The bike, part by part
Tap a tube or a part: what it is, where it runs from and to, which angle belongs to it, what longer, shorter, steeper or slacker does, whether you can swap it and how race, endurance and gravel bikes build it.
Tap a part
In the picture on a tube, the saddle, the bar or a wheel, or below on a name. The measurements that are not parts — stack, reach, wheelbase and trail — are under “Measurements”.
These are exactly the names in a geometry chart. The numbers in the picture are those of a real size 58 of the chosen type.
Frame
Seat tube
The tube that holds the seatpost.
- From where to where
- From the bottom bracket up and back to the seat clamp. Its angle to the horizontal is the seat angle.
- Seat angle
- 73.2° on the Race 58 in the picture. The angle between seat tube and horizontal, measured at the back. The chart gives the length from the centre of the bottom bracket; for your position it hardly matters, since you set saddle height at the post.
- What more or less does
- Steeper: At the same height the saddle moves forward, closer to the bar: you sit shorter and more over the bottom bracket. One degree is about 12 mm at 72 cm saddle height.
- Slacker: The saddle moves back: you sit longer, the bar is further away.
- Swap it?
- No, it is part of the frame. You can move the saddle on its rails, about 2.5 cm, or use a post with more or less setback.
- Across bike types
- Race, endurance and gravel bikes are almost the same here: at the back they hardly differ. Small frames often have a steeper seat angle than large ones. Modern mountain bikes are much steeper, often 75 to 77°, so that you sit over the bottom bracket when climbing.
Race 73.2° · Endurance 73.4° · Gravel 73.4°
One size 58 each, from real bikes; how it is worked out is below.
Frame
Top tube
The upper tube of the main triangle, between seat tube and head tube.
- From where to where
- From the top of the seat tube forward to the head tube. If it drops towards the back, the frame is called “sloping”.
- Effective top tube
- 580 mm on the Race 58 in the picture. The effective top tube: level from the seat tube axis to the top of the head tube. This lets you compare frames with and without sloping.
- What more or less does
- Longer: More room between saddle and bar: you sit more stretched. But a longer top tube can also come from a slacker seat angle alone; that is why people compare reach today.
- Shorter: The bar comes closer, you sit more compact.
- Swap it?
- No, it is part of the frame. You set the distance to the bar with the stem.
- Across bike types
- At the same size gravel bikes are often a little longer than race and endurance bikes. Mountain bikes have a strongly sloping top tube to leave room above the frame; trekking bikes come with a low step-through almost without a top tube.
Race 580 mm · Endurance 579 mm · Gravel 593 mm
One size 58 each, from real bikes; how it is worked out is below.
Frame
Down tube
The thickest tube on the bike.
- From where to where
- From the bottom bracket up to the head tube, just above the fork crown.
- Measured
- The geometry chart does not list it: where it sits follows from stack, reach and head tube.
- What more or less does
- Thicker: Stiffer against twisting when you accelerate and steer; it carries the largest forces in the frame.
- Thinner: Lighter and a little more flexible. You notice it when you accelerate, not when you sit.
- Swap it?
- No, it is part of the frame. This is usually where the bottle cage sits, on gravel bikes often a second one underneath.
- Across bike types
- Similar on every type. Gravel and mountain bikes often have a guard against stone chips underneath.
Frame
Head tube
The short tube at the front in which the fork turns.
- From where to where
- At an angle at the front of the frame, from the fork crown to just below the stem. Its angle to the horizontal is the head angle.
- Head angle
- 73.0° on the Race 58 in the picture. The angle between steering axis and ground. The chart gives the length from bottom to top edge.
- What more or less does
- Longer: The bar sits higher, the stack grows: you sit more upright. This is the main difference between race and endurance.
- Shorter: The bar can go lower, you sit more stretched. Higher is then only possible with spacers or a rising stem.
- Slacker: More trail and a longer wheelbase: the bike runs more calmly straight ahead and turns in more slowly.
- Steeper: Less trail: the bike turns in more eagerly and asks for a calmer hand.
- Swap it?
- No, it is part of the frame. The bearings inside, the headset, can be replaced; only special headsets, made almost only for mountain bikes, change the angle.
- Across bike types
- Race bikes have short head tubes and steep head angles. Endurance and gravel bikes are taller and a little slacker at the front; with more fork offset the trail stays similar, and what makes them calmer is mostly the longer wheelbase. Mountain bikes are much slacker still, often 64 to 68°, for calm descending.
