Speedometer Types
The following speedometers are fitted to the following cars. Unfortunately
I do not have details of all the speedometers calibrated in kilometers per
hour. Speedometers can be identified by the maximum speed given on the scale
and by the number of revolutions per unit distance, which given by 3 or 4
digit number immediately below odometer to the right, under the tens or units.
Austin A30 Saloon (AS3). Circular speedometer - max speed 70 mph.
Gearbox speedometer drive ratio: worm/pinion = 2/9. Tyre size 5.20x13. Rear
axle ratio crown/pinion = 36/7 (5.14:1) ( to car 1018), 41/8 (5.13:1) ( 1019
on). Mph speedometer does 1040 revs per mile. (wormxgearxratchet= 1x20x52).
Austin A30 Saloons (AS4,A2S4). Trapezoidal speedometer - max speed
70 mph. Gearbox speedometer drive ratio: 2/9. Tyre size 5.20x13. Rear axle
ratio 41/8 (5.13:1) (to car 43420): Mph speedometer does 1040 revs per mile
(1x20x52). Rear axle ratio 39/8 (4.88:1) (43421 on). Mph speedometer does
980 revs per mile (1x20x49).
Austin A30 Commercials(AV4,AP4). Trapezoidal speedometer - max speed
70 mph. Gearbox speedometer drive ratio: 2/9. Tyre size 5.90x13. Rear axle
ratio 43/8 (5.38:1). Mph speedometer does 1040 revs per mile (1x20x52).
Austin A35 Saloons (AS5,A2S5). Trapezoidal speedometer - max speed
80mph. Gearbox speedometer drive ratio: 5/13. Tyre size 5.20x13. Rear axle
ratio 41/9 (4.55:1). Mph speedometer does 1600 revs per mile (1x32x50). Kph
speedometer max speed 140kph, 1000 revs per mile.
Austin A35 Commercials (AV5,AP5,AK5,AV6,AP6). Trapezoidal speedometer
- max speed 80mph. Gearbox speedometer drive ratio: 5/13. Tyre size 5.60x13.
Rear axle ratio 41/9 (4.55:1). Mph speedometer does 1550 revs per mile (1x31x50).
Kph speedometer max speed 140kph, 960 revs per mile.
Austin A35 Van (AV8 - 1098cc engine). Trapezoidal speedometer - max
speed 80 mph. Gearbox speedometer drive ratio: 5/13. Tyre size 5.60x13. Rear
axle ratio 38/9 (4.22:1). Mph speedometer does 1408 revs per mile (1x32x44).
Austin A35 Van (AV8 - 848cc engine). Trapezoidal speedometer - max
speed 80 mph. Gearbox speedometer drive ratio: 5/13. Tyre size 5.60x13. Rear
axle ratio 39/8 (4.88:1). Mph speedometer does 1632 revs per mile (1x32x51).
The Speed Indicator.
The Distance Measurer. (Odometer)
Speedometer Problems
The only routine attention specified in the workshop manual is to ensure
that the speedometer cable is adequately lubricated. However at high mileage
and great age, problems invariably develop with the speedometer head itself,
most of which are actually quite trivial and can be readily corrected. The
main problems are:
Speedometer Needle Oscillates Wildly but Silently at Higher
Speeds.
Speedometer Noisy.
If the speedometer is screeching, but the needle continues to indicate at
least approximately the correct speed, the noise is due to lack of lubricant
in the main input bearing. Cycle oil or WD40 can be injected into the bearing
from the outside, though only with difficulty by placing the speedometer
face downwards, and it preferable to dismantle the unit and oil from inside.
When oiling from the outside it is difficult to ensure that enough oil has
been applied to the input bearing to be effective without a major risk of
oil reaching the aluminium disc, where it may eliminate the necessary clearance
between the magnet and the disc and cause oscillation problems.
