Each rider can decide their own best saddle height. Then remember it to save time setting up new bikes in future. A good starting point for inexperienced riders is that when sitting on the seat with hips level and a pedal rotated to be as far from the seat as possible that knee should be straight but not locked straight.
The seat-pin – also known as the seat-post – telescopes into the top of the seat-tube. The top of the seat-tube is clamped around the seat-pin by a threaded bolt or a quick-release mechanism. The clamp may be part of the frame or a detachable ring that slides onto the seat-tube. Loosen the clamp to start adjusting the saddle height.
When you think you have found the right saddle height, lock it in place.
Make small adjustments and give the rider some time – a few days or a couple of weeks – to get used to the new position before going back or making further adjustments.
The seat-pin needs a minimum amount of overlap with the seat-tube. This is usually marked on the seat-pin with a set of vertical lines and the words ‘minimum insertion’. 2.5 times the diameter of the seat-post is a rule-of-thumb if the mark is missing.
If you can’t get a comfortable position without exposing the safety limit replace the seat-pin with a longer one. There’s a wide variety of seat-pin sizes, their diameters vary in increments of 0.2 mm. Getting the right size is critical so the best practice is to take the one you’re replacing to the bike shop.
An aluminium seat-pin can lock itself into a steel frame by a chemical process known as ‘bi-metallic corrosion’, ‘cold setting’. To avoid this remove the seat-pin once a year, wipe both surfaces then apply a coat of anti-seize grease to the inside of the seat-tube. If the bike is run without mudguards in wet and salty conditions – the salt can come up from the road, or from the rider’s sweat – the surfaces need to be checked and greased more frequently, maybe as often as once every three months.