Many riders I know, they bring along in their bladder bags all sorts of spare parts for their bikes. It's actually a good practice. You take care of yourself, and at the same time you could perform good deeds by providing for others (or make big money for yourself). To put it in another way, you're not a nuisance for others. Being kiasu or geeky, that's a thing you won't be thinking about when you suffer a puncture but without a spare tube or the humble patch kit (with pump/CO2 canister, of course).
I'm a roadie, I don't do bags
For most of the parts, they are lightweight, so they don't really feel that much. They won't take much space in your bladder bag, too, for they are usually small in size. Personally, parts of the size bigger than my palm cannot be considered into my spare parts list. Crankset, or rims, for instance. Besides, you have to bring along special tools for them too. Oh yes, bear in mind, you have to carry along the necessary tools. Otherwise, they're useless unless if someone else in the group carries them. With this in mind, be sure to look for spaces where you can stuff the spares when you're shopping for a (new?) bladder bag.
If you're a roadie who still want to ride lightweight offroad, be sure to carry lots of spare cash. I'm selling my spares at a premium. No kidding. Of course, you can still bring your spares in your saddle bags.
Items
I usually carry with me these spare items, for my insurance:
- Spare tube (also a pump and patch kit with levers)
- RD hanger
- Brake pad
- Rotor bolts (can also be used as bolts for your shoe cleats)
- RD cable (can also double up as mech brake cable)
- Snap-on chain link
- Spare links
- Necessary tools (multi tool should do the trick)
- Snack bar (optional)
You see, they are small in size and weight, but big in value. The biggest of them all is perhaps the spare tube. Followed by the hand pump. These, however can be opted out. You may just patch the punctured tube. Hand pumps can be replaced by CO2 canisters. Even if you're a weight weenie, you can still have your insurance.
A guy I know carries with him all sorts of spares, he seems like a mobile bikeshop. Ayub's the name - flip over to the 3rd issue of Cycling Malaysia magazine, he's the one wearing the white Cap Ayam tee on the Plentong Epic Ride pictorial. Whenever the guys in the group are in need, he's the one who provides. And no, don't ask him for a spare crankset. By the way, he sells his spares :)
RD Hanger
There are various designs for the rear derailleur hanger. 'Various' seems to be a bit understated. Perhaps 'hundreds of them' gives a pretty solid picture. That being said, there are also hundreds of frame designs using different hanger designs. Check 'em out:
link. I know bro Ayub has a universal hanger, but I'm not quite sure how it works (or where he got it) - have to check with him. It certainly doesn't look like this one:
link.
May I suggest for you to have your own spare hanger. That would save your knees from grinding your way back to base single speed, should you unfortunately busted the hanger. The next time you're visiting your favourite bike shop for a chat (yeah, right), be sure to bring along your current hanger. The best, is to bring along your bike. Get yourself a perfectly matching RD hanger.
Chain Links
I'm too lazy to use the chain tool nowadays. Hence, I'm using a snap-on chain link. There are two brands commonly found in the bikeshop - SRAM's PowerLink, and KMC's Missing Link (google 'em up). At around RM10-RM15 per link, they're not cheap, mind you. However, if time is money, I don't mind spending a little money for saving me time (and headache) especially on the trail and much more, on my way to the office. Easier for me to remove the chain for cleaning, too.
Albeit, I still carry spare chain links, and my multi-tool includes the chain breaker tool. I use the links for mending other people's chains, and use the snap-on for myself. I told you, they're not cheap.
Tools
Yes, you need them. Get yourself at least a basic multi-tool. You can do away the ones with chain breaker (like mine), if you're using the snap-on chain link. You can also leave your hand pump at home if you're using CO2 canisters. However, I never leave the tire levers behid. Although using kevlar beaded tires, I prefer to use the levers. I found that at times the beads are quite stubborn to be removed from the rim.
If I could offer an advice, please, get yourself a good tool from a reputable brand. It would be long lasting, durable, and most important of all, reliable. Do enough research for the reviews of the particular product, and you'll save your money on potential replacements of inferior items. Reviews may be in the form of findings out of riding buddies' experience, or the internet (where else?). Spend some time, save some money (again, if time is money).
Enjoyable Suffering
Mountain biking, for some, at times are testing your limit. Endurance, power, physical, mental, and most of the time, your patience. You're exhausted, your parts are busted, and now you have to suffer the ride home with a lesser bike than the one you got in with. I choose to avoid the strain, so I choose to carry with me those things. Tawakkal is not gambling. It is being prepared, and leave the rest to Allah. At least when bad things happened, I know that I did my best to avoid them in the first place. Yes, we go offroad for enjoyable suffering. But they don't have to be strenous.
See you on the trails!