Bottom bracket width.

tyred

Legendary Member
Location
Ireland
I have an old Soviet-made 3 speed with what is known as a Thompson bottom bracket more common on continental bikes with a 40mm X 70mm threadless BB shell and press-in cups. I want to put this bike back on the road as it's pretty decent to ride and a very practical utility bike and great for pottering about on. However the bottom bracket is knackered.
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I can buy the replacement press-in cups and the cones easily enough from Germany or the Czech Republic so I could rebuild it to original cotterpin specification easily enough.

However, for very little more money I could buy square taper bottom bracket spindles or even sealed bearing units. I have a spare single 46 tooth Stronglight chainset so a conversion to sealed bearings would seem a sensible upgrade.

My question is that my current cotterpin bottom bracket spindle is 140mm in length.

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Is there any way of me being able to calculate what width of square taper bottom bracket spindle would give me the same chainline? Or will it be trial and error? Obviously as a hub geared bike it needs to be more accurate than on as derailleur bike. A quick look shows 127mm, 132mm and 136mm bottom brackets available and there are probably other sizes if I looked a bit harder. This is the type of thing I'm looking at - https://www.ebay.co.uk/itm/Sunrace-...900271?hash=item2cda69612f:g:-CIAAOSw-b1d6Rhe
 

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Ajax Bay

Guru
Location
East Devon
If you have another bike (;)) with a cartridge square taper BB in (and you know or can measure its spindle), fit the single 46 tooth Stronglight chainset to the right hand side and measure the chainline with that BB/chainset combo. You know what chainline you need (sprocket on the old bike) and that will allow you to work out what length cartridge BB (pressfit?) you need. Every 2mm longer spindle shifts the chainring 1mm right.
 

rogerzilla

Legendary Member
You know the one in the eBay link isn't square taper? This one is: https://www.ebay.co.uk/itm/124356925263

Maybe thin cups exist that press into your shell and take a BSC or Italian-threaded BB but the adaptors would try to rotate when tightening, so bearing fit would be vital.

You can adjust the chainline at the back by quite a lot with most hub gears, so you could get it about right at the front then fine-tune at the sprocket.
 
OP
tyred

tyred

Legendary Member
Location
Ireland
You know the one in the eBay link isn't square taper? This one is: https://www.ebay.co.uk/itm/124356925263

Maybe thin cups exist that press into your shell and take a BSC or Italian-threaded BB but the adaptors would try to rotate when tightening, so bearing fit would be vital.

You can adjust the chainline at the back by quite a lot with most hub gears, so you could get it about right at the front then fine-tune at the sprocket.
I used the wrong link in my post as there is a sealed cotterless one listed too. If the cottered one came in 140mm form to match my own I would happily use it with my existing chainset. I just fancy a fit and forget sealed bottom bracket as this design is prone to taking in dirt in my experience so needs frequent rebuilds with clean grease (or you destroy it like I did!).
 
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