Now most good things come to an end and often by unpredictable means. In the mid-1980s Scammell were working on a large MoD tender for a vehicle called DROPS. This was to be 1500 trucks and we hoped it would secure our future for years to come. Scammell won this business and everyone was happy, for a while. At about this time the Government became tired to tipping cash into the Leyland Empire and constructed the Leyland – DAF merger. For merger read, give-away. In order to persuade DAF to take Leyland trucks away, all manner of incentives were offered, including the DROPS production at Leyland. DAF took the bait and Scammell shut in 1987. The other thing that went up the road was the production of the Contructor 8 and Roadtrain 6×2 vehicles. This left a valuable piece of real estate and a collection of special vehicle products.

Now in the way of things, if a company has worked for a while and then shut, there must be some value in it somewhere, interested parties gathered around.

In the end it was a small company called Unipower who took over the special vehicles, primarily to complement their own range of fire crash tenders. Unipower was an interesting company in itself with quite a bit of history, principally in logging tractors. They were at the time based in Molesey in Surrey and made only crash tenders. The new Unipower company was set up in Watford, a stones-throw from Tolpits Lane in a new building with about 50 people, mostly ex- Scammell.

The initial business was to be selling spares and making whatever vehicle sales could be extracted from the designs that were available. The most promising vehicle was S24. Now this was a pretty special heavy-duty design, which apart from the few that were made into wreckers and heavy haulers, you would never see, particularly on the road. However the Scammell S24 became the Unipower S24 and in my case became the most interesting of all, the 4×2 S24 wrecker (of which more at a later date).

This was a fairly hand to mouth existence for Unipower, but eventually a light appeared at the end of the tunnel in the form of a large (100+) tender for military trucks for the MoD. This would be the kind of thing, if successful, that would help secure the future (where have I heard that before?). The contract was called BR90, which stood for Bridging for the 90s. Now like most things this product would be worth a book all to itself as would the history of military bridging which goes back to the first soldiers having to cross a river in order to fight (or run away from) other soldiers. Over the years the British Army had developed mechanised bridge systems that could cross gaps that could carry more, bridge wider spans and be erected quicker, BR90 was the latest and best. To describe how it works in detail is not easy, but in essence a cantilever rail is pushed out over the river in the same was that sections are added to a fishing rod. The difference is that these sections are about 10 metres long and made from large aluminium box sections. They also span up to 40metres. When the launch rail, as it is called, hits the far bank the actual bridge sections are suspended from it and pushed out to form a bridge that can carry tanks and all sorts. I won’t describe how this kit works, but suffice to say it is big and heavy and mounted on a truck that must get down to muddy riverbeds.

The trucks that carry the BR90 kit are of three types all based on the same chassis. The TBT or Tank Bridge Transporter, operates on its own and carries large sections of bridge which are launched by a modified Challenger tank. ABLE or Automotive Bridge Launch Equipment is the kit which pushes out the bridge sections and BV or Bridging Vehicle is the same chassis with a flat bed body which carries the extra bridge sections for ABLE. One ABLE and two BVs travel together. The BR90 vehicle was a 3m wide 8×8 with big 24R21 tyres. It was very big, very mobile and very heavy. 37 tons gross and 19 tons on the front bogie. Maybe now you can see where this story is going.

The BR90 chassis development was going very well, the whole design was completed in less than a year and the Army was very pleased with the prototype test vehicles as they were with the bridging equipment. Unipower eagerly anticipated a nice big order. By this time my involvement with the concept design of this vehicle was over and I was looking at other things in my capacity as Design Consultant to Unipower. However there was a problem. The MoD specification required the trucks to be suspend towed, from the front, by in-service vehicles. We knew that the Foden heavy 6×6 wrecker could only lift about 8 tons and the lift of a laden BR90 at the front eyes was 14.5 tons. We said that we couldn’t help it if the Army’s wreckers were too small, the BR90 couldn’t be made any lighter, but would they like to buy some bigger wreckers. No they wouldn’t, thank you. MoD wouldn’t place the order until the problem was solved. Eyes start to turn in my direction.” You are supposed to know about recovery, sort it”…. I pondered.

