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I have an old bike to get me from the station to work and back at the far end of my commute. It was never up to much, but anything good would be stolen in no time as well as degrading from being stored outside in all weathers.

It's got 3x6 gearing, a freewheel system. Recently, when I shift into the big ring while accelerating reasonably hard I sometimes feel a lack of resistance to the pedals, for 2 or 3 pedal strokes. It's not the rough, sudden feeling of a chain skipping, and besides only happens just after shifting up. It doesn't feel like it's tried and failed to shift up, because there's less resistance than in the middle ring, and also that tends to catch and release. It feels like something is slipping - but what? Unfortunately I haven't been able to replicate it at times when I can look down and back to see if the rear derailleur is doing something odd. It's happened more often when I'm already in the smallest sprocket, but today I deliberately shifted up while in a middling sprocket.

It's not slipping as freely as when my freehub on another bike packed up, and in that case it was equally likely in any gear.

I'm not very familiar with freewheel systems, and that might be making me inclined to blame the unknown. I've had a few but whenever I've tried to work on them they've been rusted and impossible to remove - that's why this back wheel came from another cheap old bike.

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  • Functionally, freewheels and cassettes work identically from the viewpoint of the derailleur, so that won't affect things here.
    – Criggie ♦
    Nov 4, 2022 at 22:17
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    @Criggie yes, but not knowing what to blame, I'd like to look to see if the RD is jumping around. It should simply make one move to give up chain to the big ring, anything else would indicate that the gears aren't changing properly, but if the RD behaves the slippage must be in the freewheel (matching my impression from the feeling)
    – Chris H
    Nov 5, 2022 at 9:37
  • I have experienced similar effect with freewheels that had gummed up lubricant. (The non-engagement was more random than specifically after a certain shift, though). I had been using cheap grease that used some solvent that evaporated rather quickly. To test and temporarily fix, I squirted cheap mineral oil (3 in one knock-off) into the freewheel, that diluted the grease a bit and allowed the pawls to move. The problem would eventually return, until I re-packed the F.W. with good quality grease.
    – Jahaziel
    Nov 10, 2022 at 11:25
  • @Jahaziel good thinking, I'll give it a try. We have a spray can of 3-in-1 in the lab, but I'm more likely to bring in a big can of GT85 tomorrow.
    – Chris H
    Nov 10, 2022 at 12:03

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I finally managed to replicate it in daylight, on a quiet road, when I wasn't rushing for a train. It did turn out to be the freewheel slipping, but the symptoms were very different to a failed freehub I've experienced before - it only happens on shifting up into a high gear, and rather than banging the back wheel against the ground, downshifting normally fixes it immediately if it didn't fix itself within a few pedal revs. There's also more resistance than when it was the freehub, but that could just be the far worse components and maintenance on this cheap outdoor bike. My suspicion is that the pawls are springing slowly and once engaged they're fine - though I haven't been able to get it to slip by building up speed then freewheeling in a high gear, which is something I do a bit on the route, but not much as it's pretty flat

The plan is to see if I've got enough bits to build a replacement back wheel, as swapping it is doable outside work, unlike trying to strip it down.

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    @Michael absolutely sure. It can be a little reluctant to mesh as well, but that's a very different feeling (and sound - this issue is as good as silent). I still didn't rule it out until this morning when I could look down and see the sprockets spinning far faster than the spokes.
    – Chris H
    Nov 10, 2022 at 10:15
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    I had to fix a coworker's freewheel that would occasionally fail to latch. Turns out he'd ridden it so long that the pawl's sharp corner had been rounded off by all the coasting, and the spring was not strong enough to push a pawl into the recesses fast enough, if the wheel speed was high. So it was more likely to engage when the cassette was spun up slower, over about 5 seconds. The fix was to open the freehub, not loose any bearing balls, pop out the retainer/spring and remove pawls. Each was restored/sharpened with a file or belt grinder, lube refreshed and reassemble.
    – Criggie ♦
    Nov 10, 2022 at 10:36
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    @Criggie It does sound like similar symptoms though this wheel came off an old bike that probably hadn't been massively used. But the first step is to get it open, and that means getting it home to the workshop. I can't run fast enough to catch my train without a bike at this end, so it's far better to swap the whole wheel. I have a hub that should be compatible with the frame, currently on a 20" wheel. I have a rim laced to a front hub, but hopefully the spokes will do. I'm just not sure if they've got the same number of holes. And I was planning on building a different wheel next
    – Chris H
    Nov 10, 2022 at 11:19
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    @Noise I might well do change the freehub to gain the next spare, but this bike lives a train ride (on which I have to book the bike space in each direction) away from home, and without it my day gets a whole lot harder. So I'd have to get the bike or wheel home one day, fix it in the evening, and take it back in the next day. That means having the right parts on hand and it not being rusted solid like the last one I tried to get into. It's so much easier to build a wheel from spares. Spray oil should stop it getting worse.
    – Chris H
    Nov 10, 2022 at 14:41
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    Your life, your time, Christophe. If we were friends and you brought it into my shop, I would probably spin it off for no charge. Leaving you free to spin the new one on by yourself. Fortunately you live quite a way away!
    – Noise
    Nov 10, 2022 at 17:43

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