This month’s tech column is going to be about chain tension. But before I get on to that I should probably ramble on about something pretty much unrelated for a while. In this case I want to get you to take a minute to think just how awesome bicycles are. I was awestruck the other day watching the aftermath of the London tube and bus bombs. People were talking about their fear of getting back on the underground but saying they had no option… No option!?!? It’s the middle of summer the tube network only extends about 8 miles from the centre of London anyway and London is virtually flat. Why not get on a damn bike?! Do you know what the statistics for terrorists bombings of bicycles are? I had dreams of seeing the roads of London suddenly full of cyclists cruising down the empty bus lanes, saving their tube-fare and getting some much needed exercise… Sadly it never happened. But remember that that bike of yours can do more than just tricks.
Anyway back to chain tension. We have all experienced the dilemma pretty much every time you take off your back wheel and come to put it back on. Sure you can run a Halfords/Fly Cobra inner-tube, but sooner or later you will need to take your rear wheel off, and when you do you will have to set the position of your back wheel just right to get the chain tension you want. You will also need to be thinking about getting your tyre central in the stays of course. After some wrestling you get it set and then you have to check it by cranking the pedals round a few times, because the chain tension varies! Why is that? Why do we have to adjust and re-adjust the wheel position so that the “tight-spot” isn’t too tight and the “loose-spot” isn’t too loose? Why is there even a tight and loose spot in the first place?
This is going pretty badly isn’t it? Can you tell that we are rapidly approaching deadline and I am struggling to write anything worth reading here… Time to play the “old school” card…
If you just force the cups in then the pressures can be quite incredible. The cartridge bearing can get significantly compressed by these pressures and may not spin as well as it should.
Un-even chain tension is usually down to one of 3 things. Your front sprocket isn’t quite central. Your rear sprocket isn’t quite central. Your chain is stretched unevenly.
In the first case; if your sprocket is loose on the BB axle then it will inevitably move off centre. This means that the sprocket moves backwards and forwards as you pedal, which has the same effect as constantly moving your back wheel about in the dropouts. The second cause isn’t nearly as bad as it used to be. Cassette hubs are usually pretty well machined and sealed bearing hubs and modern freewheels also tend to spin much more centrally than old loose ball hubs which had a tendency to be machined off centre.
Chain wear or stretch is the most neglected cause. Believe it or not chains “stretch” over time, the pivots wear and become loose and tricks like disasters only make it worse. So instead of ten, half-inch long links adding up to five inches they might be five point one inches long, and to complicate matters the next ten might not be stretched as much. As these different length links move round the drive train they can be pulled to the right length as they run over the sprocket teeth (or allowed to sink deeper on worn teeth) and this makes the chain length (and therefore tension) vary.
First get the bike upside down and stable. You can check the chain tension by wiggling the chain up and down but if the tension is set up OK you should just be able to feel the extra resistance as you move the pedals. Now move the cranks to the position where the chain is tightest. Remember where it is. In my case it is with the right pedal at the top (or bottom when the bike is upside down). Next give the cranks several turns and check again. Is it still in the same position? Do it a couple more times to double check.
If the tight spot is always in the same place (eg. the chain is tight every time the right pedal gets to the top of it’s stroke and it happens every turn, and the loose spot is opposite) then the problem is with your front sprocket. If on the other hand it changes every few pedals; so one minute it might be with pedals up and down and next it is when they are level, then there is a problem with the chain or rear sprocket.
Of course very often there is a problem with more than one part of the drive-train. If you have done some hard “rocks” then you might have stretched the chain un-evenly, but it is also very likely that you have mashed in a few sprocket teeth and moved the sprocket off centre too. In this case you will need to spend a little time figuring it out, maybe there are two tight spots sometimes, maybe the tight spot is uber tight on one occasion and not as bad on the next rotation.
So now you know roughly what is responsible for the problem, but what can be done to “fix” it?
Well first off you need to be realistic about how good it can ever be. Pathetic as it may seem, it is pretty much impossible to get perfectly even chain tension. You are always going to have a bit of a tight spot so if you were hoping to get rid of it entirely then forget it. Having said that, if your chain makes a horrible “crunking” noise as you go through the tight spot, and/or almost falls off in the loose place then you do need to try to improve matters.
If your tight spot moves around then look to your chain first. A broken chain is such a nasty failure that it is generally worth spending a tenner on a new one if you are at all worried about your current one. The rear sprocket is the other possible cause but it is only likely to be a serious problem if you run a free-coaster. Freecoaster drivers can become distorted and this leads to all kinds of weirdness.
But the most likely scenario is that your tight spot will be consistent with crank position and the front sprocket will be to blame, so look at it. If your teeth are mangled to hell on one side or it is obviously bent then it may be time for a new one. On the other hand it is surprising how many people simply install it wrong. Before you take it apart it is worth making a few marks on it to remind you where the chain was in contact with it when the tight spot occurred. Most sprockets need a “top hat” spacer to fit between the sprocket and the crank axle. This should nearly always be fitted with the ÒbrimÓ of the top-hat on the bottom bracket side of the sprocket rather than the crank arm side and it MUST be the right size. It should be a good tight fit into the sprocket and nice and snug on the axle. If it visible wobbles on either of these then either get a new one or try to shim it out with some baked bean tin. Many people use a pop can to make a shim but this soft aluminium isn’t nearly as effective as the steel of a baked bean tin. The shim should be even all the way round the spacer but if this isn’t possible then you need to put it at the back at the point when you normally get the tight spot. In other words, if the tight spot occurs when your right pedal is at the top (as mine does), then the sprocket is obviously at its furthest forward point when this happens, so shimming the opposite side to the part the chain is on will help bring it back to the centre.