thetray

I am using a Cateye Astrale, and haven't noticed any problems other than my average speed reading is lower since I switched from a Supergo computer. The only change that I can think of is that the magnet for the supergo comp was mounted very near the rim, whereas the new magnet is mounted about halfway down the fork. Could this have any affect, or could it be that the supergo was calculating differently? I am wondering if there is a magnet position that will give a more accurate reading on speed and miles. Thanks for any insight.

townandcountry

I've been wondering the same thing. My magnet is on the fork just below the rim level. I've seen others near the hub. My guess is that the supergo was calc'ing differently. Wheel size is a better measure, I think. But I'm not a rocket scientist. Hopefully more insight will come in.

Gonzo Bob

It's probably not the magnet position, but the wheel size setting in the computer that is making the difference.

The sensor on the fork only counts revolutions per second and that does not change at different positions on the fork. However, for one of my computers, the sensor has difficulty sensing when it is positioned out by the rim. The speed of the magnet is too high and it does not trip the sensor consistently. But in that case, I do not get a steady readout.

Rev.Chuck

You should do a roll out on your bike, pump up the wheel, find a spot longer than eight feet, mark a spot on the floor, line up your valve stem, roll your wheel 360 and measure the distance. Now input this in to your computer. The reading may still not be the same as your old computer but it will be accurate.

dobber

In millimeters........

All the tables should be more or less the same, computer to computer. The big difference is that not all tire sizes are as stated and inflation pressure and weight can change.

The Preacher's method is about the best you can get. Improve it slightly by actually sitting on the bike and having somebody wheel you along. But you can still expect some inaccuracy, in the <1% range.

As far as position on the spoke, there might be a miniscule effect from being further out from the hub. Velocity increases as you move furthur out, so the magnet will move past the sensor faster. If the sensor pickup is marginal or at the edge of its range, you may miss some.

madpogue 

You mentioned a difference in average speed, but not in overall distance or instantaneous speed.

If overall distance and instantaneous speed are different, then it's likely an inaccuracy as mentioned above, wrt. wheel circumference. Of course, if you select the same, even if wrong, circumference for both computers, they should be mutually consistent, albeit possibly inaccurate.

If overall distance is the same, then the two computers are logging ride time differently. This may be attributed to each computer's auto start/stop feature. When you stop, the computer takes some number of seconds to determine that your speed is zero. If one computer uses a different time interval (say, eight seconds versus four), or if one computer subtracts that time interval out of total elapsed time and the other doesn't, that would contribute to a difference in average speed. If your ride involves a lot of stops, this difference will be compounded.

thetray

I have a feeling that the Supergo computer was just crap. I had to do a roll-out and enter the measurement, but the computer never seemed to hold on to the data too long (battery problems). I got the Cateye to get a more reliable computer w/cadence. I guess I'll just have to use the lower average as motivation to get faster!

kevntri

If your wheel size is set the same, you may be skipping a revolution every once in awhile.

Don Cook

Rev. Chucks advice is solid. It is usually the recommended method that comes in the computer instructions. The only thing that I would add is that you should be mounted on the bike (if possible) as you roll the bike forward one complete wheel revolution. And do it 3 or 4 times to make sure that the measurement that you get is repeatable to within a millimeter or so.

It makes absolutely no difference where the magnet is mounted relative to the wheel hub or the rim.

a2psyklnut

A friend of mine did this and although I haven't tried it, it makes pretty good sense.

Go to a high school football field with a running track. Calibrate your computer and stay in one lane and start doing laps. Adjust your setting up or down to adjust to an exact mile.

Seems like a bit excessive compulsive behavior to me, but then again, I don't really care how fast or far I go, I just enjoy the ride!

Retro Grouch 

If that's your plan, I'd do some checking to be sure you have a 1/4 mile track and not a 400 meter track.

ronyex

The instructions on my protege computer say it's best to mount the magnet 1-2 inch above the wheel axle,and the sensor should be 1-2mm apart from the magnet level.