A smart trainer is not at all essential to enjoy using your bike on a trainer with Zwift and would not be my top priority. If you need to keep the cost down I'd probably skip that to start with and prioritise the bike.
- Choose your bike first. If you're only using it on the trainer there's no advantage to light weight, good brakes, lots of gears, aerodynamics, etc. And if you are going with a direct drive trainer like the Kickr you don't even need a back wheel....... However, if you plan to race, you'll need to get outside from time to time and you'll need a bike to race on. I don't know Decathlon's range but in general they're a good place to get good value for money on most things and their bikes look okay. You should be pretty safe with Cartman's advice if you can stretch to that. If you're getting a direct drive trainer you need to know what cassette it will have on it. Most important is the number of sprockets (speeds).
Then you want a way to use Zwift. There are 4 options. I've tried all 4 and spent a lot of time with two of them. The options are:
- Any old wheel-on trainer, a rear wheel speed sensor and Zwift
- Wheel-on trainer with known accurate power curves, a rear wheel speed sensor, and Zwift
- Any trainer, a power meter, and Zwift
- A smart trainer, and Zwift
Zwift has power curve data for a huge number of trainers. This means you can tell Zwift what trainer you are using and it can use the wheel speed from your sensor to tell you a power number. The same applies to many other software training software options like TrainerRoad. In my experience, the estimated figure can be very inaccurate and inconsistent for most trainers, but for a couple it works really well. Option 1:
is the cheapest route into Zwift or other power based indoor cycling, but I think you'll find it frustrating and restrictive because you won't be able to rely on the power figure which dictates everything. Option 2:
can be nearly as cheap as option 1 and would be my recommendation if you can find the right trainer second hand. The only trainer I really trust for this is a Kurt Kinetic Road Machine or Rock'n'Roll fluid trainer. Their fluid resistance unit is very temperature stable which is essential. I used mine with virtual power for 18 months. My data was always very consistent and seemed realistic. Perceived effort, heart rate, and displayed power always tallied (HR will vary a bit with fatigue, illness, heat, etc, but can still be used once you understand that). When I bought a power meter after 18 months, I compared the estimated power from Zwift with my measured power and was amazed to find it was within 1-2% at all times except during sprint accelerations and the very start of warm-up, and even then the error was relatively small (~10%). If you decide to go this route, pick a tyre, tyre pressure, and number of turns of clamping pressure and stick with it. If you change these each session the numbers may change too. I used 100psi in Felt TTR tyres (that came on my bike) and exactly 3 full turns of the clamping knob from first contact. Option 3:
gives you reliable (for most PMs!) and you're not relying on the trainer for power data. It's just providing resistance. Any trainer will do, although I'd recommend a fluid unit because they feel better IMO. As an added benefit, you will also have the PM when you take the bike out on the road. Option 4:
gives you power data from the smart trainer which replaces the need for a PM, although you obviously won't have power data on the road (not necessarily a big deal) and accuracy for some trainers is worse than many PMs (1-2% is good, 5%+ is not). The benefits of smart trainers over a PM and dumb trainer is terrain simulation and ERG mode. Terrain simulation is where the trainer will match the resistance to hills or descents on Zwift. You can do pretty much the same manually by changing gears on a fluid trainer if you like, though of course you'd have to watch the gradient to know when! I don't feel the need for this feature but I can see how it might be enjoyable and add to realism on Zwift. you can train equally well with or without it. ERG mode is where you tell the trainer what power level you want to hold and it forces you to do so by increasing resistance if you pedal slower and vice versa. I don't like ERG mode, and I see little advantage to it. A lot of people use this to control their effort while they watch films meaning they don't have to watch their power figures. I've tried it and don't like it. I far prefer to control my own power. It's more realistic and feels better to me.
It seems like you already have a GPS watch? If so, I'd stick with that for now and upgrade when/if you can afford it or feel it's needed. Just get a speed/cadence sensor that you can pair with it and an ANT+ dongle for your computer so you can send data from your sensors to Zwift (not essential if you decide to get a PM or Smart trainer).
If I were you and budget is tight I'd go with a second hand Kurt Kinetic Road Machine, if you can get one, plus the speed/cadence sensors. If that frees up some money for a better bike, I'd get a better bike, rather than a power meter or smart trainer. Either of those can be added later if you decide you want them.
I've got two very capable bikes and a rake of other equipment. Yet I'm still using a dumb trainer. I do have a good power meter now, but it worked almost as well without that. I considered getting a Tacx Neo last year but decided not to bother for now, because I honestly don't think I'm missing a lot.
I hope this is helpful and not too long and confusing!