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The impact of tilt on keeping a train on the track or do you need tilt to go around a corner faster?

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krus_aragon

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Just my attempt at an analogy, imagine a single bogie supporting a cylindrical weight. I could turn the weight by 8, 90, 180 or 360 degrees around its centre of gravity and it'd make no difference to what the bogie 'feels'.
A minor clarification, but the thrust of your argument is good.​
 
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hexagon789

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So are you trying to tell me a non-tilt train of max speed of 100 mph would not be trying to jump the rails if doing 120 mph on those same curves? Whether it's centre of gravity or rotation around an axis . I'm not saying it would but that's what's trying to happen.

Any non tilting train could run at any of the EPS speeds on the WCML in perfect safety, it might not be as comfortable was passengers but it would be perfectly safe - it has to be.

If running as EPS speeds was only safe with tilt then everytime the tilt deconfigured on a 221 or 390 that would be putting the train in a potentially dangerous situation but it doesn't because tilt is purely for passenger comfort by trying to balance out the increased forces on passengers from taking curves at faster speeds.
 

The Planner

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There were discussions in the past about harmonising 221 and 390 SRTs due to the perceived difference being manageable. but it didn't materialise in the end.
 

Bald Rick

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So are you trying to tell me a non-tilt train of max speed of 100 mph would not be trying to jump the rails if doing 120 mph on those same curves? Whether it's centre of gravity or rotation around an axis . I'm not saying it would but that's what's trying to happen.

I can tell you from personal experience that a non tilting train travelling round a curve with a max speed of 100mph for non tilting trains at 125mph will not derail.

I’ve been on a Pendolino going around Weedon at 125mph when the tilt system failed. For a brief moment we were tilting (slightly) the wrong way through one of the curves. I arrived in Birmingham fine, although some of my papers had had an unwelcome introduction to coffee.

The engineering safety margin allows for a pendolino to be on full tilt, wrong direction, slightly over speed, with an unfavourable strong crosswind, and the train will not derail.
 

LNW-GW Joint

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Any non tilting train could run at any of the EPS speeds on the WCML in perfect safety, it might not be as comfortable was passengers but it would be perfectly safe - it has to be.
If running as EPS speeds was only safe with tilt then everytime the tilt deconfigured on a 221 or 390 that would be putting the train in a potentially dangerous situation but it doesn't because tilt is purely for passenger comfort by trying to balance out the increased forces on passengers from taking curves at faster speeds.

I should think NR has a view about the excess track wear and tear of non-tilt trains running at EPS speeds.
I imagine regular excess speeding would punish the track (and maybe OHLE) more than a 390 would.
The WCML knows all about the rapid deterioration of the infrastructure with intensive use.
 

Bald Rick

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I should think NR has a view about the excess track wear and tear of non-tilt trains running at EPS speeds.
I imagine regular excess speeding would punish the track (and maybe OHLE) more than a 390 would.
The WCML knows all about the rapid deterioration of the infrastructure with intensive use.

The track doesn’t ‘feel’ any difference between a tilting train and non-tilting train at the same speed.
 

ashkeba

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Strictly speaking yes anything that tilts has an axis of rotation, but it’s important to distinguish the location of the axis. On a motorcycle, the axis of rotation is at the point of contact between the tyres and the road surface, however for a tilting train such as the pendolino, the axis is presumably through the centre of the passenger cabin so is a metre or so off the ground.
Can anyone show us something credible to say the axis is that high, please? Alstom's videos about the Pendolino technology on youtube make the axis look much lower than that.
 

Railperf

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If I recall correctly..the non EPS speeds also relate to braking capability. Pendolinos have good 100mph+ acceleration and superior braking to a Class 87 with Mk3 coaches and a DVT.

