speedo drive ratio... ???

[JonnoSkye83]

hi all,
just wondering...  my trike has yamaha front forks and wheel (xt600 if i remember correctly)
it also has its original speedo drive unit. ive seen a few aftermarket speedos on fleabay that state...  "60mph = 2240rpm" on them,  does anyone know if i were to buy one if it would show my correct speed?  (90/90-21 front wheel/tyre)

[brock]

   I reckon that if you spin that front tyre/wheel combination at 2240rpm you'll be doing 153mph 

    533.4mm dia wheel

    90x.9 = 81 (tyre profile ) x 2 = 162

    = 695.4  overall dia. , x 3.142

    = 2185mm circ , distnce travelled per rev

    x 2240 = 4894400mm per min , = 4894.4m per min

    x 60 = 293664 m per hour

    / 1.09 ( metres to yards ) = 269416.5 yds per hour

    / 1760 = 153.07 mph


          it's late , someone check it

[Manky Monkey]

I checked it -made absolutely no sense at all to me. 
I assume the rpm is referrring to engine revs, not wheel ones. For those without a rev counter, it gives a rough guide to how hard the engine's working?

[JonnoSkye83]

i was wondering if it was something to do with the speed the speedo drive is turning the cable...  cable spinning at 2240rpm = 60mph...  if i knew the ratio the speedo drive reduces to i could probably work it out...  thats if 2240rpm is indeed cable speed...

 

[Manky Monkey]

The chrome mini speedos have been around for years & are notoriously innacurate -the needle bounces around so much they usually read plus or minus about 30mph. More for show, to keep the boys in blue happy, than for any serious use.

[brock]

      "I assume the rpm is referrring to engine revs, not wheel ones"

  I thought that the Yamaha units were driven from the little round thingie with a sticky out bit that goes on the side of the wheel and gets held by a bumpy bit on the bottom of the fork leg. I thought I had one in the shed to check he ratio , I'm guessing there's a little bevel or worm drive box in there, however , can't find it.

   This from an American Yamaha site :-

"All Japanese Motorcycles are equipped with mechanical speedometers of 2240 RPM= 60mph! This will work on all Japanese bikes with cable driven Speedo! Universal on all Japanese Bikes - Honda, Kawasaki, Yamaha, Suzuki."

    trust it if you will

[Cabman77]

It`s late for me and I`ve had a hard day but the mini speedos are one of two ratios just can`t remember what they are( 2.1to1 keeps coming into my head but I wouldn`t trust that) but the lower of the two suits most jap bikes. I had one on my last triumph/cycle haven chop with a jap front end and it was deadly accurate and steady. I`ve ridden with others that were unreadable but that one was spot on.