Author Topic: Short Crank Arm Purchase Tips?  (Read 12660 times)

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Offline NEIL FROM BROOKLINE

Short Crank Arm Purchase Tips?
« on: September 09, 2010, 07:03:42 am »

Hi everyone,
I have a question for the list.  I am very seriously contemplating purchasing a triple chainring with 140 or 145 mm crankarms. These would be TruVativ Isoflows. Are there any things that I should be aware of before purchasing that I might not have thought of?
--For example, might I need to get a new bottom bracket?
--Will I need to raise my seat?
--If I do raise my seat, will that negate the benefit of the shorter crankarms?
--Does raising my seat put me at risk of knee pain on the back of my knees?
--I went for a bike fitting earlier this summer. Should I get a new fitting if I buy the new crankarms?

I ask that forum members please provide their thoughts & recommendations.

Here is some background that may be helpful. My reason for making this purchase would be to take some strain off my arthritic knees. My plan is to initially mount the chainrings on my K2 mountain bike/ city bike that I use for commuting.  If the crankarms produce a noticeable drop in inflammation of my knees, then I will mount the crankarms on a touring bike that I will thereafter purchase (likely a Trek 520 or Bianchi Volpe).  (If the short crankarms do not help my knees, I will be buying a recumbent for touring but that is a story for another day).  Currently, I use a Sugino 26X36X46 triple chain ring with 165 mm crank arms.
Thanks,
Neil

Offline rvklassen

Re: Short Crank Arm Purchase Tips?
« Reply #1 on: September 09, 2010, 10:11:30 am »
Shorter crank arms will reduce the travel of your legs and the maximum speed at which they move, but at the same time they'll reduce the mechanical advantage.   I don't know whether shorter or longer would be better for your situation.  But getting to your questions:

1) Yes you should raise your saddle by exactly the distance by which the crank arms are shorter than what you had.
2) You should also raise your bars by the distance you raise the saddle.

Assuming you move the bars up and not in or out (i.e. if you have a threaded stem just raise them, but with a threadless, you need a stem that is not only at a different angle but longer, so the saddle-to-bars distance doesn't change) you should now have essentially the same fit, other than the amount of travel of your legs.

Some places may be able to let you try different crank arm lengths before you buy.  Going down by 20 or 25 mm is pretty extreme.  I know it only sounds like 2-2.5 cm...

Offline BrianW

Re: Short Crank Arm Purchase Tips?
« Reply #2 on: September 09, 2010, 01:37:00 pm »
If you want to try shorter cranks without initially investing in new cranks, you could use crank shorteners that bolt onto your existing cranks. These are commonly used on tandems to allow shorter (child) stokers to pedal, but would also work for what you want. An additional benefit is they will allow you to try different lengths to see what works best for you.

See http://www.tandemseast.com/parts/cranks.html#Crank%20Shrotners

Offline whittierider

Re: Short Crank Arm Purchase Tips?
« Reply #3 on: September 09, 2010, 02:26:14 pm »
Quote
--For example, might I need to get a new bottom bracket?
not if the new crankset uses the same type

Quote
--Will I need to raise my seat?
yes, by the amount you shorten the crankarms

Quote
--If I do raise my seat, will that negate the benefit of the shorter crankarms?
no, because the shorter crankarms will reduce the amount you bend your knees.  Hopefully you'll be bending your knee little enough to avoid the knee angle that hurts it.

Quote
--Does raising my seat put me at risk of knee pain on the back of my knees?
no, because if you set it correctly, your leg extension at the bottom of the turn will remain the same as what you had before you went to the shorter crankarms.

Quote
--I went for a bike fitting earlier this summer. Should I get a new fitting if I buy the new crankarms?
maybe, but I'm not sure you'll find a fitter who will cooperate with your intentions.  (More on that further down.)

Going to crankarms that short will make it nearly impossible to reach the ground with your toes while seated, so putting a foot down will pretty much require getting off the seat.  It shouldn't be a major problem, but it's good to think about these things ahead of time.

Contrary to popular belief, shorter crankarms will not decrease your power; because what you lose in torque, you gain in cadence, and power is torque times cadence.  Several studies have proven the reality.  In this article for example, it says of Jim Martin, Ph.D. of the University of Utah, "His studies of 16 bike racers of various heights doing maximal sprint power tests of under four seconds duration on cranks of 120, 145, 170, 195, and 220mm showed no statistical difference between crank lengths."  Obviously the ultra-short ones would have resulted in cadences you cannot achieve on the long ones.

The main thing then is to get something that feels comfortable, does not injure your knees (it is typical to need shorter ones as you get older), and lets you achieve good posture and movement on the bike.

The difference between 175 and 170 (the range of most brands) is only about 3%, whereas the range of height in the general public is almost ten times that much (and age and previous injuries will further widen the required range of crankarm lengths).  Put a short rider with a belly on long crankarms and (s)he won't be able to get into an aerodynamic position at all.  It's a cryin' shame that the industry that is so obsessed about fit tries to shoehorn the entire population into such a narrow range of lengths.

Since each leg won't be coming up as high at the top of the turn, you may not need to raise your bars.

There are very few manufacturers that make crankarms less than 165mm.  DaVinci and Specialites TA (which you can get from Peter White Cycles among other distributors) are two I know of, but neither one has 140 or 145.  DaVinci has a triple-hole one with 130, 150, and 170 for kids as they grow, normally for use on the back of a tandem with a raised BB above the normal one, but I think you can use them on a single bike too.

Offline ducnut

Re: Short Crank Arm Purchase Tips?
« Reply #4 on: September 10, 2010, 12:06:57 pm »
I'll second Peter White.