The problem was of course the same as picking up any heavy weight from one end, lift harder or use leverage. The vehicle couldn’t be made lighter and we couldn’t lift any further forward that the front eyes as all there was in front was a bit of cab and fresh air. An objective was placed that if the lift could be reduced to 10 tons, then an archaic piece of kit called a Dummy Axle could be used to pick it up and tow it away. Now the dummy axle is an ingenious device, which was devised in the 1950s to enable the Army wreckers of the time to pick up in-service vehicles, which were too heavy for them. Same old story. I won’t try to describe the Dummy Axle in detail, but it is in effect a single axle towed crane and spacer bar which can lift a truck when coupled up to a pulling truck and then swivel the load from behind the axle to in front to imposed load on the towing hitch.

It became clear that what we had was a kind of wheelbarrow, wheel(s) at the back, load in the middle and lift at the front. Furthermore, what was needed was some longer wheelbarrow handles. My mind went back to my time at Wreckers International. Somebody asked me one day to describe how to use coach beams (hands up who knows what they are). I didn’t, but I discovered that they were a pair of box section beams that fitted over the front axle of a coach by means of hooks, reacted up against the chassis (when coaches had a chassis) by means of timber packing and protruded out of the from the front of the coach to allow lifting by a twin boom crane with a truck hitch. Quite clever, but largely obsolete by that time. However, I dug out the sketch that I made at the time of the coach beams and thought that this was what we needed, in principle, but for a different reason.

Now 4 or 5 tons of empty coach axle is not quite the same as 14.5 tons of BR90 and whereas a pair of 6” box sections did for the coach, the BR90 needed something much stronger. When I worked the sections out it needed to be about 18” deep, this was going to be massive and very difficult to rig up. I set about tapering the section down so that the biggest section was where the maximum bending was and it started to look a bit better. Basically I didn’t use the axle hooks of the coach beams, I used tension rods to go up to solid blocks bolted under the chassis. Also instead of the timber packing, I fixed the top of the beams to the front lifting eye. The two beams were cranked to come together at a point in front of the truck where the lift was 10 tons. As each beam weighed several hundred kilograms, they were fitted with small jockey wheels under the balance point so one man could push them around.

How it worked was like this: The beams were off-loaded from the carrying vehicle, fortunately the BV has a very large truck crane for lifting the bridge sections and this was ideal. Our man pushes the two beam halves under the BR90 casualty where the top plates can be pinned to the lifting eyes. As this is near the point of balance, he then only has to push down to allow his assistant to pin the two tension roads to the far end of the beam. At this point the jockey wheels are removed and the two beams pinned together at the front. All of the pins, which were quite big, use the same size nut whatever the thread, which is tightened with a special spanner. It was about this time that this contraption needed a name and it was christened Unibeam. The spanner was called Unitool.

As the design progressed, it became clear that the Unibeam could be used in another way, apart from with the dummy axle. If there was always at least one spare BV around, could the beams be hooked up to a BV and towed that way? Now the BV had a NATO type tow hook, but that couldn’t take 10 tons imposed load, but it the hook has removed, and that isn’t too difficult with the Unitool, then a specially designed tow hook could be slotted into the same mounting hole and the beam hooked onto it. In fact the tow hook mounting was redesigned with two horizontal holes which located the new towing hitch via two pins and nuts. So how to lift it all up, bearing in mind that the Dummy Axle had it’s own crane to lift with. Well if you had a handy truck with a crane on, then that would do the job. If not it was devised that two of the hydraulic jacks supplied as standard issue with the trucks could be used to jack the beams by locating in some special welded on sockets. To support the whole issue when blocking the jacks, a pair of sliding stifflegs were provided with pins to push through and hold the truck up. In fact in its final form, a special steel box was devised that could be used as a jacking support instead of bits of wood, that could be turned on edge three times to pack the jack up. We called this the Unibox. When I say we, I must acknowledge the help of my co-designer, Andy Tucker, who took my pencil layouts, transformed them into CAD drawings and got the whole idea to happen

So the Unibeam was produced in such a way that it could be used with either the BV or the Dummy Axle. A demonstration to the Army and MoD was arranged. The outcome was a success in that both methods of towing and lifting worked well. The Dummy Axle lifted the load easily and quickly, but towing with what was, in effect, a double trailer was a bit of a performance. So although the jacking took a bit longer, the two-vehicle system was much better to tow and the BV actually made a very nice towing vehicle with its 400-bhp engine and auto gearbox. A few small modifications were made (including Unibox) and a full-scale trial was organised for the production kit. As it turned out, due to shortage of staff, I was sent to test the Unibeam, including all the setting up and jacking. That really was joining up the design, development and test cycle.

