Safe Speed Forums

The 'normal' reaction you gave as an example of the downward force only acts at the points of contact. If you weigh your self by standing with one foot on two seperate sets of scales you will get two readings which add up to your entire weight, you will not get your entire weight displayed on both scales.

As the points of contact are the important factors in friction you can see a HGV is actually worse for increasing friction with the road surface in relation to it's horizontal force than a car would be on fewer points of contact.

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The Morningstarr*

Increasing surface contact would seem to be the simplest solution to stopping HGVs quickly.

But how far will manufacturers go? Catapillar tracks?

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The Morningstarr*

although this may be a soultion, its not practical to do, as you would only be able to do this on new vehicles and not ones already registered.

for stability, most trucks and trailers already have the tyres on an axle at the furthest distance apart. this follows from trailers on super singles tyres ( large single wheels on each end of each axle) thru to twin wheeled axles and front steering axles. Due to the current trend, you wouldnt be able to increase the offset of the wheel at all , as the inner edge of the tyre only just skims the suspension arms now, and the outer edge is usually just within the legal requirments of HGV width,so your limited to whats already available.
To change every existing HGV to be able to accomodate wider wheels and tyres would be very cost ineffective, and would'nt get thru legislation im afraid.

As for using wider tyres, the HGV i drive has has "super single" tyres fitted to the front axle. Although this may increase the allowable front axle load weights, its had a detremental effect on steering, whereby the vehicle tends to "wander" more than previous vehicles on normal tyres did. In fact, i now find myself on the point of aqua planing in heavy rain due to this,and thats even when i have the front axle loaded at 8 tons !!. in fact im at the point of getting the fleet engineer to revert them back to the older style ones. i will post the sizes later for reference

So what are you saying exactly? That a vehicle with 10 wheels will brake at 4/10ths the rate of a vehicle with 4 wheels? If it was simple physics that's what would happen - but of course it's nonsense.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
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as help toward's explaining the effect of wider tyres on an HGV, normal size tyres are 315/60/22.5 , whereas the ones thats wider and been fitted to a truck i drive are 385/55/22.5. ( tread width by height profile by rim size)
in effect ,
the original tyres on the steering axle have a contact width of 10 and a quarter inches with the road.

the newer wider tyres on the same steering axle have a contact width of 12. and a half inches with the road.

a difference of 2 and a quarter inches , which doesn't sound a lot but believe me it does have a massive effect on the handling.

as mentioned earlier, its to much of a negative effect as the steering wanders and struggles to correct itself when straightening up after a bend/corner.
Theres a mass of spray from the new tyres when its raining, also when its wet I've found that the steering goes light due to the aqua planing effect from the nearside thats running at the edge of the roads camber where the water settles.
More effort is required to turn the steering wheel at very low speeds when manoeuvring.
"Should" i get a front wheel blow out , I'm expecting great difficulties in trying to keep the vehicle in a straight line whilst waiting till i come to a stop.

The only thing in its favour is the fact that it can bear a higher load than normal 315/60/22.5 tyres.

IMO , theres not enough good to counteract the bad by going to wider tyres on HGV's .least ways not at the moment

And so it is with all vehicles. Motor manufacturers know a thing or two about picking the best tyre size for a particular application.

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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

Well, yeah! pretty much! Have a look at a truck carrying a really heavy load like a railway locomotive or a crane. They put lots of little wheels under the trailer! Tracks are a bad idea because of road damage - they can't steer other than by skidding and they churn up the tarmac. They also make a lot of noise. Multiple wheels are the next best thing to keep road damage down whils tstill supporting the load and, of course, stopping it!

No. I'm saying your comment about increased mass slowing a vehicle quicker is nonesense.
I'm actually trying to show you that that is what the case would be if your claim was true.

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The Morningstarr*

There's your answer then.

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The Morningstarr*

I certainly never said any such thing.

