New to Powertap on KK Trainer

[William s]

HI everyone
I know this has been done before but I am struggling to figure it out. I am aware that the power output should be can be a little below normal (outside) when using a trainer.
However compared to the kurtkinetic road machine powercuve my powertap is reading an average of 60watts below.

FTP training has been done at 32kph reading approx 260w, yet at the same speed the PT reading is 200w. 260 is consistant with SS training done outside.

Am I doing something wrong? or do I just need to HTFU!!!

ps yes I have zeroed the PT with garmin edge 500 on every use.

 

[RapDaddyo]

The speed on your KK trainer is a function of press-on force and tire rolling resistance in addition to power output. You can't really tell much by speed alone. Even if you were to calibrate your trainer with your PT with a repeatable press-on force, speed will change with tire wear or if you use a different tire. The power required to turn the wheel at a given RPM is highly sensitive to total resistance. Also, the trainer needs to warm up about 10 minutes before any calibration is possible.

 

[William s]

Thanks for responding
so what you are suggesting is that the power output (reading) from the PT is a function of the force required to turn the wheel and the resistance applied to that wheel.
Does this in turn suggest that the PT use on a trainer is useless? I am confused with this idea at the moment.
I thought the KK road machine was supposed to have a fluid density to recreate the sensation (force) closely to that of riding on the road.
Rappdaddy not being critical in anyway of your explanation I am trying to understand what I should/could do to either
A) get reliable readings which are similar to what I get outside.
B) get some middle ground where I know that x wattage is equal to y wattage outside.

cheers guys

 

[RapDaddyo]

Your PT should be reliable on the KK (or any other trainer) because it is measuring torque and cadence at the hub and this is independent of the source of resistance (e.g., gravity, wind, rolling resistance). The KK trainer (especially the model with the heavier flywheel) does provide a realistic road feel. But, the speed that corresponds to a given power output is a function of the resistances on the trainer (press-on force + tire rolling resistance). If you always ride with your PT wheel, you don't have to worry about it. Just do your rides based on your PT and ignore the speed. If you want to do some rides without your PT, then you will want to calibrate the KK with your PT after you have warmed up the KK. Just be aware that your calibration is valid only so long as you can replicate the press-on force and that you are riding the same make/model of tire with a similar amount of wear. You may be surprised at the emphasis I am putting on the tire, but here's an anecdote. A training buddy came over to use my CompuTrainer for a VO2MAX test. I set my press-on force with a torque wrench, so I am able to set it very precisely. After my friend got warmed up, I began to adjust the press-on force to synch the CT with his PT hub and was astonished to find that his tire required twice the press-on force as my tire. I couldn't believe my eyes, but numbers don't lie.

 

[William s]

Ok Thanks Mate
so the key here is press on force. Increase this until I get realistic numbers (similar) to what I am getting outside. So currently at 60watts below, that indicates there is not sufficent force to generate the numbers required ( I think).

If that is wrong then I need to see rule #5 and get on with it. I just thought that at that big of a difference I am doing something wrong.

 

[RapDaddyo]

No, you have it right. You can experiment with press-on force until your speeds are similar to what you get outside at the same power output. Just be aware that the correlation will work only with the same make/model of tire and same tire pressure. If you change the tire, you will need to re-calibrate the press-on force.

 

[Ade Merckx]

Hi William, I wouldn't really trust that power curve from KK especially given the fact you have a Power tap. Some years back I was convinced I'd ridden for 60mins at 300 watts (KK Christmas Cracker Power Computer). How wrong I was. Now that I have a Powertap 3G I know from gearing, speed and powertap that 60mins was probably only about 260watts. Grr! So as good as the KK trainer is don't trust that power curve.

 

[Bigpikle]

ignore what you think is going on with the KK and just use numbers from your PT. I have both as well and all that matters is what the PT is telling you that you're doing. It takes most people a little while (and a massive BIG fan) to get close to your outdoor numbers indoor. I still cant quite hit the same after doing it for a couple of years.

 

[Alex Simmons]

Originally Posted by RapDaddyo .

Your PT should be reliable on the KK (or any other trainer) because it is measuring torque and cadence at the hub and this is independent of the source of resistance (e.g., gravity, wind, rolling resistance). The KK trainer (especially the model with the heavier flywheel) does provide a realistic road feel. But, the speed that corresponds to a given power output is a function of the resistances on the trainer (press-on force + tire rolling resistance). If you always ride with your PT wheel, you don't have to worry about it. Just do your rides based on your PT and ignore the speed. If you want to do some rides without your PT, then you will want to calibrate the KK with your PT after you have warmed up the KK. Just be aware that your calibration is valid only so long as you can replicate the press-on force and that you are riding the same make/model of tire with a similar amount of wear.

Jeez, setting CT press on with torque wrench! Have you replaced the hand knob with a bolt? How do you do that?

I must admit I'm pretty handy at getting it where I want by feel and giving the wheel a spin by hand to see how it reacts, certainly getting it near enough to sensible (between 2.5 and 3 is what I aim for and then I find my unit is aligned with my SRM). Usually cold I have it ~ 3.2 - 3.4 and once warmed up it comes down to my preferred range.


Quote: Originally Posted by RapDaddyo .

but here's an anecdote. A training buddy came over to use my CompuTrainer for a VO2MAX test. I set my press-on force with a torque wrench, so I am able to set it very precisely. After my friend got warmed up, I began to adjust the press-on force to synch the CT with his PT hub and was astonished to find that his tire required twice the press-on force as my tire. I couldn't believe my eyes, but numbers don't lie.


I've thought about this analogy a little more as I've used the same one myself on this issue, and I think we need to be careful when using the example of the sensitivity of the CT's press on force and how that impacts reported power on a CT (versus actual power), when compared to how press on force might affect the speed-power curve on a fluid trainer like the KK.

The CT being electro-mechanically braked has different issues in it's RRC v power relationship than a non-electro braked (e.g. fluid) trainer would. I suspect that a KK's resistance would be less sensitive (but not insensitive) to press on force difference than a CT is.

It requires further experimentation in my view, but that's an outcome I'm hypothesising.


What I think would be helpful though would be a roll down calibration of some kind for software that uses speed-power curves to give proxy power readings - so that the relationship can be adjusted according to that roll down variance.

 

[RapDaddyo]

Originally Posted by Alex Simmons .

Jeez, setting CT press on with torque wrench! Have you replaced the hand knob with a bolt? How do you do that?

I machined a custom adapter from aluminum and use a standard low-range analog torque wrench. I can send you the specs if you want to have one built down there. Any machine shop could knock it out in an hour. In fact, I can send you a 3D model and you can give it to a CAD-CAM shop and they can knock it out with a few keystrokes.