Deciding which mountain bike size you should choose is no longer as straightforward as it used to be. Here are some helpful guidelines to get you on the right sized bicycle.
We used to size mountain bikes by seat tube length, measured from the bottom bracket to the top of the seat tube where the seatpost is inserted. These lengths and general bike proportions were largely based on the equivalent sized road bicycles, which of course have a very different purpose and aren’t needing to be as agile off road. Fast forward a couple decades and we have realised that having a longer reach (bottom bracket to headset length), lower top tubes, and shorter seat tubes (paired with dropper seat posts) makes a mountain bike much better suited to handling technical terrain!
By the general shortening of mountain bike seat tubes across the board any particular person is now likely able to fit on 2 or more sizes of a given mountain bike model quite comfortably and choosing the right size has thus become a bit more complex. In this article we are going to give you some guidelines and information that will help you make a better decision when it comes to which size mountain bike is best for you and your needs.
Being In Between Mountain Bike Sizes
I often find myself (at 178cm) between sizes when I look at the sizing charts of most modern mountain bike models. I’ll typically be at the top of what is recommended for a size medium and the bottom of what is recommended for a size large. This has certainly been a frustration for me but it’s just something I have had to adapt to. I now tend to look at the size charts as guidelines and try not to get myself worked up about not having my “Goldilocks” size. Sigh..
There are of course brands such as Atherton Bikes or even Trek that are offering more size options in their popular models to fill those ‘gaps’ between sizes. Atherton Bikes are quite the anomaly actually, being able to do an outrageous 22 standard sizes per model due to their unique frame construction! These kinds of changes to bike lineups can certainly help those who just want to look at the size guide and purchase the model that it prescribes for them but it doesn’t necessarily answer the question of which mountain bike size is right for you. After all, there are perks to upsizing or downsizing.
The Advantage Of A Larger Mountain Bike Size
Upsizing on your bike frame is becoming increasingly popular for the average rider, particularly as seat tubes have shortened to allow this. This is generally because the most notable characteristic that you will get from a large frame is stability and thus a feeling of security in technical terrain.
Having a longer platform between your hands and feet means that your inputs to the bike don’t need to be as precise and the effect of the terrain on the platform will also be less aggressive. Think of resisting being pushed over while in a normal push up position versus whilst having your hands on the ground just a ruler’s length in front of your feet. Maybe not a perfect example but you get the point.
Humans seem to rather enjoy going fast on bikes and the kind of stability you get from a larger bike makes straight line speed through rough terrain notably easier to achieve. Sounds pretty sweet right? You might now be thinking, what then are the disadvantages of upsizing? Well… you do sacrifice some agility in most cases. A bigger bike will more often require bigger inputs to move around on the trail. If you would describe your riding style as “playful” and “lively”, upsizing may not be your best bet. If you tend to enjoy a smoother, more deliberate, and more direct ride, then having a large bike will compliment those characteristics. Larger bike frames do also tend to be more comfortable for extended periods of seated riding.
The Advantage Of A Smaller Mountain Bike Size
Why might one downsize then? As we have just noted that large bikes are seldom the most agile, it goes to say that a smaller bike offers increased agility on the trail. Like a fighter jet being inherently unstable in the air (warning: rabbit hole), and being able to make rapid directional changes and manoeuvres as a result, so it is with bicycles.
Think about slopestyle and BMX bikes, their agility is owed largely to their small dimensions. It is for this reason that we are seeing a trend with pro enduro racers opting for bike sizes smaller than what their respective brand’s size chart would recommend them. Rider inputs have a greater effect on the bike meaning less effort is required to make the bike do what is desired (although that works in converse with precision). The shorter wheelbases make for tighter turning circles.
Now we aren’t all even remotely near as skilled as Jack Moir (who, at +190cm tall, infamously began the 2022 Enduro World Series on a size small Canyon Strive). When it gets towards the extremes, there is going to be a lot demanded from the rider either in terms of precision (on a small bike) or effort (on a larger bike).
After starting out on a Large Slakline from Pyga bikes a couple years ago, I opted for a change to a medium frame that offered me increased agility from the bike and I found that suited my riding style better (I thoroughly enjoy cornering and nibbly technical moves). When I was tired, the medium bike was easier to throw into a corner or hop over a feature but it certainly wasn’t going to look after me as much as the bigger bike. That is the trade-off you get though.
I’ve been riding a medium Giant Trance X for a few months now (also a bit smaller than what their size chart would necessarily prescribe for me) and likewise have been pretty stoked with the decision to go with the more agile package. You ultimately need to decide for yourself if the more stable platform of a bigger bike or the increased agility of a smaller bike is going to make you enjoy the riding experience more!
The Sweet Spot
What about the sweet spot? You get online and take a look at the size chart of the brand new (or second hand) bike that you’ve had your eye on for a while and what do you know, you’re smack bang in the middle of (let’s say) the medium! Easy choice then? Well it could be!
In that situation you’re likely going to end up with a well rounded and balanced ride that doesn’t have you teetering on a knife’s edge but neither requires massive inputs to get around the bends. That may sound like your sweet spot, but it also may not. You may still decide that extra stability is what you would prefer, or conversely the smaller bike is the option that excites you. It is up to you, the rider, to pick the bike for you. But before you go, I have one last nugget of wisdom.
Always Try Before You Buy A Mountain Bike
That’s right. Sizing charts vary from model to model and brand to brand, and even year to year! Before you purchase your next mountain bike, be it new or second hand, make sure you get to swing a leg over it first to get a good feeling for the proportions before you open the wallet.
When you ride the bike does it feel a bit cramped, does it feel too cumbersome? Ride it while seated but also do some turns and other normal riding inputs while standing up out of the saddle (as you would on a descent). Try it against another size of that model if you can to feel the differences and then decide what will suit your proportions and riding style the best! Trust me, it is well worth the time and effort.
Frame sizes considered, the suspension travel questions begs to be answered next… Ever wondered what the ideal trail bike might be? Well you can read about that here!
