Frame geometry

The angles and lengths that decide how a bike rides: stable or nimble, easy to climb or eager to descend. Every model publishes them in a geometry chart.

Also known as: geometry, geo, bike geometry, geometry chart

Key facts

Origin
the angles and lengths of a frame, from the geometry chart

The numbers that matter

  • Head angle — how far the fork leans out. Slacker (63–65°) is stable at speed on steep ground; steeper (67–69°) steers quicker.
  • Seat angle — steeper (76–78°) puts you over the pedals on climbs.
  • Reach — the horizontal distance from the bottom bracket to the top of the head tube; how much room you have standing.
  • Stack — the height of the front end.
  • Chainstay length — shorter is playful, longer is planted.
  • Wheelbase — the distance between the axles; longer is calmer.
  • Bottom bracket height — lower corners better, higher strikes pedals less.

Long, low, slack

Over the last decade mountain bikes have grown longer in reach, lower at the bottom bracket and slacker at the head. Combined with dropper posts, that is why today's bikes descend so much better than older ones of the same travel.

Related articles

  • Dropper post — A seatpost that drops at the push of a lever on the bar and rises again on its own. The saddle goes out of the way for the descent and comes back for the climb, without stopping.
  • Wheel size — Mountain bikes roll on 26′′, 27.5′′ or 29′′ wheels. Bigger wheels roll over obstacles more easily and carry speed; smaller ones are quicker to turn and throw around.
  • Cross-country (XC) — Mountain biking as an endurance sport: as much up as down, on light and efficient bikes, where fitness wins races. The only mountain-bike discipline at the Olympic Games.
  • Enduro — Racing the descents and riding the climbs: timed downhill stages linked by untimed transfers you pedal up yourself. Also the name of the long-travel bikes built for it.
  • Attack position — The standing stance every descent starts from: pedals level, knees and elbows bent, weight centred over the bike and eyes up. From here you can react to anything the trail throws at you.
  • Mullet — A bike with a 29′′ front wheel and a 27.5′′ rear: the big wheel rolls over the trail, the small one keeps the back end agile and out of your way on steep ground.
  • Suspension travel — How far a wheel can move on its suspension, in millimetres — the first number in any mountain bike's description. More travel absorbs bigger hits; less travel pedals and weighs less.