Bicycle Mechanics - Without a caliper, how can I tell how long my crank is?... seriously

Is it written anywhere on it? I have a 2005 Kona Jake that is all stock. It has a TruVativ Elita Triple crankarms.

Any way to tell?

No markings on the back of the arm?

Measure it.
Middle of the crank bolt hole to the middle of the pedal bolt hole.
Measure in millimeters.

Shoud be stamped on back of left arm.

Thanks. I'll take a look when I get home. I'm going to trade them in for 160mms. Since my knees have started to bother me I've heard that shorter cranks help. Sheldon Brown's site says that unless you are riding a single speed, there is no mechanical advantage or disadvantage to crank length. This should take some pressure off.

On that note, can anyone recommend a bottom-of-the-barrel crank set that comes in 160mm? I don't want to spend more than like $150.

I'm obviously not a racer. Or a mechanic.

How tall are you?

IMO going shorter will put *more* pressure on your knees because both the lever and the sweep are shorter.

To take the pressure off your knees, learn to use less gear and a higher cadence.

Check this out - it's a pretty good deal for just over $100

http://www.rivbike.com/products/list/cranks_bottom_brackets#product=12-190

How tall are you?

IMO going shorter will put *more* pressure on your knees because both the lever and the sweep are shorter.

To take the pressure off your knees, learn to use less gear and a higher cadence.

5' 11"

But my knee won't be raised as high or in as large a circle. That should take the pressure off, correct?

Although I agree that higher cadence is a given. I think that is what Sheldon Brown's was getting at when he said that crank length alone does not offer mechanical advantage, since it depends on what gear you are turning.

Check this out - it's a pretty good deal for just over $100

http://www.rivbike.com/products/list/cranks_bottom_brackets#product=12-190


Very cool!

Remember that longer means a bigger circle, but it also means more leverage.

I have weak knees, and I am 6' I ride 170 or 172.5 on pretty much all of my bikes, and I do fine. When I don't do fine is if I ride in too high a gear.

You could well have a condition where shorter cranks may be helpful, but 160 is a real short crank for someone our height...

When I ride, I shift down until the pedaling feels real easy, then I shift down one more... Following this method, knee problems are a thing of the past... at least from riding. Stairs are a different story. :twitchy:

5' 11"

But my knee won't be raised as high or in as large a circle. That should take the pressure off, correct?

Although I agree that higher cadence is a given. I think that is what Sheldon Brown's was getting at when he said that crank length alone does not offer mechanical advantage, since it depends on what gear you are turning.

At 5'11" you should be on a 170, maybe a 172.5.

What are you calling pressure? Remember, the knee isn't passive. It's channeling all the force of the muscles above it. That's what I'm calling pressure. Decrease the force and you decrease the pressure. If your problem is a range-of-motion issue, then you'd be trading one form of pressure for another and I don't think you'd see much of a benefit.

Sheldon is right, but like more than a few of his points it isn't clearly stated. If you assume the same gearing, then the longer arm alone does offer more mechanical advantage.

If you assume the same gearing, then the longer arm alone does offer more mechanical advantage.

Thanks again for the response. Also, bear with me if I'm a little dense on the mechanics- that's exactly what I'm trying to figure out. Doesn't "more mechanical advantage" mean "it takes more force to turn?" In my mind, longer cranks are equivalent to biking in a larger gear.

Also, as you said, the pressure on the knee is really the pressure of the quad muscles that run over the kneecap. I think that the further you bend your knee, the greater the pressure is on the top of the kneecap when you straighten it back out (Because of the leverage caused by the angle). So while I don't have a range of motion issue, I would assume that to a certain degree small loops with the feet are easier on the knee.

So once again I'm not trying to dispute anything, just trying to have it make sense in my head.

I put 152 mm cranks on my wife's bike, because she has arthritis and trouble bending her knees. They were about $30 with riveted chainrings (3) for a childs bike. They didnt have the Shimano bumps and profiling to help the shifting but the shifting didnt seem too bad. I also had to put on a new BB with longer axle. I forget the number of teeth on the chainrings but if they arent smaller, you may have to go for bigger cogs at the back to compensate for the reduced leverage at the pedals.

