Friday, 18 October 2013
Sometimes something strange happens whilst you are riding. The top chain loosens when you stop peddling, and if you look it's because the rear cassette is continuing to move a bit after you've stopped. Alternately, and a bit more upsetting, when you leap majestically on, start peddling and everything goes round except the rear wheel. Often this can be followed by a squark, a slight topple and you sprawled across the ground as you've not antipated this rum behaviour from your trusty friend.
Cause from Effect
The cause, a gummed up freehub. Simple actions like taking it off the wheel, thoroughly lubricating, then returning is only going to hold it for a little while. It needs to be taken apart, properly cleaned, regreased and reassembled.
Looking at Wikipedia there's a good diagram of what the mechanism is (or roughly, bikes have more than one red pawl!). The spring or space between the light green inner and turquiose outer has become gummed up.
This means the red pawl sticks in against the green inner (allowing the rear cassette to not power the wheel and you to fall over) or the red pawl sticks out against the turquiose outer (allowing the chain to keep moving).
Let the Maintenance Begin!
This is where it get's a little tricky, especially if you're British. I do scan for help on how to do bike maintenance quite regularly and know where to look. Also, Google is quite good at finding things. However, finding a site that covers this is quite difficult.
Well, I was at my workshop with a whole load of others enjoying being outdoors. And I had a bike I was maintaining for a charity for which I work. A good friend and trained engineer wandered (and wondered) in looking for things to do and I said "Let's play with a freehub!". Well, we scanned again for web pages. Not even the great Sheldon Brown's freewheel disassembly page (or here) seems to have the exact details.
Well by now my mate had the cassette off, an easy "year 7" task shown in many places.
Now comes a a slightly sticky bit which is also quite well written up, but I thought I'd share my mechanism (well, the one my mate did with my kit!). I have a "travel" bike tool box that has a good lid with edges that means upsidedown it acts as a good tray. Get spanners set on either side of the axle and give it a turn to get it loose. Now turn the wheel to being horizontal and put this tray under the wheel axle. Finish by unscrewing the axle preferably pulling it up and eventually out. This will leave the hub with loose bearings, some of which will have dropped into the tray.
Freehub on wheel with bearings showing
Now knock the remaining bearings out into the tray. If you don't have a tray, a cleaning cloth makes a good replacement. You can buy cushions or pillows that do the same thing. Move the bearings tray to somewhere safe.
A 10mm Allen key is all that's needed to remove the freehub fixing bolt as Sheldon Brown says.
Unscrewing the Freehub Fixing Bolt from the Wheel
Now you have the freehub. What a thing of beauty it is. This is what it looks like from the wheel bearing side. You can just make out a small notch in the inner assembly. Ah. Hmm.
Freehub from Wheel bearing side
And here it is from the side that attaches to the wheel. The cog on this side neatly slots into the opposite grooving on the wheel. Like a Connection. I'm going to call it a Connecting Cog.
Freehub from Connecting Cog side
The Connecting Cog is part of the inner mechanism, it that it goes round separately from the cassette mounting but with the notched bearing holder on the other side.
Now the fun begins. We've still not found any websites helpfully telling us how to take this bit apart. We're distinctly rueful as we know there's going to be numerous bearings, probably a lot smaller, and possibly some springy things, likely to fly out at any moment of disassembly. We've tighened and pulled and twisted lots of parts to no effect. Actually that was pretty good, this bit of a bike is pretty indestructible.
Then, a Swedish page pops up. Yes, Swedish. And then a Polish one! Wahoo! Er, neither of us spoke either language. But hey hoo, there's a picture and I think it's obvious we need to focus on those notches.
The Notches are the Key
So what to do? We didn't have any kind of tool that'd deal with that. Well, we did have a well stocked workshop and quite a bit of rough metal odds and sods. My mate hack-sawed a piece to fit. The steel has to be pretty tough, it's going to twist something super tightened.
Well, we tried, then thought and tried left-handed, that is reverse to how you expect it to loosen (the adjective confuses me as I'm left handed). With the Connecting Cog firmly in a vice and a good long pair of pliers it started coming undone!
Tool Pushed into Freehub
Once it was loose it was quite easy to move, holding the Connecting Cog and the makeshift tool in the notches. Again, we transferred work to in the tray. Eventually it came loose and some bearings meandered, but in the tray.
And this is what you get. An Inner Hub with pawls, which are the moving rachet parts. The bearings here are sitting in new grease, but many will be here in the old stuff.
Inner Hub with Pawls
This is the notched section turned the other way up showing the bearings at this end (again in new grease).
Inner Hub Lock Ring and bearings (notches on underside)
And this is the Outer Hub on which is mounted the cassette. Note the circular rachet facing in on the inside.
Outer hub with circular rachet facing in
Well, the next bit is pretty easy. A good clean up, oil up, and test the pawls.
Inner Hub with cleaned pawls
Then re-grease, especially where the bearings are so they can be stuck in place. These are all in the above pictures. The unit goes back together easily after that, although still worth using the tray.
Again, we re-mounting and returning axle bearings to the wheel, the tray is useful.
Hub on Bike with Bearings
Finally, tighten the axle cones until it moves freely but doesn't shift through the mounting. Again, this is shown in many places.
Full Hub on Bike
Now, searching for that tool. It's meant to be a Shimano Cassette Hub Race Remover. But they seem awkward to find. Even in a Google a TL-FH40 search doesn't seem to give many places to buy one.
Then, a French Blog with Park Tool gives a different ID. And in Google a FR-2 Park Tool Search gives a lot more places! After finding them in a Random Place and another Random Place there's loads of places in Ebay that will give them, with the cheapest here.
And subsequently, I found posts about lots of different Freehubs and a slightly strange-but-useful-pictures blog about cleaning without taking apart. Not really sure about the amount of chemicals being used once here, though.