Six Wheelers, the third way

In the beginning, it was explained to me what four and six wheel wreckers are all about. I have written about this before so I won’t labour the point. Basically a wrecker chassis is like a see-saw, one end is the imposed load on the lift forks, at the other end is the front axle weight and in between is the point about which it all pivots. On a four-wheeler, this is the centre of the back axle. On a six-wheeler with a bogie suspension, it is the centre of the bogie. So allowing for the fact that the underlift boom must be behind the last axle, the six-wheeler has a long rear overhang and hence must have a long distance to the front axle (wheelbase), unless you have a very heavy front axle.

At the time I really started to think about all this, in 1988, this was all pretty much accepted wisdom,T he vast majority of heavy wreckers were four-wheelers, with a few six and eight-wheelers. And why not? Vehicles were still running on trade plates and four wheelers did the job very well. However, even then the winds of change were blowing, so I got to thinking about this accepted wisdom about four and six-wheelers and wheelbases because it was very clear that a wrecker with a seven metre wheelbase was really a bit of a handfull

When I feel the need for some serious lateral thinking, I find a good long walk or a long drive with no distractions is most productive. At the time I was involved in car trials and happened to be driving home to Stevenage from Cornwall after the Lands End Trial on Easter Sunday. Well that was a long enough journey for anyone, particularly in a very modified and noisy Hillman Imp. So the thinking went something like this:

The crane folding boom heel cannot go any further forward than the axle bowl of the last axle, but does a wrecker have to be double drive, not really, 6×2 would be quite satisfactory. I knew that vehicles had been built with mid lift dead axles and that these relieve the rear axle of load although they also take some off the front in proportion to the axle position. A tag axle would be better and at least there is no axle diff. bowl.

So we can pull the boom heel forwards a bit now, but why stop at the axle? I had seen mid axles on short 6×2 tractors with a bend to clear the propshaft, why not a horizontal bend to clear the boom heel, in fact why not have a special axle made with a square joggle in the middle?

Well why stop there, why not remove all of the middle of the axle and leave it in two pieces. An independent axle. I didn’t know if there was such a thing, but when I got home I checked it out and found a thing called Indair. This was a split trailer axle with air suspension and a kind of trailing wishbone arrangement. No a lot different to the back of my Hillman Imp really, just a bit bigger. I got a drawing of this suspension from the manufacturers, Rubery Owen Rockwell and started to lay this in on a normal 4×2 wrecker. Up to this point I had really seen the object of the exercise to find something to add on to all the 4×2 vehicles to relieve the rear axle loads if legalisation ever came in. As a start I used the DAF 2800 chassis, as this seemed by far the most popular 4×2 chassis.

The first point to decide was how close to put the tag axle to the drive axle, at the time you could have a bogie weight of 16270 kg  at 1200 centres. Now it is normal to put the crane down boom about in line with the back of the rear tyre. What became clear was that the boom heel was still miles away from the drive axle when this point was reached. So next question, why do the bogie tyres, or even wheels, have to be the same size? I found some quite dinky tyres that would fit the Indair hubs and just about carry the load of about six tonnes. Now I have to admit that great big 12R22.5’s and little 265/70R19.5’s looked a bit funny next to each other, although it is now quite common on things like refuse wagons. The outcome was I could pull the crane right through to almost a 4×2 position.

In terms of weights, my reasoning was that if a 4×2 weighs about 12 tonnes, we add about a tonne of extra axle giving a solo weight of about 13 tonnes. If the rear bogie was about 16 tonnes and the front about 4, i.e. about 20 tonnes GVW, then we have the possibility to carry 7 tonnes imposed, have a decent front axle weight for steering and braking and stay within C&U. This was the objective, but how to work it all out? A regular 4×2 or 6×4 is fairly straightforward to work out the weights for; it is all a matter of the seesaw and moments. However when there are 3 axles all suspended in different ways, how do we know were the pivot point is? On a wrecker when you put load on the forks, the back goes down and the front goes up, in this case both back suspensions go down, but by different amounts. This looked like some hard sums! Whilst pondering this, I remembered that the Scammell Roadtrain 6×2 was similar in principle. Now this is a very clever and simple 3-axle leaf spring suspension based on an air gap in the second axle. I was never quite sure who invented this at Scammell, there are five names on the patent, but it was my predecessor as Advanced Vehicle Engineer, Richard Stone, who had to work out how to do the sums to work out the axle weights. I recalled (and had kept a copy of the calcs) that five sets of simultaneous equations had to be set up and solved longhand. I adapted the general principle to find the notional point on the chassis frame of the wrecker which actually didn’t go up or down, in effect the pivot point. Then all I had to do was solve the equations without making a mistake. Twenty pages of hard sums later I came up with the following axle weights in kg:

These figures are with a standard Mk2 Interstater boom retracted, the lift capacity is slightly less for fully extended in the usual way. However we can see that the figures are pretty closed to that required, almost 7 tonne lift and almost 4 tonne left on the front axle. It looked as though this would make a good vehicle.

All of this work was sponsored by Wreckers International with the intention that a prototype would be built and trialled, unfortunately the company didn’t last long enough and the project stopped. I understand that the configuration in essentially the same form has been built subsequently and that it works very well. I think the vehicles that have been done are all new, built from scratch and not conversions, so I still wonder what has happened to all of those 4×2 vehicles, do they just carry less load now?

So I think this proves that you can have a “legal” vehicle which is a sensible size, if you try hard enough, not a 4×2 and not a 6×4, a sort of third way.

Originally published in Professional Recovery Magazine on 9.8.00