Bike chain slipping over gears

Factual Questions
1

Occasionaly when I apply a strong downstroke to my pushbike pedals, taking off from a standstill, the chain slips over the back gear cluster. This is very dangerous, a while back I nearly came off. Now I tend to be overly cautious.

I thought it might be a stuck link or dirt. I degreased and cleaned the chain and gears and reoiled, but the problem is still there. At one stage I used a teflon based lubricant, which I thought may be the cause of the problem. When I degreased the problem diminished but was still there.

Recently my brothers new bike started doing the same thing.

The bike shop staff are useless. Anyone know what causes this problem?

2

I can’t explain the physics, but I’ve been thrown from the same bike twice for the same reason. Both times required a brief visit to the hospital. If the bike is quality, and your size is not abnormal, you may have just gotten a lemon. If either of the former is the case, get another bike. There are only so many adjustments you can make to a bike that is not for you.

3

A dirty chain would have been my first guess to.

If your bike has indexed shifting (shifter nas detent for each gear instead of moving smoothlly without clicks), it could just be the shifter adjustment problem. Put the bike in some gear near the middle, and look at the rear derailleur from behind the bike. Is the derailleur lined up with the appropriate cog? If not, turn the adjustment dial on the derailleur or shifter. (SRAM setups have adjustment on the shifter, Shimano has it on the derailleur.)

If that doesn’t fix the problem it might be a worn out chain and/or cassette. How old are they?

4

One more possibility - the derailleur hanger (the part of the frame to which the rear derailleur is bolted to) may be bent. It should be fairly obvious if it’s bent. If that’s the case, I wouldn’t try to fix it myself - bike shops have tools to bend it back properly.

5

While I’m not certain, I don’t believe this is an adjustment problem or a bent hanger. What it sounds like to me is scr4’s last guess - worn cogs on the freewheel/cassette. If a bike is ridden a lot, the front edges of the teeth on the cogs wear, and eventually this can lead to chain skip, particularly if you replace the chain. A new chain/old freewheel combination will frequently make a bike unrideable. Such wear is usually visible to the naked eye. Examine the cogs, and if the gaps between the teeth look elongated, and the teeth come to more of a point than they did new, then this is most likely the problem. The solution is to replace the chain and freewheel or cassette. Make sure the replacement is compatible with your drivetrain, or your indexing won’t work.

6

scr4 The first thing I did when this happened over a year ago was to buy a new cluster and chain. This fixed the problem temporarily. That is when I got back my confidence and nearly came off.

I have bar-end gear shifters which aren’t indexed. I have looked to see if the gears line up with the chain. You may be right, perhaps the gear is moving itself, not being indexed. However, my brothers bike is indexed and his is having the same problem (he has those brake lever shift gears - whatever they are called - I avoided them, because I assumed they would be more likely to fail).

I will check the derailleur hanger - I havent thought of this. Does that mean the derailleur is not parallel with the chain?

7

My bike has a similar problem, my most recent theory is that the chain is slightly long as the derailer seems fairly coiled, I’m thinking that maybe the derailer isn’t puting adequate tension on the chain and that when I ride over a bump with pressure on the pedals it skips.

8

I don’t have a particular remedy for your problem (seeing as it has been nigh on 20 years since I was a serious bike-rider), but can I suggest you try to get hold of a book called Richard’s Bicycle Book? It was published in the late 70’s. but is a veritable goldmine of all sorts of useful bike info. Richard was a complete bike NUTTER, and took his riding and his machine very seriously. I am absolutely sure he would have an answer for you.

Check out the second-hand bookshops, especially the weird ones. Someone, somewhere will have a copy for ya.

9

A very good resource when it comes to bikes is Sheldon Brown. He writes on the subject of jumping/skipping/autoshifting

(bolding his) (source)

Do have a look around there.

How many km have you done with that chain? If it’s a mountain biike it might be worthwile to change it rather frequently, some people say every 2000km. (If you don’t change the chain, it will wear down the sprockets, and you will have to change them all. If you keep changing the (much cheaper) chain, you won’t wear down the sprockets anywhere near as much.)

