How accurate is this?

If your interest is in doing high-intensity efforts, I think there is a protocol that will get you in the ballpark for your efforts. Each part of the protocol is relatively important.

1. Warm up the trainer ~10mins before beginning any high-intensity effort. Reason: as the trainer heats up, you will get "drift" of the power/speed relationship.
   
2. Use the same setup each ride (trainer resistance, tire, tire pressure).
   
3. For each duration(s) you want to do your high-intensity efforts (e.g., 3min, 5min, 20min), find by trial and error your maximum sustainable speed for that duration.
   
4. Ride your high-intensity efforts at 90% of the maximum sustainable speed identified in #3 (e.g., if your max speed for 5mins is 25mph, ride your efforts at 22.5mph).
   

This protocol will enable you to ride your high-intensity efforts at an appropriate intensity because it assumes a linear power/speed relationship only within a narrow range (100% to 90%). What it will not do is give you an accurate estimate of watts. For that, you may need to get a friend with a PT or SRM to ride your trainer at your speeds with the same setup. But, what's more important, (1) riding your high-intensity efforts at an appropriate power or (2) knowing what power you are riding your high-intensity efforts at? I'd say (1) trumps (2).

If you live in a hilly area you can use google earth maps to get elevations distance and then you can calculate rise/run. Then you can go to http://www.analyticcycling.com/ForcesPower_Page.html enter all the info including weight and average speed to get watts. I have a hill that gives me my 15 minute power close by if I want 20 minute power I can take my mountain bike with full paniers. If I want 30 minute power I ride up a Mt auto road.

Thanks guys, RD you make a good point. Just becasue I know my power output doesnt mean that it is going to make me more powerful.

RapDaddyo said:

This protocol will enable you to ride your high-intensity efforts at an appropriate intensity because it assumes a linear power/speed relationship only within a narrow range (100% to 90%).

Actually, you're assuming that the resistance of the trainer is constant over that range, which would mean power increases with speed alone (ie, linearly). That may be true for some magnetic trainers, if the magnetic resistance is set high enough that rolling resistance is ignored, but for a wind or fluid trainer it definitely won't be.

I'd say ~95% of max sustainable speed would be a more useful target, but that really depends on the type of trainer one is using.

frenchyge said:

Actually, you're assuming that the resistance of the trainer is constant over that range, which would mean power increases with speed alone (ie, linearly). That may be true for some magnetic trainers, if the magnetic resistance is set high enough that rolling resistance is ignored, but for a wind or fluid trainer it definitely won't be.

I'd say ~95% of max sustainable speed would be a more useful target, but that really depends on the type of trainer one is using.

Actually, I was basing it on the power/speed curves in this study geocities.comtrainerpowerOpen ↗. While most of the curves are non-linear, they are sufficiently linear within a narrow range to use a simple algorithm. You're right, though, that 91-95% might be a better benchmark to attain 90% of the power.

RapDaddyo said:

Actually, I was basing it on the power/speed curves in this study geocities.comtrainerpowerOpen ↗. While most of the curves are non-linear, they are sufficiently linear within a narrow range to use a simple algorithm. You're right, though, that 91-95% might be a better benchmark to attain 90% of the power.

They may look relatively 'straight' within the range of a couple points, but that's not the same as being linear in the sense you're using it. Since you're referencing your speeds back to zero, the graph would have to be linear back to zero in order to use it as you're intending. Since the slopes of most of those curves are much steeper at the high end than a linear curve of power vs. speed would be, that's just not the same thing.

Even the bold red line looks pretty straight in a narrow range, but it's so steep at the high end that you get ~150w increase with less than a 10% increase in speed. That 150w increase is from a ~500w starting point, so that represents about a 30% increase in power at that point.

I have one of these trainers, so I did a test ride today after my road ride to get you some data. I intended to get data out to 400W, but I blew a tire after about 25mins and didn't feel like warming up the trainer again. I'll try to finish the test in the next few days. I used the middle resistance setting, which is what I use because it gives me the broadest range of power targets without changing the resistance. The numbers below are watts, speed (mph) and watts/speed. I warmed up the unit for 10 mins at 150W first. All the numbers represent 1min constant power efforts.
Watts Mph Watts/Mph
150 7.6 19.74
164 8.2 20.00
176 8.6 20.47
190 9.3 20.43
205 9.8 20.92
214 10.1 21.19
237 10.9 21.74
It looks as though you could add ~2 basis points to get your power targets based on speed. For example, if your max power for a given duration is 11mph and you want to ride your intervals at 90% of your max power, you could ride them at ~.92x11mph=10.1mph. This relationship may change >250W. I'll let you know. Hope this helps.

