If you’re just getting into MTBs, the last thing you need is to have to make really important decisions based on limited information right up front. But that’s exactly what you have to do when you buy a bike. You don’t just have to get the right bike for your budget, you have to get the right size too. Get one too small and you’ll be all scrunched up and your knees’ll explode from over-articulating. Get one too big and it’ll feel like a supertanker and you’ll probably end up bruising your tender bits on the top tube.

Unfortunately, things are made more complicated by the sheer variety of bikes available. There are two main difficulties. First, there’s no standard way of measuring bikes. Some makers measure from the centre of the bottom bracket to the top of the seat tube, some from bottom bracket to the center of the top tube and some don’t measure at all and just call them Small, Medium or Large. So already you’ve got the possibility of one “18in” bike being taller than another one.

It doesn’t end there. Some manufacturers make long bikes, some make short ones. Even if two bikes are the same size (bearing in mind that the same size might have different names…) one could easily be an inch longer than the other.

Confused yet? We’ll try and guide you through the minefield… Bear in mind that this is for all-round bikes – “specialist” bikes for DH, trials or what have you have their own idiosyncracies.

Standover

The “traditional” way of sizing an MTB is to stand astride it, shoes on, feet flat on the floor and check that you’ve got at least three inches of clearance between the top tube and your bits. This worked Okish ten years ago, but these days you’ll encounter high bottom brackets on full suspension bikes, extravagantly sloped or curved top tubes and all sorts of other weird anomalies that make it less than satisfactory. It’s still obviously worth checking that you’re not going to clothesline yourself on the frame if you come off forwards, but standover isn’t the primary consideration.

Reach

Reach, however, is. Reach is the distance from the seat to the bars, or the length of the “cockpit”. This is an easier one to come up with a useful rule of thumb for, and here it is. Sit on the seat (with the seat at the correct height) and put your hands on the bars with your arms straight. Your arms should be at about 90 degrees to your body, with arms and body at about 45 degrees from vertical (or horizontal…). You’ll need someone else (or a mirror) to check this.

If the bike feels a bit long or short, you can move the seat fore and aft a little to compensate. Don’t go mad, though. If you end up with the seat right back, the bike’s probably too short and vice versa. You can swap handlebar stems for slightly longer or shorter ones too, but again, if you need to go more than 10mm or so either way try the next size bike – big changes in stem length have correspondingly big changes in handling.

Seat height

We mentioned you’d need the seat at the right height to check the reach, but what’s the right height? There’re all sorts of equations and measurements and ratios of seat height to leg length knocking around, but the simplest way is to get on the bike, turn the pedals so one pedal is pointing straight down and put your heel on it, wearing the shoes you’ll be riding in. Your leg should be completely straight at this point, which equates to slightly bent with the pedal under the ball of your foot.

If the seatpost won’t extend far enough to get the seat to the right height (they have a minimum insertion/maximum height line on them that wants to be inside the frame), the bike’s too small. Conversely, if you find the seat’s right down near to the frame, it’s too big.

Bear in mind that some bikes have higher bottom brackets than others – you might not be able to comfortably reach the ground when in the saddle. If this unnerves you, just drop the seat half an inch from the ideal position until you’ve built confidence.

Bar height

The final variable in fit is bar height, with the key thing being how far below the saddle they are. XC race bikes will have very low bars, recreationally-oriented bikes higher ones. On most all-rounders you’ll probably find the bars fall an inch or two below the seat. The quill stems of old allowed you to move the bars up and down to find a comfy spot, but most bikes now have threadless headsets and clamp-on stems. Some manufacturers thoughtfully leave spacers below the stem, allowing you to lower it a bit. Otherwise you’re looking at swapping stems or bars – bars can have anything from zero to a couple of inches of rise, and can make a big difference.

Of course, if you’re a first-time bike buyer, you’ll probably find that it all feels a bit weird anyway. It shouldn’t feel actively uncomfortable, though. It’s a good idea not to fiddle too much at first. If the bike falls within the basic parameters here, then it fits. Ride it as it is, get used to it and then start to think about making small changes.

We’ll look more at customising the fit of your bike and adapting it for different purposes in future episodes…