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Tag axle brake chambers
Would like to know why Prevost used such small brake chambers on the tag axle of my vintage. It is an XL45 VIN R1001788. From memory I believe tag axle weight is about 13000 lbs and steer is about 14000 lbs. The steer uses a 30 service chamber while the tag used a 16 service and 24 parking. The drive also uses a 30 service and 36 parking. Seems like we would want the same 30 service chamber on all axles. Wondered about this for the last six years. Now I wonder why I waited so long to ask. Thanks.
Oh BTW, I was at the service center in Saint Nicolas in 6/16. They replaced the tag lift arms on both sides and a lift chamber on one side. So I know they saw my tag brake chambers but didn’t say anything. I’m running 30/30’s there.
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13 Replies
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I’m only guessing … my understanding for most vehicles is that the front axle takes on a very significant amount of the braking load; the rear axle(s) taking must less of the load. The drive axle has four tires vs two on the tag, hence the larger capacity brake chambers on the drive axle. Unless you have ABS on the tag, its possible under heavy braking over a long distance (which should create some weight transfer to the front), the tag axles tires might have a tendency to lock up.
We’ll see how accurate my presumption is.
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Dan, Since you and Dick are both race car guys I am sure you realize the front brakes are heavier than the rear for the reason Dick suggests.
The Prevost is the same. Weight is transferring forward under hard braking. On our coaches you can be sure the braking system was developed and tested to provide the maximum braking which means the individual axles have brake forces that apply as much braking power as possible without locking a brake prior to any other one. The shoes, pads, drums or discs are all sized to accomplish hard braking without any premature lock up.
ABS now controls that to an extent, but keep in mind if it has a fault it will not function so you do not want a system that will require you to modulate the braking because one axle is prematurely locking and reducing the braking ability.
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Yes, got the weight transfer thing. Do have the ABS. But don’t know if there are sensors on the tag. Can’t see in there right now cause it’s down on the stops. Been running 30/30’s on the tag the last 30,000 miles and have never sensed that a tag locked up, and have never seen the ABS light on the dash come on, but it does work because it lights on start up.
The one time I could smell the brakes was after a steep down hill approach to Cochrane Alberta (suburb of Calgary) shortly after which we arrived at the rv park and got out of the bus. I sure did not like setting the parking brake that hot. At drivers schools we teach not to set the parking brake after coming off track.
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Dan: Since I’ve never been in a situation to get the brakes hot doing down a steep hill and then immediately have to stop, I haven’t yet worried about setting the parking brake on hot brakes. If you’re around home in a couple of months, you’ll likely be able to test the ABS.
We went to the Packer game Christmas Eve last December with the coach as we started our trip to La Crosse, Vonore, Virginia, and Florida. It snowed from home to Green Bay. Three inches of wet snow on the road as we approached the Oneida Casino by the airport. There is a series of traffic lights we had to go through on STH 172, all of them were red. No traffic between me and the lights, so I had an opportunity to “test” the brakes on a slippery surface. At slow speeds, I had no trouble locking up and sliding the front tires; probably not more than I would have had with a regular car. Getting the green, I could spin the back tires if I fed the throttle in too quickly … LOTS of torque.
The coach on the slippery surface reminded of driving the 750hp Heilig-Meyers Ford with slicks during a pretty good rain storm.
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Dick, no ABS? You locked?
If you have ABS, how did the pedal feel?
As we know in a car it feels like hammering the bottom of ur foot.
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Without getting under to see I don’t know if the tag has ABS or not. I do know 30/30 seems larger than what is on the tag when it comes from the factory so you have become a test driver.
I don’t know if you would know if you locked the tag wheels up from the driver’s seat because you might not hear them sliding.
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Dan: I have an inoperative ABS system. I have the ABS light on all the time. I’ve been told by Jeff and Prevost-Jacksonville that the something is wrong with the sensor(s) which are apparently buried in the rear axle. Estimate to fix was $3,500 so I’ve left it as is. Patina? We have a 1998 chassis. I’m told the later model chassis have ABS system where the maintenance/fix of the sensors is much easier to get at and hence the labor bill is lower.
I don’t think I have ABS on the front axle, just rear. I could lock up the front tires on the wet snow. I don’t know what was going on in back, couldn’t feel it. Given the wheelbase of the coach, if the drive axle moved over a foot I don’t know if I could sense it.
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For reference while we’re on the subject, just read through the ABS manual that came with the bus. It is a Rockwell 4 channel Wabco system. It only senses the drive and steer positions, not tag. It is two half systems where one half operates on the right steer and left drive, and the other half operates on the left steer and right drive. And the failure of one half does not affect the other half. When a sensor senses imminent lock up, it sends a signal to the electronic control unit (ecu) which in turn causes air pressure to be exhausted through a ABS valve which is usually mounted to the frame between the relay valve and the brake chamber.
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DICK: If your abs light is on, it’s probably safe to assume the entire system is inoperative. Don’t know that for a fact, but that’s the safe way to go.
I have gotten into abs activation on my current bus. No drama. I did not sense anything in the pedal but could tell from the seat of my pants wheels were locking/unlocking.
CAUTION TO ALL: Under extreme slippery conditions, it is possible to inadvertantly disable abs (unless newer coaches have gps speedometer capabilities…not sure). If you jump on the brakes hard initially and lock all wheels immediately, the vehicle thinks you are stopped. It has no speed reference and will not unlock any wheel. If you experience this, it is necessary to release (at least partially) brake pedal pressure in order to get at least one wheel to turn to reactivate system.
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Dick, my ABS light stays on all the time as well, and you are correct as well, its about $3500 to fix it. Jeff and Prevost as well say that you have to pull the rear end apart and put pickups on the axles, then you can put external sensors on the reared and the system will work. You still have regular brakes, but no ABS, thus far no problems but at sometime in the future will have it changed over.
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ABS is a nice system to have, but for a lot of years it was not even available and we were able to safely brake our coaches.
I used our first coach for 10 years in the snow belt and the roads were often snow or ice covered. Without ABS I just had to be sensitive to the brakes and if they started to lock up I had to modulate the pressure on the brake pedal. On dry roads there was no problem at all.
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I have an abs sensor light which i removed the light from the panel.
Jax Prevost diagnosed as a right differential axle sensor failure which was no longer available. The fix is to add tracking rings and sensor to axle to give the sensor signal.
Didn’t like the price. Still looking to get a sensor from a part out unit.
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Ha. I have the same code indicating a problem with the right rear differential sensor. Seems a common issue. The sensor is actually accessible on the differential housing and could easily be replaced. Maybe the issue is the internal signal ring.
Anyway, part of the ABS system is still working despite that code. I was braking pretty hard for a turn-off with some gravel on the road when I heard what was like a machine gun below my feet. My first thought was “That sounds expensive!”. It turned out it was the air-operated ABS system pulsing. It operates at a much lower frequency than a hydraulic ABS system in a car. You’ll definitely know when it kicks in.
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