Pushing vs pulling performance

[mervyn72]

A question for drivers but makes me wonder. Is there a difference in acceleration or other performance parameters for locomotives pushing from the rear or pulling from the front. Eg class 91 or class 37 with a DBSO. Also any for DMU/DEMU/EMU

 

[najaB]

Not a driver, but I can't see any reason why there would be a difference in *gross* parameters as it is the same amount of tractive effort and the same amount of mass to be moved.

What I suspect is that there's a different driving style required, and as a result the fine-scale parameters will change. It's an excellent question and I'm looking forwards to hearing from some drivers.

 

[PHILIPE]

A question for drivers but makes me wonder. Is there a difference in acceleration or other performance parameters for locomotives pushing from the rear or pulling from the front. Eg class 91 or class 43 with a DBSO. Also any for DMU/DEMU/EMU

What Class 43s work with a DBSO ?

 

[route101]

Arent DVTS lighter , (though still a lot of weight?

 

[najaB]

Arent DVTS lighter , (though still a lot of weight?

Yes. Though it doesn't really matter if they are at the front being pushed or the back being pulled, their overall contribution to the mass of the train is the same.

I suppose it might make a difference on a contaminated railhead if you have the lighter DVT being the leading vehicle rather than the heavier locomotive?

 

[Cliveblackpool]

On a straight no difference.
Power fed to traction motors on train no difference
Pulling on a curve, wheels drag on inside of curve (which is shorter)
Pushing on a curve, wheels drag on outside of curve
Pulling is v slightly more efficient?

 

[robbeech]

There are differences on overrun and on acceleration and braking too. When pulling, accelerating pulls the carriages apart to their maximum tolerance, when pushing it pushes them together. I believe the tolerances are very small nowadays unlike earlier designed carriages but there are differences on how they react when the train changes between accelerating and overrun and or braking.
What real term effects this has I do not know as I don't drive them. I assume one has to brake on a down hill gradient in a different manner in some situations if you have accelerated up it.

 

[AndrewE]

In the '80s or '90s we were told that you got better performance propelling because the rails had been dried or polished by the train first, whereas with the loco pulling it suffered from loss of adhesion on a greasy rail.

Of course that may have just been propaganda to help increase acceptance of the DVTs on the WCML.

I may have mis-remembered: it may have been to explain why APT power cars needed to be in the middle of the train rather than adjacent to one of the driving trailers - to ensure the same performance in both directions.

 

[dgl]

The APT power cars were located where they are due to the safety rules not allowing A 25kV bus across the top of the train (like on the TGV) and having 2 pantographs (one at either end) causes problems with standing waves being built up in the wire at high speeds causing contact problems with the rear pantograph.
H2nce the 2 power cars had to be located next to one another.

But anyway back on topic...

 

[AndrewE]

The APT power cars were located where they are due to the safety rules not allowing A 25kV bus across the top of the train (like on the TGV) and having 2 pantographs (one at either end) causes problems with standing waves being built up in the wire at high speeds causing contact problems with the rear pantograph.
H2nce the 2 power cars had to be located next to one another.

But anyway back on topic...

Yes, but they were together in the middle of the train meaning 2 sets of onboard staff and half of the punters didn't have access to the guard. They were deliberately not positioned together adjacent to one of the DVTs at one end (adjacent to one of the driving trailers,") which is what I said. Exactly on topic, in fact.

 

[341o2]

In the '80s or '90s we were told that you got better performance propelling because the rails had been dried or polished by the train first, whereas with the loco pulling it suffered from loss of adhesion on a greasy rail.

Of course that may have just been propaganda to help increase acceptance of the DVTs on the WCML.

I may have mis-remembered: it may have been to explain why APT power cars needed to be in the middle of the train rather than adjacent to one of the driving trailers - to ensure the same performance in both directions.

Maybe a bit off topic, but this reminds me of the County Donegal, where icy rails were dealt with by two railcars in tandem. The leading unit slipped, but burnt off the ice for the second unit to grip the rails

 

[robbeech]

Maybe a bit off topic, but this reminds me of the County Donegal, where icy rails were dealt with by two railcars in tandem. The leading unit slipped, but burnt off the ice for the second unit to grip the rails

A little like how wsp (on some units) allows the front wheels to slide thus creating a better surface for the remaining wheels for better braking.

