Many thanks to those who have emailed with information about double front shock absorbers, in response to my wanted post. Unfortunately, gentlemen,you missed the point that I am looking for a shock absorber whose arms connect to the ends of the axle, rather than the more inboard standard Austin locations. Pigsty used to produce these, but have stopped pending introduction of a new design. Meanwhile I am trying to overcome the problem of axle tramp at speed (over 65 mph!) on my Ulsteroid, in preparation for the Vintage Revival at Montlhèry.
Robert do you mean the set up on page 185 of the green book. I have a similar version of this and I believe it is a Pigsty one.
From memory this does not work as the geometry of the dampers and the axle conflict resulting in a locked axle. One side needs a link allowing the other to act as a very short control rod.
I have run the green book setup very successfully on my special for over 30 years now. Provided you incorporate rubber bushes in the ends if the arms the change in length created by their arc seems to be of little consequence with the very small amount of moment required for an effective sports/racing setup. I have found it desirable to run the dampers very tight with a comparatively stiff spring otherwise there is a tendency to jack the inside rear wheel in fast tight corners. As mentioned by Paul a link in one end may be preferable for a road car with more standard springing as suspension movement will be far greater. I don't think the long arm setup would provide terribly effective damping but it would fix the roll centre, which is a failing of the other arrangement.
I have seen a picture of a double shock absorber setup with two friction units, one each side of centre, each unit having shortened control arms to suit the original links to the axle. These should be much more effective as shock absorbers, but no benefit for sideways control. Robert, is this what you were originally referring to? Someone in England was advertising them for sale, but I can't remember where I read it.
I made mine by cannibalising a couple of standard front shockers and getting a bit of a 4"wide strip of mild steel to make the wider centre plate. It was carefully constructed by putting the outer arms in situ and cutting the centre plate to fit! Not difficult if you have a hacksaw, and angle grinder and some big files. The wider centre plate is a useful mounting platform for the timing strut!
On re reading your original post, the problem is axle tramp over 65 mph, I doubt the problem is entirely to do with damping. Have you checked wheel balance, play in steering joints and king pins? even slight play in the KP will cause tramp. Is toe and castor correct? What size wheels and tyres are you using, are the wheels true, and what pressure in the tyres?
Many thanks to all who have contributed to this thread, which strayed well beyond my original request. I have now agreed the purchase of the shock absorber I wanted. I already had in mind to check the other points which have been raised, and look forward to enjoying the Vintage Revival again.