How to use the comparison tool
- Choose a starting point for each bike. You can start from a typical model (race road, endurance, gravel, XC or trail) or from My numbers.
- Copy the numbers from the manufacturer’s geometry chart in the size you are looking at: stack, reach, head angle, seat angle, head tube, chainstays, BB drop, fork offset, wheel and tire. Wheelbase is optional: we calculate it, and if you enter it you will see both figures so you can check you copied everything correctly.
- Add the stem (length, angle and spacers) if you want to see where the handlebar really ends up. It is what changes your position most between two similar bikes.
- Look at the drawing and the difference table. Bike A is orange with a solid line; bike B is blue with a dashed line. Both are aligned at the bottom bracket, the fixed point every geometry number is measured from.
Save to My bike stores the stack, reach and chainstays of bike A in your My bike profile. The page address holds both bikes: copy the link to share the comparison.
What each number means
Stack. Vertical height from the bottom bracket to the center of the top of the head tube. More stack means a higher handlebar and a more upright position.
Reach. Horizontal distance between those same two points. More reach means the handlebar is farther away and you are more stretched out. Together with stack, it is the way to compare sizes across brands, because it does not depend on how each brand measures the seat tube.
Head angle. The angle of the head tube relative to the ground. Steeper (73°) gives quick steering, typical of road bikes; slacker (65°) gives stability on descents, typical of mountain bikes.
Seat angle. The angle of the seat tube. If the manufacturer lists an effective seat angle, use that one. Steeper moves your hips forward relative to the pedals.
Head tube. Its length. With the same stack, a longer head tube usually means a bike with a taller front end.
Chainstays. The distance from the bottom bracket to the rear axle. Shorter is more agile; longer is more stable and leaves more room for wide tires.
BB drop. How far the bottom bracket sits below the line between the axles. More drop means a lower center of gravity and a more planted bike.
Fork offset. How far forward the front axle sits from the steering axis. Together with the head angle and the wheel, it sets the trail.
How we calculate it
We put the origin at the bottom bracket, with x pointing forward and y pointing up.
- The top of the head tube is at (reach, stack).
- The steering axis runs down and forward at the head angle h. The front axle sits on it, shifted perpendicular by the offset, at the height of the BB drop.
- The rear axle is at chainstay distance from the bottom bracket, also at the BB drop height: its horizontal distance is √(chainstay² − drop²).
- The wheel radius is half the rim diameter (ETRTO) plus the tire height, which we take as equal to its width.
That gives the derived numbers:
Trail = (wheel radius × cos h − offset) ÷ sin h
Front center = reach + distance along the steering axis down to axle height × cos h + offset × sin h
Wheelbase = front center + rear center
Front center is the horizontal distance from the bottom bracket to the front axle: it tells you how much room there is in front of your feet and how your weight is split. For the handlebar, we go up the steering axis by the height of the spacers and from there add the stem at its angle relative to the perpendicular of the head tube. We do not count the height of the stem itself (half its clamp, about 20 mm more).
Manufacturers round their numbers, so the calculated wheelbase can differ from the listed one by 2-5 mm. If the difference is bigger, check that you did not mistype a number when copying.
Typical numbers by bike type
The typical models the tool uses: average values for a size M (54-56 on road bikes) in each category, not from any specific brand or model. All in mm.
| Type | Stack | Reach | Stack/reach | Head angle | Trail | Wheelbase | BB height |
|---|---|---|---|---|---|---|---|
| Race road | 555 | 395 | 1.41 | 73° | 57 mm | 991 mm | 267 mm |
| Endurance road | 590 | 380 | 1.55 | 72.5° | 56 mm | 1,003 mm | 268 mm |
| Gravel | 590 | 385 | 1.53 | 71° | 69 mm | 1,035 mm | 281 mm |
| XC mountain | 610 | 440 | 1.39 | 68° | 102 mm | 1,141 mm | 319 mm |
| Trail mountain | 625 | 470 | 1.33 | 65° | 125 mm | 1,229 mm | 340 mm |
Trail by head angle and offset
With a 700×28 wheel (339 mm radius), in mm:
| Head angle | 40 offset | 45 offset | 50 offset | 55 offset |
|---|---|---|---|---|
| 71° | 74 | 69 | 64 | 59 |
| 72° | 68 | 63 | 58 | 52 |
| 73° | 62 | 57 | 51 | 46 |
With a 29×2.4 in wheel (372 mm radius):
| Head angle | 40 offset | 44 offset | 50 offset | 55 offset |
|---|---|---|---|---|
| 65° | 129 | 125 | 118 | 113 |
| 67° | 114 | 110 | 104 | 98 |
| 70° | 93 | 89 | 82 | 77 |
A wider tire also adds trail, because it makes the wheel radius bigger: going from 28 to 35 mm on a road bike adds about 2 mm.
How much fit changes move the handlebar
With the typical race road geometry (73° head angle):
| Change | Forward | Up |
|---|---|---|
| +10 mm of spacers | −3 mm | +10 mm |
| Stem 10 mm longer | +10 mm | +2 mm |
| Flipping a −6° stem (to +6°) | −7 mm | +22 mm |
In other words, with stem and spacers you can make up a 1-2 cm difference in stack or reach between two bikes, but not more without changing how the bike steers.
Common mistakes
- Comparing sizes by letter or seat tube length. One brand’s L can have the reach of another brand’s M. Compare stack and reach.
- Looking only at reach. A bike with more stack “shortens” your position even with the same reach, because the handlebar ends up closer to your body.
- Forgetting the stem. Two nearly identical frames with 90 and 110 mm stems give very different positions.
- Mixing up actual and effective seat angle. If the seat tube is curved or offset, the effective angle is the one that matters for fit.
Choosing a size
The official size charts from Canyon, Trek, Specialized, Giant and other brands are collected in bike size charts by brand, and handlebar and stem diameters and widths in handlebar and stem sizes. If you do not know what size to look for yet, start with the bike size calculator using your height and inseam, then set your saddle with the saddle height calculator. If you are deciding what kind of bike to buy, the Which bike should I buy? quiz helps you narrow it down.
Frequently asked questions
What are stack and reach on a bike?
They are the height (stack) and horizontal distance (reach) from the center of the bottom bracket to the top of the head tube. Together they tell you where the handlebar starts and, unlike letter sizes, you can compare them across brands.
What is bike trail and why does it matter?
It is the distance on the ground between where the steering axis meets the ground and where the front tire touches it. More trail gives slower, more stable steering; less trail makes it quicker and twitchier. Road bikes run around 55-60 mm; trail mountain bikes, over 100 mm.
How much does a spacer change your position?
With a 73° head angle, every 10 mm of spacers raises the handlebar about 10 mm and brings it about 3 mm closer. A stem 10 mm longer moves it about 10 mm forward.
What stack-to-reach ratio is comfortable?
Above 1.5 the bike is tall and short (endurance, gravel, city); below 1.45, low and long (racing). It is a quick reference: always compare stack and reach separately.
How do I compare two sizes of the same bike?
Choose "My numbers" for both bikes, copy each size from the manufacturer's geometry chart and look at the difference in stack and reach. If one size feels short and the other long, stem and spacers can make up 1-2 cm (about 3/8 to 3/4 in).