Race 73.0° · Endurance 72.2° · Gravel 71.4°
One size 58 each, from real bikes; how it is worked out is below.
Frame
Seat stays
The two thin stays from the seat tube down to the rear axle.
- From where to where
- From the seat tube, just below the top tube, down to the rear dropout.
- Measured
- The chart does not list them: their length follows from chainstay and seat tube.
- What more or less does
- Dropped: Many new frames join the stays lower on the seat tube: the rear triangle gets smaller, the post stands more freely and gives a little more over bumps.
- Thinner: They flex a little more without softening the drivetrain: the chainstays carry that.
- Swap it?
- No, they are part of the frame.
- Across bike types
- Endurance and gravel bikes often make the stays especially compliant. On a full-suspension mountain bike they are part of the rear end that moves with the shock.
Frame
Chainstays
The two stays from the bottom bracket to the back; the chain runs along the right one.
- From where to where
- From the bottom bracket to the rear axle. The chart measures from bottom bracket centre to axle centre.
- Chainstay
- 409 mm on the Race 58 in the picture. Bottom bracket centre to rear axle centre, measured along the stay.
- What more or less does
- Longer: Longer wheelbase: the bike runs more calmly, and there is room for wide tyres and mudguards.
- Shorter: The rear wheel comes closer under you: the bike gets more agile and responds more directly when you accelerate.
- Swap it?
- No, they are part of the frame. Some gravel bikes have a flip-chip dropout that makes the stay about 1 cm longer.
- Across bike types
- Race bikes are short, endurance bikes a little longer, gravel bikes longest because wide tyres need room. Trekking bikes are longer still, so that your heel clears the panniers.
Race 409 mm · Endurance 421 mm · Gravel 431 mm
One size 58 each, from real bikes; how it is worked out is below.
Frame
Bottom bracket
The bearing the crank turns in, and the point almost every measurement starts from.
- From where to where
- At the bottom of the frame, where seat tube, down tube and chainstays meet.
- Bottom bracket drop
- 70 mm on the Race 58 in the picture. The bottom bracket drop: how far the bottom bracket sits below the line between the two axles.
- What more or less does
- Lower: You sit lower between the axles: the bike feels more planted, and the pedal touches down sooner in a corner.
- Higher: More room for pedal and chainring over roots and rocks; the bike feels a little less planted.
- Swap it?
- The bearing wears and gets replaced. Where it sits is part of the frame.
- Across bike types
- Gravel bikes drop the bottom bracket further than race bikes: their bigger wheels lift the axles, so the bottom bracket stays at a similar height above the ground.
Race 70 mm · Endurance 73 mm · Gravel 80 mm
One size 58 each, from real bikes; how it is worked out is below.
Frame
Fork
Holds the front wheel and turns with the bar.
- From where to where
- From the head tube down to the front axle. At the top its steerer sits in the head tube and carries the stem.
- Fork offset
- 42 mm on the Race 58 in the picture. The offset (rake): how far the axle sits ahead of the extended steering axis, measured square to it. Plus the axle-to-crown length.
- What more or less does
- More offset: Less trail: the bike turns in more eagerly.
- Less offset: More trail: it runs more calmly straight ahead and straightens itself more strongly.
- Longer: The front comes up, the head angle gets slacker. That is why you only swap a fork for one with the same axle-to-crown length.
- Swap it?
- Yes, if it fits the frame: steerer, axle-to-crown length, brake and axle have to match. On a mountain bike it is a suspension fork.
- Across bike types
- Endurance and gravel forks have more offset than race forks: that makes up for the slacker head angle so the trail does not grow too large.
Race 42 mm · Endurance 48 mm · Gravel 52 mm
One size 58 each, from real bikes; how it is worked out is below.
Components
Seatpost
The tube that carries the saddle; it sits in the seat tube.
- From where to where
- From the seat tube up to the saddle clamp. How far it sticks out sets your saddle height.
- Measured
- Diameter, length and setback: how far the head sits behind the axis of the tube, usually 0 to 25 mm.
- What more or less does
- Further out: The saddle sits higher and, because the seat tube is angled, also a little further back.
- More setback: The saddle moves back, as with a slacker seat angle.
- Zero setback: The saddle moves forward, as with a steeper seat angle.
