From top tube length to bottom bracket height, geometry can reveal plenty about how a bike will ride and whether or not it will fit you. Here’s what you need to know…

1. Seat Angle

Geometry Seat Angle

This is the angle of the seat tube relative to the ground. Modern bikes favour steeper angles that put the saddle directly over the bottom bracket, making pedalling easier and more efficient. Putting your centre of gravity further forward also helps with climbing, making it harder for the bike to loop out. An angle in the low 70s is ideal. At mbr we measure the ‘actual’ seat angle.

2. Bottom Bracket Height

Geometry Bottom Bracket

The distance from the centre of the bottom bracket to the ground. A low BB makes a bike more stable by lowering your centre of gravity and bringing it closer to the contact patch of the tyre. Too low though and you might start clipping your pedals on things. So how low is low? Long-travel full-sussers need higher BBs to accommodate all that potential movement, but hardtails go as low as 300mm.

3. Chainstay

Geometry Chainstay

Or, more accurately, the rear centre. This is the horizontal measurement between the centre of the rear wheel and the centre of the BB. Short back ends aren’t necessarily a good thing because they make a bike loop out more easily on climbs and, contrary to popular belief, don’t help it to corner.

It’s a complicated issue, but together with the front centre, the chainstay length determines where you are on the bike (central, further back, further forward). There’s no right or wrong here, but greater length can help a bike to feel more stable descending, and also help keep the front end down when climbing. As a rough guide, 450mm is the norm on most 29ers, 435mm on 650b bikes.

4. Wheelbase

Geometry Wheelbase

Add the chainstay and front centre measurements together to get the wheelbase. All other things being equal, longer bikes are more stable at speed. Anything approaching 1,200mm in size large is a long bike.

5. Reach

Geometry Reach

This is a hard one to measure, because you need a plumbline to do it well. Reach is the horizontal distance between the top of the head tube centre and an imaginary vertical line that runs though the BB centre.

It’s useful for sizing because it eliminates the variation in seat tube angles and isn’t affected by wheel size. It’s not perfect though, because the length of the head tube and head angle can skew the measurement. Most manufacturers list reach, so you can compare one bike to another. On a size large, 435mm upwards is a decent size.

6. Top Tube

Geometry Top Tube

We measure the ‘effective’ top tube, which is from the top of head tube centre to the seatpost centre, measured horizontally. This is more informative than the actual distance along the top tube. Either way, though, top tube measurements are pretty unreliable for bike fitting because seat angles vary from bike to bike. The down tube or reach are much better measures.

7. Head Angle

Geometry Head Angle

The angle between the ground and the head tube (and therefore the fork) is important because slack angles, where the fork is raked out and closer to being parallel to the ground, slow down a bike’s steering response but make descending easier. Steeper angles make a bike feel better climbing and on flat terrain.

As a general rule, you can expect trail bikes to have 66-68° head angles, XC bikes too have higher angles, enduro bikes around 65° and downhill rigs in the low 60s. It’s worth noting, though, that 29ers tend to have steeper head angles than 650b bikes.

8. Front Centre

Geometry Front Centre

This is the distance from the centre of the front axle to the middle of the BB. Two very disparate bikes can arrive at the same figure — a short frame with a slack head angle could be identical to a longer frame with a steeper head angle — but the fit and handling would be very different. 770mm is long on a size large but it does depend on travel and wheel size.

9. Down Tube

Geometry Down Tube

The down tube measurement is an effective tool for working out a bike’s true size and whether it will fit you. If you’re buying a new bike, measure the old one from the centre of the BB to the bottom of the head tube, right in the middle. Just be careful if your new bike has a different wheel size to your current one, as the numbers won’t correlate.

  • Chopalot

    My new xc bike will have 75deg seat tube angle and 65deg head tube. It climbs like a Billy goat and is great on descents too. Not sure if low 70s seat tube angle is considered optimal for xc anymore.

