Safe Speed Forums

ABS is standard on many makes of truck, but i don't know if it is now a "standard" .

trailer brakes locking up are usually a sign of a faulty load sensor on the trailer. The load sensor determines what pressure to apply to the actual brakes on application of the brake pedal. Its a simple device thats fitted to the chassis rails on a trailer, with a pendulum connected to an axle that is either opened or closed depending on how close the axle is to the chassis. this in turn is a simple ratio balancing valve that lets either 1:1 air pressure thru or say 5:1 .
the load sensor is open to all the elements of the road though, namely rain,salt,snow ect ect . It is part of the MOT test which means it only gets independently tested once a year.

regarding crude suspension mentioned in earlier posts, most modern trucks and trailers ride on air these days as opposed to steel leaf springs, and the this is controlled by load levelling valves, similar to load sensing valves. in effect the system is an active suspension , forever altering front to rear and side to side balancing.
supply of air isn't a problem either, as there are several storage tanks for both truck and trailer which the compressor can keep satisfied once charged. the exception to this is if the brakes are applied on and off down a long hill and the demand exceeds supply. if you empty the tanks below 8 bar, the brakes will lock on until supply can meet demand again . this is why if you see a truck starting up in the morning it will be ticking over for a few minutes before it moves, or the driver will be revving the nuts off it to charge the system quicker. until the pressure reaches approx 8 bar the spring brakes will not release.
ABS is good forward development , but isn't the complete answer as its not foolproof .again, the sensor and the ABS cog are open to the road elements

I've never fully understood HGV brakes. I thought that the braking was done by dirty great springs that pulled the brake shoes into contact with the drums and that the air pressure actually held the shoes OFF the drums against the springs. The idea (such as it was explained to me, at least!) was that if the air pressure failed, the brakes would go ON rather than OFF. All that made sense to me until you mentioned the 5:1 (or whatever:1) ratio that was controlled by the load sensing valve on the trailer. Surely this implies that the air pressure actually APPLIES the brakes and that the more pressure, the harder the braking effort? Also, on the air-sprung trailers, why can't they just do what Citroen used to do on their cars and use the pressure in the suspension units to work the brakes? That way, the more pressure in the units (because there is more load on the axle), the more braking effort can be applied. The less pressure, the less braking effort - always in perfect proportion to the load on the axle...

...it's too simple isn't it? There MUST be some reason why it can't be done!

Paul, sorry I missed this thread for a while!

The problem with the "F=Mu * N" equation which implies that the heavier the load on the tyre, the greater the braking force available will be, is that tyres are very non-linear in this respect! It happens with cars too. One tries to keep the tyre working within the (pretty short) linear part of its curve at all times but they are such complex things (full of air for a start so not a homogenous material at all!) that this isn't always possible. Also, Green V8S' point about compounds sounds like it might well be true. Truck tyres will be optimised primarily for long life rather than high grip.

Friction force is almost negligible, the wheels are running in bearings designed to take the weight of the vehicle. Friction is deliberately being reduced as efficiently as possible. The slowing friction would be the one applied by the brakes, the vehicles mass has no effect on this.

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The Morningstarr*

You say 'very non linear' but I'd be amazed if the total deviation from the ideal was greater than +/- 10% in any given self similar set of road applications.

Given the scope of other variables (tyre compound characteristics, road surface characteristics, gradient, brake design and temperature, etc, and of course all the human factors) I'm pefectly comfortable suggesting that 'linear' is a very fair approximation.

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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

We were talking about the friction between tyre and road which sets the real world limit to braking ability.

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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

The friction between tyre and road is only taken into account when skidding.

Braking is the slowing down of the wheels turning in the wheel bearings, if the force acting down onto the road surface isn't sufficient the vehicle may skid. The heavier the vehicle the greater the force acting downwards, but the force on the horizontal axis is also greater in proportion. if the vehicle has a greater number of axles the downward force is more evenly spread and so is actually reduced.

A HGV must have an increased risk of skidding as the horizontal force is increased by it's mass, but it's downward force does not increase with the same proportionality as if it was only on four wheels.

Rigpig's post was correct, the heavier the mass of the vehicle, the greater the braking force required to stop it. Road surface friction does not come into the equation unless you are talking about a skidding vehicle and hopefully we are talking about stopping without skidding?

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The Morningstarr*

Noooooooo!!!!!!!

It's !!!ALL!!!! about friction between the road surface and the tyre! The vehicle skids when the amount of retarding force exceeds the frictional force that the tyre can generate on the road.

I'll try and find a graph. I think I have one for a car tyre (pretty old one though) but not a truck tyre. You're right though, in that any tyre (car, truck or whatever) will obviously be designed to try and stay in the linear region of its graph. The point I was trying to make is that we can't just say that the more downward force you put on a tyre, the more stopping force it can generate. It's only true up to a point. I still feel, however, that centre of gravity height is the biggest single limiting factor with HGV stopping distances.

So why can't we stop in -say- 20 feet from 60mph?

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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, but mass has no effect on friction. Friction is associated with surface area.

