Okay, so you've decided you want to start training seriously and want to start gathering ride data. Or you're a data junkie and just want to start seeing what's going on during rides and monitor your progress. There are different metrics that can be measured using different devices, but the foundation of them is the cycling computer.
A cycling computer is a device that acquires and records data from riding. The most basic metrics are speed, distance and ride time. Some of the more high-tech options allow many workout aids to be programmed and record additional metrics such as heart rate and power, and some use GPS as a means of tracking speed and the route(s) that you've ridden.
Speed, Distance, Time
The basic information that many recreational riders and amateur racers like to monitor is speed, distance and time. This information is used to track how many miles or hours per week you ride as a gauge of fitness improvements or simply because you like to know the information.
Some people like to use the speed as an indication of fitness, but speed is quite dependent on the surrounding conditions, so it's more variable than other forms of data that can be used for such a measure. If you use speed as a testing format for gauging fitness, use the same route you've done before and compare times. This is usually pretty easy since most people will ride in the same area most of the time.
Speed is often measured using a magnet attached to the wheel's spokes and a sensor on the fork or chainstay that measures how often the wheel turns a revolution. This also means that you need to know the circumference of the wheel to know how far the bike travels in one revolution of the wheel. Most sensors have a guide to approximations that can be used if you don't wish to find the wheel circumference, often done by performing a roll-out test.
GPS is another more expensive option that records distance and speed. It isn't quite as accurate, and some have issues in areas where there are a lot of quick and subtle elevation changes -- like in switchbacks -- as the data points taken for determining this information may cut off some parts of the corners. This is usually fine for road riding but has caused some issues for off-road riding where there are more inconsistent lines.
Most GPS cycling computers can be used with a wheel sensor, in which case the GPS only records the route and the speed, and distance is measured by the sensor. Also, the more data points that the GPS records, the more exact the overall data will be; some units have options to change the sampling intervals.
Heart Rate Monitors
People who start tracking heart rate are most often people who have decided they want to start getting more serious about their training. Tracking your heart rate allows you to get information on how your body reacts to stress it experiences. This is great information to know because it is less dependent on surrounding conditions than speed.
Heart-rate monitors vary some, but for cycling most gather all of the information of regular cycling computers and add heart rate in. Some designed more for running do not record distance, just time and heart rate.
Heart rate information helps give a more unbiased view on how hard your rides are instead of just gauging difficulty by perceived exertion. When you start to tire, you will often find that your heart rate will drop even though it feels like you're trying hard. This is especially the case when you've "hit the wall," bonked, etc.
Heart rate monitoring is limited somewhat in usefulness in that it is best used when your efforts remain constant and in the aerobic ranges. If you are working in the anaerobic range -- specifically short, hard efforts only lasting a few minutes or less -- your heart rate may not react quickly enough to detect the full effort. Despite this limitation, most of your riding will be within the scope of what heart rate can accurately cover.
Measuring your heart rate is great for pacing since it is more dependent on your body than the other metrics mentioned thus far. This means that you can start figuring out zones of effort, called heart rate zones, to figure out how hard you are riding and how hard you should be for pacing.
Measuring resting heart rate is also useful as a higher-than-normal resting heart rate can indicate over-training, illness or other issues where your body might need a little extra recovery time.
Your heart rate can be obscured some by outside forces like caffeine. If you are an avid coffee drinker you will probably see heart rate spikes early in a ride if you finished the morning cup and immediately went for a ride. This is fine but it could lead you to think you are giving more effort than you think when reviewing the data at a later point.
Power meters have been around for a quite a while, but only recently have they become more available to the public as prices come down and more useable options become available. Power meters are still much more expensive than other devices used to measure efforts.
Power meters measure instantaneous workload, meaning they tell you exactly how hard you are working at the time you're doing the work. This varies from heart rate as heart rate measures your body's reaction and not exactly the instantaneous workload. Heart rate has a tendency to lag a little bit behind sudden hard efforts, which is where power meters find a very useful place. They are still just as useful for monitoring all of the other efforts for which heart rate is useful.
