GUIDE AND GLOSSARY: BIKE GEOMETRY EXPLAINED
Bike geometry is full of nuance, and for the uninitiated, understanding geometry charts filled with rows of numbers can seem like a dark art. Just because two different bikes are called the same size doesn’t mean they will fit the same or even ride the same. This guide will explain the basics behind bike geometry measurements and how they affect your bike’s fit or handling characteristics.
Armed with this knowledge, you can learn how to choose a new bike based on geometry by looking at geometry charts or by using our new Frame Comparison Tool.
Learn More: How To Compare Bike Geometry
Bike Geometry Diagram
- A: Effective Top Tube Length
- B: Head Tube Angle
- C: Seat Tube Angle
- D: Seat Tube Length
- E: Chainstay Length
- F: Bottom Bracket Drop
- G: Wheelbase
- H: Front Center
- I: Fork Rake
- J: Standover Height
- K: Head Tube Length
- L: Stack
- M: Reach
Litespeed Frame Comparison Tool
A: Effective Top Tube Length
Top tube length is measured from the center line of the seat tube to the center line of the head tube, but we don’t measure the top tube itself (see the image above). Most modern bike top tubes are sloped, so “effective” top tube length measures the horizontal distance from the head tube and seat tube to provide a consistent measurement across different sizes, models, and brands.
Effective top tube length should fit your torso and arm length. Too long and you’ll feel stretched out. Too short and you’ll feel cramped. Taller riders generally need longer effective top tubes, and shorter riders generally need shorter effective top tubes. However, most riders will still need to dial in the length of their bike with a longer or shorter stem.
B: Head Tube Angle
Head tube angle is measured in degrees, with 90 degrees being vertical. As the head angle decreases, it becomes "slacker." As it increases, it becomes "steeper."
Head tube angle influences a bike’s handling characteristics. Bikes with slacker head tube angles feel more stable and planted, while steeper head tube angles feel more agile and responsive to steering inputs.
Increasing stability makes long rides less taxing, and it improves confidence on rough roads and terrain. Increasing agility makes it easier to make instant direction changes in high-speed riding and racing scenarios. Road and gravel bikes usually have head angles in the range of 69-73 degrees, depending on their intended use:
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Litespeed Road Race Bikes: ~72-73°
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Litespeed Endurance Road Bikes: ~70-72.5°
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Litespeed Gravel Race Bikes: ~70-72°
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Litespeed Gravel Adventure Bikes: ~69-71.5°
Mountain bikes have a more diverse range of head tube angles, and they usually correspond with the bike’s suspension travel and riding intentions. A shorter-travel cross-country race bike might be between 67 and 69 degrees; a trail bike between 66 and 68 degrees, and an enduro bike between 63 and 66 degrees. As you can tell, the categories often overlap.
C: Seat Tube Angle
Seat tube angle is measured in degrees, with 90 degrees being vertical. As the seat angle decreases, it becomes "slacker." As it increases, it becomes "steeper."
Seat tube angle generally impacts your riding position. Riders with shorter legs generally need to sit farther forward, relative to the bottom bracket. This position is easier to achieve with a steeper seat tube angle. Riders with longer legs might want a slacker seat angle to sit farther back. Seat angle is important, but your riding position can also be addressed by adjusting your saddle position or seatpost.
D: Seat Tube Length
Seat tube length generally measures from the center of the bottom bracket to the top of the seatpost. This is the old-school way to size a bike, especially for road bikes. In the past, a size 54cm frame would measure 56cm from the bottom bracket to the top of the seatpost.
Many modern bikes, however, have sloping top tubes, meaning seat tube length generally doesn’t match the listed size (e.g., a modern 54cm bike might only have a 50cm seat tube), and it has lost a lot of its relevance. As a result, Litespeed generally uses general sizing terms (small, medium, large), rather than numeric sizing (52cm, 54cm, 56cm).
E: Chainstay Length
Chainstay Length is measured from the center of the bottom bracket to the rear axle.
Chainstay length influences a bike’s handling characteristics. Bikes with longer chainstays feel more stable and planted, while bikes with shorter chainstays feel more agile and responsive to steering inputs. Longer chainstays can also help increase tire clearance.
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Litespeed Road Race: 41.5cm chainstays
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Litespeed Endurance Road: 42-43cm chainstays
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Litespeed Gravel Race: 43.5cm chainstays
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Litespeed Gravel Adventure: 43.7cm chainstays
F: Bottom Bracket Drop
Bottom bracket drop is measured from the axles to the center of the bottom bracket. “Bottom bracket height” is a similar measurement, describing the same thing, but it’s affected by tire size, so bottom bracket drop is preferable.