Probably TPE Class 397 will hopefully have equal braking capability to a Pendolino.
In Italy ETR 500s as non tilt stock were supposed to have a lower centre of gravity and lower track forces which were supposedly designed to allow them to run at up to 10 percent faster through corners than conventional non tilt stock. But looking through the Italian sectional appendices they are still limited to speed range C rather than P (Pendolino) speeds.
On the adriatic route where there are a few short sections of C rated track at 200km/h (124mph) the ETR 500s accelerate too slowly compared to tilting ETR 485 to save any meaningful additional time.
Hence higher speeds need to be matched by good acceleration and braking to/from those speeds.
 

hwl

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If I recall correctly..the non EPS speeds also relate to braking capability. Pendolinos have good 100mph+ acceleration and superior braking to a Class 87 with Mk3 coaches and a DVT.

Probably TPE Class 397 will hopefully have equal braking capability to a Pendolino.
In Italy ETR 500s as non tilt stock were supposed to have a lower centre of gravity and lower track forces which were supposedly designed to allow them to run at up to 10 percent faster through corners than conventional non tilt stock. But looking through the Italian sectional appendices they are still limited to speed range C rather than P (Pendolino) speeds.
On the adriatic route where there are a few short sections of C rated track at 200km/h (124mph) the ETR 500s accelerate too slowly compared to tilting ETR 485 to save any meaningful additional time.
Hence higher speeds need to be matched by good acceleration and braking to/from those speeds.
The speed restrictions either side of Shap (and others) are related to how good the friction brakes are, 125mph vs 100mph is 56% extra energy to get rid of....
Reducing vehicle weight is probably key to improving braking allowances.
 

edwin_m

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A tilting train on a non-tilt speed restriction, speeding! This is the extreme but what my point was could happen.
The Talgo 250 Hybrid has a passive tilt system. Each coach is hung at ceiling level from frames that extend upwards from the bogies. Rather like some fairground rides the body, and with it the centre of gravity, swings outwards on a curve, making it somewhat more likely to overturn than a non-tilting train of similar weight and dimensions. However the Talgo is very low-slung so the C of G is likely to be lower than in most other trians. But the derailment seems to have been initiated by the diesel power car, which is likely to have a much higher C of G. I'm not sure if the power car tilts anyway.

I did say that video was extreme.

So going round a left hand curve and the stock buffeting towards the right, there must be a point where it gives way and wants to leave the left rail, it may not be as low as 20 mph overspeed, but there must be a pivotal point.
Yes there is, but an active tilting system doesn't have any significant effect on the speed at which this happens. There will be a small reaction force against the outer rail from the actuators rotating the body inwards, but this is likely to be tiny compared to other forces seen in curving.

Aren't all tilts rotating around an axis? Is that not what tilting is? The main things not like riding a motorcycle is that the maximum tilt is only 8 degrees and the axis is between two sets of wheels. I do not see why this means the CoG would not move.
In a motorcycle the line of force between the wheels and the road must go through the tyre contact patches otherwise it will tip over. On a curve the rider must move the CoG inwards (in relation to the line followed by the wheels) to allow this.

A car or non-tilting train won't tip over as long as that line of force is somewhere between the left and right wheels. The C of G stays in exactly the same place relative to the wheels. With an active tilt system the axis of rotation goes through the C of G so the overall balance is not affected. You could in principle have an active tilt system that moved the C of G inwards, which would increase the overturn speed. This would need much more powerful actuators, and probably also an even smaller body profile so the upper parts of the body don't infringe the gauge on the inward side. Since the maximum speed is set by passenger comfort and is well below the overturn speed, there is no point in doing this.
 

EE Andy b1

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Thanks @edwin_m and other, yes my bad and got that all a**e about t*t.
So were not coming off and tilts for comfort!!
 

ac6000cw

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But the derailment seems to have been initiated by the diesel power car, which is likely to have a much higher C of G. I'm not sure if the power car tilts anyway.
That is also my understanding.

I *think* the BR APT-P electric power cars (in the middle of the train, non-passenger carrying) didn't tilt either, as there is no benefit and it complicates keeping the pantograph in the correct position relative to the OHLE.
 