If the speedometer squeals and the needle oscillates wildly, falls off or
disappears off the full speed end of the scale, the magnet may have physically
caught the aluminium disc. This suggests that the inner bearing between the
input shaft and the aluminium disc may be extremely badly worn. This, and
damage to the hairspring as a result of the accident, may make the speedometer
difficult or impossible to repair.
Odometer Does Not Work.
Speedometer Does Not Read Correctly.
Removal From Car.
Unscrew the two large screws on the rear of the speedometer case, while carefully
holding the black background behind the needle. Draw the works from the case,
taking note of the order of assembly of the rubbers and washers, which varies
between speedometers. Do NOT remove the needle or disturb its adjustment
on its spindle, even if you intend to recalibrate, as the current setting
will be a helpful starting point.
Disassembly of the Head.
If lubrication of the input bearing is all that is required, oil both ends
of it, giving time for the lubricant to penetrate, cleaning off any excess.
The speedometer can then be reassembled.
Otherwise, observe the shape of the hairspring before you do anything which
may upset it, so that you know the correct shape on reassembly. Then, with
the needle resting on its stop, gently mark the edge of the aluminium disc,
so that you know the original calibration position. Remove the needle from
its spindle (this can sometimes be difficult: hold the edge of the disc and
gently twist the needle about its spindle until it comes off, taking care
not to strain the hairspring). With a very small screwdriver, preferably
a watchmaker's screwdriver, remove the two screws holding the black background
plate.
Remove the small spring behind or to the side of the odometer that holds
down the odometer pawl, taking care not to lose it in the process. Remember
where it is attached, as this varies between speedometers.
Unscrew the four screws which secure the front plate to the rear casting.
Carefully separate the two parts, taking care of the hairspring in particular,
as the aluminium disc will only be attached by the spring once the two parts
are separated.
Overhaul of the Input Drive.
Early speedometers can be easily dismantled further by undoing the
pair of screws which lie under the magnet, and the retaining plate behind
the brass wormwheel can then be slid to one side and removed from the spindle.
The main input bearing can then be easily cleaned and lubricated. Wear in
the retaining plate can be largely compensated by replacing it the opposite
side up. Clean and lubricate the main bearing with a mixture of oil and grease
and reassemble the input shaft into its housing, and screw back the retaining
plate.
Later models are less easily dismantled because the retaining plate
is riveted in place, so disassembly should be avoided if possible. The input
bearing can be oiled from the two sides with the retaining plate left in
place. If it is decided to dismantle further, the rivets that secure the
retaining plate can be persuaded to come away by drilling the turnover from
the back, and then easing the retaining plate up with a small screwdriver.
The rivets can be glued back afterwards or replaced with small screws. On
these speedometers the retaining plate lies between the magnet and the brass
worm. Although it is not normally necessary and recalibration will be necessary
if it is done, longitudinal play can be reduced by reducing the clearance
between the magnet and the wormwheel. To do this, place the magnet on a firm
surface and tap the end of the rod, or tap down the wormwheel. Do not overdo
this, the shaft must turn freely, and the relative longitudinal position
of the magnet on the shaft must not be changed much. If the input bearing
has been removed, it can be lubricated with oil and grease before reassembly.
Clean the wheel and shaft which meshes with the brass worm, lubricate it
and be careful to replace it in the correct hole in the rear casting. Clean
(and repair or renew, if necessary) the pawl and secure it with the spring
retaining clip.
Carefully clean the bearing in the centre of the magnet with spirit. Do not
reoil it unless a very tiny quantity of clock oil can be used, as ordinary
oil here becomes sticky and causes oscillation.
Reassembly of the Speedometer Head
Replace the front section onto the rear, making sure that the aluminium disc
continues to rotate freely at all times and that the front plate and rear
casting are correctly aligned, and not, say, upside down. Replace the four
screws. The free play in the aluminium disc can be adjusted using the small
screw to the right of the spindle on which the pointer will fit. Screwing
in will reduce the play. Do not overdo this, because the play will reduce
in very cold weather as a result of differential expansion and the speedometer
will stop working.