Bottom bracket drop influences a bike’s handling characteristics. Bikes with lower bottom brackets (i.e., they have more bottom bracket drop) have a lower center of gravity, so they feel more stable and planted, while bikes with higher bottom brackets (i.e., they have less bottom bracket drop) feel more agile and responsive to steering inputs (and have more pedal to ground clearance).
For most modern road and gravel bikes, bottom bracket drop has essentially settled in the 6.5-8cm range. Anything outside of that is fairly extreme.
G: Wheelbase
Wheelbase is measured from the front axle to the rear axle.
Wheelbase is essentially the combination of head tube angle, fork rake, front center, and chainstay length. Like many of these elements, it influences a bike’s handling characteristics. Bikes with longer wheelbases feel more stable and planted, while bikes with shorter wheelbases feel more agile and responsive to steering inputs.
H: Front Center
Front Center is measured from the center of the bottom bracket to the front axle.
Front Center is essentially the combination of reach, head tube angle, and fork rake. Just like wheelbase, it influences a bike’s handling characteristics, but it is also a handy measurement to see if a bike will have toe overlap if you can compare it to a known bike.
I: Fork Rake
Fork rake, also known as “offset,” is the distance between the front axle and the steering axis of the fork.
Fork rake is important because it affects “trail.” If you draw a line through the bike’s steering axis to the ground, trail is the distance from that point to where the front wheel touches the ground. Trail is what makes a bike's front wheel self-straighten when it is moving forward. Think of the wheels of a shopping cart. They want to straighten when you push the cart forward. This same phenomenon occurs on your bike. It helps you stay upright when riding.
Reducing fork rake increases trail. Increasing fork rake reduces trail. Increasing trail increases straight-line stability. The front wheel feels harder to turn but also harder to knock off line. Decreasing trail increases agility. The front wheel feels easier to turn and it can make a bike feel more agile.
Fork offset for road and gravel bikes usually ranges from 40 to 55 mm, generating trail figures from 50 to 63 mm depending on the frame’s head tube angle. ~57mm of trail is considered by many to be an ideal combination of stability and agility, and our fork rakes aim for this sweet spot.
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Litespeed Road: 4.5cm Fork Rake
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Litespeed Gravel: 5.0cm Fork Rake
J: Standover Height
Standover height is a measurement from the ground to the top of the bike’s top tube (usually measured at the center of the top tube).
This measurement essentially tells you if you can straddle a bike’s top tube with your feet touching the ground. Too much standover height and you won’t be able to comfortably put a foot down. This can help you determine if a bike will feel too big. Too little standover height isn’t really an issue, but it might indicate a frame is too small.
K: Head Tube Length
Head tube length is the length of the head tube measured from top to bottom.
Head tube length has a major effect on stack height. Comfort-focused endurance bikes generally have longer head tubes, which increases stack height. Race-oriented bikes generally have shorter head tubes, which reduces stack height (see below).
L: Stack
Stack height measures the vertical distance from the center of the bottom bracket to a virtual horizontal line from the top of the head tube.
Higher stack provides a more upright and relaxed riding position. Lower stack provides a lower and more aggressive riding position.
Novice riders or riders focused on endurance, comfort, or technical off-road riding usually feel better on bikes with more stack and a more upright position. Also, riders with shorter arms and torsos or limited spinal mobility may also prefer more stack.
Many advanced road riders and racers tend to prefer bikes with short stack measurements to get their handlebars in a lower, more aggressive position for improved aerodynamics and cornering.
Stack height can also be adjusted by raising or lowering your bike stem. Most riders will need to adjust their stem to dial in their preferred stack height when fitting a new bike.
M: Reach
Reach measures the horizontal distance from the bottom bracket to the head tube. This measurement extends a vertical line straight up from the bottom bracket and measures from that point to the center of the head tube.
As the name implies, reach is the distance from your body to the bars, and its effects are similar to effective top tube length. Too long and you’ll feel stretched out. Too short and you’ll feel cramped. Taller riders generally need more reach, and shorter riders generally need shorter reach. However, most riders will still need to dial in the length of their bike with a longer or shorter stem.
It’s worth noting that reach also influences a bike’s handling characteristics, because it influences the front center (which combines reach, head tube angle, and fork rake), which ultimately influences the wheelbase of a bike. As a result, bikes with longer reach feel more stable, while bikes with shorter reach feel more agile.
Reach is an important measurement for off-road cycling, because so much riding is done out of the saddle. Modern mountain and gravel bikes have trended toward longer reach numbers to increase stability and confidence on rough off-road terrain.
