I'm sure it depends on the specific model rollers, but my
4.5" Alloy Kreitler Rollers with the
Killer Headwind unit provides enough resistance for
my needs. Here are some numbers from a recent interval workout (measured via a PowerTap):
Fan 1/3rd open:
180watts = 16.3mph
325watts = 22.6mph
Fan closed:
180watts = 18.6mph
325watts = 25.4mph
As you can tell from the speeds @ 325 Watts, I still had bigger gears available. Even when doing Level 6 efforts, I have never needed to open the fan more than 1/3rd (opening the fan vent increases resistance). But it's nice to know I have that headroom if I end up making massive improvements at some point!?
The Kreilter's resistance also feels more "road-like" than any trainer I've tried. I think it's the combination of the 4.5" aluminum drums and Headwind fan that gives it the right mix of rotational inertia and resistance. Not to mention the quality of these rollers is topnotch. They're solidly built and very smooth. I've had my set for 13 years! I highly recommend them!
Although I'm still intrigued by the ability to stand and sprint on the
E-Motion Rollers. It's difficult to stand and sprint on "standard" rollers, so if I have to do a Level 7 workout indoors, I'll do them on my Cycle-Ops Fluid2 trainer.