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      04-30-2012, 04:39 PM   #1
mobilejo
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Odd swinging sensation in car at low speeds?..

Since I had the car, i've had this issue - hard to explain but basically when slowing down from any speed, the final part just before it stops (lets say from 20mph-ish downwards) the car has a sensation that I can only describe as feeling as though there is a pendulum in car swinging back to front making the car feel as though as it jerks forwards, pulls back, jerks forward, pulls back etc until it stops.

Its a low frequency sensation - each 'forwards/backwards' type motion occurs probably every 1-2 seconds.

Another way to describe it would be to say it feels like you are sitting on a swing going back and forth. There is nothing felt in the steering wheel, its more in the seat (suggesting rear of car?), and its not left/right, its forwards backwards.

It also does it when accelerating from very low speeds and stops once you past 20mph-ish, which makes me rule out breaks.

I have a manual, and it does it when slowing down in gear (i.e. only push in clutch right before stop), it does it when clutch is pushed in before I even start slowing down, and it does it in neutral.

Its a manual 2005 320d on what I think is SE suspension. I have changed the following but this little issue has persisted since day 1 and has been the same regardless (not got better or worse):

engine mounts
gear box mounts
new wheels & tyres
new upper front control arms
new front drop links
new engine pulley
de-flapped
oil/filter/air filter change
remap
CDV delete

Any suggestions?
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      04-30-2012, 04:53 PM   #2
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Im don't know but things i would check;

Bushings; Subframe + and control arm ball joints
Geometry set up.

The issue sounds very strange though
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      04-30-2012, 05:02 PM   #3
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Kangarooing on deceleration can be EGR valve... Is it maybe that?
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      05-01-2012, 04:09 AM   #4
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Hmmm, I don't think so - the clutch is depressed when it does it so the engine isn't affecting what the wheels are doing. Its definitely a mechanical reaction - suspension/brakes/bushes something like that.

Its started doing it pretty much every time I brake to stop now, and when I reverse...
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      05-01-2012, 04:13 AM   #5
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Prop-shaft doughnut.
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      05-01-2012, 04:15 AM   #6
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Quote:
Originally Posted by xenon View Post
Prop-shaft doughnut.
Ah ha! Never though of that!

Is this something you have seen? How would I check it?
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      05-01-2012, 04:24 AM   #7
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If the doughnut mounting holes have worn in an eccentic pattern with elongated holes, then at lower speeds or when the propshaft is unloaded, it can reciprocate around the logitudinal axis causing the side-to-side "feeling". At higher speeds or under load, the spinning smooths the imbalance gyroscopically. The only real way of telling is disassembly.
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      05-01-2012, 04:28 AM   #8
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Quote:
Originally Posted by xenon View Post
If the doughnut mounting holes have worn in an eccentic pattern with elongated holes, then at lower speeds or when the propshaft is unloaded, it can reciprocate around the logitudinal axis causing the side-to-side "feeling". At higher speeds or under load, the spinning smooths the imbalance gyroscopically. The only real way of telling is disassembly.
brilliant explanation and sounds to make perfect sense - it feels fine at high speeds, but 'swing-ey' (not a real word I know) at low speeds.

I will get it on the ramps and check.

Is it a big job to change - I have the use of ramps and tools etc and I'm a confident DIY'er.
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      05-01-2012, 04:33 AM   #9
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On ramps it's a piece of piss and the doughnut is only about a score.
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      05-01-2012, 04:35 AM   #10
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Quote:
Originally Posted by xenon View Post
On ramps it's a piece of piss and the doughnut is only about a score.
Thanks alot for your input - if its cheap and easy, I may as well do it just to see.

Will let you know how I get on.
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      05-01-2012, 06:52 AM   #11
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Quote:
Originally Posted by xenon View Post
Prop-shaft doughnut.
+1, The half shafts / diff / prop / gearbox output are about the only bits left that are consistently involved in all the scenarios you mention.


Maybe a half shaft issue though, as that is slower than the prop and off centre would more likely cause a fore and aft rocking?

As the diff ratio is about 3.5:1 the prop shaft will be spinning 3.5 times faster than the wheels....


At 20mph, about 8m/s =

wheels / half shafts / hubs / diff input etc about 4 revs per sec

prop / gearbox output about 14 revs per sec
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      05-01-2012, 07:24 AM   #12
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Agreed DB, could be any of those but I suggested the propshaft as a) it has enough mass to feel through the rest of the car and b) the doughnut can fail, as it did on my e46. The half-shafts don't really have any wear items other than the CV joints, which would also manifest itself through noise (??)
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      05-01-2012, 07:52 AM   #13
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Quote:
Originally Posted by xenon View Post
Prop-shaft doughnut.
No need to be rude Ian. he only asked.
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      05-01-2012, 08:08 AM   #14
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Quote:
Originally Posted by creepy coupe View Post
No need to be rude Ian. he only asked.
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