Back to the 25kv rail, these rules now seem to be defunct of course.

 

[randyrippley]

I have vague recollections of an article in Modern Railways around 40 years ago which discussed research into this in relation to the APT design. Apparently the weight in front of a propelling locomotive can enhance tractive effort and reduce wheel spin by increasing the downforce on the track of the locomotive wheels. However there was a tradeoff against (if I remember correctly) the reduced/irregular power delivery caused by slackness in the couplings during compression causing wheelslip/spin
The conclusion they came to was that the power car / locomotive was best placed third in the formation as that gave the enhanced tractive effort due to the increased leading weight, but avoided the worst of the coupling complication.
That would have been carried out with 1960/70s technology, views may well have changed now

 

[gimmea50anyday]

There are differences on overrun and on acceleration and braking too. When pulling, accelerating pulls the carriages apart to their maximum tolerance, when pushing it pushes them together. I believe the tolerances are very small nowadays unlike earlier designed carriages but there are differences on how they react when the train changes between accelerating and overrun and or braking.
What real term effects this has I do not know as I don't drive them. I assume one has to brake on a down hill gradient in a different manner in some situations if you have accelerated up it.

This is most noticeable on t+t services as the trailing loco is often snatched by the lead loco causing it to then whiplash back into the stock. You can se this happen on HSTs but much less pronounced if the trailing power car is the one providing ETH as the lead car has a few more HP to play with.

The distinctive "thump" when mk1 emu stock took power caused by both the motor bogies taking up the slack then the motor carriage both pushing and pulling its load some couplings naturally stretched, others compressed until momentum balanced out the forces.

 

[TheEdge]

In my experience, which I openly admit is only station limit moves up to the grand total of 25mph in 90+DVTs, the control tends to be much more precise when driven from the loco than the DVT, which seems to result in the snatching and banging. I believe this is due to the way the TDM works.

 

[KevinTurvey]

Back in pre-Pendolino days in Autumn, often the Liverpool - London services would have the loco on the Southern end in front of the DVT.
I always assumed this was to help with getting up the slippery bank out of Runcorn as this only seemed to happen with the Liverpool trains - is this correct?

 

[Stampy]

It's noticable on the 91's when heading Southbound with the 91 on the rear when it brakes....

You do get a bit of a "snatch" as I presume the loco "brakes" marginally before the "carriages" actually do?

 

[dk1]

In my experience, which I openly admit is only station limit moves up to the grand total of 25mph in 90+DVTs, the control tends to be much more precise when driven from the loco than the DVT, which seems to result in the snatching and banging. I believe this is due to the way the TDM works.

Ironically I was asked the very same question at Liverpool St last night & replied then that there is no difference power wise. Only two I can think of is obviously it's rougher through kicking DVT leading & also in wet weather (90s would slip on Brighton Beach) it has less adhesion issues with the loco on the arse end.

 

[AndrewE]

Back in pre-Pendolino days in Autumn, often the Liverpool - London services would have the loco on the Southern end in front of the DVT.
I always assumed this was to help with getting up the slippery bank out of Runcorn as this only seemed to happen with the Liverpool trains - is this correct?

I believe that was when the TDM remote control had failed, so a loco had to be put on the front. Don't know what happened when it got to Euston though, presumably another loco dropped onto the Liverpool-bound front, as in pre-DVT days.

 

[Rob F]

Yes, but they were together in the middle of the train meaning 2 sets of onboard staff and half of the punters didn't have access to the guard. They were deliberately not positioned together adjacent to one of the DVTs at one end (adjacent to one of the driving trailers,") which is what I said. Exactly on topic, in fact.

This is a very good point and one that had not occurred to me before. Why were the APT power cars not both positioned at the same end of the train? Only one of them would have needed a driving cab.

Rob

 

[daikilo]

This is a very good point and one that had not occurred to me before. Why were the APT power cars not both positioned at the same end of the train? Only one of them would have needed a driving cab.

Rob

Neither APT-P power car had a driving cab, both of which were on the end trailer cars. As I recall, it was decided at the time that pushing about 14 trailers in tilt-mode at above 125mph was too big a risk-step.

 

[Rob F]

Neither APT-P power car had a driving cab, both of which were on the end trailer cars. As I recall, it was decided at the time that pushing about 14 trailers in tilt-mode at above 125mph was too big a risk-step.