He thinks outside the box that the industry is stuck in. He has extensive knowledge in bicycle travel and applies common sense ideas. Give him a call about what you're thinking. He'll know exactly what'll work with what and have the hardware to make it happen.

Offline NEIL FROM BROOKLINE

Re: Short Crank Arm Purchase Tips?
« Reply #5 on: September 10, 2010, 11:12:41 pm »
Shorter crank arms will reduce the travel of your legs and the maximum speed at which they move, but at the same time they'll reduce the mechanical advantage.   
Hi,
Thanks for your reply. You state in your response that my legs' maximum speed would be reduced. Would you please explain why they would spin slower & not faster?
Neil

Offline NEIL FROM BROOKLINE

Re: Short Crank Arm Purchase Tips?
« Reply #6 on: September 10, 2010, 11:15:08 pm »
If you want to try shorter cranks without initially investing in new cranks, you could use crank shorteners that bolt onto your existing cranks. These are commonly used on tandems to allow shorter (child) stokers to pedal, but would also work for what you want. An additional benefit is they will allow you to try different lengths to see what works best for you.

See http://www.tandemseast.com/parts/cranks.html#Crank%20Shrotners
Hi,
Thanks very much for recommending crank shorteners. I had never heard of them before. These could be a great interim step. I will certainly research them.
Best,
Neil

Offline NEIL FROM BROOKLINE

Re: Short Crank Arm Purchase Tips?
« Reply #7 on: September 10, 2010, 11:31:10 pm »
Hi Ducnut,
Thanks for a very comprehensive reply. By way of background, I talked with Peter White by telephone about six weeks ago. His recommendation was that I get the shortest possible crank arms to accommodate my creaky knees & specifically stated that 150 mm is the absolute maximum size crank arm that I should use. As of the time I called him, the smallest size he sold was 155 mm.
Best,
Neil

Offline whittierider

Re: Short Crank Arm Purchase Tips?
« Reply #8 on: September 11, 2010, 12:58:10 am »
Quote
Thanks for your reply. You state in your response that my legs' maximum speed would be reduced. Would you please explain why they would spin slower & not faster?

No, foot speed will remain approximately constant (this has been proven in lab tests), meaning RPM naturally goes up with shorter crankarms.
« Last Edit: September 11, 2010, 03:45:47 pm by whittierider »

Offline ducnut

Re: Short Crank Arm Purchase Tips?
« Reply #9 on: September 11, 2010, 09:33:09 am »
Shorter crank arms will reduce the travel of your legs and the maximum speed at which they move, but at the same time they'll reduce the mechanical advantage.   
Hi,
Thanks for your reply. You state in your response that my legs' maximum speed would be reduced. Would you please explain why they would spin slower & not faster?
Neil

The foot isn't covering as large of diameter circle with shorter cranks, therefore, it's covering less distance, at the same RPM.

For example: If you have equal RPM, one crank is 20' long and the other crank is 60' long, the diameter of circle the 60' crank covers (~188' versus ~63') is much larger/longer. So, to maintain the same RPM, your foot would have to cover much more distance, in the same amount of time (one minute), as the shorter crank. This is why it's easier to spin a short crank at a higher RPM.

This is an extreme analogy, but, I hope it illustrated the point.

Offline NEIL FROM BROOKLINE

Re: Short Crank Arm Purchase Tips?
« Reply #10 on: September 11, 2010, 10:21:24 pm »
Hi rvklassen, Thanks very much for your explanation!

Offline BrianW

Re: Short Crank Arm Purchase Tips?
« Reply #11 on: September 13, 2010, 10:13:26 am »
Another option I forgot to mention is to have cranks custom-shortened. There's a guy on the Web that offers this service. I haven't used him myself, although I've heard good things from others in the tandem community.

See http://bikesmithdesign.com/Short_Cranks/shorten.html

Brian

Offline NEIL FROM BROOKLINE

Re: Short Crank Arm Purchase Tips?
« Reply #12 on: September 15, 2010, 09:37:31 pm »
Hi briwasson,
Thanks for the tip. According to the website, the vendor usually recommends shortened cranks only for recumbents.
Neil

Another option I forgot to mention is to have cranks custom-shortened. There's a guy on the Web that offers this service. I haven't used him myself, although I've heard good things from others in the tandem community.

See http://bikesmithdesign.com/Short_Cranks/shorten.html

Brian

Offline NEIL FROM BROOKLINE

Re: Short Crank Arm Purchase Tips?
« Reply #13 on: September 15, 2010, 09:45:03 pm »
Hi
I finally had a chance to call Tandems East today. I thought the reply would be of general interest to the list. According to the vendor, the way crank arm shorteners work is that they effectively cut the crank arm length in half. So if you have 175 mm crank arms now, the crank arm shorteners will produce a 87.5 mm crank arm. For my purposes, that is too low. :(
Thanks,
Neil


If you want to try shorter cranks without initially investing in new cranks, you could use crank shorteners that bolt onto your existing cranks. These are commonly used on tandems to allow shorter (child) stokers to pedal, but would also work for what you want. An additional benefit is they will allow you to try different lengths to see what works best for you.

See http://www.tandemseast.com/parts/cranks.html#Crank%20Shrotners

Offline BrianW

Re: Short Crank Arm Purchase Tips?
« Reply #14 on: September 16, 2010, 09:00:41 am »
Hi briwasson,
Thanks for the tip. According to the website, the vendor usually recommends shortened cranks only for recumbents.
Neil

Huh, didn't see that before. I wonder if that's new.