Speedo drives
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From: Grimly Curmudgeon on 22 Jul 2010 16:34 We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember wessie <putmynamehere(a)tesco.net> saying something like: >Salad Dodger <salad.dodger(a)idnet.com> wrote in >news:lt3h46lthi7d64s7sbvdeqqtqkk5462eam(a)4ax.com: > >> 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? Erm... dunno for sure, never measured it. Doubt it, frankly. Too many manufacturer quirks to make that likely. >> 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. Otoh, using an already-made item from the parts bin is possible for a bike manufacturer and it's cheaper to simply alter the electrical characteristics of the speedo head by changing a resistance (likely).
From: The Older Gentleman on 22 Jul 2010 16:58 Mark Olson <olsonm(a)tiny.invalid> wrote: > The Older Gentleman wrote: > > Salad Dodger <salad.dodger(a)idnet.com> wrote: > > > >> 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? > > > > 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. > > As far as the speedometer needle goes, you are right, but the odometer/ > trip meter is controlled strictly by gearing and cannot be adjusted by > fiddling with the magnetic bits or the spiral return spring. 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. -- BMW K1100LT Ducati 750SS Triumph Street Triple Honda CB400F Suzuki TS250 Suzuki GN250 chateaudotmurrayatidnetdotcom Nothing damages a machine more than an ignoramus with a manual, a can-do attitude and a set of cheap tools
From: Mark Olson on 22 Jul 2010 17:27 The Older Gentleman wrote: > 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. Oh? There used to be a 10 mile stretch of road marked as an odometer calibration route not far from me, the signage is not there anymore but the mileposts are still where they have been for many years. Typically I find odometers to be within a percent or two, it's quite rare to find one off by more than that. On the other hand, speedometers are usually much more optimistic, I've seen them read as much as 11% over true. Obviously this is deliberate, as the odometer calibration is nearly always perfect yet the speedo never is, and IME speedos never under-read unless they're broken.
From: The Older Gentleman on 22 Jul 2010 17:35 Salad Dodger <salad.dodger(a)idnet.com> wrote: > 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. Probably similar as long as the wheel diameters are similar. -- BMW K1100LT Ducati 750SS Triumph Street Triple Honda CB400F Suzuki TS250 Suzuki GN250 chateaudotmurrayatidnetdotcom Nothing damages a machine more than an ignoramus with a manual, a can-do attitude and a set of cheap tools
From: Salad Dodger on 22 Jul 2010 17:39
On Thu, 22 Jul 2010 22:35:45 +0100, totallydeadmailbox(a)yahoo.co.uk (The Older Gentleman) wrote: >Salad Dodger <salad.dodger(a)idnet.com> wrote: > >> 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. > >Probably similar as long as the wheel diameters are similar. Not very likely, then. |