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Crank Arm Offset


Stretched@Birth
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I was swapping out some Shimano Deore cranks for Icon cranks, assuming that the "offset" is the same on all crank arms. Spin the crank and "thunk", it touches on the chain stay protector. Removed the protector, so now it clears the fram, but by a ball hair...

 

So, the question is, do crank arms come in differing "offset", or are there varying length BB's available? Can I put in a spacer to space it out a little? A ball hair is too close for comfort for me...

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there are many different lengths of BB's

107,108,110,113,115,118mm the list goes on.

 

Doubt that a spacer can be put on, there will be less room for the crank to grip, might be dangerous.

(dependent on what BB and crank you using?)

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there are many different lengths of BB's

107,108,110,113,115,118mm the list goes on.

 

Doubt that a spacer can be put on, there will be less room for the crank arm to grip, might be dangerous.

(dependent on what BB and crank arms you using?)

Well, the BB needs to be replaced in any case, will measure it up, and ensure I get one that's about 5mm longer - thanks for the feedback!

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By changing the BB you will change your chainline which may result in shifting problems. Search the web and read up about it or get help from somebody who understands the principles.

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Ja, get a bigger BB. After 25 years riding, my lbs discovered mine was also a ball hair out - the chainring was fine when the bike was on the stand, but rubbed the chainstay when riding. Now that it's fixed, it's like a completely different bike.

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By changing the BB you will change your chainline which may result in shifting problems. Search the web and read up about it or get help from somebody who understands the principles.

Thanks Big H, it's going on my SS though, so just need to ensure the chainline is right using the spacers in the SS conversion kit...

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I recently replaced my chainset and discovered afterwards that spindle length is important. Just check the manual what the required length is.

 

In my case I originally had 124mm BB and needed a 113mm

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Lets get the pedantry out of the way first. Crank already means arm. So all you need to say is crank. If you want to refer to the left or right crank, you say left or right crank. :)

 

Forgive me, I go even crazier when someone talks about DVD's.

 

OK, crank offset is called Q-factor. A Q-factor of 200mm is the distance beween the two cranks, measured from the outside of each crank, where the pedal would seat. Note that this does not take into account the thickness of the crank so the absolute figure cannot be used to predict crank clearance. However, we assure thickness as that of the length of the thread on a pedal.

 

If your cranks touch the chainstays, the solution is to use other cranks.

 

The BB is not the place to fiddle with the Q-factor. On modern hollowtech cranks like that supplied by Shimano and compantibles, the crank axle/spindle is designed for a BB width of 75.5mm. Some BBs are 68mm and therefore the BB cups require up to three spacers to make the dimensions work with the standard crank axle length. MTB BB cups are supplied with three spacers. If the BB shell is a certain width, you use X-number of spacers to get to the desired overall width.

 

Now, if you fiddle with the BB width by putting more spacers in than is called for, the left crank on Hollowtech will not have enough axle to engage properly.

 

If you fiddle with BB width by manipulating a square taper BB, you mess with the chainline, which is the neutral line from the middle front chainring to the middle back sprocket. (Between the small and big blade for road bikes).

 

Therefore, the only foolproof remedy is another crank with wider Q-factor.

 

However, you can push the limits and compromise on perfect chainline, if you want. It is not a train smash. It just leads to quicker wear of drive train components.

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OK, another nice sticky from JB.

 

 

What I noticed on my setup was that fitting my chainset to my old square taper BB (124mm) threw my chainline out, as soon as I replaced it with the correct BB (113mm), chainline was in a happy place again and did not need to set anything except the FD.

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Lets get the pedantry out of the way first. Crank already means arm. So all you need to say is crank. If you want to refer to the left or right crank, you say left or right crank. :)

 

Forgive me, I go even crazier when someone talks about DVD's.

 

OK, crank offset is called Q-factor. A Q-factor of 200mm is the distance beween the two cranks, measured from the outside of each crank, where the pedal would seat. Note that this does not take into account the thickness of the crank so the absolute figure cannot be used to predict crank clearance. However, we assure thickness as that of the length of the thread on a pedal.

 

If your cranks touch the chainstays, the solution is to use other cranks.

 

The BB is not the place to fiddle with the Q-factor. On modern hollowtech cranks like that supplied by Shimano and compantibles, the crank axle/spindle is designed for a BB width of 75.5mm. Some BBs are 68mm and therefore the BB cups require up to three spacers to make the dimensions work with the standard crank axle length. MTB BB cups are supplied with three spacers. If the BB shell is a certain width, you use X-number of spacers to get to the desired overall width.

 

Now, if you fiddle with the BB width by putting more spacers in than is called for, the left crank on Hollowtech will not have enough axle to engage properly.

 

If you fiddle with BB width by manipulating a square taper BB, you mess with the chainline, which is the neutral line from the middle front chainring to the middle back sprocket. (Between the small and big blade for road bikes).

 

Therefore, the only foolproof remedy is another crank with wider Q-factor.

 

However, you can push the limits and compromise on perfect chainline, if you want. It is not a train smash. It just leads to quicker wear of drive train components.

 

Thanks Johan

What if he is using say a British Shimano Octalink BB with a spindle lenghtg of 110mm (mistakenly) wouldnt changing to a 113mm solve the problem?

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Sorry that I upset you Mr Bornman, will never refer to a crank arm again... Q-factor was the answer I was looking for, I always assumed that the offset was standard. Lesson learnt, and again, it's logical. Lucky I'm not too concerned with chain line, as it is on an SS. The crank does pass the chainstay, it's just very close (3mm), but it can work.

 

Thanks for your wise input as always.

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I personally like the word crank arm. Its perfectly descriptive and specific. It means that I'm not referring to any other part of the crank inadvertently i.e. the crank spiders or any crank threads or the crank shoulder,or the crank end.

No apologies from me.

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  • 3 weeks later...

Update on this post, incase anyone was interested. Ended up changing out the 110mm bb to a 113. Then added a spacer (around a 2mm spacer) on the drive side thread of the cartridge and now have the required spacing and the left and right cranks equally spaced. Juggled the spacers on the SS conversion kit to sort out the chain line, and all is good in the world...

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