Bottom bracket/Crank taper help

[BADGER.BRAD]

I have just built a single speed MTB for my lad from an old cheapo bike with very dodgy gears. At first I retained the full cassette and front chain wheel set up ( 3 chainwheels riveted together) But on his request and my pursuit of the simplest cheap bike possible have separated two old cassettes used the spacers and converted the rear cassette to having only one chain wheel.The next job is to replace the front chain wheels with a single chain wheel set up ( without costing any money ) the problem is I have tried four crank assemblies 3 of which put the chain line out so far as to be outside the frame and the original which works but cannot be separated as this would leave me with the large ring and leave the gearing too high. My question is if I were to try and source a suitable crank/single chainset assembly how would I know what taper/crank would work with the bottom bracket I am already using (there seems to be no makings in the bottom bracket !) The bike has now been painted the mattest black I have ever seen and looks so ugly it's beautiful ! I'll try and add a photo tomorrow.

Thanks for any help

Brad

 

[Deleted member 1258]

Do you know the BB make? The taper is likely to be JIS or ISO, the JIS taper is the most likely.

http://sheldonbrown.com/bbtaper.html

 

[RecordAceFromNew]

I have just built a single speed MTB for my lad from an old cheapo bike with very dodgy gears. At first I retained the full cassette and front chain wheel set up ( 3 chainwheels riveted together) But on his request and my pursuit of the simplest cheap bike possible have separated two old cassettes used the spacers and converted the rear cassette to having only one chain wheel.The next job is to replace the front chain wheels with a single chain wheel set up ( without costing any money ) the problem is I have tried four crank assemblies 3 of which put the chain line out so far as to be outside the frame and the original which works but cannot be separated as this would leave me with the large ring and leave the gearing too high. My question is if I were to try and source a suitable crank/single chainset assembly how would I know what taper/crank would work with the bottom bracket I am already using (there seems to be no makings in the bottom bracket !) The bike has now been painted the mattest black I have ever seen and looks so ugly it's beautiful ! I'll try and add a photo tomorrow.

Thanks for any help

Brad

With the large ring in front is the gearing too high even with one of the larger cassette sprockets at the back?

It is usually easier to source bb for specific chainset than the other way round - there are numerous varieties of chainsets but very limited number of bb lengths, and chainsets are usually more expensive than bbs.

If you want to use one of the other chainsets, once you have decided which you want to use, firstly you need to decide what chainline you want to achieve. For a mtb the front chainline to aim for should probably be around 47.5mm. Since most mtbs have rear dropout distance of 135mm, to achieve e.g. that at the back you might want to position spacers such that the centre of the rear sprocket is around 25mm from the drive side lock nut.

There are various ways to figure out how to make a specific chainline up front. You can usually find out which bb will give a particular chainline from the chainset manufacturer's spec. (and deduce from that), or alternatively you can measure the actual (excessive) chainline and the length of the existing bb and from those two figures deduce what would be the right bb to achieve the desired chainline (noting spindle length increments are not necessarily distributed symmetrically - if unsure ask here).

Regarding bb sourcing the actual replacement may depend on what type of bb you have. If it is a traditional cup and cone one you might be able to just get the right axle, see the first table here for choice of lengths. Alternatively you can replace the whole bb with a cartridge one - some are pretty inexpensive (rpm, un26 etc. from Chain Reaction e.g.). BTW it is more likely that you have JIS taper. You need to measure the bb shell width.

If you are using the outside ring of a triple you also need to ensure no clash between chainset and frame if you want to make the chainline of that ring to be less than 57mm, since many triples are designed for 50mm chainline (for the middle ring).

 

[BADGER.BRAD]

Thanks for all the help, much appreciated. I will measure it a little later as I wish to service my commuter bike in the workshop later. It is a cheap cup and cone arrangement ( no name or numbers) which in all fairness is now working fine with the original front chain set used on the middle ring but doesn't look very nice. If I separate this ie: drill the rivets holding it all together I'll end up with the big ring but to then get the chain to line up the rear sprocket needs to come right the way over to the frame and then I cannot get the clearance to use a large enough sprocket to obtain reasonable gearing ! I do have a old cartridge BB which I changed some time ago with a shorter axle to suit one of the chainsets I was trying to use ( knowing my luck it won't fit) but I think from what you say my best idea is to change the axle for a shorter one.

Thanks again for the help.

Brad

 

[BADGER.BRAD]

Here's a few pics of the beast ! I think the angle of the photo from the rear makes the chainline look out but this is just a result of the angle of the photo. I went to visit my dad earlier and then to the local tesco express about 6 or 7 miles in all and it seems to work fine Hopefully for the local chavs/ thieves it is too uncool !