Check out the blog posts on TrainerRoad.com. They go through this a far bit.

If I'm doing sweet spot or threshold work for an hour indoors, then I definitely think that it's worth at least 1.5 hours outdoors, unless I'm able to hold my target wattage for an hour outdoors without any interruption.

As a roadie, I had to train about double the hours indoors for the same results.

I'd say that makes sense, but is probably the inverse of what's true for triathlon.

Road racing is long periods of minimal effort punctuated by brief - and extremely critical - periods of maximal and supra-maximal effort. Road racing is about making breaks, sprinting, climbs, etc. It's highly stochastic. I cannot imagine a road race with a VI of less than like 1.25.

Triathlon, on the other hand, is about minimizing that sort of variance. Triathlon bike legs should have VIs of less than 1.05, something that is essentially unheard of in road racing.

Triathlon training is especially suited to that monotonous, consistent, middle-zone intensity that is largely absent from road racing. This is a big part of why big group roadie rides are not particularly useful to triathletes. They aren't very specific to the demands of triathlon. Likewise, I can also see how this would make trainer riding less useful to a roadie, because the most specific demands of road racing are things that can only be practiced with a large group where tactical decisions need to be made all the time.

I've never been a big fan of any of the equivalency rules of thumb - like 1hr on the trainer = 90min outside. I think you can use any one of several various training load metrics - kj, TSS, TRIMPS, etc - to calculate equivalency. What's clear is that you can compact workouts down on the trainer because getting to a suitable stretch of road for a given workout is not an issue. And terrain is not an issue. If you have a very specific workout, it's inevitably easier to do it inside than outside. And, in that sense, you can usually make more of a given block of time, simply because you don't need to worry about cars or stoplights or uphills or downhills or anything else. So you can get the same amount of work done in an hour on the trainer that would normally take you 90min outside, but that doesn't necessarily mean that math always applies. It's simply true that it can be true.

---

"Non est ad astra mollis e terris via." - Seneca | rappstar.com | FB - Rappstar Racing | IG - @jordanrapp