# Thread: Confused - basic physics!

1. ## Confused - basic physics!

Hi, I've been going around in circles on this one - excuse the pun.

My cycle computer doesn't say how far along the spoke the sensor should be mounted. All it asks for is the wheel circumference.

As far as I understand it, the outside edge of a wheel travels a lot faster than it does near the centre, so where the sensor is mounted will give you a different speed and distance reading.

Are my basic suppositions correct, and how can I find the correct location for the sensor...or will you tell me that the difference is so small that I should get a life?

2. The sensor counts the number of times the magnet passes it. Once per wheel revolution, regardless of magnet/sensor position. It then it uses the wheel diameter you input to figure how far your wheel traveled in that revolution.

3. Originally Posted by edoz
The sensor counts the number of times the magnet passes it. Once per wheel revolution, regardless of magnet/sensor position. It then it uses the wheel diameter you input to figure how far your wheel traveled in that revolution.
Thanks...so the magnet doesn't pass the sensor more times per minute if it's nearer to the hub, then?

Sorry, this is a real mental block for me at the moment...but it is after midnight!

4. Originally Posted by snowcatsmiling
Thanks...so the magnet doesn't pass the sensor more times per minute if it's nearer to the hub, then?

Sorry, this is a real mental block for me at the moment...but it is after midnight!
The actual speed of the magnet itself doesn't matter, because the computer only counts passes. It compares the number of magnet passes to the elapsed time since the first pass, and figures the distance traveled vs time. It only passes one time every revolution. You tell the computer the diameter, and it figures out how far the wheel travels in one revolution. It's called rollout, and say it's 91 inches. Every time the magnet passes the sensor, the computer knows the bike just traveled 91 inches. On the second revolution, the bike has traveled 182 inches. Etc...

5. Due to differences in tire size and pressure, it's best to get on the bike and measure how far it rolls in one revolution. Then, enter the circumference into you computer.

6. Originally Posted by Sh4wn
Due to differences in tire size and pressure, it's best to get on the bike and measure how far it rolls in one revolution. Then, enter the circumference into you computer.
Most computers do this calculation for you. Just enter the code from the table for your specific tire size and you're good to go.

7. Thanks, guys. It gave me a table that says 26 x 1.75 makes 202, so that's what I put in, but out of curiosity I'd like to measure it!

I'll contact CatEye and ask them how many psi they are assuming in order to arrive at the figures on their table

8. Originally Posted by SenorSerioso
Most computers do this calculation for you. Just enter the code from the table for your specific tire size and you're good to go.
Close enough for some maybe, but not close enough for me, in my experience. I always checked the actual roll out.

Now I only use a computer on the road bike. I use a smart phone on the mountain bike.

9. Originally Posted by Sh4wn
Close enough for some maybe, but not close enough for me, in my experience. I always checked the actual roll out.

Now I only use a computer on the road bike. I use a smart phone on the mountain bike.
Yes, my CoPilot on the phone gives me an approximate live speed, but this computer came with the bike so I'm just trying to work it out and get it set up right because it's there.

10. If you do a roll out measurement, let us know the before and after circumference numbers.

At least you'll have accurate numbers. Seems like the GPS always rounds off the turns.

11. Originally Posted by Sh4wn
If you do a roll out measurement, let us know the before and after circumference numbers.

At least you'll have accurate numbers. Seems like the GPS always rounds off the turns.
Just did it. The computer's table said 26 x 1.75 = 202. Running the bike along a tape measure on the ground produced 207, and running the tape measure's graduated edge around the tyre's centre line produced 209. This is with a Marathon Plus tyre with around 240 miles' wear.

Guess I'll reset the computer to 209 before I go to bed! I wonder if the real on-the-road reading should be more as the tyre stretches with my weight...

Thanks for the suggestion

12. As stated above the wheel size is counted by the computer, of course a new tire will read a bit different from a old tire, so it may be best to measure your tire, put that measurement in the computer, then check your computer against a gps, or a car/truck by setting up a measured mile. Good Luck, but then you could add another magnet or two on your wheel, and fool your computer into thinking you're Superbiker..LOL

13. As others have stated, the magnet passes the magnet once per revolution regardless of where it is located. However, the magnet will move at a faster linear velocity near the outer edge of the tire than near the hub -- further to go so it travels faster to make that same revolution. This causes the magnet to induce more electricity into the sensor pickup, making it more reliable and making it possible to increase the clearance between the magnet and the sensor. This is why my Cateye Strada recommends mounting near the rim.