Yes, but why. BR were always at pains with the 225s that there was no difference between pushing and pulling. Indeed, I think all the speed record runs (all be it with a shortened formation) were made with the DVT leading.

Rob

 

[edwin_m]

Yes, but why. BR were always at pains with the 225s that there was no difference between pushing and pulling. Indeed, I think all the speed record runs (all be it with a shortened formation) were made with the DVT leading.

As suggested above, an APT would have about 14 trailers, so about twice as long as a 225 set and with twice the tractive effort, inertia and resistance. So the compressive forces with two power cars at the rear, which influence the consequences and possibly the likelihood of derailment, would also be twice as much as for a 225 set. I don't think the presence of tilt would directly influence the likelihood of derailment but taking curves faster might worsen the consequences if it did happen.

 

[robbeech]

It's noticable on the 91's when heading Southbound with the 91 on the rear when it brakes....

You do get a bit of a "snatch" as I presume the loco "brakes" marginally before the "carriages" actually do?

This all depends on the scenario directly before braking.
If the train is overrunning (motor wise, not going too far onto a platform) and there is a downhill gradient then it would not be as noticeable. If the train is accelerating on a downhill gradient and then a brake application is made then the loco will snap back and potentially jolt the carriages as the brakes come on. If the train is going up a gradient then you'd need much more sudden brake force to cause this, one that would cause other passenger discomfort more so than this jolt.
Trains with a separate locomotive brake will use this gently(when pulling) first to "bunch up" the carriages before applying the train brake to slow the train down.

 

[theageofthetra]

The APT power cars were located where they are due to the safety rules not allowing A 25kV bus across the top of the train (like on the TGV) and having 2 pantographs (one at either end) causes problems with standing waves being built up in the wire at high speeds causing contact problems with the rear pantograph.
H2nce the 2 power cars had to be located next to one another.

But anyway back on topic...

OT I know but who imposed this & allowed it in France?

 

[jonathan01n]

A question for drivers but makes me wonder. Is there a difference in acceleration or other performance parameters for locomotives pushing from the rear or pulling from the front. Eg class 91 or class 37 with a DBSO. Also any for DMU/DEMU/EMU

Just my thought- The train mass is always constant regardless in pushing or pulling mode.
F=(u)R
u=Coefficient of Friction
R=Resultant force of the train- i.e mg
If the coefficient of Friction on that particular track is the same, and the total mass of the train is the same. The Frictional force should be the same.
F=ma
The train mass is at a constant and the force is the same in either pushing or pulling mode the acceleration should be the same.
Other things to consider is the air resistance. The DBSO seemed blunt and so it may accelerate a bit slower? Mk3 DVT with class 90 and Mk4 DVT with class 91 looks alike so the effect may be smaller, I think the lack of pantograph on the DVT may also have an effect? I wish someone can do a bit more explanation though.

 

[edwin_m]

OT I know but who imposed this & allowed it in France?

Not allowing a 25kV power line through the APT when it was OK on a TGV may have had something to do with the limited loading gauge in Britain. It was later allowed for the Pendolino but I'm not sure what changed for that to happen.

 

[najaB]

It was later allowed for the Pendolino but I'm not sure what changed for that to happen.

Twenty years.

 

[najaB]

Other things to consider is the air resistance. The DBSO seemed blunt and so it may accelerate a bit slower? Mk3 DVT with class 90 and Mk4 DVT with class 91 looks alike so the effect may be smaller, I think the lack of pantograph on the DVT may also have an effect? I wish someone can do a bit more explanation though.

I seem to recall that air resistance is a minor factor at speeds under 70mph or so.

 

[Rob F]

As suggested above, an APT would have about 14 trailers, so about twice as long as a 225 set and with twice the tractive effort, inertia and resistance. So the compressive forces with two power cars at the rear, which influence the consequences and possibly the likelihood of derailment, would also be twice as much as for a 225 set. I don't think the presence of tilt would directly influence the likelihood of derailment but taking curves faster might worsen the consequences if it did happen.

So you are suggesting that pushing IS more dangerous than pulling and the risk increases as the train gets longer? i.e with a shorter train the risk is acceptable and with a longer one it isn't.

I have never seen this suggested in any literature before and there are some pretty long push-pull formations in use on the continent.

Rob