Sorry, another Kurt Kinetic Post!

Ade Merckx

All I want to know from those of you with power tap/SRM's is, assuming a fifteen minute warm up and no wheel slippage; and accurate circ measurement are the KK road machine trainers trainers generally within 10% of your PT/SRM readings. I don't mind if they're are 5-10% off at threshold but anymore would be really dissapointing. Cheers

frenchyge

Short answer: Yes.

Longer answer: As someone who rides with a PT, I don't normally look at or think about the KK power curve *while* I'm riding. Recent rides without my PT have made me look into this more closely, as there seemed to be a noticeable discrepancy between the two. A torque test of my PT has led me to believe that the PT is at fault (~7% low), and the KK power curve would have matched within 1-2% the readings with this fault taken into account. I've started going back through my ride files to try to find when my PT may have gone out of calibration, and here's what I've come up with so far:



Disclaimer: this is just a first pass for me, and I'm not offering this up as a scientific test. These were just avg power and avg speed pulled from some intervals to see if I could identify a point in time where a major shift occurred in the PT. Since I ride with a PT, I don't do coastdowns, monitor tire pressure, or take any steps to ensure a consistent *speed* between rides, and that's why there's such variation between the points. As another poster has already pointed out, averaging power over an interval of varying speeds adds it's own issues of non-linearity, but I've not had a chance to go back and adjust for that. At the high-end, those yellow points are ~7-8% below the KK curve, which matched the error on my torque test very closely.

See short answer for less details.

Ade Merckx

Cool, thanks Frenchyge. Yesterday night I suddenly had an attack of the 'Oh No's...what if my 'real' FT is really only 250watts? ...I rode with the fast group, full of very strong 2nd cats, on a training loop today . I got dropped on the final 40 second power climb but drilled it in the VO2max area for five mins and finished 4th out of the 12 riders. . Anyway todays test means all things considered equal with more threshold power and some anaerobic gains who knows I might become quite a good road racer.

frenchyge

Nice job! Leave it to the road to make things clear.

It's pretty easy to tell the difference between an FTP of 250w or 300w if you do some competitive road rides, as those are really 2 different classes of rider (assuming equal weight). If you're tearing people up and finishing in the selected lead group on fast rides, then it's unlikely that your FTP is 250w unless you are pretty small.

Still, if you're really interested in consistent training and testing using rear wheel speed, it's a good idea to take the steps to ensure equal tire pressure and clamping force between workouts.

Yojimbo_

Check out this link for a comparison between the KK "power meter" and a PT SL hub.

Looks pretty good to me.

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

Ergoman

While a properly set up and adjusted KK may be accurate within 10%, the problem is not accuracy, it's consistency. Getting the same clamping pressure, wheel pressure and temperature all the time is a near impossibility, and as a result, KK curves/power meter/speedo readings are going to be wandering all over the place. The data shown in the chart provided by the previous responder indicates that 10% variability with the KK is certainly attainable, even at higher power ranges where the system should be warmed up.

I don't care if my power reading is accurate to within 25% so long as it's consistent workout to workout. 10% is a huge number when it's the amount your power meter has the potential to vary from day to day. For me, working at FTP for an hour is a very, very hard day while 90% FTP is just a good workout and 110% is a heart attack. If your power meter has than kind of variability and you have little control or knowledge of it, you're probably better off just going on perceived exertion or with a heart monitor...or biting the bullet and buying a real power meter.

frenchyge

Don't forget that my PT is also showing ~7% low during static torque tests as well, at least during the timeframe of the yellow data points.

In fact, there's only 1 blue data point thats 10% from the calculated curve, and that's with my taking *no* steps to ensure consistency in tire pressure or clamping pressure between rides.

Edit: Actually, I take that back. I typically follow the guidance in the instruction manual with regards to clamping pressure (typically 2.5-3 turns of the knob once contact is made with the tire, and then a 'squeak' test and possibly another half-turn if needed).

Ergoman, I've read your post about the potential for error from a cold trainer, which frankly I think is of little consequence since I'd expect pretty much every does some kind of warmup before really putting down the power. What did you find is the potential variability from the power equation for different amounts of contact pressure and/or tire pressure at ~19-20mph?

Piotr

Yes, I would say within 10 % of PT. What I find a bit worrisome is that it's not consistent within the same ride, so it's not a question of clamping or tire pressure. My power relative to speed will sometimes drop from one 20 min block to the next, so I'll ride the first at 20.2 mph and the second at 20.5 mph to keep the same power. It's frustrating to see the power in the 240's at 20 mph when it should be 258W. Due to KK's reputation as having a very consistent power curve I always reset the torque on my PT, but it doesn't always help. Hmm... I try not to let that ruin my workout though and just follow the PT.

My advice: use the KK "power" for a solid reference, but don't be surprised if later intervals get a bit easier.

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Ade Merckx

Thanks for the reassurance guys. Frenchyge, just as an experiment I clamped the tyre on so hard yesterday that you really would struggle to even force the tyre to slip. Then I did a full on 100-105% 2 x 20. Result: 320watts and 307watts. Easily the hardest 2 x 20 I've done in four months. In fact it felt like I was riding in 53/15 rather than my normal 53/17

kant314

so are you saying that clamping the tyre harder makes the PE of the session harder or are you saying that it increases the wattage reading ?

thanks,
John

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Ade Merckx

If you want higher wattage readings don't clamp the tyres so hard. I generally check that the tyres won't slip but I'll openly admits its hard to get that consistency especially when the tyres are slighly mosit or a bit grubby

frenchyge

You use the KK computer, rather than a powermeter, right? The harder you tighten it, the more rolling resistance you should have, the slower your wheel should turn for the same rider output, and the lower your KK computer should read (relative to actual rider output). It doesn't make sense that your displayed power would be higher.

FWIW, cleaning the tire and resistance wheel with degreaser will do wonders for reducing slippage without increasing the clamping force, especially if the tire has been ridden outside. Just make sure it dries completely.

kant314

Interesting.
I always clamp mine as hard as it will go. Untill I can't turn the knob any more.
That's probably too hard, I reckon.

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Ade Merckx

Oops my bad Yes I agree 100% with what you said. Yep I only have the KK power computer. Yesterday I just wanted to see what would happen if I clamped the wheel real tight. As I said the PE on the session was up (I reckon my KK FT is now 320watts which is what I tried to hold for both 20's; very difficult). The fact that it was so hard is I feel attributed to the increased rolling resistance as you mentioned. In terms of perceived exertion I had to keep it at 100-105% FT just to see the numbers I got. In most 2/3 x 20 I don't go nearly as hard to see 300watts av. Sorry hope that make sense.