|
Gear ratio It is often misunderstood, but gear ratio is not determined by power. Gear ratio is determined by speed and cadence. For example, when climbing a hill, the minimum gear ratio needed by a heavily muscled rider who struggles with hills is actually lower than that required by a slim climber who can ascend smoothly. Suppose the lowest speed you reached on a course you've ridden before was 14 km/h. Converting this to meters per minute gives 14,000 / 60 = 233 m/min. Dividing this by the wheel circumference (approximately 2.1 m) results in a wheel rotation speed of 111 rpm. To maintain a cadence of 70 rpm, you'll need a minimum gear ratio of 111 / 70 = 1.59. If you're using a 50-tooth outer chainring and a 34-tooth sprocket, your gear ratio is 1.47, which means you can stay in the outer ring while climbing. However, if your largest gear is a 30-tooth sprocket, the gear ratio of 1.67 is not sufficient to maintain 70 rpm. Alternatively, if you need to use the inner chainring, you can opt for a lightweight 11-25 tooth sprocket. Personally, I prefer to stay in the outer ring whenever possible to avoid the risk of dropping the chain and to minimize shifting losses. If the speeds shown in simulations are used, allow a 10%-20% safety margin because thay do not account for subtle undulations in the terrain. |