Chainwheel Swap question

Gasman

Old enough to know better, too old to care!
I ride a rather ancient 12 speed tourer. Since I hardly ever get onto the big ring and could often use some more low gears it occurred to me to swap the current double chainwheel for a MTB triple, purchased cheap off Fleabay (or rescued from whatever I can find dumped in the local undergrowth). By way of experiment I took the chainwheel off my MTBSO pootler and tried it on the tourer. Both bikes have square taper spindles but the triple wheel didn't seem to sit far enough onto the tourer spindle. The chain could go on the small ring but it seemed the front shifter would struggle to reach the middle and had no chance of getting to the outer ring.

Any advice from the assembled panel would be appreciated.
 

rogerzilla

Legendary Member
It's a lot more complicated than that. You'll need a BB of similar spindle length to the one on the MTB and almost certainly a new front mech with a deep cage. And a new rear mech if your existing one has a short or medium cage.

At least your front shifter is probably friction, so should do the job as long as it doesn't run out of travel.
 

Big John

Guru
I agree with rogerzilla. It's not just a case of switching chainsets but if you have the time and the budget there's a nice little project for you 👍
 

T4tomo

Guru
getting hold of a double chainset with smaller chainrings would be a better option. you then probably only need to twiddle the FD height a bit, although you may need a different length BB, depending on what chainset you swap for
 

rogerzilla

Legendary Member
110 BCD chainsets can have quite a small inner ring. SR used to make a freaky touring double that could take a REALLY small inner ring, but it was a unique BCD and never caught on.
 

Sharky

Guru
Location
Kent
To establish the starting point, what are the number of teeth on the chain rings and its BCD and the number of teeth on the largest rear sprocket. There may be some options available to you before doing major upgrade. Is the rear a cassette or screw-on freewheel and how many sprockets?
 

Chris S

Legendary Member
Location
Sparkhill
I'm not disputing what the previous posters have said but your chainring may have been nearer the frame because you might have put a JIS crank on an ISO spindle:
"If you install an ISO crank on a J.I.S. spindle, it will sit about 4.5 mm farther out than it would on an ISO spindle of the same length."
https://sheldonbrown.com/bbtaper.html
 

Ajax Bay

Guru
Location
East Devon
If you wish to pursue this, I'd recommend procuring a touring double and the BB with the recommended spindle length.
Your bike likely has cup and cone threaded bottom bracket and a long spindle (which is why when you put the triple on it's too far out).
You'll need to check measurements but most likely it'll be BSA threaded 68mm BB shell so best BB replacement is a UN-300 or a Stronglight BB (both 'cartridge' square taper).
Could go for a triple (as per OP) but one with bolted on chainrings and a shorter BB spindle (see above). Replace the large ring with a chain guard and just use the two smaller rings. Shifting would be better with a front mech with a cage designed for a tiple, but you'd probably get away with just dropping the current mech down the seat tube half an inch.
 
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OP
Gasman

Gasman

Old enough to know better, too old to care!
" ISO crank on a J.I.S. spindle"

That was one of the few things I absorbed from the avalanche of information when I Googled 'bottom bracket spindle sizes'!

Current chainwheels are 52/42, rear wheel has a 14/28 6 speed freewheel block. BB was cup and cone but I replaced it with a cartridge setup back in...in....well, a while ago.

I had a quick look around for a double chainwheel but they all had a large wheel much the same as I have. The only smaller setup had 150mm cranks. I feel a little more research is required.
 

Ajax Bay

Guru
Location
East Devon
If you replaced it with a cartridge BB then it will very likely be a JIS spline.
I looked at the Spa Cycles when constructing my last post but saw that the double tourer was £70, so didn't recommend. Sure they had some cheaper ones in stock only a month ago. They have got cranksets at £27 and a 48t and a 38t rings (110 BCD) for £6 each (and add in chainring bolts x5).
The OP has a 52-42. They'd notice a 48-38 and don't want to go too low with that 14t smallest sprocket on the rear. But cheaper to get a triple working and forego the large ring thereon.
 
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