A good collection of spare tyres, although you could also call this having tyres for all conditions!
Have a spare chain for the main MTB (XO1 eagle, I bought two when i first went eagle 3 years ago, still haven’t worn the first one out!)

Apart from that, I’ve cleared out most of the old stuff on eBay this last year.

My most used bike is of a certain vintage, so I do have a stock of stuff you can’t get any more (or you can, but with difficulty/expense when you actually need them). I have two complete sets of Middleburn chainrings and about 10 sets of Hayes semi-metallic pads for the stroker gram. Once they’ve all been used up, it might be new bike time… If I see something at a good price that I know I will definitely use, I’ll usually buy it if I can and stash it in the loft until I need it.

I always have spare cables, hoses, olives, barbs, pads. I have chainrings and other slightly used parts or the occasional bargain I’ve picked up for one of my kids builds. Otherwise, no real stockpile – unless you count a stash of parts that were cutting edge between 1988 – 1997 or so.

I have bought a couple of pairs of 29+ tyres because they are hard to get and I fear they may disappear altogether. So far they are still in their packaging but I wonder if storing them like that longer term will wreck them an I will end up with a creased and distorted tyre…?

When it became apparent that finding parts was going to be a problem – so, summer 2020 – I started buying stuff that I knew I’d need for both my bikes. So now each has a spare chainring, a cassette or two, a chain, brake pads and cables. Most have been sourced at below RRP by doing a bit of sniffing around (the exception: GRX chainrings). Got a few tyres knocking about so have only bought one DHRII and DHF. Thankfully, most of the stuff I need is pretty standard.

I keep a single spare of most stuff to avoid down time and have done so for about 10 years.

Rear mech, chain, chain ring, brakepads, wheel, BB, headset, pivot bearings, spare rim, front and rear tyres, some spokes and nipples (6 per wheel usually) a front and rear tyre, grips, cable inners and out, hydro hose, fork and rear shock service kit, shock bushes.

I am down a GX cassette at the moment, but both bikes were given new in Feb 2021 and I refuse to pay the outrageous prices that are currently about.

I try to keep a spare of consumables

Only getting into mtb recently so don’t have anything other than chainlinks and a tyre for the MTB. But chain cassette hanger etc for my road bike

I like my roadie running absolutely mint, so I change the chain fairly often

yes, always have well stocked chains / pads etc.
in the last few years, kept stock of derailleurs to cover bikes,

tend to buy tyres when on offer, and i dont like paying postage so may as well get extras

I have 6 bikes in our house all running 11sp drivetrains, with same size disc, Shimano brakes and Maxxis tyres. So, my spares box has about 3-5 sets of everything waiting to be fitted. As i fit something, i source another to put back in the box.

It does mean my spares box is worth over a grand however, so not a cheap endeavour.
It’s paid off in the current supply chain drought though.

I just checked stock with online sellers within China and Shimano Microspline 12spd cassettes are virtually gone everywhere. Some sellers still with some stock have upped the price 33%.

Sram XD cassettes are still available. No problem with hyperglide cassettes

Time to start stocking up compatible cassettes (Wuzei and TRW), or looking into swapping freehub, shifter and mech. Having 148mm MS freehub boost wheels is starting to become a problem!

Currently building a second bike up and using the exact same bits that are fitted to bike #1 for drivetrain and brakes.

After the hassle of getting a 12 speed shimano chain in the summer I’m keeping some spares in…