Doctor Diesel

Dear Bill

UK's Rising Fuel Prices Puts Economic Pressure On Haulage SectorI wrote to Bill Shepherd, veteran HGV driver, some of whose memoirs were recorded in issue 312:

Bill, with your history as a “knight of the road” can you perhaps tell me why some HGVs can crawl past others, when the speed governors are all supposed to be set at the same speed?

Is it just “pot luck” if you have one or two mph advantage over another truck, or are there other ways of gaining an effective speed advantage, so that you can overtake over the two to three miles that it seems to many frustrated motorists?

Helpful information on this seemingly daft business would be welcome.


Dear Doc, always willing to help if I can. There are a number of reasons why it can happen:

(1) When I retired the speed limiters were meant to be set at 85kph, or about 53mph. Some companies had theirs set at 50mph – it was said that there would then be no chance of getting caught speeding on ordinary dual carriageways, where 50mph is the speed limit for HGVs. All tachometers must only register in kph, by the way.

(2) The speedos on all vehicles fitted with tachometers must be recalibrated every 24 months. Some companies will have it done with tyres with not much life left on them, which they may keep just for that job, then they go back to brand new set of boots, with maybe another inch of tread all around, and it’s these larger tyres that could give you a 2mph gain. Many years ago, I was told that the tacho on coaches and lorries had to be set to within two per cent at 60mph.

(3) Wind on high sided boxes can have two effects. If you are running empty, even from quite a way back, you can be dragged quite quickly up to the vehicle in front. If it’s another box, you don’t know if it is loaded or not, but if you have caught it up very quickly, you guess that it is loaded and you set out to overtake.

You may get three quarters of the way past when the wind, vortex, and slipstream hit you and drag you back; that’s when you find out that it wasn’t the load that was holding him back – it was the wind. Sometimes you can defeat that by dropping a couple of gears.

The last lorry that I drove before I retired was a Mercedes lorry and trailer. It had 16 forward gears, and two for reverse. Essentially a four-speed box, each gear could be split with low and high, making eight gears, and then you had a range change, and went through the same 4×2 gears all over again. Empty you would start in three or four low, four high, range change and into five low. The high/low you could preselect and the range change was a flick switch on the front of the gear stick, and the high/low was a flick switch on the side.

Yes, there have been times when, like most other drivers, I’ve messed it up. In the days when I drove the old Bedford TK (with five-speed overdrive box, and an Eaton two-speed coach axle) the vehicles that had the hardest job getting past me were small vans, like Transits and Sherpas. I can’t imagine “White Vans” being held up like that these days, can you? My slipstream and wind protection had the same effect on them as the high-sided lorries mentioned before.

I have seen, although not very often, a row of lorries get to the bottom of a hill. If the front one is loaded, the driver has used the left hand indicator to warn the driver behind that he is likely to lose a lot of his speed, and to overtake, giving the driver behind the chance to mirror and signal before you move over, and pull over to lane two, before the first driver loses speed. Very helpful.

Well Doc, I hope that answered, or helped to answer, the question that you asked me. I’ll leave you with a 1960s smile… In the 1960s, lorries were taxed on their unladen weight. So before you took the vehicle to a weighbridge, all unnecessary things were removed – spare wheels, passenger seats, long range fuel tanks and even, as I mentioned before, change to very worn, but legal tyres.

But the fleet engineer said that the van was still 56 pounds heavier than the last time that it was weighed for tax purposes. The back doors had been removed, as it had a curtain as well, and it didn’t need both. So what else is there? Someone shouted “Let two of the tyres down – there’s 95 pounds in each of them”. He didn’t realise that it was pounds pressure, not pounds weight! Hehe!

Bye for now Doc,

Bill Shepherd

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