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In the days of yore, pretty much all you needed to know about bike fit involved standing over the bike’s top tube to make sure you had a couple of inches of space between the hard metal frame and your soft crotch bits. This is known as “stand over height,” and it’s still used as the starting point for ensuring a good fit. You’ll often see recommendations, such as this found a Cannondale owner’s manual:
“To check the correct stand over height, straddle the bike while wearing the kind of shoes in which you’ll be riding, and bounce vigorously on your heels. If your crotch touches the frame, the bike is too big for you. Don’t even ride the bike around the block.”
The manual goes on to describe the minimum amount of space needed, with four inches of clearance or more recommended for off-road riding. That’s a good starting point, but considering that a “professional bike fit” is an actual service that can cost hundreds of dollars, there’s much, much more to it.
Before you hire someone to palpate your ischial tuberosities, throwing a leg over a bike and checking the stand over height IS the best way to get a general sense of if a bike will fit you or not. Having more space between your soft crotch bits and the frame is generally better, and I’ll bet that most riders use this and this alone as a marker for proper bike fit.
I’d also bet that most of these riders are absolutely fine. Is their bike sizing optimized? Could handling or efficiency be improved? Sure, but for the average rider, a “good enough” fit is probably good enough. However, if you’re spending long hours on the bike and getting a sore neck, or you find that you lose traction on climbs, some tweaks could really help.
And speaking of tweaks, bike and frame designers have been making changes in tube lengths and angles since riders first started looking for better off-road performance. It’s very interesting to see what “standard” frame geometry looked like say, 30yrs ago compared to today.
But, before getting into some of those differences, let’s cover what is meant by frame geometry.
Tubes and Angles
We could dive into Euclid’s theorems and axioms to start down the path to geometric enlightenment, but all you really need to know for bike geometry is a few key frame lengths and a couple of angles.
Most frames use the seat tube length to determine the frame’s size, be that Sm, Md, Lg, or Xl, or an actual measurement, say 21 inches. Interestingly, or annoyingly, bike measurements swap randomly and indiscriminately between Imperial and metric measurements, so don’t be surprised to see a frame with one measurement in inches and another in millimeters. Another annoying aspect to frame measurements, specifically seat tube lengths, is that some frame manufacturers will advertise the seat tube length from the center of the bottom bracket (BB) to the actual top of the seat tube, and others will measure from the center of the BB to the center of the top tube where it meets the seat tube.
Two frames could be listed as “18 inches” but one could be two or more inches smaller than the other which goes back to stand over height.
Top tube (TT) length is also important to the bike’s fit as this can help make the bike’s cockpit more upright or more stretched out. With some frames’ sloped TTs and/or their seat tube angles, the actual tube length is only part of the story as the frame’s effective TT (eTT) may be a better indicator of how stretched out you will feel.
The eTT is a horizontal line from the center of top of the head tube to the center of the seat tube or post.
Enter The Slacker
Prior to the late 90’s, top tubes were relatively short and stems were long. Then Gary Fisher introduced the Genesis geometry which promoted longer TTs and shorter stems. This standard was the precursor to modern long/slack frame designs and continues to evolve today.
Fisher’s Genesis geometry also showed the benefits of shorter chainstays, measured from the center of the BB to the rear axle. This promised to get your weight better positioned over the rear wheel to improve traction, and moving the rider’s weight back would also help make lifting the front wheel over obstacles easier.
What about seat tubes?
Seat tube angle plays a role in getting the rider’s weight back over the rear wheel too, but there’s a point in which, on steeper climbs, the weight could get so far back that the bike will have a tendency to wheelie up and loop out, requiring the rider to lean forward more. Because bike seats have rails that allow you to slide it forward or back a bit, you can adjust the bike’s effective seat tube angle slightly.
There’s no “perfect” seat tube angle, nor is there a perfect chainstay length, but these two frame attributes will definitely affect a bike’s climbing performance and stability.
Another part of the frame’s design that contributes to the stability of the bike is the BB height measured from the ground to the center of the BB. Lower tends to be more stable, as this lowers your center of gravity, but lower also increases pedal strikes and impairs clearance when riding technical features. Because BB height can be changed by different tires on the same bike, 2.1” tires giving a lower BB height than 2.6”s, bikes will also often advertise BB drop—the distance down from a horizontal line from both axles down to the center of the BB.
Head Tubes & Steering Impact
The front tube of the bike, the head tube, is angled in a manner that affects how quickly the bike steers. Steeper angles yield faster steering but this can be unstable, especially at high speeds.
