I have shown that there is no significant difference in accuracy between doing calibration on a 25-m rather than on a 300-m course when using rim readings in spoke intervals. The secret of using the 25-m course is to coast rather than ride along it, so as to avoid wobble and follow the line of the course very closely.
I create the 25-m course along a straight white line on the edge of the road by measuring 82 feet 0.25 inches plus zero offset with the loop at the beginning of the tape over an imbedded nail. (Temperature correction can usually be ignored, but measurement should be done while cloudy or in the shade.) Seated on the bicycle, I start rolling by pushing the right pedal down to the bottom of its travel and keep it there while coasting. Occasionally I touch the ground with my left foot as necessary to keep accurately along the white line.
Replicate determinations in each series of four were all within one or two tenths of a spoke interval from one another. Also the % difference of the calibration factor differed from that from a determination on a 300-m course done at the same time by 0.03%,0.05%, 0.04%, and neg 0.04%. (I am ignoring 0.06% found in the case where measurements on the 300-m course showed variability.) Raw data can be found in the Excel file at:
I still think it best to use only one calibration course at home with pressure monitoring. However, if one is set up away from home using the short version has the following advantages:
1. A good site is much easier to find.
2. The measurer can do the job very quickly by himself.
3. Measuring just one length of tape means there is less room for error.This message has been edited. Last edited by: Neville,
This method does yield an accurate calibration for the bicycle as moved in a straight line with rider weight present. It does not replicate the way the rider actually rides when measuring the race course. The basic principle of calibration assumes that the rider will ride as he or she calibrates. Few riders would ride the course with the same degree of straightness as used in the proposed calibration method. I believe this would result in some degree of undersize in the resulting measured courses, because the resulting calibration constant would be smaller than one obtained through normal riding.This message has been edited. Last edited by: Pete Riegel,
Would coasting a long a 300 m cal course in the manner Neville describes for his 25 m ride answer some of Pete's concerns? If the 300 m course was then ridden in the "normal" way the results of the two could be compared to see if the difference in the two riding styles was significant for a particular rider.
Why would you want to calibrate by coasting when you will almost certainly be pedaling during your measurement ride?
Jay, not sure whether you were referring to my post or to just the question of coasting during calibration, period. If Neville's coasting for 300 m gave a similar constant to pedalling for 300 m then (start up wobble aside) it might give some credance to Neville's proposed method (at least certainly if Neville's the one riding).
Pete and Jay:
You are missing the major point of my findings:coasting over the 25-m course does in fact give the same calibration factor as that from normal riding over the 300-m course.
I tried normal riding over the 25-m course and not surprisingly the factor was 0.25% high
I see what Pete's and Matthew's concern is, but I think Neville has happened upon two distances where different methods of riding produce the same statistical wobble.
If a rider rides along a 300m course, his counts per km differed by such a miniscule amount from a 25m course which was coasted along, that a series of "coasted" cal rides would yield virtually identical "constants". In his rides, his 300m rides yielded a 479.74 clicks/km, vs 479.75 clicks/km in his 25m rides.
This small test would indicate that riding a 25m course very carefully (not varying the route by more than 4") yields the same wobble percentage as riding a 300m course normally. If this can be replicated by at least 5 other riders, then I think this approach should be examined further. Mathematically it makes sense, and logically it makes sense.
And practically, too. I could see this being very useful for a validation ride where you were coming into town solo, and had a hard time finding a 300m straight road near the race course.
Measurers with very little effort can check their performance using the method next time they do a regular calibration.
I must elaborate that I did all the measurements over the 25 m going in the same direction with my left foot dangling nearest the center of the road:I felt most comfortable pushing off with the right foot and NC does not put shoulders on most of their roads.One could alternate measurement direction by reversing feet or always using the right foot if there is a wide paved shoulder.
I have just tried the method in NJ where, unlike in my home state of NC, shoulders are generous, so I was able to take measurements in both directions. A 300-m course at 49 deg with a tire pressure of 620 kPa gave a calibration factor of 479.157 rev/km (no SCPF). A few minutes later at 54 deg and 630 kPa I got readings over the 25-m course of 31.1, 31.0, 31.0 and 30.9 spoke intervals. Number of spokes were 32 and whole revolutions were 11. Calibration factor was 478.750 rev/km and after adjustment to 620 kPa (plus 0.133 rev/km for negative 10 kPa) was 478.883 rev/km. The % difference from that of the 300-m course was minus 0.057.
The result on the 25-m course was almost identical to the one I got 3 weeks earlier on 3/14/08 in NC. At 63 degrees and a pressure of 620 kPa, calibration factor without the SCPF was 478.906 rev/km. Again this differed from that from a 300-m course by minus 0.06 %.
Note that no meter is required for calibration using the new method, and once the number of whole revolutions have been manually counted, there is never the need to do so again.This message has been edited. Last edited by: Neville,
I'll throw my tuppence in, for what it's worth. I'd be a little skeptical of a cal course that short, just because of the possibility of a single wobble, bump or whatever throwing the results off.
Granted, this is probably what they said back when cal courses were a half-mile or more. But I still see the possibility of a small error being carried over, that would be "smoothed out" with a 300-meter course.
