MTB Geometry (Part 1)

What’s inside this week:  
1. Numbers leaving you baffled? Get familiar with MTB Geometry.
2. Gadget of the week, a staple in the newsletter.
3. Shorts - understand Trail ratings and don’t get caught out.
4. Good hack, or is it whack? Tips and tricks from myself or readers.
5. MTB video of the week. All stoke, no woke.

Mountain Bike basics - Geometry.

Remember those school days when you were measuring angles in math class, wondering if you’d ever use that knowledge in real life? Well, guess what, Geometry is not just for classrooms, it’s crucial for bicycles!

Geometry refers to the collection of key measurements (lengths and angles) that define a particular bike configuration. These parameters have major influence on how a bicycle handles, as well as ride quality and comfort. Keep reading to find out why your bike rides the way it does.

Understanding geometry is important when buying a bike, but you don’t need to be an expert, your LBS (local bike shop) should be very helpful in explaining a particular bike to you. If not, drop me a message, and I’ll gladly help.

We’ll split it into two newsletters, to keep you sane.

Our understanding of geometry used in bicycle design is also constantly evolving and changing, and differs for different riding disciplines. So, even if I go into detail - some of this will still sound like a mess.

These are the most commonly charted geometry attributes:

Head Angle - the angle of the steering axis/fork and the ground. This angle significantly affects the bike's handling characteristics. A slacker (more relaxed) head angle, typically found on downhill and trail bikes, provides greater stability at high speeds and on steep descents. Conversely, a steeper head angle, common on cross-country bikes, offers quicker, more responsive steering suitable for climbing and manoeuvering tight trails.
TLDR: A laid-back ride, a slacker head angle = steady on the gnarly descents, steeper = nimble steering.

Reach refers to the horizontal distance from the centre of the bottom bracket to the centre of the top of the head tube. It is a critical measurement that influences how a bike fits and handles, particularly affecting the rider's position and comfort when standing up on the pedals. A longer reach generally provides a more stable and roomy feel, while a shorter reach can make the bike feel more agile and nimble.
Reach is the most misunderstood term in mountain bike geometry these days in my opinion - this measurement really only comes to full effect when you are standing up, and is only a part of the whole picture when sitting down. When seated, you also have to consider the seat angle and horizontal top tube measurement, for a better idea of how the bike will fit you.
TLDR: Determines how stretched out or compact you’ll feel standing up. Short reach = compact; long reach = roomy (when standing up).

Top tube - the horizontal top tube measurement, also known as the effective top tube length, is the horizontal distance from the centre of the head tube to the centre of the seat tube. This measurement helps determine the bike's fit and rider positioning. It's especially useful for comparing bikes with different frame designs and top tube slopes, providing a consistent reference for sizing and ensuring the rider has an appropriate reach and comfortable stance while riding. This is a more appropriate measurement for bike sizing than reach, since this is relevant to sitting down (as mentioned above).
TLDR: Defines your cockpit space in seated position. Longer = roomy feel, shorter = tight and snappy handling. The true measure of your bike’s fit. Balances comfort with control.

A higher Stack height (vertical measurement from centre BB to the top centre of your head tube) puts you in a more upright, comfortable position, ideal for long rides and providing better control on steep descents and tech trails. However, it can make the bike feel less aggressive when you need to get low and put the power down. A lower stack height gives you a more aggressive, aerodynamic stance, perfect for attacking climbs and technical sections, but it can be harder on your back and less comfortable over long distances.
TLDR: Your vertical comfort zone. High stack = upright and comfortable; low stack = aggressive and race-ready.

*If you’ve read to here, we consider you a hero! We are heading into the lesser understood, but good-to-know geometry next.

Bottom Bracket Height: A lower bottom bracket height (this is the measurement from the center of your BB, to your axle height) gives you that "glued to the trail" feel, enhancing stability and cornering by lowering your center of gravity. It’s perfect for railing turns and feeling connected to the terrain. However, it comes with a higher risk of pedal strikes on rough, rocky trails. A higher bottom bracket height offers more clearance, reducing the chance of hitting obstacles, which is great for technical, rocky sections, but it can make the bike feel a bit less stable and connected in fast, flowy corners.
TLDR: Lower BB = planted feel; higher BB = extra pedal clearance on rocky trails.

Fork Offset/Rake: This is how far the axle is offset from the steering axis. A shorter offset gives your bike a more stable, planted feel, perfect for charging down steep, gnarly descents without feeling like you’re going over the bars. But it can make the steering feel a bit slower, so you might need to put in more muscle to whip through tight turns. A longer offset makes for quicker, more responsive steering, great for weaving through switchbacks and navigating twisty trails, but it can make the bike feel a bit more twitchy and less stable at high speeds.
TLDR: The secret sauce to steering. More offset = agility, less = stability on rougher rides.

And now you know just enough to get yourself in awkward geometry debates. Next week we’ll finish up with Geometry and get into some riding techniques and maintenance after that.

Thanks a lot readers! This is issue number 3, and we really need your help to reach more riders. Please share this with your MTB buddies and on the MTB Whatsapp groups that you’re part of. For every three referrals, we will email you a helpful Bike Maintenance Log to keep your bike running better for longer!

Gadget of the week: Ryder Luberetta chainlube system.
This is a patented chain lube applicator, that dispenses a drop on each link. No mess or wastage. The wax Ryder chainlube with a citrus tackifier component (that comes with the Luberetta) is great, and I’ve been using it for a few years exclusively now. Try it for yourself.

Q. Trail ratings: Green, Blue, Red and Black - what do they mean?
A. Trail ratings are color-coded to indicate the difficulty level of mountain biking trails, helping riders choose routes that match their skill level:

1. Green (Beginner):
   Smooth and gentle trails with minimal obstacles. Perfect for beginners or those looking for a relaxed ride. Expect wide paths, gentle slopes, and minimal technical features.

2. Blue (Intermediate):
   A step up in difficulty with moderate slopes, tighter turns, and a few obstacles like rocks, roots, and small drops. Suitable for riders with some experience who are comfortable with a bit more technical terrain.

3. Red (Advanced):
   These trails are for experienced riders who are ready to tackle steeper gradients, technical features like larger drops, rock gardens, and tight, challenging sections. Expect a mix of natural and man-made obstacles that require good bike handling skills.

4. Black (Expert):
   The most challenging trails, designed for expert riders. These are steep, technical, and often have large jumps, significant drops, and very rough terrain. Black trails demand advanced skills, confidence, and a willingness to take on serious challenges.

5. Double black (Expert):
   Like double the trouble. Steer clear if you are not very comfortable on black trails. Steep, rough, difficult. Gnarly.

Video of the week:
South African DH racing veteran Stefan Garlicki follows Finn Iles down Whistler in this great video (that you need to fetch a beer for). The riding, filming, editing and music is next level!

About the author:
Hello, I’m Francoin, an Industrial Designer in the cycling industry based in Somerset West, South Africa.

Tools, lights, accessories and bicycle design is my daily graft.

This newsletter is to share some of the experience & industry insight that I pick up along the way, with fellow cyclists. If you are new to the sport, or been riding for many years, I plan to give you something of interest in every edition.

Thanks for subscribing and Happy trails!

FV

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