To stop a vehicle safely you must slow it down at such a rate that it doesn't cause the vehicle to overcome friction and skid. you can do this by increasing friction through greater surface area in contact with the road and then increasing the braking force applied to the wheels. I would imagine increasing the surface area of the vehicles road contact is a stupid idea and impractical. So the braking system is probably designed with that in mind, that is why it takes so long for a large heavy vehicle to stop.

If you are using the normal reaction as an example of increased mass causing more friction you will find that a HGV has a proportionaly less downward force in relation to a car. In a car the mass is spread over just two axles rather than five or six etc.

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The Morningstarr*

Mass itself technically doesn't, but the downward force it causes (i.e. weight) clearly does, and weight is proportional to mass. The distinction between weight and mass is only really useful when the pull of gravity is variable.

(I have just tried it out on my desk to make sure the weight does make a difference. When I push down hard on an object it is much harder to slide it.)

the size of an HGV's tyres is related to the load carrying weight of the vehicle and not the amount of road surface it can cover

i may of misunderstood your explanation, but see if this ties in ,
an average car weighs around 1.5 tons and an articulated HGV weighs anything up to 44 tons
the cars 1.5 tons over its 4 tyres = a load of approx 375 kg per tyre
the HGV's 44 tons over 20 tyres ( normal 3 axle unit and 3 axle trailer) = a load of approx 2 tons per tyre
an average HGV tyre has about twice the contact surface width of a car tyre, so i guess the HGV tyre still imposes a greater pressure per square inch than the car's does .Even when the HGV is unladen , the weight is in the low 20 tons , so its still around 1 ton per tyre, then half it to allow for the increased surface contact area, leaving 500 kg per tyre, still a higher force than a car tyre.
The other problem with an HGV's braking system is that there is so much imbalance between all the brake shoes and drums . you would have 12 different drums , 1 per each end of each axle, and to balance all 12 up in perfect harmony with each other would be nigh on impossible. This is due to general wear on all 12 pairs of brake shoes and drums. Even if all components were brand new , i don't think you would get them all to work in perfect co-ordination with each other to get an "even" load on all the wheels at the same time.
A car is relatively simple in comparison , mainly due to it being a hydraulic system which gives reasonably equal pressure to just 4 drums/disc's and is easier to balance up, whereby the HGV relies on air instead

But that's pretty much EXACTLY what manufacturers do to get more grip - fit wider tyres!

But in context to this discussion the mass would have a less proportionate effect as it is spread over several axles.

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The Morningstarr*

but as the mass of the 2 vehicles is totally differant you get;
car 1.5 ton over 2 axles = 750kg per axle
HGV 44 ton over 6 axles = 7300kg per axle .........

almost 10 times the mass per axle than the car

but for the benefit of balance to the point raised , a trucks braking surface per wheel on the shoes and drums is probably in the region of 8-12 x the surface area of a cars

Nope. Weight is a constant irrespective of the number of points of contact. With more points of contact each has less downwards force to work with.

10 times 1 pound is the same as 1 times 10 pounds.

It's another effect that cancels itself out. You don't get better or worse braking to any significant degree by changing the number of wheels.

Fitting wider tyres give a little more grip in the dry but a lot less grip in the snow.

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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

yes....but no,but yes,but no ( sorry ) you let air out of a tyre in sand and snow to increase traction . oiverall effect is it makes the tyre wider .

i have to say, i am really enjoying this brainstorming/discusson thread . thank you everybody who is participating

having just ahd a new truck for work, im finding that disc brakes all round on the tractor on an HGV are far superior to the old drum and shoe brakes we have had for years . I feel they certainly give more confidence to the driver

I'm not a mechanic, so I don't know the system involved in braking, but I think you are on the right lines to why HGV's are harder to stop.

The post I was replying to suggested that the increased mass of a vehicle cancels out increased horizontal force by increasing downward vertical force.

If a car of 1.5 tonne has a horizontal force 1.5 x acceleration(a) it's downward force is 1.5 x gravity (9.81) divided by the number of wheels(4) and they are proportionate.

A HGV has a horizontal force 44a, and downward force of 44 x 9.81 divided by 20

So as a ratio the car is 1.5 horizontal:36.8 vertical
the HGV is 22 horizontal:21.6 vertical

As you can see the relationship between horizontal and vertical force in a HGV shows the downward force is less proportionately than what a car has.

If you ignore the spread of the force over the axles you will find that the ratios are the same, which still shows that the increased downward force is simply proportionate to the increased horizontal force and an HGV's weight does not make it stop easier than a lighter car.

As the HGV actually exerts less force proportionately on it's road contact a system is required which slows the wheels sufficiently and is coupled with a tyre which has maximum resistance to skidding. As people have already pointed out, you can stop the wheels but it's the skidding which becomes the problem. I should think the braking system on HGV's is more than capable of stopping the wheels from turning it's just balancing that with anti-skid technology.

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The Morningstarr*

well i was right on 1 point then
LOL

thank you for the explanation, i think i understand the part i misunderstood earlier now