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
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
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.
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.
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.
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
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
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
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
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.