One of the reasons a smart trainer is so versatile is that you can set it up to operate like a magnetic trainer, a wind/fluid trainer, or an ergometer. The modes available depend on the trainer and/or app you are using but fall into the following categories.
Level mode or fluid mode
In level mode, which is sometimes called fluid mode, your smart trainer acts like a fluid resistance trainer. As you ride faster, the resistance ramps up according to a preset power curve designed to replicate the experience on a wind or fluid trainer. When controlling a smart trainer with an app, however, you can typically adjust the power curve to change the resistance at a given speed. This is useful when you want to change the gearing you’re using while riding the trainer. If you adjust the power curve to be lighter/lower, you will be using bigger gears and turning the flywheel (via the rear wheel or cassette) faster. This leads to greater momentum, like you’re pedaling on flat ground at high speed. If you adjust the power curve to be heavier/higher, you’ll experience more resistance at lower speeds and be using smaller gears. You’ll have less momentum, meaning the flywheel will slow more quickly in response to even small dips in power output. This will feel more like climbing a steep hill, where you experience a slight deceleration between each pedal stroke.
When to use level mode
Some smart trainers default to this mode when the trainer is plugged into a power source but is not connected to a cycling computer or app. While this at least lets you do something with the trainer, it is becoming increasingly rare that a person has access to electrical power but doesn’t have access to at least a phone app.
The primary benefit of using this mode is that you have to control the power output and effort level yourself, just like you do out on the road or trail. If your workout calls for holding 300 watts for 20 minutes, you will have to bring your output up to that level and—more importantly—hold it there yourself. If you lose focus, your power drops, and you have to work to get back up to your target power. If you drive it too hard, the resistance increases and pushes you over your target power. This might not seem like much of a problem unless you are riding at the limit of your sustainable power. Pushing 10 percent over that target power for too long is likely to cause excessive fatigue and require you to reduce your power output before the end of the interval.
Resistance or brake mode
If a smart trainer has a resistance or brake mode, it means the resistance will increase linearly with speed. This is essentially a way to replicate the friction-based resistance from indoor cycles that use brake pads or a belt to create friction against the flywheel. On those bikes, the tighter you turn the resistance knob, the more friction is applied to the flywheel. On a smart trainer, this mode is often represented as a percentage, from 0 to 100 percent resistance. Because this is least similar to the way cyclists experience changes in resistance during outdoor cycling, this mode is not used very much on smart trainers.
When you use erg (ergometer) mode, the trainer’s resistance is controlled by an external source, either an app or a cycling computer. Let’s say your workout for the day calls for three 20-minute efforts at 250 watts, separated by 10 minutes of easy spinning at 125 watts. Using erg mode, the trainer will try to keep the resistance at 250 watts during the intervals, no matter the cadence or gearing you are using. And when an interval is over, the trainer will automatically lower the tension and set your target power to 125 watts.
Some apps and cycling computers also allow you to adjust the target power manually, meaning you can notch your target power up and down incrementally whenever you want. This can be useful if want to use erg mode but don’t have access to a prebuilt workout. However, it is more convenient for prolonged intervals and moderate intensities because you have the time to make adjustments. It is difficult to accurately and quickly change power output from 150 to 400 watts for a 60-second interval, and then it’s even harder to release the tension at the end of the interval when your legs are screaming and you’re cross-eyed and panting uncontrollably.
Shifting gears doesn’t do you much good when you are in erg mode, because the trainer will quickly react to keep the resistance constant. The one factor you truly have control over in erg mode is your cadence. Mathematically speaking, power output is the product of force and angular velocity (cadence), which means there is an inverse relationship between force and cadence. If you begin riding an interval at a cadence of 90 rpm and increase it to 100 rpm, to keep the power output at 250 watts, the trainer will reduce the resistance so the force you have to apply to the pedals decreases. On the other hand, if you drop your cadence from 90 to 70 rpm, the trainer ramps up the resistance because you need to use more force to produce 250 watts at a slower cadence.
When to use erg mode
From a training perspective, erg mode is very useful for executing structured interval workouts. Using online training tools, cyclists can build and download workout files that define exactly when and how much the target power output should change. You can use structured workouts with or without erg mode. When you use them without erg mode, you will get a notification that it is time to increase or decrease your power output, but it will be up to you to respond. When you combine a structured workout with erg mode, you essentially hit “start” at the beginning of the workout and let the smart trainer control the changes in resistance automatically.
