Nexus 7 coaster brake stuck on, can’t remove hub inner from shell.

[ElCani]

Well... I’ve been servicing our fleet of IHG bikes. It’s been interesting learning a bit about how they work, and having successfully stripped and re-lubricated (with Landrover swivel SFG) my Nexus 7 equipped damcykel (basically a Swedish ‘Dutch’ bike) and moved on my partner’s. Everything went well until the test ride after reassembly...

Gears worked fine, but then as I turned slowly and sharply, the back wheel locked-up and I realised I’d forgotten all about the coaster brake reaction-arm clip (deliberately left unsecured while trying on the bike stand). What a clucking idiot. I didn’t even consciously back-pedal, but must’ve applied some back pressure to the pedals as I made the very slow turn.

Anyway, the result is that the coaster brake is jammed on and the wheel will no move forwards or backwards. The hub internal will not slide out of the shell, I assume because the brake shoe is jammed against it. I have tried hitting it with a hammer, but not that hard. What’s the best way of releasing the brake?

[Brucey]

if you refit the wheel in the frame, and secure the reaction arm correctly, turning the wheel backwards (and/or pedalling forwards) should release the brake and thus the internal. But events such as this can generate some spectacularly high forces in the mechanism; I'm not sure what is likely to break first though.

cheers

[ElCani]

Thanks, I've tried that with no luck but will try again. Everything happened at such a low speed, but I understand that the forces are high.

[ElCani]

Absolutely no joy with this, even with further percussive engineering. Are there any other possible routes to un-stick the brake or for not-too-destructive removal of the hub internal?

Thanks.

[mjr]

No idea if it'll work, but I think I'd try clamping the brake shell in an inverted-Ω-shaped vice, grab the axle flats below with some small mole grips (thinking that it can't rotate the grips through the vice body) and turn the wheel, hoping that the larger radius results in higher torque at the hub than the chain on the driver, but mechanics was never my best bit of maths. Without a wheel in front of me, it's not at all clear to me which way I should rotate or whether this does any good!

[Brucey]

IIRC the brake mechanism in the N7 coaster hub is the 'expanding roller' type rather than the 'axial wedge' type. Once the brake is on, the brake shoes, reaction arm and hubshell are liable to be tightly locked to one another. On reflection I think that the best way of unlocking a jammed brake of this type is to turn the driver forwards whilst holding the wheel.

To apply sufficient torque without risking damage to the gear mechanism it would be best to use a direct drive gear ratio. Unfortunately there is no such thing as a direct drive in a Nexus 7 hub which means that whichever gear you select there is always some reaction torque on the axle and/or load in the gear train. You'll get the maximum torque on the brake actuator in gear 1 but the wheel has to be in the frame so the NTWs can do their job.

Last gasp would be to use a long chain whip on the sprocket, in gear 1, with the wheel in the frame, holding the wheel. But this may result in much higher torque loads than normal on the gear train and this may simply break the hub. Unfortunately I don't think there is any other way of getting out of this jam; leastways I can't think of one.

FWIW the kind of jam you have is made quite likely if the brake shoes are 'critically worn' i.e. are close to the point at which the brake is so worn that the brake doesn't work any more. Once you get it apart, examine the shoes carefully; they may be indented, cracked, or simply badly worn. In any event changing them isn't a bad idea.

cheers

[ElCani]

Thanks. I’m going to dump this in the “one-day” pile and move on with my life.

[Brucey]

actually I have had a further think and there may be a way. Because the brake is a roller (radial expansion) type brake rather than a conical wedge type, it ought to be possible to remove the reaction arm from the left side of the hub. If the reaction arm moves slightly, easily, then it ought to come out. If it doesn't, it should be 'persuadable' nonetheless.

Having done that the brake shoes should be facing you. The rollers may also be visible. Anyway the brake shoes are made of zinc alloy and are quite soft; a few strategically drilled holes may weaken the shoes enough to allow the jam to be released and/or the shoes to be broken up in situ. Simply clean up the swarf, replace the brake shoes (which may have been necessary anyway, they don't last indefinitely) and the job's a good'un….?

cheers

[ElCani]

Yup, the reaction arm comes off with no problem. I will try what you suggest!

[Brucey]

You've probably seen this already but if not it might be useful in showing the layout of the parts.

https://si.shimano.com/pdfs/ev/EV-SG-7C30-3276.pdf

SG-7C30. Other N7 coaster hubs similar.


I don't know if it might be possible to remove #3 in situ and therefore be able to withdraw the axle and some of the gear parts before using the drill?


Good luck!

cheers

[ElCani]

Update: removed all but the LH gear cluster/brake (the circlip that holds everything together was accessible) but drilling the brake shoes was a no-go. Drill made basically no impact. But... after collecting various other non-functioning Nexus 7 hubs and mixing and matching different parts, My partner now has a fully functioning bike again.

[Brucey]

'success' comes in many colours!

cheers

[ElCani]

'success' comes in many colours!

Indeed, if I hadn’t made the initial dumb mistake and then spent many hours messing around with greasy, rusty hub internals and wonky wheels I would’ve missed out on this slight feeling of satisfaction, hehe.

There were a few interesting results before I got to working hub, namely one that ‘drove’ fine (no slipping) except with oddly mixed up gears (some seemingly duplicated, some missing) and one that had no drive at all in gears one and two about half the time these were selected. As all this took place over a few sessions, with new broken hubs entering the fray at different points, I’m sadly at a bit of a loss as to which parts are responsible for which faults...

[Brucey]

N7 hubs (in most versions) have the 'old' system for locking sun pinions, i.e. there are inwards-facing pawls within each sun pinion, which engage with various features on the axle. These pawls are usually reliable, because they use stainless steel pawl springs. However they can start not to work properly because of corrosion; the inside of each sun can trap water and the pawl can go dead sticky, often without suffering permanent damage.

Often there is a second set of pawls engaged as a backup, i.e. so that you don't lose drive completely should the chosen sun not lock. The backup pawls are overrun when the hub is working normally, but are used should the selected pawls not engage for some reason. This results in gear duplication, missing gears and so forth.

The latest versions of the N7 hub have a different internal (which still fits inside the same hubshell I think) having axle pawls that pop out of the axle, more like N8/Alfine hubs do. These hubs are more transparent in operation; you can tell from the axle assy more easily if any given sun pinion will lock or not. I don't know if one arrangement is actually more reliable than the other or not though.

cheers