COUNTING THE MILES (1)

from the BA7C Newsletter with many thanks

It is unlikely that the accuracy of the mileometer was ever intended to be better than “give or take 3%” but, if there is a serious mismatch between miles traveled and recorded miles, there is a chance that the fault lies within the gearbox or you may have the wrong instrument fitted. Apart from the early three speed gearboxes, all gearboxes have a worm and pinion on the output shaft intended to match the turns per mile (TPM) of the propshaft with the TPM required by the speedometer to make it notch up exactly one mile. If you want to check the system out, these notes may help.

 Speedo.  Quite a bit of useful information was pulled together by Ron Burchett in the 60s and this has appeared on the Motorpages web site www.austinseven.org.au. Also in the Technical Pages are dial face profiles which can be used to restore your speedometer once they have been downloaded to a printer. The TPM (revs/mile) for the more common speedos are:­

3360 revs/mile             Smiths CA & A - belt driven off tailshaft

2240 revs/mile             Smiths PA —as fitted with most 3 speed boxes— and also Smiths PN

1040 revs/mile             Smiths rotating drum, as fitted with most of the earlier 4 speed boxes.

Some speedometers have the TPM figure printed on the face of the dial. If in doubt, fit the speedo ca­ble to a hand drill and count the number of turns for a tenth of mile reading on the milometer. 

 Propshaft:  The TPM of the propshaft will obviously vary with the size of the tyres and the back axle ratio. Bill Williams’s book “Austin 7 Specials” includes several tables which enable calculations to be made of road speed versus engine speed for various combinations of wheel size, axle ratio etc. His table of Tyre Constants, reproduced from manufacturer’s data (and presumably for new tyres) shows the number of revs/mile made by the various sizes of tyre. The tyre constant is the circumference of the tyre divided into 63,360 (inches in a mile). The figure will vary slightly between manufacturers and between old and worn tyres. You may need to do your own measuring, including counting the number of turns made by the speedo cable when the car is pushed along for a few complete rotations of the wheels. Some of the quoted tyre constants below may prove useful:

  Tyres:

  Speedometer Gears:  Examples:- assuming that a three speed box will drive through a 9:44 ratio axle and a four speed box will drive through an 8:42 axle and both are on Avon Sidecar tyres:­-

 TPM of propshaft on 3 speed car will be 782 x 44÷9 =3823

Required TPM of a PA speedo =2240
Ratio of reduction gears in gearbox should be                 3823÷2240 = 1.71

Correct gears are 4 start worm with 7 tooth pinion   7÷4 = 1.75

  TPM of propshaft on 4 speed car will be 782 x 42÷8 =4105

Required TPM of a drum type speedo                           =1040

Ratio of reduction gears in gearbox should be    4105÷1040 =     3.95

Correct gears are 2 start worm with 8 tooth pinion   8÷2 = 4.00

  Other ratios are known, e.g. 5 start on 9 teeth. If you know where they should be fitted, please drop a note to the editor. Mismatching of pinions chews up the brass worm.  Sets of new gears may be avail­able from Vince Leek. His article on this topic gives more detail in Chris Gould’s “Building Ulsters”.  If the miles clock up OK but the speed appears wrong, the fault will probably lie with the instru­ment’s magnet.  Fixing this is tricky.  Anyone who can describe how to do it, please write in.

Article taken from the Bristol Austin Seven Club’s excellent newsletter with many thanks.

COUNTING THE MILES (2)

By Vince Leek

Identifying the speedometer Head TPM
There are two ways of identifying the TPM of the speedometer head. The most simple way is to identify the number printed on the dial. For example, the TPM for most Boxes and Rubys is 1040 (Nippys are 1000). However, unless you know the history of the instrument you cannot be sure the correct dial is fitted, and consequently the most accurate method is to rotate the speedometer drive (in the appropriate direction) counting the number of turns required for the mileometer to register 1/10 mile. Multiply this by 10 to give the TPM and repeat several times, taking the average, to ensure you have not miscounted. The job can be speeded up by driving the speedometer cable with a hand brace, taking into account the gearing of the brace. Having carried out the operation you will now know the TPM of the speedometer head.

Identifying the gearbox speedo drive TPM. (Best done with two people)
To identify the TPM produced by the speedometer drive shaft from the gearbox proceed as follows: -
1.      Disconnect the speedometer drive cable from the speedometer head and fix a pointer/indicator to the inner cable.

2.      Accurately measure the radius of a rear wheel and tyre to the nearest half inch, with the tyre correctly inflated. (Measure the diameter horizontally and divide by 2)

3.      Place an identifying mark, e.g. chalk, on the rear wheel tyre, and with the car out of gear push it in a straight line until it has completed six revolutions.

4.  Count how many times the speedometer inner cable has rotated. Take this to the nearest quarter turn.

Use the following formula to identify the gearbox output Speedometer Drive TPM.
    TPM = 1680 X No. of cable turns
            Radius of rear wheel in inches

Ideally the speedometer TPM will be the same as the gearbox output, but if this is not the case it is sometimes possible to juggle with the speedometer drive gears within the speedometer gearbox to put things right. The ratio of these gears is equal to the ratio of the propshaft TPM to speedo TPM. Hence you will need to know the propshaft TPM.

Identifying the propshaft TPM
1. Measure the circumference of the rear wheel in inches with the tyre correctly inflated. (A).

2. Identify the differential ratio. (B).

3. Use the following formula to identify the prop-shaft TPM.
Prop-shaft TPM = 63360 x B
                                    A
The speedo drive ratio required will be the same as the ratio between propshaft TPM and speedometer TPM.

For example: Ruby saloon
Rear wheel (400 x 17) circumference = 79.5"
Rear axle ratio = 5.25
Inches in mile = 63360
53360x 5.25    = 4184
     79.5

The Ruby speedometer head has a TPM of 1040. Ratio of prop-shaft TPM to
Speedo TPM is therefore 4184 = 4:1
                                         1040

Hence this is the reduction ratio required from the gearbox. The most common combination in 4-speed gearboxes is a "two start" drive gear and an eight tooth pinion (i.e. Ruby) giving a ratio of 4:1.

Vince Leek (DA7C)