You have a limited budget, and you want to get the best bike you can for the money. If you're not sure what to look for, you could be mislead by features that seem to make the bike better, but in fact really don't count for much. Here's what to check out in your bike search.
As before, we start with the frame. You should be trying to get the best frame you can, within reason. There's no need to spend 500 quid on a frame alone, but you definitely shouldn't be contemplating getting a cheap frame, just so that you can have an (equally cheap) suspension fork on it.
Most mountain bikes are made of steel or aluminum. There are two main sorts of steel: high-tensile, and chromoly. High-tensile, aka hi-ten, is pretty much garbage. Don't buy a bike that has any part made of hi-ten steel, whether in the main frame or in the fork. Although there are different sorts of alumimun, there's not much to distinguish them at this price range.
Nowadays aluminum frames predominate for the price range we are talking about. This is mainly the result of pressure from the bike buying public. I have been told by bike shop owners that MTB customers much prefer alu bikes over steel, and this is why most manufactorers have abandoned steel for mid-level bikes and above. However alu doesn't have any clear advantage over steel. It can be made perhaps a bit lighter, which is an advantage for top end off-road racing bikes, but is pretty much irrelevant for the midrange bikes we're talking about here. I think that steel has gotten a bad reputation because of high-tensile steel. People remember how lousy their first, cheap, hi-ten bike was, and think that that poor quality is true of all steel bikes. This is not at all the case, and a few daring manufacturers (notably Marin) continue to offer high-quality bike frames made from steel.
For both steel and alu, swaged or butted tubing, which has thinner walls in the middle (for lightness) and thicker walls at the ends (for strength) is better than straight-gauge tubing (which is same thickness throughout).
One thing you should look for on your frame is the ability to attach things to it. It should have rack/mudguard mounts at the rear dropouts, and rack mounts on the seat stays. You may think this is not needed, that you'll never want to use a rack or full mudguards on your bike, but your needs may change over time. For example, if you get really into off-road cycling you may decide to buy a full-suspension bike, and this one will be relegated to riding to work, or for touring, in which case a rack and mudguards are pretty much essential.
Another thing that's a good idea to look for, if you think that it's likely that you'll really get into off-road riding, are disk brake mounts, both on the main frame and on the fork. These provide better stopping in the wet and eliminate worn out rims through braking, and thus are a good upgrade for the serious off-road rider.
If your frame is aluminum (as most MTBs are nowadays) it should have a replaceable derailluer hanger. Crashes often knock the derailluer out of place, and aluminum doesn't tolerate being bent back into shape (it is likely to break if bent back, while bending back a derailluer hanger on a steel bike is not a problem).
If you've decided that you want a suspension fork, you'll want to know which ones to look for. Generally, sad to say, the more expensive the forks are, the better they are. The more expensive ones are stiffer, easier to maintain, and more "plush" (absorbing bumps of all sizes better). There's quite a variety of fork materials and internal construction, and it's changing all the time. I haven't kept up with the changes, having gotten a suspension fork that I really like! Often MTB magazines run a buyers' guide to forks, so it's best to get this info from them.
While the frame is the heart of the bike, the wheels are what makes is a bicycle! Most MTB wheels look similar, but when you get into the details, you'll find the difference between wheels that will fall apart after one month, vs. ones that will keep spinning happily until you finally wear out the rims through braking.
Rims. The rim is the outer metal hoop of your wheel. They are made of aluminum. (At least, all the ones in this price range should be. For very expensive bikes you can get ones made of carbon fiber, and for exceedingly cheap bikes they are made out of steel, which is to be avoided at all costs!) Alu is light and provides a good surface for brake blocks to bite into, and they shed water quickly when wet to give you good braking shortly after the brakes pads meet the metal. In cross section, the rim will usually be something like a U shape: the tire attaches to the prongs of the U. The bottom part of the U is often flat on MTB rims, but can be curved, sometimes even almost pointed, like the bottom of a V. Generally, the more curved or pointed the inner part of the rim is, the more strength it will have, but it will also be heavier. One thing that's not so easy to see is that better rims will have an additional bit of metal across the bottom of the U. (Rims with this bit of bracing are called box section if they have a flat part at the bottom of the U, and are called aero section if the bottom part is more V shaped, and are called open section if they lack this bracing.) This bracing greatly adds to the strength of the rim, so much so that it's not a good idea to buy rims without this extra bracing, even for road use. To find out if the rim has this extra bracing, you must take the tire off. If the bed of the rim has wide circular holes in it, then it will be a box or aero section rim. If instead you can see, underneath the rim tape, small lumps, these are the heads of the spokes attached directly to the rim, and it is an open-section rim.
The braking surface of the rim should be plain metal, no colors or anodizing or anything like that. Colored braking surfaces badly affect braking. (The exception is for ceramic rims, which do have a coating on the braking surface, but you won't find these on bikes in this price range.) Some manufacturers put little grooves in the braking surface. This improves braking until the edges of the grooves wear off, but then is the same as for any other rim. I wouldn't pay extra for it. Colors on rims are fine as long as they aren't on the braking surface.
Spokes. Generally, MTBs have 32 spokes per wheel. More is not needed (except for perhaps downhill racing or loading touring off-road), and fewer makes for a somewhat weaker wheel for not much weight savings (although some racers do consider this weight savings significant and get wheels with fewer spokes). Spokes should be stainless steel. (Some fancy wheel have spokes are made of aluminum or other materials, but these offer very little benefits over ordinary steel spokes, and they cost alot more and are much harder to replace if you break one.) The best spokes are double butted, which means thinner in the middle than on the end. Butted spokes make for a lighter but stronger wheel (stronger because they stretch slightly when subjected to brief but large impacts, thus absorbing the hit, rather than giving in and buckling the rim). Black spokes are OK, as long as they are still stainless. The best spoking pattern is the traditional three-cross, as on the Kona Blast below. You can get radial spoking on some MTBs, but this is for looks only: it puts much more strain on the hubs for no benefit.
