I am thinking about buying a smart trainer in order to be able to get a good workout while my kid is asleep. However, I am curious about how they work. I know that there are things such as zwift and trainer road, but can the trainer be used in smart mode without paying for these services? Are there free interval workouts? I am specifically thinking about the kickr snap if that matters.

Yes, the options that I know of would include a Wahoo Elemnt which will FOR FREE let you load in routes and it will control the trainer to simulate them. The Wahoo Elemnt and most Garmin head units will also allow you to simply +/- the resistance and simulated gradient of your smart trainer. As for computer options, you can use Golden Cheetah to both import and send workouts to your smart trainer (think intervals and such) and create you own to control it for you. Although that software, while free, can be a bit tricky.

However, in the end, you'll probably end up with a Zwift subscription like the rest of us

And there is a free basic Wahoo app.

But just pay the few bucks a month, annoying as it is. The non-subscription options are all flawed in various ways for most people. Actually the subscription ones are flawed too, but in different ways.

There are at least two free apps to run structured workouts on an ANT+ smart trainer.

GoldenCheetah is free and open source software that will control your smart trainer to do any workout you'd like. It's available for PC, Mac and Linux via free download. (As mentioned by firebird.)

Indoor trainer features include support for:
Hurts Ergo will control your ANT+ smart trainer from an iPhone or iPad. Download or build your own workouts at ErgDB.

Awesome! Thanks for the replies! So, to open another can of worms: I don't have a power meter for my bike. Would it be better to buy a power meter for my bike and a dumb trainer, or a smart trainer but no bike power. I can ride outside year round where I live. The trainer is for getting a workout while my kid is sleeping.

Why not just pay for Zwift of TrainerRoad and get the full benefit of the smart trainer?

What are your goals? A PM + good dumb trainer is better if you care about riding outdoors, want the full benefit of power training (pacing, exertion tracking), and are willing to ride a dumb trainer consistently (which most people don't like). On the other hand, if you don't care how fast you are, don't race, and just want to keep riding outside when you can and indoors when you can't, the smart trainer would probably "feel" better to ride when you have to.

The thing is, when I got TR in the first place, using VirtualPower from my dumb trainer, I suddenly hated riding outside and not getting all the metrics. (Fortunately, the great power meter price drop started about a year later.) Would that be you?

Those are good questions to think about. I think I am more inclined to dumb trainer + PM.

I ended up finding a great deal on a used Kurt Kinetic Road Machine. It seems like they come with a skewer new to replace your normal skewer. Since I would have to buy one of those anyway, would it be better to just use my existing skewer for the trainer and buy some titanium skewers for the bike, or is the trainer skewer important?

Assuming it is the same as with my Kinetic dumb trainer, the skewer is provided in case the one you have doesn't work, due to being the wrong shape or something. No need to buy one if your current one works. Also, mine came with two different interfaces for the wheel clamp. One has a slot, but not the other. The slotted one is the one I always use, since it accommodates my quick release lever.

(I have no idea how it could be made to work with thru-axle.)

There are different caps for thru-axles.

If you use a non-trainer-specific skewer, make sure it's really secure in the trainer. You do not want to find out mid-interval that the trainer doesn't grip it well and fall over. Also, if your current skewer is "nice", you may want to have a dedicated one to take the scratches. It's honestly easier to just use the trainer skewer; it shouldn't even take ten seconds to swap.

Oh and the KKRM is a great trainer. I keep meaning to pull mine out and stick it in front of the work laptop for conference calls.

Quick question: Has anyone with the KKRM used the inride "power" sensor? If so, do you have an opinion on how this compares to virtualpower calculated by TrainerRoad from a speed sensor?