Thread: Spindle length
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Old 03-07-18, 12:50 AM
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dddd
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SkyDog75 is correct about the asymmetry.

The industry term for an amount of "offset" usually refers to the right end of the spindle protruding that amount (perhaps stamped as +1.5) further from the bb centerline than the left end of the spindle.This is quite different however than offsetting a symmetric spindle by that same 1.5mm, which results in the right end of the spindle then protruding 3mm further than the left end.

So we can use a symmetric cartridge bb an put a spacer under the fixed cup in some cases (as when the adjustable cup has no mini-flange interfering with it's further insertion into the bb shell), but must be aware of what (as in how much) we are trying to accomplish in terms of how far that both crankarms are positioned out from the bb shell faces.

The previously quoted figure of "4.5mm added to each end" of a JIS spindle's actual length to reflect it's effective length is incorrect.
The actual difference in overall effective length between same-length JIS and ISO spindles is very close to 4.5mm, so only one-half that amount is actually added to each end.
I believe that the error that I cited was due to someone using an assumed 2-degree included angle for the taper instead of the actual 2-degree angle off of the spindle's centerline (or 4-degree included angle), which would cause an error equal to 2x.

As for the tapers themselves, ISO is the taper used on all square-taper Campagnolo cartridge-style bottom brackets, of which many examples can be commonly found for use as a dimensional reference.
Oddly enough, the Sugino Mighty spindle taper is identical to this ISO standard.
And even more oddly, the Mighty bottom brackets were often a very good substitute for earlier Campagnolo bottom brackets, even though those pre-cartridge Campagnolo bottom brackets had a slightly wider-thicker spindle taper than the later ISO cartridge units.

I adopted a reference-point measurement method for locating a taper thickness measurement at a fixed and uniform distance from the end of the spindle, equal to the thickness of a caliper's jaws (typically 1/8"), since a caliper and spindle end can be easily pressed against any flat horizontal surface for easy field comparisons between any two or more spindle tapers. One would not want to be trying to measure the taper's thicknees at the very end of the spindle since many spindles have these corners rounded off. Note here that the actual thickness of one's caliper is not what is important if the same caliper is used for collecting all measurements, since we are only comparing thickness of the spindles (and then perhaps using a little trigonometry to equate these measurements to a spindle's effective length).

Pictured below is the practical field measurement method for comparing spindle tapers, where the thickness of the caliper's jaws defines the distance that each spindle taper is measured from it's end of the spindle.
The difference in thickness can then be equated to the difference in the effective length of each end of the spindle, i.e. how far that the crankarms are positioned out from the bb shell.

The formula to use when equating X (spindle end length difference) to 2Y (difference in overall thickness of the spindle ends) is: X = Y / Tan 2 Degrees. Note that is Y is only one half of the 2Y thickness difference since there are two tapers contributing to the thickness of the spindle end. Each flat is 2 degrees away from the centerline. 2Y is thus the thickness difference one measures directly.

Each .1mm of thickness increase thus adds 1.43mm of effective length to each end of the spindle.

A JIS spindle measures roughly .2mm thicker than an ISO spindle which adds about 2.85mm to the effective length of each end of the spindle, or 5.7mm to it's overall effective length.
I encourage anyone who is messing around with different tapers to do some of their own comparative measurements using a cheap digital caliper as shown.


Last edited by dddd; 03-07-18 at 01:15 AM.
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