Lorry (Truck) Brakes

[Famine]

Out of idle curiousity I started playing around with numbers in braking distances While there's plenty of information out there for us car drivers, I couldn't find much in the way of articulated lorry (truck, big rig, whatever the hell you call them) braking.

Without taking braking force into account (or rather keeping it the same as that for a car), the figures suggest that a 7.5 tonne (7.4 ton) lorry would take more than a mile to stop from 80mph - which just doesn't seem right to me (especially given that the same figures suggest a shorter stopping distance for an average car at 230mph)

So... ;D

Does anyone know what kind of braking force you can expect from lorry-brakes? Most cars you can expect to pull 0.9G or more under full braking on a flat, level, dry surface.

 

[daan]

http://www.rospa.com/drivertraining/factsheets/pdf/stopping.pdf

Page 8 has stopping distances on it

 

[Famine]

I caught that one on my searches earlier but...

Their cars beat the Highway Code's stopping distances... And I'm more interested in working out the typical stopping distance of an HGV (7.5 tonnes or more) at 70mph than lower down. I'd need three or more reference points - and I'd assume that their figures all exclude the Highway Code's 0.6s "thinking" time...

 

[Dorifto]

My dads Bobtail has airbrakes, but I'm not sure of the stopping distances.

 

[Duke]

From what I've found, stopping distance is theoretically independent of mass. Try this page here. But that's obviously belied by real life. Here's the text of another short article I dug up:

WASHINGTON, (June 10, 2004) -- The U.S. National Highway Transportation Safety Administration is poised to recommend that heavy-duty brakes attain 30 per-cent shorter stopping distances. The rules may also address the topic of mandated brake maintenance.

Current regulations, which are mirrored in Canada, call for a stopping distance of 355 feet from 60 mph for a tractor and unbraked trailer at a gross weight of 52,000 pounds. Under the anticipated NHTSA proposal, the distance would drop to 248 feet under the same conditions. Jim Clark, Commercial Vehicle Systems engineering manager at Dana, says OEMs need a 10-per-cent margin below the minimum to be comfortable with variations in performance, making the target stopping distance about 224 feet.

Getting there requires more brake torque, and the only place to get it is on the front axle. Tractor drive axles, says Clark, already are at the torque limit, requiring ABS to deliver stopping power in a full application. The front axle, in contrast, conventionally has only a 15-x-4-inch drum brake because of its 12,000-pound static loading. But since it actually sees up to 21,000 pounds in dynamic loading under heavy braking, the front axle can easily load a 16-1/2-x-5-inch drum brake.

One option is to combine drums on the drives with air disc brakes on the front axle. That's what PACCAR divisions Peterbilt and Kenworth are doing, offering the Dana 22.5-inch air disc brake as an option for front-axle braking.

While brake manufacturers agree that new combinations of available brakes will be needed to meet the regulation, the disparity in brake maintenance programs and the variability in quality and consistency of offshore brake components -- especially linings from South America, China, and India -- mean that trucks in service may well fail to meet the criteria of even today's regulations.

Ron Armer, president and CEO of friction material manufacturer Brake Pro, says the agency needs to consider whether the real issue is stopping distances of new trucks or stopping distance for trucks in service. Some fleets, he says, are top notch at maintenance. Others are terrible, and not helped by poor-quality friction materials, inferior rebuilt replacement shoes, and corrosion caused by aggressive chemicals used on winter roads. Regardless of the mandate, he says, poorly maintained vehicles won't stop even to today's requirements, let alone any shorter distance.