You may have come across many confusing articles about bike fit. Bike fit has 2 main goals, to get you more efficient in your pedalling and also more aero. A great bike fit is money well spent. You'll have practically every part of your body measured including your flexibility. Some tweaks may be needed over time, but this is your number one way for getting a faster bike split. This even benefits your run as you'll have fresher legs to outrun everyone you passed on the bike.
2. Don't Wear Loose Clothing
Flappy hangy clothing causes a lot of drag. Race singlets and suits are seriously the only way to go. Nothing flaps. And besides, they make you feel faster. Loose clothing can slow you down about 1sec every kilometre. Doesn't sound much but 40secs over a 40km course is a lot!
3. Practice Drinking Aero
When you grab for your water bottle, it's often because you're feeling a bit stuffed and need a rest. Don't be tempted to sit up and take a break. This will ruin your rhythm and aerodynamics and cost you a few seconds each time. Know where your bottle is without looking for it, grab it without breaking your rhythm, drink fast, put it away. Drinking like the Boss.
4. Knees Together!
Imagine if you were on your bike in front of a mirror. Your aim is to make yourself the smallest when looking from the front. The more frontal surface area, the more you air you have to break and push. The most disruptive part of cycling are your legs which are constantly moving and pumping. Keeping your knees close to your bike frame will reduce your drag and also benefit from the air that has already been broken by your front wheel handlebars. And besides, it doesn't look cool to ride with your knees out.
5. Get Faster Wheels
Do not sweat the weight of your bike. The weight of your bike only has an effect on a hill. The percentage of time you are riding a hill in a race is minimal so forget spending the big bucks. BUT... if you want to spend some money, spend it on wheels. Deep dish wheels. This is where you can save some great seconds. We have no preference really for wheels, but get something that's in your budget and is quality. Don't buy a disk wheel, you'll waste your money when you have to keep leaving it at home because it's too windy to ride. Expect to cut 1-2mins off your time by spending this money.