You’ll see head tube angles that are lower on dedicated downhill bikes, say 63 degrees as opposed to closer to 70 degrees for some cross country bikes. Overall the bike industry has adopted “slack” head tube angles for trail bikes in the mid-60’s.
The length of the head tube will affect riding position, with longer head tubes giving a more upright position. It’s possible to still get an upright position on a bike with a relatively short head tube length if that’s what’s wanted, with spacers under the stem, a stem with some rise, riser handlebars, or all of the above.
There are less options for getting a low, aero, position on a bike with a long head tube however, so that’s just another factor to keep in mind when looking at bike measurements is an aggressive position is your preferred riding stance.
A somewhat newer measurement, called “stack,” helps people compare just how high the top of the head tube will be. Stack is the vertical distance from the center of the BB to the top of the head tube. That same distance measured horizontally is called “reach,” and that allows you to compare how stretched out you’d be on different mountain bikes, before taking longer or shorter stems into account.
One other thing to mention is that most of these frame measurements, especially head angle and seat tube angle are presented with a specific fork in mind. If a frame is designed to be used with a 120mm travel fork, but instead you use a 130mm fork, you’ll slacken the angles slightly.
Enough to be noticed? Maybe not, but there are other ways your fork affects the bike’s handling.
What the Fork?
You’ll find many (many, many) different fork designs, but two-sided telescoping suspension (or rigid) forks are most common in mountain biking. Despite any design differences, they all essentially put the front wheel’s axle somewhere in relation to the steering axis, usually just in front of it.
The distance between the front axle and the steering axis is known as the fork’s rake or offset. And, the rake helps determine the “trail.” No, not the trail you’re riding, but the distance from where the steering axis would hit the ground to the tire’s contact patch.
More fork rake leads to less trail, which offers quick steering, but less rake, and more trail gives more stability. If that’s not confusing enough, remember that trail will constantly be changing when, well, you’re on the trail. When you’re coming down a steep downhill and transitioning to flatter ground, the tire’s contact patch may be in front of the steering axis, and front axle, leading to negative trail and a tendency for the front wheel to want to flop over, sending you over the bars.
The best way to visually demonstrate the stability of more trail (tire’s contact behind steering axis’ virtual intersection with the ground) is with a shopping cart wheel or similar.
Slide Rule, Protractors, and a Helmet
You don’t need to bring a scientific calculator with you to the bike shop, you can just throw a leg over the top tube, make sure you’ve got enough “safe space” and take a test ride. You’ll feel if the bike is lively or sluggish, twitchy or stable.
But, knowing how all of these factors come together to make a bike feel the way it does is important when you want to make changes. Do you ride tight, twisty, technical trails and you want faster steering? If so, maybe you need a bike with a steeper head angle or a fork with more rake.
Do you smack your pedals constantly on rocks? Maybe you need a frame with less BB drop (or shorter cranks).
Or maybe you’ve been riding a medium Kona Kahuna, which you like, but you want to upgrade and get a medium Canyon Neuron CF8, what’s the difference? The medium Canyon has a 51mm taller stand over height and a 25mm longer seat tube. The Canyon’s stack is only 3mm taller than the Kona, and its reach is actually 7mm shorter.
I think the take-home message here is that with the reach and stack being fairly close, the bikes might feel similar, and the Canyon even has chainstays that are 10mm shorter, perhaps making it more playful, but the stand over height is 2 inches taller than the Kona, and that might feel too close for comfort.
Building off the Frame and Fork Foundation
Assuming you’ve got a frame that’s the right size, how do you get the seat and handlebars in the right position to also fit? Well, there are many ways to get an “approximate fit,” to get you in the ballpark for optimum performance and efficiency, but they’re simply starting points and suggestions.
You may try one of these suggestions and not like it, and that’s okay. Everyone, and every body, is different. Many fit suggestions will have to balance different outcomes. For example, if you may want a more upright riding position, to alleviate neck or wrist pain, but you may lose some aerodynamics on the flats or lose some stability when climbing steep hills.
Getting the right position requires lots of trial and error. Some riders who have been doing this long enough will have a drawer full of stems of varying lengths and angles because we might try one, then decide that we want something different after a season of rides.
Fit can change because our bodies change too, whether that’s because of improved flexibility or an injury or a myriad of other reasons, so again, re-establishing how you can ride comfortably may take trial and error, even after years of time in the saddle.
