Safe Speed Forums

Comment in "Green-amber" made me think of this.

Why *Can't* HGV's brake (Decelerate) as quickly as Cars?

What are the limiting factors?

Why cannot this be overcome so that *all* vehicles sharing the same road space are able to Brake at a similar rate?

Time after time HGV related accedents seem to involve HGV's ploughing into the back of stationarry/slow moving traffic because they cannot stop quickly enough, such accedents almost always involve fatalities

What can be done? (Ie Engineeringly possible)

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"The road to a police state is paved with public safety legislation"

HGVs have done a great deal of catching up in the last 20 years.

All normal road vehicles with normal loads tend to cluster around 0.9g maximum braking effort. The fundamental limitation is the tyre/tarmac grip, and deceleration achieved is independent of weight.

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Paul Smith
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Tch Tch! Dusty! You obviously haven't been reading your Highway Code! Didn't you know the stopping distance from any particular speed was the same for ANY road vehicle (from F1 car right up to 40 tonne artic)?!

Seriously though, I agree with Paul that trucks have come a very long way in the last 10 years. ABS and disc brakes are common now. The problem is that as you brake, some of the vehicle's weight transfers off the back and on to the front. The extent of this depends ONLY on the height of the centre of gravity above the road surface and the amount of deceleration. Trucks will always have high centres of gravity (compared to cars) so they will always transfer more of their weight forward. Simple physics would suggest that the amount of grip you get is proportional to the force pushing the two surfaces against each other. If that was the case (and it IS to some extent) this wouldn't matter because whatever braking effort was lost on the rear axle(s) would be gained at the fronts. In reality, however, tyres are non-linear devices and above a certain amount of load, you don't get much (if any) increase in grip.

I can't think of any way round that really. Also, I'd be keen to know out of the number of HGVs that ploughed into the back of something, just how many of them were braking and from what distance back. I sometimes wonder whether some of these accidents, at least, are caused by drivers being asleep (or nearly so). Also, I'm sure we've all seen artics following each other along the motorway leaving the sort of gap between each other that I wouldn't even like to try and parallel-park into! I don't suppose that improves their chances much!

i would think the majority is caused by tiredness



if theres only a 10 foot gap between you and the truck in front , you shouldnt have as much impact speed when you hit it as if you had a 20 feet gap LOL

and to answer the original question, max weight trucks weigh 44 tons, your average car weighs 1.5tons , although trucks have more axles and disc/drum brakes than a car, its still not enough to be as efficient as a car.

EDIT = an afterthought , the auto braking system being developed my mercedes cars wouldnt be a bad idea for trucks

As this was a rhetorical question, should anyone be answering?

The reason why I called it a "Retorical question" is that I do have an engineering baclground and understand the issues. Im just wondering if anybody else manages to come up with something I havnt already thought about.

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"The road to a police state is paved with public safety legislation"

the weight carried

the brakes are on the truck but not the load. if the load is properly secured, it will push the truck onwards. the added danger with artics is the load sending the trailer out to the side which can and does have disasterous consequences. add a petrol tanker to the equation and you are looking at a controlled stop taking far longer than anything else. although the tank is broken into segments, that is a lot of liquid which cannot be strapped down so it will rock the entire vehicle. i have heard of many cases where the truck has stopped but the liquid 'catching up' has actually moved it forward several feet.

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Richie

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Newton's second law:

Force = Mass x Acceleration

Deceleration is of course -ve acceleration, thus the force required to decelerate two vehicles down to a stop from a similar speed will depend, amongst other things, upon the mass of each.

BUT increases in mass also increase friction force which in turn increases maximum braking force and exactly cancels out the effect you're imagining.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I'm not imagining anything.

The effect that you believe to be true is very much contested, certainly with respect to the braking force benefit gained from increased mass. And this in turn is dependent upon whether the vehicles is skidding to a halt, or coming to a controlled stop.

