Crank Length, How do you decide?

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Crank Length, How do you decide?

Postby Tomcat65 » Wed Sep 27, 2017 8:06 am

All of the factors that come into play on crank length include; inseam measurement, height, leverage, cadence vs. crank length, knee pain, pedal to ground strikes, chainring size, overall gear ratios, etc.. These all come down to what feels right. My riding experiences have become more about analyzing fitment and different types of riding. Those experiences have shown me something I thought was interesting, and I wonder if you guys have noticed anything similar?

On my road and gravel bikes, I am much more aware of cadence and maintaining a steady run, as I get older, I wish more and more that I had a power meter so I could pace myself better, and work more on my endurance over longer rides. But this is where I noticed crank length has an effect on me. I ran a 175mm crankset on the gravel bike for a month. I thought it might help me with my climbing and power on take-off. But the effect on my cadence and overall endurance was disappointing. Maintaining a defined cadence was more difficult for me, and eventually knee pain became a problem, along with lesser, but noticeable pain in my right hip. I took a break from the gravel bike, and the pain went away. I put the 170mm cranks back on, same chainrings, and I've been fine since.

On the MTB's, it's a completely different story. Cadence on my Schwinn Ascension is only an issue when I'm on long climbs, and trying to stay out of the red. I ride my son's Mongoose Deception some, it has a 170mm crank. Similar gearing to my Ascension. The 170 cranks are less likely to cause any pains on the long climbs, but the short bursts of power on undulating trails with steep, short climbs are more difficult. I know you're probably thinking that I'm just not choosing the right gear, but all things equal, the 175 crank seems to have an edge when I need to deliver power on the trail, with no noticeable difference in pain or soreness after a ride. The longer cranks do have a certain feel of tall stair stepping that agrees with me, but probably indicates something different to other riders.

The heavy steel frame Mongoose Malus fatbike is another issue altogether. I've had 4 different crank arm set-ups on it now, and each one had an effect on me and my effort. Starting with the stock 170mm single chainring, steel arm crank, There was something strange about the way that crank felt. It was an uncomfortable, uncertain feel. I suspect that it was flex. The smallish crank arms were not very strong, and in a peak burst of power situation, they felt odd. I wonder now if it was twist from the pedal leverage, or just plain flex messing with my power points on the pedal stroke. Either way, I changed to a 170mm aluminum crank with thick castings and the weird "soft" feeling went away. Far more rigid, the crank gave me a more immediate, stiff feel when I delivered power. The bike became more responsive to my effort, and at times, would spin or slip slightly under hard effort. While that reads like loss of control, it was actually more manageable because it was now doing what I expected, much more predictable. Just for kicks, I had a 165mm aluminum crank laying around, and I slapped it on for an afternoon romp. It didn't last long. The small cranks and heavy bike, with my long legs were a complete mis-match. I could not find a gear low enough, and even in my lowest gear, I could not deliver power to my satisfaction in climbs, bursts of power, or even on flat road in take-off or sprint. That's when I went back to my experience with the Schwinn Ascension, and decided to try a 175mm crankset. The difference was brilliant. Power, endurance, predictability, and sensitivity on the fatbike were all improved immediately. So much so, that I was able to determine appropriate new chainring sizes and the result has been transforming. This bike went from kind of a pig, to very capable, in one simple mod. Of course this all further convinced me of my theory of different crank arm length for different types of riding.

