Shopping for a new mountain bike is an exciting prospect to be sure. But how does one know what size frame to ride? It can be a daunting task to say the least to find that perfect fit. There are more options than ever for a mountain biker today, when he or she begins to consider which bike to buy.
Before size frame can be considered, one has to determine what the bike will be used for. Is it going to be the weekend gravel muncher, or the daily downhill dominator? There are bikes at both ends of the spectrum, and a veritable cornucopia of options to choose from in between. Wheel size is currently a hot button in the industry, with many widths, and diameters to muddy the waters. After a person decides the type of bike that most closely fits his or her riding style, it is time to determine a size.
A properly sized mountain bike frame is key in making certain that you get many comfortable hours on the trail. Similar to the number of bikes out there, there are no shortage of methods that can help a rider make that first step in the decision making process. A few of the measurements that are close at hand are your own personal measurements. Height and inseam are the two that most directly relate to bicycle fitment.
It is traditionally stated that a rider should be able to stand over his or her bicycle with about an inch of clearance between the pelvic bone, and the top of the frame. This measurement is becoming less and less accurate with modern mountain bikes, and the geometry that the companies are building into them to handle the extreme terrain that we as riders are throwing at the bikes. It a number of charts that can help narrow the decision are easy to find online. About.com has an article, seen here that is a generic height to frame size chart.
With a few simple measurements the correct size mountain bike frame seems to be an easy decision. However, while using these types of charts, there is something extremely important to keep in mind. This fact is that it is crucial to be comfortable on your bike. You might be lucky to find your perfect size and fit with a chart like this, and they are no doubt a helpful tool for a rider early in the decision making process. But, the more likely scenario is that it will be close, but not perfect.
I suggest making a trip to your local bike shop (LBS) and trying out a few different sizes. You may find that the proportions of your body (legs, and torso for example) are not stamped out from a generic human shaped mold. My legs could be longer, or shorter than a person taller, or shorter, or even the same height as myself.
Measuring the frame, and gasp… some math
Physical frame measurements are the next step in the process. There are websites that show you fairly accurate measurements to make that will land you very close to that proper size. Bikeradar.com has a page dedicated to these measurements and using them to help make your frame decision. However when making these measurements, it is helpful to know what it is that you are measuring. I will attempt to clarify some of these potentially mystifying terms.
- Top tube length – The measurement from the point on the bicycle where the stem on the handlebars meet the fork, to the point on the bicycle where the seat post recesses into the frame. On mountain bikes, this is usually not a horizontal line due to the sloping geometry designed to handle the rugged terrain. One can see where this would be an issue.
- Seat tube length – The measurement from the point on the bicycle in the center of where the pedals are housed (further explained below) to the point on the bicycle where the seat post recesses into the frame. Another measurement that is not exactly vertical due to the unique geometry of mountain bikes.
- Bottom bracket – While not mentioned on the above listed web site, it helps to know what this is to clarify some of the terms that I will use next. The bottom bracket is the cylindrical housing that contains the bearings to make smooth pedaling possible. The center of this cylinder is a common place to make measurements from.
- Head tube – The cylindrical bearing front frame section housing bearings that makes smooth steering and control possible, the handlebars are indirectly attached to this part of the frame by the stem.
- Down tube – The down tube is the frame measurement from the bottom bracket to the head tube.
The next few measurements are neither listed on the Bikeradar site, nor are they necessarily easy to make. These two measurements are gaining popularity in the mountain biking world, and it is a good idea to know what they are.
These two measurements are correlate directly to how the rider feels on his or her pedals in relationship to the handlebars. The stack is a vertical line drawn from the bottom bracket upward. The reach intersects the stack with a line drawn horizontally rearward from the head tube.
Knowing these measurements can bring clear focus to the size mountain bike that a rider needs. If we look at a picture of this we can see that if we connect the two lines that the stack and reach represent, a right triangle is formed with the longest side being a line drawn from the bottom bracket to the head tube. This “comfort line” is the most critical measurement.
Some lucky riders (professionals for example) have the opportunity to try a wide variety of makes and models of bikes. These riders may find a bike that “just fits”. If a rider takes the diagonal measurement on the bike that he or she is most comfortable on it could be assumed that any bike that has a measurement close to this third line would be comfortable for him or her. This should be true for nearly all riders. And it is important to note that this line is not gender specific.
With all of these measurements to be taken, finding a frame might pose an intimidating afternoon with a tape measure. This alone could scare away some people. Your LBS can help you with these measurements, and most shops will be more than happy to take that time with you. There are a number of benefits of having these measurements taken at your LBS. One is that if you buy the frame from them, some shops keep files for their customers for future reference. Another positive is that when it is time to fit the bike to you these will have already been taken, and the fitting procedure will be that much easier.
I’ve measured everything now what?
The modern person looking for a mountain bike, or any bike for that matter, will no doubt be tempted to make their decision and purchase online. While this might seem to be an attractive avenue to pursue, and might actually be convenient for certain riders, I personally would not purchase a bicycle without actually trying it out to see how it felt. The online customer can usually find most of the measurements listed above on the manufacturer’s website. Keep in mind though, without knowing how a specific size frame felt prior to purchase, the risk involved would be great considering the price point of the modern mountain bike.
It just doesn’t fit!
If it comes down to it, and you are between sizes, there are a few things that can be swapped out to better accommodate your specific measurements. Crank arms (the arm between the pedal and bike) come in different lengths, and could be a potential way to dial in fit on your bike. Another swappable item is the stem. This connects the steer tube to the handlebars and could be swapped out to either lengthen or shorten how far forward the handlebars are positioned. The handlebars themselves come in various geometries. A longer or shorter back sweep and rise could, and should be taken into consideration to achieve the most comfortable hand and arm position to reduce fatigue, and improve control.
The riders’ saddle can also be adjusted forward and aft to get the riding position just so. These items are usually adjusted at a LBS, or in the rider’s home, and will be secondary to choosing frame size. It just pays to remember that these other pieces of the puzzle come in play while striving to achieve the proper fit.
There’s gotta be a better way!
Bicycle fit is rapidly becoming its own industry, with high tech computers and motion capturing sensors that attach to the rider’s body to exactly tell a store, or fitter, the perfect size and fit for that individual rider. Sessions where someone does this for you are usually extremely accurate, and worth the time if that precise level of fit is required. This usually is in a professional racing setting. However if the rider desires that fit, or has unsuccessfully tried every other option, then it might be the only way to ensure that the bike and rider are in perfect harmony. This level of accuracy does require time, effort, and a high level of professional skill. These reasons justifiably dictate a premium cost at this time. It is just another thing to consider when choosing a frame size.
In summary, it is important to find the correct size mountain bike frame to reduce fatigue, and ensure that the rider is comfortable and in control at all times. Several measurements take play in the decision making process, and taking these measurements will help to find that perfect mountain bike size. I feel that however important these measurements are, it is more important to find the size frame that feels the best to the individual rider. Remember, each of us most likely have different requirements and specifications for our particular shapes and sizes.
Finding the right feel can potentially be difficult, especially if the make or model of bike you are seeking out is not available to you. The professionals at your local bike shop can help you find a bike that feels right, or can suggest another shop that can better serve you. In the end it is most important that we as riders ride. To make sure that we ride, finding a frame size that feels the most comfortable to us goes a long way to ensure that the bike does not end up neglected.