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Post by dave1800 on Jun 13, 2011 5:55:15 GMT
As many of you know I am compiling a technical guide to complement the workshop manuals.
One of the not uncommon issues I have seen but not found any satisfactory explanation of either the cause or the solution is when you see a car that has a sagging rear end - despite being level from side to side and the height at the front being normal.
Does anyone have any experience of sorting this out please? (I will ignore comments about removing the 100kg load from the boot!)
I have seen some reference to the cause being air in the system but is this the cause and does evacuating the system provide a cure? Or is it just tired hydrolastic units maybe?
Thanks for your help
regards
David
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Post by ahctog1 on Jun 15, 2011 20:36:55 GMT
Apparently it is caused by the bushes in the trailing arm, over time they turn and "grip" the trailing arm and hold them lower than they should be. Mk 1's didn't have this problem due to a roller bearing arrangement.
One sort of cure is to slacken the bush and raise the rear of the car, and then re-tighten, this sounds a bit hit and miss to me. Assisters are another solution commonly used... or replace the bushes and set them up as new.
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Post by dave1800 on Jun 16, 2011 2:52:03 GMT
Apparently it is caused by the bushes in the trailing arm, over time they turn and "grip" the trailing arm and hold them lower than they should be. Mk 1's didn't have this problem due to a roller bearing arrangement. One sort of cure is to slacken the bush and raise the rear of the car, and then re-tighten, this sounds a bit hit and miss to me. Assisters are another solution commonly used... or replace the bushes and set them up as new. Thanks for the information. I know that setting up the slipflex bearings can be tricky. I took one of my cars to a BL main agent as they had the corrrect tools and I wasn't confident enough to replace it - I recall it was quite expensive at the time. (It had failed allowing the wheel to rub the inner arch and messed up the handling but the rear end was level). They screwed up and had to fit a second bearing. I have seen Mk1 cars sag too, so presumably there is another cause as well. Maybe air trapped in the system, but I would have thought it would have found its way to the highest point in the system ie the front units unless it can get trapped somewhere? Thanks again David |
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Post by ahctog1 on Jun 17, 2011 20:00:53 GMT
I think the early type were "sealed for life" unlike the mini, and so when water got in they collapsed.
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Post by dave1800 on Jun 18, 2011 10:37:45 GMT
I think the early type were "sealed for life" unlike the mini, and so when water got in they collapsed. Thanks Yes the Mk1 cars had tapered (steel) bearings and no grease nipple so once the seal failed the bearing followed. I am just curious though as to whether anyone has experienced rear end sagging due to air in the system - not being evacuated after a repair or depresurisation - and whether evacuating the air sorts out the problem? regards David |
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Post by tommydp on Oct 13, 2011 22:53:41 GMT
Hi all:-) I'm picking up this thread again, both to keep the forum active and to shed some light on issues with the hydrolastic.
For some reason the left hand side of my crab seems stiffer than the other side, even though pumped up to correct height. I have used a proper pump, evacuating the system for air before pumping up. Strange thing is, sometimes it seems fine, bouncing softly over rough surfaces. Other times it seems more stiff. Typically it gets better the more I drive. I believe the slipflex bearings are fine, there are no tyre marks on the inner wings etc. Also, when it appears stiff, the car will feel a bit shaky at low speed, up to around 30- 40 kmh.
I believe the rear end sits somewhat lower than it should, too, especially on the left. I'm wondering if I have fitted the wrong (the short) pushrod to the left hand rear assembly. I believe the short pushrods were used with the thick seating/ locating ring mounted on to the displacer and the longer pushrods with the thin type of rings.
I suppose there could be an issue with either of the displacers too, which I'll never be able to find out. Strange things these displacers, I once pumped up an 1800 on wich the front raised, but nothing happened to the rear end! Finally the front displacer's hose burst...
I think I'll remove the left hand drive shaft coupling to see if the driveshaft is making some kind of pressure to the suspension, as the big flat engine mount is already giving up so perhaps the power unit is becoming twisted. I'll then see if the left hand side becomes more softly suspended. I'll then depressurize and check the slipflex bearings. As mentioned earlier, I notice the left hand driveshaft has an up/ down movement while turning the whell around, it's running out of true (if my vocabulary is correct...) I believe this movement is transferred to the chassis when the suspension is stiff on this side, making the car shaky/ hard bouncing. The shake seems to follow the driveshaft movement.
Regarding the driveshaft I've tried to mount a driveshaft onto a spare gearbox to check the running of the shaft. It seems to run out of true anyway, but I suppose this would ideally not be noticed due to the suspension. That is if the suspension works correctly.
