Which is better for training, cadence or speed?
With the sufferfest both are displayed does it make sense to use both while training?
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Cadence is useful to know if you want to optimize you technique and produce the most speed for effort. If you plot you speed vs cadence on a steady state ride, you can find you optimal cadence. A trained rider will normally rider around 80-90rpm - a novice needs time to perfect the technique to spin this fast efficiently. For a novice a a cadence meter can make a lot of sense.
In a given gear, cadence can be a proxy for speed (and vs versa), so only one is needed. Both take a mental load off the rider (What gear am I in, how fast am I riding, is my cadence about right....).
However for general fitness, which it appears you are aiming for, a heart rate monitor is more useful than either of the speed or cadence. For many years, before the advent of power meters, heart rate was the gold standard for training. Monitor are cheap and effective as a way to control training sessions and measuring progress.
The ultimate is a power meter, however power meters cost a lot of money, and to make maximum use of the investment require quite a deep understanding of exercise psychology and physiology as well as a good understanding of cycling. The target market is more the competitive club and pro riders than the weekend warrior and fitness enthusiasts.
I'm not very familiar with indoor training programs like Sufferfest. However, in general, if you're doing structured workouts indoors, it's better to target your efforts using power, heart rate, or perceived exertion. The latter is the least optimal, but it can be enough if you're just looking to improve fitness. You'd need some sort of workout plan that supports targeting effort via perceived exertion.
A rough example of a structured workout might be warm up, then ride at 70-80% of functional threshold power (FTP) for 3 minutes, then ride at 100-105% of FTP for 2 minutes, then repeat several times, then cool down. Again, you can target this by heart rate zones.
While outdoors, for any given power output, your actual speed can vary greatly depending on the wind, the road's gradient, and other factors. If you want to do structured training outdoors, you would want to target a certain level of effort through power output, heart rate, or perceived exertion. Briefly, power output is how much power you are putting into your cranks or rear hub, and it's probably the optimal metric to target. This is usually measured through strain gauges attached to your crank arm, in your pedal spindles, or in the hub, or through a smart trainer that has its own strain gauges in the resistance unit. All these are expensive.
Your heart rate is an indicator of how hard your heart is working. It's definitely usable for structured workouts. It does lag your effort, i.e. when you start an interval, your heart needs several seconds to catch up, whereas your power output would respond instantly. Perceived exertion is subjective, and probably less accurate for targeting purposes than heart rate.
Importantly, I just remembered that Trainer Road can estimate your power from a) your speed, and b) the trainer's power curve. That is, Trainer Road believes they know the amount of resistance many trainer models provide at any given speed. Thus, you could just get a compatible trainer and a speed sensor, and do structured workouts that way. Trainer Road's site lists compatible trainers, and it describes what you might do for trainers that aren't listed as compatible.
Power estimated this way won't be as accurate as power measured from a dedicated meter. However, you merely need a consistent estimate of power to do structured workouts.
I've observed that entry-level cyclists tend to prefer lower cadences. More experienced cyclists prefer higher ones. Performance-oriented cyclists may benefit from learning to pedal hard at 90-100 revolutions per minute or higher.
Strictly speaking, I don't think cadence is required if you're mainly looking to work out indoors and you don't need to optimize cycling-specific fitness. If you opt for virtual power as described above, a basic bike computer and speed sensor will do.
If you want it, you will need to get a separate cadence sensor. I haven't investigated Trainer Road's hardware requirements, but I suspect they will need a speed sensor that transmits through Bluetooth to connect to your tablet. If you added a cadence sensor, it would probably need to meet the same requirement. If you want to take your bike outdoors, you'd need a computer unit capable of receiving through Bluetooth (and/or ANT+, a different wireless protocol). Companies like Garmin and Wahoo make these sensors. I believe some bike companies (eg. Trek/Bontrager, Specialized) may make them also.
Many bike computers are wired to speed and possibly cadence sensors, or use a non-Bluetooth wireless protocol. I don't think Trainer Road would work with them. You need to make sure your hardware is compatible to take this option.
The only reason I'm focusing on structured training is because you said you were training indoors. I would normally recommend riding outdoors. Structured training there is unnecessary unless you want to maximize cycling-specific fitness. Moreover, if you were still doing Crossfit or another strenuous structured workout program, you would be better off using cycling for active rest.
In theory, you can also do unstructured indoor workouts, i.e. just ride the trainer, don't target any specific effort levels. After a while, this can get terribly boring. I used to race, but I never managed to get more than an hour on the trainer in the winter. People had (and still have) trainer parties at someone's house, which broke up the tedium, but it still wasn't that fun.
For indoor cycling, I think most people will benefit psychologically from some sort of training structure and/or some sort of group interaction. That is, regardlesss of cycling-specific fitness, I think most people will need something to keep them engaged in indoor cycling, since you would lack the stimulation of the outdoors. Thus, you can definitely consider Trainer Road, or you can check if other cycling programs will estimate virtual power. You could also consider a membership to an indoor cycling studio as an alternative - cost permitting, naturally.