Quoted message said:There was some discussion awhile back about this frame fatigue test. One of the criticisms was
about the magnitude of pedal loads.
I have some SRM data from several people on my site (including myself) that might shed some light
on the subject of "real world" pedal load magnitudes.
Please keep in mind that the SRM unit averages power over one pedal revolution, so it is
pretty reasonable to say that the peak pedal loads are on the order of two-times that
indicated in the plots.
tinyurl.combc8hOpen ↗
Kraig:
This is certainly some interesting data and I hope people look at it and think about it. As Damon
has/had on his website, "One measurement is worth 50 expert opinions."
What I thought after looking at it and thinking it over for a bit:
1. I think in general the assumption that twice the average force is equal to the peak is probably
a reasonable one, maybe even conservative.
However it it may be in particular cases where the average load is high, this is not because the
peak load is high but rather because the load was better distributed about the pedalling cycle. For
what its worth, when I make peak effort, it comes from a concerted effort to pedal in full circles
rather than because I pushed as twice as hard as normal.
2. But assuming that this factor of 2 is the correct assumptions, there are certainly some peak
loads that would then be in the 1300 newton range.
You do not indicate how long these rides were and with several overlapping rides is difficult to do
an analysis. Your hilly training ride seems to be the clearest and I will make the assumption that
this ride lasted about an hour, probably longer but for this sort of rough analysis, an hour is
probably OK.
Now I count about 20 (21 points, I think) points that are above 600 Newtons average load, 1200
Newtons, assumed peak load.
Dividing that into the 200,000 cycles, it would take 10,000 hours to achieve this many fatigue
cycles at this load level, which would probably be somewhere over 150,000 miles of "hilly training
with sprints."
3. In previous threads, not the most recent one but one probably about a year ago, I suggested that
a more realistic test would be one which used a broad spectrum of loads that accurately
represented the loading history.
What I am seeing here, says that likely these peak loads are a significant (even major) contribution
to the fatigue failure in frames but that other lower level but higher frequency of occurance loads
should also taken into account.
One could do analysis, knowing the S-N curve for the material of a given frame, using the frame test
data to put set determine the location on S-N curve and then use a histogram such as Kraig has
provided to determine the fatigue lives of different frames of different materials under this "real
world" loading spectrum.
4. It is important to remember the assumptions made when working with data. In this case the
principle assumption is that factor of 2. It may be that these outlying points are the result of
unique situations either in pedalling or in the data acquisition.
I have also assumed that indeed these points represent all the points in the ride and are not some
average values or something.
5. I think that as usual Kraig has upped the level here and I would like see the some significant
input on this thread.
I certainly have my point of view but I am also certainly open to being wrong on this, Kraig has
provided some "preliminary" data that raises important questions and has certainly made me rethink
my position on this.
I am interested in what Kraig has to say, he just the data out there without much comment.
Jon Isaacs
