Formulae to Program Cycle Computer to Read Metres Minute

[velocette]

Hi to all "Home Machinists" I have been restoring a vertical bandsaw and have added a worm drive gearbox to the drive system
for metal cutting.

Planning on adding a variable speed DC motor to be able to cut at between 20 metres a minute to 70 metres a minute.

what is the formulae to find the circumference to program into a "Cycle Computer" to be able to display Metres Min as
Km Hr I. E. 30 Km hr or 3 decimal 0

Eric

 

[kvom]

1 km/hr = 1000m/60 min = 16.67 m/min

20 m/min = 1.2 km/hr
70 m/min = 4.2 km/hr

A cycle computer has a sensor that measures RPM and uses that to convert the wheel diameter to speed. You need to know the RPM range of the bandsaw plus the diameter of the drive wheel. Assume a diameter of D. Circumference is D*pi, If R is rpm, then SFM is D*P*R. The cycle computer will display km/hr, and you can multiple that by 16.67 to get SFM.

 

[lensman57]

Hi to all "Home Machinists" I have been restoring a vertical bandsaw and have added a worm drive gearbox to the drive system
for metal cutting.

Planning on adding a variable speed DC motor to be able to cut at between 20 metres a minute to 70 metres a minute.

what is the formulae to find the circumference to program into a "Cycle Computer" to be able to display Metres Min as
Km Hr I. E. 30 Km hr or 3 decimal 0

Eric

Hi,

The trick is to fool the computer by specifying a wheel size that would then result in the speed to be displayed as a number that could be read as rpm. If you googled the subject a few links would come up that could put you on the right track.

Regards,

A.G

 

[mklotz]

I typed cycle computer tachometer into Google and the very first hit was:

http://www.instructables.com/id/Tachometer-made-from-a-bicycle-speedometer-cycloc/

which discusses the arithmetic involved.

 

[rhankey]

Bike computers simply count wheel revolutions, and multiply the revolutions by the programmed wheel circumference to derive distance travelled or speed/hr. With most bike computers, you need to tell it the circumference of the wheel, while others allow you to enter the diameter of the wheel. A slight complication is that bike computers typically only accept wheel sizes that work out to between a minimum of 20” to 24” up to about 29” maximum. But you could get a little more flexibility for your use by changing the display between MPH or Km/h. Most don’t give overly accurate readings at much more than roughly 45mph for a roughly 27” wheel. Between wheel size and displaying miles or Km, you should be able to get a nice unit of measure to work with, so long as your worm drive doesn’t spin too quickly. Once you have found the optimum wheel size setting for your use, be sure to write down the setting somewhere, as you have to re-enter the wheel size when you replace the battery in the display unit. You can get wired or wireless computers. With wireless, you need the display to be within about 1’ of the pick-up unit, but the motors might interfere with the wireless signal.

Robin

 

[lensman57]

I typed cycle computer tachometer into Google and the very first hit was:

http://www.instructables.com/id/Tachometer-made-from-a-bicycle-speedometer-cycloc/

which discusses the arithmetic involved.

Hi,

That is correct, I believe that magic wheel size to enter is 166.66 for Kph reading and the resulting figure displayed is 1/10th of the actual rpm. Some of these computers have a slightly more input range for wheel sizes but they are all seem to be around £5.00 to £7.00 mark in the UK, so you could perhaps afford to experiment with them.

There is also a sort of surface mount tachometer available in the US called SanDec which reads the rpm directly but my search did not come up with anything in the UK.

Regards,

A.G

 

[velocette]

Hi thank for all the valuable input this has my old grey head head on overtime.

I now realise that I have not supplied enough information by trying to over simplify with my posting

((Wheel diameter is 360 mm or 1.131 circumference @ 30 metres minute = 26.525 RPM ))

Right now back to the problem (( Have taken a break to watch the sun bring a new late autumn day to Hawkes Bay ))

No success so far with the Math or to put it clearer not so much the math as my understanding of it.

Cycle Computer is a "CatEye Velo 5" for tyre size from 1020mm circ to 2333mm circ.

This works well as a Tachometer fitted to my drill press programmed to 1667 mm circ wheel.

