Brompton One Piece Chainsets

[djnotts]

Does anyone know if the "holes" in pre-spider crank Brompton chainsets are at a BCD which matches any standard 'rings (to bolt on an inner ring for manual shifting)?

[SA_SA_SA]

I think some are at 86mm (eg the old Stronglight size available from Spa) and from very rusty memory some may be/also be 122mm but don't quote me

[djnotts]

Thanks for that.

[rmurphy195]

Somewhere on Sheldon Brown there is a method for measuring BCD, so you can select a suitable replacement from Spa's parts list. If its an odd size so the replacement ring cannot be obtained, or costs an arm-and-a-leg, then (asssuming you have the square-taper BB), you should be able to fit a replacement chainset - a nice light alloy one! - with a single chainring.

[djnotts]

Somewhere on Sheldon Brown there is a method for measuring BCD, so you can select a suitable replacement from Spa's parts list. If its an odd size so the replacement ring cannot be obtained, or costs an arm-and-a-leg, then (asssuming you have the square-taper BB), you should be able to fit a replacement chainset - a nice light alloy one! - with a single chainring.

Yes, if I had one to hand easy to find the bcd and then to find if there are or ever have been any to "fit"! And fitting say a compact double only requires estimating axle length. I'm simply trying to guestimate relative costs of converting a 3 speed to a 6 speed by various methods - and whether or not savings on buying a 3 rather than a 6 would be lost by conversion expenditure!

[SA_SA_SA]

Are you buying 2nd hand?
The modern Brompton crankset is a 130mm BCD spider (why couldn't they have chosen 110mm.....).

[djnotts]

Oh yes - always let someone else take the first depreciation hit on bikes - altho' Bromptons drop less than most on first re-sale. I have one with the new spider cranks, but casually looking for a second to tinker with!

....and the current 130 bcd spider arms are shaped for only one ring, bolted on the inner face.

[Gattonero]

Does anyone know if the "holes" in pre-spider crank Brompton chainsets are at a BCD which matches any standard 'rings (to bolt on an inner ring for manual shifting)?

I'm not sure this is a good idea, those one-piece chainsets use a full size ring pressed onto the crank. It can flex a lot so a double chainring may be used only for a leisure bike and light rider.
Also bear in mind that if the crank has had some good use and/or heavy rider, the joint between the crank and the ring may be weak by now.

[Gattonero]

The modern Brompton crankset is a 130mm BCD spider (why couldn't they have chosen 110mm.....).

Because with a 130mm BCD you can fit a 39t chainring.
On a 16" wheel a 39t gives a very low gear (3.3mt or 41 inches) if combined with a 16t sprocket.
Not meant to be rude, but lower than that, you better walk?

[djnotts]

The modern Brompton crankset is a 130mm BCD spider (why couldn't they have chosen 110mm.....).

Because with a 130mm BCD you can fit a 39t chainring.
On a 16" wheel a 39t gives a very low gear (3.3mt or 41 inches) if combined with a 16t sprocket.
Not meant to be rude, but lower than that, you better walk?

Well.....may you never find out that for even soft hills 20" gear is all that will let you ride up - and that walking is just as physically draining.

[Gattonero]

The modern Brompton crankset is a 130mm BCD spider (why couldn't they have chosen 110mm.....).

Because with a 130mm BCD you can fit a 39t chainring.
On a 16" wheel a 39t gives a very low gear (3.3mt or 41 inches) if combined with a 16t sprocket.
Not meant to be rude, but lower than that, you better walk?

Well.....may you never find out that for even soft hills 20" gear is all that will let you ride up - and that walking is just as physically draining.

Special requirements require special measures. This is not what goes on with a mass-production bike, as everyone seems ok with the 50t ring, or the 46t when it needs to.
Given that you can obtain a pretty low 41" gear with a 39x16 on those 16" wheels, the use of a very common 130mm BCD chainset makes more sense than using a 110mm one and trying to find a decent 50t or 54t ring for it.