Bike Frame: If you’re thinking of buying a new bike or a first bike, understanding the key measurements that define your frame is important to ensure that you get a machine that fits you and works for the riding you intend to do.
And if you’re planning to rent a bike, it’s useful to know your frame’s key measurements, so you can make sure that your rental will be comfortable to ride and can be adjusted to fit. Not all manufacturers measure frame sizes the same way, so you need to look at a few fundamental measurements to ensure that you’re comparing apples with apples.
Key numbers that define your bike
We’ll tell you below how to measure your bike’s:
- Top tube length
- Seat tube length
- Reach and stack – and why they’re important numbers
- Chainstay length
- Front centre
- Seat tube and head tube angles
- Bottom bracket drop
- Bottom bracket height
What you’ll need to measure a bike frame
- A tape measure
- A clinometer to measure angles (there are free smartphone apps you can download)
- A long spirit level (or you can use the clinometer app and a straight piece of wood)
- A plumb line (or you can improvise with string and a couple of blobs of Blu Tack)
Whether road or mountain, most bikes are now measured in metric units, but you may find some manufacturers that still size mountain bikes in inches. Some brands, such as Moots, even mix the two!
We highly recommend you stick to metric units to keep things consistent. If you really must, you can always divide centimeters by 2.54 to switch to inches.
You will usually find a geometry chart for all sizes of a current frameset on a maker’s site. If your bike is still an existing model, it’s worth taking a copy of this because it will be more accurate than your measurements and likely a handy reference down the line.
How to measure top tube length
Once, bikes all had horizontal top tubes. Now many bikes’ top tubes will have a slope. If you look at a bike geometry table, it will usually include the entire top-tube length. But for a consistent measure, regardless of the top tube angle, you need to measure the horizontal maximum tube length, called the effective full tube length or virtual top tube in many geo charts.
That’s the horizontal distance between the head tube’s centreline and the seat post’s centreline. Measuring it clearly will mean using your spirit level or the clinometer app to ensure that your measure is horizontal.
Many manufacturers size road bikes by top tube length. That’s not true of mountain bikes, where the frame size is usually marked S, M, L, etc. That’s a system used for some road bikes, too: Merida’s road bikes go through S, S/M, M/L, and L.
Of course, this measurement is up to each brand’s interpretation – Ridley’s size S frames have a top tube around 54cm, which is equivalent to many brands’ size medium frames.
It’s worth noting that not all brands measure virtual top tube length in the same way either.
Colnago, for example, records the horizontal distance from the head tube to a vertical projected up from the top of the seat tube, so it doesn’t take account of the further backward projection of the Seatpost, and its numbers will come out smaller than other makers’. A 50s Colnago is equivalent to a 54cm top tube.
How to measure seat tube length
Seat tube length is the straight line distance between the center of the bottom bracket and the top of the seat tube.
Again, it’s trickier than it sounds: some bikes like the Trek Madone have a considerable extension of the seat tube above the top tube junction. In contrast, others use a seatmast, so comparing with an alternative’s dimensions isn’t easy.
Plus, mountain bikes in particular often have a kink in the seat tube, so you don’t want to follow the line of the pipe itself, which will be longer. Line up your straight edge with the bottom bracket center and the top of the seat tube and measure along with this if you’re not sure you’re following the right line.
How to measure reach and stack
So we’ve seen that top tube and seat tube lengths are a bit of a minefield if you want to compare frames. For more consistency, most manufacturers will now show reach and stack values for their bikes.
These have the advantage of being independent of frame design and measure the perpendicular distances between two key contact points: 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.
In brief, the reach is the horizontal distance between the two. To measure it, you’ll need your spirit level again.
Attach a plumb line to the end of the level. If you’re using Blue Tack, make sure that the blob at the end of the string is fairly symmetric and your string hangs down straight; otherwise, your measurements may be off.
How to measure wheelbase
Your frame’s wheelbase is the distance between the front and rear axles. It’s a key determinant of a frame’s ride quality and will vary with frame size too.
It’s fairly easy to measure, although you need to set the fork straight ahead, or your measurement will be incorrect.
Like reach and stack, it’s worth repeating the measurement several times to make sure you get the same number. Accuracy will also be increased if you measure the wheelbase on both sides of the bike and take the average because this will compensate if the fork is not quite straight.
How to measure chainstay length
Chainstay length is one of the two components that make up your wheelbase and, again, contributes significantly to your frame’s handling characteristics. A-frame with shorter chainstays will typically feel more lively than one where the stays are longer.
The chainstay length is the straight line distance between the center of the bottom bracket axle and the center of the rear dropout, so it’s fairly easy to measure with a ruler.
How to measure seat tube and head tube angles
The seat tube and head tube angles are two of the most important factors to handle, with more upright tube angles typically leading to more agile handling. Your clinometer app will come in useful here.
If you’ve got a straight seat tube, you can measure the seat tube angle by lining up your smartphone and reading the number of the
clinometer app. Make sure your bike is vertical and standing on a horizontal surface for an accurate reading.
If there’s a kink in your seat tube, you’ll need to use a straight edge to follow the line between the bottom bracket shell and the top of the seat tube, then line up the phone with this.
Most newer bikes will have tapered head tubes, so the angle of the front of the head tube will not be the same as the angle of its centreline.
You can get close to the latter by holding your phone at the angle of the centreline or by using a straight edge to line up with the centers of the top and bottom of the head tube.
If you have straight fork legs without an angle at their crown, the angle of the legs will be the same as the head tube angle so that you can measure this instead. Again, it’s important to have the bike standing vertically.
You can also measure head tube angle by lining up the clinometer with the steerer extension above the head tube.
How to measure bottom bracket drop
The bottom bracket drop is the difference between the height of the wheel axles and the centreline of the crank axle.
You can measure it by finding the height of the rear axle and the height of the bottom bracket, then subtracting one from the other. It’s another key measurement quoted by bike brands on their geo charts.
How to measure bottom bracket height
Finally, the bottom bracket height is the distance from the ground to the centre of the bottom bracket shell. So that’s quite easy to measure, although be careful to keep your bike straight upright for an accurate reading.
Unlike bottom bracket drop, it will be affected (slightly) by your tyres too, so inflate them to your usual running pressure.
So now you’ve got all the measurements you need to size up your frame. Keep your numbers somewhere safe though: you don’t want to have to repeat the process.