Shimano Roller Brake: No User Servicable Parts

If you have to cut through a product with a hacksaw, it’s probably never going back together again.

I was fortunate enough a couple of weeks ago to win an Ebay auction for a Shimano Nexus 7-speed hub gear unit. It was a bit grubby and didn’t come with the shifter, but I did manage to get it with the roller brake unit thrown in.

Ordinarily you can get the Nexus hub without the brake unit and a little bushing-cum-cover for the blank side of the hub, but I have a different idea. I think I can use the brake mount to fit a hub generator to.

I’ll probably go over this in more detail once I acquire the needed dynohub to gut and show some of the reasonably minor mods I’ll be making to the Nexus hub itself (all reversible I expect, so don’t panic). But for now I just need the plate that meshes with the gear hub.

The middle bit with teeth is what I'm after

The other side of the brake was brown from overheating, and the drum’s aluminium heat-sink was pretty warped too, so there wasn’t much guilt in tearing it open. Plus the frame I have is made for rim-brakes (even if it wasn’t, I’d rather not put that extra force on the spokes of using hub-braking).

I clamped it carefully in a vice, after finding there wasn’t enough clearance to simply pry up the three retaining lugs and release the brake drum, and used a hacksaw to cut across the fold of the lug. I cut through enough that it folded back and the drum got a lot freer.

A lucky fluke, the tab stayed attatched but folded aside

That didn’t quite get it loose enough though, so I had to do the same to another tab until it finally popped free.

These things are not meant to be taken apart. They’re sealed units that are only available for replacement whole.

Parts apart

From here it just took a few taps with a leather mallet to knock the steel drum out of the heat-sink. It was just a press-in fitting with some mating teeth.

The prize!

The 4 holes, btw, are for the dust-seal which I’d already removed before cutting began.

Hopefully I’ll attach this to a custom aluminium enclosure I’ll be housing the guts of a Sturmy Archer Dynohub in. It’ll be a lot easier than designing and making my own generator from scratch, and I’m pretty sure it’ll fit once all the heavy steel shell is removed. But anyway..

The mechanism of the roller brake is interesting. It’s similar to a drum brake, but the brake pad is spring-retained. The “nut” in the middle turns when braking is applied, pushing the rollers in a carrier outward as they’re usually against the flats of the nut, but get pushed to the higher corners. This makes the rollers push the brake pad outward in all directions against the brake drum. I thought there might be some braking force applied against the rollers directly, but the brake pad doesn’t move relative to the rollers.

Brake actuator
Roller carrier and rollers

Like the hub gear, these brakes need an anti-rotation/non-turn washer on the axle as that’s what it uses to anchor itself, not to the frame directly.

Unless I’m missing something, calling this a roller brake seems mostly a way of avoiding calling it something as old-fashioned as a drum-brake. But that’s essentially what it is, though with a clever mechanism for increasing brake-pad contact area.

It’s probably about the same contact area as rim-brake pads, only without the benefit of as much mechanical advantage on the wheel.