chgofan78 Posted May 17, 2009 Report Share Posted May 17, 2009 Last night around 6:30p.m., New Flyer 4140 was turning off of Michigan Avenue onto the Congress Loop to finish its southbound run. As it was turning, the bus looked as if it were about to hit a light pole- maybe because the turn was made too wide and as a result, the bus blocked northbound Michigan Avenue traffic for about an hour. The driver referred to the bus as being locked. What causes a bus to become locked- especially a newer model? Quote Link to comment Share on other sites More sharing options...
Guest Posted May 17, 2009 Report Share Posted May 17, 2009 Last night around 6:30p.m., New Flyer 4140 was turning off of Michigan Avenue onto the Congress Loop to finish its southbound run. As it was turning, the bus looked as if it were about to hit a light pole- maybe because the turn was made too wide and as a result, the bus blocked northbound Michigan Avenue traffic for about an hour. The driver referred to the bus as being locked. What causes a bus to become locked- especially a newer model? He was probably referring to the brakes, a safety feature buses have. If a bus air brake system loses air pressure, typically below 90 psi, the brakes automatically are suppose to lock so that the bus cannot move. When a problem occurs, many times a bus operator will try to build up air pressure but as the bus brakes are used the pressure does not build up. Problems like this may cause a bus brake system to lock up. This happens quite often in the winter time. Quote Link to comment Share on other sites More sharing options...
MT0851 Posted May 18, 2009 Report Share Posted May 18, 2009 He was probably referring to the brakes, a safety feature buses have. If a bus air brake system loses air pressure, typically below 90 psi, the brakes automatically are suppose to lock so that the bus cannot move. When a problem occurs, many times a bus operator will try to build up air pressure but as the bus brakes are used the pressure does not build up. Problems like this may cause a bus brake system to lock up. This happens quite often in the winter time. Indeed it does. Running alcohol thru the air system is supposed to prevent it from happening, however on newer buses you can't run too much alcohol due to the types of hosing used, the alcohol tends to dry the hoses up and it cracks them. Another thing I was thinking is that maybe the joint locked up due to a false"max angle" reading from the computer, which would disable the accelerator. Did the bus dump its air shortly before coming to a stop? If so that's a indication that the Maxi was automatically applied which would most likely be caused by a drop in air pressure. Quote Link to comment Share on other sites More sharing options...
jajuan Posted May 18, 2009 Report Share Posted May 18, 2009 Indeed it does. Running alcohol thru the air system is supposed to prevent it from happening, however on newer buses you can't run too much alcohol due to the types of hosing used, the alcohol tends to dry the hoses up and it cracks them. Another thing I was thinking is that maybe the joint locked up due to a false"max angle" reading from the computer, which would disable the accelerator. Did the bus dump its air shortly before coming to a stop? If so that's a indication that the Maxi was automatically applied which would most likely be caused by a drop in air pressure. This second explanation is sounding a more likely cause given the mention that the bus appeared it would hit a lightpole from turning too wide. Perhaps the driver was trying to compensate for this while moving the bus more into the turn. When I rode bus 4126 on the 146 early Wednesday afternoon, the driver had started the turn from State onto Lake a bit too wide and pulled the bus into a tighter turn as he was completing it. It had looked like he was going to get stuck behind a parked car on Lake or maybe even strike it, though this time around the cars parked on Lake for once weren't parked too close to the intersection as they sometimes are. I didn't think the articulation joint on an artic could bend as much as this one did from the driver compensating for starting his turn so wide. Quote Link to comment Share on other sites More sharing options...
artthouwill Posted May 18, 2009 Report Share Posted May 18, 2009 Could another culprit be the power steering? That is not to discount the loss of air pressure theory which is quite plausible. But if the steering locks up in the course of a turn, it would make for a wider turn. I have driven a bus where the steering radius was better for turning left than turning right. Quote Link to comment Share on other sites More sharing options...
santran Posted May 18, 2009 Report Share Posted May 18, 2009 On all artics the bus has a joint lock. If the bus is turning to sharp the bus will lock up as a safety messure, so the joint is not damaged. Some older units had an over-ride but the new ones do not. Quote Link to comment Share on other sites More sharing options...
