How do Artics go so fast

To the truck experts out there, how do artic tractor units go so fast ? As they have big diesel engines these can't rev very high ! so is it gearing in the gear box or rear axle, I would love to know the respective ratios if possible only on a general rule of thumb I can understand various manufacturer's have different methods.

They have about 16 gears, it takes them a long time to get up to speed though.

Are yes I know they have lots of gears but are the higher gears over drive so the output of the gearbox is going faster than the input to the gearbox ?

What I am interested in is if the engine is only capable of say 2500 RPM how can the truck do 70 ish MPH, what are the gear ratios. Do they have low ratio diffs or is it in the gearbox ?

Gear ratios will vary greatly from truck to truck depending on weight carried, tire sizes to be run etc!

The main ingredient of modern trucks speed is the power of the engines. I remember less than 10 years back a friend buying a Foden for heavy haul use 50-60 tons and speccing a 400bhp CAT engine. Was about the most powerful available at the time. Nowadays I believe 500bhp is not uncommon in run of the mill 40t artics.

Contrast this with military vehicles from not too long back and 200bhp would've been unheard of.

What I'd give for 200BHP in my Ward La France!

And a few spare gearboxes...

Sheer power, in a nutshell............ Most of the bigger artics have direct drive top gears, rev to about 2000 rpm, have an axle ratio around 6:1, give or take a little bit.

Overdrive gearboxes to my knowledge, were more common on trucks of the fifties and sixties, mainly due to the increased speed limits introduced during that era, and it was another way of getting a higher top speed.

Over the years, trucks have benefitted from higher power to weight ratios and engines that produce a lot more torque, mainly from turbo charging.

I used to drive an artic in the seventies that had a gross weight of 24 tons, with a 150 hp engine. (Around 6.25 bhp per ton) I then had another 24 tonner with a 180 engine in it. (7.5 bhp per ton) I drove a 32 tonner with a 240 engine in the eighties. (7.5 bhp/ton). More recently a Daf with a 330 engine running at 41 tons. (8.05 bhp per ton) I now drive a Daf 85 430 running at 44 tonnes (9.77 bhp per ton). Are you beginning to see a pattern here?

There is an ever increasing demand for power in the truck industry, mostly because of the strangling time restrictions all the drivers hours legislation is creating. We've had to go for higher power trucks to get through the days work. The 430 shaved half an hour off of a journey, just because of the extra power and speed. This is why you will see Scanias, Dafs and Volvos with badges on the side or front with 460, 530, and some with 600. 600 hp at 44 tonnes equates to 13.6 bhp per tonne. Better still, drop the trailer and you have a 600hp Volvo artic unit running at about seven and a half tonnes or 80 bhp per tonne. Try outrunning a solo artic unit at the traffic lights. Most cars will take a few hundred yards before they start to pull away from the truck.

It aint all fun though......... trucks are allowed to do 60 mph on motorways........ so why the fu........ are speed limiters set to 56 mph :???

90% of the foreign trucks romp past UK trucks :shocked:..... for some strange reason. :???

It's not such a strange reason, they pull the fuse as I'm sure you know.:whistle:

i love mine when ive took tanker off back and just unit can leave a few cars standing at lights ...:shocked:

Sheer power, in a nutshell............ Most of the bigger artics have direct drive top gears, rev to about 2000 rpm, have an axle ratio around 6:1, give or take a little bit.

 

Give or take quite a bit Will, most high powered (380+) tractor diffs are between 3:1 and 4.5:1. Together with far larger tyres than cars this is why trucks are potentially quite fast.

Give or take quite a bit Will, most high powered (380+) tractor diffs are between 3:1 and 4.5:1. Together with far larger tyres than cars this is why trucks are potentially quite fast.

I'll have to have a look at ours just out of curiosity, cos I always thought they were "Lower ratio" than that.

Jus' goes to show though....... even a smart ar*e like me doesn't know everything. :cool2:

It's not such a strange reason, they pull the fuse as I'm sure you know.:whistle:

What does that do then............ :whistle: :n00b:

Artics go so fast due to the size of the engine, gearing and weight. If it's just a tractor unit, then 400+ BHP is a lot of power for a 6 tonne tractor.

Volvo make a 700BHP truck.

I used to have a 460bhp V6 Megaspace Merc, but my company has now got 400bhp Volvo FM tractors rated at 44 tonnes on a years lease due to the recession.

They are all (or should be) limited to 56mph. Gearing depends on what truck you have. Mine is auto with a manual option. I drove a DAF XF430 last week which was a manual. This had a range change and splitter box. Think of a normal gearbox of reverse and 4 forward gears. You have 1,2,3,4 in low range, then knock it up into high range for another 4 gears. Then the splitter splits each gear in both high and low. Without a trailer you can start in high range and block change, this will beat most cars at the lights until the limiter kicks in...:-\

I'll have to have a look at ours just out of curiosity, cos I always thought they were "Lower ratio" than that.

