Any question answered...

Oooh ta. @Clockwise Gotta be a nexus compatible one tho!

Mine came off a Nexus 8 that I'm rebuilding into a rim-brake wheel.

Is that a front or rear? Got a model number?

The practical cycles link one is.

Gotcha, that looks good too - thank you!

It's a BR-IM70-R(ear). This is the exploded diagram FWIW. SJS will sell you the missing grease port cover for a whole 99p!

Sounds good. Do you know if there’s any reason I can’t pop it on a crosscheck? Do the chainstays need to be up to the job like with disc brakes?

AFAIK there are no special requirements of the frame, you just add the p-clip thing to the NDS chainstay. It came off a completely unspecial alloy frame, so I think any steel frame would be fine. The chain stay has to take a rotational force, but I can't imagine it's bigger than your weight on it, and the actual braking force is through the dropout.

They are found on cheap-ish bikes, so I don't think there a huge amount of torque going through that arm. I don't know much about compatibility I'm afraid.

the actual braking force is through the dropout.

The P-clip pulls down on the chainstay. Given how little torque it takes to lock the rear wheel, it's not a big load; if your chainstay can't take it, you have bigger problems than brake torque reaction 🙂

Thanks all. Project shoulda-got-a-disc-tab-welded-on-when-powerdercoating-it crosscheck is on.

if your chainstay can't take it, you have bigger problems than brake torque reaction

Very true! Presumably the actual retarding force on the bike i.e., the horizontal force, is though the dropout though, as it is with all brakes.

the horizontal force, is though the dropout

Yeah, but the pull down on the stay is actually larger (roughly double) than the pull back on the dropout, because levers.

When is the next offer period for 99p Eurosport Player subscription likely?

(Or £1.99 or whatever)

Fixed colour to accurately represent ffm's standpoint and tester's.

I have found on the internet (Sheldon website) that
"a 19-622 (700 x 19C) tire would theoretically have exactly the same outside diameter as a 38-584 (650 x 38B)"
Is there anyone that have a more or less precise equivalence table between 650B and 700 wheels diameters depending on the tire chosen? I am considering switching from 700x25 to 650Bx35 and would know approximately by how much the bottom bracket would be lowered.

Thanks in advance

Is there anyone that have a more or less precise equivalence table between 650B and 700 wheels diameters depending on the tire chosen

Roll your own, just add twice the tyre section to the rim diameter (622mm for 700C, 584mm for 650B) and you'll be close enough. The actual inflated diameter is a bit more than that and affected by rim width and bead hook height among other factors, but after allowing for squish you end up more that accurate enough for judging the effect of tyre changes on BB drop.

My brain goes a bit weird when I try to work out the forces in a braking system. The brake resists the turning force of the wheel, which results in a reaction force at the dropout, but if the brake is essentially horizontal to the dropout (as in discs, drums and chainstay-mounted rim brakes) then that reaction force acts vertically (hence the issue of front wheels jumping down out of the dropouts on cheap forks with disc brakes). The force actually stopping the bike has to act horizontally, presumably as a reaction to the force exerted by the ground on the tyre.

Got a frame that needs seat tube reaming (fnar...), anyone has a good London based shop that can do it? Current choice is SeaBass but good to know any other alternatives! Thanks!

Thanks @mdcc_tester, that does result in 18mm drop
700x25 <=> 622+2*25=672mm
650Bx35 <=> 584+35*2=654mm
Considering that I currently run 170mm crank arms and I never had any pedal strike (currently running fixed), switching to 650B and 165mm crank arms woud result in 13mm pedal axis drop which does appear to be quite a bit even if it wouldn't be fixed anymore but with gears, would it be a problem? I assume that this discussion would be much easier with the BB heigh measurement but I do not have it yet. I reckon it would be quite better to run 38mm wide tire to reduce the drop

With no metalworking skill, I borrowed an adjustable reamer form a friendly forumer and did it myself. Was quite satisfying and easy (following their advice).

The problem when discussing this kind of thing is the distinction between torques and forces. The reaction arm is for the braking torque, the dropoput is for the braking force.

My brain goes a bit weird when I try to work out the forces in a braking system.

Draw yourself a free body diagram and it becomes a little easier. Could be some nice Wests entertainment if you fancy?

only 9mm drop (half of wheel diameter difference)

Is the bike recent enough that you can find a geometry chart with BB drop?
If it is in around the 70mm mark, and it's likely to be, then you'll be fine with 650x38B (there aren't any good tyres in 35).
I rode that size with a 72mm drop and while it noticed the lower centre of gravity it was more of a positive than anything else, as It really helps with confidence in cornering.

Yes indeed you are quite right miscalculation from my side! if only 9mm difference, with change of crank arms pedal axis will only lower for about 4mm which does seem acceptable but I reckon if it fits 38 would be better anyway!

@Lolo more or less old 700C bike with not much clearance currently fixed that I would to re-convert to gears with 650B tires to have a more relaxed ride