- Swap it?
- Yes, and easily, if the diameter fits; aero frames often have their own shape. With the setback you adjust what the seat angle sets.
- Across bike types
- Race and endurance bikes mostly use a setback or straight post, endurance bikes often one that gives a little. On mountain bikes, and more and more on gravel bikes, it is a dropper post: lowered with a lever on the bar while riding.
Components
Saddle
What you sit on. For your position, where it sits matters more than which model it is.
- From where to where
- On the post, with its rails in the clamp. Saddle height is measured from the centre of the bottom bracket along the seat tube to the top of the saddle.
- Measured
- Height, position forward or back (setback from the bottom bracket) and tilt.
- What more or less does
- Higher: The leg extends further at the bottom. Too high, and the pelvis rocks sideways with every pedal stroke.
- Lower: The knee stays more bent; you push on the pedal with less force.
- Further forward: Shorter to the bar, more over the bottom bracket.
- Further back: Longer to the bar, more behind the bottom bracket.
- Swap it?
- Yes. On its rails it slides about 2.5 cm forward and back.
- Across bike types
- Race saddles are narrow and often short at the nose. Trekking and city saddles are wider, because you sit more upright and more weight rests on the saddle.
Components
Spacers
Rings on the steerer below the stem.
- From where to where
- Between head tube and stem, on the steerer.
- Measured
- Usually 5 or 10 mm tall. Together with the headset cover they set how high the stem sits.
- What more or less does
- More: The bar sits higher and, because the steering axis is angled, a little further back: you sit more upright and shorter.
- Less: The bar comes lower and a little further forward: you sit more stretched.
- Swap it?
- Yes: moving them below or above the stem is the simplest change on a bike. But a steerer that has been cut does not grow back.
- Across bike types
- New bikes often come with 2 to 4 cm of spacers so you can go lower. Where the cables run through the stem, moving them is more work.
Components
Stem
Connects steerer and bar.
- From where to where
- From the steerer forward to the bar clamp.
- Measured
- The length from steerer centre to bar centre, and the angle against the perpendicular to the steerer, often −6°.
- What more or less does
- Longer: The bar moves forward: you sit more stretched, the steering gets calmer.
- Shorter: The bar comes closer: you sit more compact, the steering gets more direct.
- Rising: Flipped, at +6° instead of −6°, a 100 mm stem lifts the bar by about 2 cm and brings it 6 mm closer.
- Swap it?
- Yes, and cheaply. It comes in 10 mm steps, on road bikes usually from 70 to 140 mm.
- Across bike types
- Race bikes use long stems, gravel bikes rather shorter ones for more direct steering. Mountain bikes use 35 to 60 mm: the wide bar and the slack head angle need no long one.
Components
Bar
Where your hands rest. A drop bar has three positions: on the tops, on the hoods and in the drops.
- From where to where
- In the stem clamp. The brake levers sit at the front of the curve.
- Measured
- Width, reach and drop: how far forward the curve goes and how low the drops sit.
- What more or less does
- More reach: The hoods move forward, as with a longer stem.
- Less drop: The drops sit higher: easier to reach, so you use them more often.
- Wider: More leverage and calmer steering; narrower is more aerodynamic.
- Swap it?
- Yes. A bar with shorter reach works like a shorter stem; levers and bar tape have to move with it.
- Across bike types
- Race and endurance bikes use a drop bar, 38 to 44 cm wide. On gravel bikes it is wider and flared at the bottom, for control off-road. Trekking and mountain bikes have a flat bar, up to 80 cm wide on a mountain bike; the hands rest behind and above the clamp.
Components
Crank
The lever from the bottom bracket to the pedal; it carries the chainring.
- From where to where
- From the bottom bracket axle to the pedal axle.
- Measured
- The length from axle to axle, usually 165 to 175 mm, in steps of 2.5 mm.
- What more or less does
- Longer: More leverage, but at the top hip and knee bend more, and the saddle has to go down a little.
- Shorter: At the top the leg stays more open, and high cadences come more easily.
- Swap it?
- Yes, usually together with the chainring. If the length changes, you move the saddle by the same difference.
- Across bike types
- Similar on every type: the length follows leg length more than bike type. Small frames come with shorter cranks.
Components
Wheel and tyre
Rim, spokes, hub and tyre.
