Just installed new computer on bike...

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WhoYoDaddy

Banned
May 5, 2004
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The manual seems to want the full circumference of the wheel. I did that and inputed the equivilent of 26" in mms. It reallt doesn't seem logical as you can't even put the computer at the full circumference of the tire...and you're supposed to be able to put it anywhere on the spokes so it can't know where it's true circumference is..


Anyways, I tried their method first and I shot up to 18mph with little effort in pedaling which seemed too high. Then I tried my method of measuring the actual diameter of the sensor to the middle of the hub and used that value. Now I can't seem to get past 15mph as my max speed on my mt bike.

Now how exactly should I input the circumference and if I should use the full cir of the tire then how does the snor know the real circumference as you can place it anywhere on the spokes?
 
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boyRacer

Lifer
Oct 1, 2001
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As long as the magnet is as close to the sensor as the manual says and you selected the right size you should be fine.
 
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Brutuskend

Lifer
Apr 2, 2001
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The best way to do this is, get a tape that has centimeters on it, put the wheel on the tape with the valve stem down at the start of the tape, roll the wheel one revolution until the stem comes around again, enter THAT number of centimeters into the computer. To be VERY accurate, you SHOULD be sitting on the bike when you roll it out, since the tire will compress somewhat.

As far as I know, where you place it doesn't seem to effect it. Seems it would, but it doesn't AFAIK.
 
ISAslot

ISAslot

Platinum Member
Jan 22, 2001
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um it doesn't matter where on the wheel the sensor is It will always 'sense' at the same time
 
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WhoYoDaddy

Banned
May 5, 2004
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Well, actually I know that the instructions tell me but i don;t know why their way is right.

The speedometer works by counting the number of rotations (1 rotation = the circumference) and calculates the speed by d/t. The circumference should be measure from the sensor and the hub right?

How would knowing the entire wheels' circumference help in calculating the speed?


That's what confuses me
 
F

FrustratedUser

Lifer
Aug 16, 2001
22,505
4
81
Originally posted by: WhoYoDaddy
Well, actually I know that the instructions tell me but i don;t know why their way is right.

The speedometer works by counting the number of rotations (1 rotation = the circumference) and calculates the speed by d/t. The circumference should be measure from the sensor and the hub right?

How would knowing the entire wheels' circumference help in calculating the speed?


That's what confuses me
Click to expand...

Here we go!!

speed = distance / time

In this case the distance is the circumference of the wheel and the time is the time between 2 pulses from the sensor.

It does not matter where on the spoke you put the sensor but the further out you put it the more accdurate is the measurement of the time of one revolution.
 
ISAslot

ISAslot

Platinum Member
Jan 22, 2001
2,891
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Originally posted by: WhoYoDaddy
Well, actually I know that the instructions tell me but i don;t know why their way is right.

The speedometer works by counting the number of rotations (1 rotation = the circumference) and calculates the speed by d/t. The circumference should be measure from the sensor and the hub right?

How would knowing the entire wheels' circumference help in calculating the speed?


That's what confuses me
Click to expand...


because they want to know how long it takes to travel the circumference of the wheel
So you need the time and one revolution, hence the Sensor
 
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WhoYoDaddy

Banned
May 5, 2004
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Actually, I understand now why they need the entire circumference of the tire....


So that means I can go faster than 15 mph on my crappy mt bike! Woohoo.
 
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Brutuskend

Lifer
Apr 2, 2001
26,558
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Originally posted by: WhoYoDaddy
Actually, I understand now why they need the entire circumference of the tire....


So that means I can go faster than 15 mph on my crappy mt bike! Woohoo.
Click to expand...

Yes, especially DOWN HILL!
 
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dderidex

Platinum Member
Mar 13, 2001
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True, the sensor is only counting rotations, nothing else.

The computer is taking the number of rotations, comparing them to the internal clock (for rotations per minute) and using the circumference to display the speed. Why it's called a 'computer'.

Anyway, I find that 15 mph on a mountain bike on flat terrain is possible, but tiring after a while. I usually average around 12 mph for now. Downhill, I can hit 20+ (just talking roads, here, so no serious grade....probably 25+ on a steaper hill before I started getting nervous!)
 
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WhoYoDaddy

Banned
May 5, 2004
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Originally posted by: dderidex
True, the sensor is only counting rotations, nothing else.

The computer is taking the number of rotations, comparing them to the internal clock (for rotations per minute) and using the circumference to display the speed. Why it's called a 'computer'.

Anyway, I find that 15 mph on a mountain bike on flat terrain is possible, but tiring after a while. I usually average around 12 mph for now. Downhill, I can hit 20+ (just talking roads, here, so no serious grade....probably 25+ on a steaper hill before I started getting nervous!)
Click to expand...

Well, on my friend's mt bike I was averaging about 14mph with no wind on his mt bike according to his computer. Then today I took out his road bike and we avged about 18mph into heavy wind with him keeping up pretty good on his mt bike.

It sucked that I couldn't toally destroy him mainly cuz he's a better biker but the advantage was huge!

Well, the road bike didn't make me much better...but i guess every advantage helps
 
Kadarin

Kadarin

Lifer
Nov 23, 2001
44,296
16
81
Originally posted by: WhoYoDaddy
The manual seems to want the full circumference of the wheel. I did that and inputed the equivilent of 26" in mms. It reallt doesn't seem logical as you can't even put the computer at the full circumference of the tire...and you're supposed to be able to put it anywhere on the spokes so it can't know where it's true circumference is..
Click to expand...

Did you input 26" as the circumference? :confused:

That sounds more like a diameter. Think back to like elementary school for the formula for calculating the circumference of a circle: Pi * r^2
 
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WhoYoDaddy

Banned
May 5, 2004
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Originally posted by: Astaroth33
Originally posted by: WhoYoDaddy
The manual seems to want the full circumference of the wheel. I did that and inputed the equivilent of 26" in mms. It reallt doesn't seem logical as you can't even put the computer at the full circumference of the tire...and you're supposed to be able to put it anywhere on the spokes so it can't know where it's true circumference is..
Click to expand...

Did you input 26" as the circumference? :confused:

That sounds more like a diameter. Think back to like elementary school for the formula for calculating the circumference of a circle: Pi * r^2
Click to expand...


Nono, I calculatted the cir with 2*pi*r and yeah 26 is the diameter so no 2.
 
Zenmervolt

Zenmervolt

Elite member
Oct 22, 2000
24,514
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91
Originally posted by: WhoYoDaddy
Well, actually I know that the instructions tell me but i don;t know why their way is right.

The speedometer works by counting the number of rotations (1 rotation = the circumference) and calculates the speed by d/t. The circumference should be measure from the sensor and the hub right?

How would knowing the entire wheels' circumference help in calculating the speed?


That's what confuses me
Click to expand...
CIRCUMFERENCE != DIAMETER

The circumference of a tire with a 26-inch height is: 26*pi Or 26*3.14159265359 in inches.

ZV

EDIT: Nevermind, didn't post fast enough.
 
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