How To Measure Bike Frame? – Bike Knowledge
Russell Franklin - September 21, 2022To make sure you get a bike that fits you and is appropriate for the type of riding you intend to do, it’s crucial to understand the key measurements that define your frame if you’re considering purchasing a new or first bike.
Furthermore, knowing your frame’s critical dimensions is helpful if you intend to rent a bike. This way, you can be sure that the rental bike will be comfortable to ride and that it can be adjusted to fit you.
To make sure that you’re comparing like with like, you should take a look at a few standard measurements since not all manufacturers measure frame sizes in the same way.
What You’ll Need To Measure A Bike Frame
- A tape measure
- A clinometer to measure angles
- A long spirit level
- A plumb line
How To Measure Top Tube Length
The top tubes on bicycles used to be horizontal. Now, the top tubes of many bicycles will slope.
You can usually find the actual top-tube length in a bike geometry table. The horizontal top tube length, also known as the effective top tube length or virtual top tube in many geo charts, must be measured for a reliable measurement, regardless of the top tube angle.
That is the horizontal separation between the Seatpost centerline and the head tube centerline. To ensure that your measurement is truly horizontal, use your spirit level or the clinometer app when taking it.
Road bikes are frequently sized by top tube length. That is not the case with mountain bikes, where the frame size is typically marked S, M, L, etc. Some road bikes also utilize that system: The road bikes from Merida pass through S, S/M, M/L, and L.
The interpretation of this measurement will vary depending on the brand; however, Ridley’s size S frames have top tubes that are roughly 54 cm in length, which is comparable to the size medium frames of many other brands.
Notably, not all manufacturers gauge virtual top tube length in the same way.
Colnago, for instance, measures the horizontal distance from the head tube to a vertical projected up from the top of the seat tube rather than accounting for the seatpost’s further backward projection, so its measurements are smaller than those of other manufacturers. 54cm top tubes are comparable to 1950s Colnagos.
How To Measure Wheelbase
The distance between your frame’s front and back axles is known as its wheelbase. It greatly varies with frame size and is a significant factor in determining how well a frame rides.
While setting the fork straight ahead is required to ensure an accurate measurement, it is fairly simple to do.
Similar to reach and stack, it is important to perform the measurement several times to ensure that you get the same result. The accuracy will also be improved if you measure the wheelbase of the bike from both sides and average the results because this will account for any fork slant.
How To Measure Reach And Stack
As a result, it is clear that comparing frame lengths for the top tube and seat tube can be challenging. Most manufacturers now display reach and stack values for their bikes in order to increase consistency.
These are advantageous in that they measure the separations between two important contact points perpendicular to the frame’s design: the bottom bracket and the top of the head tube.
We’ve published a more detailed explanation of why reach and stack are important here.
The reach, in a nutshell, is the separation between the two horizontally. Again, you’ll need your spirit level to measure it.
At the level’s end, fasten a plumb line. If you’re using Blue Tack, check that the blob at the end of the string is fairly symmetrical and that the string hangs straight down; otherwise, your measurement might be off.
Align the top edge of the level’s top with the head tube’s top centerline. After that, move the level back and forth until the bottom bracket spindle’s center is intersected by the plumb line. Simply measure the distance between the head tube and the top of the plumb line to determine your reach.
Another option is to lean your bike up against a wall, measure the distances from the bottom bracket to the top of the head tube, and then subtract one from the other. However, you must still ensure that your measurement is horizontal.
The stack is the vertical distance between the top of the head tube and the bottom bracket of the bicycle. Therefore, once you’re ready to take a reach measurement, you should also be able to measure your stack by using the plumb line as a guide.
A different approach is to measure the height of your bottom bracket from the ground and subtract it from the measurement of the vertical distance from the ground to the top of your head tube.
The capture of both reach and stack requires quite a bit of finesse; if you use the plumb line method, you’ll probably need a second set of hands, and it’s worth practicing to make sure you get it right.
How To Measure Seat Tube Length
The length of the seat tube is the measurement in a straight line from the bottom bracket’s center to its top.
Again, it’s more difficult than it seems: some bikes like the It’s challenging to compare the dimensions of a Trek Madone to those of an alternative because it has a significant extension of the seat tube above the top tube junction while others use a seat mast.
Additionally, the seat tube on mountain bikes in particular frequently has a kink, so you don’t want to follow that line because it will be longer.
If you’re unsure whether you’re measuring along the correct line, align your straight edge with the center of the bottom bracket and the top of the seat tube.
How To Measure Chainstay Length
Again, the handling characteristics of your frame are greatly influenced by the chainstay length, one of the two elements that make up your wheelbase. Generally speaking, a frame with shorter chain stays will feel livelier than one with longer stays.
It is relatively simple to measure with a ruler because the chainstay length is the straight line distance between the center of the bottom bracket axle and the center of the rear dropout.
How To Measure Front Center
The front center is the second part of the wheelbase. This is the equivalent of the chainstay length, but it is calculated from the axle to the front dropout.
In addition to toe overlap with the front wheel, it again has an impact on handling. However, BMC, for instance, displays it on its geometry charts. Bike manufacturers don’t frequently quote this information.
Because neither of these measurements is taken horizontally, the chainstay length and front center are not added to determine the wheelbase.
Finding The Right Size For Your Bike
Kid And Junior Bikes
A child’s bike falls under a different category because it has wheels that range in size from 12 to 26 inches. For every junior bicycle, the uppermost tube must have a two-inch clearance.
On the other hand, smaller bikes with wheels up to 20 inches in diameter would naturally have a narrow stand-over height.
Touring Bikes
equivalent to commuter and road bikes. The geometry of the frame is where it all begins when it comes to creating a comfortable bike for you. Touring bikes frequently have longer stays (longer wheelbases) because the rider needs to be able to carry panniers on the back rack without clipping them every time the rider pedals. Though the long wheelbase may not be a problem if you have longer legs and smaller feet like I do. In actuality, some travelers ride road bikes with even shorter wheelbases.
Cyclocross Bikes
The rules for road bikes will also apply to cyclocross. Because cyclocross is a sprint sport, you will want to take a slightly less aggressive stance, but not significantly. You can choose from a wide range of options; just do a little research, get a sense of what you think you want, and test out a few of your friends’ bikes.
Cruisers, Hybrids, Or Cross Bikes
These are all essentially the same type of bike with different names. Imagine a bike that is designed for commuting that has flat handlebars, wider tires, and disc brakes. The majority will follow the small, medium, and large conventions, but you might come across some outliers. These bikes are frequently inexpensive and strangely enough capable of fitting people more comfortably outside of the typical ranges. I regularly ride large and extra large horses while working with them, and I never have any major issues.
You can proceed on your own now that I’ve covered everything you need to know about how to measure a bike frame. The Amazon Basics Self-Locking Tape Measure is a good tape measure for riders to use to simplify their work.
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