I personally think bike fit for a mountain bike is a bit less critical than for road bikes. On road bikes, you can spend a considerable amount of time on the bike in one or two positions, but off road, you’re constantly moving around.
That being said, here’s my DIY bike fit recommendations to help get the most out of your bike’s holy trinity of contact points: seat, pedals, and grips.
Let’s assume, again, that you have a frame that’s the right size and that you’ve adjusted your suspension sag according to the shock/frame manufacturer.
2. With your shoes set, hop on your bike to set the seat height. If you’re riding a dropper post, make sure it’s all the way up. Now, adjust the seat (and post’s) height via the seat post clamp, up or down, until you have your leg totally straight with your knee locked and your heel on your pedal. Check both legs. If the saddle height is too high, and your hips rock back and forth, that will strain your knees. If your seat is too low, that can also strain your knees and you’ll lose power as well. While this height is recommended for maximum pedal stroke efficiency, if you ride more technical terrain, you may want to go a centimeter or two lower. You’d have a bit more ability to maneuver around the bike, without using the dropper, and you’d likely only have a negligible decrease in efficiency. Again, trial and error. You can use this as the starting point and make any additional adjustments by feel, and you don’t need to get all complicated and use any math.
3. To adjust the seat on its rails, its “fore and aft” position, you’ll want to be seated, with your pedals level, and use a plumb bob (or just an imaginary line) from your knee to the pedal spindle. This position is called “Knee Over Pedal Spindle,” (KOPS) and believe it or not, is somewhat controversial. Yeah, controversies in bike fitting is a thing. Anyway, while there are other thoughts on the topic, this is realistically one of the easiest way to get a starting point before you move a saddle forward or back, and in my opinion is probably good enough to get most riders to the correct position. Remembering again that mountain bike riders move around more than road riders, and that riding style means we tend to slide forward and backward on the seat when needed, so using KOPS to get the seat position established is fine.The seat should be angled a few degrees down to help with mobility (and avoid an unfortunate jab to the behind when transitioning from seated to standing.)
With your pedals level, having the bump below your knee (tibial tuberosity), in line with the pedal spindle wil get you close to the perfect spot for the seat’s fore and aft position.
4. With the seat at the right height and fore/aft position, the last thing to check with the seat is its angle. A flat seat is typically recommended, and if not flat, maybe a couple of degrees one way or the other. If the nose of the seat is too high, it’s going to cause some “discomfort” in the aforementioned soft crotch bits. You’ll also have a tendency to slide off the back. If the nose is too low, you’ll have a tendency to slide forward and may have wrist or shoulder pains. And, if after all this, you still don’t feel comfortable on your seat, maybe you need a different sized seat.
5. As we consider these aspects of your bike’s fit, I should mention crank arm length. Most bikes are going to come with cranks in the 170 to 175mm length (170’s on the smaller bikes, 175’ on the larger), but I know people with proportionally longer or shorter legs, and they may benefit from longer or shorter cranks than the ones that came on the bike. This is a bit harder to put in the “trial and error” category, as there’s no way to adjust crank length, and trying different cranks can get expensive. One thing to note though is that if you have shorter legs, and/or you’re plagued by pedal strikes when you ride, you may want to consider shorter cranks. This is before you get into pedal spaces, which can add lateral length to the pedal axle should you have wider hips or need additional horizontal support.
6. And finally, it’s your grips. Your stem and handlebars determine where your grips will be and this will either make you more upright or down low. Shorter stems or stems with a rise will obviously make you more upright, as will riser handlebars, and longer stems, flat handlebars, and wider handlebars will stretch you out more. There are no real “start here” recommendations other than what your bike might come with as stock components. Your stem might also have spacers under it which can be moved around to get the handlebars (and grips), slightly higher or lower. It’s all about finding the best position for the majority of riding that you do, as the best climbing position will likely be terrible for downhills, and vice-versa. Changing MTB handlebars (width, sweep, rise) and/or changing stems (length, angle) is relatively easy (unlike some modern road bikes) and goes back to the concept of trial and error. Grips also come in different diameters to accommodate different hand sizes.
Does the Bike Fit?
Now, armed with this knowledge, you’ll be able to understand those bike geometry specifications and better be able to compare and contrast different frames. You’ll also be able to check to see if your bike is set up right for you. But don’t worry, if you have a bike that’s too big, you can always get limb-lengthening surgery. Maybe stand-over height isn’t as important as I thought.