I've used a turbo that way before, on an old bike where the gears were so knackered I couldn't change gear any more and just changed the resistance on the turbo. It was OK like that - this is a Tacx Excel (I think, it's an old model) where I can change the resistance using a control at the bars, as FB says. If you can change either the front or back then so much the better.
Using an old bike like that is a good way of making the most of it without having to bother repairing it. This old one of mine had no brakes, no working gears, a front wheel so out of true it wouldn't clear the forks anyway and frankly would have been a deathtrap on the roads. Still good for lots of turbo miles though.
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