BSLeVan

Warning, this post may be long and rambling.

I've been thinking (always a dangerous thing) about the recently revived thread started by Bill K. on crank length. For whatever reason, there are parts of the discussion and concepts about crank length that just are not making sense to me. For example, if we're talking about crank lengths within a range of 15cm of one another, aren't there other variable (related just to pedaling) that would make just as much difference. I noticed that on one bike I have pedals that the effective contact point between the cleat and pedal is higher (by maybe 5 to 8 cm) than on another bike I ride with the same crank arm length. Would this have the same effect as moving to a shorter crank arm? And while thinking about this, I realized that there are noticeable difference in the thickness of the sole between my three pair of road shoes. Would this have an impact? Now as I write this, I'm thinking in the back of my mind that some of you are probably thinking, "This guy just doesn't get it." And the truth is, I don't... although this comes as no surprise given it took me two years to make it through one year of Algebra in high school.

Now here's the real kicker. You know how sometimes you can get a song stuck in your head and not be able to get it out for a day or two? Well, I've been pondering these questions for that last two days and it's starting to be a distraction. Can anyone help?

stevetone

I'm assuming that you mean 5 to 8 mm (not cm) difference in sole thickness. That would only move the effective center of the pedal path up 5-8mm--it does not change the effective length of the crank arm nor the effective diameter of the pedal circle.

Steve

leob1

It's called a "brain worm", last night I had Play that Funky Music White Boy, by Wildcherry, stuck in gmy head for half my ride.
The length between the pedal axel and your hip joint will change with all the variables you mentioned.
I too have different pedals and swaping them bike to bike will cause me to adjust the seat height, the same is true if I wear different shoes. Were talking millemeters, but it's noticable. More noticable when the crank is at the top or bottom.
Hope that helps.

deraltekluge

The same applies for differences in how you place the pedal under your foot...closer to or farther from the toes. The center of the circle in which your feet travel is moved, but its size remains the same.

Terrierman

Which then affects other things. I ride 175 mm cranks. Am not going to fret over this. But do carry on, that other one has been an interesting thread to follow.

Longfemur

I think small differences in crank length probably don't amount to much unless you're already near the edge, like if you're very tall or short.

For example, if you're short, you are limited to 170 mm with many systems, and if you can get them, 165 mm. Let's say you have 170 mm cranks. That means that you can end up pedaling what feels like a much bigger circle than a guy who has longer legs, or than if you were using 165mm cranks. So it does make a difference that way. It makes a difference in the fore-and-aft position of the saddle, and also the height in relation to the pedal. I've certainly been able to tell the difference on my bikes, but it was never to the point that I couldn't ride it and adapt to it. I can ride 175 mm cranks, but I don't feel like I'm spinning as smoothly. They make the whole bike feel too big just because the pedaling circle is that much bigger. I've tried this, so I'm not just guessing.

If you're riding a road bike and position matters to you, then you may as well get the right cranks if available. No point fighting something that's too short or too long if you don't have to.

Allegheny Jet

My two bikes have different crank arm lengths (172.5 and 175) and I can feel the difference between them when first out. After a few minutes I seem to adapt to the bike and don't seem to ride any different. I do think that with the longer crank arms I could generate more power on climbs if I've dropped my candence to really low numbers because the leverage is longer..

BSLeVan

OK, this is the stuff the puts my brain on overload. I understand that one is creating a circle when pedaling. However, if your foot is say 10 mm closer to the axis point (bottom bracket axle), isn't the circle you're creating actually smaller? And, if the your foot is 10 mm farther away from the axis, aren't you creating a longer lever that will require your foot to make a slightly bigger circle? Or, is it the fact that you've got two axis points (one in the bottom bracket and the other the pedal itself) that makes this difficult for me to reason through? Stay with me for a minute.... So, the circle you create with the foot moving around the pedal axle has nothing to do with the length of the lever created by the crank arm length? And if this is true, shoe sole thickness or pedal contact height from the center of the pedal axle has no effect?

flian

At first I thought "Absolutely!", looking at the pedal/crank down situation. But at the top of the stroke it's longer! I guess the average crank length is the same as with no offset of contact point.

cooker

Yes, a fatter pedal or thicker shoe sole simply moves the pedal circle up at both the top and bottom of the stroke without changing its diameter. Make a slight seat height adjustment and it neutralizes the effect. Presumably someone with unequal leg length due to a short tibia on one leg could compensate by adjusting the thickness of his shoe sole with an insert or putting a shim under the pedal cleat on that side. If it's only a small discrepancy, moving the cleat backwards on the shoe might be a "good enough" solution.

On the other hand, getting a short crankset means that the top and bottom of the pedal stroke are both moved closer to the pedal spindle. Of course you would raise the seat slightly so you still get proper extension at the bottom, and the end result is you don't have to lift your knee as high at the top. This is helpful for arthritic elderly knees. The disadvantage is you (EDIT)will apply slightly force when you mash really hard (but we don't do that over age 50, do we?) However performance studies have shown that minor crank length changes have very little impact on overall cycling efficiency. The crank length really only affects power in your highest or lowest gear. In the middle of the gear range, you unconsciously correct for it by selecting a different gear!

fthomas

Thanks cooker for clearly explaining the equalized impact on the radius and ultimately same diameter of the pedal circle.

