Short Crank Issues
 

This is a continuation of: http://www.bikeforums.net/showthread...tighten-pedals

A few people seemed interested in my short cranks (100mm) setup, and I have some more background and pictures.

I have a knee flexion problem, so I used this pair of crank arms shorteners for 3 years:

http://www.hostelshoppe.com/cgi-bin/...ory=1033761548

then this pair for like 2 years:

http://www.jensonusa.com/store/produ...x?SSAID=277653

They both sorta worked but not all that well. I would get tons of noise and creaking and I could never tighten them down enough. They would dig holes into the cranks too from all the repeated stress. Once the second one got a little loose and I broke a screw in half and it fell off with the pedal. Actually I think that happened a couple times.

Here's a picture of the second one my *old* bike, and you can see the hole that the first setup dug:

http://chubot.org/share/old-bike-crank-note.JPG

So then I discovered these shortened cranks for a new bike:

http://chubot.org/share/new-bike-short-cranks.jpg

They are done by Mark at bikesmithdesign: http://www.bikesmithdesign.com/Short...s/IsoFlow.html

From the other thread, I think I just forgot to tighten them. I had a million different concerns at the time -- I had an ordeal with making the handlebars high enough to account for being 3" higher off the ground. The cranks were the 3rd thing or 4th thing on my mind and that was my first time putting a bike together.

Although, I didn't feel them being loose when riding for that 10 miles.

The work looks solid to me and you can see on the page that he's pretty conscientious about making sure the cranks have enough metal to drill around and stuff like that. Seems like he's done it many times before.

I was just surprised that the loose pedal made it fail in such a spectacular manner, but apparently people have seen the same thing happen before on regular cranks. I didn't expect the threads to come out in pieces like they did.

BUT, here's a high school physics question: Suppose that I'm using 95mm cranks (this set was 95 not 100), and a guy who weighs the same as me is using 175mm cranks.

Now we both accelerate from 0 to 20mph in 30 seconds (or whatever). How much more force am I putting on the pedals than he is?

I feel like it must be a lot more, although I don't consider myself particularly strong. I can keep up with most riders even with the 95-100mm cranks. It works fine. I can even accelerate faster sometimes just because it's easier to make these small turns from standing still.

I think that 95mm are the shortest cranks offered. So maybe the shorter the cranks are, the more force they have to be able to withstand for the same amount of power. That could certainly account for many the problems I've had with the crank arm shorteners as well.

Ummm... you should have just posted this as a new post on your original thread. I'm not sure if a moderator can join these two together or not but it's not normally kosher to have two threads about one topic. And this is certainly the same topic as your other thread.

Lemme say first off that I never took physics in high school; and I satisfied my college physics requirement by taking astrophysics. So as for expertise, I don't have any.

But I too am interested in short cranks. The shortest I've used was 5" (127 mm) but typically use something in the 140 - 165 range. I'm 6' tall and 165 lbs.

One day last winter it snowed while I was at work, and on my way home I ended up riding through about 4" of unplowed snow on a MUP. It was just a half mile, and I figured I could downshift and muscle my way through; and what happened? I ripped the left pedal out of the crank. I felt it working lose; I stopped and tightened it; it worked lose again, and this time it wouldn't tighten any more. The threads were completely stripped.

Bottom line: yes, in my experience, shorter crank arms put a lot more stress on the pedal threads. The shorter crank arm inevitably gives you less leverage, and if you apply the same amount of force to it, something is going to give. The pedal threads seems to be the thing that gives.

The solution, I think, would be steel. You can probably get steel crank arms, though they will probably be the old fashioned cottered style. Check out this ebay sale, for example. Of course it would take some fancy machine work to put double or triple chain rings on there... but it might be worth thinking about.

The guy on a normal length crankarm has the advantage of leverage ,
and can turn a higher ratio gear as a result.
on the stubby ones you have to start in a much lower gear to make up for that,
and spin it out, and Go to the next higher gear ratio, and repeat that procedure .
much like an Auto , go up thru the gears from a low one.

In your hypothetical situation I'd make arrangements to meet up at some Pub down the road ,
your companion will be waiting and one pint round ahead of you. :beer:

If you think you actually only finger tightened the pedals and never actualy pulled on them with a wrench then yeah, a spectacular failure of the sort you had becomes almost a sure thing. That would certainly solve that part of the mystery.

