Hi, welcome to the darker of the two dark sides....
Terminology first. Downhilling is riding ridiculously fast down geet big hills on bikes that ought really be taken up the hill first on a ski lift or uplift truck. Go onto Youtube and search for the Danny hart video. Unless you want to do that sort of thing competitively then I would suggest you're more likely to be interested in XC or trail riding. The two are very close in terms of kit needed, but don't require 180mm travel forks etc etc
Downhiller:
Note the burly frame, enormous travel forks, double barrel rear shock, single chainring and brakes the size of dinner plates.
A Cross Country (XC ) race bike on the other hand is light, responsive, and has fork travel between 80 and 120mm, no rear suspension. Wheels tend to be light but not too good over big rocky stuff, and the entire package tends to be light and stiff. They're built to race, to weigh not much more than an entry level road bike some as light as 9kg, but are often nervous thoroughbreds.
You can get short travel XC full suss bikes, but again, you're into short rear shocks absorbing some of the hits and keeping your rear wheel planted, but the bikes are designed to accelerate fast, take some obstacles in their stride on fast off road routes, but won't be all that comfy for all day epics.
An awful lot of MTBs sold today are nearer XC bikes than Downhillers, especially at the "entry level" end of the market. Here you're looking at trekking frames, often alu with a bewildering array of components. You can however narrow it down to stratae of componentry. Less than abut £600 and you can expect, generally speaking, a basic 100mm coil sprung fork, plain, basic wheels and drivetrain by Shimano or SRAM. 9 or 10 speed the norm now, with some 9 speed chainsets costing more aftermarket than 10 due to demand.
Shimano have a tiered system of components, Acera (m3**) Alivio (m4**), Deore (M5**), SLX (M6**), XT (M7**), XTR (M9**). Acera mech c £20 through to XTR c £250
SRAM have the same, with their X5, X7, X9 XO and XX similar pricing
On your £600 entry level bike you'll find a mix of Alivio and Deore drivetrain. Functional, but basic and heavy. A bike in this price bracket will do well as an off road bike, but you'll soon want to upgrade and swap for lighter components. Frames will indeed be 6061 or 7005 alu alloys, and the more expensive the frame, the lighter it will get with hydroformed tubing and double/triple butting shaving weight in expensive tubes. I would generally expect a sub £600 bike to weigh around the 13-14kg mark.They'll mostly have factory finishing kit rather than named seatposts bars and stems etc.
Between £600 and £1000 you're starting to see quality components. More than likely you'll find better damped coil or basic air forks, full deore or even , SLX drivetrain with a smattering of XT mechs to tempt the buyer. Same basic wheelsets, but you're into lighter hollowtech cranksets, and branded finishing kit. Expect weights to be dropping to 12-13 kg now, as these bikes will be hydroformed or butted 6061/ 7005, and lighter drivetrain.
Over a grand and you're into the quality stuff, lightweight air forks and full SLX or XT drivetrain. You'll start seeing named wheelsets more and more, and as you creep over the £1k mark you'll start to see carbon frames. Weights will be nearer to 11kg than 12.
Full suss XC and trail bikes start around this price point too, but don't expect much for under £1500. Spend a ballpark £2-£2.5K though, and you'll be into serious trail bike territory with 140mm travel Lapierres, basic Orange fives and well-equipped imports like Canyon and Rose.
I suppose this is where we start to talk about trail riding. Comfort, not speed over distance, but huge fun on the more rugged descents and trails, and this is where you tend to find a lot of specialist bikes, self builds and custom jobs.
Travel tends to be longer. Trail hardtails typically vary between 100mm and 140mm, but there are some 160mm fork HTs out there. Custom frames are de rigueur with niche brands like Cotic, Dialled, Orange, NS, Ragley, On One, to name but a few. If you have around a grand to spend you could do worse than look at On One and Ragley full builds. They are designed to be fun and take big hits on rough trails, rocks, drops, jumps and clattery singletrack. A lot of hardtails in this category are made from steel alloys. My Cotic Soul with its Reynolds 853 tubing weighs about 500g more than the aluminium Cube frame it replaced. Trail bikes often have MTB compact gearing setup. Mine has 36-22 front over 9 speed 11-34 cassette, so a spread of gears that's hugely useful. (A 42 or 4T chainring on a MTB is good for 45mph, so don't think you'll be undergeared)
Full sus trail bikes are a far bigger part of the fun bike market, with 120-150mm travel the norm. Heavier frames with rear shocks, but geometry designedd to allow climbing, with fun and planted descent is the order of the day. Typically a full susser in this category will weigh between 12 and 15 kg depending on kit. Built for all day epics and fun stuff, this is my favourite riding genre. This Canyon-framed XT drivetrained bike weighs 12.5kg
So, that's a bit of a whistle stop tour, but don't forget, MTB includes Dirt Jump, 4X, Freeride, Slopestyle, Enduro, the list goes on.