The happy ending was that the MoD finally placed the order for BR90 and even bought about twenty sets of Unibeams. So if you’re walking around an army surplus sale in twenty years time and find a set of beautifully preserved, funny shaped lumps of steels all in a great big wooden box, you’ll be able to impress everyone with your knowledge of what Unibeam is and what a Unitool looks like. The sting in the tail was that Unipower actually went bust waiting for the MoD to place the order and was bought by Alvis. All the BR90s were built in Coventry. Alvis finally shut Unipower Watford during 2000 so that’s the end of truck manufacturing in Watford, or is it? Déjà vu all over again.

Unibeam hooked up

Towing an ABLE by Unibeam

Unibeams awaiting shipment

First published in Professional Recovery Magazine on 30.11.00

Now there are a number of ways to minimise this effect, it doesn’t really matter how much comes off the front axle; it’s really a matter of what is left. The traditional method is to nail the front end down by ballasting it. In days of yore, trucks were built with a certain amount of spare metal in the front bumper area and it was quite feasible to make a heavy front bumper. This has advantages in that, due to leverage, a ton in front of the front axle will put about 1.1 tons on the front axle and take 0.1 tons off the rear. This is just what you want. The other advantage is that you can paint it in red and white stripes and it really does look tough. However, modern trucks, particularly forward control ones, have quite flimsy frames at the front and also they are all cluttered up with steering boxes, spring hangers and cab mounts. There’s not much room, let alone strength, to build in a decent ballasted front bumper.

What we’re left with is the ballast box behind the cab. Now this works exactly in reverse of the front bumper, 1 ton behind the front axle only puts 0.9 tons on the front and puts 0.1 tons on the rear axle, but this is still reasonable, but not ideal.

When you are arranging front ballast it is ideal to get as close as possible to the plated (and legal) front axle weight. Just check the plate carefully, you must be within the capacity of the axle and the tyres. Also don’t forget that the weight must include full fuel, full kit in the lockers and a person in each seat. Because of modern legislation and the availability of higher capacity front tyres, axles are now available up to 8 tons plated weight, which is a great help. Traditionally it was 6 tons on 11R22.5 tyres and 6.6 tons on 12R22.5 tyres and that was that.

The other obvious answer to this problem is two front axles, twice the capacity, twice the tyres and twice the brakes – brilliant. However, and there always seems to be a however, twin steers must be approached with eyes wide open. The normal ready source of twin steers is eight wheel tippers. You don’t have to extend the wheelbase, but make sure it isn’t too long or you will end up with an unwieldy vehicle. The second problem with eight wheelers is train weight. Eight wheelers almost never tow a trailer in this country, so most of the time the GTW box on the plate is left blank. It is the internals of the axle and gearbox that mostly determine GTW and whilst they may be up to 50 or 60 tons, it may be hassle getting the manufacturer to re-stamp the plate. Also beware of vehicles on single rear tyres, they are limited to only 30 tonnes not 32 tonnes GVW.

The final difficulty with twin steers is a bit more subtle, and it is to do with that old loading outside the wheelbase thing again. On a wrecker the front goes up and the back goes down. The rising frame at the front means that the front axle goes up most and thus off-loads most. The second axle goes up less and off-loads less. However, it is the front axle weight that has the most leverage to keep the front down. What happens is that as the load is applied, the second axle starts to carry most of the front-end load and effectively shortens the wheelbase, which is not good. The shorter the wheelbase, the worse this effect is. The eight-wheeler designer never intended it to be loaded this way.

A final problems always used to be engine power on eight wheelers, 200 bhp was not really enough. There are some much more powerful vehicles around now if you want to stick at about 400 bhp.

