Mountain Bike Geometry 101 A Complete Guide To Geometry + How It Affects Your MTB

These days, everyone is talking about reach, they're talking about offset, they're talking about trail, but why on earth do you need a steep seat angle and what exactly is long low and slack? Well, today, I've got a bike behind me here and I'm going to take you through everything you need to know about basic geometry and angles and measurements. We're going to tell you exactly what they mean in relation to your riding. Okay, so we're going to start with the basics first. Now, I've got behind me here a classic double diamond frame. Now, it's called that reason literally because it's two diamonds meeting in the middle. Now, most mountain bike frame designs today will still resemble this even if they're full suspension. You might see a bit of a linkage going on here, you might see a shock placed in the middle. They're essentially the same concept. For ease of illustration, this is what I've chosen for this video. Okay, so the first one we're going to look at is the top tube. That's this feather right up here. Simply called that because it is the top tube of the bike. Now, originally, top tubes were quite flat in their profile. You'll notice this one is quite sloping. That essentially is to give you a bit of undercarriage clearance. No one wants to be too close to the top tube when you have to get off the bike. Now, originally, the length of the top tube was quite important in terms of bike fit. Actually, by today's standards, it's very inaccurate for a mountain bike. The reason being, that is only relevant when you're sat down on a bike for comfort. That is not important on a mountain bike because the most important stuff you do on a mountain bike is when you're stood up. It's all about control. These days, we don't really talk about top tube length in terms of bike fit. However, it is good to know these things. There's two ways of measuring the top tube length. There's the actual measurement, which is measured from the center of the seat tube to the center of the head tube. Then there is the effective top tube length. The way that is measured, and this is basically because some frame designs are quite wild, so it's not always applicable to be able to measure it like that. You'll measure from the center of the head tube and you'll draw a horizontal line to where it meets with the seat tube, in fact, the seat post extending from that. That will be your effective top tube length. Again, it's not that important in terms of mountain bikes these days. It's only really a comfort-based measurement. It doesn't affect the performance on the bike, which is what you need to know about. Next up is the head tube. That's this fellow up the front here. Now, that joins the down tube and the top tube to the rest of the frame. It also houses the bearings that in turn house the steerer tube and the stem and basically the controls of the front end of your bike. Originally, these used to be quite thin. You'll notice this one's quite fat in size. It's because it houses the bearings directly into it. Original head tubes were a lot smaller and a lot thinner. You'd actually have to press cups into them into which the bearings would be pushed. It's quite a different system these days. What is quite important to notice is on older frames, in particular 26-inch models, you might see the head tubes are quite long on the bike. The reason for that is a thing called stack heights. This is the height a bike is. If you think on a bike with very small wheels, if you've got a very low front end, it's going to be an awkward position to ride in. To keep the position the same equivalent for a tall rider as it would be for a short rider, you have to have differing lengths in the actual head tube length. These days, though, which we'll get to later on, again, it's not that important because we have increased wheel sizes, which has changed things. Next up is the down tube. Of course, that goes from the head tube down to the bottom bracket area of the bike. It doesn't do anything fancy. Obviously, it's a structural part of the bike, but there's nothing else you really need to know about the down tube. It's not often referred to in terms of measurements or any sort of geometry that actually indicates the way a bike would ride. Importantly is the bottom bracket, which is housed under here. The bottom bracket is a shell that essentially meets a bit of a hub of the bike. This part of the bike is really important to be stiff and well-designed. The reason for this is when you're pedaling, all those forces go through that part of the frame. If it's weak, if it's flexible, if it's not very well-designed, you're going to get movement down here, which translates as an awkward feeling bike and also a loss of power. Something with the bottom bracket itself, the shell, there's a few different styles on the market. The classic ones are threaded into which you would thread a bottom bracket shell, but you also get press fit style bottom brackets that just push straight in. They don't require threads to be in place on that shell. You can also get some fancier bottom brackets that cost a lot more money on certain frames where those cups are actually machined as part of the frame. Whilst this is an excellent system, it's very expensive to manufacture like that, so they're not that common. The classic