Speedo drive

[veloduke]

I have a question about speedo drives;
I built a 1971 450RT from parts (which didn't all come out of the factory on the same bike); as I wanted to use this on the road, I used a Monza Jr front hub with a speedo drive. I also used a 60mm Smiths speedo (as used on my 250D).
I found that the speedo was reading slow, and the recorded mileage was low.
Now the speedo relies on a 2:1 drive from the hub, but when I measured the ratio of the drive at the wheel, it was more 1.7:1.
It then dawned on me that the Monza Jr has 16" wheels, which would explain the difference, 16" wheels having to turn more times per mile.

So my question is (finally!) is there a 2:1 drive from another hub that I can fit to the hub I have?

I'm guessing yes, since the later RTs had a speedo fitted, but does anyone know for sure (before I part with money)?

Was it only the Monza Jr that had a 1.7:1 drive?

I'm also looking out for a 450 sprocket carrier with two bearings in which was fitted to the RT - mine has only one bearing - must be from another model.

[graeme]

Hello Max

Are you going to use an RT front 21” rim? Or 18” from something else?
Could you have your Smiths speedo re calibrated to suit.
I used an Acewell electronic speedo with tach and odmeter on my RT.

Sprocket carrier from a 450 or parallel twin (wire wheel model) has 2 bearings.

Graeme

[veloduke]

Hi Graeme

I am using a 21" rim, and I recalibrated the speedometer to read (more) correctly. Ended up approx 1220rpm to give 60 mph instead of 1600 rpm.
However, the recorded mileage is still wrong, since this is a mechanical connection rather than a magnetic one. I guess I could enquire about different internal gearing, but I think it would be easier if there was a sppedo drive from another bike that I could use.

Good point about parallel twin sprocket carrier, I'll have to look, they must be more common.

[themoudie]

Aye Max and Graeme,

Please excuse my interjection Max.

Graeme, how do you find the Acewell, with monitoring the RPM?

I have an Acewell 1550 fitted to the 1974 450 MKIII and with both the points alone, points +Boyer IDU and now the Electrex Hall effect ignition, I obtain some wild fluctuations on the display. This is with both 2 and 3 turns of the sensor wire around the HT lead, or a direct connection to the coil, when that was in use and with and without the inline resistor supplied in place. Using either the 10K RPM or 20K RPM ( you wish! ) scales makes no difference either! Out of defference to the engine, I have the RPM warning light set at 7,000RPM and this blinks at me regularly.

I shall be contacting the UK supplier, Craig, for some help, but if I might have any alternative running information on a similar engine it might help the diagnosis and then the solution.

Thank you for your time.

Good health, Bill

[graeme]

Hello Bill,

My Acewell is ACE-3100
I connected it’s sensor wire to the Sachse ignition box, there is a place for it on the Sachse.

Don’t get any wild rev readings, seems steady enough.
Would I use it and feel confident on a race engine,, No.

Graeme

[themoudie]

G'day Graeme,

Thank you for your quick reply. I think I shall be contacting Craig, the UK importer to see what he recommends and maybe Electrex as well for a direct wiring solution, rather than a "wrap around" the ignition cable!

Good health, Bill