dark13star

I am considering getting a Powertap for my personal training. I don't raise, but I love data and self-improvement. Right now I use a Garmin 305 and I would upgrade to a 705. My plan is to use the Powertap with the Garmin and not use the PT head unit at all. However, I was reading the PDF manual last night and I saw stuff on periodically calibrating the PT.

Is it necessary to use the head unit periodically for this? Ideally, I would love to buy the Powertap without a head unit or cardio strap, since I get both with the Garmin, but I don't see them sold this way.

I am interested in anyone's experience using PT and Garmin. Do you need the head unit at all?

Thanks

waterrockets 

You can't calibrate a PT. All you can do is reset the head unit to understand what 0 torque looks like (called "zeroing it"). All power meters will be some % off, and to correct that is setting the "slope." You can't set the slope on a PowerTap.

I do believe the 705 supports zeroing, but we'll need someone who has one to confirm.

dark13star

Zeroing, you are right. I should have re-read it this morning, but that is what I am asking about.

umd

Yes, you can do the zeroing with the 705. And they moved the coasting auto-zero logic from the head to the hub for this purpose.

Dago

I thought you can do a static torque calibration? I do not own a PT but I just had the impression.

waterrockets 

You can measure torque readings with a stomp test, but you can't correct it. My most recent stomp test showed 99.3% of calculated/expected value. You you all watch out, I'm 0.7% stronger than I say...

bdcheung

Does anyone have that spreadsheet that's used to calculate slope?

waterrockets 

https://wattage.googlegroups.com/web/...hw4ZZRj3RjJ_-A

mattm

if there's an auto-zero feature, how often do you need to manually zero it?

i've had mine for only a month but i haven't needed to zero it yet. at least as far as i can tell.

bdcheung

lovely TY

dark13star

Thanks all. Now there is just the question of whether the 705 is getting an update. Garmin didn't show anything at CES, so I think I am likely to buy.

umd

FreddyBoy is convinced there is one that will do your laundry right around the corner. Most rational people believe it is more likely than a newer model with incremental improvements is at least a year away.

Jynx

What's the benefit of using the Garmin 705? Why not just use the powertap head unit?

dark13star

Yeah, I am a Garmin stock holder and have been going over their financials. Seems like they are really focusing on the car crowd and embedded technologies this year, so I am not sure I would count on a huge investment here. If they do update, I agree with your assessment and likely to be Q3 or Q4, if we see anything. That won't do, so I will likely grab one of the deals out there now.

GPS, of course. This is the killer feature. I don't want to go without GPS or use two devices (current 305 and head unit).

jrobe

I haven't needed to zero my Powertap Pro in over 2 years now.

bdcheung

you haven't checked the zero offset in 2 years?!

tanhalt

Actually...if "auto zero" is enabled (and it SHOULD be, if you're smart), I've found that manual zeroing is pretty much not needed.

It's pretty easy to see if your power doesn't register zero when coasting as a "check" of the zero without actually going into the "torque mode" and manually zeroing...

FreddyBoy

I typically find your posts and comments knowledgeable. (especially because you often post with charts, and try to provide empirical evidence to support your opinions)

You saw and responded to the arithmatic mistake I made on the release date but continue to propogate that mistake with nothing more than the intent to denigrate. Save the hate for that steep climb.

Anyway glad I popped in. BTW that Garmin is now OOS but listed for $400+ on PBK :-(

dark13star

I recieved two email alerts from them in the last month. Both times they had about 10 units, but I could have grabbed one if I was sure. Just stay on the email alert if you want one.

umd

No hate dude, it was just a little jab. You didn't seem convinced after that last thread that an update is not immenent. Regardless of your arithmatic mistake regarding the release date, nobody who has purchased a 705 has had it for even a year yet. I of course cannot prove that something is not going to happen, but I haven't heard any chatter about an upcoming unit like I did last time and in the absence of any proof, anything to the contrary is idle speculation. I would also say that there is a higher burden of proof to say that a new unit is forthcoming as opposed to saying one is not likely to be announced, and in the absence of any information it is far more reasonable to assume that one is not coming than is.

But carry on...

waterrockets 

Well, it's a nice pseudo-geek-out feature, but the killer feature (the real geek-out feature) is measurement aero drag and rolling resistance with power. Those two are much more useful than maps, if your goal is to get faster. ibike FTW.

dark13star

So I guess I missed this on BF. There were rumors at CES this year, but I didn't buy them. Garmin's revenue recognition model is to amortize costs over the product's lifespan and, as you mention, it hasn't been out for all that long. Garmin has a lot of core technology that they can amortize across their entire product line, but there is a lot that probably went into the 705 alone, if only firmware development costs. I think they need an 18-24 month cycle for the unit to show appropriate margins.

pharding

How does one measure aero drag and rolling resistance?

umd

The iBike estimates power using external forces, such as measuring windspeed. In order for this to work, one of the things you must do is calibrate it through various procedures. One of these is a coastdown test, which establishes rolling resistance. Normally the iBike would be used to caclulate the power, but if you know the power because you are also using a powertap etc., then you can use it to caculate other variables, such as the aero drag.