 

Jus' goes to show though....... even a smart ar*e like me doesn't know everything. :cool2:

Some of the new Mercs are geared for a theoretical max of 90mph, only needing 1150 or so rpm for 55mph.

What does that do then............ :whistle: :n00b:

For the non truckers amongst us it disables the speed limiter. At one time on Scania's if you turned the ignition off and then on whilst driving this had the same effect, needless to say this was soon changed by the manufacturer.

For the non truckers amongst us it disables the speed limiter. At one time on Scania's if you turned the ignition off and then on whilst driving this had the same effect, needless to say this was soon changed by the manufacturer.

All modern trucks are controlled by computers, so it's near on impossible to disable the limiter.

All modern trucks are controlled by computers, so it's near on impossible to disable the limiter.

Exactly, and some even throw a wobbler if you engage Aberdeen overdrive or hold the clutch down for too long.

Give or take quite a bit Will, most high powered (380+) tractor diffs are between 3:1 and 4.5:1. Together with far larger tyres than cars this is why trucks are potentially quite fast.

Ahhh thats what I wanted to know, so gearboxes only go as far as direct in top gear 1:1 so the ability to do say 100 MPH is only down to diff ratio and tyre dia although these are almost all the same size these days arnt they ?

This has interested me due to my GMC that has the engine governed to 2750 RPM, top (5th ) is slightly overdrive and rear axle is 6.6:1 only does 45 MPH.

If you were to swap your 7.50x20s for 11x22.5s, R3, you could get almost 60 out of the poor old girl!!

If you were to swap your 7.50x20s for 11x22.5s, R3, you could get almost 60 out of the poor old girl!!

But the problem then would be that the gearing has been raised right through the gear range and climbing hills might be a bit slower........the GMC engine not being blessed with 300-400 bhp as in modern lorries :-D

yes I know all that about tyre dia, I am not interested in upping the gmc speed I was just using that as an example of the engine speed with gearbox ratios and diff ratios to give only 45 MPH.

So the way modern trucks are able to go so fast is down to the fact of having a low ratio diff !!!!!! and not over driving the gearbox !!!

Do all the modern diesel engines only rev to 2000-2500 RPM ish ?

yes I know all that about tyre dia, I am not interested in upping the gmc speed I was just using that as an example of the engine speed with gearbox ratios and diff ratios to give only 45 MPH.

 

So the way modern trucks are able to go so fast is down to the fact of having a low ratio diff !!!!!! and not over driving the gearbox !!!

 

Do all the modern diesel engines only rev to 2000-2500 RPM ish ?

Depends on the size of them.......... we have some ERF's with Cummins 6.0 litre 'B' series engines that rev to about 2900. My old favourite on the fleet has a Perkins Eagle TX that only just about makes 2000. That one is around 12 litres.

My 18 tonne ERF with a 235bhp Cummins B is a joy to drive. It sits at 56mph at 2000rpm and does about 60mph down hills, but then the limiter won't give the power back at the right time for the next uphill! It doesn't seem to make any difference if it is loaded or not, as long as it sits at 2000rpm then the turbo keeps it there uphill and down dale.

The easiest way to make a truck go faster is put Irish plates on it and drive it at a weekend from wales to Dover....

The easiest way to make a truck go faster is put Irish plates on it and drive it at a weekend from wales to Dover....

Didn't you know they've all got an exemption certificate.....must be similar to the one BMW drivers have:whistle::rofl:

I assume the reason they are kept to 56mph is for fuel economy :???

I assume the reason they are kept to 56mph is for fuel economy :???

No, it's an EU directive. The speed limit for truck on a motorway is 60mph, but you only reach that downhill, unless the truck has been "turned up"

I worked for a company that thought it would be good to reduce the top speed down to 52mph for fuel economy reasons. It doesn't work. Yes the engine uses less fuel on the flat doing 52 rather than 56, but when you hit hills doing 52 because the engine is at lower revs you then have to change down a cog (when you might not have needed to with more RPM), if loaded to keep momentum up. The revs are then high and this uses more fuel. We also had problems with the trucks auto boxes, as these are designed to work at 56. We found that they wouldn't change into top gear because the truck wasn't going fast enough. Also remember that driving for three hours at 56mph you will cover 168 miles. To drive for 3 hours at 52mph you will cover 156 miles. That 12 miles difference means you take longer to reach your destination, using fuel.

The reason big engines are being used now is because they have massive torque to prevent gear changes on hills and low engine revs at max speed reducing fuel used. Most now use Ad-Blue which is a urea solution sprayed into the exhaust system to reduce the emissions, but Scania and MAN have gone down the EGR route, which in my opinion is better and cheaper.