Overhaul of the Odometer.
Early Speedometers. Press the axle at the ratchet wheel end to release
the spring locking clip which is entangled with the ratchet wheel, and rotate
it until the locking tab is clear of the pillar. The spring clip can then
be slid off the ratchet wheel. The axle is now free to slide out. The components
in order from the right hand end are: 1) The spring locking clip, 2) Ratchet
wheel (with its teeth such that the pawl will pull the wheel round), 3) brass
collar washer (plain), 4) small washer with tab to engage with axle, 5) large
washer with a tab to engage with axle and another tab to operate a spring
catch, 6) washer (type 4)), 7) number wheel, 8) washer (type 4)), 9) number
wheel, 10) washer (type 4)), 11) number wheel, 12) washer (type 4)), 13)
number wheel, 14) washer (type 4)), 15) number wheel, 16) medium washer with
tab to engage with axle, 17) coil spring, 18) dish washer with engagement
tab with axle (main curvature towards spring). Reassembly is the reverse
process, with the axle threaded through one of the pillars, with the groove
at the ratchet end. Take care not to damage the spring catches. It is easiest
to put the ratchet wheel on first and work towards the spring end, as this
avoids the washers sticking in the axle groove. Make sure that the number
wheels and the large washer 4) are the correct way round. The latter should
have the spring disengagement tab pointing in the same direction as the tab
on the left of '5' on the wheels. Slide the spring clip back over the ratchet
wheel and make sure that it engages in the slot in the axle, and then rotate
it until the tab on it engages with the locking hole.
Later Speedometers. The basic idea is the same as the early type,
but the details are different. Remove the small spring retaining washer from
between the pillar and the ratchet wheel, by pressing the axle and disengaging
the spring washer lug from the pillar and sliding the washer out. The axle
is now free to slide out. The components in order starting at the right hand
(ratchet) end are: 1) spring lock washer, 2) ratchet wheel (with its teeth
such that the pawl will pull the wheel round), 3) spring, 4) large washer
with a tab to engage with axle and another tab to operate a spring catch,
5) small washer with a tab to engage the axle, 6) number wheel, 7) washer
(type 5)), 8) number wheel, 9) washer (type 5)), 10) number wheel, 11) washer
(type 5)), 12) number wheel, 13) washer (type 5)), 14) number wheel, 15)
washer (type 5)), 16) collar, 17) washer (type 5)). Reassembly is similar
to the early models.
Adjustment of Odometer.
Recalibration of Speedometer.
In-car Calibration.
so that 120 seconds to travel 1 mile gives 30 mph. If the two speeds give
much the same error then the pointer can be moved so that it reads correctly,
and the speedometer glass and chrome surround refitted. It is not worth being
too fussy about the accuracy, as speedometers are never very accurate, if
only because the size of the tyres varies between manufacturers, through
wear and with the speed of travel. Speedometers are normally calibrated when
new to read at 30-33mph at 30mph, and 60-63 at 60mph. Provided the driver
knows what the errors are he can make allowances. If the errors at the two
speeds are very different, so that a suitable compromise seems impossible,
then it will be necessary to recalibrate more carefully, and this is difficult
on the car.
Bench Calibration.
Final Assembly.
Modification of Odometer.
Conversion of Speedometers for Different Axle Ratios.
Otherwise, the required number of revolutions per mile or kilometre can be
calculated using the formulae:
provided that all these variables are known. The effective tyre circumference
can strictly only be determined for a particular car by experiment. However
it can be estimated from the nominal tyre measurements using the formula:
For example, a 185x13, 60% aspect tyre has the
circumference = 3.142x(2x185x.60/25.4 + 13) = 68.31 inches,
and using the previous formula with an axle ratio of 38/9 and speedometer
drive ratio of 5/13,
the revs per mile required of the speedometer = 1760x36x(38/9)x(5/13)/68.4
= 1506.