threaded bottom bracket shells are really probably still the best option. Seat tube, that is the one that essentially divides the front half of the bike and the back half of the bike. You'll notice that the seat tube itself houses the seat post which you sit on, and the height of the seat tube therefore actually correlates to the sizing of the bike. Now, this used to be much more important on bikes because you would have literally a 20-inch seat tube would be on a size 20-inch bike and so forth, and you would still see the same thing on road bikes today. On mountain bikes, modern geometry is starting to change things a bit. It's done to size bikes in length rather than height, which actually is the correct way. We're going to get to that when we talk about specific measurements and geometry a little bit later in this video. These are the seat stays. These are the upper rear stays that connect the rear axle to the main frame, and these are the chain stays, so the opposite. These are the lower parts of the frame. Now, typically, you'll see short chain stays give a nice agile handling and longer chain stays give more stable handling, but we're going to talk about that next. Right, so now let's get into some specific angles and measurements. Now, we've had a bit of an overview of a typical frame layout. Now, first up, let's talk about the seat tube angle, also known as the seat angle. Now, this is the acute measurement when you put a line all the way through the seat tube straight through the bottom bracket to the ground between that and the ground, effectively. Now, originally on rigid road frames and early mountain bike frames, a slack seat tube angle was seen as a good thing. The reason for that is it takes more weight off your hands when you're sat down because you're putting your weight further back on the bike. Now, if you are touring or you're gravel riding or something like that, then this could be a good thing. However, for modern mountain biking, it's not really that good because you don't spend that much time sat down. You don't need the comfort from the frame anyway because most bikes have suspension in the form of bigger tires and also in rear suspension and front suspension you find on modern mountain bikes. Now, in terms of road bikes, as I just listed there, you have those slacker seat tube angles for comfort, which we used to see on touring bikes and stuff. Now, the complete extreme of that would be a time trial bike, which they're nearly vertical, the seat tube angles. What that does is engages different leg muscles, stronger muscle groups that can give more power over a shorter time, and it basically loads up your arms to be in an aerodynamic position on the bike. Now, it's a really efficient use of the body, but only for a short time, and it's not comfortable at all. If you look at a typical road bike or a typical mountain bike cross-country frame today, it's somewhere in the middle of those. The reason for that is it gives the best of both worlds and it makes a nice transition between the seated and the stood up positions. Therefore, your muscles work equally as well sat down and stood up, but you're using different muscles all around the bike and it's a generally efficient position to be in. Now, these days, you might hear steep seat angles being referred to as a good thing on mountain bikes. Now, this is something that we know is not comfortable on road bikes because of that extreme angle, but on a mountain bike, this is a really good thing because it puts you further between the two wheels when you're climbing on a bike. The advantage of that is you get traction on the rear wheel, which, of course, you need, and you also give weight on that front wheel. The reason that is a good thing is you're not fighting the front wheel trying to climb, you're not putting yourself in an unnatural position. You can just sit there and keep on cranking and the bike is going to go where you want it to. As soon as the front wheel goes light when you're climbing, you have to really load that front end of the bike and it becomes an additional chore when you're climbing a bike. Modern mountain bikes tend to have a steeper seat tube angle for this very reason. We still haven't found the limit. They're getting steeper and steeper. Essentially, what you have to remember with mountain bikes is our riding position changes constantly. Therefore, we need to rely on our stood position and our seated position as two different things. We can't rely on it like we do on a road bike, whereas it's all about efficiency. It's very different on mountain bike terms because it's about handling. You might also hear the term effective seat angle. This is a way of calculating the seat angle on a bike if you have a peculiar frame design that doesn't have a consistent seat tube. The way to measure it essentially is still through the bottom bracket axle at the bottom and through the middle of the seat post at the top, irrelevant of whatever is going on in the middle of the frame. You take that measurement, not what is going on anywhere else. It's that simple. There's nothing else you need to know about that. The effective seat tube angle is really where your weight is on the bike, despite the optical illusion of some frame designs. Next up is the head angle of the bike, also known as the steering angle. If you draw an imaginary line through the