- From where to where
- Around the axles; the axles are the reference points for wheelbase, chainstay and bottom bracket drop.
- Measured
- The rim by diameter (700C or 28 inch is 622 mm), the tyre by width in millimetres.
- What more or less does
- Wider tyre: More grip and comfort at lower pressure. The wheel gets taller: trail and bottom bracket height grow by a few millimetres.
- Bigger wheel: Rolls more easily over obstacles; smaller wheels are more agile.
- Swap it?
- Yes, as far as frame and fork have room for the tyre.
- Across bike types
- Race bikes run 28 to 32 mm, endurance 30 to 35 mm, gravel 38 to 50 mm. Mountain bikes are mostly 29 inch with 2.2 to 2.5 inch tyres.
Measurements
Stack
How high the top of the head tube sits above the bottom bracket.
- From where to where
- From the centre of the bottom bracket straight up to the height of the top of the head tube.
- Stack
- 591 mm on the Race 58 in the picture. Vertical, in millimetres. Together with reach it says where the cockpit starts on the frame.
- What more or less does
- More: The bar sits higher: you sit more upright.
- Less: The bar can go lower: you sit more stretched and flatter.
- Swap it?
- A frame measurement. After that you can still adjust with spacers and the stem angle, a few centimetres together.
- Across bike types
- At the same size endurance bikes are taller than race bikes, gravel bikes a little taller still. This is the difference you feel most when you sit.
Race 591 mm · Endurance 614 mm · Gravel 624 mm
One size 58 each, from real bikes; how it is worked out is below.
Measurements
Reach
How far the top of the head tube sits ahead of the bottom bracket.
- From where to where
- From the vertical through the bottom bracket level to the top of the head tube.
- Reach
- 402 mm on the Race 58 in the picture. Horizontal, in millimetres. Unlike the top tube it does not depend on the seat angle.
- What more or less does
- More: The bar is further forward: you sit more stretched.
- Less: You sit more compact. A short reach with a longer stem puts the hands in the same place as a long reach with a short stem.
- Swap it?
- A frame measurement. The rest of the way to the hands you set with stem and bar.
- Across bike types
- At the same size race, endurance and gravel bikes are only a few millimetres apart; the stack makes the difference.
Race 402 mm · Endurance 396 mm · Gravel 407 mm
One size 58 each, from real bikes; how it is worked out is below.
Measurements
Wheelbase
The distance between the two axles.
- From where to where
- From the rear axle level to the front axle.
- Wheelbase
- 1008 mm on the Race 58 in the picture. Horizontal, axle centre to axle centre.
- What more or less does
- Longer: Calmer straight ahead, especially fast and downhill; more room for tyres and feet.
- Shorter: More agile in tight corners and more direct.
- Swap it?
- It follows from chainstay, frame and fork; only the fork can be swapped.
- Across bike types
- Race bikes are shortest, gravel bikes longest. Mountain bikes are much longer still, often over 1.2 m.
Race 1008 mm · Endurance 1034 mm · Gravel 1069 mm
One size 58 each, from real bikes; how it is worked out is below.
Measurements
Trail
How far behind the point where the extended steering axis meets the ground the tyre touches down.
- From where to where
- On the ground below the fork: from the point of the steering axis to where the front tyre touches down.
- Trail
- 62 mm on the Race 58 in the picture. It follows from head angle, fork offset and tyre. Makers state it with the tyre the bike comes with.
- What more or less does
- More: The bike runs more calmly straight ahead and straightens itself more strongly; it turns in more slowly.
- Less: It turns in more eagerly and quickly, but asks for a calmer hand.
- Swap it?
- Not a part. A wider tyre adds a few millimetres, a fork with more offset takes some away.
- Across bike types
- Road bikes are close together: endurance and gravel bikes make up for their slacker head angle with more fork offset. Mountain bikes have far more, often over 100 mm.
Race 62 mm · Endurance 61 mm · Gravel 63 mm
One size 58 each, from real bikes; how it is worked out is below.
Where the numbers come from
The bike in the picture is a size 58 from the page “Race, endurance or gravel?”: stack and reach as on the frame map; seat angle, head angle, wheelbase, chainstay and bottom bracket drop the mean of six real bikes each. Fork offset, trail and effective top tube follow from these, the trail with the same tyre as in the geometry lab. Stem, spacers, crank and bar are the parts a bike of this type usually comes with from the shop.