10

How many miles have you done on your bike? The chain could have “stretched” - it’s not actual stretching, but is a lengthening caused by wear inside the rollers in the links of the chain. If this has happened, a new chain might fix the problem, but if you’ve been riding with a stretched chain for any length of time then it’s likely that the chainwheels and sprockets will also have worn excessively due to the chain not fitting the teeth properly. I’d find a different, non-useless bike shop too and see what they think.

11

Good links, Popup. The OP’s problem sounds like auto-shifting to me since he says he’s replaced the chain and sprocket. The link explains about this. In indexed shifters it can also be caused by improper shifter adjustment.

By the way, the twist shifters like the SRAM Gripshift ™ work fine, at least the high-end ones. I have SRAM Centera and Rocket shifters on my bikes and haven’t had a single problem so far.

Also there’s an updated version of Richard’s Bicycle Book - I think it’s called Richard’s 21st Century Bicycle Book. I don’t know if it’s as good as the old one. (I have the new one but haven’t read much of it, and I’ve never read the old one.)

12

Oil does not free a link that has been twisted.

**
I think, though, that your most likely culprit is a tight link, this is very common.
If you make a bad change, like uphill and with lots of weight on the pedals, the chain will twist and the result will be that a link binds slightly at that point.
When the stiff link goes through the derailler, it will cause it to jump and kick the chain, which is enough to jump the chain from the teeth of the sprocket.
**

This is not always easy to spot, to do this, stand the bike upright and set your gear so that the chain runs dead straight from your chainring to your sprocket, it’s best to do this on the inner chainring for a double chainset, or the middle chainring on a triple chainset.This will mean you will have to select probably the fourth sprocket in.

Now stand at the side of your bike, and steadily wind the pedals backwards, don’t go mad, just a nice steady even pace, quite slowly.

Whilst you do this, observe your derailler, it it kicks just slightly, the chances are that you have a stiff link.

If you are sharp enough you might just see the kink in the chain, the link moves one way and then is moved the other way by the derailler jockey wheels.

You can do things like use a drop of nail varnish to mark where you think the stiff link is, it helps when you look even closer to observe exactly which on is doing it.

There are a couple of ways to solve this, you could fit a new chain, which is what you did, and what I think solved your problem first time around,

If you know your way around a rivet extractor you can clamp it on the offending link and push the sideplates a teeny tiny bit, but you really have to know what you are doing.

or,

You could take the chain off, find the stiff link you already marked, and twist the chain at that point.

If the sprockets and chain have less than a cuople of thousand miles in them this would be my main suspect.

These index gears unfortunately encourage folk to change gears when pressing hard on the pedals, before index levers you had plain old friction levers and you had to back your weight off a little to get a smooth glitch free change, or predict when you would be likely needing to change gear so it didn’t happen in the first place.

13

I am pretty sure I have Richards bike book - with the bloke with a beard and hand knitted wooley poloneck jumpers. I will dig it up.

14

Thanks for the help everyone. Great link popup. As scr4 said it is probably autoshifting (in my bike at least). In my brothers case it may be the improper index shifter adjustment.

I checked the derailleur hanger. It looked to be aligned well.

I have had the current chain/cluster for around 18 months, which which works out at about 5000km. So I might have to stop being stingy and buy another set. First I will try casdaves diagnostic procedure.

From the Sheldon Brown article, it sounds very much like ‘Unintentional upshifting’. Since I have a large frame (custom built bike), the frame may be flexing. One of the first things I did was tighten the friction shift bar-end lever, so its not that.

The solution may be to grease the cable guide.

I suppose it is good practice to get out of the habit of applying a lot of torque in high gear, which is what I tend to do when taking off at the lights. I always use the large cog at the front and dont often use the lower gearing cog at the back. Reading Sheldon Browns site, it not only puts strain on the bike but on your own body (and you run the risk of crushing your nads/perineum).

All that should keep me busy this Satuday morning.