Shimano bottom bracket compatibility

[TomC211]

Hi all, I recently bought a Trek 6500 SLR second hand. I was having a few issues with chain slipping and replaced the gear cassette and chain as these were pretty worn but now need to replace the crankset as well as the slipping continued (some teeth were pretty worn on this as well). The crankset that was on it was a Shimano Deore LX one (fc-m563, a three speed one) as this is a pretty old crank finding chain rings to work with the 94/58mm BCD has been hard and looks very expensive! So I think I’m going to opt for a new crankset. The bottom bracket that the bike is fitted with is a Shimano BB-UN54. It has a few measurements on it, the first is 73mm which I think is the shell length and the 107mm which I’m guessing is the spindle length? Does anyone know what cranks would be compatible with this bottom bracket? Any help would be great!

[Brucey]

most folk in this situation would use a different BB that suited the new cranks. Often it is both cheaper and better to buy new cranks and BB together. It is well worth looking at cranksets from spa cycles. A 110/74 x5 BCD triple might be the weapon of choice; no shortage of replacement chainrings here and nor is there ever likely to be, such is the ubiquity of this format.

[TomC211]

most folk in this situation would use a different BB that suited the new cranks. Often it is both cheaper and better to buy new cranks and BB together. It is well worth looking at cranksets from spa cycles. A 110/74 x5 BCD triple might be the weapon of choice; no shortage of replacement chainrings here and nor is there ever likely to be, such is the ubiquity of this format.

Hey Brucey thanks for the response!

How would I know what length bottom bracket to go for in this case? On Sheldon Browns website I found that the crank set originally on the bike had a chaniline of 47.5 with a 107mm bottom bracket. Should I aim to get the bottom bracket and chainset combination to reach this same figure or is there some "play" in this figure? I've found an option that I'm looking at using a Shimano Altus FC-M371 chainset (annoyingly a riveted one!) with a Shimano BB-UN300 thats 73/113mm with a 50mm chainline (closest I seem to have found to the 47.5mm. Would something like this work or with some more searching a non riveted combo with similar specs from Spa? Sorry for the rookie questions this is my first time venturing into the bike repairs world!

[geomannie]

To answer your question to Brucey, a Spa Cycles TD2 chainsets might be a good option. If you look at their website they specify the BB spindle length (116mm) and then recommend possible BB purchases.

https://spacycles.co.uk/m8b0s109p3052/S ... cral-Rings

[slowster]

There is usually some leeway when it comes to chainline for derailleur geared bikes. However, to state the obvious, it is best to try to keep as close as possible to the manufacturer's recommendations and/or to keep the chainline the same when replacing components, because it is not always easy to predict whether a change will work (satisfactorily) or not.

You have an MTB chainset. The TD-2 triple sold by Spa is a road chainset, so would require a wider than usual bottom bracket axle to give the same chainline as an MTB chainset.

Your chainset probably has 44/32/22 teeth. The Spa chainset can be assembled with 44/34/24, which is probably a good enough match, but many off the shelf Shimano triple MTB chainsets come with 48/38/28, and you might not like having a significantly higher bottom gear.

On Sheldon Browns website I found that the crank set originally on the bike had a chaniline of 47.5 with a 107mm bottom bracket.

According to Shimano it's 50mm - https://si.shimano.com/en/pdfs/si/F330D ... 00-ENG.pdf.

NB You will see from Brucey's post here - viewtopic.php?t=105385, that the 107mm and 110mm bottom brackets have the same right hand axle stickout, and so give the same chainline (110mm just gives a bit more clearance between the left hand crank and the left hand chainstay).

If you search online, you can find other 42/32/22 chainsets (175mm crank length), but it is likely that they will require a different bottom bracket length, e.g. https://www.sjscycles.co.uk/chainsets/s ... 22t-170mm/. That would be my preference over a chainset with riveted chainrings, unless you could buy the latter extremely cheaply (not so easy now that Brexit has made it much more difficult to buy from Germany, where the online sellers have a wide range of stock of such components at low prices, e.g. https://www.bike-discount.de/en/bike/bi ... x-8-speed/).

[TomC211]

To answer your question to Brucey, a Spa Cycles TD2 chainsets might be a good option. If you look at their website they specify the BB spindle length (116mm) and then recommend possible BB purchases.

https://spacycles.co.uk/m8b0s109p3052/S ... cral-Rings

Thanks for the response geomannie!

That option from spa looks like a really nice crankset but is a bit too expensive for my budget at the moment as I’ve got some other work I need to do on the bike as well to get it up and running well

[TomC211]

If you search online, you can find other 42/32/22 chainsets (175mm crank length), but it is likely that they will require a different bottom bracket length, e.g. https://www.sjscycles.co.uk/chainsets/s ... 22t-170mm/. That would be my preference over a chainset with riveted chainrings, unless you could buy the latter extremely cheaply (not so easy now that Brexit has made it much more difficult to buy from Germany, where the online sellers have a wide range of stock of such components at low prices, e.g. https://www.bike-discount.de/en/bike/bi ... x-8-speed/).

Thanks for the response slowster!

I really like that sun race option as well. I’m assuming to fit this I’d be searching for a 73x122mm bottom bracket, seems to be loads of options for this. Also how would this work comparability wise with the front deralier that I currently have which is a shimano deore lx? Would it just be a case of fitting the bb and crankset and trying to tune the deralier to shift with the new cassette, and then buying a new shifter if it’s not compatible?

[slowster]

SJS state "Manufacturer’s recommended B/B size: 68 x 122mm*...*This is a general recommendation and is meant as a guide only, it is NOT guaranteed to be correct for every single application."

What is not stated is whether that gives 47.5mm or 50mm chainline (and I think the information may not even be on Sunrace's own website). According to Bike Discount (https://www.bike-discount.de/en/sunrace ... 2/22-175mm) it should give a 50mm chainline.

In any case 2.5mm is unlikely to make a critical difference, and I think at worst you might need to slightly raise or lower the front derailleur. Note that Shimano did not increase chainlines of its MTB cranks to improve shifting; it did so to allow bikes to be built with clearance for wider tyres. As long as whatevever chainline you have/end up with works for you and your bike, that is all that matters.

That Sunrace chainset is marketed as 9 speed, but should be OK for 8 speed (and probably 7 speed or 10 speed). If you search the Bike Discount website you will also find a cheaper 8 speed Sunrace chainset.

NB Historically the chainsets on bikes like yours usually came as standard with 175mm cranks. There has been a trend towards shorter cranks since then for average height and especially shorter riders, and 170mm is more common now. Some of the chainsets listed by Bike Discount are 175mm, some 170mm, and some are available in both lengths. However, choice and availability in the UK is nowhere near as good.

As for bottom bracket, I would get either the Shimano UN300 or Tange Seiki LN-3922 in 73mm x 122.5mm/122mm size.