Another derailleur problem - changes gear if pedalling hard!

Mosschops2

Afternoon All

My hybrid has just acquired this problem. When pedalling really hard - e.g. when standing up to get away from lights, round roundabouts etc, the gears slip down a couple of notches. Which is not just annoying, but also dangerous.

It seems like a basic enough thing, but I'm not sure either what the exact problem is, or what to do about it!

Apologies for being clueless, thankful for any / all assitance!!

Mosschops2

Can nobody help me??

nicklouse

dave what do you mean by

the gears slip down a couple of notches

changes gear? cranks move and wheel does not? does it happen in all gears or just one in particular.

and this may have been better posted in your tech section?

Mosschops2

Morning Nick

(Is it polite to say Morning Louse?)

Fair point, was not clear enough!

When pedalling hard, the gears change to a lower gear. So for example when standing up on the pedals, when the gears drop by one or two, this means that the pedal you're standing on drops like a stone, and I end up at best sitting down, at worst with two feet on the floor straddling the crossbar.

Also re. tech section - fair enough! But is the sort of question that might have appeared in beginners before!!

nicklouse

ouch. case 2.

it sounds like the freebody/hub is on the way out (if you are on a gear system with mech).

but again it could be that the chain and rear cog(s) are so worn that the chain is jumping.

(if it is hub geared i have no idea )

Basically for the pedal to drop to the floor with the speed you describe there has to be a failure somewhere. the drive pawls and ratchet or the chain jumping. the gears changing would only make the pedal feel harder. (easier for the chain to fall down the cassette than up).

sorry about not being more helpfull but without a full spec (or picture) it is hard to "fix" but i hope i have got you looking in the right area. LBS time really as it needs to be a hands on fix.

RufusA

Mosschops2 wrote:

Afternoon All

My hybrid has just acquired this problem. When pedalling really hard - e.g. when standing up to get away from lights, round roundabouts etc, the gears slip down a couple of notches.

Sounds like something is worn or bent - how many miles has the drive train done, and have you replaced anything recently?

My old "racer" did almost exactly this and it was caused by a stretched chain and worn teeth on the rear cog. Only happened in my favourite gear though so I learnt to change down before lights until I replaced the bike!

Can also be caused by a sticky link in the chain, or a bent cog, poor tension in the rear derailleur etc.

The only way to tell is to inspect the chain and cogs very carefully looking for wear / damage.

HTH - Rufus.

HungryCol

Theoretically, it could be that the bottom bracket is too long and the chainset is set too far out from the frame. This would mean that the chain wants to line up with the rings on the chainset when there is pressure applied from the chainset and in order to do this it skips down. Therefore I would imagine this skipping of gears under pressure would most likely happen in 1st, 2nd or 3rd gear? New chain and worn sprockets would exacerbate this problem or a misfitting chain (8 speed chain on a 10 speed cassette)

Could be load of bosh but it makes sense in my head!

Mosschops2

Thanks to all for the messages!!

I guess I'll take it to my friendly LBS and get them to check it out.....

Something may have worn - it is unfortunately (sorry Cathrine) a Dawes Discovery 201.
(therefore no hub gears!!!)

So far ( in 8 months / 1000 miles?? not the longest commute ever known to man!) I've already had to replace the brake pads, the gear and brake cables, and the tyres. And the bell.

I'm well up for completely replacing the brakes - as I don't like the way that they are sprung off the frame (two sprung pieces of wire oppose one another - and it's not possible to get them to exactly match, so one brake pad wears more than the other. Grr.)

And now the chain and or gearing is worn.

Not impressed with my Dawes :evil: ..... gives me an excuse to go to the LBS tho!!

domtyler

Sounds like your bike has converted itself to an automatic Mossy, why complain? Normally an automatic gearbox is more expensive than manual.
________
Dodge Charger (B-Body) History

McBain_v1

Have you checked out Shedlon Brown's website on gearing? He's got some excellent advice about fine-tuning your mech to get the most out of it. I referred to this when solving a pesky problem on my beloved 1983 Raleigh 531c - the gears weren't indexed properly, it all came down to incorrect cable tension :?

xover_runner

I agree it's probably cable tension on the rear mech. Try adjusting the tensioner 1 turn one way, test it and try the other way if it didn't fix the jumping. Exactly the same thing happened to me on Saturday (first ride after a major service) and I fixed it as described

Positron

I experience a similar effect on an old and not very good steel frame (Falcon Banana). I believe it can be due to the force applied when standing flexing the frame sufficiently to alter the total length of the cable run so it wants to change gear.

Any such problem would be more apparent if the gear indexing isn't quite set exactly.

LBS sounds like a good idea (before purchasing super stiff carbon fibre I suppose?!?)

Good luck, p.

Mosschops2

Thanks once more for the additional advice guys!!

"Further investigation" has lead me to believe that this happens most in 4/5/6 gears (rear mech, out of 7). I do not believe it is chain slippage, but may simply be that the gear set up needs modifying. I'll definately check out Sheldon Brown's site - which I have perused rather than used in the past, and visit the LBS over the weekend!!

cyclone

For your pedal to drop down as fast (And dangerously) as you say, it sounds like chain slip.
Just changing up or down gears shuld not cause the pedals/cranks to move to any extreme.

It may also be that your rear mech has been bent or has loose pivots resulting in it wobbling and being pulled in when you pedal hard.

What Rear Mech and shifters do you have?
Also, have you replaced the chain recently?

Finaly. Where did you buy it?
If it's only 8 month old it still under warranty so long as it's not usual wear or crash damaged etc.

Mosschops2

Good call cyclone. However I think it is the gears - there seems to be a lot of gear inches difference between 20th and 18th (or whatever!)

The reason I say that is when I sit down and pedal slowly, the gears click back up through the one or two that were lost. If it were chain slippage this wouldn't happen.

I think on your advice I'll pop into the LBS first, see if they won't help me out. (Bought it from the LBS btw). (And never replaced the chain - only done 1000 miles)!!

acorn_user

If you only get slippage when you are pushing hard, then the cahin is probably worn out. Replace the freewheel at the same time, because it is probably worn too. The problem comes if your chainset is worn too, which is entirely possible.

As for your brakes, it sounds like you have v-brakes, and they are supposed to look like that. The Sheldon Brown or Park Tool websites will help, but I recommend you buy the Haynes Bike Manual if you want to do any of your own maintenance.

Gear slipping can be caused by a flexing bottom bracket. I doubt that this is the problem here.

The big gap is probably because you have a megarange freewheel or cassette btw.

Drivetrains wear out, especially if not oiled well. It sounds like there is nothing out of the ordinary with your bike, and I certainly wouldn't blame Dawes for it! [Specialized suffer from chain wear too you know =]:) ]