The following information has been taken from the Cycling Time Trials website and I would say the advice is pretty good and makes sense for many other cycling disciplines. https://cyclingtimetrials.org.uk/articles/view/29
A guide to clothing for time trials
Text originally from the Easterley Road Club.
The use of specific cycling clothing is not essential, however it does make life more comfortable, especially in winter.
The first items of clothing to buy would be cycling shorts, shoes and crash hat.
Proper cycling shorts (rather than so called ‘cycling’ shorts available in sports shops) have an insert in the crotch area. This is usually padded and has flat seams to aid comfort. This insert used to be made from chamois leather although most are now from man made materials, however they are still referred to as the ‘chamois’. It is best to go to a cycle shop to obtain these. No underwear is worn under cycling shorts as they have seams which negate the benefits of the chamois. Because the chamois is next to the skin, it should be kept scrupulously clean to avoid any infections. Shorts do tend to ride down so it is worth purchasing a pair of clip on braces to hold them up, or you can obtain shorts that have straps that go over your shoulders called bib-shorts.
Cycling shoes have a stiff sole that aid the rider when pulling up on the pedal and do not transmit pressure from the pedal to the riders feet. Cycling shoes usually have the facility to attach shoe plates which lock on to the pedal. Do NOT use cycling shoes designed for the road however unless you are using toe clips and straps, or clipless pedals, as the soles of these shoes are made from plastic and will slip off the pedals very easily. Mountain bike shoes tend to have an aggressive knobbly sole and are suitable for ordinary pedals or specific mountain bike pedals (see SPD pedals). Most of the modern shoes are Velcro or buckle fastened, but if you are using shoes with laces (or trainers), ensure that the laces on the right foot are short or tucked into the shoe as they can become entangled in the chain and chain ring when pedalling.
Riding without a crash helmet is not illegal but we would strongly recommend its use. Most serious injuries to cyclists are head injuries and it makes sense to try and avoid these if at all possible, although no crash helmet can give 100% protection. When choosing a crash hat , the most important criteria is the fit. It should be a snug fit with no excessive twisting movement allowed. Most hats have pads supplied to tailor the fit exactly. Look for helmets that comply with the usual safety standards such as BSI, Snell, ANSI etc. Good makes to look out for include Giro, Bell, Specialized and Met. Generally speaking, the more money you spend on a helmet, the more ventilation it will have which is an important consideration for summer riding and racing. If you intend to race, please note that an approved helmet is compulsory for all BC and BCCA events, although they are optional for CTT time trials. If you do hit your head during a crash, you should replace the helmet immediately, even if it does not look damaged. They are designed to absorb the impact and once they have done so they will be weakened.
For winter riding, ensure that you have sufficient clothing as you will suffer from a higher wind chill than when you are walking or jogging. Wear gloves and a hat (if you are not wearing a helmet). A headband will keep your ears warm if you are wearing a helmet. To keep your feet warm and dry, a pair of overshoes will help. Wear cycling bottoms (or track suit bottoms that are not too flared in the leg) to keep legs, and especially your knees, warm. Remember that plenty of thin layers will keep you warmer than one or two thick layers as they trap air between them and you can regulate your temperature more accurately by taking off or adding thin layers. Try to include a ‘thermal’ base layer next to your skin that will wick away the perspiration and avoid that ‘clammy’ feeling.
Bright clothing will help to get you noticed, especially in poor light. Fluorescent coloured tops are especially good around dusk. For night time riding, add clothes that have reflective or Scotchlite strips. You can purchase these to sew on to clothes or you can buy reflective ‘Sam Brown’ belts.
Purchase a good quality waterproof as you can never depend on the British weather! Ideally this would be one of the breathable fabrics such as Gore-Tex, however they can be expensive!
Fit mudguards to your bike if you are riding on the roads. This is for your comfort and the comfort of the riders behind you. The spray from the wheels will make your feet and back, as well the riders behind – cold, wet and dirty if you do not have them.
Most cycling tops have two or three pockets on the back that enable you to carry food and other essentials. This is especially useful if you are doing longer races.
If you do intend to race, please note that you must wear the club design in all events under BC jurisdiction. For open time trials, you can wear club clothing or any top that does not have advertising or company logos other than the registered sponsors of the club. This rule does not apply to club only events. You may wish to consider purchasing a skin suit for shorter time trials, road races, and track events. This is a one piece combined top and shorts that helps to reduce wind resistance and are more comfortable than separate items. In any bunched race, you should always wear a T-shirt under your outer racing top because in the event of a crash, the two tops will slide against each other, helping to reduce cuts and abrasions. For the same reason you should also wear gloves or (track) mitts.