Dipping

Philip Chidlow · Post by **Philip Chidlow** » Fri Jun 12, 2009 12:39 pm

This might well have been discussed before but (as usual) the search function didn't quite narrow it down enough for me.

Why does my BX exhibit the following?:

When I stop (it's an auto BTW) in traffic, keeping it in 'D' or indeed 'N' on the level sometimes (this reduces the vibration - but that's another story), and I have my foot firmly on the brake, the rear will often sink. Releasing the brake causes the rear to jump upwards (much to the surprise of the car behind no doubt). It rides fine on the rear and there's no issue with brakes. The rear arm bearings are new, as are the spheres.

What's wrong?

kiwi · Post by **kiwi** » Fri Jun 12, 2009 1:22 pm

Theres nothing wrong thats normal

Basically the Brake pressure valve is taking the pressure away from the suspension. When you release the Brake the Suspension repressures!

Its great because it also helps to adjust the Handbrake when you keep the foot on the brake and catch and release the handbrake a few times.

I use the feature of back end rearing up to surprise people aids removal of tailgaters in the traffic for some reason

RichardW · Post by **RichardW** » Fri Jun 12, 2009 8:33 pm

I'm not sure the rear sinks after you stop - that might be your imagination - unless the car is creeping forward a bit, which causes abit more rear sink. What is happening is that the rear is below the normal height, so the height corrector opens up to try to correct the height - except it can't because the brakes are on stopping the arms rotating, so the pressue just builds up instead. Once you release the brakes, the arms are free to rotate, and the pressure in the system causes the car to jump up, before the HC catches up, and recorrects the height downwards. You can see the same by moving the height lever in the car with the brakes on!

Philip Chidlow · Post by **Philip Chidlow** » Fri Jun 12, 2009 8:42 pm

Thing is this is the fifth BX auto I've had and although I have experienced this effect before it seems more pronounced in this BX. So long as it's not symptomatic of anything to worry about I guess I'll live happily with it...

Thanks for the interesting feedback chaps.

jonathan_dyane · Post by **jonathan_dyane** » Fri Jun 12, 2009 8:45 pm

It is possible that you have a little trapped air which is exacerbating the characteristic, would recommend bleeding the rear brakes.

Aerodynamica · Post by **Aerodynamica** » Fri Jun 12, 2009 9:01 pm

RichardW wrote:I'm not sure the rear sinks after you stop - that might be your imagination - unless the car is creeping forward a bit, which causes abit more rear sink. What is happening is that the rear is below the normal height, so the height corrector opens up to try to correct the height - except it can't because the brakes are on stopping the arms rotating, so the pressue just builds up instead. Once you release the brakes, the arms are free to rotate, and the pressure in the system causes the car to jump up, before the HC catches up, and recorrects the height downwards. You can see the same by moving the height lever in the car with the brakes on!

-agreed! this is the cause but the reason it's not a commonly observed in every BX (or other hydraulic Cit for that matter) becaue it's accentuated bu automatic transmissions. I also find this happens in my CX C-matic.

I think it's because when the car is trying to 'creep' it has pushed the rear height down a very small amount despite the brakes holding the trailing arms.

Of course, you should not sit in traffic with the foot brake on!

Philip Chidlow · Post by **Philip Chidlow** » Fri Jun 12, 2009 10:23 pm

I put the car into N when the car is stationary for more than a few seconds - at lights for instance, but do hold the car (if it's on a slope) with the footbrake. Handbrake if I'm stopped for any longer period of course.

I'll have to consider bleeding the system then.

Mike E (uk) · Post by **Mike E (uk)** » Fri Jun 12, 2009 10:35 pm

I will suggest bleeding the rear brakes too.

If I don't drive my BX for a couple on months, it gets air in the rear brake/suspension circuit somehow.

Bleeding the air out stops the rear kicking up when the brakes are applied.

Mike

Philip Chidlow · Post by **Philip Chidlow** » Fri Jun 12, 2009 10:45 pm

Mike E (uk) wrote:I will suggest bleeding the rear brakes too.

that's what I meant do'oh

Jaba · Post by **Jaba** » Fri Jun 12, 2009 11:14 pm

Without wishing to be controversial can I expose that urban myth.
If you apply the brakes and then lower and/or raise the suspension the car's height will respond despite the brakes being on. The weight of the car and/or the hydraulic pressure will overcome any friction that is trying to hold the rear arm height at its current level. Try it.

