kmulder

Greetings all,

I've been having a debate with my friend and we haven't been able to agree. Also, with a search, I wasn't able to find an answer to the question...(almost all results showed people asking the right length for their height)

Me: Longer cranks are better for faster acceleration and climbing. Since the arm is longer, you have more torque. However, at speed, longer cranks require faster pedaling, which would be less beneficial for TT, etc.

Friend: Shorter cranks are better for faster acceleration and climbing. Since the arm is shorter, you spend less time "turning over." At faster speeds, longer arms are better because the additional torque helps produce more power.

I KNOW at any given speed longer cranks = faster foot speed. We both agree on this. But we can't agree on what benefit/consequence this has.

Thanks!

magimum

well i think what is most important is if the cranks are the right size for you. in a physics point of view, more torque means more acceleration and smaller cranks are better for top speeds because the tangental speed will be smaller than that of a longer crank. in other words i think you are right.

Falchoon

On the same topic but a slightly different question; is it true that on TT/tri bikes you are supposed to run longer cranks - if so why? So if I run 172.5mm cranks on my normal road bike should I put 175mm on my TT bike?

fazzman

https://www.sheldonbrown.com/cranks.html Some good info

jrobe

"So if I run 172.5mm cranks on my normal road bike should I put 175mm on my TT bike? "


Take a ruler and measure 2.5mm. What difference could that possibly make? I could see how someone could tell a difference between 165 and 175 cranks but the smaller increments seems almost irrelevant.

TrippleB

you need to use cranks that fit your body geometry.

2.5mm on the crank arm changes your pedaling circle by over 3%

by giving up some leverage you can 1) significantly reduce the travel distance of your knees and 2) open up your hip angle which will not only improve efficiency buy allowing you to use more of your quad and glute muscles, but will also reduce lower back pain and open up your diaphram for better breathing while staying in a more aero position. sometimes all these things can be improved by a simple 2 degree shift in angle which is more than provided by this 3% change.

yeah i'd 2.5mm can make a massive difference

given my body geometry, i can pedal at a smoother, higher cadence with shorter cranks. this allows me to adjust to speed changes more fluidly (i.e. accelerate), thereby climb faster, ride in the wind faster, and maintain more consistant pack position.

kensuf

Just curious to see if you've tried? I definitely notice a difference between 170 and 172.5 cranks.

clausen

I'm the opposite. I prefer the 172.5 over 170. I can already spin fairly well keeping 95-100 on flat terrain, 105 if in a pack and it gives a little more leverage on hills with lower 80-85 cadence where I need it.

carpediemracing 

If you search for me and "175" and/or "170", you'll find that I regularly have a crank length debate with myself.

longer cranks = more leverage. I find them extremely helpful on short power climbs, 100-400 meters, 6-10% or so. Usually I need to turn some decent wattage to climb them, 500-900 watts. Standing on long cranks helps, and I can stay seated more and just grind.
= more muscle groups/fibers. you spread the load more.
= lower cadence. kills you in a gear limited situation (juniors, track, descents).
= better acceleration from slower speeds due to leverage.
My useable rpm range is 70-120 rpm, prefer 85-95.

shorter cranks = higher cadence. I find I can spin higher easily, using lower gears. Helps place load on aerobic system, saving muscular load for stuff like sprinting.
= less leverage. I found myself standing more jumping out of turns, because I couldn't accelerate as well as I did with longer cranks.
= more cornering clearance. I feel I can pedal deeper/later into a turn, or accelerate earlier. I don't do the out of saddle, push bike around, pedal in turn thing because I feel less smooth and less predictable.
= better acceleration at top end due to higher rpm window.
Range 90-120 rpm, prefer 105-110.

To me the short cranks are like low displacement, high rpm engines. Peaky, smaller useable rev range, easier to push over the edge, but properly utilized, better. That means if I'm fit, shift properly to maintain a small cadence window, and don't blow up (which drops me out of useable rev range), they're better. ultimate top speed higher.

Long cranks are like a big displacement, lower revving engine. Wider, lower usable rev range, broad torque curve. Geared right, you can go just as fast in general, but you will tax your muscles more than with short arms. Don't have to stand as much. Can't go as fast when descending or gear limited.

Keep in mind that BMXers usually use 180s, and they only have one gear and they hit prob 25-30 mph. It's possible to spin long cranks for short distances.

I've bounced between 170 and 175 for a few years. 2003-2007 175, 2008 170, 2009 175, Aug 2009 170. After the 170 change I had one bad race (first ride on 170s after the swap), did well for first time on track the following day, then had a good race the next week until I crashed with 1/2 lap to go (first time I made it to the last lap in 2009).

In 2010 I'll be using 175s until after Bethel (150m finishing hill, 53x17 min 53x12 max during race, 30 mph sprint). I'll move immediately to 170s for the rest of the season, emphasis on track (I have 170s on my track bike) and flat crits.

Heh this means I ought to swap crankarms to 175 as soon as I can.

cdr

OVERthetop

I have ridden 175 mm cranks since 1883. I'm 6' 3" with 35 inch inseam.