cta_44499_FG Posted May 18, 2009 Report Share Posted May 18, 2009 Indeed it does. Running alcohol thru the air system is supposed to prevent it from happening, however on newer buses you can't run too much alcohol due to the types of hosing used, the alcohol tends to dry the hoses up and it cracks them. Another thing I was thinking is that maybe the joint locked up due to a false"max angle" reading from the computer, which would disable the accelerator. Did the bus dump its air shortly before coming to a stop? If so that's a indication that the Maxi was automatically applied which would most likely be caused by a drop in air pressure. There is supposed to be a switch that unlocks the "max angle", or it lighter terms hinge lock and allow the bus to finish straightening out. I've seen this happen before, and have had to use it one time from experience of a false reading (mind you, I wasn't making that wide of a turn). Air pressure has mysteriously dropped for no reason many times, but I have yet to see it happen on the New Flyer series or the Metros. This has happened to me on the NovaLFS countless times. Winter of course, is another culprit for this. During the winter, from North Park Garage to about Foster/Western....there were three NFs stuck in the streets with frozen air lines. Quote Link to comment Share on other sites More sharing options...
trey824 Posted May 19, 2009 Report Share Posted May 19, 2009 There is supposed to be a switch that unlocks the "max angle", or it lighter terms hinge lock and allow the bus to finish straightening out. I've seen this happen before, and have had to use it one time from experience of a false reading (mind you, I wasn't making that wide of a turn). Air pressure has mysteriously dropped for no reason many times, but I have yet to see it happen on the New Flyer series or the Metros. This has happened to me on the NovaLFS countless times. Winter of course, is another culprit for this. During the winter, from North Park Garage to about Foster/Western....there were three NFs stuck in the streets with frozen air lines. by any chance does anyone know if our standard 40 footers have "locked up" before or can they???? Quote Link to comment Share on other sites More sharing options...
cta 5555 Posted May 20, 2009 Report Share Posted May 20, 2009 U mean low air pressure? Then to bad u were not on bus tracker last friday morning looking at the 65. U colud of saw 1077 at Grand and Western sitting there for a hour. That was me. (Well nothing else new me broke down with a nf)The new flyers are getting to be good for that. Having low air pressure. Quote Link to comment Share on other sites More sharing options...
Anonymous Guy Posted May 26, 2009 Report Share Posted May 26, 2009 Sorry to be a bit of a bus newb, but can anyone explain the phenomena of buses losing air pressure? About a month ago I boarded a (non-Chicago) New Flyer D60LF that seemed to be experiencing something similar to the air pressure concerns brought up earlier. When the bus pulled into the stop, it immediately let off all of its air pressure. After I had boarded, the driver explained to all the passengers that he would need to stop the bus for a minute or so to build up air pressure. As the driver pulled away from the curb, the bus moved at a very slow speed and again let off all of its air pressure at the next stop. This continued for about two more stops until the driver explained that the bus had locked up and we would have to wait for the following bus on the same route. Also, this happened on a day with about 65-70 degree weather, so winter chill wasn't an issue here. Quote Link to comment Share on other sites More sharing options...
speedracer1407 Posted May 27, 2009 Report Share Posted May 27, 2009 Sorry to be a bit of a bus newb, but can anyone explain the phenomena of buses losing air pressure? About a month ago I boarded a (non-Chicago) New Flyer D60LF that seemed to be experiencing something similar to the air pressure concerns brought up earlier. When the bus pulled into the stop, it immediately let off all of its air pressure. After I had boarded, the driver explained to all the passengers that he would need to stop the bus for a minute or so to build up air pressure. As the driver pulled away from the curb, the bus moved at a very slow speed and again let off all of its air pressure at the next stop. This continued for about two more stops until the driver explained that the bus had locked up and we would have to wait for the following bus on the same route. Also, this happened on a day with about 65-70 degree weather, so winter chill wasn't an issue here. I can't tell if you're wondering why losing pressure causes the brakes to lock, or if you're specifically asking why a bus would dump all its brake air pressure for no apparent reason. So I'll try to cover both, so we're on the same page. All air brake systems, truck, bus, or train, are designed to be "fail safe." That is, a loss of brake system air pressure will cause the brakes to engage, which either stops a moving vehicle, or prevents it from moving in the first place. On a train, moving the brake lever actually reduces pressure in the main brake pipe, causing a sliding valve to move, which opens a pathway for a separate reservoir tank to dump its pressure into the brake cylinder, which pushes a piston, which applies the brake shoe to the wheel. This system is considered "fail safe" because a leak or failure of the main pressurization system (compressor, brake pipe, etc) will cause an uncommanded brake application and will stop the train or prevent it from moving. A bus (and truck) system is similar except that the service brake application and fail safe application are separate elements of the same system. Pressing the brake pedal opens a valve, which releases air from a reservoir tank, which fills a brake cylinder, which acts on the brake shoes of a drum brake. Without a fail safe system, a loss of brake air pressure would leave the bus without any braking action. But each brake cylinder also has a separate compartment with a very stiff spring that is directly connected to the brakes. This compartment MUST be filled with pressurized air from the main brake system at all times to prevent the spring from applying the brakes. When the main brake system pressure gets too low, the spring brake compartment will also lose pressure, and the spring will be allowed toto apply the brakes and will stop the bus or prevent it from moving. I have no idea why a bus will unexpectedly empty its air when coming to a stop. I suspect, however, that a bus will never dump ALL its brake system air. Rather, I suspect that low air pressure will trigger a safety system that dumps spring break air once the bus has stopped, preventing it from moving again untill normal system pressure is restored. I hope some mechanics or others who are more knowledgeable about air brake systems (my weakest subject) can either confirm or correct what I've explained here. Quote Link to comment Share on other sites More sharing options...