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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 main purpose of lots of small wheels is to keep the height down on the low loader. Obviously if you had large wheels,loading the locomotive would be a lot harder , and the overall height on the road would be to high to get under bridges.

another thing is axle weights unless your STGO registered ( special types goods order) , you have to comply by the gross,train and axle weights of your vehicle and the ministry, being a max of 44 tons in the UK.
most small wheeled axles are very low rated for weight , as the small tyres can't carry as much load as large tyres. Hence you see low loaders with more than the standard 3 axle configuration of its larger wheeled counterparts
A 4 axled trailer with small wheels causes more road damage than a 3 axled trailer with large ( normal size) wheels, even if they are running at the same weight. This is down to the fact that trailers dont actually follow the tractor unit round corners, the first 2/3 axles do but the last 3/4 axles just drag sideways round corners whereas the 3 axled trailer will only have 1 axle dragging . the point of pivot isnt in the middle of all the axles as you might expect.
The way round this is to have rear steering axles on the trailer, but then you reduce the load carrying capacity on the axle, as steer axles dont have the capability to bear as much load due to the moving parts for the steering.
The exception to all of the above is specialised heavy haulage trailers, the likes of the modular ones made by Cometto , which are multi units attached to each other for either length or width . these are VERY heavy duty things and also VERY expensive

Road damage increases with the 5th power of axle weight. (This means that small increases in axle weight lead to big increases in road damage.)

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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 increased mass doesn't have as much downward force in relation to it's horizontal force as a car does. The braking involves stopping the wheels, the mass has no effect on this whatsoever.

Friction is a highly complex phenomena and is related to surface area and surface tread, the downward force would be overcome with momentum and has little or no effect on friction.

Braking and skidding are two different things entirely, you can't reduce skidding by increasing mass. I think it's been witnessed how heavily loaded HGV's skid more than lighter cars.

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The Morningstarr*

They are the ones I was referring to!

I agree with everything else though

If a vehicle is ten times heavier it NEEDS ten times the braking FORCE to slow AT THE SAME RATE. F=ma.

Fortunately that is (approximately) what we get.

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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 really struggling to get my head round this! The braking doesn't just involve stopping the wheels. If it did, we'd be able to get away with cycle brakes on our cars! The mass of the vehicle has (nearly) EVERYTHING to do with how easy or difficult it is to stop!




At least I can agree with the first half of this sentence but the second half baffles me. There IS no "momentum" in the downwards direction. Momentum is mass x velocity and as it isn't moving downwards, there is no velocity in the downwards direction so there can't be any momentum in the downwards direction either. The only momentum the vehicle has (if travelling forwards) is in the forwards direction.



Volvofl10 might correct me on this but I'd have thought that the majority of artic trailers that skid are actually more LIGHTLY loaded ones. I don't know about this for sure but I'd have thought it would be more common to see an empty trailer skid than a fully loaded one?

Mole
I would agree with you entirely . Ive never seen a fully freighted trailer skid in recent years, the ones i do see are normally empty or very lightly loaded trailers ( lightly meaning upto 5 tons of freight) .
you do see the odd one lock a single wheel up on one side , but not a full axle lock up

That's what Rigpig was saying, when you said the mass cancels it out.

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The Morningstarr*

To clarify, the mass has no effect in stopping the vehicle. The mass is part of the momentum carrying the vehicle forward it doesn't aid in braking in any way.

Yes, and this forward momentum overcomes the downward force.
If that is the case this is possibly caused due to the weight being spread over so many wheels.

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The Morningstarr*

The mass DOES have a profound effect on stopping the vehicle! If it has a big mass it is harder to stop than if it has a small mass! We can, at least agree that it doesn't aid braking though - completely the opposite in fact!

If forward momentum "overcame" the downward force, the vehicle would rise up and fly! The two things are completely different and separate and act in completely different directions. They can therefore be treated independently. The downward force is the vehicles mass times the acceleration due to gravity. The forward momentum is the vehicle's mass times its velocity. They have different units so you can't add them together or take one from the other.

I could agree with the last statement if it referred to the (lack of) weight spread over so many wheels. The trouble is, it needs that many wheels to carry its weight when fully loaded. Come to think of it, I've seen plenty of rigid body trucks with multiple axles - some of which don't touch the ground when the lorry is unladen. I've never seen an artic trailer like that though - do they make such things and would it help?