Now there is a good way around these problems and that is if you only want a 6×2, start with a tractor chassis. If you want to get around the tilting frame problem, the best answer is to lift the second axle. This way you have two axles to carry the front-end weight in the solo condition and as the load is taken off the front when lifting, the second axle can be lifted. This in effect converts a 6×2 into a 4×2 and most of the weight is concentrated on the first axle. Braking and steering are now as good as a normal solo vehicle. However, don’t forget that extra weight that you can carry has to go somewhere and that somewhere is the rear axle.

All this is really a preamble to the main story about how I designed a vehicle like this. At the time I joined Wreckers International, which was about the same time as the demise of Scammell, my previous employer, there seemed to be a seemingly inexhaustible supply of donor chassis to make into interesting wreckers, mostly in the Experimental Department of Scammell. At the time I started this exercise, the chassis had already been chosen as a suitable mount for one of the first of three mark 3 Interstater cranes. The customer was Dave Marks of Albany Recovery in Birmingham and it was intended that the vehicle would be somewhat of a showpiece.

Now the history of this vehicle is rather interesting. It was basically a Leyland (Scammell) Roadtrain 6×2. The story of how all the vehicle manufacturers designed there offerings for the UK 38 tonne market is an interesting story in itself, suffice to say that even though Leyland (and interestingly DAF) had decided that all you needed was an extra trailer axle, Scammell decided that what was needed was a whole new vehicle. Those people who remember the Scammell Trunker will recognise this as “déjà vu all over again”.

Although the Roadtrain 6×2 was the last 38 tonner to appear, it was a very clever and unique design. It had two main characteristics, firstly it’s very short wheelbase, this restricted the GVW to 20 tonnes, but made it able to couple up to almost all existing trailers. The second thing was the unique patented air-gap mid axle suspension. Short 6×2 tractors tend to suffer from traction problems over humps and when part laden. The Road train 6×2 had normal Roadtrain front and rear suspension. The second axle had a slipper spring with an airgap in the rear slipper bracket. The clever bit was that part laden it acted like a 4×2 with the drive axle taking most of the weight. When it started to reach it’s limit, the air gap closed and the second axle rapidly started to take its share. The hard bit was the mathematics of working out the weight distribution as it loaded up.

Now even though all this worked well in practice, the truck could not compete in the higher GVW category of 22 tonnes because of the short wheelbase. The vehicle in question was an experimental long wheelbase version converted to see how the air-gap suspension worked. The vehicle was actually extended between axles I and 2. Nothing much came of the project and there it stood until it became destined for a new life as a wrecker.

Now the first job was to extend the wheelbase to about 5 metres, however this was done between axles 2 and 3 and gave a vehicle with three equally spaced axles, which looked a bit funny. I then approached this vehicle and attempted to put the rather lonely looking second axle back where it belonged, up at the front next to its brother. This required a certain amount of ingenuity to get the steering geometry right, but a few Constructor 8 parts seemed to get it all correct again.

The new crane was fitted to the chassis with an extra slice in the spool valve to work two hydraulic rams to lift the second axle. The general design involved two levers mounted at each side of the chassis with a pivot at one end, a ram pushing upward at the other end and a cable going down to the axle. The cable meant that when the axle was on the road it could bump and articulate as normal. Now in order to work out the forces involved, it is essential to know the spring rate. I referred to some old calculations that I had brought from Scammell, unfortunately these were some early ones, which used a soft spring. The production vehicles had a much stiffer spring to make the suspension work better. This required a mid way redesign of the lifting arms. Eventually it all worked well and was quite a treat to watch the front end drop when the axle was lifted.

The way the axle weights worked out was as follows:

Loads in Kg.

You can see that the front axle could have been a bit heavier still, but with a decent load on the hook, the front axle is still very reasonable. The outer axle wheelbase was 5480 mm which still makes for a quite compact vehicle. The rear axle load is obviously very high, as it is on any 4×2, but the Leyland heavy hub axle is a very substantial design and copes well. I am told that the vehicle is going strong and now has a double drive rear bogie, i.e. an 8×4. Apart from a bit of bogie scrub, I’m sure this works very well.

Well there you are two axles to carry the load solo and one when it’s laden. Almost the best of both worlds.