This link might help: http://www.citizen.org/autosafety/fuele ... fm?ID=8264

David Greene of the Oak Ridge National Laboratory has studied the vehicle size-weight-safety question for many years and has concluded that "the more carefully one controls for confounding factors [like driver and environmental characteristics], the more the ‘weight effect’ fades away, or even reverses." As he points out, there is no correlation between NHTSA crash test performance and weight. Moreover, Greene found that braking distances increase with vehicle weight, suggesting that lighter vehicles may avoid crashes better than heavier ones. For all these reasons, he was unwilling to support the majority opinion in the NAS study.(19)


My bold BTW.

Even if they can brake at the same rate

1.5 ton car @ 10 mph
40 ton truck @ 10 mph

Which is going to hurt more when it hits the back of your stationary car?

Such relatively small effects as DO exist have to do with centre of gravity, load stability and suspension design and ABSOLUTELY NOTHING to do with Newton's second law.

See: http://hyperphysics.phy-astr.gsu.edu/hb ... t.html#nor

Normal Force

Frictional resistance forces are typically proportional to the force which presses the surfaces together. This force which will affect frictional resistance is the component of applied force which acts perpendicular or "normal" to the surfaces which are in contact and is typically referred to as the normal force. In many common situations, the normal force is just the weight of the object which is sitting on some surface, but if an object is on an incline or has components of applied force perpendicular to the surface, then it is not equal to the weight.

With friction proportional to weight we can see that greater mass means (equally) greater force is available to poke into Newton's second law.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Whichever law applies, I'll put my mortgage on Mr Greene's studies rather than your theories thank you very much.
Which, in the context of the orginal question, suggests that vehicles with more mass take longer to stop.

If you want to claim that it has anything to do with the broad application of Newton's second law, I'll take that bet. Thanks.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Claim? This is an informal discussion, not a meeting of Scientists UK.
I offered a suggestion as to why bigger trucks take longer to stop than smaller vehicles. Which, in the context of Newton's second law, means that there are bigger forces involved because the mass is bigger.
If you MUST have the last word, then fill your boots below.

Easiest house I've ever won. Please send it recorded delivery to the usual address.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I don't know how the braking distances with trucks compare with cars under different conditions, but if they do typically take longer to stop this seems to imply that either the brakes aren't capable of exploiting all the grip available at the tyre (which seems unlikely) or the coefficient of friction of the tyre is lower (which seems entirely likely to me, given that the contact pressure is so much higher presumably requiring harder compound tyres to give an aceptable tyre wear rate).

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Peter Humphries (and a green V8S)

Couple that with:

- much cruder suspension
- much higher CoG
- difficulties in brake balance caused by unknown loading
- load stability issues

- possible unsprung weight issues
- possible brakeforce delivery issues (time to build pressure in a large system)
- issues with articulation

And I think we have a fair picture. But trucks aren't far behind cars these days, except perhaps when the load has its say. I think ABS has been a big thing because it lays to rest the unknown load demons.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

my road to work often means driving behind HGVs, far too often i see their back wheels locking up on breaking. more recently i was alongside the back end of a HGV on a bend (2 lanes per side dual carriageway) and his back wheel locked up and started to smoke -fortunately the other wheels kept his traction, still not an ideal set up!

- I was speaking with a HGV driver recently about this and he was saying how poorly the maintenance is on a lot of HGV's and their trailers +the associated problems with varying loads.

–could this be a contributing factor to them ploughing into the back of people?

Imagine the massive difference in braking force required from rear trailer wheels when empty to when fully laden. A difference of at least 6:1. This has always been a big engineering problem and lightly loaded or unloaded trailers often lock wheels under modest braking.

However, the proper technical solution is ABS and it is now being installed on all new vehicles (I believe, anyone confirm?). ABS is a complete solution.



Unlikely. Unladen trailers are the ones locking up and they don't need a whole lot of brake force because they are (comparatively) light.

And I've certainly seen no evidence that HGVs plough 'into the back of people' more than any other vehicle class.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety