The dyno is extremely simple, primitive even. But it works very well, is accurate and repeatable. The only downside is that it doesn't give a direct hp reading, you have to work it out from the rpms and torque reading. I have a spreadsheet set up so that all I have to do is punch those two numbers in and it spits out the hp figure, corrected for the current weather conditions (entered at the start of the session).
The absorbing element is a bloody big ventilated disc brake, 380mm and 15kg, so it's bigger than a Chev flywheel and twice as heavy. It's mounted on a hub that spins on needle bearings around the spindle. The caliper is free to rotate around the spindle too, and is only restrained by a torque arm exactly 12" long that is connected via a cable to the torque gauge. This gauge is just a dial type spring scale, just like those of the older mechanical dynos. If you wanted to get fancy a load cell would work just as well.
I used this style instead of an inertia style because I wanted to be able to run at full load for an extended period, something an inertia dyno can't do. I also didn't want to deal with the inconsistencies that go with a tyre on a roller. The absorbed energy is converted to heat, just like any other dyno. With short runs - say 10 seconds or so - I don't even have to run a blower on the brake, I just let it spin a bit in top gear and the natural circulation of air through the disc is enough. For longer runs and higher power levels I direct an air blower into the "inside" of the disc, and this allows it to run loaded indefinitely. But for normal tuning work, by the time I've changed the jets or timing or whatever it's good to go again. It's never been hot enough to even turn bluish yet, though it's only had to deal with around 60hp. The only thing I might do is put a shroud around the brake so I can direct the hot air outside where it won't affect the engine output.
But like I said, it works very well, cost under 2k and took a couple of half-arsed weekends to build. I'm very happy with it. I'm half tempted to build another bigger version for sixes...
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