Basic Mountain Bike Geometry and Fit
Geometry: A lot of people make a big deal about a frame's geometry and for good reason - because it effects bike fit and handling. Changing a bike's geometry can make a bike more capable at climbing or descending; turning; more or less stable; more or less comfortable; and more or less efficient.
There are a lot of variables to consider when designers develop a frame and, like chaos theory, changing one little variable can drastically effect the outcome of the overall fit and handling of a bike. The good news is, as long as you are within the ballpark of frame size and type, you can change what I'll call the "effective geometry" of a bike without cutting up the frame and welding (or gluing) it back together. I call it the effective geometry because you are not changing the geometry, just the angles in relation to the rider. This is accomplished by changing or adjusting some of the parts that bolt onto the bike. I'll touch on those areas as I explain them in relation to the frame.
For beginners, disecting a frame's geometry can be a little too far in the weeds when choosing a bike. View the following as an extremely generalized guide to help you understand why one bike may feel different than another and give you an idea as to what people are talking about when they throw around esoteric terms while discussing the mystery of frame design.
For the most part, certain categories of bikes have a specific type of geometry and each is within a pretty close range of one another. Cross country bikes are pretty close to other cross country bikes' geometry and same goes for all mountain vs AM and down hill compared to other DH bikes. But cross country bike geometry is a lot different than a down hill bike's geometry. Manufacturers tweak their designs within each category to give specific types of fit and handling because some people prefer a certain type of geometry over another because of their body dimensions or riding style.
There are four basic areas of a frame's geometry that will effect the way it rides: head tube, top tube, seat tube and chainstay. See diagram below:
Let's start with the front of the bike.
The head tube. There are two measurements to be concerned with: the head tube length and the head tube angle.
Head tube length can be important if you want a really aggressive (low) handlebar position or a really relaxed (high) handle bar position. You can alter the handlebar position with more or less spacers on the steering tube, but only within limits (the steering tube is the part of the forks that goes through the head tube and connects to the stem and handlebars). So, for the most part, a short head tube, with no spacers, will allow you to get lower than a longer head tube with no spacers.
Since the number of spacers you can use has limits, some people need or want a longer head tube to get a sufficiently upright position, just as some prefer the short head tube to get a sufficiently aggressive riding position. Some bikes with a short head tube ship with a long steering tube and a lot of spacers to allow you to determine how aggressive of a riding position you want. Others will cut it low assuming you want an aggressive riding position.
Note: You cannot lengthen a steering tube once it has been cut - you must buy a new set of forks, so, make sure you like the handlebar position before you go cutting off that steering tube! If the bike comes with a short head tube and steerer tube and you want a more upright position, you can buy a set of riser bars. Or, if it's too high, you can go the other way and cut the steerer tube or buy a set of flat bars. Because 29ers' head tube sits higher than a 26ers' head tube, some people are slamming the stem and/or turning it upside down with flat bars to get the aggressive riding position they are used to having with a 26er.
This begs the question, "what's the difference between an aggressive and relaxed riding position?" In short: aerodynamics, leverage and comfort. The lower you are, the more aerodynamic with greater leverage to pull up on the front wheel (and arguably less comfortable and less stable) compared to more upright. So, it's a trade and depends on what is more important to you.
Head tube angle. For the most part, the more a bike looks like a chopper, the more stable it will be while riding in a straight line. This is why downhill and all mountain bikes have "slack" HT angles - for stability. The less chopper-like the bike gets, the faster it will steer and turn. Careful though, not enough head tube angle and the bike will be twitchy (difficult to ride with no hands) and you'll also likely to be over the bars often.
Depending on the category of riding a bike is designed for, the angle of the head tube will be within a certain range. XC bikes have a steeper HT angle compared to All Mountain (AM) and Downhill (DH) bikes. Generally, entry-level XC bikes will be a little more slack than race XC bikes.
