What is the Rockrider 520 ST seatpost diameter and clamp size?
If you own an older or newer version or Rockrider 520, now called ST (Sport Trail), these are the sizes for the seatpost and seatpost clamp which are not specified on the Decathlon website:
Rockrider 520 ST seatpost diameter and clamp size?
For the seatpost, the size is 27.2 mm (millimeters) (you need a 27.2mm seatpost).
You can get a Truvativ Stylo 27.2 mm seatpost. I had one and had no issues with it.
It looks and performs much better than the stock Rockrider 520 seatpost.
For the seatpost clamp, the size is 28.6 mm (this was confirmed by the customer service from Decathlon UK) (you need a 28.6mm seatpost clamp).
I’ve asked them by email because this spec was unknown, not being specified on the decathlon website.
If you plan to upgrade your seatpost and get a non-quick-release clamp, now you know the correct size.
Do not go for the cheap seatpost clamps made in China as they might not be reliable and quite dangerous to use.
Rather, you should go for a good brand, a bit more expensive, but they will last you a long time.
Finally, Rockrider 520 is a very entry level bicycle, you should not invest a lot of money in it.
The stock seatpost and seatpost clamp do a great job for their purpose.
If you really want something better, an idea could be to sell your Rockrider 520, put more money and buy a new bicycle, used or new.