Jump to content

Cheap and easy rev counter

chris w

By chris w
in Boat Equipment

 Share

Featured Posts

chris w Veteran II

chris w

Posted
Posted

There was a thread on here somewhere in the last few days where someone was querying the idle speed on a new engine install as their rev counter indicated only 600rpm and the manufacturer stated 850rpm. It was suggested that the tacho may need recalibrating for the new engine - but how does one calibrate it? That needs another calibrated tacho!!!

 

It then occurred to me that one could use a cycle computer to measure rpm quite easily and this could be used to measure either engine revs or indeed alternator revs.

 

Without the maths, here's how to do it.......

 

Mount the magnet on the crankshaft pulley with the detector on the engine block, set the wheel circumference on the cycle computer to 1667mm (or, the same thing, set the wheel diameter to 531mm) and bingo. It does NOT matter how far away from the centre of the crankshaft pulley you glue the magnet; the result will always be identical and accurate.

 

Switch the bike computer to speed in km/hr and simply multiply the answer by 10. So if the cycle computer says 93.5km/hr,the engine is rotating at 935 rpm and so on.

 

For those interested, the maths is as follows:

 

Let C = circumference in mm, R = Revs per minute and S = speed in km/hr

 

It should be obvious that C x R is the distance travelled (in mm) in one minute

 

So 60 x C x R/ 106 is the distance travelled in one hour in km (ie: km/hr!) and this is equal to the speed S

 

So S = 60 x C x R / 106 km/hr

 

Rearranging this equation, we can state that R = S x 106 / (60 x C)

 

So to get the 10:1 relationship above we need to ensure that R = 10 x S or, in other words, that 106 /(60 x C) = 10

 

So if 106 /(60 x C) = 10

 

this means that C = 106 /(60 x 10) = 1667mm (This is equivalent to approximately a 21" cycle wheel diameter so no problem with the computer's range being able to handle it)

 

I tried it with a cycle computer I own and even though the speeds in km/hr are way above what one would expect from a bike, the internal electronics have a VAST range; mine allows circumferences from 1mm up to nearly 4000mm! and speeds up to warp levels! The beauty is that you now have a cheap and very accurate tacho which can be used to check out the main tacho or fitted to a boat without a tacho. A wireless version will obviate the need to run wires up to the control panel too and they only cost about a tenner for a wired version and a couple of quid more for the wireless version.

 

I glued the tiny magnet to the pulley using B&Q "Serious Stuff" glue.

 

Chris

chris w Veteran II

chris w

Posted
  • Author
Posted (edited)

I am humbled................ :lol: I even contributed to that thread (in a comment about reed switches) but had totally forgotten about it. Hopefully the maths will at least be useful to some.

 

Chris

Edited by chris w
boatchad Contributor

boatchad

Posted
Posted
There was a thread on here somewhere in the last few days where someone was querying the idle speed on a new engine install as their rev counter indicated only 600rpm and the manufacturer stated 850rpm. It was suggested that the tacho may need recalibrating for the new engine - but how does one calibrate it? That needs another calibrated tacho!!!

 

It then occurred to me that one could use a cycle computer to measure rpm quite easily and this could be used to measure either engine revs or indeed alternator revs.

 

Without the maths, here's how to do it.......

 

Mount the magnet on the crankshaft pulley with the detector on the engine block, set the wheel circumference on the cycle computer to 1667mm (or, the same thing, set the wheel diameter to 531mm) and bingo. It does NOT matter how far away from the centre of the crankshaft pulley you glue the magnet; the result will always be identical and accurate.

 

Switch the bike computer to speed in km/hr and simply multiply the answer by 10. So if the cycle computer says 93.5km/hr,the engine is rotating at 935 rpm and so on.

 

For those interested, the maths is as follows:

 

Let C = circumference in mm, R = Revs per minute and S = speed in km/hr

 

It should be obvious that C x R is the distance travelled (in mm) in one minute

 

So 60 x C x R/ 106 is the distance travelled in one hour in km (ie: km/hr!) and this is equal to the speed S

 

So S = 60 x C x R / 106 km/hr

 

Rearranging this equation, we can state that R = S x 106 / (60 x C)

 

So to get the 10:1 relationship above we need to ensure that R = 10 x S or, in other words, that 106 /(60 x C) = 10

 

So if 106 /(60 x C) = 10

 

this means that C = 106 /(60 x 10) = 1667mm (This is equivalent to approximately a 21" cycle wheel diameter so no problem with the computer's range being able to handle it)

 

I tried it with a cycle computer I own and even though the speeds in km/hr are way above what one would expect from a bike, the internal electronics have a VAST range; mine allows circumferences from 1mm up to nearly 4000mm! and speeds up to warp levels! The beauty is that you now have a cheap and very accurate tacho which can be used to check out the main tacho or fitted to a boat without a tacho. A wireless version will obviate the need to run wires up to the control panel too and they only cost about a tenner for a wired version and a couple of quid more for the wireless version.