This season I purchased a new bike (Trek Madone & Sram Red) and unfortunately had to temporarily use 172.5 mm cranks. The bike shop was out of stock in 175mm so he let me borrow until 175 mm cranks came in. After making a slight change to account for the 172.5 cranks with the saddle height I began to use them.

Climbing: I enjoy climbing. I rode 172.5 mm cranks and did not enjoy the lack of torque. Every time my cadence fell below 80 rpm I could really feel the lack of power on the hills. Standing is how I prefer to climb and most of the time I range from 80 to 60 rpm.

Even when I was spinning 80 to 100 rpm in or out of the saddle it was clear that I could make more power in all the rpm range with longer cranks when climbing. I could remain in the saddle and I could not use the rpm effectively. I would watch my rpm suffer when climbing. I had to work harder to go up the same hills. My heart rate would be higher and speed was lower. I still could be OVER the top of the hill before the same cyclist friends but it took more effort to do so.

Sprinting: I was also not surprised to experience the lack of torque with the shorter cranks when sprinting. I could still beat the same cyclist friends in a sprint, but could really tell that I was never gaining speed as quickly. The top speed was down. I could spin faster but could not get as much MPH.

Throughout the the entire rpm range from the initial jump at 95 rpm to 135 rpm was too limited.
By the time I got up to spinning where I should be making most power it felt as if I was in the wrong gear the entire time.
Every up shift was too slow initially and with not enough movement for my legs to get really reaching the way they made most power. My legs were still going around, but it was like I was forced to take too little steps to get there.

Paceline: Paceline work was still disappointing. The pulls were shorter and slower. It felt like I was in a crosswind.

I finally did get the 175mm cranks installed. What a contrast. I slightly lowered my saddle. I smiled on the first ride. I returned to the shop and I told the bike shop owner that the 172.5mm cranks were "Too tall, too tippy and too tiny". The 175mm cranks clearly allowed for me to make more power for my leg length.

challaday

Wow, and I thought I was old. I'm already in the geezer generation according to my kids.

TrippleB

that's why i said that choosing crank size is 100% all about the fit to your body

kmulder

Wow, thanks all for the answers! I've had another question pop up while reading them.

Several have mentioned that shorter cranks = higher cadence.

Am I correct in assuming that this means it is easier to maintain a higher cadence than with long arms? Because at any given speed, shorter cranks = higher cadence. So is a higher cadence with shorter arms better than a lower cadence with longer arms?

kensuf

I just wanted to say that it's great to see you talking about gearing up for next season already!

shuffles

Uh, I think you need to check your math. The circumference of a circle of radius 172.5mm (1083mm) is less than 1.45% smaller than that of a radius of 175mm (1099mm).

175s would give you marginally more leverage when going uphill, while making it marginally more difficult to spin at high speed.

165 to 175 might be noticeable, but I sincerely doubt if 2.5 mm would be noticed by anyone.

kensuf

I've switched back and forth from 170mm to 172.5mm cranks and I notice it. Seriously.

LesterOfPuppets

I used to ride 175s on my MTBs all the time. But in 1996 I got one with 170s. Always had 170s on the road bikes, so that's ALL I rode 'til a few months ago.

A few months ago got 175s on one of my MTBs and thought, "this sucks trying to spin these big circles" But then tried to climb a STEEP hill that I always try but fail to top with 170s and conquered it no problem.

I JUST got 172.5 for the road bike and it's the best thing I've ever done! I'm just not smooth enough on 170s. 172.5 seems just right for my body and my cadence.

shuffles

Maybe, but 2.5 mm is less than 1/10 of an inch, so it may be a placebo effect

kensuf

No, it's not.

Seriously.

I typically ride 12-15 hours a week, roughly 11,000 miles a year, and I kind of know how my bike feels and handles and every little nuance and niggle.

With 170mm cranks I can spin faster, but don't have as much force/leverage. And by "spin faster" I mean my cadence will run in the 110-115 range without my thinking about it. With 172.5mm cranks my cadence typically runs 100-105, but I have more leverage and notice it in sprinting and climbing.

FLBandit

I've ridden 175 and 170 and I like the 175 better. Of course, I'm a big heavy guy and I like the extra torque.

timeedgevxr

Here's an option ...if you're in the market for cranks, get the Stronglight cranks that are adjustable. I swapped my Campy Record std crankset out for a Stronglight compact wjen I moved to the mountains. I wa sblown away - these cranks are awesome. They have an oblong insert that the pedal bolts to. Set one way they are 170's - turn them around and they're 175's. They also come with a 172.5 insert. I've swapped them around a few times and I love the 175 set up (I'm a masher).

If you are really sweating this and you still want great cranks, check out the Stronglights

Howzit

- 170mm - For sprinting
- 172.5mm - For normal multi-purpose road racing
- 175mm - For Time Trials and Climbing.
- 172.5mm - For climbing if you are a natural climber

Leg length doesnt matter

Just ride 172.5mm you should be fine. You are not a Pro.

there you have it.

carpediemracing 

heh I can't wait to get back on the bike It's sitting right there about 8 feet away from me. 170s still on it though. I need two working arms to get the cranks off. Or at least I'll wait until I have two working arms.

cdr