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I am looking to buy a new road bike from Decathlon, for general usage and maybe some longer rides in the summer. I don't know what size should I get because I am about 165cm (5'5) tall and am in my upper teens, and this means that I will definitely grow. The internet says that the medium sized bikes are suited for the minimum of 5'6 (167/168cm). Should I get the bigger one, and if it is too big what should I do?

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    Consider - how tall are your fully-grown aunts/uncles ?
    – Criggie
    Jun 21, 2023 at 5:24

5 Answers 5

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What a "M size" means pretty much depends on the model of the bike (not even the brand, and some brands have also different sizing between men and women frames). So the best recommendation is to go the website and check on the product page.

Now most brands are having guides where you enter your height and sometimes the inseam, and that will recommend the proper size.

I checked for the Triban RC120, the "S" size is from 165cm to 173cm, "M" is from 173cm to 182cm. For the Rockrider 520, 165cm to 173cm matches a "M" size.

And more specific to Decathlon, given you can check the inventory of a store online, it's certainly worth checking to see if they have your sizes in the inventory and try the bike in the store before buying. You can also ask for an in-store delivery, which would ease the return in case it doesn't fit.

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My experience, when you are close to the boundaries between sizes as I am, you pretty much have to try the bike because there are a number of factors in addition to height that contribute to bike fit. By straight numbers, yeah it might look like you fit ok, but you just don't know for sure.

I would probably look at the bigger size, you can always do things like move the seat forward on the rails, install a short stem to make micro adjustments and conversely, get a bike that's a fraction on the small side, move the seat back, longer stem etc.

However, these types of things aren't going to be significant enough if you've got the wrong size.

You can't go wrong with a proper bike fit if there is someone in your area offering that service

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You're probably about 16-18 years old so you're still growing.

So go for the larger size if you're part-way between sizes. Or be prepared for this bike to only fit you for a few years, if that.

To some extent you can use a shorter stem and lower the seatpost to improve your fit for now.

If the larger bike feels really bad right now, then stop and reconsider. But a little minor inconvenience now will help in the long run.

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    It is not uncommon to already stop growing by this age. I did. I am also significantly lower than my father. It is quite unpredictable. Jun 21, 2023 at 8:27
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I'm not yet too sure about the answer for your age group, but for someone in the first few stages of moving to 700C road bikes and 27.5"/29" MTBs, I am sure that the right approach is to size up.

If you have any doubt at all, buy the next size up and the shortest stem you can find. Replace the stem with the shorter stem for the first phase of using the bike. (Ask the shop to do it when you're buying the bike; it will take them two minutes and if you're buying the stem and bike from them, they will generally not charge you labor.) That "phase" might be as brief as 6, 9, or at most 12 months. Replace the stem back to the longer one when needed. (It's easy to do it yourself, but to make sure the handlebar is held tight enough, you need a tool that's not cheap: a torque wrench.)

If you happen to buy the next size up and find that it is not yet suitable—because, for example, there is little or no top-bar clearance—then keep the bike in your garage and hop on it once a month. Continue to use your older bikes in the interim. Once you can ride the new bike, use it and quickly sell the bike it replaces. You will then get maximum use of each bike size, and you'll always have good bike fit.

Incidentally, the standard wisdom is that used items in good condition can never fetch as much as 50% of their original price on the second-hand market. Some standardized, branded items (IKEA furniture, say) will slightly break this calculus, but good-quality bikes in good condition are easily worth more than that on the used-bikes market.

The preference for one-size-up is doubly important if you're buying at the end of summer rather than at the end of winter (and you will stop biking over winter).

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Bike fit is difficult. There are some online calculators that give some idea.

One thing you never want to do is to use a calculator where you can't yourself specify "saddle-to-handlebar drop", i.e. how much lower the handlebar is than the saddle is. Some calculators may assume racing use, meaning the handlebar will be very low. Then when you move the handlebar up, two things happen:

  1. Handlebar becomes closer to bars
  2. Your riding position changes

Usually these effects don't cancel each other exactly out.

There is no ideal saddle-to-handlebar drop. Some more athletic riders prefer more of it, less athletic riders may prefer less of it. Generally most road bikes have too much saddle-to-handlebar drop these days except if you're a racing cyclist who wants lots of it.

Any calculator based purely on rider height is crap. Don't trust such calculators. Calculators need detailed information about your inseam and arms.

A calculator would ideally output:

  • Stem length (affects ideal reach)
  • Frame reach
  • Frame stack
  • Saddle height
  • Seat tube angle
  • Seatpost setback (affects ideal seat tube angle)
  • Needed standover clearance
  • Crank length

Fortunately, modern compact frames with sloping top tubes and long seatposts almost always have enough standover clearance so that's not a problem.

What most cyclists do is that they somehow determine which bike fit is good. They may try various sizes on a bike shop (works only if the bike you're buying is available from a bike shop which usually isn't true since wannabe-racers only buy wannabe-racing bikes and reasonable bikes have been displaced from everywhere except maybe some rare online store), or use experience from a previous frame. Maybe that previous frame is slightly off, so they may need to use a seatpost with a unusual setback and non-standard stem length or even a non-standard stem angle (so-called "riser stem") on the previous frame.

How bike fit works:

  1. Firstly, you have to set your standing position. This means handlebar is at a certain vertical height and horizontal distance from the bottom bracket. This is why "stack" and "reach" are by far the most important frame parameters, they determine exactly this standing position while not saying anything about the sitting position. You may compensate wrong "reach" by a different stem length to a certain limit where the stem becomes so unusual it affects steering geometry too much, and you may compensate wrong "stack" by a stem with an unusual angle since the spacers on modern threadless headset systems only allow maybe 15mm or 20mm of adjustment plus 20mm from flipping a 6 degree stem.
  2. Then you put the saddle to the position where it feels the most natural. This means you need a certain seat tube angle with a compatible seatpost setback. You also need to put the seatpost to the correct height. Fortunately saddles have fore-to-aft adjustment so as you lower or raise the saddle slightly, you may compensate by moving the saddle forwards and backwards

Usually (1) is by far the most difficult. Reasonable frames allow lots of adjustment for (2), as long as the seat tube angle is not very unusual, a frame when used with a seatpost of the setback for which it's intended allows lots of adjustments for the sitting position (2), but very little for the standing position (1).

Since you may still grow, I'd say buy a slightly too large bike that still has enough standover clearance. It will have a stem that's too long for your current use. Switch to a shorter stem and save the too long stem. Since modern bikes have too much saddle-to-handlebar drop, and since you may still grow, it means the handlebar is actually very likely to be at the correct level. When you grow, try the original stem in a configuration that puts the handlebar to the maximum possible level, if it's still too low then buy a 17 degree stem. You probably won't need a longer seatpost after you have grown but that can be purchased for not unreasonable money.

I originally bought my Surly Long Haul Trucker frame based on an online calculator without ever trying the complete bike. Then I built the complete bike, and the fit was just right.

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