Taller people running 165mm (or 170mm) cranks for trail riding?

[Damian Sanders]

Anybody 5'10 or above running 165mm cranks on a trail / AM bike that you pedal up hills? Thoughts on the subject? Looking to give a low bike better pedal clearance.

[Dantheman]

I am neither tall nor currently running short cranks. But, the info I found about using short cranks while searching for cranks for my son's bike has me thinking that the short crank idea has legs. As a 5'-6" semi-midget with a 29" inseam I am seriously considering trying out some 150ish mm cranks. Cost is the only reason I haven't tried it already.

[toast2266]

As someone that's not all that tall but has experimented with short-ish cranks, I notice the lost leverage when cranking up steep stuff. I'm aware of the slightly smaller circle when spinning, but it doesn't really bother me too much. I suppose whether the loss of leverage is worth it really comes down to how low the bike is and what kind of trails you're riding.

[evasive_MT]

I went to 170mm cranks a few years ago when I built a bike with a lower BB. I was aware of it for a couple weeks, but haven't given it a 2nd thought in years.

I'm 6'3" with short legs. I forgot my cycling inseam years ago, but I usually buy 34" jeans.

[shirk]

Studies have been done and there really is no loss in power in going to the shorter cranks.

If the bb is low and you get pedal strikes switch to 165's and don't worry about it.

[davep]

I always thought the issue with shorter cranks isn't losing average power, it's more about losing the amount of energy you can put into a single power-stroke for bursting over short techy things (wheelying up-over a ledge, for example) Short cranks are great on road bikes & DH bikes where there's more high-cadence spinning, but if you go TOO short you might be hindered on short power-burst moves.

Plenty of tall guys with bad knees run 170s on trail bikes so that's not too short. I haven't tried climbing on 165s.

[stuckathuntermtn]

Better for your knees, huh?
Hm....

[Damian Sanders]

I've read everything I can google about the subject. Seems to be pretty popular with the triathlon crowd, and studies show max power is not inhibited. But what about normal pedaling output?

The loss in leverage from 175 to 165 is like going from a 32t front ring to 34t. About a 6% change. So if you're going to a smaller crank and proportionally smaller sprocket, to maintain the same power output and bike speed you apply the same force to the pedals at the same pedal speed. This ends up being a higher cadence - say 95 rpm vs. 90 rpm. (torque decreases and rpm increases)

If you change just the crank arms and maintain the same cadence (and bike speed and power output), you have to apply roughly 6% more force to the pedals, while the pedal speed has now decreased by about 6%. (torque and rpm at the bottom bracket stay the same)

So the question becomes, are we as comfortable and powerful with a less leg extension, and a higher cadence or more pedal force at a slower pedal speed? From what I'm read, people are more comfortable with a smaller leg motion (a shorter pedal stroke), but can be less comfortable if they have to apply more force.

Another thing to consider is that you're supposed to raise your seat by as much as the crank is shorter, and you then may need to change your bar and seat position. Obviously this is more critical for road riding. At the upper part of the stroke, a 10mm shorter crank means your foot comes up 20mm lower, probably pretty noticeable.

I'm concerned that when out of the saddle on technical terrain, this means 10mm less seat clearance if it is raised. I don't ride with a dropper, and virtually never lower my seat because my local riding has no sustained steeps. Will it be possible to not lower the seat, and just not extend your leg as much when pedaling? Does that make sense?

I had 177.5mm and 178mm cranks on my XC bike in the mid to late 90', but have ridden 175mm cranks since about 2000. I've got 170mm cranks on my old school road bike, and noticed a slight difference but they seemed to work pretty well. I've never had 165mm cranks and can't recall any impressions of possibly riding them on demo DH bikes.

[juanbendedknee]

I am 6'2" and I'm rocking 170mm RF Next cranks. Love them. Not sure if I'd do 165 'cept for DH maybe, but who really knows? Maybe they'd be awesome? I was tempted to get the SixC ones, as they come in 165.