As I got older I switched from 175, then to 172.5, now I'm riding 170. I'm in my mid-50s and 5'11" and I did notice a lot less stress with the shorter cranks - and it seemed to take some strain off my hamstrings, too.

With most things like this, it's probably different from one person to the next. It did help me.

You want cheap! I just installed these-
http://www.nashbar.com/profile.cfm?sku=20985

See my thread- Read my initial post and then skip to yesterdays to avoid a lot of tedious reading. Basically, that'll be before & after.
http://www.bikeforums.net/showthread.php?t=361045

It amazes me how people without bad knees try to advise someone that has bad knees.
For those people, it's really simple- BAD knees don't BEND as much as GOOD knees!
The whole torque leverage thing is bogus. You just use a lower gear! Power to the rear is theoretically the same. In reality, it's better because you are pedaling MUCH more efficiently.

You want cheap! I just installed these-
http://www.nashbar.com/profile.cfm?sku=20985

See my thread- Read my initial post and then skip to yesterdays to avoid a lot of tedious reading. Basically, that'll be before & after.
http://www.bikeforums.net/showthread.php?t=361045

It amazes me how people without bad knees try to advise someone that has bad knees.
For those people, it's really simple- BAD knees don't BEND as much as GOOD knees!
The whole torque leverage thing is bogus. You just use a lower gear! Power to the rear is theoretically the same. In reality, it's better because you are pedaling MUCH more efficiently.

Rock on. I think I'm going to buy that.

Before you go changing cranks, you ought to make sure your saddle height is correct and that the fore/aft is properly set. The last set of cranks a 5'11" person with a normal inseam should be on is a 160mm. That's plain dumb.

If you shave 12-15mm off the crank and leave your saddle where it is, you're going to have a completely different set of problems. Sure, you don't have to raise your knee as high, but a shorter crank will also compress your knee at the bottom of the stroke.

It's a bad idea. The problems you're describing sound far more like a poorly positioned saddle than something to do with "crank force." I have a complete POS right knee and I would never consider riding anything else other than a standard 172 or 175. A 160 is for tiny folk.

If it would work for you, much cheaper and simpler to learn to spin faster in easier gears.

"More mechanical advantage" from the longer crank arms means your leverage ratio is higher so it takes less pressure to move through a longer arc of the crank circle.

If you've got tricky knees that don't like to cycle (sorry pun not intended:D) through a larger diameter because the top of the circle is bending your knee too far then you want to go with a shorter crank arm. If it's not a mobility issue but rather a "contained force within the joint" issue where you're trying to reduce the load your knee sees then a longer crank and running an easier gear with higher pedalling RPM is the way to go. The longer cranks and higher spin RPM will greatly reduce your kneed loading.

You want cheap! I just installed these-
http://www.nashbar.com/profile.cfm?sku=20985

See my thread- Read my initial post and then skip to yesterdays to avoid a lot of tedious reading. Basically, that'll be before & after.
http://www.bikeforums.net/showthread.php?t=361045

It amazes me how people without bad knees try to advise someone that has bad knees.
For those people, it's really simple- BAD knees don't BEND as much as GOOD knees!
The whole torque leverage thing is bogus. You just use a lower gear! Power to the rear is theoretically the same. In reality, it's better because you are pedaling MUCH more efficiently.

Wrong. Lower gear doesn't make the pedalling circle any smaller.

Wrong. Lower gear doesn't make the pedalling circle any smaller.

And I said that where?

Sheldon is right, but like more than a few of his points it isn't clearly stated. If you assume the same gearing, then the longer arm alone does offer more mechanical advantage.Also with the same gearing at the same speed longer arms will require less force on the pedals to generate the same power. Thus causing less pressure on your knees and requiring less force from the muscles, thus fighting off fatigue and soreness longer.

