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I want to buy a mountain bike for the summer to ride with my friends, go on shorter rides (max 6 hours) and use in town. I am approximately 175 cm tall and 16 years old (so I might grow a bit). My inseam is about 80 cm.

I went to my local dealer and he recommended a size M and told me that I could buy a size L as well, because it would suit me better over time (he was ~2 cm taller than me and he rides a size L). I am now thinking about buying a size L (19”) MTB. Should I go ahead with it, or are there any significant disadvantages?

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  • At 16 years old you will absolutely still be growing. Depending on your genetics, you may stop growing around 18, but will probably fill-out over a few more years after that.
    – Criggie
    Jun 12, 2021 at 14:10

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If you are new to mountain biking I would suggest the medium. Most novice riders I have loaned a bike to feel more comfortable on a slightly smaller frame than a slightly too large frame. When riding in terrain they have never experienced the smaller frame tends to be more nimble and not being overly concerned about your crotch hitting the top bar instills a little more confidence. I assume you are looking at an entry level bike. Two things are likely to grow in the next two years your body and your skill level. In a year or two your skill level will likely exceed the bike and its components. At that point upgrade to a bike that matches your size and your skill level.

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My advice would be to buy the medium, especially if the bikes you're looking at buying are 29ers. Your height and inseam place you the upper range for size medium, however you need to grow 2½-4cm to place you at the low range of the large size. In addition many 19" MTB 29ers have stand-over heights (distance from the ground to the top of the middle of the top tube--basically where you would be if you stopped the bike, got off the seat and stood on the ground straddling the bike) between 31 & 32 inches. Conventional wisdom of bike fitting says you should have 1-2 inches clearance between the top tube and your crotch when standing flat footed on the ground. Your inseam calculates out to 31 inches.

Now all that being said, the absolute key to this is how you feel--your comfort--when riding a particular bike. There is such a wide range of choices of bikes and the geometries of these vary quite a bit as well. Thus, a bike that is sized medium whose geometry is race oriented and more "slack" and with a top tube that is highly angled may feel too small for a person with your dimensions while a more city or trail oriented frame that sets you more upright and is also a size medium may perhaps feel just right. The point is that all these numbers and calculations and conventional wisdom of proper bike fit mean absolutely nothing if the bike doesn't feel good to ride. I suggest borrowing a friend's bike for a few hours to a day, if possible, and ride it and see how it feels. Note the frame size and the general class of mountain bike (XC, trail, all-mountain/enduro, and downhill) if the particular bike classifies itself. Ride and see how it feels. Then ride another bike that maybe differs in size and/or geometry to see how that feels. At this point it's possible to either eliminate a size or style or note a few aspects to further experiment with. Keep in mind, that one bike can have a wide range of set-up options from tire and rim choice to seat height and front-back difference, handle bar, suspension travel, etc which will offer a little variation of one bike's fit. So at this stage you're somewhat feeling out and creating a pool of possible candidates.

A third aspect of choosing a bike is determining where and how the bike is ridden. There are better choices for a bike that is to be ridden in mostly urban areas of pavement and groomed trails compared to a bike that will be ridden in unpaved mountainous terrain and bike parks featuring wild single track. You needn't be too focused on getting this absolutely right either since a general use mountain bike can be set up to handle a very wide range of terrain and conditions. And while a road bike with narrow tire pressured to a 100psi would be a damn poor choice, it will roll right down the mountain trail just as a slacked out full-suspension mountain bike will. Gravity works. On one, you're likely be miserable and feel out of place and on the other the ride will feel like you're one with the mountain. Choosing the right bike for your typical riding conditions and style is an aspect of a bike purchase but think of it as selecting something "more right" than needing it to be perfect.

Here's my concrete opinion with the given facts of the question: You should look to the pool of medium sized, hardtail, XC or trail bikes offered by a quality maker (Specialized, Trek, GT, Santa Cruz, Cannondale, Pivot, Canyon, and others). Basically, the brands of bike that are sold through true bike shops. Avoid department store bikes as the diminished quality and substantial weight of these types of bikes detract from the joy of riding, are sometimes poorly or incorrectly built up, and these types of stores lack knowledgeable support specific to bikes and bicycling. The bike you choose should be aluminum frame because you can get more for your money compared to a more expensive base of carbon fiber. Travel of the front fork suspension should be 120mm or more. Air forks are lighter and more tunable but coil forks have their place. It's most important to have the sag and rebound set-up correctly for your riding weight. Pricing of a new, good quality, aluminum, entry-level hardtail MTB can be $1000-1500 at the time of this writing. Less than this you begin to run into manufacturers using inferior components--especially brakes and suspension forks--to shave cost. If your budget doesn't permit such an expenditure, look to the used bike market where a five year old bike can bought for several hundred dollars less while maintaining (sometimes even exceeding) the quality of componentry of a new bike. The latest tweaks to modern geometry will be absent, but at the entry level it's questionable such changes have any meaningful effect. Certainly won't be any night and day difference to a beginner.

Here's a few good references via a Google search of "buying a first mountain bike."

Outside Online-Buying Your First Mountain Bike

DIY Mountain Bike

MTBer.com 5 Things You Need to Know

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    I disagree with the sizing recommendation. 175cm and growing on a medium is quite a tall rider, and I’d only recommend that for someone who explicitly likes riding smaller frames. The large, especially at this price point where geo isn’t very radical, will be just fine. Your stand over numbers seem quite wrong. On my current MTB, stand over is 694mm on a large, 80mm lower than your estimate. That won’t be a problem.
    – MaplePanda
    Jun 12, 2021 at 17:01
  • Because of my suggestion to get a hardtail, and according to some specs I looked at, typical stand-over heights of large hardtails are very near 800mm or 31.5" and some 820mm and above putting SOH >32 inches, I felt a medium size (SOH of hardtails very near 30") is most appropriate for entry level experience. It also may be more fun as skill and competence develop to have a greater ability to throw around the smaller frame. In addition, it's my belief that it's best and safest to fit a bike to a person's current size as opposed to an expectation of growth which is unknown.
    – Jeff
    Jun 12, 2021 at 18:39
  • @MaplePanda Full suspension bikes, which is what I assume you mostly ride based on what I recall of your writings, do have smaller stand-over heights. A large Stumpjumper 29er's SOH is 762mm or 30" so it would make more sense with a full squish to maybe go to the larger size when the average proportioned rider's size falls in the in between range. At 5'8" it puts this person at the upper range of medium but not quite to the beginning of the large range which begins @ 5'9"
    – Jeff
    Jun 12, 2021 at 18:53

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