Bikers are always looking for that elusive moment when the bike disappears—when any niggles fade away, when body and machine are perfectly in sync, and when flow is achieved. A properly fitted bike is critical to this process. Nevertheless, getting the correct fit on a new bike, or improving the comfort of an old one, may be a difficult task. Knowing how to read geometry charts and why one bike fits differently from another may help you find your way out of the mud.
There are several bike manufacturers that size their bikes in the same way as t-shirts are sized—small, medium, and large. Others (mostly on mountain bikes) utilize imperial measurements, such as 17 inches, 19 inches, and 21 inches. It’s common for road bikes to have their sizes listed in centimeters rather than inches. The nominal size of any bike may imply many different things depending on the manufacturer, further complicating matters. Bianchi’s racing geometry road bike is 55cm in size, whereas Giant’s is 47cm, thus an effective medium-sized frame is 55cm in Bianchi and 47cm in Giant. According to Bianchi, their size comes from the projected length of the seat tube (what they call a 55cm is really 52cm in that dimension); Giant’s top tube slopes dramatically and has an integrated seat tube clamp, both of which add up to a small 47cm seat tube length. You must be perplexed, right?
Using the effective top-tube length as a general reference will allow you to get an accurate idea of what size you need. The most challenging part of measuring a bike is dealing with the different top tube angles, but utilizing the effective top-tube length (ETT) helps standardize the procedure.
The second basic guideline is the height of the front of the bike; the headtube length is usually used here (HT). This is a rough guide since it does not take into account variations in headset and fork styles. This is far from perfect, but if you don’t care too much about your bike fit, it may be a decent place to start if you want to feel comfortable on your bike. An example of this would be an external headset.
For many riders, however, more precision is needed than what can be obtained by measuring the top tube and head tube lengths. Having a different bike fit may be due to nagging ailments, excessive mileage, or just a desire to be consistent. Understanding two dimensions: stack and reach, will be your most useful tool here. In an X–Y coordinate system, the bottom bracket serves as a fixed point for this dimension. Another way to say it is as follows:
The vertical space between the bottom bracket and the top of the headtube is known as the stack.
A bike’s reach is measured horizontally from the bottom bracket center to the top of the headtube center.
When we speak about stack and reach, we often refer to the distance between the top of the bike’s frame and the ground it sits on. The same principles apply, but measure from the top of the handlebar’s center to include the stem’s length and angle, as well as any external headset cups, cone spacers, and steerer spacers if applicable.
Because they take into consideration the complicated interplay between headtube and seattube angles, as well as fork length and rake, these measurements are useful when choosing a bike.
When it comes to reach and stack dimensions, an OEM frame manufacturer’s experience shows how critical these may be.
Head-tube Stack Reach\sXS 480 525 105 510 383.4\sS 510 540 120 522 385.2\sM 540 550 155 557 384.8\sL 570 565 190 588 385.1\sXL 600 580 215 612 392.5
According to the top-tube and head-tube measurements, you’d anticipate a smooth development through the sizes; the leaps don’t seem to be very dramatic, and there appears to be a frame choice to accommodate riders of various heights. There is only a 10mm difference in reach between the lowest and the biggest sizes, so riders on tiny frames will have to reach farther than those on large ones. This will need stems that are crazily short or long, which will negatively impact the bike’s handling. Not only will the sizing be confusing and lead to a poor fit, but there will also be little to no consistency in how the bike rides from one size to the next because of this.
The following is a more balanced size chart (taken from Specialized’s Roubaix model):
Stack Reach\s49 445 514 125 528 378\s52 475 536 145 547 379\s54 495 547 165 564 380\s56 515 565 190 590 387\s58 540 582 225 622 392\s61 565 601 245 643 398\s64 600 616 260 657 409
The stack and reach have logically increased, and there are no amusing inconsistencies as in the prior example.
While stack and reach is a helpful tool for determining the proper bike size, it’s even more so when it comes to reproducing a bike fit from one bike to another. Measurements may be entered into online calculators such as bikegeo.net, which is helpful if you’re in the market for a new bike and don’t have a real bike to compare against. However, if you’re riding two bikes, it’s time to get the tape measure out. Just imagine that in addition to your cross-country and road bicycles you’ve also got a sore and uncomfortable back. Despite the fact that your bikes have the same listed sizes, you may feel more at ease riding the road bike than the cross bike. Using the methods below, you’ll be able to compare the stack and reach of your bikes and see what has to be changed to make them fit more uniformly.
measuring the height of the stack and the distance it reaches
Bicycle 1 should be positioned in the corner of a room, diagonally across it (rear tyre touching one wall, handlebar touching the other).
The distance horizontally between the back wall and bottom bracket’s center should be measured. Record. IMG 9244 s
From the floor, find the center of the lowest bracket and record the distance vertically. Record. IMG 9250 s
Do not measure vertically, but rather horizontally. In order to get the true value, subtract the value from measurement #2. This is the distance you’ll have to reach with your hands on the handlebars. IMG 9252 s
From the ground, find the center of the handlebar and record the distance in meters. To get the actual measurement, subtract the third number from the total. This is what your handlebars will look like after they’re all put together. IMG 9255 s
Bike 2: Perform the same five steps as Bike 1, then compare the results. When comparing the two bikes, bike 2 has a 2 cm longer reach and a 3 cm higher stack. IMG 9256 S
As long as you follow the same concepts, you should be able to figure out how high the seat should be, as well as how far back it should be.
Changing one or both of these parameters, or moving the fork steerer up or down, may be all that is required to bring everything back into balance. For the levers, you may use the distance from the center of the handlebar as a reference. To make bike 2 fit the same as bike 1, we lowered the stem position and shifted the seat setback a little. This is the final result: IMG 9265 s The size disparities between your bikes have finally been identified and corrected, so now you’re all set to ride out into the future in more comfort!