Size Guide

Getting the right size for your bike can be tricky, especially when buying online. However, if you follow our simple tips you should be able to find the right Bird for you quickly and easily.

How to size a bike

There are 3 key size elements to consider on your bike.

  1. Effective Top Tube length – how will the bike feel when sat down?
  2. Reach – how will the bike feel when stood up?
  3. Minimum viable dropper seatpost height – if buying a dropper how much drop can you get away with.

Where to start?

Its helpful if you can to measure up your current bike to get a good starting point, or find the geometry numbers online for your old bike, and use those. However, you can probably only find 1 of these numbers at best, the Effective (sometimes called horizontal) top tube length, or ETT, and even then thats not a complete picture, so a tape measure is often the best way.

If you don’t have an old bike for comparison, that’s no problems, just check out our sizing guide and go with that. While everyone is different, on average the size guide will put you where you need to be.

Take some measurements

Firstly, if you have an old bike you are replacing, take some measurements. You will need:

  1. A bike to measure
  2. A tape measure
  3. A pad & pen/phone/computer to write down your numbers

Measuring your Effective Top Tube Length (ETT)

The technical definition of and ETT is the distance from the centre of the top of your steerer tube to the centre of your seat post, however we will measure from the centre of the stem cap instead. It gives almost the same size and is just easier to measure.

Take your tape measure and measure the distance from the centre of your stem cap to the middle of your seatpost, ensuring the tape measure is horizontal when you do it. It can be easier to lean the bike against a wall on a flat surface to allow you to use the bricks of the wall as a guide to show whether the tape is horizontal. This measurement is your ETT, make a note of it.

Once we know your ETT, we’ll correct it for your stem length and bar width. The size we really want to know is not the ETT, thats actually a little useless, but the corrected horizontal difference from grips to seatpost. This defines more accurately the fit. To do this we need to do some simple calculations.

  • Measure your stem length. Stem length is the distance from the centre bolt of your top cap to the centre of the bars. Stems are usually measure in 10mm increments, and many have it written on them. You should expect stem length to be between 10 and 100mm.

Now you know the 3 numbers you can work out your corrected ETT for your old bike. The formula is:

Corrected ETT = Your ETT measurement + your stem measurement.

For example it might be: 600mm + 70mm = 670mm.

Now use this corrected ETT figure to work out the Bird frame that suits you best. Our frames are designed around a 40mm stem, but don’t forget that you can choose anything from a 32mm to a 50mm stem on our bike builders.

Corrected ETT – Bird Stem length = Bird Target ETT

Example from above : 67mm0 – 40mm = 630mm, which is a ML size on our latest Aeris frames.

 

Dropper seatpost length

The maximum dropper seatpost length is calculated as follows:

Measure the centre of the BB to the rails of your saddle.  Subtract the length of the seat tube for the size of the frame you are considering, and then subtract 75mm from this measurement. The result is the maximum length of dropper seatpost that you can run.

Example:  640mm saddle rail to BB centre measurement.  If considering an ML Aeris, this size has a 440mm seat tube. 640-440-75 = 125mm maximum reverb length.

 

Size Reach ETT Min Recommended height (CM) Min recommended height, Ft Inches Max Recommended height (CM) Max recommended height, Ft Inches
Aeris 145 2017 S 433 580 149 4 11 165 5 5
Aeris 145 2017 M 458 605 161 5 4 174 5 9
Aeris 145 2017 ML 481 630 170 5 7 180 5 11
Aeris 145 2017 L 506 655 178 5 11 193 6 4
Aeris 145 2017 XL 528 680 188 6 3
Aeris 120 2017 S 422 580 149 4 11 165 5 5
Aeris 120 2017 M 447 605 161 5 4 174 5 9
Aeris 120 2017 ML 470 630 170 5 7 180 5 11
Aeris 120 2017 L 495 655 178 5 11 193 6 4
Aeris 120 2017 XL 517 680 188 6 3
Zero AM 2015 S 416 595 159 5 3 172 5 8
Zero AM 2015 M 436 615 167 5 6 177 5 10
Zero AM 2015 L 453 635 174 5 9 190 6 3
Zero AM 2015 XL 478 660 183 6 1 0 0
Zero TR 2015 XS 400 575 152 5 0 165 5 5
Zero TR 2015 S 425 600 160 5 3 173 5 9
Zero TR 2015 M 447 625 168 5 7 178 5 11
Zero TR 2015 L 472 650 178 5 11 193 6 4
Zero TR 2015 XL 495 675 185 6 1