While there’s a lot to know when it comes to bike frame geometry, understanding the basics goes a long way. Read on to get familiar with bike sizes and dimensions, to help you choose the right size for the way you ride.
Wheelbase refers to the horizontal distance between the centre of the front and rear axles. A bike’s wheelbase affects steering speed, responsiveness and stability.
A longer wheelbase allows natural flex and compliance, resulting in a more comfortable and stable frame. Urban bikes, touring bikes, and entry-level mountain bikes are designed for comfort and stability, and generally feature longer wheelbases.
A shorter wheelbase creates a stiffer, more responsive frame and significantly reduces flex and compliance. Performance bikes (road or mountain) are designed to be responsive under power, and usually feature short wheelbases.
This is the distance between the centre of the bottom bracket to the centre of the rear axle. It plays a role in determining a bike’s ride quality and responsiveness under power.
Long chainstays add flex, so they reduce the amount of shock transferred to a rider. As a result of having some flex, they track (hold a straight line), but don’t steer as quickly as shorter models. Chainstay length is not the only factor in how a bike handles, but in general, long chainstays are found on urban bikes, touring bikes, and entry-level mountain bikes.
Chainstay length is critical in transmitting power to the rear wheel. Short chainstays feature less flex, so less energy is lost when pedalling. The result is better acceleration under power. Short chainstays are generally featured on performance road, mountain and cyclocross bikes.
Head tube angle
The angle of the head tube affects a bike’s steering and riding position. The slacker the angle, the more effort required to steer. The steeper the angle, the less effort required to steer.
Touring bikes and mountain bikes generally feature slack head angles. Touring cyclists rarely need to change direction quickly, so they benefit from a comfortable and stable ride. Mountain bikers benefit from slower steering to increase control on rough surfaces and to hold a steady line through technical terrain.
Performance road and racing bikes have steep head angles. The ability to rapidly change direction is important when racing or riding in a peloton.
The length of the head tube also influences how a bike feels to ride. More length offers an upright and comfortable ride, less length feels aggressive and positions you for racing or hard riding.
Top tube length
You’ll see “top tube length” or “effective top tube length” on manufacturer’s bike geometry info. Most top tubes aren’t perfectly horizontal, so “effective top tube length” isn’t the length of the tube, but the horizontal distance spanned from the head tube to the seat post. What is describes is how stretched out you’ll be on the bike.
Comfort-oriented bikes feature a shorter top tube combined with a slacker head angle. Performance-oriented bikes often combine a longer effective top tube length with a steeper head tube angle.
Imagine your bike has wheels one inch in diameter. A wheel this small isn’t much bigger than the obstacles it has to roll over, so significant momentum is lost as it navigates them. Now imagine your bike has wheels that are ten feet in diameter, like a monster truck. These enormous wheels will lose very little momentum when navigating obstacles. Large-diameter wheels (700C, 26, 27.5, or 29-inch) can steamroll over obstructions, and have less rolling resistance than small-diameter wheels (20 to 24-inch). But larger diameter wheels, also take more power to initiate momentum and get rolling, so they can feel more slugish than smaller wheels.