Aspect ratios are not constant for older types of tyre, so the calculation
in this case can only be approximate. Table 1 gives the MPH speedometers
corresponding to the common tyre sizes and axle ratios when the speedometer
drive ratio is 5/13. The table is internally consistent, and reflects the
revolutions per mile of pre-AV8 vehicles and all Sprites and Midgets. AV8
A35 vans and MkII A40's (1440) have speedometers with fewer revs per mile
than suggested in this table.
Table 1. Speedometer Requirements for Common Axle Ratios and Tyre Sizes.
Alternatively, the standard speedometer can be modified. The design of Smiths
odometer ratchet wheels seems to be fairly standard, and it is therefore
possible to use one from another speedometer to modify an A30/35 speedometer.
One of the ratchet wheels from a 1098cc Midget speedometer is suitable to
make a speedometer giving 1472 revs per mile, for instance. Once the appropriate
worm, gear and ratchet wheel have been selected to give the required number
of revolutions, the speedometer can be calibrated from the odometer to match
as already described.
Rescaling the Speedometer
Conversion of A30 Gearbox to 5/13 Speedometer Drive Ratio.
Identification of Differential Ratios
Figure 1 Diagrammatic cross-section of A30 Speedometer Head
1 Indicator needle
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Whilst great care is taken to ensure that the technical information and advice
offered is of the highest standard, neither the author, nor The Austin
A30/A35 Owners Club, or the Officers of the Austin A30/A35 Club accept
any liability at law for any death, injury or loss whatsoever arising from
the use of advice contained in this article.
How Does The Speedometer Work?
This problem tends to get progressively worse until it occurs at lower and
lower speeds. It is normal for a small (up to 5mph) oscillation to occur
in time with the point at which the odometer is pushed by the pawl, and it
seems impossible to remove this oscillation entirely even under carefully
controlled test conditions with play reduced to the minimum. The 803cc
speedometers are much more prone to oscillation than the later ones, because
the forcing frequency introduced by the odometer driving mechanism is lower,
and is near to the natural oscillation frequency of the spring and disc at
road speeds between 40 and 70mph. Larger oscillations without any associated
noise are usually caused by dirt on the pivot bearing between the magnet
and the disc shaft. These oscillations will be worse if there is excess wear,
giving abnormal play between the shaft and the bearing in the magnet centre,
or between the input shaft and its retaining plate. This wear may have been
caused by excessive end thrust on the input shaft arising from the use of
a speedometer cable with the wrong nipple design. Cases of severe wear may
prove uncorrectable.
A tiny periodic click is normal and is caused by the pawl on the ratchet
wheel, and cannot be eliminated. Any rasping or screeching noise should be
investigated immediately. If it develops on a journey, disconnect the speedometer
drive - you will probably want to anyway! (It is legal to drive a car without
a working speedometer if it has broken on the journey, or if steps are being
actively taken to get it repaired. You could still be prosecuted for speeding,
however, if you exceeded the speed limit.)
This is usually caused by failure of the pawl mechanism. There is a small
spring which holds the pawl onto the ratchet and the odometer will not work
without it. The pawl can become worn and fail to engage with the next tooth
of the ratchet when it returns for the next cycle. Skipping figures usually
only occurs after an overhaul, and is caused by bent control springs or failure
to ensure that the number wheels are correctly locked into position.
If the odometer correctly records the distance travelled, then the speedometer
requires recalibration. Otherwise, the wrong type of speedometer has been
fitted for the combination of rear axle ratio and gearbox speedometer
drive ratio being used.
Speedometer Overhaul
As a general rule, if the odometer is working satisfactorily, the odometer
wheels are best left alone. Most failures only involve the pawl and the ratchet
wheel which are subject to wear, and loss of the pawl spring. These can be
repaired without dismantling of the odometer completely. The set of spring
catches behind the odometer can become bent, usually as a result of hamfisted
overhaul of the odometer wheels, and they may then allow the wheels to turn
round all the time. These spring catches can be bent back into place without
dismantling the odometer. For completeness, and because odometer modifiers
will need the information, dismantling of the odometers is given below, but
care must be taken not to damage the spring catches.