However, one can change the "effective" HT angle (slackness/stability and steering speed) by increasing or decreasing the length of the forks. Not all forks are adjustable. However, some are designed and built to allow a rider to change the fork length (travel) by +/- 20mm. This is typically accomplished by altering the travel, or, the distance a suspension fork can compress. A segment of adjustable forks even have a way to adjust the travel (and therefore fork length) with a couple twists of a knob. When you increase or decrease the travel, you are raising or lowering the head tube. In doing that, you are increasing or decreasing the "effective" head tube angle and changing the angle of the forks. Comparatively longer travel will make a bike more slack. Be careful though - if you change the travel more than +/- 20mm, you will void the warranty because you are putting additional stress on the head tube and you might cause it to break.
Another way is to buy a special "adjustable" headset. The headset is the compilation of bearings and spacers in the head tube. An adjustable headset allows one to adjust the steerer tube angle within the head tube +/- 1.5 degrees. It might not sound like a lot, but 1.5 degrees in either direction can really change the way a bike rides.
It is also possible to speed up or slow down the steering or twitchiness a couple of ways without changing the travel or HT angle. Putting a longer stem on a bike and/or widening the bars slows the steering down just as a shorter stem and/narrower bars speed it up. This is like the difference between a small steering wheel on a sports car and a big steering wheel on an 18-wheeler. It doesn't actually change the ability of the bike to steer quicker or slower, it just feels like it.
Changing either the stem or handlebars, or both, will also change your effective reach and put you in a more or less aggressive riding position. Longer stem means you have to bend over farther to reach the grips. Wider bars have the same effect. This may or may not be what you want. You can counter this by moving your saddle forward or backward to compensate for the change in reach. Again, you have to be careful making changes because moving your seat position also changes your effective seat tube angle and, in an essence, also changes the effective top tube length.
Which brings me to effective top tube length.
They call it that because the actual top tube length is the measurement of the top tube from the middle of the seat tube, through the middle of the top tube, to the middle of the head tube. The effective top tube length is the measurement of the middle of the seat tube, parallel to the ground, to the middle of the head tube. The reason these two measurements are different are the same reasons Pythagoras has his own Theorem. More on the importance of top tube angle in the discussion about seat tube length and stand over height later.
A shorter or longer ETT will dictate how cramped or stretched, or how upright or aggressive the cockpit feels. If you are a person with long legs and a short torso, you'll likely want a shorter ETT. This is also desirable for people who prefer a more upright seating position. The seat tube angle also plays a part in the ETT measurement. The more relaxed seat tube angle, the longer the ETT. Generally, an entry level bike tends to have a shorter ETT and a more relaxed seat tube angle because this allows a more upright seating position. This makes the front triangle of the frame shorter than a race bike which typically has an upright seat tube and longer ETT to give an aggressive riding position.
Nevertheless, you can customize cockpit length by moving your seat +/- 1.5, inches, which is a common adjustment. In extreme cases, some people opt for a "laid back seat post" which is a seat post with a backward bend in it to give a little more room in the cockpit. This can give another inch or two of rearward seat adjustment. *I've also seen people use a laid back post to bring the seat forward, but I don't recommend you do it.
When moving your seat forward or backward, you are changing the "effective" seat tube angle (a term I think I made up a long time ago) which is responsible for the position of your butt in relation to the bottom bracket. This measurement is important because it will dictate the weight distribution of the bike while peddling.
Moving your saddle forward shifts your weight forward, which can make climbing a little easier by keeping the front tire from popping up on you. However, it can also reduce traction during climbs and, on the flats, it will add a little more pressure on the wrists and hands and makes popping a wheelie a little more difficult. Moving your seat back shifts your weight*back, which, conversely, *puts more weight on the rear wheel and increases traction. This can also make climbing while in the saddle a little easier, but it can also cause you to loop out on steep climbs in low gears.
Lastly, a more relaxed effective seat tube angle is a little more comfortable as it will soak up bumps a little better by letting your seat post flex instead of sending the force of a bump straight up the seat post and into your back.
