Did I choose the wrong size bike? I don't know much about bicycles, and I have relatively little experience with them.  I bike for fun, or to get around the city, not for sport.

Recently I bought a BTWIN Rockrider 340 mountain bike, in size M. My height is 168 cm.



After going out for a ~35 km ride, I found it somewhat uncomfortable and I got a back ache.  The handlebar seems too low.  It's much more comfortable and relaxing to just touch it with my fingertips instead of holding it properly, as I don't have to bend down that much.

How can I tell whether I got the wrong size bike for my height or whether I simply did not use the correct posture?

In the country where I am currently, exchanging the bike after such a ride is probably not possible, or very difficult, so it is really a last resort to try.  While I did try the bike before buying, the store was in the city, on a high floor of a multi-story shopping centre, so there was very limited space and I didn't realize the problem. While people on this site might not think much of this bike, for me it was a fairly pricey one, mostly having had second hand bikes before. Therefore I would really like to make sure that I did not get the wrong size, or otherwise correct the problem somehow.

I got this bike for getting out of the city and going to places inaccessible by public transport, mostly rocky dirt roads.
People tend to think of fit as something for which there is a single solution, i.e., a particular combination of frame size, seatpost height, stem length, and so forth, all of which will create the optimal size bike for them. In reality, it isn't that straightforward; proper fit can be achieved within a range of values. For example, I have a few different bikes which I ride. One is a road bike with a 58 cm frame (flat top tube), one is a more modern road bike with 53 cm frame (sloping top tube), and so forth. How is it that I can ride a collective 16,000 km on these different-sized bikes and be comfortable? Because I have adjusted the combination of frame size, seatpost height, and stem length within a range that works for me.

You can achieve the same; it's not difficult.



You may wish to consider this fit calculator to help you figure out what the ballpark range of values is for your body. You'll need someone to help you, but the numbers it gives you will be a decent starting point as you fine tune the fit of your bike. You can use them to do an informed adjustment (if necessary) of your seatpost height/length and likewise your stem.

In addition, you can find many videos on YouTube and elsewhere which can help to inform your process:

Correct saddle height for a mountain bike
Mountain bike fit
Etc.

Assuming that the people at the bike shop sold you the proper size frame, you may need to increase your fitness a bit, and then be sure the reach is appropriate, and the seatpost height. In other words, you may need to ride the bike a bit to get a better idea of what the real root of the problem is. 35 km may not seem like a long way, but for a lot of people it takes them some time to get comfortable going that kind of distance: time to strengthen their core, their legs, and everything else that can ache at first.

After some saddle time, I think you will have a much clearer idea of what needs to be adjusted.