Smiths Electronic Tacho Error

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robs ss said:
Anyone know what the number of teeth on the front and rear pulleys of the cNw belt drive is?
Requires a fair bit of disassembly to count!
Cheers

Edit: Just looked on Matt's site (should have done that first) 32 front 70 rear
Click to expand...
Within a decimal point or two it’s the same ratio as standard I believe?
 
So...
A. 32 to 70 (primary) equals 2.1875 reduction
B. 21 sprocket to 42 rear equals 2.0000 reduction

Total reduction (A x B) equals 4.3750 (ie engine revs per rear wheel rev)

Therefore 3500 rpm (to choose a convenient number) equals 3500/4.3750 = 800 wheel rpm
800rpm equals 48,000 revs per hour

Measured tyre circumference is 2.106m (this won't allow for centrifugal enlargement of that value at speed)

48,000 x 2.106 = 101.1 km/hr (divide by 1000 for km)

The Smiths, when reading steady and "usual" has 3500rpm equating to 62mph (100km/hr)

So it appears the Smiths (when behaving) is correct with only a 1.1% error - and that could be the "larger" tire at speed, although 800rpm isn't really spinning that quickly.

(I really have to get out more and find something to do..... ;) :rolleyes:)

Cheers
 
Last edited:
robs ss said:
So...
32 to 70 (primary) equals 2.1875 reduction
21 sprocket to 42 rear equals 2.0000 reduction

Total reduction equals 4.3750 (ie engine revs per rear wheel rev)

Therefore 3500 rpm (to choose a convenient number) equals 3500/4.3750 = 800 wheel rpm
800rpm equals 48,000 revs per hour

Measured tyre circumference is 2.106m (this won't allow for centrifugal enlargement of that value at speed)

48,000 x 2.106 = 101.1 km/hr (divide by 1000 for km)

The Smiths, when reading steady and "usual" has 3500rpm equating to 62mph (100km/hr)

So it appears the Smiths (when behaving) is correct with only a 1.1% error - and that could be the "larger" tire at speed, although 800rpm isn't really spinning that quickly.

(I really have to get out more and find something to do..... ;) :rolleyes:)

Cheers
Click to expand...
Rob,
I have checked my electric speedo with 2 different GPS units and found the unit to be within 1-2 mph off which works fine for me.At slower speeds it is spot on.
Mike
 
robs ss said:
So...
32 to 70 (primary) equals 2.1875 reduction
21 sprocket to 42 rear equals 2.0000 reduction

Total reduction equals 4.3750 (ie engine revs per rear wheel rev)

Therefore 3500 rpm (to choose a convenient number) equals 3500/4.3750 = 800 wheel rpm
800rpm equals 48,000 revs per hour

Measured tyre circumference is 2.106m (this won't allow for centrifugal enlargement of that value at speed)

48,000 x 2.106 = 101.1 km/hr (divide by 1000 for km)

The Smiths, when reading steady and "usual" has 3500rpm equating to 62mph (100km/hr)

So it appears the Smiths (when behaving) is correct with only a 1.1% error - and that could be the "larger" tire at speed, although 800rpm isn't really spinning that quickly.

(I really have to get out more and find something to do..... ;) :rolleyes:)

Cheers
Click to expand...
Rob, what rear tyre are you using out of interest ?
 
@ROB

I think you need to measure loaded tire circumference. You can do this by sitting on the bike, placing a chalk mark on the tire and the pavement, then "walking" the bike with your weight on it for one revolution of the wheel, marking the pavement again, then measuring the distance between the marks on the pavement.

I do not think the tire overcomes the loaded tire radius by centrifugal force at speed. By think, means it is only a "gut" estimate. Perhaps you can get someone to visually observe the tire profile when riding.

Slick
 
FWIW; General Motors, in truck specifications of the 1950s, published the rolled radius of the various tire size options when loaded to capacity. It's quite different from the published diameter of the tire, as one would expect. That would give buyers (specifiers) information on what gear ratios and engines they would need to operate in their terrain. Both gear ratios and engine sizes came in 10% increments, i.e. slow slower and very slow. Nortons, on the other hand can be specified to be fast, faster and very fast. My 850, two up, could accelerate from 50 MPH to 80 MPH in the distance it took to pass a loaded GMC log truck, back in the day when I did such things. :)
 
texasSlick said:
@ROB

I think you need to measure loaded tire circumference. You can do this by sitting on the bike, placing a chalk mark on the tire and the pavement, then "walking" the bike with your weight on it for one revolution of the wheel, marking the pavement again, then measuring the distance between the marks on the pavement.

I do not think the tire overcomes the loaded tire radius by centrifugal force at speed. By think, means it is only a "gut" estimate. Perhaps you can get someone to visually observe the tire profile when riding.

Slick
Click to expand...
A tyre can grow considerably at speed, as I have learned to the tyres detriment before when I’ve mounted mudguards too close !
 
Fast Eddie said:
A tyre can grow considerably at speed, as I have learned to the tyres detriment before when I’ve mounted mudguards too close !
Click to expand...
Agree, but the comment was it oesn't change effective rolling diameter, if this is the case: "I do not think the tire overcomes the loaded tire radius by centrifugal force at speed". An interesting thought.
 
Mart UK said:
Agree, but the comment was it oesn't change effective rolling diameter, if this is the case: "I do not think the tire overcomes the loaded tire radius by centrifugal force at speed". An interesting thought.
Click to expand...
It must take quite a lot of force to make a tyre grow. So to my mind, that force could well be enough to counteract the weight on the tyre and increase the effective diameter.

Under hard acceleration I can imagine the rear being compressed. But at high / top speed I think it could be different.

But I’m really not sure how we can test this theory… unless someone wants to get inventive with their Go-Pro !?
 
I remember seeing films of a car tire at the maximum rated speed for the tire. While a car puts a greater load on the tire than a MC, the speeds were well in excess of 100 mph. According to the films, the tire remained flattened at speed where it met the pavement, and bulged towards the rear at the point the tire left the pavement. EX. if one viewed the tire moving right to left, and the tire contacted the pavement at 6 0'clock, the tire bulged at 5 o'clock. The behavior of the tire was frightening, it made me buy only the best tires after that.

Slick
 
This is an electronic tachometer thread , correct ?
My biggest concerns have been the drop outs from a poor battery , poor worn switch , stuff like that.
Generally speaking the abysmal horrid photocopied instructions made my life a pain for at least a year until I found help wiring it all up properly.
Accuracy I no longer care about.
 
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