Tacho Check

[Kiwi_Roy]

The alternator on the V11 turns at crankshaft speed. It has 14 poles (7 pairs of poles), so if you have a multimeter with a frequency range or dedicated frequency meter its a breeze to check the engine speed.

Connect the frequency meter to the two yellow wires coming from the alternator and measure the frequency.

Multiply the frequency by 60 to get cycles per minute, then divide by pairs of poles and you have the exact RPM

 

RPM = Hz x 60 / 7

 

Another way to check if you don't have a frequency meter would be to remove the alternator cover and make a mark on the rotor (spinning part)

View this mark in the light of a florescent fitting, at certain points the mark will appear to stand still (stroboscopic effect)

In parts of the world where you have 50 Hz power the light pulses at 100 Hz or 3000 times per minute, (for 60 Hz power pulses at 120 Hz or 3600 times per minute).

The mark will never actually stand still in one place but at 1500 and 3000 (900, 1800 & 3600) will appear to move slowly.

A single tube florescent will give you the most visible results.

Sorry, you can't use a battery powered one.

 

I haven't actually tried but a super bright LED powered from a small transformer with a suitable resistor should also work as a strobe light at half wave it will pulse at 50 Hz (60 Hz) or if you rectify the AC with a bridge rectifier it will be 100 Hz (120 Hz)

[fotoguzzi]

gotta love lectricity,

 

[68C]

This thread (and the recent speedo threads) reminds me of how we used to determine the safe speed in each gear when you don’t have a rev counter, most medium sized Brit Bikes didn’t of course.

 

As our old bikes tended to disintegrate above 6000rpm this was always taken as a safe limit – shock horror when the Commando was allowed to run out to 7000rpm!

 

Conveniently 6000rpm is exactly 100 revs per second. So if we remove the spark plugs, put the bike in top gear and push it for exactly 100 engine revolutions and measure the distance travelled in feet we can say that at 6000rpm the bike will cover the measured distance in one second. You may remember from school that 100mph is about 147ft/second so be prepared for a long push. You could push for ten revs and multiply the distance by ten but it may not be so accurate.

 

As one statute mile is 5280 feet and there are 3,600 seconds in an hour we can now calculate the speed in miles per hour at 6000rpm:

 

distance travelled x 3600 ÷ 5280 = speed in mph @ 6000rpm

 

This is even easier in Kilometres as of course there are 1000metres in a Kilometre.

 

distance travelled x 3600 ÷ 1000= speed in kph @ 6000rpm

 

Or more simply: distance travelled x 3.6 = speed in kilometres @ 6000rpm

 

Repeat this in each gear and mark the edge of the speedometer to indicate the safe limit in that gear. Of course most speedometers are optimistic so there tends to be a built in 10% error on the safe side.

 

If you draw a graph of ‘distance travelled’ against ‘speed in mph/kph’ ( try a vertical scale of 0 - 150mph and horizontal of 0 to 200 ft) and plot your data you can draw a straight line from zero to that point. Extend the line some distance then divide the line between zero and your point into six to indicate the thousand rpm jumps, carry on past your point for the higher revs. You can now move up your diagonal line to find the safe rpm for your bike, track across to the speed scale and you have your safe speed. You will need a different diagonal line for each gear. I have drawn the graph and saved it as a jpeg but could not figure out how to put it into this post. I don’t have a web page to upload it to.