Just like heart rate, power monitoring is valuable for unbiased information about what your body is doing versus how you are feeling; power meters are better at doing this than heart-rate monitors, though. Let's use weight lifting as a metaphor: you may be able to lift 200 lbs 10 times on most days, but you are only able to lift it seven times on a particular day. This kind of info can be seen with a cycling power meter; if you are used to putting out a certain number of watts (units of power) but aren't able to get the power numbers out, it may be time to take it easy.
Power meters are best used, in my opinion, for intervals -- especially shorter intervals -- which is where heart-rate monitors are less useful. Power meters are also great for pacing in rides like time trials, as they allow you to directly gauge your current efforts to make sure you're not going too hard at any moment during the effort. They do this more accurately than heart-rate monitors, as well, as power meters can't be fooled as easily as heart-rate monitors.
One of the best pieces of information that is obtained from power meters is what you do during races. This is incredibly useful because in training you want to train for the efforts you will experience during a race. Maybe you push too hard to get up climbs and/or at wrong spots during the climbs, or maybe you work too much at the front of the peloton and don't have the power at the end of a race for a race winning effort.
Power profiles have become commonly shared amongst athletes and on websites that track this information so people can see what other athletes are doing. For the 100th edition ofthe Tour de France, trainingpeaks.com has shared information about some of the riders' power profiles to see just how much work the pros are doing during the race.
Heart Rate with Power
Currently, this is the best way to train for cycling. Using both heart rate and power together tells you a great deal of information about how the body is reacting to efforts. You get the effort measured by the power meter at the time of the effort and you get to see how the body reacts as measured by the heart-rate monitor.
When is this useful? When you start training consistently, you'll see a trend that when the power goes up, so does the heart rate. This is referred to as coupling. If you set a pace of 130 beats per minute (BPM, measurement of heart rate) and you expect to see a power number of about 200 watts, if you are in good shape you will be able to keep putting out 200 watts without your heart increasing or your power dropping. Decoupling is when the two start to diverge over time and is a sign that you are either out of shape, have extended your efforts past what you can handle, or that you ran out of energy (bonking or hitting the "wall").
What Do I Do with All This Information?
While all of these devices and the data they acquire are great tools to see what your body is doing, they are only as good as your (or a coach's) ability to analyze them. There are several programs available to help you analyze this information, some free and some that require you to buy the program or subscribe. This is a place where coaches can be a great benefit as they can analyze everything for you and gather the information that you may be missing.
Keep in mind that it takes time to figure out what this information means. If you don't want to spend the time analyzing information after the ride and just want to ride, a lot of the data isn't going to be really helpful. On the other hand, there is a wealth of knowledge that can be obtained from this information, so much so that there are books written specifically on the topic of training with these devices -- mostly power meters though since they offer up the most complex data. The longer that you record data the more useful this data is.
Many people will stop their data capture at heart-rate monitoring mostly because of the cost of power meters, but some have purchased power meters and decided they didn't want to analyze the information and chose to stop using them. The information can quickly become overwhelming from power meters, or even heart-rate monitors, if you don't really want to spend the extra time finding information within the data. It can quickly get quite scientific and the devices were actually first created for the purposes of sports science.
Be realistic with yourself before getting any of these devices in terms of if you are going to actually sit down and use them. There are plenty of examples of graphs and data sets available online that you can look at that will give you an idea of the complexity of the information that can be obtained.
Using a power meter with a heart-rate monitor is kind of like bringing a science lab with you out on a ride; there is that much information being recorded. This also gets some people lost in the numbers, meaning that they may limit themselves during a competition because their device tells them to. There's still a human component to cycling, so don't get lost in the numbers.
Above all else, you still want to have fun in the sport. Keep riding and let the numbers be what they will; there is no device that measure a rider's smile.