How to change gears on a lorry?

How do you change gears on a twin speed gear box lorry? ie how do you change up and down through the gears, and whats the differnce when the lorry is laden and unladen, how do you switch between diff speeds to work your way thro the gears again, etc? cheers for your help
14:22 Sun 27th Aug 2006
 
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Here in the U.S., at least, most larger trucks that still have two transmissions are powered by diesel engines. There's an optimum RPM (revolutions per minute) at which the engine is designed to operate. Here, it;s usually about 1500 RPM. The trick, then is to keep the engine speeds within that range. The most damaging thing one can do to any engine, but especially a large diesel is to "lug" it,,, that is make it operate at too low of RPM's. Therefore, one starts from a stop with both transmissions in lower gears and as the pseed of the truck increases, the gears in the "top" transmission isn't shifted up until reaching a continous maximum RPM of, again, around 1500. The "top" tranny is shifted up through most of the gears attempting to maintain the 1500 RPM range, then the "bottom" transmission is shifted up one gear and the "top" transmission is shifted down to the lower gears and the whole process starts over again. Experienced truck drivers can skip through some gears without the need to shift though every gear available... Most trucks these days have automatic transmissions, by the way...
In the UK, most have a 4 or 5 speed gearbox (like a car) but also have a range-change, in series with the gearbox.

So you go Low-1, Low-2...Low-4, High-1...High-4. (High-1 being the same gear as Low-1, but with the range change switched) Yes, you have change gear and range simultaneously at one point, though the range changers are usually presettable, and won't switch until you depress the clutch, so it all happens seamlessly (you hope).

Twin speed diffs seem to be rare on new trucks, they have been replaced with a splitter gearbox (which is in series with the above). So each of the 8 gears above has a high and low.

Unless very heavy, it is rare to go through every one of the 16 gears, you usually leave the splitter alone and change 'whole gears'. Splitters are presetabble, also - flick the switch and declutch and it changes.

I find splitter's great benefit is in getting just the right gear to get up hills.

When empty, you can usually change up 2 whole gears at a go.

I once had a Volvo tractor that with no trailer would easily pull away in High-1. Zero to 30mph in one gear!
most foreign truck ,volvo,man,mercs seem to have a synchromesh box,so apart from more gears,it's similar to driving a car,when you change will depend on weight and terrain. The "british" I drove with twin splits,SA ,Foden have a kind ofconstant mesh box,similar but a little more forgiving than the old 'crash' box. Once you get used,you won't even need your clutch,apart from starting/stopping. As Cats says,unless your going up a steep hill with a load,then you can skip a few. When your changing down,if your wagon wont go faster than 30 say in third,then you won't be able to get third going down if your going 35,then you won't be able to select that gear on a non-syncromesh box. Just a matter of practise and getting used to engine noise. If it's a crash box,then check with rev counter what speed the wagon will go in each gear,then you'll know you can't change down if your going faster

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Related Questions

More closely, how does a splitter work on a lorry? I dont get it, all you do is flick a switch and put the clutch in and they change to the other gears...how does this work?

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