 

I glued the tiny magnet to the pulley using B&Q "Serious Stuff" glue.

 

Chris

 

Chris why not just use a cycle computer with a cadence option?

 

Wouldn't that give you the rpm without the maths?

chris w Veteran II

chris w

Posted
  • Author
Posted
Chris why not just use a cycle computer with a cadence option?

 

Wouldn't that give you the rpm without the maths?

I haven't checked it out, so you may be right, but I suspect it wouldn't read high enough to give engine rpm. Cadence on a bike would be around 80 -120 rpm max.

 

Chris

Guest

Guest

Posted
Posted (edited)

Interesting idea. I have wondered about a 240v lamp running off the inverter (or shorepower) at 50hz and a tippex mark on the crank pulley. That is how I used to calibrate my (dare I say it) gramophone.

Edited by Guest
alan_fincher Veteran II

alan_fincher

Posted
Posted
Brilliant!!

Apparently in a follow up version he is stupid enough to try enquiring about a galvanic isolator.....

chris w Veteran II

chris w

Posted
  • Author
Posted (edited)
Interesting idea. I have wondered about a 240v lamp running off the inverter (or shorepower) at 50hz and a tippex mark on the crank pulley. That is how I used to calibrate my (dare I say it) gramophone.

Hmmm. The lamp will be flashing at 100Hz. How would you know which multiple of 100Hz your engine is running at when the tippex mark is stationary?

 

You would have to make 2 tippex marks accurately positioned so that they appear stationary at the desired revs. If my maths is correct, as an example, a 4" diameter pulley with 2 marks exactly 2" apart on the outer edge would appear stationary to a mains lamp at 960rpm.

 

Chris

Edited by chris w
paulcatchpole Rising Star

paulcatchpole

Posted
Posted
Apparently in a follow up version he is stupid enough to try enquiring about a galvanic isolator.....

 

Worse still, a new alternator...

 

"Need an insulated return with that? How about a controller?..."

 

PC

Amicus Rising Star

Amicus

Posted
Posted
Interesting idea. I have wondered about a 240v lamp running off the inverter (or shorepower) at 50hz and a tippex mark on the crank pulley. That is how I used to calibrate my (dare I say it) gramophone.

I did something like this years ago. I used one of my 2D work lights and worked out a pie chart in Excel, different colour for each segment, printed it and glued it to a CD disc then attached that to the pulley with Velcro tape. Used it to adjust the idle on me van. Fact, thinking about it now, I made two, one for cold idle and one for warm idle.

John Orentas Veteran II

John Orentas

Posted
Posted

During my racing days I bought a DIY electronic rev counter.. The so called state of the art ones at the time were grossly inaccurate and the mechanical movement would fall in bits after a couple of laps of a circuit.. The concept was quite clever as it had no moving parts at all, thirty two LED's were arranged around the circumference of the dial, they would progressively illuminate as the revs went higher..

 

Never did get the bloody thing to work.

OptedOut Community Regular

OptedOut

Posted
Posted (edited)

Did a similar set up to check my lister max RPM, worked out the surface speed for the hand start rachet drum at 1500 RPM and just held the front wheel of my fold-up bike against it.

Adjusted engine to get the right MPH on the bike speedo.

 

Edit: The old bike speedo I used would not work at anywhere near the range of the new ones it would appear.

Edited by OptedOut
OptedOut Community Regular

OptedOut

Posted
Posted

Some info from my Fitter/Turner days, handy for pulley ratio/diameter etc.

 

N = 12*S

P*D

 

D = 12*S

P*N

 

S = P*D*N

12

 

N = Revs/min

D = Diameter in inches

S = Surface speed in Feet/min

P = 3.142

chris w Veteran II

chris w

Posted
  • Author
Posted
Edit: The old bike speedo I used would not work at anywhere near the range of the new ones it would appear.

 

Modern bike computers typcally will indicate up to 199.9 km/hr which means that one can monitor revs up to 1999 rpm

 

Chris

chris w Veteran II

chris w

Posted
  • Author
Posted
During my racing days I bought a DIY electronic rev counter.. The so called state of the art ones at the time were grossly inaccurate and the mechanical movement would fall in bits after a couple of laps of a circuit.. The concept was quite clever as it had no moving parts at all, thirty two LED's were arranged around the circumference of the dial, they would progressively illuminate as the revs went higher..

 

Never did get the bloody thing to work.

This is the same idea I used to create an input to the automatic delta/star alternator switcher (on another thread). With the advent of programmable microprocessors, doing such stuff is (electronically) very simple nowadays.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share
Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.