Trainers: Fluid or Mag

I'm looking at getting a trainer because I have next to no time to get on the road and want to start up with Carmichael's Time Crunched Training Program. Part of the program is to ride do high intensity intervals at low RPM. I like the idea of a fluid trainer being the quietest due to training at night with a young one in the home. Also that I don't have to modify the resistance as I'm riding, but my concern is whether I can do high intensity training at low RPM on a fluid trainer. I've read that the resistance increases when the fluid thickens. And that the fluid thickens by the speed of the wheel in the chamber, so I assume that spinning at a high gear would allow for this, but I just want to ensure this is possible before ordering one.

• I haven't looked at the specifics of the TCT high intensity low rpm workout. How high is high intensity, how is it measured, and how low is the rpm? Commented May 17, 2014 at 3:55
• Also, in order to give a good answer to your question, it would be good to know what speed you can maintain when climbing, the slope of that hill, your cadence on that hill, and your total weight. Knowing those things will help to determine whether a fluid (or magnetic) trainer can provide the appropriate load. Commented May 17, 2014 at 4:54
• Good points! I would be ideally riding at, or just below lactate threshold. As for climbing, depends on the slop. 15% grade would be about 12kph (yeah, not that fast) at about 75rpm. In this case, I'm shooting for 60-75 RPM. My regular spinning cadence is 85-95 RPM. I'm about 165" @ 6'1". I haven't weighed the bike, but I suspect it's around 17 lbs
– BenK
Commented May 17, 2014 at 5:01
• OK, thanks. The short answer is you're going to have a hard time finding a fluid trainer that will provide that kind of load at low rpm (and you won't be able to find a magnetic trainer that can provide that load at all). I'll do the calculations and provide a more complete answer tomorrow -- unless someone else wants to do the calculations and write-up and I'll upvote that. Commented May 17, 2014 at 5:35

1 Answer

Using bikecalculator.com, I have a 165lb rider on a 15% grade at 12kph (units, oy!) needing roughly 500W. I'm pretty sure that most decent trainers can generate that, including the SuperMagneto Pro that I use. (My power meter and trainer road have told me that I've hit those numbers, however briefly). Like you've indicated, the progressive resistance is generated by speed, so you need to use an appropriate gear combination (big/small) that lets you pedal slow, but spin the wheel at a greater multiple of pedal stroke.

Power Curves:

RoadMachine: http://www.kurtkinetic.com/powercurve.php

SuperMagneto Pro: http://blog.trainerroad.com/cycleops-supermagneto-pro-added/

• Right, the issue isn't whether there are trainers (either magnetic or fluid) that can create enough load -- the issue is whether they can create enough load at low rpm. "High intensity" usually means "sprint intervals" which are supra-VO2Max. In a sprint interval on the road your cadence is typically up around 120 rpm or higher and you're using a relatively low gear to accelerate up to speed. On a trainer the roller speed will need to be high, so you'll need to use a higher gear. You'll need an even higher gear still to do this at low rpm. Commented May 17, 2014 at 13:40
• Here's an example: I just hopped on my CycleTek fluid trainer with my Power Tap-equipped bike. The trainer can handle 1000 watts but in my highest gear ratio (52x12) at 60 rpm my wheel speed was only ~31 km/h, and the trainer could only produce about 300 watts of resistance at that wheel speed. I would need a higher gear ratio to produce "high intensity" power at low cadence. The alternative, of course, is not to do this at low cadence. Many fluid trainers can produce adequate load if the wheel speed is high enough. That means high power at high cadence. Commented May 17, 2014 at 14:18
• If you incorporate the clarification that a fluid trainer can produce the needed load but at high cadence rather than low, I would upvote your answer. Commented May 17, 2014 at 14:37