Single speed, city riding road, and cruisers have been a mixed bag for me. The Huffy Cranbrook comes with 165mm cranks. I bought one while I was in Daytona back in April. I rode it on the streets and bike lanes, then on the beach. It performed ok for a cruiser, but I was never really satisfied with it for anything more than laid back, low speed cruising. Knowing this is what the bike was intended to do, I passed it off as not really being an issue. But as I schemed a new plan of attack for my next visit to the beach, I realized that the crank length could have been the whole issue. So, I bought another one when I got home, lol... I have not changed the crankset, but I have confirmed to myself, that the crank length is a serious disadvantage to this bike for me. While it might be perfect for other riders, it just doesn't give me the space I need to make power like I want. It's like pulling punches, or not punching through. Each pedal stroke feels limited, and I feel less effective when I'm trying to lay in a hard line of power. The Cranbrook is a joke among the elites, but I have fun with this bike - when I do get to ride it. It's very popular in the Tomcat's stable of bikes, and I rarely even see it.
On to the single speed/fixie/city road style of riding, I'm too old to start with the fixie thing, no brakes does not appeal to me in any way. But I get the appeal it has with younger, bike addicted riders. It's a challenge like no other, but I'd not even try it with less than a 170mm crankset. Skidding a fixie takes leverage, lean forward and stop that back tire with a back pedal, I'd want all of the leverage I could get, and that makes me lean toward a 175mm crank, but my city riding experience says no. Making and/or holding cadence on a fixie is difficult to say the least, and bouncing on the seat at a cadence above my comfort zone, would only be worse with a 175mm crank. That would make me wish more for a 165mm crank to minimize the impact of that bounce when going down hills at high speed. From fixies, to city riding a geared bike, the difference is night and day. I much prefer the option of coasting without pedaling, and at least 6 speeds (and I lean toward even more gear choices.) This goes directly back to cadence. I know that the difference in a 170mm and a 175mm crank is just one centimeter top to bottom of the pedal stroke, but being a woodworker, I've built enough staircases to know that one centimeter difference in height is huge. It's easy to trip a normal person on a staircase with one step just a quarter inch higher than the rest. One centimeter makes a huge difference in how fast you can climb different stairs, and the staircase runners I know, they all have favorite stairs to climb, because they've discovered a rise/run somewhere that their stride matches. The same applies to crank length, 170mm cranksets are easier for me to spin. Simply put, it's the most satisfying crank length for me when I need to pedal non-stop for any real distance, no matter what gear I'm in, or how fast I'm going. Gears and coasting just make it 100% more enjoyable to me.

My conclusion is that 170mm cranks are best for me on the road, while 175mm cranks suit me best off road. I am aware that everyone is different, and the vast majority of folks will just ride whatever the bike came with, but for you who have experimented with crank arm length:

What length crankset are you using?
How did you determine your optimal crank length?
Do you believe wheel size or other factors apply that I did not mention?
Are you more into cadence off road than I am?
Have you ever had a crank length cause you a problem with knee pain or other pain?
Do you also run different length cranks on different bikes, and why?
Tomcat65
 
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Re: Crank Length, How do you decide?

Postby dddd » Sun Oct 08, 2017 8:25 am

Over the years with riding different bikes with different crank lengths, I have come to prefer shorter crankarms on road and off-road.

The shorter cranks are always less sensitive to any saddle height adjustment compromises, and I seem to be able to ride hard earlier in the season when using shorter crankarms.
On longer rides with rest breaks thrown in, longer cranks often leave my knee joints hurting for the first few minutes after getting back on the bike. I've even had to let myself fall well behind the group of riders I was with in order to get my knees moving comfortably again, which can take a while.
I can be much worse with suspension bikes that either lack proper damping or which have a pivot location that favors suspension squat under pedaling forces and which forces the knee joints through a greater range of motion. And where I live shorter crankarms make it easier and safer to ride over rocks that nearly block the narrow trails.

I am usually always happy with 170mm crankarms but often have issues with 175mm arms it seems.
I have a 31" inseam and am in my fifties for what that's worth.
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Re: Crank Length, How do you decide?

Postby desertguns » Sat Oct 21, 2017 10:42 pm

I really hadn't thought about it for a while. Both the Deception & Dolomite have 175mm alum crank arms. But thats a 29er & the fatties are the same OD as a 29er so clearance isnt a big concern. I'd have trouble getting use to 175mm ground clearance on a 26" wheel I think. Pretty sure the Huffy Millenial stock steel are 165 but converted to 3 speed & always street riding in town so it doesn't bother me. I went too short on the Beast & went aluminum (think it was 155-160mm) but came with a smaller chain ring up from & raised the tooth count in back. Couldn't ride it hard anymore. The torque just wasn't there. But it seemed an easier cruise on flat desert & didn't have as much trouble with the pedals hitting the ground. I'm 6' but have mutant short 30" inseam so fitting a bike is always a customization requirement, or at least a good excuse to do it. Don't get me started on stems, bars & seats!
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