Fitting the Big Dog a.k.a Can I Fit a Larger Chainring?

First published February 15, 2021

The other day I was talking to my brother, and he was telling me that he wanted to up the size of the Big Dog (outer chainring) on his TT bike. He asked if there was anything he needed to consider. This got me thinking, and I realised there are several considerations, so I’ve written this piece in case you have the same question.

When you fit a different-sized outer chainring, you have to reposition your bike’s front mech (derailleur). If you don’t, when you shift chainrings, your front mech will either hit the chainring (it it’s larger), or it will throw your bike’s chain (if it’s smaller).

Front mechs generally attach to your bike in one of two ways. The first is the band-on method: a band wraps around your frame and the front mech connects to that band. In this instance, you can, normally, move the band up or down to adjust the derailleur height. The second method is the braze-on method: a small piece of metal or carbon is permanently connected to your bike’s frame.  In this instance, you cannot move the brazed-on fixing, but the fixing does allow a small amount of vertical movement. If your bike has a braze-on fixing, check that the front mech has enough room to move up or down to accommodate a different-sized chainring.

Every rear mech (derailleur) has two constraints: the largest usable cog size (easiest gear on your cassette) and its total capacity. If you exceed your mech’s capacity, it may not function correctly. You can find the capacity of your rear mech on its manufacturer’s website. To calculate your current setup’s capacity, use the following formula:

For example, a compact chainset (50/34) with an 11-28 cassette:

If the capacity exceeds your current derailleur, not all hope is lost. Often, reach mechs operate without a problem when they slightly exceed the manufacturer’s guidelines. So, you can buy a chainring, fit it, and try it while your bike is on a workstand, suspended, or upside down, but don’t try it under load because if there is an issue, you may damage your bike. The safer approach is to buy a new rear mech. Most manufacturers provide two or three versions of their mechs: a short cage, a medium cage, and a long cage. If your bike currently has a short cage, a medium cage will probably give you the extra capacity you need.

Note: road bikes normally use short or medium cage mechs, with long cages reserved for MTBs or bikes with triple chainsets.

If both your front mech position and your rear mech capacity allow for a different chainring, it’s great news. Now, there are a couple pieces of information you need to gather, and then you’ll be ready to buy. The first is does your bike have a single or double chainring. You must buy a chainring that matches your current setup. This is because double chainring setups have chainrings that have specially designed ramping teeth, which allow your chain to move from one chainring to the other. On the other hand, single chainring setups have special teeth that prevent your chain from coming off the chainring.

When you buy a new chainring, you’ll need to attach it to your bike. There are different ways to mount chainrings, and these vary between manufactures and models. In most cases, road bike chainrings are mounted with special bolts which, funnily enough, are called chainring bolts. If your bike has this setup, you need to know two things: how many bolts there are and their Bolt Circle Diameter (BCD).

The BCD relates to the distance between the chainring bolt holes. If a chainring’s BCD does not match your bike’s chainset precisely, it will not fit and you’ll have to return your purchase. Luckily, calculating BCD is not complicated. Firstly, measure the distance in millimetres from the centre of one hole to the centre of its neighbour. Then multiple this by 1.414 if you have four holes and 1.701 if you have five holes. For example, here is the calculation for a four-hole Shimano compact crankset:

Measuring to calculate BCD

You’ve calculated your BCD, checked whether you have a single or double setup, and ensured that your mechs can handle a new chainring. It’s time to buy! There are just two final things to mention. The first, if you can buy a chainring from the same manufacturer as your current chainrings to ensure compatibility. The second is that if you are buying a larger chainring, you are likely to need a new chain because your existing one will be a little too short. That’s it, you’re ready to buy, so start clicking or pop down to your local bike shop where they can always check you’ve made the correct choice.

Photo credit: Igor Flek