What is this? (Mod Note: Air suspension related)

[7ransport]

Hi all, just wondering what this tyre looking thing is and what it does?

 

[Paul Jones 88]

Air suspension

 

[pdeaves]

That's suspension. It's filled with air (like a tyre).

 

[aleggatta]

It's an air bag, it forms part of the secondary suspension (passenger comfort). It is also worth noting that inside the air suspension is a redundant mechanical spring, so in the event of bag failure the vehicle won't smash into the bogie.

 

[7ransport]

Thanks everyone

 

[a340egkk]

If the air suspension is damaged and passengers are still to be conveyed in the affected vehicle, you can only do 45mph. If the vehicle is locked out, it's linespeed - just to illustrate that as said above, it's purely passenger comfort related.

 

[pdeaves]

If the air bag is deflated, there are sometimes restrictions on what platform the vehicle can use (as the body is now at a different height to what it should be).

 

[Ken H]

If the air bag deflates, does it cause a drop in the air pipes and cause a brake application? I assume they use the same compressors as the brakes?

 

[skyhigh]

If the air bag deflates, does it cause a drop in the air pipes and cause a brake application? I assume they use the same compressors as the brakes?

Depends how big the leak is in the air bag. If it's a small leak the compressor may be able to keep up with the loss of air but if it's a full rupture it will likely cause a drop in the main air res pressure and a brake application.

 

[CDM]

If the air suspension is damaged and passengers are still to be conveyed in the affected vehicle, you can only do 45mph. If the vehicle is locked out, it's linespeed - just to illustrate that as said above, it's purely passenger comfort related.

Those speed restrictions vary by Stock type (and whether anyone has even reported that the secondary suspension is deflated in the first place!)

 

[Towers]

If the air bag deflates, does it cause a drop in the air pipes and cause a brake application? I assume they use the same compressors as the brakes?

Just to add, in that event the air suspension would be isolated at the relevant location in order to maintain operating pressure in the air system.

 

[Kneedown]

If the air bag is deflated, there are sometimes restrictions on what platform the vehicle can use (as the body is now at a different height to what it should be).

If I remember correctly, it was 5mph through Heckington with air suspension isolated for HST's for that very reason.

Whilst on most stock the air suspension is "secondary", ie for passenger comfort as opposed to safety, on 360's (and I would imagine other Desiro units?) It is the primary suspension due to the sensitive nature of the electronic computer gubbins on board, and reduced speed of 45mph applies when isolated whether passengers on conveyed or not.

 

[edwin_m]

If I remember correctly, it was 5mph through Heckington with air suspension isolated for HST's for that very reason.

Whilst on most stock the air suspension is "secondary", ie for passenger comfort as opposed to safety, on 360's (and I would imagine other Desiro units?) It is the primary suspension due to the sensitive nature of the electronic computer gubbins on board, and reduced speed of 45mph applies when isolated whether passengers on conveyed or not.

I think you're using something different from the normal definition of primary and secondary suspension here.

As I've always understood it, primary suspension is between the axles and the bogie frame, and secondary is between the bogie frame and the body. Primary suspension universally uses some form of solid spring or rubber component, but most modern passenger trains have air bags as secondary suspension. I believe if the air suspension is deflated, the body settles down onto some sort of rubber stop so there is still some element of cushioning, in addition to that provided by the primary suspension.

It may well be that the design on Desiros is different from other units, and results in a rougher ride in the rare event of the airbags being deflated, but they are still considered to be secondary suspension.

 

[Kneedown]

It may well be that the design on Desiros is different from other units, and results in a rougher ride in the rare event of the airbags being deflated, but they are still considered to be secondary suspension.

Perhaps it was how the guy who trained us on them worded it, but that was what we were taught. Certainly it's an oddity as primary suspension is normally the one that keeps you safe on the track. Things change though, as technology evolves.

 

[TrainTrackJack]

Is this why MK3 coaches make that characteristic wheezing sound as the bag tries to keep the body level?

 

[Towers]

Is this why MK3 coaches make that characteristic wheezing sound as the bag tries to keep the body level?

Indeed, self-levelling at work!

 

[GC class B1]

Is this why MK3 coaches make that characteristic wheezing sound as the bag tries to keep the body level?

The air suspension on Mk3 coaches is different to more recent vehicles in that the air suspension is part of the bogie and they have a conventional bolster as part of the bogie. Other vehicles with air suspension have the bolster as part of the vehicle underframe and the air springs (bags) are fitted between the bogie and the body. Mk 3 vehicles have four levelling valves (2 per bogie) and i believe most or all other vehicle types have only 3 levelling valves, 1 at one end and 2 at the other end. With this arrangement of three levelling valves the two valves at one end set the height and cross level of the body and the single valve sets the vehicle height at the other end. On the Mk3 vehicles if the track is not on the same angle at each end the levelling valves will unsuccessfully try to compensate for this imbalance and the result is that valves will be emptying and filling whilst the imbalance remains. This situation does not occur with the three levelling valve arrangement.

 

[Ken H]

Dont some trains monitor air spring pressure to calculate the number of passengers?

 

[skyhigh]

Dont some trains monitor air spring pressure to calculate the number of passengers?

Some of our 158s used to have that, but I'm fairly certain it's been removed on them all now.

 

[TheEdge]

Dont some trains monitor air spring pressure to calculate the number of passengers?

They are meant to but I'm yet to see a unit that manages its convincingly. At best they only seem to differentiate between "carriage full of air" - empty, or "Tokyo Metro at rush hour" - full. They don't seem to display anything between those two states.

 

[Juniper Driver]

Dont some trains monitor air spring pressure to calculate the number of passengers?

Load weighing valves on 455's.

 

[GC class B1]

Load weighing valves on 455's.

As I understand it the air suspension pressure from the air bags that is used to determine the maximum brake cylinder pressure (so that the braking retardation rate is the same whether the vehicle is empty or has passengers) is also translated to an electrical signal that is interpreted as the number of passengers.

 

[Desiro123]

I think you're using something different from the normal definition of primary and secondary suspension here.

As I've always understood it, primary suspension is between the axles and the bogie frame, and secondary is between the bogie frame and the body. Primary suspension universally uses some form of solid spring or rubber component, but most modern passenger trains have air bags as secondary suspension. I believe if the air suspension is deflated, the body settles down onto some sort of rubber stop so there is still some element of cushioning, in addition to that provided by the primary suspension.

It may well be that the design on Desiros is different from other units, and results in a rougher ride in the rare event of the airbags being deflated, but they are still considered to be secondary suspension.

Desiros do have a rubber stop in the middle of the air bag, in case it deflates. Secondary suspension consists of the airbags, plus lateral and yaw dampers