You will often see people mountain biking in t-shirts. I prefer to use cycling jerseys. They are a snug fit and don't flap all over the place in the wind. They also have a zip front which you can open if you are too warm. Most have pockets in the back which can be used to carry essentials such as food that you can nibble away on as you cycle along. The main thing is to wear something that is not going to snag in anything and that allows plenty of movement. Comfort, warmth and breathability are the qualities that you are looking for.
Cycling jerseys are specifically designed for the purpose. There is every conceivable option with cycling jerseys, some come with vents to keep you cool others come with thermal lining. Some even incorporate organic coconut shells to absorb odor! They are also sewn in such a way as to match the contours of the body for better fit. Proper cycling gear just makes the trip much more enjoyable.
Trousers and Shorts
Tracksuits really are not the best thing to wear on a bike. Firstly they flap around in the wind and in general are too warm. They also have a tendency to catch in the chainset. They catch and bunch around the saddle area and make it generally uncomfortable. A better option, if you don't want to reveal your legs is to use cycling tights. Tights come padded and unpadded. You can get fleece lined, and other engineered fabric tights or plain lycra ones, depending on whether you are riding in the winter or summer. Tights are warm, allow plenty of movement and don't catch in anything. They also come with bibs if you prefer. These are straps which come up around the shoulder to keep them perfectly in place.
If you really want to show off your pins don't be tempted to use a pair of football shorts. First of all, I don't like football, but there are better reasons than that. Football shorts are too short for cycling in, and your legs will end up rubbing off the saddle until they are raw. Instead try to use cycling shorts or mountain bike shorts, that cover at least an inch or two below the saddle. Cycling shorts are tighter fitting and usually are bibbed. Whereas mountain bike shorts are a bit more casual looking and generally have pockets. Or you can wear the mountain bike shorts over the cycling shorts or tights.
There are lots of thermal base layers available that can be worn underneath your cycling gear. They are well worth investing in because they really do keep you toastie warm in the winter. They are also cheeper than wearing two or three layers of jerseys and t-shirts. and do a better job at keeping you warm. You can get arm warmers, knee warmers, leg warmers, long johns, the list goes on. Generally thermal shirts an shorts are about half the price of cycling jerseys and shorts and seem to be twice as warm.
A wind proof Jacket is an absolute must if you are on the side of a mountain. Cycling jackets usually come in some type of wind protective material they are snug fitting so they don't flap around in the wind and yet they allow movement and they really keep in the heat. They also have pockets at the back to carry essentials. Mostly these jackets are not waterproof so you may need to carry an additional waterproof jacket that you can put on in the event of rain. The main purpose of the cycling jacket is to keep you warm whilst still allowing the skin to breath.
Gloves & Glasses
It is a good idea to wear gloves when you are mountain biking. There are a few reasons for this. They keep your hands warm, they give better grip, they offer protection in a crash. Mountain biking involves the upper body much more than road cycling and your hand grip is brought very much into play during descents and technical manouvers. So gloves do help you to maintain control of the bike. There are essentially two types full fingered and fingerless. Some come with armour to protect the hands from branches etc.
I would also recommend glasses when you are on the bike. Mainly because they help protect your eyes from trail dirt and fly's. I find when I don't use glasses I tend to get eye infections. They can also help reduce the effect of hay fever if you are a sufferer.
Glasses come with a variety of lenses usually clear, yellow and tinted. The yellow lenses help to enhance visibility in dull conditions.
Runners are really not the best thing for cycling in. The reason is that the sole is too flexible and it offers no support when you are standing on the pedals. They are just not designed for cycling. Long term use of runners on a bike usually leads to foot cramp. A good pair of cycling shoes really is a must have if you are going to cover any distance. Many of them have straps to keep shoe laces out of the way. They are also water repellant so your feet keep dry and therefore warmer. The sole is a hard plastic which does not flex much thereby giving the foot lots of support when pedaling. This also translates to give more power and more efficiency. Cycling shoes can come with or without cleats. A cleat is a block which is usually made of metal that clips into a specific type of pedal. Some shoes come with a section of the sole which is removable to enable a cleat to be fitted at a later date. For beginners I would recommend that flat pedals are used until you are more used to mountain biking.
The single most important thing to get right in mountain biking is your helmet. If you mountain bike you will fall off the bike. There is no question about it. Now it may be that you just fall to the side very slowly and end up in a bush,in a fit of giggles, or you may be traveling at speed, loose control and hit a tree. Both scenarios may result in a bang to the head. Bicycle helmets come in three types, Open face, Full face and Open face with chin guard. If you intend to do a lot of downhill then you need to seriously consider a full face helmet or one with a chin guard. Cross country is better suited to open face helmets due to ventilation problems that you get with a full face helmet. Full face helmets tend to be a lot heavier and warmer than open face helmets.
Buying a Helmet
First thing to check when you pick up a helmet to buy is that it conforms to reputable international standards. Since Ireland is in the EU we are governed by the European Standards. However, unlike the European Standard, the American Standard specifically tests for downhill suitability.
European Helmet Standards
The european standard which governs recreational cycling is EN1078. Ireland is bound by law to implement this standard. It covers recreational cycling and skateboarding.
American Helmet Standards
The American standard is set out by ASTM (American Standard for Testing and Materials) They have two catagories for bicycle helmets. One is similar to EN1078 and governs the suitability for bicycle helmets for rollerskating and recreational cycling. This is ASTM F1447-06. The other standard recognises the requirement for additional protection for the face and chin, when the helmet is being used for down-hill mountain biking. This standard is ASTM F1952-00. Perhaps Europe will follow the lead by the Americans in this regard in the near future.
Bicycle Helmet Safety Institute.
For more information on helmet fit and advice please visit the Bicycle Helmet Safety Institute. This is an excellent site and well worth a visit. I have not detailed fitting instructions because the safety institute covers this section and is updated regularly. Be sure to check out the photo's section detailing crashes!
If downhill is your thing then you are well advised to use full body armour. It is possible to get individual elbow, shin, knee, and back protectors but there are also full armoured suits available which give additional rib and shoulder protection. Not only will they help to protect you if you do crash but they will also help to give you more confidence on the bike. You tend to crash less if you ride confidently. There is the additional advantage of course that they make you look like the Terminator.