However, I mounted mine where the spokes cross, not too far from the hub. The angles and everything worked out so that I can put the sensor very close to the magnet. It made for a solid installation and it works great even at slow speeds.

I recommend measuring the distance of several rotations then dividing to average the distance per revolution. I have a 100' tape I used.

Also, your weight should be on the bike. The effective diameter is less when loaded, especially when running low tire pressure. As I recall I got about a 16" difference total over 90' loaded vs unloaded.

Ideally, you should have someone hold the bike up and push you down a premarked straight line. It is impossible to ride perfectly straight. But on the other hand you don't ride perfectly straight on the trail either

14. Originally Posted by DennisF
As others have stated, the magnet passes the magnet once per revolution regardless of where it is located. However, the magnet will move at a faster linear velocity near the outer edge of the tire than near the hub -- further to go so it travels faster to make that same revolution. This causes the magnet to induce more electricity into the sensor pickup, making it more reliable and making it possible to increase the clearance between the magnet and the sensor. This is why my Cateye Strada recommends mounting near the rim.

However, I mounted mine where the spokes cross, not too far from the hub. The angles and everything worked out so that I can put the sensor very close to the magnet. It made for a solid installation and it works great even at slow speeds.
Most computer sensors are just reed switches. As the wheel magnet passes the switch it pulls the switch closed momentary, completing a circut to send a pulse for the computer to count. The electricity is supplied by batteries.

The main reason to mount the wheel magnet close to the rim is to minimize the distance between computer and sensor with wireless models.

15. Originally Posted by DennisF

I recommend measuring the distance of several rotations then dividing to average the distance per revolution. I have a 100' tape I used.

Also, your weight should be on the bike. The effective diameter is less when loaded, especially when running low tire pressure. As I recall I got about a 16" difference total over 90' loaded vs unloaded.

Ideally, you should have someone hold the bike up and push you down a premarked straight line. It is impossible to ride perfectly straight. But on the other hand you don't ride perfectly straight on the trail either
What he said.

With your weight on the bike your effective circumference will be smaller. Using the unweighted circumference will cause the computer to display faster and farther numbers than actual.

16. Most computer sensors are just reed switches. As the wheel magnet passes the switch it pulls the switch closed momentary, completing a circut to send a pulse for the computer to count. The electricity is supplied by batteries.
Ok, so if that's the case then linear velocity won't matter. A slower velocity would actually be better -- too fast the reed switch may not have time to react :-).

And, the only reason to mount the magnet close to the rim is to get the transmitter closer to the receiver. However, mine is located about opposite the rotor, close to the BOTTOM of the fork slider, and I am still well within the 70cm distance limit stated in the manual (29" wheel). The closer the better I guess.

Anyway, the magnet hast two tabs (one on each side) and I couldn't see not using both of them. Also, my rims are off-center dished, and the distance from the slider to the spokes is pretty large near the top.

Thanks for the enlightment.

The thread is old, but the consensus at that time does seem to be that Cateye uses reed switches. I will listen and try to hear it.

That does raise another concern. What happens if you happen to park the bike with the magnet near the sender? Wouldn't that drain battery in a hurry? The manual does not mention this.

17. Originally Posted by SenorSerioso
Most computers do this calculation for you. Just enter the code from the table for your specific tire size and you're good to go.
those table measurements are crap. They're for lazy people who won't do a rollout.

Tires vary a lot from brand to brand and model to model within the same "listed" size.

Plus, the pressure you use will change that rollout. Higher pressures will result in a larger circumference, lower pressures result in a smaller circumference.

The only computers that do this calculation for you are GPS enabled computers, and not all of them will.

18. I listened to my Cateye and it definitely has a reed switch. You can hear it close and open as you roll the magnet by it. HOPEFULLY it is just sending a digital pulse to something rather than actually powering up the transmitter.

19. This thread helped me! Thanks to all of you!

20. Originally Posted by DennisF

Also, your weight should be on the bike. The effective diameter is less when loaded, especially when running low tire pressure. As I recall I got about a 16" difference total over 90' loaded vs unloaded.

Ideally, you should have someone hold the bike up and push you down a premarked straight line. It is impossible to ride perfectly straight. But on the other hand you don't ride perfectly straight on the trail either
Wow, something else I didn't think of. Looking forward to doing it again, and trying to do properly once I receive my new wheels next week which will give a different profile bcoz I think the rims are narrower...

Thanks guys!

One thing I have found is that this particular computer occasionally freaks out and gives extreme averages or freezes up in some other way, needing a hard reset. I suppose you get what you pay for, or I wonder if they're all equally cheap electronics until you get into silly money...

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•