UKC

So we had a baby last year and I did the only reasonable thing which was to impulse purchase a road bike of FB market place a week later, having never ridden a road bike before (lot's of MTBing though). I rode it twice then put it on the wall in the garage...

Skip to last night, I went out on it again - had an absolute blast, really enjoyed the change to MTBing - but 2 or 3 times in the space of an hour something weird happened with the pedals, and only when I was going downhill, fast, in the highest gear (aka the last time I want something weird to happen with the pedals). It felt like they lost all traction with the gearset and kind of free-span really quickly for half a rotation, so my feet slipped off (not got clip-ons).

Does anyone know what this could be or if it's an easy fix, or if I'd need replacement parts?

Many thanks!

the pawls inside the free hub are sticking and not engaging with the teeth sounds most likely. You’d need a chain whip and cassette tool to replace the free hub so if you don’t have these tools then local bike shop would be more than happy to do it for you. It’s fairly quick and inexpensive to change the free hub 

Chain slipping off the smallest sprocket. Try indexing the gears (google for it & follow the step by step guide, it involves adjusting the rear mech’s outward movement by turning a very small screw).

If that doesn’t work, probly your chain or sprocket teeth are knackered and you’ll need to spend money.

Did it feel as if the chain disengaged or slipped on/between the gear(s)? might be worth checking the alignment on the rear mech before you take things apart.

You say going fast? How fast?

What is your highest gear?

E

Probably around 40kph ish I guess? Feels fast on a dainty little road bike compared to a burly full-susser anyway! Especially when you go over a pebble haha!

It did very much feel like the chain slipped, I thought the chain had come off both times but I looked down and it was fine and then the pedals worked again, and the mech(s) are making a bit of noise when I pedal in the outer gears so maybe it is to do with mech alignment. I'll look up indexing thanks!

My guess would be what Wilkie14c said, this happened to me although it was less intermittent, I found it mainly an issue when starting off: push on the pedal and nothing happened.

Advice above on potential causes is good. Until you've got the problem sorted be very wary of pedalling hard, particularly out the saddle climbing.  If it slips when all your weight is on one pedal it's very likely to cause a crash.  In my younger years I skimped on maintenance, a trip to A+E and a big scar on my elbow nudged me to change attitude

Yeah, on my commuting bikes in the past I've had some nasty bashes where either just wear on the drive train or accumulated gunk on it has made the chain skip and it throws you forward quite violently if you are for example pulling away having left it in much too high a gear! I remember catching the inside of my thigh on the stem one time. God it hurt, and perhaps the most impressive bruise I've ever seen on myself over the next weeks!

elliot - I've been riding my old road bike quite a lot for daily exercise in recent weeks. I've looked after it so it is in good nick and it hasn't been used so much in recent year but it has done maybe 4 or 5 000 kms and I've really had very little issue with the components it came with beyond changing tyres. I got it on a really good sale 9 years ago I think because in part it had a triple chainset when already 90% of serious road bikes only had doubles. Anyway I suspect I've ridden huge amounts in the middle ring at the front because now when I go into one of the lower gears at the back it quite reliably will skip off the middle cog at front down into the smallest. Not what you want if you're putting the power on! But I've sort of got used to it and tend to go to the granny ring the front more often. If your bike is well used it might be something similar which you either need to replace (probably) most of the drive train to fix or just learn to live with it. Or if it's an indexing issue, some watching youtube videos and fiddling with your bike following the instructions might fix it. If you don't have a bike work stand being able to hang it somewhere so you can spin the pedals really helps.

I agree. The rotation of the wheel is causing the freewheel to creep round when freewheeling and creating a slack bit of chain I would guess. Well diagnosed!

Once had a similar issue in a nearly new bike. Disconcerting slip that happened (usually when standing on pedals powering up hills.) Guy in local bike shop diagnosed it as an issue with the bottom bracket... Worn bearings from an incorrectly fitted unit, I think it was. He fixed it very cheaply rather than replacing the whole thing and I've used him for all servicing ever since.

Just something to think about if it recurs after you've tried fixing chain alignment, free hub etc.

Worn chain/chainring/cassette would be my first guess, along with free hub problem.