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Old 05-23-05, 10:20 AM   #1
bboseley
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Pace v Average Speed

I have a Cateye 7 computer that performs well – but I can’t seem to get it calibrated exactly. I suspect a cordless is not intended to dead on. On a premeasured bike trail, when I cross the 20-mile marker, my Cateye will show about 19.2. Does this throw the current and/or average mph off also? In which direction?

Anyway, my real question concerns average mph. First, when riders go out and talk about riding a 20mph pace – won’t the average mph be somewhat less? Even a pace-line will have occasion to slow for various obstacles, but they can still say they are riding a 20mph pace. The computer on the other hand is going to penalize them on average mph.

Lastly, and maybe this isn’t all the information needed, but say I am beginning the last 5 miles of a 40 mile ride. The computer shows me at 14.8 Ave mph. Based on my ride yesterday, it seems I would need to maintain virtually 19 mph all the way in to bump the Ave to 15. Actually I did almost that, and was perturbed to see my final average at 14.9. As George is fond of saying – “This is hard work!”

By the way, anyone with a Cateye know if I need to adjust the circumference number up or down?
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Old 05-23-05, 11:09 AM   #2
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Quote:
Originally Posted by bboseley
I have a Cateye 7 computer that performs well – but I can’t seem to get it calibrated exactly. I suspect a cordless is not intended to dead on. On a premeasured bike trail, when I cross the 20-mile marker, my Cateye will show about 19.2. Does this throw the current and/or average mph off also? In which direction?

Anyway, my real question concerns average mph. First, when riders go out and talk about riding a 20mph pace – won’t the average mph be somewhat less? Even a pace-line will have occasion to slow for various obstacles, but they can still say they are riding a 20mph pace. The computer on the other hand is going to penalize them on average mph.

Lastly, and maybe this isn’t all the information needed, but say I am beginning the last 5 miles of a 40 mile ride. The computer shows me at 14.8 Ave mph. Based on my ride yesterday, it seems I would need to maintain virtually 19 mph all the way in to bump the Ave to 15. Actually I did almost that, and was perturbed to see my final average at 14.9. As George is fond of saying – “This is hard work!”

By the way, anyone with a Cateye know if I need to adjust the circumference number up or down?
Tyre size as measured and not from the quick data given on the sheet will give you a computer that is set for your tyre size. The correct tyre circumference, or incorrect, will affect all the readings for the computer.

To my thoughts, a 20mph pace will ocacsionally drop below 20mph, hence lowering that average---but it will also occasionally go above the 20mph so should even itself out over a ride.

I have used Cateye cordless for many years now, and other than breaking the sensor on the Cateye 2's, have never had a problem with them so wise choice on your part. As to whether they are accurate enough, I think they can be when correctly set up, but there will always be a minor discrepancy on any computer.
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Old 05-23-05, 01:32 PM   #3
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Assuming the trail marker is correct, then your computer calibration is too low. Yes, this affects both your mileage and your speed - both current and average. If you did the 20 mile trail in 60 minutes, it would 'only' register 19.2 mph average speed.

The best way to calibrate is to do a rollout. Start with your valve stem at the bottom of the wheel, above a crack. With the bike weighted normally, roll the tire 2-3 times, stopping with the valve at the bottom again. Measure the distance and divide by how many revolutions you did. If you measured in ft/inches, you'll have to convert to cm or mm, depending on which number your computer wants. (it'll be obvious - 210 vs 2100, for instance.) If you can't be bothered to do it right, then just guesstimate that you're about 5% low, and bump up the currently-entered number by that much.
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Old 05-23-05, 02:02 PM   #4
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I have two wireless Cateye computers (CD300 & new Micro wireless) plus a third other brand on my third bike. They all agree within a tenth of a mile when taken on the same course. In each instance (and they have three different sized tires 700x19, 700x23 & 700x25) they were simply calibrated using the scale given in the instruction books. Over or under inflation will certainly affect the accuracy. The most accurate way is to mark the bottom of the tire, place that mark on another mark on the road, and with your full weight on the bike, and the tire inflated to what you run it at, measure the distance travelled by the bike in millimeters, and use that as your calibration figure. Some simply roll the bike - that is insufficient. You must have the weight of the rider in the matrix.
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Old 05-23-05, 02:53 PM   #5
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Don’t assume that measured trails are accurate to the inch, foot, yard or mile.

