175's to 190's or 200's like Miguel Indurain used to use there would be a much more noticeable difference.

My bike have cranks of 165mm (tandem), 170mm (touring and folder), and 175mm (road). I switch between them frequently and hardly notice any difference at all. If I pay attention then I notice that my cadence is naturally a little higher on the 165s and a bit lower on the 175s - but it really doesn't matter since I just choose the gears accordingly. If cranklength mattered to most riders I'd expect that a wider range of choices would be more readily available.

The change between 170mm and 175mm is ~2%. Not a big deal to most recreational riders, the world to people who ride at the highest levels of competition. When I was riding on the National Team years ago, they tried to move from 170mm to 172.5mm, it killed me. Felt like I was dead on the bike. These days, I doubt it's a big deal to me, but back in the day....

I notice a big difference between 170 and 172.5 mm cranks. I'm most used to 170 mm cranks, but recently a couple of bikes I've bought have come stock with 172.5mm, including the front of the tandem. I didn't expect such a small difference to be noticeable. I've adjusted the seat height to get the same leg extension at the bottom of the pedal stroke with both crank lengths. However, with the 172.5 mm cranks, I feel like the seat is too low because my legs become too folded up at the top of the pedal stroke. So, I try putting the seat up slightly, but that makes the extension at the bottom of the pedal stroke too long, so I reset the seat height based on this, but then it again feels too low at the top of the pedal stroke. I've changed the 172.5 mm cranks I had on one of my singles to 170 mm, but I believe that changing the ones on the front of the tandem would not be so easy (they are 2008 FSA Gossamer cranks), so I have to live with my leg being bent more than I would like at the top of the pedal stroke.

I was amazed at how noticeable this 2.5mm difference is, but when I thought about it I realized that this must be doubled when thinking about how close the pedal is to the seat at the top of the revolution, when the seat height is set based on the distance to the bottom of the pedal stroke. Changing from a 170mm to a 172.5 mm cranks is therefore similar to moving your seat down by 5mm (the effect is not quite the same, but it is sort of similar), and to a 175mm would be like dropping it by 1 cm, which is something that most people would notice, I think.

From 170 to 172.5 the difference at the top (or bottom) of the pedal stroke is .1 inches(2.5mm), not .2 inches (5mm) measured from the center of the BB. The difference in the lengths of the crank doesn't cause it to double at the top and bottom! Your foot probably moves more that in your shoe! ;)

I, and most others I know, was taught to measure and adjust saddle height based on the heel placed on the pedal with the crank arm parallel to the seat tube - the saddle should just contact the seat of the rider (some pressure required in the case of a suspension seat post). If you use that method, the top of the stroke is, in fact, 5 mm higher with the 172.5 than it is with the 170.

And interestingly enough, folks do detect a 1mm difference in saddle height - I think TG mentioned that, and my stoker certainly notices when her seat post is off by 1mm. The post is calibrated in cm, and it's a matter of partially hiding one of the calibration lines by the fancy work at the top of the seat post.

I guess it depends on your size. I'm 6'1" and I regularly shift from 165mm on the fixed-gear bike I normally ride to 175mm on the tandem, and I hardly notice any difference. (Shorter cranks are a real advantage with fixed gears. They let me spin at very close to 60 kmh in 42x16 on 8% descents.)

If you currently use 170mm cranks and you switch to 175's, you must lower your saddle by 5mm so that your maximum leg extension remains the same (so that you don't develop knee problems). It's a mistake to leave your saddle in the same place with a crank length change.

However, with a saddle 5mm lower and a crank 5mm longer, your foot will be 10mm closer to your saddle at the top of the stroke. Being 6'1", this hasn't bothered me; in fact, I hardly notice it. If you are short, it might be noticeable.

Where I find crank lengths useful is when riding with a less-experienced cyclist who is stoking. Most are not used to the high rpm's of experienced cyclists. By putting 175's on the front and 165's on the back, for a given crank rpm, the longer cranks require a higher foot speed (as the shorter cranks require less pure foot speed), so it helps deal with the perceptions of how fast the foot is moving when spinning and possibly allows the stoker to handle a sightly higher pedal rpm than she's used to.

