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Gee whiz, the thought of bobtailing a tractor with no brakes on the front axle stresses me just thinking about it! Throw in some rain for a good time!
I think the idea (of ācontrol reasonsā) was that if you locked the brakes, you could still steer because you didnāt lock the steering wheels. Thankfully, that issue has been resolved by putting ABS on commercial trucks.
Yeah, that idea stresses me out too, because if Iām driving a loaded 80,000 pound semi, I want the brakes to work on every wheel.
āWhy donāt all vehicles have air brakes?ā
Because they donāt need themā¦They can stop very quickly now, tires right at the limit of adhesion. The existing anti-lock brakes in universal use today would be almost impossible to improve uponā¦ Air -brakes make sense on large trucks and railroad trains but not on passenger vehicles and light trucksā¦
Dont think they āBobtailedā all that much,no load,no money-Kevin
ā¦ and even when an 18-wheeler is bobtailing, itās got 10 wheels touching the ground. If two of those wheels donāt have brakes, it probably wonāt make much difference in braking distance.
@Whitey: How much weight is really carried by 8 of those 10 wheels, when all the heavy stuff is over the front axle? Braking force is proportional to the % of weight on the braked axle, times the static friction coefficient between rubber and asphalt.
I might be wrong, but I think the weight on a bobtailing semi is pretty well distributed. All that hardware that makes up the tandem axle weighs enough that itās at least better balanced than an empty pickup truck, and with eight wheels you get eight contact patches in the back, so some of the weight would shift from the rearmost axle to the front wheels on the tandem axle as the frame flexes. Also consider that, unless you have a cabover design, a semi with a sleeper cab is roughly twice the length of two large sedans, so the engine isnāt right on top of the steering wheels. Itās more behind them than on top of them.
I agree that having no brakes on the steering wheels is pretty nuts though, for the same reason you wouldnāt want only rear brakes on a car: the weight shifts forward when you hit the brakes.
Prior to 1975, big rigs (semi-trucks) were not required to have front brakes and many of them did notā¦Today, most big rigs have a limiter switch in the cab for the front brakes to eliminate any chance of locking them up during wet weatherā¦It reduces front braking effort by 60%ā¦
Some mid-range trucks had āair over hydraulicā brakes, where an air booster replaced the vacuum booster used on cars. Vacuum boosters are limited to 14psi boost pressure, atmospheric pressureā¦But an air booster can have much more force in a smaller packageā¦Another factor, diesel power produces no āfree vacuumā like a gas engine doesā¦
A ādryā (empty) semi truck flatbed weighs upwards of 4-1/2 tons. Thereās weight on the rear wheels.
Hereās a bit of brake trivia. Steam trains used to have vacuum brake. There was no need for a vacuum pump. Steam was sent to a venturi to create vacuum for the brake system. It had simplicity in its favor. Although vacuum pressure differential was limited, force=pressure x area. Increasing the size of the brake chamber makes up for the low pressure.(some vehicles today use a dual diaphragm brake booster)
However, diesel locomotive came along and a vacuum pump was needed. This was when compressed air took over. The ability to work with a smaller brake chamber that isnāt limited by altitude (vacuum brakes donāt work well at high altitude) made air brake a clear choice.
Thats pretty light for a truck Same,my current torture oven weighs13.5 tons empty,I think most heavy duty tractors weigh upwards of 8 tons ,bobtailing(tandems of course)-Kevin
The question being about weight on the rear tires, I was referring to just a flatbed without the tractor. But yeah, even just a trailer can be well in excess of 4-1/2 tons.
The trend now is to save weight anyway possible on the tractor or trailer and some concepts work out mighty well-Kevin