This repeats the original post hopefully to save you from having to skip back to it.

""Hi to all I have been restoring a vertical bandsaw and have added a worm drive gearbox to the drive system
for metal cutting.

Planning on adding a variable speed DC motor to be able to cut at between 20 metres a minute to 70 metres a minute.

what is the formulae to find the circumference to program into a "Cycle Computer" to be able to display Metres Min as
Km Hr I. E. 30 Km hr or 3 decimal 0""

Thank You all again for your help and advice will keep banging away at the problem

Eric

 

[rhankey]

Eric,

Barring any math or transposition mistakes on my part at this late stage in he day, here are the numbers I think you are wanting:

At 2333mm for the largest circumference you can program into a bike computer (which is roughly a 29” diameter wheel), to have the bike computer display 10MPH, your magnet will need to pass by the pickup unit 114.97 times a minute (5280 / (2333mm / 25.4 / 12 ) * 10 / 60). In other words your “wheel” will need to be spinning at 114.97RPM to register 10MPH.

To get the display to show 10KmH with the same wheel circumference, your magnet will need to pass by the pickup unit 71.44 times a minute (3280 / (2333mm / 25.4 / 12 ) * 10 / 60). In other words your “wheel” will need to be spinning at 71.44RPM to register 10KmH.

So, if your “wheel” is only spinning at let’s say 25PRM as you indicate, it will only register 2.17MPH (25 * 10 / 224.97) or 3.50KmH (25 * 10 / 71.44).

Most bike computers will show instant MPH or KmH speed with one decimal (eg 12.3), and Average speed at zero decimals (eg 12) which is going to reduce the accuracy you are going to see.

If you wanted to have 10KmH = 10RPM, you would need to mount your magnet to a gear that is spinning 2.857 times quicker than your actual “wheel” (71.44 / 25) or 4.598 times quicker for 10MPH = 10RPM (114.97 / 25).

Robin

 

[Tin Falcon]

the way i see it plug in the wheel circumference. Let the computer read in kph and make a conversion chart to show meters per min.
the alternative is let it read rpm and make a chart to show meters per min

This works well as a Tachometer fitted to my drill press programmed to 1667 mm circ wheel.

1 rpm would equal 1.131 Meters per minute so
20 M/min would be 17.683 rpm
70 M/min = 61.892
now if we plug this into a simple ratio proportion formula:
17.683 = 1.667
20.000 X

cross multiply and you end up with 17.683X =33.340
divide both sides by 17.683 to solve for X you end up with x=1.885 meters so if all this is correct you should be able to set your wheel circumference to 1885 mm and have a direct read out.
I am a little rusty on my math so if I am wrong please someone let me know but this should work.
tin

 

[rhankey]

A couple more thoughts came to mind since my last post…

If you are using the KmH scale on the bike computer, you could space 3 magnets equally around your wheel, and be able to program a a slightly smaller wheel circumference into the bike computer such that 25KmH = 25RPM. If you want to use he MPH scale, you would need to space 5 magnets equally around the wheel at which point a slightly smaller wheel circumference would allow the bike computer where 25MPH = 25RPM. In fact, if you wanted the computer to be very responsive to any speed fluctuations of your “wheel”, you could equally space 6 or even 7 magnets around the wheel, and program progressively smaller wheel circumferences such that 25MPH = 25RPM. This is just a simple game of math and ratios, for which you now have enough formulas to play around with. Remember, the bike computer simply counts how many times a magnet passes by the pickup unit, and each times assumes the “wheel” has travelled the programmed circumference, from which it can then derive speed.

The second option is as simple as it gets and I am kicking myself for not having thought of it sooner. Many bike computers have a “cadence” option, which gives a second magnet and pickup unit to monitor pedal revolutions per minute. If you got a bike computer with a cadence option, you simply attach a magnet to your “wheel”, and the cadence setting will display the actual RPM. No fuss, no muss, done!

If want your bike computer to be more sensitive or responsive to fluctuations of RPM, or accurate to a tenth of an RPM, you will need to mount additional magnets to your “wheel”, and divide the RPM display by the number of magnets, or change the wheel circumference setting if using MPH.

Robin