A New Seat For "Little Beastie"
Construction details of my "5th wheel" Seat

ModelWorks 4 inch Scale Burrell Agricultural Traction Engine 

I had fooled by the apparent satisfactory static weight distribution on the flat as I thought a front axle weight of 140 lb was sufficient to keep it in contact with the ground.  What I now know is that 140 lbs can be used up very quickly and severely limits the operation of the engine.   My studies show that the original front axle weight of 260 lbs enables the engine to be driven on slopes, soft ground and accelerated without the front lifting.  What is significant is that while 140 lbs is easy to "use up", 260 lbs allows combinations of slope and acceleration beyond what I would want to subject my engine. 

It was clear that I could not continue to sit on the engine and my thought turned to making a riding trolley.  Whilst at Boconnoc I tried Steve Traill's conventional two wheel driving trolley and did not like it as I found it a stretch to reach the controls and that my legs hit the tender whilst cornering. 

My thought turned to a novel driving trolley I saw at the Guildford Rally.  This consisted of a chassis running on castors that was attached to the engine with a semi rigid joint such that it keeps position to the engine centre line even during turns. The answer was to use jock wheel(s) so that the driving trolley could be fixed in X&Y to the engine.  Some form of joint would be required to allow the trolley to move in Z so that the drivers weight fell on the jockey wheel.

The simplest joint possible would be a tube running inside a slightly larger tube.  The inner tube would resemble a bar stool with a seat at the top and a jockey wheel at the bottom. The outer tube would be rigidly attached to the Burrell by means of a simple frame.  This design never gelled as I was relieved when I hit on the idea of using a pair of swing arms.

My original thoughts were that both swing arms would need to be parallel and of equal length but my design options expanded when I realised that this was not necessary.   The seat needs to be free to swing up quite a long way.  The concern is that if the links don't swing un sufficiently and lock the weight of the Burrell will be transferred to the jockey wheel.  I arbitrarily decided that 5 inches would be sufficient  and  to date this has been sufficient except when I tried to winch the engine onto my trailer.  Normally I remove the seat but on this occasion as the front wheels went up my steep ramps I had to stop in a hurry when I noticed that the jockey wheel was being compressed.  Note the steepness of my ramps is such that the bottom of the tender only just misses the ground.

It is tolerable for the seat to lock up when it swings below its nominal position as the engine is able to momentarily take the drivers weight.  As I don't want this to happen in normal running I aimed to ensure that the seat could swing 2 inches below its nominal position.

While I have laboured the point it should be noted that the seat needs to swing up a lot more than it does down.  I therefore optimised my link lengths for upward travel.  The exacts geometry was found by trial and error using computer simulation however this could have just as easily been done using cardboard templates and pins. 

Ideally the driver should sit over the jockey so that the link arms just provide small forces to balance the driver on top of the jockey wheel but what if this is not the case.  I was intrigued to discover that if the drivers weight was in front of the jockey wheel that the loading on the Burrell's front wheel would be increased.  This is the opposite to simple single axle trolley with conventional linkage where sitting in front on the trolleys axle will cause the front wheel loading to decrease.

To double check this finding I made a simple model of the linkages using Lego Technic.  When the hand from the sky pushes down on the seat the front wheels remain firmly on the ground an the back wheels lift!  The reason is quite simple:- when weight is applied in front of the seat wheal the seat's vertical strut tries to tip forward placing the upper link arm in compression and the lower link arm in tension.  This has the effect of push down on the front wheels.
 

Trial fitting the lower swing arm and frame

To ensure rigidity I fabricated most of the seat from 20mm square section steel tube.   The Jockey wheel is mounted on a rectangular frame made from 25mm square tube.

The lower swing arm is attached by M8 shoulder bolts screwed into this frame.  As with my passenger trolley  I fitted internal bushes inside the frame to provide meaty M8 threads.  As I did not have any suitable shoulder bolts I made these by screwing a 1/2" outside diameter sleeve onto a standard bolt.

Rather than fit internal bushes for the jockey wheel mounting I welded M10 bolts at suitable places inside the tube.

I plan to mount a tow hitch on the back of the jockey frame which will expose the lower link arm to significant side loads as the drawbar can be up to 45 degrees to the Burrell.  I therefore kept the lower link arms short and included a cross brace made from 35x20mm rectangular section tube.  There were compromised here as the jockey frame gets in the way of the brace and the link arms need to be long enough to provide 5" of upswing with the seat remaining nearly vertical without hitting the back of the Burrell.

See the seat in action