Kurt Kinetic computer

Ergoman

I've got thousands of miles on a KK, and I've used it with the KK "power meter", an Ergomo, a Polar CS600 and a PT. I've run comparisons among the power meters and temp tests and clamping pressure tests on the KK.

Bottom line: With the exception of the KK "power meter", all the real meters are relatively consistent to within a few percent across the board. What they have conclusively shown me is that the KK is a fine exercise device but that the power that it takes to turn it varies considerably with setup and temperature.

As a typical example at a room temperature of 68 degrees F and a ball park correct clamping pressure, when cold at an indicated 16mph, the KK requires a real power of about 175 watts to turn it. When warmed up, at the same speed it takes more like 155. At 19mph cold, power required is about 265 watts, warm about 230. Again, all this assumes accurate clamping pressure. Clamping pressure can cause the results to vary in the range of about 30 watts.

With too much clamping pressure and a cold KK, power could be off as much as 75 watts at 19mph, which would be an error of over 30%. I was being generous when I said 20%.

phantoj

The calculated power from Kurt's equations is 231 W at 19mph.

Extra power to turn the cold resistance unit would explain why the beginning part of my workouts hurt so much.

kant314

thanks for that Ergoman,

that's pretty damning stuff!
For what it's worth, I bought I Trek rear wheel cycle computer the other day.
It's a Trek Incite 11i
http://www.evanscycles.com/product.jsp?style=12359
Its really nicely made and the difference between this and the KK computer is night and day.
However, one thing I've noticed is that with the exact same wheel calibration size put into the computer, the KK gives a higher speed read out compared to the Trek computer.
Up to about 10mph they are pretty similar, but when you get to around 15mph the KK comp is about 0.3 mph faster than the Trek which is about 7 watts or so at that speed.
However one you get to over 20mph the discrepancy is over 10watts.
Now I don't know which computer is right, if any of them are, but just going by looks and build quality alone, I would plump for the Trek.
But who knows.

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daveryanwyoming

It could, as could the very normal warming up many folks find they need when doing long SST/L4 intervals during training. Even after a moderate warmup a lot of folks(myself included) find it's a good idea to ramp into L4 efforts a bit below target power and then ramp the power up after five to ten minutes. I get my best 20 and 30 minute power numbers using this approach even after warming up. That approach works best whether I'm riding indoors or out and when I try to hit my target power right from the start I tend to hurt a lot more in the early going until my legs have opened up.

That's different than a full out time trial where I'll get forty to fifty minutes of warmup including some short L4 and L5 efforts prior to lining up for the start. In that case I'll go out very close to my target power but still try to err on the low side.

YMMV,
-Dave

frenchyge

Really? Don't most people spin an easy gear for a few minutes at the start of a trainer ride before really worrying about their power output anyway?

My typical 10-min warmup (~60% FTP) with a couple high-cadence spin ups is more than enough to get the KK trainer warmed up and into the 'accurate* range' for the computer.

* assuming you've taken some measures to ensure consistent rolling resistance.


FWIW, all those damning problems that Ergoman mentioned will still be issues with your new computer.

wiredued

Polar power meters on indoor trainers have issues and Ergomos have issues indoors and out. I believe the Powertap results but does any one actually workout cold without a warm up? If coast down is 13.4 seconds from 20mph and the KKR is warmed up isn't it fairly accurate?

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phantoj

The biggest issues I see with using power from a KK are (in decreasing order of importance):

1. There is a published calibration curve for a KK trainer. But you have no way of knowing how closely your KK trainer follows this calibration curve, unless you test it yourself. How do you know your trainer didn't get overfilled by 10% or it ended up with a slightly less efficient impeller shape due to manufacturing tolerances? Self-calibration is not a bad idea if you know someone with a power meter.

2. Rolling resistance issues, primarily clamping force against the rear wheel, but tire selection and pressure also probably affect things.

3. Temperature effects are unknown. How long does the KK take to warm up? Once warmed up, it it stable, or will resistance drift downwards during an extended effort (sure doesn't feel like it does). Does resistance rise again between intervals?

frenchyge

IMO, the fact that they actually publish an equation, rather than a vague power chart (like other manufacturers) actually gives me more comfort that quality assurance steps are taken to promote consistency between units. Plus, the fact that they tuned their resistance unit design to match the power generated by a road model would be pretty much a waste of time if they didn't then try to make sure individual units conformed to the design.

Edit: actually, in reading the website, it looks like they pulled the equation from empirical test data from a group of production units, rather than trying to manufacture units to a design equation -- even better.

Yep. Not a KK issue, however, and completely user-controlled. It's a valid criticism only because of the extra steps that KK has undergone to make their units consistent, and the additional user expectations associated with having a power equation (vs. chart). It's understandable, though, since they do set the expectation that their power computer will be usably accurate. I'd move this one to #1 if users aren't taking steps to ensure consistency.

Believe I covered that one in post #27, above.

Edit: if anyone is getting the feeling that maybe this has been discussed before, you might check out this thread: http://www.cyclingforums.com/showthread.php?t=380967

kant314

Yes , I know.
I only mentioned it to add weight behind the doubts surounding the veracity of the KK power data.

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frenchyge

Ok, but I think the doubt surrounds the method of using rear wheel speed to *calculate* power in general. That's why I'm not sure why you would abandon the flimsy KK computer for a beefier model which uses the same dubious methodology.

phantoj

Thanks, I missed that.

kant314

Simply because I'm on my third KK computer since November and all have broken and proven completely unreliable.
At least with a better computer I can get a consistent idea of how much I'm improving, even though I don't know the exact numbers in terms of wattage.
What's the alternative? Buy/rent a powertap? I don't really want to buy a cycle accesory that's worth more than my bike. .

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phantoj

For those who have KK Power Computers, how does it calculate the "average wattage"? Is it really just taking your average speed and calculating the power required to hold that speed? That would be a weak method that would underestimate average power.

Or does it track total energy expenditure and divide that by total time?

Yojimbo_

I am reposting the information I provided earlier, since judging from the discussion in this thread it looks like no-one has bothered to read it.

http://www.planet-x-bikes.com/triat...age&PAGE_id=189

This link is a comparison between the KK computer output in Watts and a PT SL hub. The guy describes his methodology, presents results, and discusses uncertainties. The comparison looks damn good to me, and there is no way it's off 20 or 30 percent.

Don't you people warm up anyways before you start intervals?