The stopwatch and slide rule will again be needed. In addition, the input
shaft will have to be rotated at two or three steady speeds within the range
required by the speedometer. If an electric drill is used for this, it must
be a reversing drill. I use such a drill, together with a controller, such
as is normally used to dim domestic electric lights. Make sure the controller
can cope with the power of the drill ( a 500 watt model is usually adequate).
The controller will only give a steady speed on the drill if there is a bulb
of at least 60 watts in parallel with the drill. An old speedometer cable
is used to connect the drill and the speedometer. Proceed in a similar way
as for the road test. All this can be done without replacing the speedometer
in the case, if the speeds are pencilled in on the black background plate.
When timing, it is possible on the bench to use the pawl pulls on the ratchet
wheel instead of the mileage readings, if you know how many teeth the ratchet
wheel has. This is often marked on the plate behind the odometer, otherwise
they will have to counted. So if the wheel has 50 teeth, each click represents
1/50 mile. Record enough so that the errors in stopping and starting the
stopwatch can be ignored. Produce results for two or three speeds as for
the in-car test. If all the speeds are in error by the same number, then
adjust the pointer accordingly. Otherwise, adjust the pointer so that the
lowest speed is correct. If the highest speed is then too low, there is
insufficient magnetic coupling between the magnet and the aluminium disc.
To correct this the two 'U'-shaped pieces of metal on the magnet must be
moved away from the aluminium disc. (Yes I do mean away - these 'U'-shaped
pieces draw the magnet's magnetic field near them into themselves, and make
less available to the disc, so moving them away from the disc increases the
field available to the aluminium disc.) Conversely, if the highest speed
is too high the 'U'-pieces must be moved nearer the disc. If it is impossible
to change the magnetic coupling enough, then the magnet itself will need
to be moved a little on the input shaft, but avoid this if possible.
All this is going to need quite a lot of trial and error, and a really bad
case is better cured by looking for another speedometer. There does not seem
to be a serious shortage of old speedometers.
The speedometer head is reinstalled in its case, remembering to fit the
appropriate rubber washers. Remember to clean the inside of the glass. Refit
the scale and then the glass and chrome surround. Do not attempt to fully
press down the tabs. It is unnecessary, and will probably result in damage
to the chrome. Besides, any future maintenance will be easier.
Speedometer Modifications
If the rear axle ratio tyre size, or gearbox speedometer drive ratio is changed,
then the speedometer ought to be changed to correspond. Where the new combination
is one used in one of the A30/35 models with the standard tyres or the usual
radial equivalent (145x13 for 5.20x13, 155x13 for 5.60x13, 165x13 for 5.90x13)
it is simply a question of locating the appropriate speedometer. The basic
speedometer data given earlier will help here.
Ratio
5.20x13
145x135.60x13
155x135.90x13
163x13
5.38
1890
1830
1790
5.13
1800
1750
1710
4.88
1720
1660
1630
4.55
1600
1550
1520
4.22
1480
1440
1410
3.93
1380
1340
1310
3.73
1310
1270
1240
In practice, it will be necessary to choose the nearest speedometer actually
available. Where originality is unimportant, it should be quite easy to find
a speedometer from another type of vehicle which will have roughly the right
number of revolutions per mile.
2 End float adjustment screw
4 Hairspring
6 Odometer pawl
8 Odometer wheel retaining spring
9 Front plate
10 Front plate retaining screw
11 Aluminium disc
12 Permanent magnet
13 Worm gear
14 Input shaft
15 Input shaft retaining plate
16 Rear casting
17 Screws to hold outer casing
18 Odometer drive pinion
19 Spring retaining clip
20 Bearing
(c) R H Johnston, 1998. Material may not be published for profit
without permission.