You're asking about the gearing between you and another guy of the same weight but with the two crank arm lengths.

Let's consider this as if you are a piston in an engine. You and he are both the same weight. Let's think of this as the two pistons both have the same bore and compression ratio and you both just ignited the same amount of fuel and air so the chamber pressure pushing you two pistons down is identical. But you have a shorter throw crank arm while he's got a long throw arm. Two things happen. One is that you cannot apply your piston pressure over as long a time because of the short throw. Second is that with the shorter throw you are not going to extend as much torque into the crank as the longer throw.

So for the same RPM the longer cranks are a win-win and short cranks are a lose-lose. So where do you make it up so that this combo becomes both useable and practical? I gather from reading a little at the Bike Smith Design site that a number of specialty riders are enjoying the shorter cranks. Folks such as yourself with flexure issues and regular riders of odd HPV's of various forms where a higher cadence is desireable. It's through using a higher cadence that you can make up for the low torque lose. In fact if you are going to match the other rider's/piston's performance you'll HAVE to use a higher cadence. So as fietsbob suggested you'll find that to keep up you'll be using lower gears and likely more of them and spinning out in them more like motorcycle rider doing a sporty ride and constantly shifting gears to stay in the power band. Meanwhile the guy with the regular length cranks will be more like a tractor with lots of torque available if he should slow down for any reason and want to grunt his way back to speed.

Now if you feel that you're matching their speeds and you're not having to spin like the dickens then it has to be because you're using more leg pressure and working harder than they are. If you are spinning like the dickens then you'd be able to keep up decently with "normal" leg pressure. The trouble is that the human leg and nervous response time does not like to work at truly high cyclical rates. The feedback begins to lag and the leg ends up still pushing down when it should be lifting and not pushing soon enough as it comes over the top of the arc. With practice you can work on this and make the timing more of a pattern that you follow instead of reacting to your perceived foot position. In effect with time and practice it can become an almost instinctual action sort of like the folks that do that western quick draw shooting or how a martial artist will react to block an incoming attack based on where it's coming from and other visual cues that triggers a well practiced virtually instinctive reaction.

If you are managing to do this you're likely using a cadence that few of us could match without some considerable practice to make it effective. You've traded knee flexion angle for speed of knee movement.

I agree with your whole post, and am quoting just this bit because it does, in fact, describe the way I ride. But bear in mind, the BikeSmith normally cuts 22 mm off a crank; so 175's become 153's, 170's become 148's, and so on. If you've tried cranks of these lengths you'll agree with me that it's a subtle difference. Since moving to shorter arms, I've found my cadence has gone up, but probably not more than 10%. You are right about the disadvantage(s) of "spinning like the dickens," but now you're talking about the extreme; somewhere there's a dividing line between a high cadence, and spinning like the dickens. And, in my experience, a shorter crank arm with a higher cadence is preferable to a longer crank arm and a lower cadence. So, where to draw the line? I don't know. OP's 95 mm crank arms are significantly shorter than any I've tried, but obviously he's been doing it long enough to know that it works for his legs.

I find it very odd that over the last 50 years crank arms have tended to get slightly longer, while at the same time there have been significant advances in bicycle gearing, both in internally geared hubs and derailleur systems. If people were using the improved gearing systems to their full advantage, I believe the tendency would have been the opposite, toward shorter cranks, at least for road usage.

Discussions of crank arm length often employ an argument based on body size; that is, human adults range from 60 to 80 inches or whatever, so why don't cranks range correspondingly, such as from 6 to 8 inches or whatever? Of course we all know that they don't. The bicycle industry has basically decided that every one can make do with crank arms around 170 mm, and this is good because it significantly reduces the cost of bicycles of all sizes. As long as everyone can use circa 170 mm crank arms, then everyone's happy. And, it turns out, almost everyone can. The reason for this is, I think, that we are all accustomed to the same range of motion. From the age of two or three we all walk up and down stairs, which are standardized at pretty much the same size --well, closer to 180 mm I think-- and this is what determines our accustomed range of motion. Body size doesn't really enter into it.