The wheel needs to be properly tensioned in order to stay together. If the spokes are not tight enough, they will rattle loose as you ride, and then spokes will start to break since the few spokes that have remained tighter are taking most of the strain. The best way to check for properly tensioned wheels is to pluck the spokes. For the front wheel, they should have an even pitch. For the back, the right side (the side with the gears on it) should have a much higher pitch than the left side. This is because the right side spokes are shorter and have a higher tension than the left side spokes. However, the right side spokes should all have the same tension (hence the same pitch), and all the left side spokes should have the same pitch, but it will be lower than the right side, and usually lower than the front spokes as well.
Hubs. Hubs go around, so the best thing to look for is good bearings that let them go around with as little resistance as possible, and good seals to keep them going that way. You can spin the wheels of a bike you're thinking of buying to see how freely they spin. One simple test is to attach a spoke wrench to a spoke at the rim. If the wheels slowly drifts down so that the wrench is at the bottom, it's got good bearings (note that it may not be completely at the bottom, as the valve weighs more than other parts of the tube, and the rim join also weighs more). If the wheel doesn't move at all, it's not as good, but still may be plenty fine when you're riding it. Ask your bike shop if you're uncertain about rim quality. As I mentioned in the previous section (What sort of bike should you get?), if you think that you may get seriously into off-road riding, it's worth buying a bike that has disk-ready hubs, so upgrading to disk brakes will be easier and cheaper.
Tires and tubes. I suppose I ought to say something about tubeless tires. They are fairly new on the scene, and not many bikes come with them (certainly none in this price range). I have never used them, so I can't comment on them from a personal perspective. Right now, they look to me like something that doesn't have any clear advantages over the conventional approach, except for racers who are concerned about every last gram. (For example, if you get a bad puncture you'll have to put a tube in anyway.) But in a few years time they may have proven their worth so much that finding a MTB with normal tires will be as difficult as finding a MTB with rigid forks!
Back to normal tires. They come in a huge variety of tread patterns, widths, compounds, etc. For on-road use you want a smooth tire, with width between 1.25" and 2". For off-road use, the choice is bewildering. Generally, narrower tires are better in the mud (there's less area for the mud to cling to), but wider tires have more cushioning and grip. Thus the best tire for you depends on your riding conditions. Some tires have a Kevlar bead (the bead is the thing on the very edge of the tire that keeps it on the rim) rather than a steel one. These reduce the weight of the tire with no drawbacks other than being a bit more expensive, so are a good idea if you want to prevent your bike weighing too much.
Tubes are mainly distinguished by the valve type (presta or schraeder) and width. For width, get them to match your tires. I prefer presta (aka French valve) to schraeder (aka car tire valve) as I find them easier to pump up. There's not much to say about tube materials, other than that I have always gotten along well with plain black rubber ones of normal thickness.
Back to the beginning
You should look carefully at the components on the bike. These are things like brakes, brake levers, shift levers and derailleurs (also known as gears). Generally, the higher spec components last longer and are easier to maintain than the lower ones. They are also more expensive. Getting a good frame should be your priority, but quality of components can help you choose between bikes that have similar frames.
The transmition (shift levers and derailleurs) are usually made by Shimano, so it's useful to know the grades of Shimano components. From highest to lowest, they are
Shimano Alivio logo
You should be looking at getting something at the Alivio range or better. Often manufacturers mix and match components groups. For example a bike could come with an Alivio front derailleur and a Deore rear derailleur. This is sensible, as it puts the better components where they are most needed and keeps the cost down by putting in somewhat cheaper components where less is demanded of the part.
Some bikes now come with SRAM transmition components, which are also good. Generally, the larger the number in the Gripshift component's name, the better quality the component will be (for example, 9.0 is better than 7.0). SRAM are best known for their twist shifters. There are two main varieties of SRAM shifters: the ESP (1:1) system which requires their own rear derailleur, and 2:1 system, which works with Shimano rear derailleurs. The Shimano-compatible system relies on the rear derailleur moving about 2mm for every 1mm of cable pulled, while the ESP system has the rear derailleur moving about 1mm for every 1mm of cable. Of these two I much prefer the ESP system, as I find it too easy to shift more gears than I indended to with the 2:1 system.
Shifters come in two main styles: twist shifters or lever shifters. It used to be that SRAM made the twist shifters, and Shimano made the trigger shifters, but now SRAM make some trigger shifters and vice versa. Most people prefer one or the other, but you won't know which one you like until you've tried them. Try to test-ride bikes with both kinds so you'll know which type you'd like to have on your bike. If you find your dream bike, but it comes with the wrong kind of shifters, don't despair. The bike shop will often replace them with the other kind for a small fee or even free.
For brakes and brake levers, Shimano, Dia Compe, Gripshift, and Avid are brands to look for. Tektro brakes, while not being top-notch, also do the job pretty well. Never buy a bike with plastic brakes.
Often bike companies have their own line of parts. For example, you'll find Specialized components on Specialized bikes and CODA components on Cannondale bikes. It's sometime hard to tell how good these parts are. Often they are excellent quality and value (especially for Specialized and CODA parts), but sometimes they aren't quite a good quality as you might like. There is no general rule here, so ask someone you trust about these parts if you're worried about it.
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