How do I work out what length bottom spindle, is it this way

[swordheart]

hello all I am new here so I'm not very knowledgable as to what parts are called so please bare with my

I am moving all the parts brake wheels handlebar gears cranks from one frame to another, I am fine with all the easy stuff but my question is with the bottom spindle length the cranks attatch to, I am not sure that the spindle and cranks go together so need advice on getting new sealed cartridge with regard to the spindle length.
This is how I plan to work it out am I correct in doing it like this.

I have 6 gears on the back wheel and 3 on the front crank, so I was thinking if I measure from half way between the 3rd and 4th gear on the rears to the centre of the bike frame say it 40mm should I get a spindle that puts the middle gear on the front crank 40mm from the centre of the frame ?

I am probably talking bull but hey ho if you don't ask you don't find out

Phil

 

[Retro Spud]

Hi Phil

to be honest I couldn't be ar5ed to work it out that way, as there is a lot of archived info available

Are your cranks branded if so they will most likely be in a data base, but to ensure your get the right info you'll need to know the model number which should be stamped on one of the legs

check out Sheldon http://www.sheldonbrown.com/bbsize.html#shimano

hope this helps

 

[CassidyAce]

Re:

Welcome. You're on the right lines.

Park Tools has a lot of useful information about chain line (and much else). Check here: https://www.parktool.com/blog/repair-he ... -section-3

 

[ishaw]

Re:

Most cranks have a specific spindle length, so if you know if what you have, various online sources like Sheldon brown and others will tell you what you need to know.

Don't forget that it's not just axle length, you need the BB shell width which should be 68 or 73mm.

 

[swordheart]

Re:

Hi thanks for replies, my crankset just says PRO - 170, I suppose its just a bog standard cheapo set.

 

[focomat 1c]

Re:

Hi Phil

Your well on the way to sorting your problem . You just need to do a few more measurements to find the solution

Typically your chain line ( measured from your existing rear wheel as you have already done ) will be about 47.5mm
from the frame c/line

If you now measure the distance from your middle ring c/line to the outside face of the crankset , you will get a figure of about 27mm

Now insert the square taper of the spindle into the crankset , and measure from the outside face of the crankset down to the
visible end of the square taper , you will get a figure of about 13.5mm

The calculation becomes : 47.5 + 27 - 13.5 = 61.0 ( the distance of the end of the square taper from the frame c/line )

Multiply x 2 to get the correct spindle length , in this case 122mm

The width of the required unit can be measured directly from the frame as mentioned above . I have never come across any
data relating to the insertion depth of square taper spindles into older cranksets . The numbers you have found
relate to the length of crank arm ( measured spindle c/line to pedal c/line ) in your case 170mm

Don't get too fussy about chain line , bearing in mind that most of the time the chain will not be running straight anyway .
However do bear in mind that while you have 18 gears in theory , it would be wise to limit yourself to just 9 gears in practice .
Each chainring lining up with the 'nearest' 3 rear sprockets , saving wear to both chain and gear rings

Hope this is of some help

 

[FluffyChicken]

Re:

post some goo d pictures fo the crank, also the length anf some pictures of the bottom bracket.


PRO might actually mean something.
170 is the crank arm length in mm

 

[swordheart]

Re:

Excellent thanks for the help its much appreciated, I now have it in my head as to what I need to do

As for pics of crankset its very difficult to do as the crankset is black and I can't get any detail into any of my pics.
Thankyou all