Mountain bikers go through kit faster than most riders. Mud, grit and the odd crash see to that, which makes them easier to buy for than you’d think.
Cleaning is never wasted. A UK mountain bike gets washed after almost every winter ride. Cleaning kits, brushes and bike-safe cleaners get used up and replaced all year.
Protection is a kind gift. Knee pads and a good helmet are things riders often put off buying. Knee pads are sized by leg measurement and helmets by head circumference, so check the brand’s size chart.
Tubeless kit fits most bikes. Most modern mountain bikes run tubeless tyres. Sealant and a plug kit don’t depend on wheel size, so you can buy them without checking the bike.
Tyres need a size. Mountain bike tyres are marked on the sidewall, for example 29x2.4 or 27.5x2.6. Match it exactly, or pick a different gift.
Shoes and pedals need care. Flat-pedal shoes are fine if you know their shoe size. Clip-in shoes and pedals depend on the cleat system they already use, so leave those unless they’ve asked.