  • dickens

    I ride a tallbike (31″ bb ht.) and it’s easier to balance than lower bb bikes. Wanted to share what I read that helped me to understand why. Imagine trying to balance a broom in your palm. Now imagine balancing a toothpick the same way. The longer broom handle offers a longer moment to make balance corrections. Two other notes:A. Really nice how your exchange of initially opposing viewpoints was handled by both of you with grace and respect for another, and B. My bike kicks ass! Be well!

  • Wes Rademaker

    Hi James Smurthwaite,

    Thanks for the recommendation That’s what I Thought ,
    What’s Your Opinion on the Monarch Plus 2016 with High-volume -chamber Is this a good option for a 112 kilo Rider or Would I go With an Vivid Air or X-fusion Vector Air or a Fox DX2 air 2016 models

  • James Smurthwaite

    We’d definitely recommend the XL then, our tester was 6ft2in and he said it fitted him perfectly

  • Wes Rademaker

    I’am 6,3 ft and inseam is 34 Inch Inseam My reach On XC from Center-Saddle to center-handlebars is 28.75 Inch

  • James Smurthwaite

    Hi Wes, how tall are you?

  • Wes Rademaker

    Hi There ,

    I ‘am Looking for a new Enduro bike and can’t figure out what size is best anyone?

    The bike that I have in mind is a YT Capra Comp1 AL Some say I need A Large others say go with the X-Large
    Sizes S M L XL
    A Oberrohr (horizontal) 556,9mm 582,0 mm 608,2 mm 626,3 mm B Reach 398 mm 420 mm 443 mm 458 mm
    C Stack 587,5 mm 596,6 mm 605,6 mm 614,7 mm
    D Sitzrohr 400 mm 450 mm 480 mm 510 mm
    E Kettenstrebe 430 mm 430 mm 430 mm 430 mm
    F Lenkwinkel 65 ° 65 ° 65 ° 65 °
    G Sitzwinkel (effektiv) 74,8 ° 74,8 ° 74,8 ° 74,8 °
    H BB Drop 4 mm 4 mm 4 mm 4 mm
    I Radstand 1149,4 mm 1175,6 mm 1202,8 mm 1220,0 mm
    J Steuerrohrlänge 100 mm 110 mm 120 mm 130 mm
    K Überstandshöhe 755,9 mm 753,7 mm 766,6 mm 768,5 mm

    I ride aggressive but like a long feel to bike but not to much and comfy as wel
    Also like to use it for racing and Dh runs
    My XC Bike is 20 Inch top tube 58 effective Seattube 58 effective
    I have a downhill-bike Beone Woodbumble in A Large
    Any Advice

  • Leevi

    You are correct. Sorry for my ignorance. I’ve studied surfers and obviously it’s a completely different principle again. As a lesson to forever recite; always think before you type. Thank you for being patient and averting your rage upon me.

  • dstamat

    It is not uncommon for myths and legends to take a life of their own and become so tightly woven in our belief system that it makes it almost impossible to get rid of. It is impossible for any system to become both stabler and nimbler. It is a contradiction if you understand the principle. A stable system cannot be (very) nimble and a nimble system cannot be (very) stable. Just because a car becomes stabler when the CoG is lowered it does not mean that the same would apply to a bicycle. A bicycle is an inverted pendulum (like a metronome). You may want to look this up.

  • Leevi

    Well no, it’s not the opposite. As was written in the article, and from my own knowledge, the lower the center of gravity the more balance you gain.
    “As the CoG gets lower your bicycle becomes *more* stable and at the same time nimbler.”

  • Eot Lemac

    I always measure the downtube. It gives me a good idea of how the bike will ride and whether or not it will fit me.

  • dstamat

    It is kind of a cycling myth that a lower bottom bracket makes for a stabler ride. It is quite the opposite. A bicycle follows the physics laws of an “inverted pendulum”. The higher the center of gravity the stabler and more sluggish it gets, i.e. it takes longer to lean from one side to the other. As the CoG gets lower your bicycle becomes less stable and at the same time nimbler.