One of the main parts of the bike which I’ve still to get to grips with is the front mech, aka front derailleur, or if you’re wanting to use the real French spelling, dérailleur. In other words, the thing that changes the gears on your chainrings. I have to be honest, the thing has constantly had me worried and it rarely works very well on my commuter. When I got it, back when I hadn’t a clue about bikes in the slightest, I got a bike with a triple chainring merely because that’s what the bike came with. If I was to now choose what I have on it, it would have a double. Indeed, as and when I change the groupset on my bike, I’ll go for a Tiagra groupset with a compact double, since I’m not expecting to whizz along above 30mph, and a cassette of 11-28 should allow for pretty much all conditions.
But I’ve already gone off subject, so let’s get back to the front mech.
My front mech currently does not like to go in to the big chainring. It’ll happily go into the granny ring which I don’t believe I have ever used, except by accident. But trying to get the bastard into the big chainring is either hit and miss, or 100% miss.
As with all things bike, I’ve tended to be a bit scared when it comes to areas that I’ve not touched before. But then, what typically happens is that I then discover how to maintain or replace a part, and realise that, so long as you have the right tool, it’s actually pretty basic. The front mech is no exception.
This video pretty much sums up how simple it is; but I do warn you, there’s more to it than this:
That video does mention that the front mech is a braze-on. This has baffled me for months, if not years, because I knew you also got clip on (aka clasp on) mechs, and also you can get braze-on mechs, and I wasn’t sure how on earth you told the difference. Also, I wasn’t sure at all how on earth these things attached to a bike.
So, that video has a braze-on. I have the impression from many bike-related web sites that this is considered the standard, but if that is the case, then why are two of my three road bikes not equipped with them? Maybe I’m unlucky, but I’d guess that braze-on is either new, or for higher-spec bikes. Probably both. The alternative is that you get front mechs which quite simply have a ring attached to them, which wraps around the seat tube, down the bottom. Or, as I have now discovered, you can get “adapters” which allow you to attach a braze-on mech to your bike if your bike frame does not come with the braze-on thingy. And when they say “adapter”, they really just mean a ring which wraps around your seat tube, and you attach your front mech to said adapter.
The only additional thing which might confuse you, when looking at clip-on mechs, is that they come in different sizes – 28.6, 31.8 and 34.9. What on earth are these? They are simply the diameter of your seat tube, which is why the braze-on ones don’t come in sizes, since they don’t have a ring to go around your seat tube. But how do you know the diameter of your seat tube? Well, remember when you did mathematics at school, and they told you about pi, and when to use it? That’s right, you’re quite possibly going to find a use for that for the first time in your life. Measure the circumference of your seat tube, and divide by pi (3.14159). Bingo. If your seat tube is 100mm, you need a 31.8. If it’s 110mm, you need a 34.9. And if you have a slim 90mm seat tube, you guessed it, 28.6 is for you.
I’m now going to go buy a new front mech plus gear cables, and have a go at replacing all of that lot. This could render my commuter out of action.