Speedo drives

How do they work, exactly?

I know the wheel goes round, and it drives a worm thing which makes a
cable go round which makes a needle sweep majestically across the face
of one's speedo, but ...

Is the rate of spin of the cable the same on all (most) bikes at the
same road speed?

i.e. is all the conversion done in the hub drive?

 

I know I can make my speedo more accurate by changing the gearbox thing in
the wheel hub. Apparently the one from the RT model is better at maths than
the one on the R-GS.

This suggests BMW speedos are generic items and that any adjustment for
wheel size is done by altering the gear ratio in the wheel hub. I bet other
manufacturers follow the same principle.

 

Not necessarily. What happens is that in the speedo, the twiddling cable
generates a magnetic field which pulls the speedo needle round the dial.

So I imagine you could recalibrate a speedo either by changing the
gearing (ie: at the wheel end) or by generating a weaker or stronger
magnetic field (at the instrument end).

The rate of spin won't necessarily be the same, either, because
obviously a larger diameter wheel will rotate slower for a given road
speed than a smaller diameter one.

There again, wibbleflip etc.

 

Yebbut, BMW do seem to have more commonality of instruments than most
manufacturers.

 

That's true. I hadn't considered the odometer. There again, the question
concerned the speedo rather than the odo.

We hear a lot about speedo error, but not about odometer error, yet it
affects one's perception of fuel consumption, for one thing.

I suppose nobody's ever really been arsed enough to measure it.

 

The question arises following my successful acquisition of a CBX lower
yoke ans teering stem.

Now, if I[1] can successfully transfer the CBX stem to another (say,
Bandit 12) yoke set, it makes fitting a front end dead easy.

I'll need a wheel driven speedo to make the CBX dial work, but I was
wondering as to the likelihood of the readings being in the same
ballpark.

There's always the Zumo and Tippex method, I suppose.

Of course, there's then the problem of the arse end, which a B12
swinger won't solve.

Metmachex are somewhat "pricey", though they will make a swinger to
take any rear wheel you specify.

 

Probably similar as long as the wheel diameters are similar.

 

Not very likely, then.

 

There are still a few "Mile 0" "Mile 1", etc, markers around that I
see. IME, odos are usually pretty close, too.

The street wisdom on speedo error is that manufacturers don't want to
be liable for speeding tickets (or some such) and deliberately set
speedos to over-read. Dunno if it's true, but I wouldn't doubt it. My
Hondas are around 7% off.

 

They do vary. I remember Yamaha RD350 and power-valve ones were tested
by the magazines and turned out to be astonishingly accurate - like bang
on at 60 and maybe 1 mile an hour out at 70.

ShiteOldGuzzi ones varied, as you'd expect. A friend's V50 actually
under-read badly: showed 60 at 70 (according to Plod) while my Spada,
as, er, kindly calibrated by French Plod, was showing 85mph at a
speed-trapped 135kph.

 

More than that. It's also a legal requirement that they have kph
markings as well as mph, and my Ducati just has mph.

There was a bit of a stink in the press a few years ago, because some
over-zealous MOT testers were refusing to issue MOT certificates to
Ducatis so equipped.

 

Well, mine has never been failed, but apparently the fact that "it came
like that" is immaterial. Under EU rules, all mph speedos made since
(whatever date, but quite a few years back) should also have kph
graduations, and Ducati sort of forgot )

 

(The Older Gentleman) gurgled happily,
sounding much like they were saying:

Probably more to do with the subtle detail that the speedo isn't included
in the MOT at all...

 

Indeed. It's not even covered under the 'if it's fitted it has to work'
catch all.

 

C&U then. You're right.

 

I know of one young lady who was refused her bike test because the
examiner noticed she only had MPH on the speedo - this was in Chiswick
a few years back. The bike was an import and the speedo face had been
replaced by a company in Southend. The examiner claimed that as the
bike didn't conform to British road traffic law he couldn't allow the
test to take place.

--
Lozzo
Versys 650 Tourer, CBR600F-W racebike in the making, TS250C, RD400F
(somewhere)
BMW E46 318iSE (it's a car, not one of those 2-wheeled pieces of shite
they churn out)

 

Like 700 quid for a polished one. Ouch.

What I'm now thinking is, rather than try to assemble all the bits
necessary in a piecemeal fashion, my best bet would be to buy a scabby
old B12 or XJR1300, ransack it for the bits I need(want), and Ebay the
remainder.

I've seen a big Yam for just over a grand, and forks, yokes, wheels,
tyres, calipers and discs would come to that easily, bit by bit.