 Poster: A snowHead
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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@Hells Bells, this is one reason why mini-bus or vans are better suited to such roads. They do not need hydraulic retarders, and the braking systems of such vehicles can be inspected yourself. I was very conscious of this when driving up and down to certain resorts such as Alp d'uez, Les deux Alps, Val Thorens, Val d'Isere and Tignes where enormous coaches drive up and down at speeds I would not dare contemplate.
To have a hydraulic retarder in such poor repair that the brake pad overheated would suggest that it was not inspected recently. (or it was damaged)
https://en.wikipedia.org/wiki/Retarder_(mechanical_engineering)
Regular use of a vehicle on such roads without inspecting brake condition or having a roll cage fitted to your vehicle is lunacy. Either that or go to resorts such as Serre Chevalier for which it is much easier to access with an enormous campervan.
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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Hydraulic retarder doesn't have brake pads as far as I know.
A vehicle of that size usually has the normal friction brakes but going down a gradient of that severity it'd normally be using a non friction retarder to control it's kinetic energy within a reasonable pace.
This prevents the friction brakes being over heated, and in effect has two separate systems to give redundancy. If the driver was aware of a problem with the primary non friction brake then it should have been brought to a safe halt and investigated prior to descent.
Running almost any vehicle down that route at increased speed would fry the friction brakes if they were the only device in operation.
If the vehicle was not fitted with non friction retarder, then you'd expect to run it down there at low speed in a low gear using engine braking as it's primary means of controlling pace. This reserves the normal brakes for emergencies as they are left in a cool state.
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You need to Login to know who's really who.
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"....The inquest jury heard the French report concluded the brake failed as the pad had been "completely destroyed by excessive heating" due to the "poor condition of the hydraulic retarder"."
I would guess that at the inquest, there would have been an expert witness to come to such a conclusion. (unless the translation into English is incorrect)
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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No, I think it's you thats misinterpreting it. Reads to me that the hydro' retarder was in poor condition and meant the friction brakes were working harder, overheated and failed.
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You'll need to Register first of course.
You'll need to Register first of course.
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That sounds right, normal operation would be to go down with hydraulic retarder in operation to generally control speed.
Normal foot brake that operates the conventional pads will "mostly " be left alone.
If primary hydraulic failed then you'd probably have a couple of clear attempts in the friction brakes to bring the vehicle to a halt. If you ignore the primary failure and tried to get down just on the brake pedal, then it would very quickly heat that system to a point of saturation and wouldn't stop the vehicle.
So "poor condition of hydraulic retarder" means they haven't maintained what is the primary system to retard a vehicle of that type on a gradient of that severity and length.
The driver would have been aware of the failure at some point, he should have been driving without heating up his footbrake system so that he had reserve capacity in the event of that failure. This method would allow him the safety margin.
Say he went down a couple of hairpins of road initially using the footbrake and found it was too severe to realistically control pace, then switches in the hydraulic system, he's already pre heated the friction brakes, taking away his capacity to safely arrest in the event of unexpected hydraulic failure.
In other words, operation procedure, if not adhered to has already scuppered his emergency choice.
Significantly overheated friction brake systems won't stop anything (known as brake fade) even if you push it till your eyes pop out, sounds like that is what he was doing. That's why you'd go with a strategy of keeping the friction brakes cool, in case of this eventuality.
You'd also check your hydraulic braking effect as soon as you can, that road is a severe test and requires very cautious approach driving something like that.
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| adithorp wrote: |
| No, I think it's you thats misinterpreting it. Reads to me that the hydro' retarder was in poor condition and meant the friction brakes were working harder, overheated and failed. |
That sounds like how I interpreted it. Maybe you mis-interpreted what I said?
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The inquest is actually taking place in Berwick-on-Tweed, as the driver was a UK citizen. The inquest was quoting the French investigators.
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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@Hells Bells, I also posted the link from our local newspaper, in the original thread on this, about someone who survived the crash, but with terrible injuries :-
http://snowheads.com/ski-forum/viewtopic.php?p=3134445#3134445
The comments about the hydraulic retarder are pretty damning but unless you have a knowledge of that particular vehicle it is not certain when the driver would have been aware of the severity of the problem. Certainly on some of the newer vehicles I understand the driver can have no control over which part of the braking system is doing what, all the driver does is press the pedal and the computer decides how much the retarder does, how much the engine brake does and how much the friction pads do.
I remember following a loaded drawbar skip lorry, ie a rigid truck with a loaded skip on towing a trailer with another loaded skip on, down the 21 bends one day. I did not need to contemplate overtaking it, and I think the brake lights came on twice during the whole descent. Whilst that's great for not wearing the brake pads out an unintended consequence can be that the continuous light braking can glaze the pads and when they are really needed their efficiency may be much reduced.
Just from travelling up and down there a lot, I have always thought the driver might have had the option of "ditching" the right hand side of the vehicle to slow it down. (There is a gulley along the right hand side below a retaining wall that slopes up and away from the road).
Easy to say with hindsight though.
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Interesting that in addition to hydraulic retarders there are also electromagnetic retarders (using eddy currents as the braking mechanism).
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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@Alastair Pink, Telma electromagnetic retarders have been around for a long time, got a very good reputation. I think the biggest people in the hydraulic side are Voith (?). They reckon it should be possible to do 90% of the braking with the retarder, which means an awful lot less disc and pad replacements.
Quite shocking how little of that coach is left after the blaze, and to take hold so quickly. Presumably as a result of the brakes being so hot ? The engine compartment at the back (?) looks relatively undamaged.
The co-drivers comment about being thrown backwards down the coach sounds strange.
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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@Alastair Pink, those devices, Eddie current, are pretty amazing things. They use them on the big fairground rides, those ones that drop straight down a column to a stop in particular. Spectacular performance and reliability.
@skitow, I know what you're saying about automation, then system should cope with failure in its intended use envelope though, that's why they design them with at least two independent methods of arrest. It should have been offering alarms or other indications of non function to the driver. You can see when they start them that the various systems check out with lights displayed then extinguished, plus various confirmation audio alerts. Something odd is there in the first place.
Also, glazed pads don't overheat brakes, they don't offer enough friction to put heat into the metal discs or drums. Overheated pads and discs is from too much time at friction, leading to the total assembly saturated with heat and unable to generate any further retardation. Conventional brakes are after all a device for converting kinetic energy into heat, nothing more. Like anything, they have a capacity above which the transaction breaks down. They need to be cooled in the wind between application to reinstate more of the capacity after absorbing the heat you've just put in.
There's also a fundamental change to friction materials brought about some time ago with the removal of asbestos, they were bonded in resins to construct the pad material. It's the resins that would glaze if not bedded in properly. The removal of asbestos brought about sintered materials almost universally. They don't ordinarily glaze as resins are not present. Further to this though, unlike asbestos systems, the pads and discs wear more as a system so both of the surfaces are effectively renewed during use.
I can't believe the vehicle algorithms would be designed to allow one system to compromise another if under a computer's control. Even the most basic cars that were originally installed with ESP systems have vehicle attitude yaw sensors as part of their operating strategy.
Can't believe that the vehicle specification would lead to failure.
Guess that's why they've grabbed the statement of failure to maintain as the principle cause, you'd have to ask if the vehicle flagged alarms and had they been disabled.
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