I recently lowered my EF, by around 75mm or so, using Pedders gear mostly. I also installed a front camber kit and had Pedders re-set all the front end geometry after lowering.

Since then, the car has a couple of bad vibrations:
1) at highway speeds - between 90kmh - 110kmh, a rhythmic vibrations like a "vmmm... vmmm... vmmm..." (yes I know it's hard to describe a vibration using only as keyboard!). When I lift of the throttle, the thrumming drops away, when I gently squeeze the throttle back on, it comes back. I had this before in my XE track car: a bent/out of balance driveshaft. But this vibration occured only immediately after lowering.
2) when off the gas, and coasting down from say 60kmh (like when approaching a stop light), a nasty vibration builds up as you coast down past 50kmh, that's enough to rattle the coins in the ashtray. A vibration much like when you touch one of those rumble-strip white lines we have on the edge of some highways here to alert drivers who're drifting off the road (only not quite as bad as that). If I slip the car into neutral when it's vibrating, the vibration disappears.

I've heard it's a problem in the rear driveshaft universal joint, with the pinion angle becoming too great for the rear U/J. And the U/J 'binding'?

BUT, here's the thing - the guy at Pedders reckons it's nothing to do with that, and that my diff head itself is stuffed, and that it has nothing to do with the car being lowered. He said that's just pure coincidence.

Anyone got any knowledge or fixes on this one?

Any input much appreciated.

Thanks all.

my suggestion is take of you rear wheels and have a look at the bolts, some pedders places have been known to miss a couple of bolts here and there

Hi,good suggestion, but I did the install myself and am pretty sure it's all there! I'm really careful with that stuff - especially because my little kids ride in the car with me.

But your comment on Pedders is interesting: I had them looking for the vibration problem last week, they had jacked the car up at some point - I noticed when I went underneath it the one of the rear lowering springs was subsequently completely out of it's seat!! Not good.

Have you checked wheel nuts?
Sounds simple but it is very common.

Thanks - but no, wheel nuts all good.

maybe something else, have u check the tailshaft bolts??

That's all been checked, and is all good. Thanks.

it'll be that the pinion angle is all wrong now because you've lowered it. my EB does it. i just live with it

I had a bad vibration when coasting to a slower speed, mainly at 100km/hr and slowing down to past 50km/hr. Vibrations dissapear once I apply a bit of throttle.

Turned out to be uni joint on the tailshaft. I pulled it out and had a look and it was way out of wack. One of the pinions on the join was super clean...ie it was sliding in and out of it's slot. I put my spare tailshaft in and vibrations dissapeared.

U/J could have been already dying on you (if it hasn't ever been replaced before) and putting more strain on it after lowering could have killed it further. I suggest you get the uni joint checked and replaced as necessary. If it gives way it can be quiet destructive and dangerous. Best to check it...shouldn't cost too much.

1) Will probably be tailshaft balance or uni joint. But since it was silent before lowering, id say a uni is worn and is now under stress. Would probably go away if the pinion angle was right, but you'd still have a trashed uni.

2) Will be pinion angle from extreme lowering. Common issue when lowering an EF (or any Falcon with that design suspension), and there's lots of threads on it. Don't think there is much of a fix with that you have right now though. Just have to live with it.

Unless you dig up an EL diff and EL XR control arms and install them.

Or if you get the precise measurements, you can drill a new mount point for EL XR arms in your current diff and use EL XR arms. Will get it mostly fixed with these 2 options.

Jesus Christ mate, 75mm!!! It must be flat to the boards, haha. Third party spring manufacturers like King springs, for instance, offer 2 kits, low (32mm) and superlow (50mm), they don't offer ultra low 75mm springs for a reason, they mess up the front and rear geometry of the car way too much for the standard linkages and other attachements to compensate. Listen to all the mofos who are telling you your pinion angle is out of whack. I did my car 32mm and it bottoms out way more than I thought it would. I'm glad I never got superlows at 50mm. I'd have no sump or lower cross-member left!!! lol As it is I dinged the bottom of the sump taking a drive on the winery roads in the upper hunter around Cessnock

Uni joints is good to do. I'd also first look at the tailshaft tunnel. See any scrape marks there? If so, the tailshaft was probably hitting/rubbing it at the speeds described which means your pinion angle is incorrect. I'm not sure if there is much you can do if this is the case (with an EF) either than raising the car again. With an EL diff, you are at leat able to relocate the upper arms to adjust the angle a little.

It's not that bad - it passed a Warrant of Fitness (but the testers could hardly speak English, so maybe that had something to do with it!

The rear springs are Pedders and they're not 75mm lower, but the fronts are reset standard springs, very low, and frankly, it's too low in the nose and has too much rake. But it's done now so I'll leave it as is and try and cure the driveline issues.

It's still got it's standard Fairmont alloys on - I have a set of 17" polished ROH's ready for it but don't want to put them on until I get the vibration prob's mostly sorted. Unfortunately I do most things on the car on a bloody shoestring budget - otherwise I'd just send it to a top suspension specialist and say "just fix it".

No scrape marks but without doubt, the pinion angle is now way out of whack.

What a pain in the a***. Starting to think about getting the rear lower arms customised to be adjustable so I can control the diff location/pinion angle. But that was never in the plan when I lowered it on the cheap!!!!

There wouldnt be any scrapes on the tunnel as the nose is pointing that far down, it wont lift enough to hit the body. You'd have to be hammering it and carrying some decent weight in the back.

However, fixing the angle by making the upper arms longer (or putting in XR arms) will tilt the nose back up, and then it'll start hitting the body regularly.