Mechanical speedo converters.

Hi all,

I Google stumbled on those little in-line mechanical speedo converters
the other day. Working on the assumption that we aren't lucky enough
to get the tidy Mph marked dash[1] for someone's Piaggio Skipper
that's awaiting the verdict of an insurance company at our local
dealers, that the converter might be an easy way to do the job
(Daughters bike is marked Kph only with some randomly applied shitty
MPH stickers).

I *think* I have the choice (enough space) of the top or bottom
fitting one but wondered:

Do they do what they say on the tin (not had an import bike before).

Would there be an advantage to top or bottom fitting if I have the
room for either?

All the best ..

T i m

[1] They said we could have it for 20 quid (IF they get to keep the
bike). I can get a converter for 25 ..?

 

/// from

T i m

 

Yes. They're not wildly accurate, mind.

 

Ok, so maybe worth a punt and if the real thing does eventually come
along (a mph speedo), I can stick it in me bits box?

Oh, ok. Are you saying they aren't a good mathematical division (kph >
mph) or that because of how they work it somehow effects the accuracy
of the speedo mech etc?

I mean, given that they *are* the right divider ratio (~1.6:1 is it?)
and that they run smoothly (so they don't cause the speedo to
fluctuate) wouldn't it then be just down to the speedo anyway?

I guess the speedo mech won't be spinning as fast so won't cover as
wide a rage as it might previously so is less accurate. Is that
something to do with it, like when using an analogue multimeter?

Whatever the outcome it can't be worse than the shitty stickers
(potentially huge parallax error to start with)!

All the best ..

T i m

 

Well, speedos aren't that accurate to start with, and the relatively
small number of gear teeth on the convertors, and the fact that they
aren't machined too finely, allows more error to creep in. Could just as
well creep in in the 'right' direction, mind. But they're certainly
'good enough'.

True.

 

NAHAY? The Older Gentleman's mathematical skills are legendary.
A reduction gear of 18/29 would underread by 0.1%, which is accurate
enough for anybody. What gears they actually put in a converter could
be completely different though.
How do you calculate the ratio of an epicyclic gearbox? More caffeine
required!

 

Which is what I've just said. But remember, as I pointed out, you do
need accurate gear machining for accurate readings to be achieved. Which
you're unlikely to find in a £5.99 widget.

Quite

 

What are you smoking? Accurate machining doesn't come into it.
Just count the teeth. If it's not worn enough to start jumping
it will work as designed.

 

Agreed, that looks pretty good to me. Thinking on what the old boy
said though it could (also) be an issue re the reduction of the
speedo needle movement when used with the converter. It'll move
18/29ths of its original design so will be compressed anyway hence
less easy to read 'detail'. In the real world it will (should) be much
better in a general readability sense so better than what's there
right now! [1]

Ok ..

Pass and awww, is it a baby epicyclic .. cute ;-)

Indeed! ;-)

All the best ..

T i m

[1] I was going to do some GPS guided on road calibration, strip the
dash down and get the Letraset out .. and still might if it all goes
belly up.

 

FWIW they are about 30 notes and *look* nicely made / engineered?

http://www.worldofpower.co.uk/acatalog/Speed_Convertoers.html

All the best ..

T i m

 

*Shrug*

OK, if you say so.

 

I had an aussie Ford Falcon XR6 (a bit like a Cosworth on steroids, with a
4L engine and lots of Tickford goodies) fitted with one of these converters.

The result was pretty accurate with an indicated 70mph being an actual 68mph
according to a gps on a straight road.

It also had the added advantage as the speed restrictor is factory-set at
240kph (~150mph) and the pick-up is in the speedo head, so with the
converter fitted it wouldn't cut-in until 240mph; though I only ever got an
indicated 176 on a hi-speed test track.