volvofl10 wrote:
Mr Morningstarr* wrote:
I would imagine increasing the surface area of the vehicles road contact is a stupid idea and impractical.
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
Mr Morningstarr* wrote:
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.
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
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.