Well I'm not sure:-) If well functioning, the hydrolastic is marvellous, but if not.....
It amazes me to look at photos of crabs nowadays, and look at the big difference of trim heights, and difference between front and rear heights etc. We know the correct front height, but what is correct for the rear end? Is there an answer? I try to find out from pictures from when the cars were new, but there are differences here too!
I see that Tony W can no longer supply rear assisters. Pity, I wanted to get a set as I plan to get an old caravan..
Regards, Tommy
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Post by Penguin45 on Oct 14, 2011 0:18:47 GMT
I think your describing a typical "soggy" rear displacer.  This picture is from when the car was literally just finished - it had moved 10 yards. It's sitting tail up and slighty low at the front. By the time I had driven a couple of miles to the garage for the MoT test, it had levelled to the right height at the front and very slightly raised at the back, which is as it should be. I assume that the system had settled due to the best part of four years lack of use. A bit of bouncing about got it all moving again. Last year, the car was taken on a 2,500 mile charity run by some good friends. I saw the car about halfway through and it was very low in the rear. However, the car was heavily loaded. When the car was returned, it immediately settled to it's previous position. I would suggest that there was nothing wrong with my rear displacers. The manual only offers two references - front wing height and system pressure. If the wing height is correct and the system pressure is about right and the car is still low at the back, it can really only be a displacer issue. Chris. |
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Post by indianajones on Oct 14, 2011 1:31:21 GMT
I believe my 18/85 sits the same as Chris's, ever so slightly higher at the rear then the back. Not sure if that's correct, but it seems fine to me.
-Andrew
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Post by dave1800 on Oct 14, 2011 1:33:41 GMT
The mysteries of hydrolastic! One thing to consider is that the slipflex bearings are preloaded when fitted which I suggest will not only affect the damping but also the ride height at the rear by effectively increasing the weight needed to lower the rear. Over time I imagine this preloading will weaken but possibly not the same on each side. It has also been suggested that the slipflex bearings can become oval leading to a sagging rear and this can be (temporarily) overcome by rotating the bearing. All this of course will have a knock on effect on the pressure needed to achieve the correct height at the front! Maybe better to set the pressure accurately as per the BL specs instead of the height and see what is happening.
It is important to evacuate any air out of the sytem as this will give strange results usually the rear sagging.
I believe Tommy has evacuated the air so differences from one side to the other could be down to the slipflex bearings or the hydrolastic units themselves.
With the Mk1 cars the conventional bearings can wear but this should be apparent.
As far as the out of true drive shaft, I have also experienced this, it is especially noticeable if you jack the car up and drive the wheels, the affected side shakes like crazy. The solution? I gave up and substituted a different driveshaft but you also need to check the bush and bearings where the shaft enters the diff.
Any one got any more thoughts?
regards
David
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Post by kelsham on Oct 14, 2011 8:14:54 GMT
Strangely, I am suffering the same syptoms on my Rover 100 cabriolet. slight vibration when cold, and then sometimes settles down and runs nicely. I had been blaming the nitogen spheres.
Like Tommy i have been playing around with hydrolastic pressures. There is not much information on these cars.
What difference does a small amount of air make to the system?
Regards Kels.
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Post by dave1800 on Oct 14, 2011 8:25:45 GMT
As you know air compresses easily whereas water or in this case hydrolastic fluid does not. My only experience has been that the car (in this case a "wet" mini) feels less taught if there is air in the system and you can never be sure where it will end up as the fluid moves backwards and forwards and you drive up and down gradients with the trapped air trying to find the highest point. As you say there is little information anywhere, but there is the recommendation to ensure any air is evacuated regards David Strangely, I am suffering the same syptoms on my Rover 100 cabriolet. slight vibration when cold, and then sometimes settles down and runs nicely. I had been blaming the nitogen spheres. Like Tommy i have been playing around with hydrolastic pressures. There is not much information on these cars. What difference does a small amount of air make to the system? Regards Kels. |
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Post by kelsham on Oct 14, 2011 9:49:59 GMT
I recall talking to a mechanic some years ago. He worked in a Rootes garage. He admitted to having a customer who was complaining that his crab rode too low.
Because they did not have access to a pump they gave the system a shot of air with an airline. I am not suggesting anyone does this.
Regards Kels.
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Post by dave1800 on Oct 14, 2011 11:26:57 GMT
He invented Hydragas! ;D I recall talking to a mechanic some years ago. He worked in a Rootes garage. He admitted to having a customer who was complaining that his crab rode too low. Because they did not have access to a pump they gave the system a shot of air with an airline. I am not suggesting anyone does this. Regards Kels. |
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