Tacx Flux S Review

Every trainer is a set of decisions about where the money goes. The Tacx Flux S made an unusual one: it spent it on the flywheel. At 6.7 kg the physical flywheel was heavier than several trainers costing considerably more, and Tacx simulated around 25 kg of inertia on top of it. What got cut was everything downstream of the mechanics — a ±3% accuracy claim, a 1,500 W ceiling and a 10% simulated gradient limit.

That was a defensible trade when the Flux S was current, because the alternative at the money was a wheel-on trainer, and losing the tire is worth more than any accuracy figure. It is a harder trade to defend now, because current entry-level direct-drive trainers have moved the electronics on considerably while the used Flux S has become very cheap.

±3% is the number that decides this

Three percent is about seven and a half watts at 250, and it is the loosest published claim in any trainer we would still recommend. Whether it disqualifies the Flux S depends entirely on one question: does your number ever have to be compared with someone else's?

If the answer is no — if you train alone, follow a plan built on an FTP tested on this trainer, and judge progress against your own history — then ±3% costs you nothing. Structured training runs on repeatability, not absolute truth. A trainer that reads consistently high still grades your intervals correctly against each other and still tells you honestly whether you are getting fitter. Our accuracy guide is the full version of that argument and it is the most useful thing an indoor rider can internalize.

If the answer is yes — Zwift racing against riders on ±1% hardware, a coach's plan built around outdoor power meter data, comparing indoor and outdoor sessions — then ±3% is a real handicap and no amount of flywheel mass compensates for it.

The 10% gradient ceiling is the specification that actually bites

We usually argue that gradient ceilings are marketing, because 20% and 25% are numbers almost nobody meets. Ten percent is different. Ten percent is a gradient that appears regularly on Zwift routes, and if you ride with trainer difficulty at 100% you will meet it.

What happens then is not a failure. The trainer simply stops getting harder, so a virtual 14% ramp feels like a 10% one — the climb flattens out rather than throwing an error. Many riders never notice, because trainer difficulty is commonly set at 50%, which halves the simulated gradient and keeps you inside the limit on almost everything.

But it does mean the Flux S is a poor choice if climbing is why you ride indoors, and it is one of the reasons the cheaper, newer Elite Rivo and its 18% ceiling reads as the more modern product despite a lighter flywheel.

What the 6.7 kg flywheel gets right

Flywheel mass is what carries your momentum through the dead spots at the top and bottom of each pedal stroke, and it is the specification most riders actually feel from the saddle. At 6.7 kg the Flux S is heavier than the Kickr Core 2's 5.4 kg and substantially heavier than the 4 kg Elite Rivo, and Tacx layered simulated inertia on top of the physical mass.

The result is that a Flux S rides better than its price and its accuracy figure suggest. It is settled at low cadence, it does not feel stop-start when you push a big gear, and it is quiet — as every direct-drive trainer is quiet, because the tire is out of the equation and what remains is your own drivetrain. If someone offers you one cheaply, the ride is not the reason to say no.

Buying one used: what to check

Freewheel body first. Direct-drive trainers use a specific freehub standard, and a Flux S set up for a Shimano 11-speed cassette will not accept a SRAM XDR or a Campagnolo cassette without the right body. Establish what is fitted before money changes hands, and read our cassette guide on matching the cassette to your chain — an 11-speed chain on a 12-speed cassette shifts badly and wears fast.

Then listen to it under load. A dry or failing bearing is audible and it is the main way these trainers die. Run a spindown calibration if you can — the Flux S needs calibrating manually and a unit that will not complete one is telling you something. Finally, confirm the power supply is present; replacements exist but are an irritation.

Who should buy it

Buy a used Flux S if the budget is the binding constraint and you want direct drive. Nothing else at used-market money gives you that flywheel, and the jump from a wheel-on trainer to any direct-drive unit is the largest single upgrade available in indoor cycling — our direct drive vs wheel-on comparison puts that in perspective.

Buy new instead if the price gap is small. WiFi alone is worth a lot — it removes the most common cause of indoor training frustration — and an 18% or 16% ceiling removes the Flux S's one genuine functional limit. The budget smart trainer ranking covers the current field, and if you are still deciding between tiers, our cost guide lays out what each price band actually buys.