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