Bicycle frames: How to measure bicycle frames
How to measure bicycle frames: our complete guide on bicycle dimensions
If you are considering buying a new bike or your first bike, knowing the key dimensions that define the frame is very important to ensure that you get a machine that is right for you and for the type of riding you intend to do. If you plan to rent a bicycle, it is useful to know the key dimensions of the frame so that you can ensure that the rented bicycle is comfortable to ride and can be adjusted to fit. Accordingly, this article will tell you how to measure bicycle frames.
Not all manufacturers measure bicycle frames size in the same way, so you need to look at some basic measurements to make sure you compare apples to apples.
The key figures that define your bike
We will tell you how to measure your bike below:
• Length of upper tube
• Seat tube length
• Covering and stacking-and why they are important numbers
• Chainstay length
• Front center
• Seat tube and head tube angle
• The bottom bracket is lowered
• Height of bottom support
What is needed to measure bicycle frames
• Tape measure
• Inclinometer to measure angle (you can download a free smartphone app)
• Long spirit level (or you can use the inclinometer app and a straight piece of wood)
• A plumb line (or you can improvise with a rope and a few drops of Blu Tack)
Most bicycles, whether road bikes or mountain bikes, are now measured in metric units, but you may find that some manufacturers still measure mountain bike sizes in inches. Some brands, such as Moots, even mix the two!
We strongly recommend that you stick to metric units for consistency. If you really must, you can always switch to inches by dividing centimeters by 2.54.
You can usually find geometric diagrams for all sizes of the current frame set on the manufacturer’s site. If your bike is still the current model, it is worth making a copy because it is more accurate than your measurements and may be a convenient reference.
How to measure the length of the upper tube
Once, bicycles had horizontal upper tubes. Many bicycles now have a slope on the top tube.
If you look at the bicycle geometry table, it will usually contain the actual top tube length. But for consistent measurement, you need to measure the horizontal pipe jacking length regardless of the pipe jacking angle, which is called effective pipe jacking length or virtual pipe jacking in many geographic charts.
This is the horizontal distance between the centerline of the head tube and the centerline of the seatpost. Measuring correctly means using your level or goniometer app to ensure that your measurement is truly level.
Many manufacturers use the length of the top tube to determine the size of the road bike. This is not the case for mountain bikes. The frame size is usually marked with S, M, L, etc. This is also the system used by some road bikes: Merida’s road bikes pass through S, S/M, M/L, and L.
Of course, this size depends on the interpretation of each brand-Ridley’s S size frame has an upper tube of about 54cm, which is equivalent to many brands of medium frames.
It is worth noting that not all brands measure the length of the virtual top tube in the same way.
For example, Colnago records the horizontal distance from the head tube to the vertical distance projected upward from the top of the seat tube, so it does not take into account the further rear projection of the seat post, and its number will be smaller than other manufacturers. A Colnago from the 1950s is equivalent to a 54 cm upper tube.
How to measure seat tube length
The seat tube length is the linear distance between the center of the bottom bracket and the top of the seat tube.
Again, this is trickier than it sounds: some bicycles like Trek Madone have a seat tube that has a considerable extension above the upper tube joint, while other bicycles use a seat mast, so it is difficult to compare the size of alternatives.
In addition, especially mountain bike seat tubes often have kinks, so you don’t want to follow the line of the tube itself, which will be longer.
Align your straight edge with the center of the bottom bracket and the top of the seat tube, and measure along this, if you are not sure if you are following the correct line.
How to measure range and stacking
So we have seen that if you want to compare frames, the length of the top tube and seat tube is a bit like a minefield. To be more consistent, most manufacturers will now display the coverage and stacking values of their bikes.
Their advantage is that they are independent of the frame design and measure the vertical distance between two key contact points: the bottom bracket and the top of the head tube.
We posted a more detailed explanation of why coverage and stacking are important here.
In short, the range is the horizontal distance between the two. To measure it, you will need your spirit level again.