When adjusting your seat, make sure your "sit bones" rest on the back of the seat. This will take some pressure off your pelvis and make riding more comfortable (There are different width seats for custom fit) This will also give you some room to adjust your weight forward by sitting on the front of the saddle when climbing.
Some bikes have a top tube that is more parallel to the ground than others. This is mostly determined by the seat tube length.*
The extreme examples are XC bikes compared to trials and jump bikes. Trials and jump bikes have a steeper top tube angle to give more clearance between one's groin and the top tube. This allows the seat to be low*and out of the way (or eliminates it in the case of a trials bike). XC bikes have a top tube closer to parallel with the ground to support a raised saddle. In XC, you want to have your saddle in a position for maximum peddling efficiency when seated. This is achieved by having your leg almost at near full extension when the pedal is in the full downstroke position.
Note: The best way to find the right seat post height is to sit on the bike with the right-side crank in the five o'clock position. With riding shoes on, put your heel on the pedal spindle. If your right knee is bent, your seat is too low. If you are stretching or reaching to touch the spindle with your heel, your seat is too high.
For XC, if your seat tube length is short and you have long legs, you will need a long seat post. If your seat post sticks up too far past the top of the seat tube, it will break, or it will break the frame. Seat posts are marked with a maximum height. Do not exceed this height. If you must exceed the height, you need a larger frame.
If you are short and your seat tube is long, you won't be able to stand over the bike without your crotch painfully straddling the top tube - which is where the "stand over height" measurement comes from.
A lot of attention is given to stand over height, which is important, but, arguably, less important than ETT; because, hopefully, you spend more time riding your bike than straddling it. It gets the most attention because most manufacturers sell frames in sizes according to seat tube length and stand over height. A general rule of thumb is to have two or three inches between "you" and the top tube. But this doesn't always give the best fitting frame. Worse is when they recommend a bike size according to height. Most times this is okay, but, sometimes it's really bad.
Remember the rider with long legs and short torso needing a shorter ETT? Well, paying attention to height or stand over only would possibly recommend a frame too large. Conversely, having short legs and a long torso will require a longer ETT. Sometimes this will result in a larger frame choice that will be grazing the rider's crotch when standing over. However, the cockpit*will be more comfortable when riding compared to a smaller size that gives two or three inches stand over clearance and a hump back. Of course, this can all be "fixed" with a laid back seat post, a longer stem and a set of wide bars - but it isn't ideal.
The last area of discussion is chain stay length.
This area effects stability, agility and weight distribution. The shorter the chain stay, the easier the bike will be to wheelie and the more nimble it will feel. Too short and it will loop out on climbs. Longer chain stays make the bike track in a straight line better. Taller guys riding larger bikes with a relaxed seat tube angle and a long seat post typically need longer chain stays to keep from having too much weight on the rear wheel.
Those are the four basic areas of geometry and how they relate to handling and fit. As you probably noticed, the frame is just a starting point and the parts you bolt to it can change the handling and fit.*
Now, let's take a minute and imagine an interactive graphic of a rider on a bike that allows you to*drag each of the areas we discussed and how it makes the bike and rider look. Drag out the front wheel, pull the top of the seat tube toward the rear wheel, make the seat tube longer, make the top tube shorter. You see how changing one changes almost all?
I made sure to stay away from including angles and inches because different sizes, different constructions and the parts bolted to a frame can move some bikes into different categories. Some XC bikes have slack head tubes. You can make an AM bike a decent XC bike.
The short of it is, geometry is relative. It's impossible to understand a frame's geometry without putting it into context. It's more of a comparative analysis. Get a bike based on your height and standover and category. As you ride it, you can customize it. If you find yourself wanting your bike to perform a way that can only be accomplished outside the adjustable limits of your current bike, look for a frame with that geometry. If you see a bike that you want, let us know what you plan to do with it and we can help you understand if it is appropriate. For the most part, you can just look at a bike and understand what the category is.