First published in Professional Recovery Magazine on 12.10.00

So most rigid casualties have to make do with the wrecker brakes. If you have a standard 4×2 wrecker with 6t front and rear and a laden casualty of say, 7t front and 10t rear, you have 29t gross. Lets say the wrecker has a design weight of 7t + 11t = 18t, then it probably has about 10t of braking drag to give 60%g braking.

You may say that if the wrecker does all the work, 10t of braking drag has to stop 29t of outfit which is about 35%g. Not brilliant, but OK with a bit of care, but is this the whole story?

When the designer arranged the brake system of the tractor that your wrecker was made from, he had to provide brakes for fully laden and also take account of the lightest condition (solo tractor) that it might ever be to avoid too much brake lock-up. He knew that the front axle would be about 5t solo and 7t laden. He also knew that the rear could be 2t solo and 11t laden, these were the absolute limits, give or take a bit of overloading. Note that the front does not really change much, so most tractors make no adjustment in front brake pressure laden to empty. There are exceptions, I have seen the Ford Transcontinental has two rear load sensing valves (LSV) at the rear, one to work the front brakes. Some Mercs have a similar arrangement, but as I said, these are exceptions. ABS will make this even less likely these days.

At the rear the story is different and something has to be done to avoid 11t worth of braking going to 2t worth of axle weight. The main reason for this, particularly on tractors is to avoid jack-knifing. Jack-knifing occurs when the front brakes of an artic are gripping hard on the road, the trailer is pushing on the kingpin and the drive axle has locked up. Now when a tyre locks up going forward, it looses most of its grip, forwards and sideways. Its like pushing on both sides of a hinge, hence the name, the slightest misalignment and it all folds up with usually drastic consequences. I’m sure you have all had to go and unravel the consequent mess at one time or another. In the USA, they had a much different solution to this problem; they didn’t fit any front brakes. If the front has nothing to react against, then jack-knife is much less likely. They do have tractor units with double drive rear axles so more brakes and there is plenty of room to stop out there.

Returning to the wrecker plot, what Mr Designer never considered was that his tractor would one day be loaded behind the rear axle in a way that not only gives a much higher rear axle load, but a much lower front axle load, i.e. a wrecker. If we impose 7t on our typical 4×2 wrecker, we get about 3.5t left on the front and 15.5t on the rear, so 12t+7t=19t. Now the normal average grip on a dry road is a mue of about 0.6. This is just a way of saying you can only get 60% of the groundweight in grip. This means that no matter how powerful the brakes, the 3.5t front axle can’t generate more than 2t of brake drag. OK you say, but still plenty at the back. Well Mr Designer would have given the back about 60% of 11t, i.e. 6.6t of brake drag. So now we’ve only got 6.6t+2t of brakes to stop 29t, which is about 30%g.

But, is even this the whole story, no, because when you press the brake pedal, the same pressure goes to the front and rear and you have had to back off to stop the front locking by about half, this means the rear only gets about half pressure. This leaves us with about 2t at the front and 3.3 at the rear. This leaves us with 5.3t of braking drag to stop 29t which is only 18%g. If it were a laden 6 wheeler this would be down to 15%.

The problem is not enough brakes at the back and too much at the front. In much the same way as in jack-knifing, loss of braking traction leads to loss of sideways grip of the drive axle, then loss of braking grip on the front axle of a vehicle leads to loss of steering grip. This all sounds a bit drastic, but is really the downside of the story. There is an upside and this is weight transfer. When we put the anchors on weight is pitched forward and you can feel the front dive (on most types of vehicle). Weight transfer depends mainly on vehicle weight, wheelbase and centre of gravity (CofG) height (where the weight is concentrated). Now weight transfer is good on the wrecker because it puts some weight back on the front axle. It is not good on the casualty as it put weight on the forks and offloads the front of the wrecker, however things do work in our favour this time. What counts is the height of the CofG from the point were it is reacted. Brakes work though the tyres on the road and you can’t get any lower than that, but the casualty weight is reacted through the forks and they are under the axle or even higher and are lifted up so the reaction point is much closer to the CofG. The result is only about 0.6t extra transferred onto our theoretical outfit stopping at 18%g, so it doesn’t offload the front so badly. The wrecker brakes react against the road and hence give more weight transfer. Our effective front axle weight goes up from 3.5t to 4.5t which is significant so you can actually brake a little bit harder which transfers some more weight and so on in ever decreasing amounts. This doesn’t mean you can slap the anchors on with gay abandon, but it means the picture is not quite so bleak.

