dimensioning a cartridge BB from an old axle

[bigbloke]

Hi Folks

So as regular readers of my drivel may know I have been doing up an old raleigh royale 531 frame from the
early 90s for my teenage daughter

It still has the original non cartridge BB axle.

Recently, I changed the chainset from an old Stronglight 99 triple to my own Shimano Alivio triple as its in
better nick & more in keeping with the bike's new red / black colour scheme.

Unfortunately, despite both being square tapered chainsets, the Alivio, akin to Brighton pier, sticks out half a mile and I will , going forward
need a shorter BB axle length

Im indebted to [XAP]Bob for clarifying that the middle chainset ring should align with the middle freewheel cog (but is that cog 3? cog4? or
halfway in-between on a six speed block) when we met earlier this week, but other than the absolutes that I <can> measure :

* The length of the BB tube on the frame itself
* The length of the incumbent (non cartridge) axle
* The distance between the rear of the new chainset and the BB tube
* The BB frame tube outer / inner diameters

How do I determine which of the bewildering array of cartidges will be the right one for daughter please ?

Regards

BB

[gregoryoftours]

I've had a think about it, it may be a long winded way of going about it, but this is what I've come up with:

Your current BB spindle is probably not symmetrical, but according to Sheldon Brown, Shimano BB spindles are symmetrical when installed in the frame - http://www.bikeforums.net/bicycle-mecha ... ength.html

As you're putting a mtb chainset on a road frame you will probably need a length of bb spindle different to that specified for use with that chainset, so Shimano tech docs might not be of much use in this case.

You'll first need to measure your chainline at the rear dropouts, and the current chainline at the front with the alivio chainset installed on the wrong spindle length.

http://www.parktool.com/blog/repair-hel ... e-concepts

1. Subtract the chainline at the back from the chainline at the front.
This will tell you how many mm you need to lose on the drive side in order to get correct chainline.

Then, as long as Sheldon is right you should be able to calculate your bb spindle length by doing the following-

2. Remove the Alivio crank from the existing bb spindle and measure how far the current bb spindle protrudes from the driveside edge of the bb shell (from the edge of the bb frame tube, not from the outer edge of the bb fixed cup)
3. From the figure arrived at in step 2, subtract the number arrived at in step 1. The result should be the amount of spindle you need sticking out on driveside.
4. So to get the total required spindle length for a suitable shimano bb you double the figure arrived at in 3 (Shimano symmetrical bb spindle) and add 68mm (the bottom bracket shell width).

Then you order your square taper bb - Shimano BB-UN55 is good quality and pretty cheap, so the options you'd choose would be English thread, 68 x (closest match to your ideal spindle length).

This lot assumes that the alivio crank would bed the same amount onto the shimano spindle as it does onto the existing spindle, if not you might be back to square one! But at least in that case, you can then correct the chainline by multiplying the distance it's out by 2 and adding it to the new but wrong spindle length to get the correct bb spindle length!

[gregoryoftours]

If the old chainset was correct in terms of chainline, it might be better to measure chainline with that installed at the front. Then measure current chainline with alivio crankset. Subtract the former from the latter in order to come up with the figure arrived at in step 1. in my previous post.
More interesting stuff on chainline from Sheldon here -
http://sheldonbrown.com/chainline.html

[Brucey]

you can make all this very complicated if you like but there is a simple method;

1) look up the SI techdoc for your model crankset.

2) Note the recommended BB and the chainline it gives therein

3) note how you want the actual chainline to vary from the one stated in the techdoc, and check that there won't be a clash etc.

4) Buy a different shimano BB to the recommended one, approximately x2 different vs the difference in chainline that you want. So for example if the usual BB is 115mm but you want a chainline 2mm inwards, look to buy a ~111mm BB of a similar type.

This method will give a good result about 19 times out of 20.

cheers

[gregoryoftours]

Yes Brucey's method is considerably simpler!

[bigbloke]

Thanks to All,

We now have a 119mm shimano BB fitted and it works perfectly.

the old axle was 125mm so it REALLY stuck out !

Regards

BB