All that said, I'm surprised how little attention has been given to determining what exactly is the ideal crank size for all people. Based on my own experience, it would be shorter than 170 mm.

But all this is, I think, a thread hijack. OP's reason for starting this thread was to investigate the effect of unusually short crank arms on the pedal-to-crank connection.

For the pedal-to-crankarm interface, the length of the crankarm doesn't matter, the force by which you can push down on the pedal remains the same. If it'd been the crankarm-to-BB interface, then there'd have been a difference.

I've seen the work of that guy before, and he does seem to take his job seriously. Every one can have a bad day, so flawed fabrication is of course possible, but I'd hold insufficient torque at assembly as responsible.

(bold added)

True, but with a longer crank arm, and more leverage, you won't need to push the pedal with as much force. The bicycle will move before the threads start to strip. With shorter crank arms, it takes more force to move the pedals, and since you can push down on them harder, you do. Anyway, that's my intuitive explanation of the phenomenon.

The fact is, I have stripped out the threads of a short crank arm, something I've never done with normal length ones.

Cranks are a simple lever.

So, if it take 200lbs of force to move the bike 10mph with 200 mm cranks,it will take 400lbs of force to do the same with 100mm cranks.So do the math.



If the pedal is tight to begin with,your going to have to really try to sheer a 1/2-9/16 cromoly bolt with your foot.

I agree, but in this case I think I neglected to properly tighten them. I have another pair from Mark... I will tighten them down real good and try again :)


I agree with all of the tradeoffs as you've put them -- however there is another variable: Your leg is a lot stronger extending from a short bend than it is a large bend. I don't know how much, but intuitively it's true. A physical therapist could probably answer this quantitatively.

Your leg has a different strength at different points in the bend, so you have to naturally set the gearing to accomodate the *minimum* strength.

I favor the higher gears on my bike, although some would expect me to favor low gears with 100mm cranks. My old bike was a 48/38/28 with a 14-28 cassette, and I would usually ride in the 38 6th gear or 48 4th-6th gear (I don't know the exact sprockets).

I told Mark from bikesmithdesign this and he said that is what he expects, though most people expect the opposite.

Another variable that makes it "nonlinear" is that I think it's considerably easier to keep a high cadence with a short radius than a large radius. I don't know how to quantify that either. But once you get used to making small circles I imagine you can do it pretty fast without expending more energy.

I don't think you can quite make this generalization. I think road racers generally favor longer cranks, but if they were really more efficient, why wouldn't they go to 180, 190, or 200mm cranks? Some taller riders could go even further.

I'm not saying that 100m is better of course, but I agree with rhm that there's surprisingly little reasoning/experiment about the optimal crank length.

If the answer is it's proportionally more force, 175/95, then that answers my question. Anyone else disagree?

But why would your riding style change that much? The hardest push from the rider is still the hardest push, and regardless of crank length I guess there's a few occasions on each ride when the drive train gets to see just that. (grinding up a hill, getting going from stop in a poorly chosen gear...)
There is a place for your scenario, but only if the rider actually do push harder with the shorter cranks, which we don't know. I don't think I would.

I understand your point; and I'd suggest you just try it, but instead I'll tell you my exact experience.

The shortest crank arms I've tried were 5" unicycle cranks (127mm) that I put on my Strida (an unusual folding bike, single speed, and front freewheel system, which is why unicycle cranks work). As it turned out, I had to replace the crank anyway, and chose the short unicycle cranks to see if they would let me spin a little faster, and maybe increase my top speed. I think it did increase my speed maybe a little, certainly not much; but I did notice that it was now much more difficult to get up to speed when starting from a stop. Somewhere around 5 to 10 mph I normally want to push really hard on the pedals, and with the short cranks that usual amount of push didn't seem to do much good. So to get up to speed, I necessarily pushed a lot harder.

You can get short 110bcd cranks, square taper from danscomp.com or anyother mailorder bmx.

It sounds like your general riding style is much more controlled than mine.
I don't think I've had a single commute when, for one reason or another, I haven't felt compelled to push pretty much as hard as I can for a brief moment or two. Can't see crank length changing that in any particular way.

Now, if I were to switch between a step-through frame/cruiser bike and my regular bikes, then it's no question that the former would see less force on the pedals.