Mountain biking isn’t as easy as hopping on the first bike you find and heading off the beaten track. There’s a maths to it.
You and your bike need to work effortlessly together to ensure powerful and skilful riding from start to finish.
This means sitting down and understanding bike geometry.
But don’t worry, you don’t have to hit the books any time soon. Instead, we’ve brought everything you need to know together in one neat post.
What is bike geometry?
Each bike is different, but will always be proportional to the style of riding it was designed for. After all, it’s not all about biking up and down mountains until the sun goes down. And taking a cross country bike around a dirt jump track is just going to leave you uncomfortable and dissatisfied with your performance.
So put simply, bike geometry is all the measurements on your mountain bike. Every angle, every tube, every number; it’s everything and anything that impacts the fit, feel and design.
From seat angle to bottom bracket height, we’re here to help you understand what you should be looking out for, what each number means and how to work out which bike is going to be the perfect fit for you…
1. Head Angle
What is it: The angle between the front fork and the ground.
Measurements: The lower the number, the slacker the angle. In general, 65° and lower is slack and 70° is steep.
What does it mean: The slacker the angle, the more stable the bike when travelling at high speeds. It also makes the bike a lot more comfortable and responsive during descents. Steeper angles help uphill terrain by keeping the front wheel planted securely.
In other words, attempting one of Europe’s toughest mountain climbs with a slack head angle is a fool’s errand. As is taking on a downhill trail with a steep angle. Both will reduce the steering response and leave you with little to no control of the bike.
Head angle, like all elements of bike geometry, is specific to the type of rider you are. Whilst downhill daredevils will keep the angle below 65° and enduro enthusiasts at 65°, peak pursuers and cross country riders will usually position it above 67°.
2. Seat Angle
What is it: The angle of the seat tube to the ground.
Measurements: For help powering through challenging uphill climbs, keep the angle steep at around 70°.
What does it mean: Changing the seat angle affects where you are seated on the bike.
Steeper angles push your saddle forwards, keeping it directly above the bottom bracket. This positions your body to be able to drive more power through the pedals, whilst exerting less energy – keeping you pedalling stronger for longer.
For riders looking to keep their riding position backwards, keep the angle slack and below 70°.
3. Bottom Bracket Height
What is it: The distance between the center of your cranks (the bottom bracket) and the ground.
Measurements: The bottom bracket height is effectively measuring the height of your center of mass. The lower the bottom bracket, the better – some mountain bikers get as low as 300mm!
What does it mean: The lower the bottom bracket, the lower your centre of gravity – which is perfect if you’re up against technical corners. But too low and you’re at risk of impact with rocks, roots, and other obstacles – pretty common on the rough and unpredictable terrains mountain bikers take to.
Whilst most riders desire to keep the height as low as possible, enduro enthusiasts settle for something a little higher so they can manage and control the more exaggerated movements.
4. Chainstay
What is it: The distance between the center of the bottom bracket and the rear wheel.
Measurements: Generally, long stays are around 17.3 inches, and short is as low as 14.75 inches.
What does it mean: Ultimately, it has a significant impact on the maneuverability and stability of your bike…
Short stays allow for more dramatic hops and pops as they give you superior control whilst in the air. They provide freedom in terms of body and weight position – perfect for freeriders and dirt jumpers who require more dynamic body movements.
But more length offers more stability over tough terrain and helps keep your front wheel in contact with the ground. So if you’re heading out on a climb, add some length!
5. Front Centre
What is it: This is the measurement from the centre of the bottom bracket to the centre of the front axle.
Measurements: This all depends on the rest of the geometry; particularly the frame length and head angle.
What does it mean: Lengthening the front centre helps improve the bike’s stability, whilst also allowing for a more dynamic steering response. And the opposite can be said for when it is shortened – however, this is more appealing to riders looking for short and fiery movements.
6. Wheelbase
What is it: The distance between axles (also the culmination of the chainstay and front centre measurements).
Measurements: Long wheelbases – around 47 inches – offer more stability, while shorter lengths – around 40 inches – are more agile and easy to maneuver.
What does it mean: If you’re looking to inject more life and energy into your bike handling, then try to keep the wheelbase as short as possible. This is also helpful should your riding demand quick and sudden spins.
As wheelbase is affected by both the chainstay and front centre measurements, it’s not as easy as adjusting the length to suit your needs…
If you’ve aligned your chainstay with a long measurement, your wheelbase is going to need to be long. Otherwise you won’t be able to enjoy a pleasant feel and fit.
READ MORE:
- Elliott Heap: “I perform better under pressure”
- Inside Wales’ MTB Trail Centres
- The mad world of mountain bike festivals
7. Down tube
What is it: The measurement between the headtube and the centre of the bottom bracket, taken along the tube.
Measurements: The end measurement depends on the length of the head angle, seat angle and top tube.
What does it mean: A bike’s down tube is particularly useful when establishing how well it is going to fit you. But as with the wheelbase, it is going to come down to the rest of your set-up; the longer your down tube the stiffer the bike.
8. Top Tube
What is it: The distance between the centre of the seat tube and the centre of the head tube. This measurement is taken horizontally from point to point, rather than along the top tube itself.
Measurements: Your ideal length will vary depending on the rest of your bike.
What does it mean: Although this is a dated element of mountain bike geometry, the top tube can still impact the fit, feel and performance of your bike.
Shortening this generally shortens the wheelbase, which means less stability and more agility in terms of movement.
9. Reach
What is it: The distance between the centre of the headtube and the centre of the bottom bracket, taken horizontally.
Measurements: For many riders, this is the most essential measurement to get right. And the only other geometric elements with the potential to affect it are the top tube and head angle.
What does it mean: Ultimately, the bike’s reach impacts your range of movement when out of our seat and standing up on the pedals.
The longer the reach, the more freedom you have which often means you’ll be too stretched out. Whereas, keeping it too short will make shifting your weight difficult and drive you into an overly upright position.
What the numbers really mean…
It’s all a matter of what style of rider you are – enduro enthusiasts are going to demand a completely unique geometry compared to downhill daredevils.
So before you rush to remeasure your bike, workout what you need.
- How do you want it to feel?
- What techniques and riding styles do you implement the most?
- What kind of fit are you looking for?
And then work your way through each element, tweaking as you go along to find the perfect fit for you.