P.S., Tire was Maxxis Detonator at 120psi

Thanks for the efforts RD I appreciate it. So in the middle (with your setup) you are getting 150watts at 19.74 mph. Do I have that right , what is the middle number .... I feel dumb for asking may have a AHHH moment once you explain it thanks again

Middle is mph, right is 'watts per mph'.

RapDaddyo said:

P.S., Tire was Maxxis Detonator at 120psi

That's probably why it blew. Kinda funny name for a tire. 🙂

Thanks for the clarification. I knew I was missing something simple (like the heading for the columns) I am pretty new to cycling. I have recently switched sports from ice hockey so I am used to using my legs. I have been spinning 3 min intervals on the middle resistance at around 23 mph and it is killing me. I was thinking I could never sustain this for 60 min...

so at around 10.9 mph you are producing 237 watts with the settings you are ridding and the trainer on the middle resistance RD? I may still be mixed up feel free to correct me

djk202020 said:

Thanks for the clarification. I knew I was missing something simple (like the heading for the columns) I am pretty new to cycling. I have recently switched sports from ice hockey so I am used to using my legs. I have been spinning 3 min intervals on the middle resistance at around 23 mph and it is killing me. I was thinking I could never sustain this for 60 min...

so at around 10.9 mph you are producing 237 watts with the settings you are ridding and the trainer on the middle resistance RD? I may still be mixed up feel free to correct me

I think you have it right. My unit is a couple of years old and perhaps they have changed the design since I bought it. My unit has a resistance setting device which consists of a red plastic lever and a black bracket that is intended to be mounted on the handlebar. I never change the resistance so I just leave mine on the ground. There are 7 resistance settings and I use the middle one (3 from either end). With this setting and with my tire at 120psi I get 237W at 10.9mph. The speeds are low because I use a fairly high resistance setting. This is so that I am in my small ring and a small cog at 150W and cadence of 90rpm. I don't have any need to ride at less than 150W and I want to be able to ride at many power levels without changing the resistance. Note that the watts/mph relationship appears to be increasing as power goes up. I don't know if that relationship will hold at higher power. I'll complete the test after my ride tomorrow and post the results.

frenchyge said:

That's probably why it blew. Kinda funny name for a tire. 🙂

Yeah, I've sort of had it with those tires. I like Maxxis but I have gone back to my old faithful Columbierre. Better name.😄

djk202020 said:

so at around 10.9 mph you are producing 237 watts with the settings you are ridding and the trainer on the middle resistance RD? I may still be mixed up feel free to correct me

One more point. I think the most important number is the 92% number. IOW, once you determine a max speed for a particular duration, I think you can ride your intervals at 92% of that speed and you'll be pretty much on the mark. It's likely that my watts/mph relationship may not be accurate for your trainer/tire, but it won't matter if the 92% relationship holds.

I finished the testing, but with a different rear tire (Maxxis Columbiere @ 120psi). I re-did the 237W test to see if there is a difference. There is! Note that the numbers from 237W > are with the new tire. That's the problem with trying to extrapolate trainer resistance/speed data to power.

Watts Mph Watts/Mph
150 7.6 19.74 (Maxxis Detonator)
164 8.2 20.00
176 8.6 20.47
190 9.3 20.43
205 9.8 20.92
214 10.1 21.19
237 10.9 21.74
237 11.1 21.35 (Maxxis Columbiere)
247 11.5 21.48
262 12.1 21.65
279 12.8 21.80
292 13.3 21.95
317 14.2 22.32
354 15.5 22.84
392 16.9 23.20
435 18.0 24.17

Interestingly, I think the 92% of max sustainable speed multiplier works throughout the range.

Which trainer are you using? The only one on the graph that's even remotely close to 435w at 18mph (29kph) is the Nashbar Mag 2 (Minoura). If that's the one you're talking about, then I'd agree that assuming a linear power vs. speed is probably okay. 😉

frenchyge said:

Which trainer are you using? The only one on the graph that's even remotely close to 435w at 18mph (29kph) is the Nashbar Mag 2 (Minoura). If that's the one you're talking about, then I'd agree that assuming a linear power vs. speed is probably okay. 😉

It's not on the chart. It's the Tacx Swing, the one the OP has. It has a variable resistance setting with 7 settings, so there would need to be 7 curves on the chart just for this one trainer. Isn't it interesting how much difference the tire made?

Yes RD that is interesting how much difference the tire made. I am running bontrager race x lite at 120 psi. Thanks again for the effort. At least I have an idea of where I am producing. I will do some max speed efforts today and determine my 92% efforts.

RD ... my speed for 60 min was 12.6... I have been riding for about 7 months is this a respectable starting point?