Garfield Cat

In your example, you're switching bikes and pedals. In a bike fit session, that bike is fitted with only those pedals and shoes. Heaven help, if you choose to change anything!!

Also its assumed you're not in a growth spurt like a 14 year old gymnast.

Skipper

See how much this makes your head hurt.

The thicker the pedal or the sole of your shoe, the longer the length of the oval pattern created by your feet as your pedal axles create a perfect circle around the center of your cranks.


Edit: The oval does not get longer. It becomes more egg shaped.

BSLeVan

Actually this doesn't make my head hurt. On an intuitive level, this is the very thing I think happens.

cooker

No, your foot is going to be higher at both the top and bottom of the stroke. See picture:

The black block is the pedal, the reddish block is a thick shoe sole.

The black circle is the pedal circle without the thick shoe sole.

The red one is with a thick shoe sole. It's still a circle, it's just higher.

BLIZZ

Actually the black line of the pedal should be a perfect circle, while the red line should be a slight oval as you have pictured.

deraltekluge

I really can't speak for you and what you feel, but it doesn't seem to me that a tenth of an inch difference would be significant. You may be feeling what you expect to feel, or there may be some other factor different between the bikes that accounts for a difference in feel...you'd really have to conduct a double-blind sort of test, having the crank lengths (and only the crank lengths) changed on the same bike, to be sure that it's the crank length that's responsible for any difference in feel.

You may or may not generate more power with the longer crank, but what you will do is produce more torque for the same force on the pedal...1.45% more.

Skipper

Red blocks, black blocks, thick soles or thin, the center of the crank (the bottom bracket) does not move.

The center of the red block is going to be further away from the horizontal center line of the cranks at the top than at the bottom of the crank rotation. The centers of the red blocks are going to be really, really close to the same distance from the vertical center line of the cranks as the pedal axles.

deraltekluge

It's not an oval. If you start with a circle, and move every point on that circle some fixed distance in a certain direction, the result is an identically sized circle, translated in location by that distance in that direction.

cooker

They're both perfect circles (or supposed to be).

It's going to be farther from the bottom bracket at the top, and, by the same token, closer to the bottom bracket at the bottom. So if both the top and the bottom of the circle have moved up an equal amount, it hasn't changed its diameter.

Imagine a bee settles on the top of your shoe as you pedal. Your cleat on the bottom of your shoe is moving in a perfect circle, and the bee is moving in a perfect circle an inch or two higher.

Dchiefransom

Ya know, I think Blizz is correct. In order for it to be a perfect circle, the cleat would have to stay in the same relative position between the pedal spindle and bottom bracket. For instance, in the picture above, where the red block is on top of the pedal at the bottom of the stroke, it would have to be on the bottom of the pedal at the top of the stroke, always on the side of the pedal spindle toward the bottom bracket. Since the cleat is always on top of the pedal spindle, it can't be a circle, but more like an egg.

deraltekluge

If the cleat/shoe always stayed between the pedal and the bottom bracket, you'd still have a circle, but one of a smaller radius. (Besides breaking your ankle, of course)

Start with a circle, and move every point on that circle by the same amount in the same direction. You'd also move the center of the circle by that same amount, and every point would still be at the same distance from that new center. The center of the new circle is not at the center of the bottom bracket, but a point offset from the bottom bracket...but it's still a circle.

Skipper

Doh! Why didn't I see that? Probably because it is not right.

Close your eyes. Imagine yourself sitting on top of a pedal. Now try to imagine how in the world the two orbits can both be perfect circles if one orbit is two inches higher than the other while they share the same axle. The imaginary bee's eyeballs are not always the same distance from the center of the bottom bracket. The bee's eyeballs are also orbiting the axle of the pedal upon which my shoe is resting. It is this extra orbit that creates the oval. Visualize .....

deraltekluge

Try looking at it this way...

Suppose you have 170 mm crank arms. The center of the pedal pivot will describe a circle 340 mm in diameter as the crank is turned. Assume the pedal remains level as the crank turns. The top of the pedal will be a small distance above the center of the pedal's pivot point. Assume that the thickness of the pedal is 20 mm, so that a point on its surface is 10 mm from the pivot point.

When crank is positioned so that the pedal is at its lowest, that point on the pedal surface is 160 mm below the bottom bracket center. When the crank is positioned so that the pedal is at its highest, that point on the pedal surface is 180 mm above the bottom bracket center. The distance between highest and lowest is still 340 mm.

Now, do the same thing with an additional space (the thickness of a shoe sole, perhaps) of another 10 mm. Now, the top of the path of the point will be 190 mm above the bottom bracket center, and the bottom of the path will be 150 mm below the bottom bracket center...but the diameter is still 340 mm.

In all cases, the width and height of the path described by that point will remain at 340 mm, and the path will be a circle.

Skipper

You don't get to move the bottom bracket.