Author Topic: Novice Bike Mechanic Doing it wrong  (Read 2999 times)

chops

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Novice Bike Mechanic Doing it wrong
« on: January 02, 2015, 09:44:15 AM »
Hi all,

I am trying to fix my 20 year old Giant Rincon mountain bike and I think I'm doing it wrong.  I replaced my left hand crank arm with a new one which is slightly curved out more than the original which is completely straight (both are marked 170) but I didn't think a slight curve mattered.  2 short rides later, I am experiencing significant knee and back (tailbone) pain on my left side.  I'm thinking it has to be because of the slight curve in the new LH crank arm?

GuitarStv

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Re: Novice Bike Mechanic Doing it wrong
« Reply #1 on: January 02, 2015, 10:14:37 AM »
You only replaced one crank?  Your body wants symmetry while pedaling, and cranks are usually matched to provide this.  If you have a mismatched pair of cranks one leg is likely further out from the bike than the other, which means that one side is going to get uneven pressure as you pedal.  Sounds like a recipe for discomfort, as you won't be able to optimally adjust your seat for both cranks.

Can you get the matching right hand crank and put it on as well?

chops

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Re: Novice Bike Mechanic Doing it wrong
« Reply #2 on: January 02, 2015, 11:10:02 AM »
It's odd, I only felt discomfort last night after riding it for two short trips over the past two days.

Only the left hand crank was damaged mountain biking so I figured I only could replace one - looking online it seems that if you replace the right hand one you have to buy the entire crankset. 

I was previously under the impression that one LH crank is as good as another and that they are pretty generic - but clearly if one sticks out even a bit it can cause some pain.  So I will also be looking to find a flatter/straighter LH replacement?

Otherwise I think if I have to buy a new crankset ($50?) it will quickly evolve into a fix/buy situation as I can pick up a replacement (albeit used) TREK mountain bike around here for around $75. 

Any thoughts on that?

Thanks,

Chops

Chops

poorboyrichman

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Re: Novice Bike Mechanic Doing it wrong
« Reply #3 on: January 02, 2015, 11:17:31 AM »
If there's 3 things you shouldn't scrimp on those are:

Your shoes, your bed, your bike.

I'm assuming your old crank cannot be repaired and you using old parts that cannot be sourced due to discontinuation. Just replace the entire crank set. There's no guarantee replacing the entire bike with a new second hand ride isn't going to land you with a bike with a whole new set of troubles, stick with what you have.

Are you so poor/cheap that you won't spend $50 on repairing such an important tool? Why not look if you can pick up a second hand set, or zombie some parts of an old unloved bike in a friends garage?
« Last Edit: January 02, 2015, 11:21:14 AM by poorboyrichman »

chops

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Re: Novice Bike Mechanic Doing it wrong
« Reply #4 on: January 02, 2015, 11:51:33 AM »
Thanks for your input, poorboy, but my priorities for spending actually don't fit into your "3 things you shouldn't scrimp on [being]...your shoes, your bed, your bike."  I'm happy riding whatever bike will get me around.  And I guess I really shouldn't mention how inexpensive my shoes and and my bed are ;)

I'll keep looking for a closer fit LH crank arm or maybe that TREK.


GuitarStv

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Re: Novice Bike Mechanic Doing it wrong
« Reply #5 on: January 02, 2015, 11:57:17 AM »
Many places that do bike repair would probably have a crank set pulled from an older bike that you could buy for less than 50$.  See if there are any bike co-ops in your area.

poorboyrichman

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Re: Novice Bike Mechanic Doing it wrong
« Reply #6 on: January 02, 2015, 12:00:16 PM »
Actually, my shoes and bed were cheap too so I don't even live by my own standards! Whereas biking is my hobby as well as main form of transportation, so it's something I'm happy spend a little more on from time to time.

In all seriousness I would try to source some unwanted cranks from somewhere before dropping on $75 on a new bike, if you don't have a backup bike, I'd pick up the TREK anyway, as if you have any mechanical faults your not out of action too long. I'm always looking for an excuse for a 'new' bike too ;)

shuffler

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Re: Novice Bike Mechanic Doing it wrong
« Reply #7 on: January 02, 2015, 12:00:36 PM »
I replaced my left hand crank arm with a new one which is slightly curved out more than the original ...
What you're describing is called the "Q-Factor" (or sometimes the "tread") of your pedals.

I don't think anyone would recommend having imbalanced distances on either side.  :^S

chops

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Re: Novice Bike Mechanic Doing it wrong
« Reply #8 on: January 02, 2015, 03:50:32 PM »
Shuffler  - thanks for the link!  I didn't know what to call it, you've definitely helped me explain what I was getting at for slight difference in my crank arm replacement width. 

Unfortunately, it doesn't seem like any crank arm brands (that I've seen online, at least) post their Q Factor.  This is really surprising to me even though I'm a newbie at this bike mechanic thing, because it seems pretty F'in important.

So...it looks like to the biking co-op it is!  Good suggestion GuitarStv.  I'm sure there will be some folks there that can either get me a used crank or direct me to an equal Q Factor LH crank arm.

 - Chops

kendallf

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Re: Novice Bike Mechanic Doing it wrong
« Reply #9 on: January 02, 2015, 09:00:55 PM »
I am relatively insensitive to Q factor and ride bikes with all sorts of cranks, switching frequently, with no issues.  Most cranks don't list Q factor because it's (presumably) matched on each side.  Now if you get a set that's significantly mismatched like yours, I can see it being a problem.