stem, through the center of the stem and the center of the head tube all the way down to the ground and measure the acute angle against the ground, you will get your head tube angle or your steering angle. The steeper the angle, typically, the more nervous and twitchy the handling, which is why bikes that used to have steep angles used to have longer stems because they would calm that effect down. Also, look at road cycling bikes. Look at typical road racing bikes that have very steep responsive head angles on them. To calm that steering down, they have a longer stem. There's one way around of it, but that's not the optimum way for off-road riding because we need more stability at speed, but you need low-speed response, which is why we opt for a slacker head angle with a shorter stem, so it changes your riding position. Having this position also really increases your comfort and stability. To a degree, you could say your confidence off-road because your handlebar position is further behind the rear wheel axle as opposed to in front of it, which means when you're handling on very steep terrain, it feels like you're less likely to be pitched over the handlebars. Next up is your BB height and drop, also known as bottom bracket height and bottom bracket drop. Bottom bracket height is essentially how high your bottom bracket is off the deck, off the ground, so that is that measurement there. Your bottom bracket drop is how low your bottom bracket is, the axle, so the center of the axle, below the line of your two-wheel axles. It's that simple. The lower the bottom bracket is on your bike, the more stable your bike is going to feel, especially when cornering. You're going to feel really low and in the bike. This is why 29ers are adopted by a lot of riders. It's not necessarily the size of the wheels, it's the fact that the wheel axles are so high on a 29er in relation to a normal bike. To keep your riding position the same, the bottom bracket has to be lower to return your body position. One of the effects of that is you feel like you're sat in the bike a lot more on a 29er. One of the downsides of having a low bottom bracket is you've got the stability, but it also means your feet are a lot closer to the ground, so you could bottom your feet out on rocks and roots as you're pedaling. To make the most of this, you end up having to have shorter cranks, which arguably can make things a little harder when you're pedaling a lot. A higher bottom bracket doesn't have that effect, and it also has a positive effect of making your bike feel more agile, but at the same time, it can feel a little nervous. There's a fine line between too high and too low. Some riders love them high, some love them low. There's no right or wrong. Reach and stack are the two next measurements. We'll do a stack very briefly first because it's slightly less important on a mountain bike. It's much more relevant on a road bike where your frontal view and basically keeping nice and low on the bike is more important. Of course, we have to factor in ground clearance and the amount of suspension travel we have, so the stack does change on frame. Reach is the one we're going to look at here because this is much more important. Reach is a realistic way of measuring a modern mountain bike for fit. You don't need to look at the seat tube length. The only reason you need to know that is for your inside leg measurement to make sure you can actually fit on the bike and fit a dropper post on there with enough room for it to extend and compress. What the reach does is essentially tell you the distance between your bottom bracket and your handlebars, which of course, on a mountain bike, all your important riding, the handling of the bike happens when you're stood up. Basically, you want that measurement to be nice and long if you're tall and a bit shorter if you're shorter. Now, I'm going to make this quite easy to understand. If you look at two-wheel axles on a bike, what you're looking for for any size rider really is the illusion of an equilateral triangle. Your body weight wants to be at the top in the middle of the two wheels. If you've got a bike that's got too short reach, you could be more like an isosceles triangle. The effect of that is you're going to be like a big pendulum swinging around up top and you're going to have to move your body weight fore and aft much more when you're going up and down for different reasons. When you're going up, you need to move further forward to weight the front wheel, keep traction on the rear wheel to move around, and that becomes more physical on your arms and on your legs. When you're descending, it could be terrifying if you're too far up on the bike because you've got too much weight on the front wheel. You need to return that weight to the rear wheel. If you look at the reach on a bike, this is really where you're going to calculate the correct length for you. Of course, it's different for everyone, but it doesn't mean you should get carried away looking at geometry charts. Once you find a length that works for you, you can figure things out from there. The way this distance is measured is literally a vertical line going up from the bottom bracket shell off the ground, and then that is met all the way to the top and the middle of the head tube there. You'd also get the stack height there by doing that and the reach is the