L.

I am definitely one of the people that are quite sensitive to crank arm length as well particularly on the long side...I've got a 31" insean and prefer a 172.5 length...at 175 and with the seat adjusted appropirtely I am totally uncomfortable and feel it negatively in one of my hips as the mileage goes up. I have 170's on two singles and can ride those with no real problem but prefer the 172.5's particularly on our Tandems.

Bill J.

It is my recollection that Andy Pruitt's Complete Medical Guide for Cyclists has a discussion as to why you would want to go with a particular crank arm length.

If not there, then it was one of the other cycling books I have gone through.

As I recall it has something to do with the length of your femur - not necessarily your total height. When you change the crank length I think you change the seat height, as was mentioned above, but I think you also change the seat setback (where your butt is in relationship to the bottom bracket fore/aft) as well.

If my memory is correct, I believe that once you have your butt in the right place relative to the handlebars and bottom bracket you select the proper size crank arm to go with that combination. (or maybe the reverse of that - I can not remember)

I am not certain if I have this correct. A quick discussion with a bike builder will get you a better answer.

The center of your circle is the bottom bracket. This is simple geometry, at the top of the stroke the pedal is only 2.5mm higher on a 172.5mm crank arm than with a 170mm crank arm. Think radius! At the bottom it is 2.5mm lower, at 90deg. it's 2.5mm in front or back. Your feet are 5mm further apart from each other but not 5mm higher, ie the diameter of your circle.

Like I said before, you could loose 2.5mm (.1inch) in your shoes or your shorts. Heck, your chamois is thicker than that!

But hey, if you can feel 1mm who am I to argue that. ;)

If you have a 28" inseam you can feel differences in crank length. If you keep your maximum leg extension the same, the angle of your leg at the top of the pedal stroke changes...a LOT. If you are taller it would make a much smaller difference in leg angles at the top of the stroke.

When riding the Gold Rush with YOU I remember changing my shorts in Susanville on the way back and thinking that there was a difference in my seat height.:lol: (Not all Assos are created equal.)

Andy Pruitt (the guy who wrote the book) stresses that there is a "fit range" rather than a precise fit point. I think you are on target indicating that 2.5mm or possibly a 5.0mm difference is within that range and inconsequential. If I remember correctly, when discussing this with Steve Rex, the guy who built my custom tandem, he asked me what crank length I was used to, rather than suggesting one to me. I don't think it is any sort of hard rule (femur length of X indicates a crank arm length of Y), but I do think that generally people with longer legs prefer longer crank arms.

I also think that no matter the crank length it is important to have your seat in the correct spot relative to that length. Not everyone is as tough as you and can just keep powering it out in spite of the pain.;) How many people do you know that can hit a van at 40+ MPH after they have cut them off and slammed on the brakes in front of them and then go on to race another 2500 miles?

As Homeyba knows, what is a funny feeling after a century can be excruciating pain after a longer brevet.

I also seem to remember reading that you could even out the cadence or power between a captain and stoker by varying the crank length. A powerful rider or masher gets the longer one with the spinner or weaker rider getting a shorter one.

Your results may vary.

Your chamois was was wayyyy thicker than .1inch! ;) Lesson learned, never change your shorts on a 1200k! ;) That's why I do RAAM with only one pair of shorts too...;)

That's just a genetic defect. Maybe I'm just numb to all this fit stuff... :)


I have wondered about the effectiveness of doing that. That thread had me thinking. Switching from 170's to 175's results in only a 5+% difference in foot speed. I would guess the foot speed difference might be noticeable, definitely more noticeable than pedal height difference (IMHO). I would guess for it to have a significant impact you'll need to have a greater than 10+mm difference. That's just doing the math. I haven't done that with a stoker so I don't know how it would work.

Aint that the truth, like I said, if someone can feel 1mm who am I to argue that. ;)

+1 Have you ever seen little kids riding in the back of a tandem without crank shorteners? Ouch!!