Bike Frame and Geometry Glossary: Stack, Reach and Angles

The main structure of the bike, which the fork, wheels and every component attach to. It's made of aluminum, carbon, steel or titanium, and almost always comes in several sizes.

See also: geometry, frame size, carbon fiber Geometry Comparison

The set of frame measurements and angles (stack, reach, angles, chainstays, wheelbase) that decides your riding position and how the bike handles. Two size M frames from different brands can differ by 2 cm (0.8 in) or more in reach.

See also: stack, reach, head tube angle Geometry Comparison

The frame's nominal size: in centimeters (54, 56), inches (17, 19) or letters (S, M, L). Each brand measures it its own way, so to compare across brands it's better to look at stack and reach.

See also: stack, reach, standover height Bike Size

The vertical distance from the center of the bottom bracket to the center of the top of the head tube. More stack means a higher handlebar and a more upright position.

See also: reach, head tube, headset spacers Geometry Comparison

The horizontal distance from the bottom bracket to the top of the head tube. It tells you how far you stretch to the handlebar: 1 cm more reach is roughly like a stem 1 cm longer.

See also: stack, stem, top tube Geometry Comparison

The tube that joins the head tube to the seat tube. Its effective length, measured horizontally, was the main sizing reference for years; today reach is used more.

See also: reach, standover height

The tube that runs from the head tube to the bottom bracket shell. It's usually the thickest because it carries most of the pedaling and braking loads; on e-bikes it often houses the battery.

See also: bottom bracket shell, battery

The nearly vertical tube the seatpost slides into. Its inner diameter decides which seatpost you need: 27.2, 30.9 and 31.6 mm are the most common.

See also: seatpost, seatpost clamp, seat tube angle Saddle Height

The front tube of the frame, inside which the fork turns on the headset bearings. A longer head tube gives more stack and a taller position.

See also: headset, stack, head tube angle

Each of the two tubes running from the bottom bracket shell to the rear axle. Short chainstays (about 410 mm on road bikes, 430-440 mm on mountain bikes) make a bike more agile; long ones, more stable. Their length is a key number for sizing a chain.

See also: seatstay, dropout, chain Chain Length

Each of the two tubes that join the top of the seat tube to the rear dropouts. Thin seatstays that join lower than usual filter out more vibration.

See also: chainstay, dropout

The piece at the end of the chainstays and seatstays (and of the fork legs) that holds the wheel axle. It's open on bikes with a quick release and closed, with a hole, on thru-axle bikes.

See also: thru axle, quick release, derailleur hanger

A small bolt-on part that connects the rear derailleur to the frame. It's designed to bend or break in a crash before the frame does; there are hundreds of models, so it's smart to carry a spare. SRAM's UDH standard aims to unify them.

See also: rear derailleur, dropout

The angle between the head tube and the ground. About 73° on a road bike gives quick steering; the 63-65° of an enduro bike gives stability on descents at the expense of agility in slow corners.

See also: trail, fork offset (rake), wheelbase Geometry Comparison

The angle of the seat tube relative to the ground. A steeper angle (76-78° on modern mountain bikes, about 73-74° on road bikes) puts your hips more over the pedals and helps on climbs.

See also: seat tube, saddle setback Geometry Comparison

The distance between the two wheel axles. It's around 980-1,010 mm (38.6-39.8 in) on a road bike and over 1,250 mm (49.2 in) on an enduro bike: a longer wheelbase means more stability and less agility.

See also: chainstay, head tube angle Geometry Comparison

How far the bottom bracket sits below the line connecting the two wheel axles. On road bikes it's around 70 mm: the more drop, the lower the center of gravity and the more stable the bike.

See also: bottom bracket height, bottom bracket shell Geometry Comparison

The height of the bottom bracket above the ground. It depends on BB drop and on wheel and tire size; on mountain bikes a low BB (330-340 mm, about 13-13.4 in) gives stability but means more pedal strikes on rocks.

See also: bottom bracket drop, crank arm Geometry Comparison

The distance on the ground between where the steering axis meets the ground and where the front tire touches it. More trail (about 60 mm or more on road bikes) makes steering more stable; less makes it livelier.

See also: head tube angle, fork offset (rake)

The distance between the front axle and the steering axis. At the same head angle, more offset reduces trail; 37-51 mm is common on mountain bikes and 43-50 mm on road bikes.

See also: trail, fork, head tube angle

The height of the top tube where you stand over the bike. You want a few centimeters (an inch or two) between your crotch and the tube; more clearance on a mountain bike than on a road bike.

See also: top tube, frame size Bike Size

The part that holds the front wheel and turns with the handlebar. It can be rigid (carbon, aluminum or steel) or a suspension fork, with a spring and damper.

See also: suspension fork, steerer tube, fork offset (rake)

A 12 or 15 mm axle that passes through the hub and threads into a closed dropout. It's stiffer than a quick release and always puts the wheel in exactly the same spot, which matters with disc brakes.

See also: quick release, Boost, hub

A 5 mm skewer with a cam lever that clamps the wheel into open dropouts without tools. It was the standard for decades; today you mostly see it on city, comfort and entry-level bikes.

See also: thru axle, dropout

A wider hub standard for mountain bikes: 110 × 15 mm front and 148 × 12 mm rear, versus the older 100 and 142 mm. It spreads the hub flanges for stiffer wheels and leaves room for wide tires.

See also: thru axle, hub, chainline

The part of the frame where the tubes and chainstays meet and the bottom bracket is installed. It can be threaded (BSA, 68 or 73 mm wide) or press-fit (BB86, BB92, PF30…).

See also: bottom bracket, bottom bracket drop

Cables and hoses that run inside the frame instead of outside. It looks cleaner and is more aerodynamic, but complicates maintenance, especially when they also run through the headset.

See also: cable housing, brake hose, headset

The bolted ring that clamps the seat tube around the seatpost. With carbon posts or frames, tighten it with a torque wrench (often 5-6 N·m, about 44-53 in-lb) and carbon assembly paste so you don't crush the post.

See also: seatpost, seat tube

A composite of carbon fibers and resin. It makes frames lighter and stiffer than aluminum, but it gives no warning before it fails: it's damaged by sharp impacts and overtightening, so always follow torque specs.

See also: frame, seatpost clamp