Take that for what it's worth. Y'all know more about riding cals than I do.
Just out of curiosity, I decided to try this out a few days ago. The street in front of my house has a concrete curb that extends out onto the asphalt road surface for about 15 inches. This provides a nice straight line to follow. I put two pieces of tape on the road, approximately 25 meters apart.
I practiced the riding technique one time before trying it for real for 3 trials. My wheel is marked in 100ths of a revolution, so it's possible for me to see very small differences in two successive trials. I found that there was no more than 0.002 to 0.003 revolutions difference in the readings of my three trials. For a 25 meter course, this corresponds to variation of about 0.02%, which is a good bit smaller than variations I see when I ride 100 to 300 meter cal courses.
I did this quick test just to check how repeatable rides like this are. I will check later how it compares to cal constants I get with longer rides. But it is easy for anybody to do this same quick test, even if you don't have your wheel marked up. Just put two pieces of tape on the road about 25 meters apart. Then mark your wheel where it rolls over each piece of tape. If you repeat the rides starting at the first mark you can see how close you get to the second.
Clarification: some skeptics are commenting about wobble. Neville stated very clearly early-on about using a painted stripe approx. 4" wide, and weighting the bike while rolling it forward. This is not a normal "mounted" ride, so the wobble factor is much more controlled.
I just wanted to make sure it was still clear this far along in the thread, that the technique is different than for a 300m+ ride.This message has been edited. Last edited by: Duane Russell,
In real life I don't have a wheel calibrated to 100th of a revolution. I have a road bike, which wobbles when I start up, but gets smooth, stabilized and straight at about 10 miles an hour, well after the first 75 feet.
I use a jones counter that has a bit of whiplash, if I roll forward or back when getting lined up with the start, which is something I tend to do.
I have also noticed that with the same bike and same moisture there is some variation on my cal rides depending on the wind, and more surprisingly the time of day. In the evenings, as the light drops off it takes more counts to get to the other end.
I also notice that it takes about two to four rides of the cal course before the bike and I get warmed up and the counts get consistent.
Since the cal course is 1/2 mile long, thats about 1 to 2 miles of warm up.
While in theory if you multiply anything by 1600 it is mathematically 1600 times larger. In practice, when using an intermediary measuring tool to scale it up you will just scale in and scale up all the errors. That's why blue prints are large and dimensions are printed on them to discourage scaling up from the drawing.
I have no doubt that under ideal conditions an experienced measurer can sale a 25m course up to a Marathon without introducing too much error.
The key is, our method should work for not just the experienced and careful measurer, but for almost any tom dick and harry who follow the process correctly.
Therefore the process should be one that tends to naturally lesson errors, not one that gives an opportunity for compounding them unnecessarily.
I realize that you are not talking about the recommended method for measurement, using the jones counter, but experimental methods using the operators own improvisation of marked wheel rims and electronic revolution counters. While this is technically interesting, it's not providing a bullet proof method that a man in Botswana can replicate, out of a filed guide.
As a person who works in the high-tech field I welcome appropriate use of technology, but often wonder at the lengths people go to modernize or fix something that is not broken. The point of the measurement system we use is to replicate how a rider will ride when on the course. Wobbles, looking up, and down, and all.
The less like the course it gets the less the ride duplicates the variation that the rider will have when riding the real course.
I suppose the logical next step is to eliminate the errors caused by actually 'gliding' the bike over the 25m, instead we will just have to transcendentally think about a meter.
Heck, at only 25 meters it seems to me that you would get equally consistent results with a surveyors rolling push wheel! The point is we know that even when a surveyors push wheel gives consistent results over 25m, you can't reliably scale that up into a marathon.This message has been edited. Last edited by: JamesM,
You have missed the main point that I repeated on March 17: I get the same calibration factor from the new method as I get from conventional calibration using a Jones. The main advantage is calibration can be done quite quickly at a site where it might be impossible to do a conventional calibration.
Incidentally you do not need to mark your rim in hundredths of a revolution. It is better to just number your spokes and this only takes a few seconds.
On my bike, a difference of 0.05% (half the SCPF) translates to about 6 thousandths of a revolution on a 25m calibration course. You'd better have your wheel marked accurantly and often if you want to discern such differences.
Indeed your calculation is correct, but the way you express the SCPF makes it sound awfully small as a rim reading! I prefer to think of it as nearly half a spoke interval. Actually you would need to number only one spoke since all readings would be taken there. A few smaller divisions could be placed quite quickly at that spot.
Actually it is not too much more effort to get more accuracy by swinging the tape about a single nail to create a 50-m course.
I am puzzled as to what you mean about "marking your wheel accurately and often".
On a wheel with 36 spokes, 0.006 of a revolution is about one fifth of a spoke interval.
When I said "marking your rim accurately and often" my "often" referred to the number of marks. I think you need more than 20 or 36 to be able to discern differences as small as 0.006 revolutions. But it's a small point because it's no big deal to measure out more marks on your rim.
I finally got around to trying this out for myself. I rode a 25-meter course using Neville's method and a 200-meter course with normal riding. I used rim readings on all the rides, doing my best to estimate to the nearest thousandth of a revolution.
Here's the results
|Powered by Social Strata|