ryanbytes Posted May 28, 2009 Report Share Posted May 28, 2009 I was on a bus that dumped air this morning. It was a New Flyer eastbound on Chicago. We got a couple blocks past Milwaukee when we STOPPED. I was surprised at how fast the stop was. I guess it's a testament to the effectiveness braking system. We had to wait a little bit for the air to build and then we were on our way. It was quite a moment. Nobody got hurt, we were just surprised. Quote Link to comment Share on other sites More sharing options...
santran Posted May 28, 2009 Report Share Posted May 28, 2009 On all transit buses the "pop up" feature activates at 35-50 psi. This is when the parking break activates on its own. The warning lights come on at 60-70 psi. So the pop up should never happen if the driver has any clue as to what is going on. And in that rare case the bus should be taken out of service right away. Quote Link to comment Share on other sites More sharing options...
MT0851 Posted May 28, 2009 Report Share Posted May 28, 2009 Sorry to be a bit of a bus newb, but can anyone explain the phenomena of buses losing air pressure? About a month ago I boarded a (non-Chicago) New Flyer D60LF that seemed to be experiencing something similar to the air pressure concerns brought up earlier. When the bus pulled into the stop, it immediately let off all of its air pressure. After I had boarded, the driver explained to all the passengers that he would need to stop the bus for a minute or so to build up air pressure. As the driver pulled away from the curb, the bus moved at a very slow speed and again let off all of its air pressure at the next stop. This continued for about two more stops until the driver explained that the bus had locked up and we would have to wait for the following bus on the same route. The bus most likely wasn't dumping its air on its own, if anything the air pressure was getting on the low side (most likely caused by a governor switch stuck to the unload position, or a sticky compressor unloader valve). So when the driver got to the stop he probably pulled the maxi (Which dumps the air) so he could rev the engine to try and increase the air pressure. Quote Link to comment Share on other sites More sharing options...
artthouwill Posted May 29, 2009 Report Share Posted May 29, 2009 The bus most likely wasn't dumping its air on its own, if anything the air pressure was getting on the low side (most likely caused by a governor switch stuck to the unload position, or a sticky compressor unloader valve). So when the driver got to the stop he probably pulled the maxi (Which dumps the air) so he could rev the engine to try and increase the air pressure. Not a mechanic, but there should never be a sudden loss of air pressure to the point of locking up the bus. But should it happen, there would probably have to be a break in the air lines or the air bags or possibly the compressor. The compressor is supposed to "cut out" at 120 psi. When applying the service brake there will be some loss of air pressure, pumping the brake will cause air pressure to go down quicker. A warning light should come on if psi goes below 80 lbs. At 60 psi, the bus should be inoperable or come to a stop. I once tried to have a bus towed. When Airline towing came out they tried to air up the bus (since the engine was no good) but could only get up to 15psi max, which meant there was a leak somewhere in the system. Quote Link to comment Share on other sites More sharing options...
Anonymous Guy Posted May 29, 2009 Report Share Posted May 29, 2009 Appreciate the answers, thanks! Quote Link to comment Share on other sites More sharing options...
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