Gearing Down

Most bicycles that I have bought, be they hybrid bicycles or mountain bikes, don't appear to have adequate low gearing for hill climbing with a load. Even a touring bicycle that I bought was not geared low enough for loaded hill climbing. All bikes required at least the fitting of a granny gear on the front chain rings, except for one mountain bike which had a low enough gear on the front.

The non technical explanation for low gearing, is to get the smallest cog possible on the front chain ring, and the largest cog possible on the rear gear cluster. Most bikes suitable for touring will have a triple chain ring set on the front, and an 8 or 9 speed rear cluster. The smallest cog commonly available to fit a front chain ring is a 24 tooth (24T). A small cog on the front is commonly referred to as a Granny Gear. I have seen granny gears as small as 18 teeth.

With the exception of one mountain bike that came with a 24T granny gear. all my other bikes came with a 30 tooth (30T) small cog. Hauling a load up a steep hill whilst on tour, I found that this is just not low enough. Some bikes do not lend themselves to having the smaller cog changed without having to change all of the cogs. This can get quite expensive.

Most mountain bike and hybrid style bike's front chainring sets are spot welded together, so you normally have to buy a whole complete new set of cranks and chain rings. Road bikes however, use a chainring set where the cogs are bolted together. These are easily changed with a few exceptions. Some "touring" bikes, such as the last one I bought, came equipped with mountain bike running gear, but has road style front chainrings and a road style front derailleur and shifter.

Common chain rings combinations for a mountain bike with low gearing are:

• 24 tooth;
• 32 - 36 tooth; and
• 42 - 44 tooth.

Common chain rings combinations for a mountain bike without low gearing, or a hybrid are:

• 30 tooth;
• 36 tooth; and
• 44 - 48 tooth.

Common chain rings combinations for a touring style bike are:

• 30 tooth;
• 42 - 44 tooth; and
• 52 tooth.

My last touring bike purchased came with the last combination of 30 - 44 - 52. Whilst the 52T is great on a downhill, or flat with a tail wind, the 30T low gear was just not adequate for loaded hill climbing. Before changing any chainrings there is one other consideration to take into account, and that is the front derailleur's motion range. Most manufacturers will specify a range for the front derailleur. In my case, it was specified as having a 22 teeth range. Looking at the gears, there is a difference of 22 from the small 30T to the large 52T.

In this case, fitting a 24T small gear would necessitate changing the other two chainrings as well to keep within the front derailleur's range, or getting a new derailleur with a bigger range. After a bit of thought, I decided to experiment and see what would happen. Most manufacturers advise against using gears that put too much lateral strain on the chain. What this means is do not use both the small cog on the front and the small cog on the rear, or the opposite of large front cog to large rear cog.

So keeping that in mind I ordered a 24T chain ring to suit the bolt pattern of my inner chainring. There are patterns with 4 bolt holes and 5 bolt holes. Also the diameter has to be measured. Mine was a 74mm diameter inner chain ring with five bolts.

View of front chain ring set with 24 tooth inner chain ring fitted.

I found after fitting the 24T small cog, that the change down from the 44T to the 24T sometimes caused the chain to come off. This is often referred to as "chain suck". If I took it easy when changing down it was fine. I then fitted a device designed to stop the chain coming off to solve this problem. Changing up was fine if I took it easy. Now as for the range, the cage on the front derailleur is quite long, and I found that if I used the largest 4 cogs on the rear cluster there wasn't any problems. When I changed gears on the rear down to the fifth cog the chain started rubbing the bottom of the front derailleur.

This is fine with me as I rarely use these combinations anyway. So now I have a granny gear on the front which I only use for hill climbing with a load. Most of the time the front middle chainring is used. The largest rear cog fitted on the bike is a 32T. It is then a fairly close range of gears from there down on the rear cluster.

The only size bigger than the 32T is a 34T cog. I could fit one of these, by changing the rear cluster, but it usually results in a large gap between some of the gears. I have found whilst touring that it is better to have closely spaced gears. Most mountain bikes and hybrids come with either a 34T, 32T, 30T or a 28T as the largest cog on the rear.

A view of a rear cluster that has a 34T large cog.

If there is too much of a gap between the rear gears, you may find one gear is too low, yet by changing gear it is then too hard to pedal. Or when coming up to a hill, by changing down to a gear which is too low, your speed will drop off too quickly and you lose momentum. By keeping the gears closely spaced, you can change to an easier gear without losing too much speed.

On some bikes you will find that the rear cluster is one entire unit, and others, the cogs are interchangeable. The entire cluster can be separated and the cogs then mixed and matched to your preference. Removing the rear cluster also varies from bike to bike, and may become the subject of a later technical article. Thanks again to Crank for another fine set of photographs. This time of the cluster with the 34T large cog.