Bletchleyite

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I should think NR has a view about the excess track wear and tear of non-tilt trains running at EPS speeds.
I imagine regular excess speeding would punish the track (and maybe OHLE) more than a 390 would.
The WCML knows all about the rapid deterioration of the infrastructure with intensive use.

Tilt does not affect track wear - why would it? A 390 is a heavy beast, that will have far more effect.
 

Bletchleyite

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That is also my understanding.

I *think* the BR APT-P electric power cars (in the middle of the train, non-passenger carrying) didn't tilt either, as there is no benefit and it complicates keeping the pantograph in the correct position relative to the OHLE.

Actually I think they did - there was a hugely complex mechanism designed so the pantograph didn't.
 

a_c_skinner

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Having established that tilt makes no difference to safe speeds or track forces can I ask what work has been done to establish what lateral forces are accpetable to passengers? I've found some stuff on the www about forward/backwards jerking and acceptability but found nothing about lateral forces which is what I was looking for. Where are the standards for lateral forces/cant deficiency?
 

Bald Rick

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That is also my understanding.

I *think* the BR APT-P electric power cars (in the middle of the train, non-passenger carrying) didn't tilt either, as there is no benefit and it complicates keeping the pantograph in the correct position relative to the OHLE.

Actually I think they did - there was a hugely complex mechanism designed so the pantograph didn't.

Yes the APT power cars tilted. However, when the plan was for tilt on the ECML (hence the Mk IV tilt profile), the 91s were not going to tilt. They would have been fun to drive!

Having established that tilt makes no difference to safe speeds or track forces can I ask what work has been done to establish what lateral forces are accpetable to passengers? I've found some stuff on the www about forward/backwards jerking and acceptability but found nothing about lateral forces which is what I was looking for. Where are the standards for lateral forces/cant deficiency?

BR did research into this a long time ago, which resulted in the easing of the exceptional value of cant deficiency permitted to 6 degrees. This is employed in a few areas, notably the top end of the ECML, parts of the MML between Bedford and Leicester, and (I think) the Berks & Hants route. What I don’t know is how widespread it is elsewhere. It’s use does, of course, result in higher track forces, particularly to the high rail.
 

ainsworth74

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They would have been fun to drive!
I could be wrong but I seem to recall reading that there was a plan to have harnesses fitted to keep the driver and secondman secured. Would have been like strapping into a jet fighter :lol:
 

aar0

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I assume that some of the 125 EPS limits would have been 140 anyway?
 

a_c_skinner

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BR did research into this a long time ago, which resulted in the easing of the exceptional value of cant deficiency permitted to 6 degrees. This is employed in a few areas, notably the top end of the ECML, parts of the MML between Bedford and Leicester, and (I think) the Berks & Hants route. What I don’t know is how widespread it is elsewhere. It’s use does, of course, result in higher track forces, particularly to the high rail.

Of course track forces would be greater, but unchanged by tilt or not of course. That is interesting because when Mk4/91s were first being discussed for the WCML someone said the ECML was more liberal as regards lateral forces than the WCML and there was some speculation that a similar easement on the WCML might make a deal of difference to running non-tilting trains at higher speeds. I've not noticed the top end of the ECML (which I travel from time to time) is, well, noticeable. Back on topic I still think the extra WCML trains won't tilt and will go a bit faster than we are expecting.
 

mcmad

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Having established that tilt makes no difference to safe speeds or track forces can I ask what work has been done to establish what lateral forces are accpetable to passengers? I've found some stuff on the www about forward/backwards jerking and acceptability but found nothing about lateral forces which is what I was looking for. Where are the standards for lateral forces/cant deficiency?

The construction standard for track (2102) details the limits on cant, cant deficiency and transitions (both lengths and rates of gain of cant and deficiency).

