How to measuere "inner bearing length"? 15 October 2018 at 5:54PM in Public transport & cycling 6 replies 1.2K views

tronator

Hi all,


I'd like to replace my existing 42/32/22 crankset with a 48/38/28 one.


https://www.amazon.co.uk/gp/product/B00B7084SM/ref=ox_sc_act_title_1?smid=A3P5ROKL5A1OLE&psc=1


The description says "Recommended. Inner bearing Length: 123 mm (chain line 50 mm)"


How do I measure it on my bike? I googled it, but everything I found was about inner and outer diameter of bearings. "Lenght" doesn't make sense to me. The only thing I can think of is the overall length of the bottom bracket. I don't want to remove my crankset right now to measure that, but if the chain line on my bike is 50mm measured from the center of the frame to the middle chain ring, will this crankset fit?

Nebulous2

What bike do you have? What make of cranks are fitted?

Does this help?

https://janheine.wordpress.com/2013/01/02/bottom-brackets-demystified/

tronator

Nebulous2 wrote: »

What bike do you have? What make of cranks are fitted?

It's a ~20 year old mountain bike bought in Germany. It has a Shimano crank with a sticker reading "Shimano IG chain only" and a capital A incorporated into the Shimano writing.

Nebulous2 wrote: »

Does this help?

https://janheine.wordpress.com/2013/01/02/bottom-brackets-demystified/

Not really. I don't want to change the bottom bracket. The only useful info I could gather from the link is, that the spindle length seems to be important and I think it is the same as the inner bearing length I mentioned in my first post. Is that right?



If so, then I need to remove the cranks and measure it...

wongataa

tronator wrote: »

How do I measure it on my bike? I googled it, but everything I found was about inner and outer diameter of bearings. "Lenght" doesn't make sense to me. The only thing I can think of is the overall length of the bottom bracket. I don't want to remove my crankset right now to measure that, but if the chain line on my bike is 50mm measured from the center of the frame to the middle chain ring, will this crankset fit?

You need to work out what type of bottom bracket your bike has. Then you can buy a crankset that fits the bottom bracket you have.

tronator

I remove the cranks and the left side of the bottom bracket looks like that





Does it look like a JIS square taper bottom bracket?



The spindle length is ~115mm, so not the 123mm as written in the product description of the crankset. So will this crankset fit?


Or should I replace the bottom bracket too? Might be time after ~20 years...

verybigchris

A chainset that needs a 123mm spindle will still fit on a 115mm, but the chainline will be off and chances are you won't be able to adjust the front derailleur far enough to get onto the big ring.


That looks like a standard square taper so a replacement will only cost about £12 and might last another 20 years! If you've not swapped one out before, you'll also need a splined tool to fit/remove it (about £3 on ebay), a big spanner and some grease.

tronator

Thanks, new bottom bracket it is then.



Never done it before, but I have all the required tools and there are plenty of videos on Youtube.