HGV Speed limiter - do they brake?
Discussion
blueST said:
I see a lot of HGV brake lights coming on on motorway descents. Just wondering, does the limiter apply the brakes to prevent over-speeding, or does the driver do it manually?
We do it ourselves. If you do nothing you’ll just run on faster and faster. Some lorries put themselves into an eco-roll mode, like selecting neutral, but will drop out of it and back into gear when they exceed 60mph.You can get the lorry to hold back its speed without using the footbrake by engaging an exhaust brake or retarder and making it hold a lower gear but unless you’re on a very long and steep descent which is likely to overheat the brakes many drivers won’t bother. Some aren’t even interested or curious enough to figure out how to use the various capabilities of their vehicle to get the most out of it.
Some vehicles will apply the brakes for you if you’re using distance sensing cruise control. It will maintain a set distance to the vehicle in front and engage both engine brake and wheel brakes to do so. It feels a bit strange when it happens.
The reason we hold back our speed to <90km/h is because if you exceed that speed for a continuous minute the tachograph will register an overspeed (tachograph code 30), recording the peak speed attained and the length of time spent in excess of 90km/h. This isn’t necessarily evidence of lawbreaking as our absolute speed limit is 60mph or 96.5km/h but it will show up if the tachograph is interrogated by DVSA and some employers will treat it as an infringement.
When I was on National Express, our coaches had an "overspeed" limiter that would automatically apply the retarder on a descent when the vehicle reached the selected speed. This is to try and reduce tachograph infringements and the associated paperwork. And yes, though not using the vehicle brakes, applying the retarder will bring on the brake lights
Some makes (Swedish makes mainly) apply the brake lights when the exhaust or other type of retarder are in use (doesn't need the brake pedal to be depressed because exhaust/retarders are switch operated too in modern vehicles), most annoying when all you are doing on a long motorway descent is preventing overrun and not actually reducing speed at all...ie those long hills in Leicesterhsire M1 northbound dropping down to the Loughborough turn you can have the brake lights on for several miles at a steady 55 mph and have all feet off the pedals, nothing the driver can do about it.
Some makes don't operate the brake lights for exhaust retarding only, MAN being one such, i prefer this because in my view brake lights should mean actual speed is being reduced not just maintained on a downhill.
Overspeed auto braking can usually be programmed on the menu by the driver, but its such a faff i don't bother and simply use the retarder in an appropriate gear to control speed, no brake lights showing on my vehicle unless i use the footbrake itself...note most vehicles automatically apply the engine brake tio assist deceleration when the footbrake is applied, but there is a usually a switch to enable or disable this feature.
Some makes don't operate the brake lights for exhaust retarding only, MAN being one such, i prefer this because in my view brake lights should mean actual speed is being reduced not just maintained on a downhill.
Overspeed auto braking can usually be programmed on the menu by the driver, but its such a faff i don't bother and simply use the retarder in an appropriate gear to control speed, no brake lights showing on my vehicle unless i use the footbrake itself...note most vehicles automatically apply the engine brake tio assist deceleration when the footbrake is applied, but there is a usually a switch to enable or disable this feature.
Edited by Smint on Sunday 30th April 21:10
The old dafs at our place don’t do anything for you, can set cruise control that’s about it. The new scanias you can set how many mph over the set cruise you want to go and it will use the retarder to hold it there. So could be 56mph +2 and it would hold you at 58 if you started to run away down hill. Also applies brake lights from first click on retarder were as our Ivecos lights only come on from 3 clicks.
The automatic hill descent retarder feature is an option with all the manufacturers. Some companies spec it, others do not. When I was on the bulk milk tankers my own DAF had hill descent which was just awesome for holding it at 60 down hills, especially with a part-loaded tank as the consistent speed keeps the milk level. But I had a DAF for a while without it and what a PITA ! You have to use the foot brake to limit the speed (the standard exhaust brake doesn't do anything useful on DAFs) and every time you touch the pedal it sets the milk racing up and down the tank. By the time I got down to the bottom of the hill I'd be like a nodding dog being thrown forwards and backwards until the milk settled down 
.
I do know on the DAFs with hill descent, they do activate the brake lights, but only if the braking effect exceeds a certain point. I've tested this a few times when running in convoy with other drivers and we've been quacking on the phone. The ones without hill descent and just the standard useless exhaust brake, these don't seem to do it, although it could just be because the exhaust brake doesn't provide sufficient braking effect to activate them.
.I do know on the DAFs with hill descent, they do activate the brake lights, but only if the braking effect exceeds a certain point. I've tested this a few times when running in convoy with other drivers and we've been quacking on the phone. The ones without hill descent and just the standard useless exhaust brake, these don't seem to do it, although it could just be because the exhaust brake doesn't provide sufficient braking effect to activate them.
and31 said:
I got a new Scania last week,it doesn’t apply the engine /exhaust brake at 60mph like my old Daf did.it would be possible to go very quickly indeed down a steep hill.
Has it got that crappy GPS cruise control where it stops accelerating before the top of hills and stuff, never actually doing a constant speed making other drivers want to murder you because it speeds up as they're trying to pass? s p a c e m a n said:
and31 said:
I got a new Scania last week,it doesn’t apply the engine /exhaust brake at 60mph like my old Daf did.it would be possible to go very quickly indeed down a steep hill.
Has it got that crappy GPS cruise control where it stops accelerating before the top of hills and stuff, never actually doing a constant speed making other drivers want to murder you because it speeds up as they're trying to pass? Very pleasantly surprised with how good the Scania auto is.-I might change my mind when I’m in the mud next winter…
rohrl said:
Volvo’s gearbox is reputed to be the best, but I’ve only ever driven one about twenty yards so I can’t back that up.
I had a brand new Volvo 12 years ago,the I shift box in that was good on road-absolutely terrible when proper off road.The new Scania auto I have is every bit as good on road as the Volvo I had then
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