[jono]

MHO agrees with everything in your last post, Damian. I've looked at the seat height change too, and FWIW I expect the "right" seat height will be higher by, say, 90% of the crank length reduction (exact fraction personal). If reach is really an issue you'll get a dropper, but if you're doing this to avoid pedal strikes that seems like a minimal concern.

I'd be going shorter (probably 170) with an oval ring now if the current bike weren't nearing replacement; plan to on the next one. Knees. I need mine.

[stuckathuntermtn]

All I read was "popular with the triathlon crowd" and didn't need to read anymore.
Next!

[toast2266]

From reading a bit on this stuff, the problem seems to be that most of the studies are done in the context of road bikes, where power output, cadence, and bursts of power play out differently from a mtn. bike. Doesn't mean those studies aren't relevant, just means that I think they're not 100% applicable.

[jono]

Power studies are as applicable as they need to be, they just don't address all practical concerns. It's not like your power gets made so differently on a MTB during seated pedaling. Shorter cranks mean less torque which you make up with a smaller chain ring to get the same chain tension (and chain/foot speed) at a higher cadence.

The practical question is whether you can live with (or actually prefer) a higher cadence. At the same foot speed, I'm pretty sure the only time I'd mind is when I'm ratcheting to avoid a pedal strike--the solution to which could often be shorter cranks.

Is there another situation where you want the crank speed lower?

[Dantheman]

Shorter cranks mean less torque which you make up with a smaller chain ring to get the same chain tension (and chain/foot speed) at a higher cadence.

In theory, the reduced leverage gets made up for by increased muscular efficiency at the reduced ROM. That's why the studies don't show a loss in power output.

[jono]

Remembering that power is torque times rpm, if power remains the same while rpm goes up and torque down in proportion, then a better theory is that muscular efficiency is unchanged. Since foot speed is unchanged at a proportional increase in rpm that would seem to be expected. If you actually get better muscular efficiency then we should expect a power increase.

Or: power is force x distance/time. Same force and speed (linear foot speed, that is) = same power.

[muted]

I am tall and have 170 on one bike, and 165 on another. All that matters is there is a slightly less chance of a pedal-strike-to-over-the-bars on my 165 bike, the other with the 170 is my XC bike and I don't mash the downhills as much. Discussing performance difference between 5mm, unless you are racing pro, is a waste of time. Just go ride and think of anything but your fucking cranks.

I expect this to go 5 pages nevertheless.

[forty]

6'3'' and run 170's on my dj and an sx trail that i pedal somewhat

i dont notice any power difference or desire a gear ratio change due to crank size

i do slightly notice the smaller circles being pedaled but its not a negative or positive for me

i definitely notice the smaller stance front/back with shorter cranks
when riding dh bikes with 165's i dont feel nearly as "inside" the cranks due to how close together my feet are and cant pressure front/back between them as easily for balancing things like g-outs and manhandling chunder that tries to buck you if timing gets off

the pedal clearance and slightly lower seat height make me favor 170's over 175's, but it seems thats as low as i care to go

[Dantheman]

Remembering that power is torque times rpm, if power remains the same while rpm goes up and torque down in proportion, then a better theory is that muscular efficiency is unchanged. Since foot speed is unchanged at a proportional increase in rpm that would seem to be expected. If you actually get better muscular efficiency then we should expect a power increase.

Or: power is force x distance/time. Same force and speed (linear foot speed, that is) = same power.

RPMs should only increase if you reduce the chainring size, which if IIRC they didn't do in the studies since it would add a second variable and confound the results.

[jono]

RPMs should only increase if you reduce the chainring size, which if IIRC they didn't do in the studies since it would add a second variable and confound the results.

Lots of ways to adjust rpm in a test besides chainring, just like in real life we'd just shift to the comfortable gear. Accounting for gear ratio when using a dynamometer to measure power is basic stuff, so I'm sure they managed it. That said, if you shorten the crank and hold rpm constant you're reducing foot speed. Only way for power to stay the same when that happens is to push harder, so if that's what they found it's a strong argument for shorter cranks. But in the real world we shift, so constant rpm seems arbitrary.