Erg mode offers athletes and coaches an unprecedented level of accuracy because the duration and intensity of intervals and recovery periods are preset. You can’t quit an interval 30 seconds early or take an extra minute of recovery between hard efforts. This lets an athlete focus on producing the power rather than keeping track of the interval count.
High-intensity interval workouts are perhaps the best application for erg mode because they involve a high number of short intervals separated by short recovery periods. During repeated hard efforts without erg mode, athletes lose track of what interval they’re on and have trouble watching a timer. When I want an athlete to dig really deep, the safest and most effective environment for those workouts is indoors with erg mode. In the last 30 seconds of a 3-minute maximum effort, a rider can put their head down, close their eyes, and completely drain themselves to the point that they slump over the bars and gasp for breath when it’s over. For obvious reasons, that would be dangerous and irresponsible on the road, track, or trail.
When erg mode defeats you
Erg mode is an unforgiving taskmaster. If you are in the middle of a particularly challenging workout, you might start struggling to maintain your target power output, and your cadence will drop. The trainer doesn’t know or care that you are struggling. All it knows is that when cadence drops, resistance must increase in order to keep the target power output steady. So the trainer clamps down harder, making it even more difficult to maintain your cadence. This death spiral continues until the resistance becomes so great that your feet grind to a halt.
Smart trainers and apps handle the death spiral differently, but they are programmed to recognize that you’re stuck and then release the tension so you can get your cadence back up to speed. Many of the apps that support structured workouts will release the tension, let you get your cadence back up, and give you an automatic countdown before resetting to the target power for the interval.
The disadvantages of erg mode
Erg mode can make you strong, but it won’t make you smart. Excessive reliance on erg mode creates crucial physical and mental skill gaps that can hurt cyclists and triathletes in outdoor competitions. There is a difference between withstanding the effort required to ride at a fixed output of 400 watts for 5 minutes, as set by erg mode, and having the grit and internal motivation to reach and sustain 400 watts for 5 minutes on your own. Similarly, in the last 30 seconds of that 5-minute effort outdoors, you have to keep your head up and maintain control of your bike. On the other end of the spectrum, long-course triathletes and ultra-endurance cyclists have to develop the mental fortitude to maintain their goal pace after several hours on the bike.
The accuracy that makes erg mode so effective for interval training also makes it completely unrealistic compared to cycling outdoors or even with other modes of indoor cycling. Maintaining a fairly steady power output on the road requires constant adjustments to cadence, force, and speed based on undulations in the road and changes in wind and rolling resistance. Learning to manage these variables is essential for success outdoors.
It would seem logical that time trial and long-distance triathlon competitors would go fastest by completing the entire distance at their maximum sustainable power output. In other words, if an elite cyclist can ride at 350 watts for 60 minutes on an ergometer, wouldn’t it be fastest to ride at 350 watts from start to finish during a time trial outdoors? No. The fastest way to complete a real-world course is to use more power when it will provide the greatest advantage and use as little power as possible when it makes the least difference. In practice, this might mean charging up short hills at 600 watts and coasting in an aerodynamic tuck once the speed reaches 40 mph on a downhill.
The mode that enabled the explosive growth in indoor cycling is route, or simulation, mode. This mode leverages the electromagnet to seamlessly adjust the resistance based on physical parameters of the rider and the real or virtual route being ridden, including rider weight, cycling position, bike weight, steepness of the terrain, wind speed and direction, and rolling resistance. Typically, you can just input your weight and use presets for the other parameters, but in some systems, you can tweak the details to see how those changes could potentially change your pace outdoors.
“Sim” mode is what makes popular route-based apps like Zwift and FulGaz work, as well as virtual group rides and e-racing. With the route data and your weight, the trainer can adjust the resistance to simulate what you would feel if you were actually on a specific course. And based on your power output, the system determines your speed and position on the course relative to other riders.
When to use sim mode
Simulation mode is useful for previewing a real course you will later see in competition. For instance, the Ironman World Championship bike course in Hawaii was one of the first routes created when CompuTrainer was the only smart trainer on the market. Most competitors are only in Hawaii for, at most, a couple of weeks before the race, and many first-time Ironman Kona competitors see the course for the first time the week of the race. Smart trainers gave athletes the opportunity to study the route and plan a pacing strategy based on the terrain. By adjusting variables like bike weight, aerodynamic drag, and wind direction and speed, athletes could also see how equipment changes could affect their speed and how headwinds or crosswinds could impact their pace.
Adapted from Ride Inside by Joe Friel with permission of VeloPress.