SRM: Amateur or Pro?
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Speaking of power meters, the SRM is the Rolls (or Mercedes depending on how you view cars) of PMs and within the SRM family there is the Amateur model and the Pro model. The difference being the number of electronic strain gauges involved which effects the accuracy. Or is it precision that is affected? At any rate, is the Amateur model usable? Anyone with experience out there who can tell for sure? What are the real world differences?
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Record or Chorus...Dura Ace or Ultegra?
Which are you? Must you own the "best" like the pros have? Or are you OK with "good enough"?
We all want the best we can afford and justify, I suppose. I think both (SRMs) are great, but neither are accurate at all unless calibrated properly. I had a friend that had an Amateur, but calibrated it so that his wattage was off the charts....it was effective for training (high precision/reproducibility), but had no real-world comparison to other riders (accuracy was trashed).
The only real difference is the number of strain gages. This translates to an accuracy issue of a few percent. The reports I have seen indicate that the precision is still there. That means that as long as you are using one PM for all your needs, the numbers are reproducible, but not as accurate.
OTOH, if you use a PT on one bike and an SRM on another, you want both to be accurate and precise so that a 100 watt effort is similar on both rigs.
Get it?
I would recommend the Am. model. For one, the cost is lower, and for two, the accuracy is still pretty good (does it really matter to you if you are holding 300 vs 309 watts?)
Should there be a rule about a comment being longer than a post?
Accuracy Varies
A little googling revealed this discussion about the SRM amateur. It appears that it's accuracy can vary with cadence.
I also found this review which, although a bit dated, is interesting. I did not realize that the PT has 8 strain gauges and only the SRM Science model comes with 8 gauges. Yet, I don't think the measured accuracy (or precision) of the PT is any better than the SRM Pro. The SRM Amateur is not tested in the review though. I suppose the accuracy of the amateur model would be different depending on what cadence and power you were putting on it but would never vary more than 5%?
That cycling forums post is
That cycling forums post is bs. really, does he even own a SRM? hang a weight at 4 points? he only shows he does not know how to calibrate one. and the other guy who says he knows someone who says there is a potential for non-linear torque response from a straing gage? toss in a review about a big pink stuffed bunny and that thread would not get more absurd.
Have to agree
I have to agree. First off, I'm no strain gage expert, but I know they measure deflection by means of a change in resistance in a conductor. This electrical response is linear. I don't see how using a greater or smaller number of gages would make the response less (or more) linear.
Calibration is a snap. It involves a few simple calculations on the maximum frequency response generated by a known weight hanging from the pedal. Obviously that will only happen at or very near the point where the crank arm is horizontal.
Mark Ewers
I may not be fast, but I'm 2 old 2 go slow
1. That some SRM Amateur
1. That some SRM Amateur cranks provide non-linear increases in signal frequency in response to increasing torque is well established.
2. As you might expect, SRM cranks (both Amateur and Professional) tend to exhibit "hot spots" and "cold spots" at different positions. I therefore think that the approach that Ric recommends - i.e., to test at all of the four (if you're using a square taper b.b.) or eight (if you're using Shimano Octalink) possible angles is therefore worth doing at least once, just so you know how much (or how little) your particular crank shows such behavior.
Couple of questions...
Can you substantiate that?
What are the 4 and 8 possible angles? I'm not following that. Could you clarify please?
I'm also uncomfortable with the notion of hot and cold spots. Torque is a force about an axis. It "exists" 360° around the axis on which it's applied. It's independent of orientation. Maybe I'm not understanding what you mean when you say "hot spot".
And finally, do you think 1 and 2 are related?
Mark Ewers
I may not be fast, but I'm 2 old 2 go slow
JBV Cautions Too
I noticed recently that John Verheul's Training Document (pdf doc) cautions that the Amateur model is good for steady state efforts but because of it's inaccuracy is not recommended for anything else.
Pro, Amateur, then all the others
If I had to choose from among PowerTap, Ergomo, iBike, Polar or SRM Amateur power meters, I'd go with the SRM Amateur.
SRM has been shipping bicycle powermeters since 1986. They have no reliability issues and they live up to their accuracy claims. For me, SRM wins out over all the others because their product features and capabilities match best with what I value in a power meter.
Mark Ewers
I may not be fast, but I'm 2 old 2 go slow
Which do you have?
Mark,
Which do you have, amateur or pro? Rich Strauss of Crucible Fitness is now an Ergomo dealer. Check out this discussion about different PMs. Rich chimes in with his experiences (which are many). Ergomo sounds very interesting doesn't it?
SRM Pro
I have two of the pro cranks and one PCIV. I bought them both used.
Interesting yes. Attractive... I just can't get myself there. I read that entire thread - it takes a while - and I did not find much to compel me to consider Ergomo. Admittedly, I have SRM. It's what I'm comfortable with. I understand it well and I'm confident in both what it does and how it does it.
If I had to choose any bicycle powermeter besides SRM I'd go with the PowerTap.
Mark Ewers
I may not be fast, but I'm 2 old 2 go slow
dunno...
I have had a am then a pro, then a science...
I haven't seen dramatic changes in FTP after switching from each, with that said, I had a "good am" in that it's slope didn't change much from test to test.
The main reason I upgraded my am to pro is I couldn't get good data from fixie rides (including my fixie tt bike) which was VERY frustrating.
HTH
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