Alvis Stalwart 6x6 Transmission and Wind up

All wheels on the same side are permanently connected together, with no differential between them. That is to say that on each side, the front, middle and rear wheels of that side are permanently connected together. This is what gives the Stalwart it's amazing off road capability. This system works fine when the vehicle is permanently off road. 

However, for road work, the transmission can suffer from wind up through the transmission if basic instructions are not followed, and vehicle maintenance and checks are not completed. 

STALWART "WIND-UP" 

The following notes explain what is meant by "Wind-up" of the Stalwart transmission and the ways in which this can be minimised. It should be brought to the attention of all drivers and supervising NCO's and Officers. 

1. The Stalwart has six driven wheels. The three wheels on each side are driven at the same speed and no differential action is provided, as there is in the rear axle of a conventional motor car. 

2. If the circumference of one tyre is different from the circumference of any other tyre on the same side, and if they are rotated at the same speed, then the one with the larger circumference will try to travel further than that of the shorter circumference. Thus if the larger circumference tyre is on the rear wheel station, it will try to overtake the front ones; this tends to distort the chassis until the point when something must give. Either the tyre slips, or the vehicle jumps when it hits a pothole and slip occurs. 

3. When going across country, the designed role, the wheels continually slip as obstacles are overcome and wet or loose patches of soil are encountered. On smooth metalled roads this "stress relieving" is not so easily possible. 

4. This difference in circumference is aggravated by: 

a. Poor matching of circumferences* of tyres after normal use and wear. 

b. Distortion of tyres by failing to adjust the tyre pressure (particularly on rear wheels) after loading. 

c. Distortion of tyres by overloading. 

5. Wheel changing between vehicles at regular intervals is highly recommended to achieve the best match. 

6. The faster a Stalwart is driven the more serious the implications of having unmatched tyres. Hence fast, long distance motoring on Autobahns is a serious stress on the vehicle transmission which could lead to premature failure. 

*NOTE
As a simple guide for supervising officers, the disparity of tyre circumference can be roughly estimated (ie to see if a more careful check should be carried out immediately) by checking on the difference in height on level standing, of the top of the centre of the wheel hub. A difference of 1/2" on this height indicates a variation of approx 3" in the circumference. This is in excess of the laid down limits. 

17. Remedies

Regular tyre measurement and wheel changing as necessary (see paras 18 & 19).

a. Avoidance of long fast runs on roads, especially Autobahns. Maximum distance to be travelled on Autobahn or similar type roads is 100 Km. After no more than 50 Km turn off the road and relieve "wind-up" as in c below,

b. Driving cross-country, or on rough tracks for about 2 Km to relieve 'wind-up'.

c. Warning of “wind-up” can be given by use of white parallel reference lines on wheel hubs.

18. Tyre Measurement.

Every 625 miles (NOT 1000 Miles) and whenever a different wheel is fitted, adjust the tyre pressure to "road unladen" figures, having first ensured the tyre is "cold". Then jack up each wheel, measure the circumference and record the measurements in the vehicle documents. This should be done with the issued caliper gauge, if available, otherwise a tape measure or string and a ruler will suffice.

19. Wheel Changing. 

The effects of "wind-up" are minimised if the wheels are fitted in matched pairs of the same or nearly the same circumference with the least worn (greatest circumference) to the front and the most worn to the rear. The permitted tolerances are:

a. The difference between the longest and the shortest tyre circumference on one side of the vehicle must not exceed TWO INCHES (5 cm).

b. The difference in circumferential length between the two sides must not exceed ONE INCH (2.5 cm) for each pair of wheel stations.

From the measurements taken it will be seen which wheels need to be moved to another station, or even changed with another vehicle.

Research and Development of the Stalwart drive train transmission windup

By Kit Spackman

I worked in the R&D Department at Pressed Steel Company * from 1961 to 1969, we were seen as the 'hi-tech' area of BMH ** once that grouping was formed, especially for our familiarity with strain gauges and their uses. We did a number of 'external' jobs for other parts of BMH after 1965-66, such as the drive shafts on the Austin gas turbine truck, later cancelled by Leyland (what a surprise...) and front suspension loads on the Jaguar XJ6. At around that time Alvis were having problems with windup of the transmission shafts of the Stollys and asked us to do some gauge work on the final drives. One of the other Test Technicians and I went up to MIRA and put the gauges and instrumentation on the test vehicle they had up there, a Mk2 as it had the sloping cab windows, and stayed there to run the instrumentation while they did the tests.

Applying the gauges was a REAL pain as the shafts were so short and access was 'difficult' to say the least! But we managed it after a few abortive attempts and the Alvis test driver went through his series of test runs, most of which went round the Cross Country Circuit or over the Spoon Drains. A year or so before my boss, Bob Goff, had had a 'British Standard Pothole' built outside the Cross Country Circuit, which was a 9" deep hole maybe 6 ft x 8 ft, through which we'd drive test cars to test their suspensions and structures, and on one run the Alvis bloke drove the Stolly through the Pothole. Amazingly the wind-up was less after that test and he said he was going to try it off a larger step to see if that helped, his theory being that when the centre wheel set was off the ground the wind-up un-wound itself.

And it worked, he took the Stolly off a 2 ft ledge that was a wall of one of the test sites there and all the wind-up went away, which pleased Alvis no end. I don't know what happened after that as they decided they didn't need any more strain gauging and we went back to Cowley. Rumour had it that Alvis had recommended the driving off a step kerb as a standard technique to unwind the transmission after that, but it was only a rumour at our end of the information chain.

Ref * - The Pressed Steel Co. was formed in 1926 by William Morris, later Lord Nuffield, and William Budd, the American steel pressing magnate, to build car bodies for Morris Motors. Their factory was right across the road from Morrises and the bodies were carried across by a conveyer, and nowadays the Pressed Steel plant is BMW's Mini factory.

Ref ** - In the '60s Pressed Steel was bought wholly by BMC, the British Motor Corporation, and merged with Fisher Body to become Pressed Steel Fisher. Later on BMC merged with Jaguar, Triumph, Rover, Alvis and some others, to become BMH, British Motor Holdings. And before long the whole shebang was bought by Leyland, and became British Leyland - of which the less said, the better.

Kit Spackman, from Lydney in Gloucestershire

Stalwarts were designed to carry 5 tons off road. The weight of the load brings to hull down, reducing the angles that the tractor joints mesh. When the vehicle is empty, the joints of the two tracta joint parts risk damage, due to angle that they mesh at.