As said above you need to do a roll out. But it will not be accurate if you change air pressure in the tires, gain or lose weight, add a rack to the bike, or have it loaded or unloaded…will all effect the roll out. So load your bike to normal, pump the tires and get in your kit and do the roll out. To increase the accuracy prior to the rollout use a string to mark a straight line. Most riders can't ride an absolutely straight line. So have some one push you along the line. Then measure the distance with a cm/mm tape and divide with the number of rotations. Enter that number in your computer and then release that if you don’t pump your tires…gain or loose weight… the computer is not absolutely accurate….. but what is…

And yes as answered above “Yes, this affects both your mileage and your speed - both current and average. If you did the 20 mile trail in 60 minutes, it would 'only' register 19.2 mph average speed.” Not to dig at you…but check with you math teacher. This is the truth and you really should know that if your teachers did their job.

“Pace” is subjective in cycling. When I hear a ride is at a 20 mph pace, and from experience, tells me they will ride at but not exceed 20 mph under normal ride conditions. Downhill and tailwinds will increase the actual speed and wind and hills will reduce the actual speed but they aren’t using more leg muscle.

If I’m told the average speed is 20 then I assume that they could hit 10 – 30 mph under normal conditions. So I need to be ready to ride above 20 to keep up.


“Lastly, and maybe this isn’t all the information needed, but say I am beginning the last 5 miles of a 40 mile ride. The computer shows me at 14.8 Ave mph. Based on my ride yesterday, it seems I would need to maintain virtually 19 mph all the way in to bump the Ave to 15. Actually I did almost that, and was perturbed to see my final average at 14.9. As George is fond of saying – “This is hard work!” – nope your math is off… all you have to do is cover 5 miles in .301802 hours. for the last 5 miles to get to 15 avg...I think.

“By the way, anyone with a Cateye know if I need to adjust the circumference number up or down?” - Check the web for a manual.

Cheers,
Dusk

Last edited by Dusk; 05-23-05 at 04:51 PM.
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Old 05-23-05, 04:27 PM   #6
al-wagner
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Where does doing 20 miles in 1 hr equal 19.2 mph.?? 20 miles in 1 hour or 60 min. equals 20 mph
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Old 05-23-05, 07:19 PM   #7
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If you believe the 20 mile marker, then all you have to do is bump up the number in the computer for tire size by 20/19.2 which is the old number times 1.04166666667
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Old 05-23-05, 08:00 PM   #8
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I put a piece of tape on my tire (just to get a marker closer to ground level), lined up a metal tape measure, then sat on the bike, slowly rolling it next to the take until the marker hit the bottom again. The way I did this was to use a hallway so I could hold upright by using my hands on the wall. Did numerous tests and they were the same each time so it was consistent.

I have also found that my normal ride is amazingly consistent - I hit 10 miles at almost exactly the same point every time - I expected more variation because of minor alterations in the exact line I followed. I guess the pluses and minuses evened out, so now when I put on a new tire, I see where it shows 10 miles and if not the same, I adjust it until it comes out right.

I know that weight on the bike makes a difference because I checked both ways. I don't know if minor changes matter much - at least not on a high pressure road bike tire. My weight has covered a 10 lbs range and I can't see any noticeable difference on my ride distance - lower pressure tires may vary more from less weight, I guess.

But if you do let the pressure drop, that will make a difference.
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Old 05-23-05, 08:03 PM   #9
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Oh yeah, I never trust premeasured distances. I have found the mileage markers on interstates to be pretty good (though most car odometers are wrong), but even so, I've caught a misplaced maker sign sometimes. That's easy to tell if you measure over a bunch of them - the bad one will not match.

If the trails are marked by some group, you have to wonder how careful they were.
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