Phil, it might be that one of your rear spheres is leaking nitrogen into the LHM at the rear end making it feel more spongy. What happens if you reverse and apply the brakes quickly to stop the car then release them and note how much the rear drops or rises.

The advice I was given by those that know, which I pass on without being able to confirm or deny its veracity, was to leave the car in D while stationary at lights or whatever. It is kinder to the auto box than selecting N as you are subjecting the gearbox hydraulics to less wear by not making an unnecessary change from D to N. After all it is only a manual drivers habit to select neutral to avoid having to hold the clutch pedal down.

Philip Chidlow · Post by **Philip Chidlow** » Fri Jun 12, 2009 11:24 pm

Interesting, Jaba. I will attend to the rear brakes and check the (new) spheres - ot at least Dennis at Softride can (he supplied and fitted them).

The reason I shift to N when stationary for some time is that held in D I get a lot of thrumming vibration. Difficult to describe, but it's quite intense and the bonnet shakes and so on - slip it into N and it smooths out nicely. Doc suggested a while back it might be something to do with kickdown cable fitting/adjustment but I haven't got around to investigating this symptom further. Any further ideas?

Let's not forget this is a 213,000 mile BX which has been 'driven' and yet been maintained regardless of cost throughout its life.

ken newbold · Post by **ken newbold** » Fri Jun 12, 2009 11:32 pm

I was out in the auto today and noticed it did just this! Can't say I've ever noticed it before so I'd suspect the rear spheres could be leaking. They've been on almost 4 years now, so that would seem to figure.

Way2go · Post by **Way2go** » Fri Jun 12, 2009 11:55 pm

Jaba wrote:
The advice I was given by those that know, which I pass on without being able to confirm or deny its veracity, was to leave the car in D while stationary at lights or whatever. It is kinder to the auto box than selecting N as you are subjecting the gearbox hydraulics to less wear by not making an unnecessary change from D to N. After all it is only a manual drivers habit to select neutral to avoid having to hold the clutch pedal down.

Who are "those that know" though - do they actually? Logically, keeping it in D whilst stationary creates frictional effects and heating. Clearly the box is having to dissipate unnecessary energy amongst other things. I always go to N at lights and it's benefit can be seen on the oil temperature guage. I imagine too, correct me if I'm wrong, that it probably keeps the valves and oilways with less risk of congestion if they are usually operated?

Jaba · Post by **Jaba** » Sat Jun 13, 2009 1:04 am

Way2go wrote: Who are "those that know" though - do they actually? Logically, keeping it in D whilst stationary creates frictional effects and heating. Clearly the box is having to dissipate unnecessary energy amongst other things. I always go to N at lights and it's benefit can be seen on the oil temperature guage. I imagine too, correct me if I'm wrong, that it probably keeps the valves and oilways with less risk of congestion if they are usually operated?

Ok had a sniff around and this is from the Rospa driving manual.....

When stationary in traffic, even for many minutes, it is not necessary to move the gear lever into neutral because the torque converter absorbs the engine’s propulsion force but does not transmit it all to the gearbox. No wear is taking place. In fact, more wear will take place if you engage neutral then engage a drive gear when it is possible to move off. Most gearboxes will automatically select first gear when the vehicle becomes stationary.

I had an auto box in my GTi once. Terrible power draining things. But very enjoyable around town - apart from the fuel consumption.

Vanny · Post by **Vanny** » Sat Jun 13, 2009 7:43 am

ROSPA and AIM driving manuals are not to be believed, they where last updated in oh i dont know 1902? They certainly dont reflect modern automotive technology and driving.

That said, on an auto, you only use N to coast and you really shouldn't do that. Park and Drive are the only thing that should be used (unless you spend a lot of time driving high powered autos, in which case Sport should be used in conjunction with flappy paddles). I suspect at the moment i'm spending a lot more time driving auto's than i really should (and that includes one of the best auto transmissions ever made), but that is the way of things. Autos suck.