I know, dark ages technology right?
My son's new build is going to have to have a sqaure taper BB since the only cranks at 150mm I can find are sqaure taper.
I've not used or worked on a sqaure taper setup since 2003.
Any advice on what spindle length I might need for a 73mm BB shell and a 48mm chain line?
At the moment the only way I can see it's to get a BB and offer it up, which is less than ideal if it's the wrong size.
depends on the cranks, usually 113mm for Shimano stuff.
Can you offer some up on some BBs in a shop, or similar? I've found they tighten on by 3mm from pushing on to the axle.
And yes it depends on cranks
As above, depends on the cranks: check the specification for those. If you're running the cranks with one ring fewer than they're designed for, then you may want to add 5mm to the spindle if you're omitting the outer ring, or (less likely) subtract 5mm if you're omitting the inner ring. If you can't find a spec, the suggestion of 113 is a good one as the most likely candidate and the least likely to be too far off (exact chainline isn't normally critical, though some setups can be fussier than others).
What cranks and what configuration - i.e. single ring, single & bash etc.
When I did my lad's Clash because I was running a single and bash on the Suntour Cranks I had to go with a 122mm bottom bracket to get the 50mm chain line because the single was mounted on the inside of the spider.
I had an old 113 BB that I put in the frame, mounted the cranks and worked it out off that.
If it helps SRAM NX cranks come in a 155mm option.
Thanks gents, all useful.
Dirkpitt74 - it's the same cranks as you used. After I saw your thread I sourced one. I'll be running it single ring without a bash. Looking at the pictures (it's not arrived yet) it looks like I'll be running the ring in what is effectively the outer position.
If it helps SRAM NX cranks come in a 155mm option.
Yes they do, but stock seems to be virtually absent and I'm reluctant to pay 90+ for something he'll grow out of pretty quick.
If it's these cranks then you'll get a 48mm chainline on the middle ring position with a 122mm spindle:
https://www.srsuntour.com/products/xct-jr-t312?variant=27528535619
@Scienceofficer if I get 5 minutes later I'll measure what I have to the bash guard in the outer position and work it backward for a 48mm chain line.
I bought the t202, SR XCT JR which is the 7/8 SPD version. It was a bit cheaper and I'm not planning on using the rings.
This link:
https://www.bikeinn.com/bike/sr-suntour-crank-set-xct-jr-t202/136826974/p
Suggests it might need a 118mm spindle.
I switched Suntour cranks for SRAM... and the sq. taper was different... (Japanese vs Euro)
The Suntour cranks (came with the bike) were just horrible... and not so much engineered as just what they were...and the square wasn't really square... these came with the bike though but the lack of quality put me off trying other Suntour cranks..
The SRAM cranks were not much more. ££.(maybe a tenner difference). but everything about them (for the price) was better... We needed 140mm as he was just getting onto a 24 at the time so the SRAM cranks I got were S600's in 170 or 175 that I cut down to 140mm... Everything about the S600's just felt a lot better... and they actually went onto the Square taper properly but when they did they mounted further in...
Basically a lot of that is feel... but the SRAM cranks just seemed to be nicer... despite being lowest end... (stuff was just machined properly and they weighed less as well)
I replaced the BB with a UN55 (which was 2/3rd of the weight and £15 or less) ... and actually just went into the LBS (Evans) where we offered them up... I might have paid £1 or so more... but it was easier than messing about.
If your convinced you need 150mm that's one thing.. if you can use 155mm then you get the option of SRAM external BB's...
Before deciding I'd strongly recommend you use the chart and method at the bottom of this page.
https://highpath.co.uk/crank-shortening/
It's a lot of messing and such if you go to all the trouble for the wrong size....
If you're feeling flush then there are 140 options from SJS cycles and 140 and 150 from Trailcraft...
I used 113mm with thorn cranks on lewis's blast and ripcord. currently using 155nx cranks on his scout. The suntour cranks take a slightly wider axle 118mm IIRC.
The costs and faff for crank shortening can't really compete with getting a decent thorn crank of the right length from SJS for 50 quid.
It may be that the SR cranks aren't up to the same standard, but it seems that plenty of people are happily running them and I'm not looking to provide top end MTB kit to an 11 year old that is an occasional rider, especially when budget is tight. He's got far worse on his current bike now and has got on fine.