What it is based on isn't detailed but presumably BR research from back when speeds of 100mph+ were first common place.
 

a_c_skinner

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I'm sure that will be too technical for me, but what is intriguing me is not the track/train interface but the subjective experience of passengers in the gap 'twixt normal and enhanced speeds if the train were running without tilt. There are some obvious places on the WCML where tilt is very active and plainly makes the ride much more comfortable (roughly Stafford southwards I think) but I'm intrigued by the gains on stretches where tilt is a deal less active.
 

hexagon789

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I should think NR has a view about the excess track wear and tear of non-tilt trains running at EPS speeds.
I imagine regular excess speeding would punish the track (and maybe OHLE) more than a 390 would.
The WCML knows all about the rapid deterioration of the infrastructure with intensive use.

I doubt the wear is any higher running tilting trains at EPS speeds than it would running non-tilting ones at EPS speeds, the weight on the track is the same, the axle loadings would probably be higher with tilting as the weight is shifted more to one side.
 

edwin_m

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I doubt the wear is any higher running tilting trains at EPS speeds than it would running non-tilting ones at EPS speeds, the weight on the track is the same, the axle loadings would probably be higher with tilting as the weight is shifted more to one side.
The weight is not shifted (in an active tilting system as fitted to 390s and 221s). The body rotates about the centre of gravity (or very close to it) so the effective weight distribution is unchanged.
 

hexagon789

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The weight is not shifted (in an active tilting system as fitted to 390s and 221s). The body rotates about the centre of gravity (or very close to it) so the effective weight distribution is unchanged.

In that case there's no difference in weight distribution then, it comes back to primarily passenger comfort when cornering.
 

Ken H

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Having established that tilt makes no difference to safe speeds or track forces can I ask what work has been done to establish what lateral forces are accpetable to passengers? I've found some stuff on the www about forward/backwards jerking and acceptability but found nothing about lateral forces which is what I was looking for. Where are the standards for lateral forces/cant deficiency?

They do it with cant deficiency
cant is the amount the outer rail is higher than the inner rail on a curve measured in degrees.
Passengers in a train going round the curve at the right speed will feel no lateral acceleration
If you go a little faster, the passengers will start to feel lateral acceleration. you could measure that with a plumb line and measure the angle between the uprights in the coach and the pendulum. or use an accelerometer.
That angle is cant deficiency. the amount the track isn't canted to cope with the curve at that speed.
You can remove that cant deficiency by tilting. The APT ran into trouble as passengers felt sick when their perception of up differed from what they saw outside.
You cant tilt too much because of slow trains (esp freight) that cant cope with large amounts of cant.
I think the original rule for cant deficiency was 8 degrees. But when the LM wanted to do 110mph trains they started to use 'relaxed curving rules' and 9 degrees.
The ER had a different tactic, got out bulldozers and straightened out curves.
 

Ken H

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I can tell you from personal experience that a non tilting train travelling round a curve with a max speed of 100mph for non tilting trains at 125mph will not derail.

I’ve been on a Pendolino going around Weedon at 125mph when the tilt system failed. For a brief moment we were tilting (slightly) the wrong way through one of the curves. I arrived in Birmingham fine, although some of my papers had had an unwelcome introduction to coffee.

The engineering safety margin allows for a pendolino to be on full tilt, wrong direction, slightly over speed, with an unfavourable strong crosswind, and the train will not derail.

I seem to remember that a wrong side failure of tilt (like you mention) was one of the problems the APT systems had to resolve. A wrong side tilt failure would take the APT out of gauge.
 

edwin_m

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I seem to remember that a wrong side failure of tilt (like you mention) was one of the problems the APT systems had to resolve. A wrong side tilt failure would take the APT out of gauge.
I've always had a niggling doubt about that - the reduced body profile surely means it stays within the "standard" profile at any tilt angle? Although APT had a transponder system I think it was for speed supervision not to turn tilt on and off.

Pendolino has less reduction of profile, but also less tilt angle, so the same may or may not apply.

Incidentally the APT, at least in its initial configuration where it tried to zero out the lateral acceleration, would tilt the "wrong" way if it was going round a canted curve at below the equilibrium speed.
 
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