speedo problem?

[spyke]

Hi chaps

No its not about my swimming trunks, its just that it seems that  BSA is going slower than indicated
Ive just tested it against GPS and at 40mph on the speedo its really doing 46mph.
Ive got a std gearbox with a 19t sprocket and an 18" rear wheel with a 4" tyre, and a chronometric speedo,
What makes it wrong by that amount?

Spyke

[orabanda]

It would have been calibrated for a 19" rim, and probably 3.50 tyre.

[spyke]

-Doesnt that result in the same overall radius?
-Does the speedo drive gear and the gearbox sprocket effect the reading

[RichardL]

Spyke,

What are the numbers on the face of your speedo? I think "1440" should probabaly be there. Also, don't you mean that the bike is going "faster" than indicated? When setting up a GPS speedo on my phone, it starts out with a warning that it is not necessarily accurate over short distances. Before I got a working chronometric I mounted an inexpensive bicycle speedo. I Believe the bicycle speedo is fairly accurate and was happy when the chronometric concurred.

Richard L.

[bsa-bill]

AFAIK GPS takes no account of gradient, so a bike will travel further going up hill or down dale than GPS will measure as it sees the earth's surface as 2 dimensional, I'm making some assumptions here, I'm basing this on experience with John Deere Starfire (satellite GPS system) as used on tractors accurate to 10cm, it can make some adjustment from the module on board the tractor that can measure pitch and yaw but generally if measuring with the tractor ground radar compared to the Starfire there will be about a meter difference over 100 metre run up a gradient.
Not sure if this makes your case better or worse Spike, orabanda has a good point though re wheel size

[BSA_54A10]

The speedo relies on friction and slipping discs.
After 60 years it is not beyond expectations that it might need to be recalibrated.
And yes different tyre sizes will have an affect in fact different tyre pressures will have an affect.

[iansoady]

The speedo relies on friction and slipping discs.

Not if it's a chronometric (as fitted to most (all?) A10s). These are lovely devices which do a mechanical differentiation of distance against time, with a little balance wheel ticking backwards and forwards.

I successfully repaired mine (well cleaned the big lump of grease out of it) and felt quite pleased with myself.

[RichardL]

Ian,

I was just about to say the same thing, because I didn't remember seeing any "slipping disks" in mine when I did as you, also with a very successful result. However. I found the text that follows at http://velobanjogent.blogspot.com/2008/03/smiths-chronometric-speedometer-and_21.html . Clearly, I didn't identify the clutch and I'm not sure it would result in Spyke's problems, but, indeed, there seems to be one.

Richard L.


"The escapement is mounted on the left hand side of the movement. The escape wheel is mounted rigidly to a small camshaft. and is driven through a spring loaded clutch between it and the driving gear at the base of the shaft. This gear is directly coupled to the speedometer driving cable, and provides the motive power to maintain the oscillations of the escapement. The speed of the camshaft is, of course, maintained at a constant figure according to the timing of the escapement. The clutch being caused to slip when the driving speed exceeds this figure. Next to the camshaft the main wheel assembly is seen. This consists of the three wheels known respectively as the integrator (at the bottom), the recorder wheel (in the centre) , and the stabiliser (at the top)."......"There is a certain minimum figure for accurate recording i.e. the figure at which the clutch commences to slip and at which the camshaft is driven at its proper speed. This figure is very low ? is in fact less than 5% of full scale, but it does make it impossible to start at a true zero."

[A10Boy]

John Deere Starfire (satellite GPS system) as used on tractors accurate to 10cm,

I didnt have one of those on the old Fordson Major. Didn't even have power steering 

[RichardL]

Hey, I had a Fordson tractor in 1970. I think it was a 1930, with a Model A engine and crank start.

Totally off topic.

[bsa-bill]

I started working on farms in 1960 driving a Fordson  Major it had about the same bhp as my Flash, ended my full time farm work in 2008 on a John Deere 425 bhp.
Trick with the Major on the road was to lean forward and remove the oil pan from the air filter (whilst driving) this leaned the mixture and increased engine revs.

Getting off subject here though, and an interesting subject when considering all the variables

[wilko]

The balance wheel  usually has detachable weights on it, to increase your reading remove them one at a time and check by driving the speedo with a constant speed drill bit with a piece of old cable.