Muted has it right; I was going to agree but decided to go ride instead. I only bring up the math side of this to point out that there's no hidden fly in the ointment as has been speculated. As everyone with experience (mine is 170, 172.5 & 175 FTR) has said: you won't really notice it pedaling. If you have another reason to do it, enjoy; there's no harm.

[Dantheman]

Lots of ways to adjust rpm in a test besides chainring, just like in real life we'd just shift to the comfortable gear. Accounting for gear ratio when using a dynamometer to measure power is basic stuff, so I'm sure they managed it. That said, if you shorten the crank and hold rpm constant you're reducing foot speed. Only way for power to stay the same when that happens is to push harder, so if that's what they found it's a strong argument for shorter cranks. But in the real world we shift, so constant rpm seems arbitrary.

True, lots of extra variables when you move into a real life setting. Increased muscular power and efficiency at reduced ROM is exercise physiology 101 though. Exactly why if you walk into a typical gym you'll see so many people half-squatting and shorting their pullups.

[jono]

True, lots of extra variables when you move into a real life setting. Increased muscular power and efficiency at reduced ROM is exercise physiology 101 though. Exactly why if you walk into a typical gym you'll see so many people half-squatting and shorting their pullups.

To be clear, when you say power in that context do you mean force or force x distance / time? The former gets a really clear example with shorted pull-ups, but the latter can be less so since reduced ROM means less distance.

Anyway, you basically named my reason for going shorter: I have knee damage that gets worse when firing the quad at acute angles, so I'm looking to avoid them.

[Dantheman]

Sorry, the former. I wrote "power" but meant force.

[toast2266]

Power studies are as applicable as they need to be, they just don't address all practical concerns. It's not like your power gets made so differently on a MTB during seated pedaling. Shorter cranks mean less torque which you make up with a smaller chain ring to get the same chain tension (and chain/foot speed) at a higher cadence.

The practical question is whether you can live with (or actually prefer) a higher cadence. At the same foot speed, I'm pretty sure the only time I'd mind is when I'm ratcheting to avoid a pedal strike--the solution to which could often be shorter cranks.

Is there another situation where you want the crank speed lower?

Seated pedaling at a decent cadence - yes, the studies are applicable. But on my MTB I climb stuff on a regular basis that's waaay steeper than most roads. I'm in my easiest gear with a fairly low cadence pretty often. And at least some of the time, I'll be standing up and mashing while moving pretty slowly. I pretty clearly notice the shorter crank length in this scenario. You said in another post that in the real world we shift, so constant cadence is arbitrary. But that's ignoring the scenario where I have no easier gears to go to. Most of the studies don't really address that situation, presumably because it has limited applicability on a road bike.

[jono]

You should try a road bike sometime. I run out of gears with that corn cob out back far more often than on the MTB. I think the real reason no one looks at standing is because they're idealists who think you should get a lower gear if you need to stand since it's so well established that sitting is more efficient.

Of course, the answer to your issue is the same, even though it won't get you back to sitting: lower your bottom gear as Damian described and the grunt on standing pushes will feel pretty similar to longer arms in your present gear. Spinning slightly quicker is pretty negligible since, as I mentioned, faster rpm on a shorter crank can give the same foot speed. You just have to quicken your change in direction, but a few percent is pretty easy to do.

[toast2266]

You should try a road bike sometime. I run out of gears with that corn cob out back far more often than on the MTB. I think the real reason no one looks at standing is because they're idealists who think you should get a lower gear if you need to stand since it's so well established that sitting is more efficient.

Of course, the answer to your issue is the same, even though it won't get you back to sitting: lower your bottom gear as Damian described and the grunt on standing pushes will feel pretty similar to longer arms in your present gear. Spinning slightly quicker is pretty negligible since, as I mentioned, faster rpm on a shorter crank can give the same foot speed. You just have to quicken your change in direction, but a few percent is pretty easy to do.

Yeah, I could obviously go to a lower gear, but there's a point where it doesn't make sense to go any easier. A 30/42 combo means I'm already crawling along; any easier than that and I might as well just walk.

Obviously the answer is to that issue is to get in better shape, but that's another matter. Maybe that comes back around to your first point - I should spend more time on a road bike. But really, I'd rather be fat and slow than go for a road ride.