The other major factor is pedaling form. Knees don't like on & off power transitions. It's better for them to have continuous pressure all the way around. To test this on/off variation, try riding one-legged. If you can't do it easily, then you're putting a lot of stress on the knees by applying a lot of force on the downstroke and not much on the upstroke. What ends up happening is the leg on the opposite side has to apply extra force to push up the dead leg, causing even more pressure on that knee than necessary.

BTW, I was bed-ridden for two months in the beginning due to bad back and bad knees. Doctor said to take lots of pain-killers, get lots of rest, and stop doing whatever it was that injured my back and knees. Well.. I wasn't going to stop cycling, and I had just caught the racing bug too! Finding the right PT and coach made all the difference and I haven't had any problems with backs or knees after adopting better technique.

also make sure your seat is high enough. The heel on the pedals with a straight leg is just a baseline test for the minimum height. You get better performance, efficiency and knee-comfort with a saddle slightly higher. Track racers typically adjust their seat so there's a 10-13mm gap between their heel and the pedal with a straight leg.

So the idea that longer crank=less power required finally clicked with me. However it doesn't seem like anyone is wrong here. Oh, and my bike is professionally fit otherwise. I didn't mention the knee thing to them though.

The problem when you assume the same gearing... is that I've got a triple on the bike, so I won't need to assume the same gearing. But the point is understood.

Sounds like for people with "tricky" knees, the choices are to:

1) Do nothing with the crank. Keep at a high cadence and as smooth a spin as possible. Stay in a larger range of motion (knee goes higher).

2) Go to a smaller than recommended crank, move the seat up, use an even higher gear and even higher cadence than above. Stay in the smaller range of motion.

So the only benefit would be less range of motion.

Don't really know what to do know. It's all food for thought. I think I'm going to throw that $10 crank on there and see what happens. It's hard to argue with Kapaun's logic.

So the idea that longer crank=less power required finally clicked with me. However it doesn't seem like anyone is wrong here. Oh, and my bike is professionally fit otherwise. I didn't mention the knee thing to them though.

The problem when you assume the same gearing... is that I've got a triple on the bike, so I won't need to assume the same gearing. But the point is understood.
Not power, but force. Force is the instantaneous factor that determines how much load you have on the knees. Power = force X RPM. Power is force X distance per unit time. So the idea is that to generate the same power, you can use a lower gear at higher RPMs.

And ultimate, it's the lower gears that'll save your knees. The difference in force due to crank-length difference is something like 2-3%. But the difference in force due to changing to a lower-gear is around 15-20%. Practice your spinning and use lower gears; that makes a bigger difference than crank-length.

bear with me if I'm a little dense on the mechanics- that's exactly what I'm trying to figure out. Doesn't "more mechanical advantage" mean "it takes more force to turn?" In my mind, longer cranks are equivalent to biking in a larger gear.

No, exactly the opposite. Is a longer pry bar easier or harder to push? [hint: easier]


Also, as you said, the pressure on the knee is really the pressure of the quad muscles that run over the kneecap. I think that the further you bend your knee, the greater the pressure is on the top of the kneecap when you straighten it back out (Because of the leverage caused by the angle). So while I don't have a range of motion issue, I would assume that to a certain degree small loops with the feet are easier on the knee.

Well, no and no. Some of the force is transmitted through the kneecap, and some is transmitted through the joint and some through the other connecting tendons. But forget all that. The pressure through the knee is the same pressure that pushes on the crank arm. Make one less the other is less. The angle of the knee has very little to do with it. Disclaimer: I am not an orthopedist.

It amazes me how people without bad knees try to advise someone that has bad knees.
For those people, it's really simple- BAD knees don't BEND as much as GOOD knees!
That's a range of motion issue, not a "pressure" issue. As I said, there may be reason to prefer the smaller circle, but all else equal it will make the pedal harder to push.



In reality, it's better because you are pedaling MUCH more efficiently.
Care to explain that?

The angle of the knee has very little to do with it. Disclaimer: I am not an orthopedist.

Yes it does.