Mountain bike sizing: what size bike do I need?

Posted: April 2, 2014 by kirisyko in Bike, Mountain Biking
Tags: , ,

Your frame size is the starting point for getting your mountain bike perfectly set up

Choosing the correct size of bike is one of the most important decisions you’ll make. Don’t buy until you’ve read our advice so you can get the perfect mountain bike set-up for maximum comfort and a reduced chance of injury.

A bike that fits correctly is a joy to ride, while one that’s too small can cause handling problems and be uncomfortable on longer rides. Read on for some advice on what frame size to go for, especially if you’re in any doubt about it.

We all come in different shapes and sizes, so we recommend using the information below as a starting point and a guide. This diagram will help explain the anatomy of a mountain bike.

This diagram shows the different tubes used on an average mountain bike frame: this diagram shows the different tubes used on an average mountain bike frame

Frame sizes

Ask any experienced riders about bike fit and feel, and they’ll tell you that all bikes feel and ride differently, even if the numbers look almost the same on paper.

Manufacturers’ listed frame sizes can be confusing. The traditional method is to list the seat tube length, but even that varies because some are measured to the top of the seat tube and some to the middle of where the top tube joins the seat tube. Many manufacturers simply list their bikes as S, M and L, perhaps with XS or XL at either end.

The seat tube should leave you with an acceptable standover gap (see pictures below) and usable standover clearance. To get this, stand back as far as you can while over the bike and ensure that there’s a minimum of an inch of room from the top tube to your crotch area. If you adhere to this advice then your frame should provide you with a large range of adjustment at the seatpost, which is important for finding your optimum saddle height.

Another important consideration is the top tube length. Together with seat position, stem length and handlebar position, top tube length dictates the comfort and efficiency of your body on the bike. To confuse matters further, the aspect of top tube length that matters is not the top tube itself, which often slopes, but a horizontal line from the middle top of the head tube to the middle of the seatpost.

You can see the handlebar stem and head tube clearly with this inset shot: you can see the handlebar stem and head tube clearly with this inset shot

So, where do you find out what size frame you need? Like so many other things on a mountain bike, there is no one perfect solution, because within sensible limits you can adjust your saddle, stem and handlebar to help make a slightly imperfect fit feel fine. We’d always recommend looking at manufacturers’ own size charts but here are some general guidelines:

  • XS: Bike size 13-14in: generally for riders between 5ft and 5ft 4in
  • S: Bike size 14-16in: generally for riders between 5ft 4in and 5ft 7in
  • M: Bike size 16-18in: generally for riders between 5ft 7in and 5ft 10in
  • L: Bike size 18-20in: generally for riders between 5ft 10in and 6ft 1in
  • XL: Bike size 20-22in: generally for riders over 6ft 1in

(Bear in mind that road, cyclocross and hybrid bike sizes tend to be 3 to 4inbigger for riders of the same height – something that confuses a lot of riders when looking through bike listings.)

Frame size problems

Two things to watch out for: the length of the handlebar stem and the standover clearance. Too big a frame? Could be painful – as this picture demonstrates.

Too big a frame leads to problems like this:

Get the frame too big and you could suffer from:

  • A sore back from overreaching on long rides
  • A lack of standover clearance, leading to some particularly wince-inducing experiences
  • A lack of control of the bike

Too small a frame leads to problems like this:

Get the frame too small and you could suffer from:

  • Injury on longer rides from a too cramped position
  • Potential toe overlap problems (where your foot clips the front wheel)
  • Too much standover clearance, leading to back problems when sitting down on longer rides

Beyond frame size

Ultimately, what matters most is how the bike feels when you sit on it and ride. If possible, get out there and test a few different frame sizes (many shops will let you do this) before committing to buy. Try different bikes at different places.

And as well as frame size, you’ll need to make sure your bike fits at all the main contact points too: saddle, handlebars and pedals. Once you decide on your frame size and have bought your bike, have a read of our mountain bike positioning article to make sure you get the perfect mountain bike set-up.



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