What does this sign mean: "Use of air compressor brakes is forbidden inside the city limits"

I think I got the wording right, but this sign is on some of the approaches to Portsmouth NH. I thought all large trucks use “air compressor brakes” ?

I think that it is a reference to “jake brakes”, which utilize engine compression, and can be very loud.

+1 We have a similar sign “Engine Brake Use Prohibited” (or something to that effect) on Rt.1 going into Old Town Alexandria, VA.

Does this mean that these big trucks are normally in a free wheeling mode where there is no braking from the engine?

@Triedaq ,
Pretty much yes.

Well, no, the engine isn’t ‘free wheeling’ w/o the Jake brake, it’s taking in air, compressing it on the combustion stroke, and having that compressed air push the piston back down, reducing any braking effect. When the Jake brake is engaged, the exhaust valve opens near top dead center for the piston on the combustion stroke, releasing the compressed air which prevents it from pushing the piston back down. All that wasted compression work now slows the truck.

I always wondered about “Jake Brake Prohibited” laws, and how much authority they’d hold if, in the expert opinion and discretion of the truck driver, Jake Braking was necessary to slow the vehicle with an acceptable margin of braking safety.

After all, the folks who put those signs up are generally NOT commercial drivers, and an operator of a motor vehicle has a legal obligation to operate safely…even to the point of deviating from laws, if obeying said law would create an unsafe condition (such as driving below a minimum speed in a snowstorm).

If an over-zealous town enacted a “Jake Brake” ordinance that created a dangerous condition, if obeyed, in certain heavier trucks, would a driver be cleared in a courtroom? I’d like to think so!

Using a Jake-Brake makes downshifting actually slow the truck but like downshifting with a gasoline engine the most significant benefit is on long steep descents where brakes would get hot. Driving in urban areas using compression brakes should not be necessary while crossing Idaho on I-90 there would be a chorus from all the tandems hoping to avoid the HOT BRAKE exits.

Jake brakes make a racket, and for that reason densely populated areas often prohibit their use. It is a citable offense of the trucker is caught, and they’re so noisy that any cop in the area is sure to notice.

"Driving in urban areas using compression brakes should not be necessary while crossing Idaho on I-90 there would be a chorus from all the tandems hoping to avoid the HOT BRAKE exits."

Actually, to the contrary, there are MANY MANY instances of US-routes that steeply drop speed limits as you hit town. Quite frequently, the towns are in valleys…so, after a long, steep descent to get your brakes “toasty,” you NOW have to quickly reduces speed for the town, as you continue to descend, ALL without Jake braking! That may be well and good…but if a child runs out into traffic–how much “margin” do you suppose you have left?

Laws that usurp command authority from the driver, and that prioritize aesthetics AHEAD of safety, are somewhat repellent, and I’d hope any sane jurist would agree.

There’s a “No Engine Braking” sign just down the road from where I work. My ears tell me that it’s routinely ignored.

No doubt there are exceptions to any rule but as a rule, an attentive driver should be able to anticipate a steep descent and allow the truck to coast down as he ascends the approaching hill to avoid needing to herald his approach with an extended rattling of a Jake Brake. Merchants at a small strip mall near me complained about the brake noise and the city council put up the NO EXHAUST BRAKE sign and it seemed that the truckers felt they were being dared to use them, especially loaded log trucks going to a nearby mill and the racket worsened considerably. That says a lot about the underlying conflict and ‘one upsmanship’ of the issue.

An “Jake” brake is an optional accessory. Most large fleets of tractor trailers do NOT have them.

They save wear and tear on your brakes but are not required in any jurisdiction. Therefore there should be no problem turning them off to comply with local law. They don’t shorten stopping distances because they only work on the drive wheels,.

Their only function is to keep your speed down on hills without using the brakes. A truck without them may have to go down hill in a lower gear and with less speed.

In 40 years of driving tractor-trailer for class 1 freight companies I had exactly one truck with an engine brake.

Even without a compression brake, a big diesel has much more engine braking than a car, so much so that if you are trying to stop on ice, you had better shift into neutral or put in the clutch to prevent locking up the drive wheels.

I think the signs all say “engine brake” because “Jake Brake®” is just like “Jet Ski®”, a registered trademark.

I keep thinking, why don’t they make quiet Jake Brakes? If the cops can’t tell you are using one, it doesn’t matter if you are using one.

The noise is from popping open the exhaust valve at max compression. Bigger event than normal exhaust, I guess.

Well what actually happened in my opinion was that some owners would gut the mufflers on on their diesel trucks to increase the noise(kinda ala HD) and imagine they were getting some magical power increase(usually not) and really come down on the trucks inconsideretly in populated areas,a good muffler well get rid of most of the racket(over 90% of the diesel trucks around here are equipped with either engine brakes or sometimes exhaust brakes(exhaust brakes scarcley make any racket{operates differently then compression brakes}If you want to know the history of “Jacobs” engine brakes go to the websitye,it was basically a west coast innovation,there is reallyno reason to get your brakes stinking now,in towns the service brakes are sufficient to stop safely and keep the speed down,speed plays an important part in this,the only old diesels that had any appreciable retarding effect were the detroit diesels,the rootes blower consumed so much power it would actually help retard you on a grade,the other diesels were handicapped by a term called called “airspring” and no throttle plate to turn the engine into a vacuum pump of sorts"which consumed a few HP" in Europe you can get small electric retarders that actually fit on the backing plates and create a flux whick slows you down when engaged"dynamic braking"(the same thing is used on diesel electric locomotives.

All we need now is for someone to come up with a Jake Brake for Harley Davidson motorcycles, so they can be just as obnoxious slowing down as accelerating.

“All we need now is for someone to come up with a Jake Brake for Harley Davidson motorcycles, so they can be just as obnoxious slowing down as accelerating.”

Most Harley’s sound horrible as they pull away from you. Very few have tuned exhausts so hearing a nice sounding Harley Davidson is a rare treat.

@B.L.E. Don’t encourage them! Harley owners can be obnoxious enough as it is.

Not ALL H-D owners are like this, but we can clearly hear the ones that are.

Met 5 vehicles on an upgrade a couple of weeks ago,including an 18 wheeler and a Harley,guess which made the most racket.