Bottom bracket offset

[AndyB1]

Apologies if this has been already covered, but I could not find anything using Search.

What sideways offset in the cranks is ‘normal’?
It is a rainy day so I have measured up the 3 bikes in the household to see how central the cranks are to the frame. As an easy distance to measure with digital callipers I have measured from the end of each crank to the nearby chain stay with the crank pointing towards the rear wheel.

On the three bikes in the house:
- Thorn Sherpa - unknown history but I have changed the BB and crankset in the past year.
RHS 12.5mm / LHS 16.5mm - so cranks offset to left by 2mm

- Oxford Bike Works Rohloff Tour - original parts.
RHS 21.9mm / LHS 20.4mm - so cranks offset to right by 0.75mm

- My wife’s Econic One ebike hybrid - original parts
RHS 26.3mm / LHS 11.4mm - so cranks offset to right by 7.5mm!!

Any thoughts?

[531colin]

If it doesn't give you or your wife a problem, I shouldn't worry about it.
The E bike has the biggest "error" and is likely to be the most difficult to "correct" at a guess, because theres an electric motor in there?

[AndyB1]

Hi Colin,
The ebike has a rear wheel hub motor, but it does have crank sensors, so as you say it is unlikely to be correctable…..I do intend to go and look at others at the shop and see if they are all the same though.

[PT1029]

The question you ask isn't a measurement people usually make on a bike as far as I am aware.
It is partially a function of the chain line, which is determined by the type of transmission you have (road, ATB, hub gear, gravel).
Rohloffs have quite a large chain line (55mm (?) from memory). If you replaced your Rohloff gears with single chainring derailleur gears, keeping the same chainset, you would need to move the RH crank to the left (shorter BB axle) to keep the chain line in line with the rear derailleur sprockets.

The fact that your 3 bikes have 3 different offsets probably answers the question, there is no "normal".

[slowster]

What sideways offset in the cranks is ‘normal’?

There is no such thing as 'normal' offset. What usually matters is chainline (and sometimes for particular bikes the clearance of the cranks and/or the inner chainring from the chain stays, i.e. if the bike is designed for wide tyres, especially an MTB). Crank models vary, so two different cranks may give a different chainline if used with the same bottom bracket. Most square taper crank manufacturers will specify what bottom bracket width should be used with the crank to give an appropriate or nominal chainline.

The asymmetry of different width Shimano bottom brackets is given in this post in 'Too good to lose' - viewtopic.php?t=105385. It appears that other bottom bracket manufacturers use the same dimensions. Note that these are for JIS (Japanese Industry Standard) square taper bottom brackets and cranks. ISO taper is slightly different and was mainly used by Italian manufacturers, principallly Campagnolo.

Standard (i.e. normally optimal) chainlines for derailleur gears are given here - https://www.sheldonbrown.com/chainline.html.

With a hub gear, and even more so with fixed, matching the front and rear chainlines is highly desirable/necessary, so the bottom brackets fitted by Thorn and Oxford Bike Works should combine with the respective cranks fitted to give the correct chainline for the Rohloff.

[AndyB1]

Thanks for the links Slowster.
I am happy about chainlines but I was surprised to see from one of your links that different width Shimano BB axles can have different symmetries. I had assumed that all BBs kept the pedals symmetric to the frame so that a longer axle would move the chainline (and RH pedal) out and the LH pedal out by an equal amount.

[Brucey]

Thanks for the links Slowster.
I am happy about chainlines but I was surprised to see from one of your links that different width Shimano BB axles can have different symmetries. I had assumed that all BBs kept the pedals symmetric to the frame so that a longer axle would move the chainline (and RH pedal) out and the LH pedal out by an equal amount.

I concluded that I like low 'Q', even if it comes at the expense of symmetry. Most of my bikes have been less symmetric than those you measured.