Plunger speedo gearbox ratio

[olev]

Plunger speedo ratio.
Anybody know off the top of their head how many turns does the speedo drive in the gearbox make per rear wheel rotation.
I had the missus trying to count while I turned the rear wheel but she gets cranky easy.
and vicious,   cheers

[Greybeard]

I had the missus trying to count while I turned the rear wheel but she gets cranky easy.
and vicious,   cheers

I have one like that! 

[olev]

no takers? I'm trying to set up an acewell 2853 electronic speedo that's been lying around for a few years.
I want to drive a reed switch off the gearbox speedo output.
The thing is programmed off the circumference of the rear wheel.
I think I'll have to divide the circumference by the number of reed pulses per turn of the wheel.
Can't go back to the missus.
Could set up a magnet and reed switch on one of the wheels but its not as sexy.
cheers

[chaterlea25]

Hi
I wonder if a reed switch will work at higher speeds , at 60mph the cable spins at approx 1600 rpm
That's a lot of switching cycles 
I think you need to go for electronic pulses ?
John

[Swarfcut]

Data from Avon Tyres.  Revolutions per mile, rear wheel.


                                Tyre Size                       Revs/Mile

                              3.50 x 18                          825

                              3.50 x 19                          795

                              4.00 x 19                          783

 Source, Roy Bacon, BSA Twin Restoration, page 217.

 Swarfy.


 Additional. Lying in the dirt yourself is your best option.  Persuasion to turn the wheel should be easy.

[Ted_Flash]

Doesn't an original chrono speedo have the number of revolutions per mile printed on the face in tiny writing? I can't help with the number  as mine's not original, but should be about 1600.

[RichardL]

First, I don't know if plunger ratios are the same as swingarm, but I'm guessing they are. Here's my take on it from a while back.

https://www.a7a10.net/forum/index.php?topic=701.msg4399#msg4399

Once there, you need to read some of the following posts that advise regarding applying the conditions that applied to my bike to all,or other, bikes.

Richard L.

[olev]

The rear tyre is an 18x3.50 roadmaster. The radius of the thing is round about 2030mm.
Its pretty easy to work out the revolutions per mile but that is no good to me.
If I used a magnet and switch to measure rear the wheel revs, its one rev per pulse and I just plug 2030 into the speedo and it gives me km/h.
but if the ratio is 2 pulses per wheel revolution it will need to have 1015 programmed into it.
It will probably need fine tuning with a gps or something anyhow but I'd like to get it close.
John, I think the reed switch will be ok. It came with the speedo and goes in series with the cable drive.
I've seen a couple of specs for swinging arm speedo gearboxes but am not confident they are the same.
I've approached the missus again but it doesn't look good.
She still remembers our attempts at bleeding car brakes from the old days.
cheers

[olev]

Just for the record, 47 turns of the back wheel is very close to 100 revolutions on the speedo drive.
John, you were right. The sensor turns out to be a 2 wire hall effect device which gives a current output rather than a digital switch.
It seems to give good voltage switching points across a 249 ohm resistor wired in series with the output leg but.
cheers