Service Brakes w/ Retader, Panic Stopping

[davidbrady]

Guys,

There are folks on another forum who claim that those of us with retarders, whether hydraulic transmission, jacobs brake, or both, should always leave them enabled so as to shorten our stopping distances in full on panic braking.

I've never succumbed to this belief so I thought I'd ask this group. The Prevost literature corroborates what I've always thought was true and that's that retarders are for downhill braking, and downhill speeds should be reduced such that the retarder alone can maintain the desired downhill speed. This approach keeps the service brakes cool and on standby for emergency braking.

Nowhere in the Prevost owners or drivers manuals do I see anything alluding to the idea that panic braking in a Prevost is only achieved to it's full power when the service brakes are applied with the retarder(s) enabled. It seems to me that the service brakes alone should be sufficient to achieve maximum braking power and minimum stopping distances during 100% brake pedal application.

I guess what I'm saying is that surely the service brakes are powerful enough to lock up the wheels on all the axles. I've never had to do this but I'm sure it's true for the tag and steer axles. That leaves the drive axle. Are the Knorr-Bremse Prevost drive axle disk brakes powerful enough to overcome the drive axle tire grip or can more braking power be attained by adding retarder braking on the drive wheels?

I routinely drive with the retarders disabled at the dash. I find it's easier to maintain transmission fluid temps when I'm in control of when the retarder is activated. With a Jake I find it's easier to pick up momentum on downhills if I wish to let momentum shave a tenth or so off fuel consumption on the uphills with the jake disabled.

Thoughts?

[cmillsap]

Yeah David, I read that post also, I found it very interesting. Ulterior to what I know, it was stated that professional truckers always drive with their Jake or retarder on. I don't believe that to be the case at all. I'm sure there are novice and reckless truckers (cowboys) out there that hope their Jake or retarder will save their ass. But good Professional truck drivers use it only for downgrades and when needed in low speed congested traffic conditions. I do the same. My Jake is off on wide open roads.

Having the Jake or retarder activate every time you lift the accelerator is like pressing your chassis brake pedal each time you let off the accelerator. I see it all the time while following a coach equipped with a Jake or retarder. The brake lights are constantly going on and off. I bet a lot of auto drivers following a coach with the Jake or retarder on wonders what the hell the coach driver is doing.

Obviously, I want all the braking assist that I can get in a panic stop situation. I even appreciate the added auxiliary brake that I place in my towed vehicle. I'm sure the Jake being on helps in a panic stop but the switch is in a prominent place on my dash and is easily activated. In a panic stop so short that you don't have time to turn on the Jake, an active Jake isn't going to have much effect in helping avoid the accident either.

Safe driving habits will prevent more accidents than a Jake or retarder ever will. Good defensive driving habits such as maintaining adequate space in front of you to stop and watching out for the yahoos who are depending on their Jake or retarder to stop them will do more to avoid an accident than a Jake or retarder ever will...

[Ernie Ekberg]

I saw that post on the thread I started. I always leave my Jake off during travel. I can switch it on automatically as I memorized exactly where it is located. The Prevost we are looking at has the Jake and will continue my driving habits with that. The engine and transmission are same as my bird- putting a different brand name on the coach shouldn't make any difference in how things work

[davidbrady]

Sounds good Chuck and Ernie. There's also the fact that the jake disengages at 15 mph leaving braking to a stop entirely up to the service brakes. Then there's the issue of making sure the retarders are turned off on slippery roads to avoid possible loss of drive axle traction and loss of control. All of this adds up to me turning on the retarder when I need it and off the majority of the time, which I also agree is what skilled truckers do. The transmission retarder will bring the bus to a complete stop, but the added braking provided by retarders is far from smooth and linear. Retarder braking power is dependent upon engine speed and gear selection. If a Prevost's service brakes are a finely tuned system with each axle doing it's proportional amount of braking, then additional drive axle retarding upsets this balance. I've looked at the many NHTSA braking test for transit buses and semi's and all braking requirements are based on service brakes alone. The service brakes need to get the job done in an emergency, the retarders need to get the job done on long descents - two different requirements satisfied by two different systems. Some may thing the bus stops faster when using the retarder along with the service brakes, but I think that's an illusion. The retarder on my H3 makes my service brakes feel like they're power boosted, but I'm sure w/o the retarder I can achieve maximum braking power and minimal stopping distances by pushing harder on that pedal. If you can't stand on the service brake pedal with the required force, then the driver is the limiting factor. There are lots of myths perpetuated in the Prevost community and I think this is one; be careful out there.

[CC_Guy]

I used to keep my Jake off unless needed or driving through the mountains. The reason being almost all Jake's left the factory programmed to downshift to 4th gear when activated. I never liked forced downshifting with every Jake activation. Of course, the Jake would only downshift when doing so would not over rev the engine. Still I didn't like it.

If you use your cruise control with the Jake on, the same downshift would occur when the cruise effectively let off the pedal. Again, not to my liking.

A couple years ago I had my local Detroit/Allison shop reprogram the Jake downshift from 4th to 6th. Now I leave my Jake on almost all of the time when I'm on the highway. I know many coaches have been reprogrammed as mine is now. I'm guessing those that leave their Jake on don't have their factory programming.

