How can I increase the highest gear ratio?

Question

I am looking for some higher cog front gears for an old bike I am considering. I've been away from it for some time and don't know the current number of cogs on the front ring, but I am looking to maybe put a really large ring on it. I just find that there are times I need one or two more gears to shift up to. I don't always ride in the highest gear, but I have way too much in the lower range.

Is it possible to switch to a larger front ring to boost my higher end range?

Answer 1

Some background on the front chainrings:

• Lots of bikes have a front derailleur, allowing more than one chainring. Some bikes have a single front chainring and no derailleur.
• They're called chainrings rather than cogs (which are at the back).
• There are typically either 3 or 2 chainrings on the front crankset.
• If there are 2 chainrings, there are two 'flavours': Compact (with smaller rings, eg: 50 tooth and 34 tooth) or Standard (with larger rings eg: 53 tooth and 39 tooth)
• Another important stat is the BCD (Bolt Circle Diameter). This is the diameter of the circle described by the bolts attaching the chainring to the crankset. A common figure is 130mm.

So you have a few factors to consider, and there's more information we need.

• Do you have a front derailleur?
• How many chainrings are in the front?
• What make and model do you currently have?
• How many teeth are on each chainring?
• How many cogs are you your rear cassette?
• How many teeth are on the smallest and the largest cog on the cassette?

With this information we can give you some advice. Just as an alternative, you can also replace the cassette at the back to change your gear ratios.

Answer 2

The simplest way is to take the bike to your LBS and ask them what the options are. The answer may be limited by what they have in stock and might cost more than buying parts on the Internet, but they'll do the work for you and you'll be sure that the parts they supply will work together or they'll be obliged to fix it.

To do it yourself, start by counting the number of teeth on the chainrings at the front and the cogs at the back (aka the cassette). Then you can plug the numbers into a gear calculator. That gives you a table of either gain ratios or gear inches which can easily be compared to other cobinations of rings/cogs you might consider.

Bear in mind that not all drivechain parts are compatible and it's safest to stick within the same brand/model. Also, if you significantly change the size of the cogs/rings, your existing deraileur and chain may not have enough scope to take up the slack.

Answer 3

Usually there are three types of road gearing

• Compact Double (50 / 34)
• Traditional Double (53 / 39)
• Triple

Where the numbers is the number of teeth on each ring. Very old racers (pre-90s) tended to be very different, so it depends on what you mean by old.

Most modern road bikes come with a Compact Double chainset these days, older bikes normally have closer groupings because the derailleurs at the time couldn't deal with the range. If you largest chainring on the front is larger than 53, I doubt you would need larger gearing and maybe you might think about riding at a higher cadence instead.

Most rear cassettes have 12-26 teeth on the back for systems that 7 speed or higher in my experience.

If you feel you need to change the front chainrings, you need to know the number of arms the crank arm has and the BCD. It usually says on the chainring, or as others had said ask your local bike shop.