Power meters selection - pros/cons

Discuss light weight issues concerning road bikes & parts.
rchung
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by rchung

petepeterson wrote:Keep in mind this is an internet forum and all of these posts are individual opinions and people recommend what they know/like/have. With that in mind I really have to question why people recommend hub based power these days?


In my case, there are two reasons: 1) In order to get the right aero position, my TT bike uses a different crank length than my road bike, so it's easier and cheaper to swap the PT hub than to have two different crank-based power meters; and 2) if you do CdA or Crr testing then a PT is "closer" to the physics (that is, you don't have to worry about drive train losses or distortions from non-round rings).

by Weenie


airwise
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by airwise

Tinea Pedis wrote:airwise, did you miss my post?


Sorry TP. I would personally far rather run a Powertap than pollute my beautiful bike with such monstrosities :wink:

Seriously they would rather stick out like a sore thumb and I do care about the aesthetics (possibly too much!). Maybe a pedal based system will live up to the hype one day. Right now, the main prerequisite for a power meter has to be data reliability and consistency and IME there are very few that qualify.

For me personally, I'm experimenting with relying on HR and VAM and finding it surprisingly effective for the sort of riding I do. The problem is mixing non power based data with Powetap files in GC or the like and ending up with worthwhile long term data.

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djconnel
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by djconnel

How do you adjust the torque tube? Send it back to Saris?

I just crashed 2 days ago, landing hard on my right hip, which fortunately wasn't fractured. I still can't walk (probably strained a muscle) let alone ride yet, but when I can, I'd sure love to see l-r balance. So I'll be a fan if Garmin when its out. They took the time to make sure it actually works, which I appreciate.

But I agree with Robert on reasons to like Powertap. I add 3% for drivetrain, if I care.

Sent from my ADR6300 using Tapatalk 2

airwise
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by airwise

djconnel wrote:How do you adjust the torque tube? Send it back to Saris?


Yes sadly. I'm sure much of this could be done in the field but in the UK, the distributor keeps all work in house - including even servicing the bearings.

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stella-azzurra
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by stella-azzurra

Since I took the power tap SL apart there is really nothing to adjust on the torque tube.
The torque tube sits between the bearings and has a small round magnet glued to it.
Too bad I cannot attach a pic.
I never took drugs to improve my performance at any time. I will be willing to stick my finger into a polygraph test if anyone with big media pull wants to take issue. If you buy a signed poster now it will not be tarnished later. --Graeme Obree

Grill
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by Grill

Tinea Pedis wrote:Mario, how did you establish the power 'over reading' that the P2M was giving? Not that I don't believe you, just curious on your process.

That would be a little concerning if I were an owner.


I too would like an answer to this. My P2M has been solid and consistent throughout.

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Mario Jr.
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by Mario Jr.

I had the feeling that the P2M read too much, so when I got my Rotor Power, I rigged the bike to my home trainer which has power reading. Without changing anything else, I switched the P2M to the Rotor and compared the results. Both were zeroed and calibrated. For the same heart rate, the Rotor shows app. 20W less.
I have since compared with other power meters as well including a SRM, and the Rotor power seems much in line with that.

climberevan
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Location: Tahoe, NV

by climberevan

stella-azzurra wrote:Since I took the power tap SL apart there is really nothing to adjust on the torque tube.
The torque tube sits between the bearings and has a small round magnet glued to it.
Too bad I cannot attach a pic.


Just to clarify: that's not the torque tube; that's the axle. The torque tube is actually in the hub shell, and measures the strain between the part of the shell that the spokes attach to and the part that the freehub catches when engaged. Taking it apart further than to replace bearings is pretty advanced... Saris will rebuild it to G3 specs for pretty cheap, though.

eric
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by eric

Actually the tube with a magnet is a bearing spacer (and magnet holder). But it's definitely not a torque tube!
Many hubs have a similar spacer, minus the magnet. Without it there would be side force on the bearings inner races, causing excess preload and killing the bearings quickly.

In use the spacer is stationary while the hub rotates. The magnet triggers a switch to measure rpm.

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stella-azzurra
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by stella-azzurra

Eric,

A lot of people call that aluminum tube with the magnet a torque tube. Although they could be wrong, me included, that tube does not act as a spacer between the bearings. It simply holds the magnet If you push the bearings too far in then the tube will exert a pre-load on the inner races of the bearings which cause the bearings to be tight and the hub cannot spin freely. Technically if it did not need to hold the magnet you would not need it at all.

climberevan,

the power tap hub I am referring to is an old power tap sl 2.1 hub (wired version). It's not a G3 unit.
I never took drugs to improve my performance at any time. I will be willing to stick my finger into a polygraph test if anyone with big media pull wants to take issue. If you buy a signed poster now it will not be tarnished later. --Graeme Obree

eric
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by eric

It actually is a bearing spacer. Take it out of the hub and see how long the bearings last.
I am familiar with this type of design from motorcycle hubs. You are right about preloading the bearings too much if you press them in too far (if the hub tolerance allows that- if it's correct, it won't)
There is no torque on it so it would be incorrect to call it a torque tube.

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stella-azzurra
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by stella-azzurra

Have you taken a power tap sl hub apart before?

I have taken it fully apart and put it back together about 4 times now.

The bearings can stand on their own. You cannot press then further than a certain point so they are seated.

The bearings will be fine without the spacer. I will not call it a torque tube.
This spacer holds the magnet and that is it.
I never took drugs to improve my performance at any time. I will be willing to stick my finger into a polygraph test if anyone with big media pull wants to take issue. If you buy a signed poster now it will not be tarnished later. --Graeme Obree

eric
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by eric

Yes, I have. I have an older SL with the 12mm axle.

Without the spacer the bearings will have too much preload, because the outer caps that thread onto the axle press on the inner races of the bearings to take up any slack. It doesn't matter how the bearings are seated into the hub.

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stella-azzurra
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by stella-azzurra

OK good.

The axle washer and nut will push on to the inner bearing race but will not have enough force to push on to the outer bearing because the outer race is where the bearing get pressed.
The bearing needs to be pressed right or binding will occur even before you tighten the axle nut with the washer.
I never took drugs to improve my performance at any time. I will be willing to stick my finger into a polygraph test if anyone with big media pull wants to take issue. If you buy a signed poster now it will not be tarnished later. --Graeme Obree

KevBed
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Joined: Fri Oct 10, 2014 8:18 pm

by KevBed

I will within the next couple of months be purchasing the 2015 Specialized Tarmac Pro. Which is the sl4 10r carbon frame with the ultegra 6800 components and the Roval CL40 wheels.

I will also be investing in a power meter and have been hoping to pick one up for sub 1000 dollars. The crankset is Specialized FACT carbon so that means stages is out.

I plan on racing in the spring with my collegiate team and therefore want to pick the best power meter for my budget and the smallest weight gain on the bike.

I was looking at the powertap and since I do not really have enough finances at the moment for a new set of wheels I was curious if the Roval CL40's would be ok to train and race on through the winter and into the spring races. (Live in the U.S North east) Is powertap a good choice if I don't plan on changing wheels?

Also looking at power2max which is a bit more expensive but I'm not sure if the pro's are worth the cost if I'm not switching wheels at this point.

And lastly I've been keeping my eye on the 4iii Precision power meter which has not yet been released, but seems to be perhaps an ok alternative?

Tldr; Powertap, Power2max, Precision 4iii for my current needs. Thanks for your time

by Weenie


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