Understanding Bicycle Geometry

The geometry of a bicycle frame is all about balancing tradeoffs. Every change that improves one aspect of a bike’s handling will sacrifice another. Ultimately, the perfect geometry for you comes down to what you use the bike for, your body proportions, and your personal preferences. We’ve broken down three major bicycle handling characteristics—rider position, steering, and cornering—to give you an overview of how differences in geometry affect your ride.

Rider Position

Most affected by: Stack, Reach

Your position on the bike is determined primarily by the stack and reach measurements. A higher stack and shorter reach will result in a more relaxed upright position. A shorter stack and longer reach will place you in a lower, more aggressive position.

A relaxed geometry has the advantage of being more comfortable over longer distances. The more upright position also opens up your chest, helping you breathe better on those long climbs. Finally, relaxed geometries tend to place your center-of-gravity farther back on the bike, helping maintain rear-wheel traction on slick terrain or steep climbs.

More aggressive geometries trade in some comfort for improved efficiency. A lower stack leans you downward into a more aerodynamic position, while a higher reach positions your body weight farther forward, helping you more efficiently transfer power to the pedals.

Drop bars (commonly found on road-style bikes) naturally provide flexibility in your position. Placing your hands back on the flats puts you in a more relaxed position; reaching forward to the hoods moves your weight forward into a moderate position; and down in the drops gets you low and aerodynamic in an aggressive position.

Steering

Most affected by: Head Tube Angle, Trail

The mechanics of a bike’s steering behavior are complicated. Virtually every part of a bike’s geometry contributes in one way or another to how it handles. Here, we’ll focus mainly on steering speed—how responsive the bike is to steering input (either turning the bars, or shifting your weight). Steering speed is primarily determined by the angle of the bike’s head tube, and trail. In general*, bikes with slack head tube angles and high trail will have slow steering. Bikes with steep head tube angles and low trail will have fast steering.

Bikes with slow steering require more effort to turn, but will feel more stable and are better able to hold a line at high speeds. Slow steering is a great fit for off-road bikes, where more stable handling helps you maintain control over rough terrain. Bikes with slow steering also tend to respond well to steering with your weight, rather than with handlebar input.

Bikes with fast steering will respond quickly to even light handlebar input, but tend to wander at higher speeds. Fast steering is a good fit for road bikes, where quickly dodging potholes and maneuvering in a group of riders is important.

* Since the trail measurement is derived from multiple frame geometry measurements, this description glosses over a lot of subtleties in bike handling. Low trail is usually found on bikes with steep head tube angles, and vice-versa, but frame designers can change a bike’s trail independently of head tube angle by adjusting the fork offset. Check out the sections describing the individual geometry measurements for slightly more detail.

Cornering

Most affected by: Wheelbase, Trail, Bottom Bracket Drop

A bike’s cornering abilities are mainly* determined by wheelbase, and trail. A long wheelbase and high trail will correspond to more stable cornering. A short wheelbase and low trail will make you more nimble in the corners. Cornering is closely correlated with steering, so typically bikes with stable cornering have slow steering, and bikes with nimble cornering have fast steering.

Bikes with stable cornering will lean into turns with you, and more easily track smooth lines through sweeping turns. This helps the bike flow through rough trails with greater stability and consistent control at high speeds. At low speeds and on climbs, a bike with stable cornering can feel squirrely.

Bikes with nimble cornering will be agile and maneuverable at lower speed, helping you confidently execute tighter turns. At speed, it will take more effort to follow a clean line through corners.

* Bottom bracket height also plays a role in cornering because it affects how high your center of gravity is on the bike. BB Height is usually chosen based on the bike’s ground clearance requirements, and cornering characteristics are a side-effect of this decision.

Fit

Despite all the options and tradeoffs in bike geometry, the most important aspect of selecting frame geometry is finding a bike that fits you. Your body proportions can dramatically change the performance characteristics of a bike. If you’re an inexperienced rider, it can be difficult to judge the fit of a bike based on a 5 minute parking lot ride. Don’t be afraid to ask your local bike shop employees about fit, or use a fit calculator if you’re purchasing online. Once you pull the trigger, paying a visit to a professional bike fitter is well worth the expense. Even if a frame fits you well, fine-tuning saddle position, seat post, stem, handlebars, and pedals makes a world of difference. Stack and reach measurements are a good way to quickly gauge a bike’s fit. Once you’ve found a bike that fits you well, use its stack and reach as your starting point for your next purchase.