A follow up re ticktime

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tom
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A follow up re ticktime

Post by tom »

Because some "Engineers" obviously know better, I have deleted my contribution to this topic.
Last edited by tom on Sat Aug 11, 2007 11:11 pm, edited 1 time in total.

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AndersDK
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Post by AndersDK »

I can only say YES Tom -

But where does it take us using maths to predict possible leak flow rates ?
C U / Anders - '90red16riBreak - '91GrisDolment16meteor - Project'88red19trsBreak
dead cars : '89white 16RS - '89antrasitTRDturboEst - '90white19triBreak

tom
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Post by tom »

I can no longer be bothered to help
Last edited by tom on Sat Aug 11, 2007 11:09 pm, edited 1 time in total.

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AndersDK
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Post by AndersDK »

okay Tom -

its just because understanding the systems function is more than enough for most steelsprung heads - not involving maths 8)


BTW : ever seen a flow rate that high from a leaking doseur valve that it literally makes the PR having a hard time reaching its cut out ?
I've just found one on my 19tri :shock:

On disassembly there was nothing obvious to see, except for very mucky "LHM" inside the doseur valve. All three piston plungers still had a very snug and extremely precise slide in the bore.
Bit of a mystery actually.
I want to test it with my rig-up of pump, PR and reservoir before I fit it again. Its no fun having a second go on a doseur - even living in a left hand country :wink:
C U / Anders - '90red16riBreak - '91GrisDolment16meteor - Project'88red19trsBreak
dead cars : '89white 16RS - '89antrasitTRDturboEst - '90white19triBreak

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Jaba
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Post by Jaba »

Nice to have a theory session again on Cit hydraulics.

A few thoughts have come out of this for me......so to sum up :shock:

1. If the PR had been fitted with better damping, instead of Megaclick, then the cutin/cutout would be smooth and inaudible. We would not then be able to measure it (its a male thing you know).

2. Now that might be a good way of changing the LHM - pumping it all out. Will try it next time b4 taking out the tank for cleaning.

3. The thing which worries me most about a leaking doseur is the speed at which the back end drops down at rest. If the front struts are a bit stiff too then this effect is magnified. The two are related.
This is where I am going to drift off into theory a bit. What I am suggesting here is that if the front end, with good struts, drops at its normal rate and the doseur is not completely shot then the back end will not appear to drop as quickly. So changing the struts could cure an apparently worn out brake doseur without the pain of changing it.
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Post by tom »

I can no longer be bothered to help.
Last edited by tom on Sat Aug 11, 2007 11:09 pm, edited 3 times in total.

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Post by jeremy »

Jaba - I'm not quite sure what you're saying about the front dropping but the doseur is not in that circuit. The front suspension drops through leakage through the height corrector and the struts. As in fact good struts leak very little and the car still drops the leakage is in fact in the height corrector.

Part of it is in the rear circuit to provide load compensation.

Tom/Anders - this volume theory is all good stuff. Can we develop it to the stage where accurate fault finding can be done with a stopwatch?

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Post by tom »

I can no longer be bothered to help.
Last edited by tom on Sat Aug 11, 2007 11:08 pm, edited 1 time in total.

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Jaba
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The Concorde effect

Post by Jaba »

Good stuff this. The point I was trying to make about the front struts was purely that it is a visual thing. Stiff struts do not sink gradually. They just stay where they are for far longer than they should until they drop rapidly when you get in the car or just eventually settle down with the passage of time. Usually in several stages.
So when the back drops normally or quickly and the front stays stuck up in the air. What is the real problem ?

Another test for accumulator capacity is to switch off the engine then raise the height lever and observe or even measure the car's height rising.
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Post by jeremy »

Both ends will leak from the struts - small leakage if the struts are good.

Both ends will leak from their respective height correctors.

Only the back will leak through the doseur.

The height corrector will isolate the rest of the circuit until it opens due to leakage on the suspension side - when it will keep calling for more high pressure LHM which is not available and will stay open.

I've never bothered to observe the thing but presumably the car drops (probably back first) in steps as it will drop a bit, the height corrector will open and top the suspension up again whereupon the cycle will repeat until the accumulator is exhausted. I would expect the volume of LHM in the spheres to remain constant until the accumulator is exhausted and the height corrector remains open.

Stiff front struts can be fun - like when you jog the car it drops rapidly - such as when you open the door.

An easier test for the accumulator is simply to charge the accumulator by running the engine with it at normal height for a couple of minutes - then turn it off and sit in the boot. The back should drop a long way then rise again after about 30 seconds. The car will also adjust itself if you get out, allow it to settle then get back in again and wait. This movement is often not noticed as you are starting the engine. My TD estate will sink and rise again when I fill its fuel tank full.

The delay is due to the construction of the height corrector and is intended. Early DS didn't have the delay.

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Post by AndersDK »

The pre-requisite for this discussion is the car parked, engine stopped, and left in normal height. Remember then that there are no compensation from the pump to top up pressure loss in this situation.

Effects of doseur valve leaks when the car is parked with engine stopped, is that the rear suspension - at some rate - is emptied for pressurised LHM. The rear then starts to sink. But very importantly : the rear HC also opens to try let in pressure compensating for the height loss.

The exact same effect is observed if one (or both) rear cylinders are leaking past their cylinder seals. As Tom points out also a common BX problem.

WHEN the system pressure has fallen to some 100bar, there is no more an open path from the charge circuit to the suspension circuit. Thats the job of the safety/priority valve. We are then left with a suspension system on its own, and the charge circuit then holds the remaining some 100bars for the front brakes.

IF the front struts are weeping past their seals at any rate (which they are), then the front HC will open at some point to try compensate. This instantly connects the front suspension spheres to the rear suspension spheres, by way of both HC's which now are open. The front struts are then emptied through the doseur or rear cylinder leaks, as well as through their own leaks. This is seen as the front suddenly drops if you push down on the front after parking for some time.
If you dont touch the front, it will suddenly drop when the ever so small front strut binding suddenly gives way for the weight of the front.

Applying maths to this behaviour is far from easy, as there are so many variables involved. It will be more sensible to do a couple of simple visual leakback flow rate tests at the reservoir return hoses, knowing which circuits they serves.

The T-ed hose with a black plastic hose is the return from the doseur. Always the first port of call to test on a 1. generation hydropneumatic Citroen. It will have a "rather high" i.e. not allowable flow/dripple rate first off when car is parked engine stopped, with a fast rear sink.
With engine running and fast tick interval, there will be a constant flow in this return if the doseur is the cause for leaks.

Next test is always the front struts & rear cylinders seepage leakback hose, the one nearest to the RHS headlamp. There will be flow here at any rate if the front struts or cylinders are leaking past their seals.

If these 2 tests checks out good its more than likely that a fast tick rate is caused by a flat accumulator - or the more sophisticated problems : like the PR non-return valve ball bearing needing a reseating or the FDV goosing up.

You can also set the suspension to low before the engine is stopped. This has the effect that both HC's are closed off for any pressure feed to the suspension, thus leaving the source circuit fully charged only feeding the front brakes.
This is the test described by Tom counting the number of pedal pushes before the PR cuts in with its tick.

Thats all there is to it really.
C U / Anders - '90red16riBreak - '91GrisDolment16meteor - Project'88red19trsBreak
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Re: A follow up re tick time

Post by Vanny »

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tom
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Post by tom »

PM sent.

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Post by Vanny »

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Post by tom »

I can't be bothered to help any more. If you want to know, ask Vanny.