horizontal. Nice and simple. Just a little thing to touch on with reach and stack is the measurements given by frame manufacturers are the frame reach and the frame stack. You can also have your bar reach, basically on the bike, and your bar stack because everyone's going to have a slightly different preference. I, for example, like a nice short stem and I like a really high bar. My stack and my reach accordingly will be a little bit different to what the frame manufacturer specifies. Something I found makes a quite cool way of setting up a new bike is to, once you find out your reach and stack that you're comfortable with, make a measurement from the end of your bars down to the ground and also make a measurement from the end of your bars to your bottom bracket axle. You'll find that pretty quickly you can sort the bar rise, the bar width, and the stem length, and the stem height you need on your bike to get in a position that feels very similar to what you're riding previously. It just makes the transition between bikes a little easier. Chainstay length. This is the measurement from the rear wheel axle to the bottom bracket of the bike. It's generally seen that a short chainstay length is a good thing and a long chainstay length is a bad thing. Opinions vary, but this is not accurate information. A long chainstay can give you better traction and it can put your bottom bracket closer to the middle of the bike neutralizing your position. That is a positive thing. However, having a long chainstay can weight the front wheel more, which can be a good thing and a bad thing. It can mean you get more weight to that front wheel, which is a great thing if you have a long bike. You actually need a long back end to keep things balanced, but if you're the sort of person that likes to manual and pick up the front wheel a lot, you're probably going to prefer a short chainstay, which has the opposite effect. Not so much weight on the front end because there's more weight on your rear end. It's all about balance. Don't go getting carried away with this. It is literally there's so many different factors within a bike. The next one we're going to roll straight into is the front center. That is the measurement from the bottom bracket to the front wheel axle. Again, these do correlate to each other and they correlate to the total wheelbase, which is the axle to axle. That's three measurements in one there. You'll often see chainstays mentioned and you'll see total wheelbase mentioned, but you don't often hear the front center mentioned. Fork offset is next up. This is something you'll hear talked about quite a lot because fork offset is something that people are now starting to experiment with rather than what was just specified on bikes. There's a lot of factors in this and we're going to go into it in another video soon that deals with that specifically. Fork offset, what is it? It's simply the measurement of your fork axles in front of your steering angle. If you take that steering angle pane all the way down to the ground, you draw another one on that same angle all the way through your fork axle there. It's that measurement between the two. Nice and simple. The last one you need to know about is your trail measurement. This is in direct relation to the fork offset and your steering tube angle. If you take that steering angle all the way down to the ground, the trail measurement is the measurement between where that meets the ground and your front tire contact patch. The shorter that measurement is, the more agile or perhaps nervous your steering can feel. The larger it is, the more stable your handling can feel. The reason for the offset change recent times is basically with the advent of 29-inch wheels coming in because the higher your fork axle is off the ground, it changes that contact patch there. You get a larger trail measurement naturally when you have a larger wheel because the axle goes higher. When the 29-inch wheel boom really kicked off, frame and fork manufacturers started specking forks with a larger offset number in order to reduce the trail back to the same equivalent that they were getting on their 27.5-inch wheel bikes. These days, they're actually going the other way around. They're trying to increase it again for better handling, but we're going to get to that in another video. The reason this measurement is called trail is because it trails behind the steering axis. Simple as that. Well, there we go. That was Geometry 101, I guess you could call it. All the measurements you need to know about what they are, what they mean, and what they do for your riding. If you have any questions, let us know underneath. We have three more videos planned, slightly more advanced, telling you how to make the most out of some of these measurements on your bike. They're coming up soon. If there's anything you want us to directly look at, let us know before we start scripting them. For a couple more videos, I'm going to throw you down here to an e-bike challenge with Jonesy over on EMBN. Good fun video. He's got a bit of an appearance with Olly Wilkins and Brendan Fairclough in that one. I'm going to throw you down here to Henry's video with some World Cup mechanics at Red Bull Hardline. If you want to know anything else about geometry, as I said, let us know in the comments underneath. Give us a thumbs up and hit that subscribe button.