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Speedo Irregularities

ogrilp400

Well Known and Active Forum Website User
VOC Member
Dear All,
Nigh on 40 years of fiddling with Vincents and I still come across things that mystify me. As I wrote some weeks back I am having problems with the indications on a 5" speedo. So lets go through a few things and see where I'm at.
On the thevincent.com site under instruments are some things listed.


1. "At the bottom of the speedo is a small number(1500 or 1000)"
I have 5 5" speedos here and two 5" faces. Not one of them has a number at all at the bottom of it. Two of the five are unmolested 1950s' speedos.

2. "The 1500 goes with the drive unit marked 15/12 and the 1000 goes with the drive unit marked 2/1"
I have 5 smiths right angled wheel mounted drive units. All of them are three turns of the input shaft, to two turns of the output shaft. Four of them have 25791 stamped in the bottom plugging plate. The smiths factory information shown on the velobanjogents blog site shows that Vincent only ever used the smiths 25791, 1.5:1 wheel mounted drive units on all the B and C series machines. Only 41/13 drive gears where ever used inside of the brake drum.

3. The small right angle drive unit on the back of the 5" speedo is a 1:1 ratio with a 16TPI worm drive and 19 tooth brass gear on it.


So put all this together. My HRD with a 21" front wheel has a working circumference of 81.68". At 60mph the speedo cable will be rotating at 1630rpm. Why is it then that the 5" speedo, at approximately 60mph, is showing 108 mph? I have just had the speedo serviced to sort out this problem and they tell me that they have tested it and it is working perfectly?.

Phelps.
 

Howard

Well Known and Active Forum Website User
VOC Member
Not sure this is helpful, but your cable will turn at 164 rpm at 60 mph not 1630.

H
 

ogrilp400

Well Known and Active Forum Website User
VOC Member
Maths was never my strong point Howard. Let me run through the figures.
The 21" front wheel on my HRD has a dynamic radius of 13" (measured) giving a circumference of 81.69". Therefore in a mile it will rotate 775.61 times. At 60mph it will cover the mile in one minute, therefore 775.61 RPM of the front wheel. The gear ratio inside the RH drum is 41/13 or 3.153 step up. This gives an input speed of the wheel right angle drive of 2445.51. There is a reduction in the wheel right angle drive of 3 : 2 or 1.5:1. This then gives an output speed of 1630.34. Have I missed something?

Phelps.
 

ogrilp400

Well Known and Active Forum Website User
VOC Member
Ken, who serviced your speedo.? Drop me a PM with the name if you like.

It was done by Ringwood speedo services. I clearly pointed out to them what the problem was. However when picking it up and putting it on the HRD I set off up Whitehorse Road and looked down to see that I was doing an indicated 90. I turned around and took it back but they were adamant that it was correct and that the problem must be in the drive mechanism. I found this most perplexing as I had just taken a 3" unit off and it was working perfectly with that on it.

Phelps.
 

Howard

Well Known and Active Forum Website User
VOC Member
Maths was never my strong point Howard. Let me run through the figures.
The 21" front wheel on my HRD has a dynamic radius of 13" (measured) giving a circumference of 81.69". Therefore in a mile it will rotate 775.61 times. At 60mph it will cover the mile in one minute, therefore 775.61 RPM of the front wheel. The gear ratio inside the RH drum is 41/13 or 3.153 step up. This gives an input speed of the wheel right angle drive of 2445.51. There is a reduction in the wheel right angle drive of 3 : 2 or 1.5:1. This then gives an output speed of 1630.34. Have I missed something?

Phelps.

No you haven't - It never occurred to me the 41/13 was a step up, and when I checked my calcs several times "you" were still a factor of 10 out so I thought you'd written it down wrong. 41/13 squared is about 10 so when I divided and you multiplied ............... Sorry.

H

ps 1630 rpm sounds fast for a cable drive, that's why I checked it, my Egli/Laverda/Smiths only turns about 620 rpm, and a Triumph would be about 400 rpm.
 
Last edited:

ogrilp400

Well Known and Active Forum Website User
VOC Member
No you haven't - It never occurred to me the 41/13 was a step up, and when I checked my calcs several times "you" were still a factor of 10 out so I thought you'd written it down wrong. 41/13 squared is about 10 so when I divided and you multiplied ............... Sorry.

H
No Problems. These figures have given me an insight on just how fast all those components are spinning as I ride down the rode at 60mph let alone what they are doing at far faster speeds.

Phelps.
 

nkt267

Well Known and Active Forum Website User
VOC Member
We got a refurbished speedo for my brothers Meteor..Zero miles on the clock..took it down for an M.O.T and in a 30mph zone it read 60mph and I was not passing all the traffic!. I took the speedo of my bike and mounted a gearbox and speedo gears on my lathe and ran it at around 550rpm. My speedo read about 30mph, and when I fitted the one of the Meteor it read just over 60mph.Distance to and from testing station 6 miles,distance on odometer 12 miles, so double speed and double mileage..Have you checked if the mileage is double??..We are still waiting to hear why..john
 
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