I wish there was 2 Jake functions. The one I just described and a Jake High function with dowwnshifting tied solely to brake pedal use. In a panic stop it would be great to have Jake High engaged.

[cmillsap]

Many owners of Bluebirds have had their Jake's 4th gear factory setting reprogramed to start with 5th gear and think it is an improvement in the way the Jack functions on a 50,000# bus.

[Itchintogo]

I think it depends on where your driving. Out here in the Rockies and Cascade mountains I am confidant you will find a very large percentage of trucks and buses are running with their jakes on all the time. Due to mountainous terrain and tight reduced speed corners is really saves on the service brakes, helps you control momentum and saves you from flipping the switch constantly. There is usually a sweet spot you can feather the accelerator to more or less coast. Of course if your following a vehicle and waiting to pass I probably would shut it off to keep the jake from cycling. Brake lights don't come on in a highway truck when the engine brake engages.

Out on the prairie I would drive with the Jake off and I think most people do. Switching it on when required. When slippery you must be cautious and this is when sometimes with a long grade and a heavy load I would engage the engine brake in stages usually 3, from low to high, gradually increasing the torque resistance on the drive wheels. Extreme icy conditions, I have come off the Coquihalla Hwy in first gear edging down the mountain with no engine brake on. When it gets that bad I generally have a rule that it is time for bed!

But more to the point. The example given was a panic stop situation. So if we are reducing speed at the greatest or maximum rate of decline then you may achieve a slight advantage by having an engine brake or retarder engaged but in my opinion it will be very minimal. There is no way a driver can safely control the vehicle and down shift to achieve maximum RPM braking power and/or the Allison transmission will be hunting and pecking and can't find the appropriate gears fast enough to achieve maximum braking power, when the wheels are either locked up or shuddering with ABS activity. The rate of speed and engine rpm loss are in a type of "free fall" during the event. It would be next to impossible to find the correct gears fast enough. For that reason, in my opinion the engine brake likely won't make an appreciable difference in stopping distance.

[Joe cannarozzi]

There is a way the Jake can help in slowing that was not discussed here. Can't respond to the other threads I do nto add content to that site.

I think u really can't compare it's use in the bus to its use in a semi that's like comparing apples and oranges because 1 has an automatic trans and 1 is a manuall.

I am fairly confident that in a truck downshifting will get u stopped way quicker when combined with braking. If u r downshifting with the Jake on the rpms wI'll drop way faster enabling g u to get down thru those gears way quicker.

So in a truck with a manual trans thats in a attempt to stop as quick as possible and the driver is agressivly downshifting with the Jake on it is been my experiance that it will get u slowed down quicker.

Bus and the automatic is very different.

With the Trans left in 6 or 5 Whate very u have and u try to stop quick and have the Jake on as soon as speed drops below the gear on the keypad the Trans does not downshift itself. I suppose there is a term for it let's call it limbo. It comes out of limbo when u step back on the gas or come to a complete stop. So no additional braking g can result from an automatic trans if u leave it in drive.

HOWEVER

The reason I responded to this thread is it got me thinking. IF , in the bus and with the automatic trans and the Jake on during a panic stop the driver assists braking by agressively manually downshifting the automatic trans IMHO that would assist in shorting the stopping distance and for the same reason as the manuall trans, the rpms wI'll drop quicker.

Or another way to explain.

In a truck if the driver did not manually downshift with the Jake to slow down quicker the Jake would be almost as worthless as having it on with an automatic and not bringing it down thru the gears with the button.

My 2 cents. Just stumbled across this site. Interesting thread.

---

I just reread the post before mine and brought another thought.

I got a question.

What would happen if in a panic while trying to stop fast a driver hit that downshift button as fast as he could right down to 1st as soon as the braking began at whatever speed that might be????

Would the Trans imidatly respond and self destruct or would each lower gear only engage when the correct speed is acheeved??

Either way I'd like to know . Either I never want to never make that mistake OR if it will not hurt the Trans I want to know it's in the arsonal if I need it.


Nother question.

I fortunately never had to make a panic stop but am curious to know if anyone reading this has ever been able to lock up the tires getting them to skid in a panic stop. I'm not completely convinced a prevo motorhome can lockup the brakes in a panic stop.

Someone please tell me that it ain't so.

[davidbrady]

Hi Joe,

Great to see you on the site, welcome, and thanks for your input!

Attached is a paper that says a fully loaded school bus, loaded to GVWR can lock it's tires on all axles. It also shows that the school bus outperforms the braking performance of a class 8 truck, also loaded to GVWR. I think this is the crux of the question: if the service brakes are powerful enough to engage the ABS on a high grip dry surface, then any additional stopping power injected by a retarder (whether transmission or jake) won't improve stopping distances and may actually lengthen stopping distances because the ABS can't correct for the retarder. I've never done it, but my opinion, FWIW, is that a modern disc brake equipped Prevost motorhome conversion can lock its wheels on a dry high grip surface. If true, then there's no need to waste time flipping switches when braking in an emergency situation. Just stand on the pedal!