I have produced a little spreadsheet, which calculates the various weight changes on our theoretical vehicle, which you can see. It shows the factors that are considered.

Download spreadsheet

What can we do to improve matters still further, well there are a number of things.

• If your wrecker has front brakes modulated from the rear, this is the last thing you need, as you pile the weight on the back it just gives you more front braking, which is not good. This needs sorting out.
• Unlike most vehicles, a highish CofG on a wrecker is good because it transfers weight forward under braking. I don’t means a vehicle that falls over at every bend, but there’s nothing wrong with a set of nice twin boom winches on a frame up front behind the cab, providing you screw them down properly.
• ABS on the front axle is good because you can lean on the brakes and make the back work without upsetting the front. If you don’t have ABS, then a LSV on the front is a good idea. I have never seen this done except on the 4×2 Unipower that I designed and it worked well.
• You may think that more back brakes would be good, well more actual brakes is good and you may be able to squeeze a bit more output out of the single axle brakes, bigger chambers, longer levers etc, but a word of warning, there is a limit to how much you can push into a brake, the manufacturers knows how much, but probably won’t tell you. Unless you really know brake systems, the best thing is to make sure your rear brakes are in absolutely top nick. Also your tyres. In this respect a rear bogie is good because it has more brakes.
• Retarders, a really good idea because they work back down through the rear tyres of the wrecker which still have plenty of grip and transfer a bit of weight. Exhaust brakes make a good noise, but don’t do a great deal on their own, but modern ones are better and they are cheap and can be combined with other retarders. Hydraulic retarders are good and more widely available now even on manual boxes; they do reject heat to the cooling system though. The best are either a Jake Brake or a Telma. The Jake brake, which is available on other engines apart from Cummins and Detroit now, turn your engine into a kind of air pump and are very effective, light, but not cheap, and makes a great noise. The Telma works like a big generator in the driveline, rejects heat to atmosphere and is heavy, but still very good.
• Probably the most important factor in avoiding braking trauma is the skill of the driver and this is, in my opinion, the main reason there are not more wrecker braking accidents.

Now I know that most recovery people are very resourceful and able to do amazing modifications to vehicles but please don’t go fiddling about with vehicle brakes if you’re not 100% sure, this is not an area for trial and error.

Finally a little story about LSV s, remember, load sensing valves. In the very early days of my employment in the civilian wrecker business, I was sent into deepest Essex to do a training with a brand new Iveco. This was the first one I had done so I was somewhat preoccupied with what I was going to say and do on the way. The training didn’t last long. I pressed the button marked boom up and it stayed where it was and a pool of oil formed on the ground. I went back to the works. On the way back I got to thinking about why this truck didn’t seen to stop very well, was it just because I was used to Scammells or was something wrong. The truck was brand new so what had we done to it. My mind focused on the rear brakes. We fitted a very thick and very stiff rear spring, which is what stops the rather sad tail down attitude that wreckers are otherwise prone to. What did we do with the LSV? When I got back the answer was – nothing, just joined it up again. This explained things, in the solo condition there were no rear brakes at all and probably not much at all until a lot of load was on the back. Now fortunately most modern vehicles have a plate in the cab, which gives the brake pressures at various axle loads. By measuring the spring deflection during proof load testing, which was also my job, I was able to calculate the spring rate and reset the LSV linkage. Of course this meant that it was my job from now on as well. I asked some operators what they thought about all this and some replied that they just tied the LSV up in the wide open position. Well at least this gave some brakes although the vehicle would be rather over-braked in the solo condition. So if you’re not sure what yours is doing, get it checked. See it it’s even working. If you’re about to ask about hydraulic brakes on small spec lift vehicles, well so did I. That’s another long story.

Just a thought, wouldn’t all this front axle braking hassle be avoided if there was a wrecker front axle that weighed the same laden and unladen? Perhaps we’ll talk about that next time.

First published in Professional Recovery Magazine on 15.9.00

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