Connect the plumb line to the end of the spirit level. If you are using Blue Tack, make sure that the spots on the end of the strings are quite symmetrical and your strings hang straight down, otherwise your measurement may deviate.
Align the top edge of the level with the center line of the top of the head tube. Then move the level back and forth until the plumb line intersects the center of the main axis of the bottom bracket. Now just measure the distance between the top of the plumb line and the head tube, and you can achieve it.
Another option is to push your bike to the wall, measure the distance to the top of the head tube and the distance to the bottom bracket, and then subtract one from the other. However, you still need to make sure that your measurements are level.
The stack is the vertical distance between the bottom bracket and the top of the head tube. Therefore, once you are ready to record your coverage, you should also be able to measure your stack size along the plumb line.
Another method is to measure the vertical distance from the ground to the top of the head tube, then measure the height of the bottom bracket from the ground and subtract it.
Both reach and stack are very difficult to capture; if you use the plumb line method, you may need a second pair of hands, and it is worth repeating to ensure your consistency.
How to measure the wheelbase
The wheelbase of the frame is the distance between the front and rear axles. It is a key determinant of the ride quality of the frame, and it will also vary with the size of the frame.
The measurement is quite easy, but you need to set the fork directly, otherwise your measurement will be incorrect.
As with range and stacking, it is worth repeating the measurement multiple times to ensure that the same number is obtained. If you measure the wheelbase on both sides of the bicycle and take the average, the accuracy will also improve, because if the fork is not too straight, this will be compensated.
How to measure the chainstay length
The chainstay length is one of the two components that make up the wheelbase, and it also has an important impact on the handling characteristics of the frame. A frame with a shorter chainstay will generally feel more lively than a frame with a longer chainstay.
The chainstay length is the straight-line distance between the center of the bottom bracket and the center of the chainstay, so it is easy to measure with a ruler.
How to measure the front center
Another component of the wheelbase is the front center. This is equivalent to the chainstay length, but measured from the axle to the fork.
Likewise, it affects the handling and the overlap of the toes with the front wheels. Bicycle manufacturers don’t often refer to it, but, for example, BMC shows it on its geometric diagrams.
Please note that the wheelbase is not the sum of chainstay length and front center, as they are not measured horizontally.
How to measure the angle of the seat tube and head tube
The angle of the seat tube and the head tube are the two most important factors that determine the maneuverability. The more upright the tube angle usually brings the more flexible maneuverability. Your inclinometer app will come in handy here.
If your seat tube is straight, you can measure the seat tube angle by arranging your smartphone and reading the numbers from the inclinometer app. Make sure your bike is vertical and standing on a level surface to get accurate readings.
If your seat tube is kinked, you need to use the straight edge to move along the line between the bottom bracket shell and the top of the seat tube, and then align the phone with this.
Most newer bicycles have a tapered head tube, so the angle of the front of the head tube is different from the angle of its centerline.
You can approach the latter by keeping the phone at the angle of the centerline, or use straight edges to align with the center of the top and bottom of the head tube.
If you have straight pronged legs and there is no angle at their crown, the angle of the legs will be the same as the angle of the head tube, so you can measure it instead. Likewise, it is important to let the bicycle stand upright.
You can also measure the head tube angle by aligning the inclinometer with the extension of the steering gear above the head tube.
How to measure the bottom support drop
The bottom bracket drop is the difference between the height of the axle and the centerline of the crankshaft.
You can measure it by finding the height of the rear axle and the height of the bottom bracket, and then subtracting one from the other. This is another key indicator cited by bicycle brands on their geographic charts.
How to measure the height of the bottom bracket
Finally, the bottom bracket height is the distance from the ground to the center of the bottom bracket shell. So it is easy to measure, but be careful to keep the bike upright to get an accurate reading.
Unlike the bottom bracket lowering, it is also (slightly) affected by the tires, so inflate them to your usual driving pressure.
So now you have all the measurements needed to adjust the frame size. However, please keep your number in a safe place: you do not want to repeat the process.
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