GearCalc
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"GearCalc is a computer programme that does what its name suggests and, as they say, a whole lot more" - Cycling Weekly
"It's good fun and a great way to optimise your gear ratios." - Cycling Plus
"hey presto - the answer's there" - Cycling & Mountain Biking Today
Riders can opt for up to nine cogs on the rear wheel. Combine this with a triple chainring and you have, in theory got a twenty seven gear groupset. In practice you will probably find that only about fifteen of these will be used. The remainder will probably be duplicates, or near duplicates i.e. different ways of making the same gear.
The more cogs you put on the back, the more the wheel must be dished to compensate, and the narrower the hub flange spacing. The result is a rear wheel that will self distruct sooner due to increased stresses.
With a single chainring chain path can only be straight in one gear, i.e. one per chainring. In all the other gears the chain is pulled sideways, causing reduced efficiency, and increased friction and wear due to the extra lateral forces. A wider gear block means extra lateral forces if the extra gears are to be used.
The most efficient transmission systems are single speed, fixed wheel set-ups. Preferably with a belt, rather than chain drives. This is partly because the chain line will be straight, and partly because the chain does not need to follow a rather tortuous path around the two jockey wheels. Obviously a fixed wheel is a bit impractical most of the time though.
Bigger cogs, are more efficient because the tight angles of rotation formed by the chain around a small cog generate extra friction. Enormous chainrings increase wind drag, and weight.
From a rider efficiency perspective, the more gears the better, and conventional wisdom suggests that rider efficiency is considerably more important than transmission efficiency. The rider can be compared to a high performance engine, with a narrow power band. Bunching gears close together allows the rider to keep within this power band. Generally speaking below about 60-75 rpm there is an increased risk of knee strains. You wouldn't attempt to pull away in top gear in the family car, why ask your body to do the equivalent? Above around 90-120 rpm most riders start to become seriously out of breath and there is more to be gained from changing up than pedalling faster. It is very common for racers to pedal at between 90-100 rpm. Racers will often build custom gear blocks specifically for particular events. The trick is to design a groupset that produces a nice simple shift pattern, with appropriate steps between the gears, and an adequate range.
The two most commonly used measures of how big a gear is are the inch ratio, and the metre ratio. Both relate to the size that a penny farthing wheel would need to be to produce the same gearing. The metre ratio is its circumference in metres, or the distance moved by the bike in one complete pedal revolution. The inch ratio is the diameter, or the distance in inches moved per pedal revolution divided by PI.
The metre ratio (MR) is defined as:
Because most of my local bike shops still use the inch ratio, I shall stick to these units: Gears that are spaced less than about 4 inches apart will hardly feel any different from each other. Racers prefer steps of 9 inches or less between the gears. Steps of around 10-12 inches are ideal for the Bristol city centre rush hour grand prix. A wider spacing is preferred here because for safety reasons it's worth keeping my fingers near the brake levers for more of the time. Also it is easier to avoid having to stop in a gear that is too high to pull away in. Steps of more than about 12-14 inches are likely to feel unacceptably large to most people.
A 700-25C tyre has a circumference of about 211.5 centimetres giving a diameter of 26.5 inches. With a tyre of this size and a fifty two tooth chainring, the step between 52/13 and 52/12 is 8.8 inches. As you can't have half a tooth this is the closest spacing possible. A 52/13 (106 inch) gear puts the rider at 27.4-29.6 mph at 87-94 rpm. The step between 42/23 and 42/20 is only 7.3 inches.
GearCalc is a program that does all these sums. Initially by generating a table of inch (or metre) ratios, speeds at upper and lower cadence, and a graph giving a visual representation of the steps between each gear. Then by collating a table based only on the gears that you are actually using, showing the size of step between each gear and the previous gear, and the optimum speed range for each gear based on a constant cadence. It also tells you to the nearest rpm what cadences these up and down gear changes correspond to. Its uses to the more competitive rider are obvious, and it makes good economic sense too. Custom made freewheel/cassettes are expensive, its worth getting the correct values first time.
Perhaps less obvious is GearCalc's usefulness to non racers: If you walk into three different bike shops and ask for a freewheel with a range of 28-14 teeth, the chances are that each shop will have at least one of these available as a relatively cheap 'off the shelf' item. It is also probable that each of these 28-14 freewheels will be different. I.e. the 'in between' cog values will differ. The differences can dramatically effect your ride. The sprocket manufacturers seem to be deciding the 'in between' values with a random number generator. Providing that the cogs generally get bigger from one side of the block to another this is considered acceptable. Most of these blocks will seem pretty naff when analysed with GearCalc, but a few are pretty good. All you need to do is enter the numbers and the differences will be immediately obvious, making gear selection a breeze.
This is by far the best gear ratio calculator for cyclists you can get - beware of imitations!
A ‘386 or better, running Windows 3.1, Windows 3.11, or Windows 95
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