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dickie wrote:I stumbled across the answer within the last week. Either on here or the other unmentionable one. It was specified in revolutions per kilometre and I'm pretty confident that it was 1000 cable revs per kilometre. I'm pissing around with my speedo at the moment so I'll measure it on mine either tonight or tomorrow.
Post by lambojohn » Tue Jan 24, 2017 6:39 pm
So 1610 revs per mile
1 mile per minute=60mph
60mph =1610rpm
30mph = 805rpm
All agreed?
Warkton Tornado No.1 wrote:Post by lambojohn » Tue Jan 24, 2017 6:39 pm
So 1610 revs per mile
1 mile per minute=60mph
60mph =1610rpm
30mph = 805rpm
All agreed?
Not quite. Based on one mile divided by Rolling Circumference (RC) & a worm wheel/worm ratio of 13/10, my calculations result in something different....
One mile = 1760 yards
Multiply by 36 to obtain inches = 63,360”
The theoretical* RC of a 3.50 x 10” tyre is 53.407”
Therefore, the number of revolutions the wheel will make in one mile is 1,186
So, if the machine travels @ 30 mph for one hour, there will be 30 x 1,186 wheel revolutions in one hour = 35,580
However, the speedo cable rotates @ 13/10 times faster. 2967 x 1.3 = 46,254 RPH
Divide that by 60 to obtain RPM.
771 RPM @ 30 mph
Or 1,000 RPM @ 39 mph
* The only assumptions I’ve made is whether my maths is right & the RC (Rolling Circumference) which should be measured...
I hope this helps.
Post by Fast n Furious » Wed Jan 25, 2017 3:48 am
Your calcs work out the Revs per mile for the speedo cable to be 1542 r/mile.
The true rolling circumference would of course be less due to tyre / road compression.
So the calculated rotation of 53.4 inches / rotation, would only need to be reduced to just over 51 inches to give the Innocenti produced figure of 1610 revs per mile.
Measuring the true RC is a conundrum.
drivera wrote:There is a simple way of doing it , get someone to drive a car in front of you at a set speed ( say 40 ) , look at what the speedo is actually reading and remember it , then after taking the speedo unit out take the glass out and mark on the speedo housing where 40 is and what the speedo actually read , take out the speedo face and turn the pointer needle to what speed you read , using fine needle pliers hold the spindle under the pointer tightly then carefully move the pointer to 40 , reassemble and you should have a fairly accurate speedo .
I saw this in practical classics car mag and have uses it on mine , it works
drivera wrote:There is a simple way of doing it , get someone to drive a car in front of you at a set speed ( say 40 ) , look at what the speedo is actually reading and remember it , then after taking the speedo unit out take the glass out and mark on the speedo housing where 40 is and what the speedo actually read , take out the speedo face and turn the pointer needle to what speed you read , using fine needle pliers hold the spindle under the pointer tightly then carefully move the pointer to 40 , reassemble and you should have a fairly accurate speedo .
I saw this in practical classics car mag and have uses it on mine , it works
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