diff nose hights

Clutch, Gearbox, Overdrive, Propshaft, Differential, Drive Shafts, Hubs.
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rustjunkie
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diff nose hights

#1 Post by rustjunkie » Tue Apr 24, 2018 2:28 pm

hi all. I have a Question regarding diff nose heights. I have a mk2 estate on a late L plate with a 2.5 s engine fitted, the current diff appears to have an early nose, is this a standard fit on an estate ? The engine mounts have been changed along with the mounting cross member to 2.5 spec engine is tilted over to the passenger side so should I have a later nose fitted ? the rear end is out the car so would like to sort it now as a pain to do when its all back in !

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Re: diff nose hights

#2 Post by Clifford Pope » Wed Apr 25, 2018 6:05 am

Surely the diff nose height depends on the relative positions of the diff mountings and the rear crossmember legs? I don't see the connection with engine tilt.

There was a thread ages ago about adjusting the diff angle to try and get more of a straight alignment with the prop shaft, but that was to do with minimising prop wear and possibly also nose bearing.

I didn't think engine spec altered the axle/diff requirements, other than perhaps highlighting the possibility of changing the diff ratio to get a higher gearing?

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Re: diff nose hights

#3 Post by Mike Stevens » Wed Apr 25, 2018 9:51 am

The diff nose height was changed quite early on in production to reduce drive line vibration I think. CW has some notes on his website about that. As Clifford says, I don't think the engine location has much relevance to it.

Somewhere there is a measurement to be able to determine the different diff nose heights, but looking at Chris's note, it seems quite obvious.
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Re: diff nose hights

#4 Post by rustjunkie » Wed Apr 25, 2018 10:34 am

Ah ok will take a look. my understanding of it was that if you have the wrong one it can alter the rear camber and height of the rear .and the engine mounting angel alters the prop alignment so it can set up a vibration .will check out Chris's website . I cant see how the nose change would affect the sub frame as its the centre of the quill shaft that moves rather than the actual mounting position. Or is that not the case.

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Re: diff nose hights

#5 Post by rustjunkie » Wed Apr 25, 2018 1:17 pm

I have spoken to Chris Witor .He said the car should have a later type nose as the early one will give to much negative camber and sit to low .but it doesn't have much effect on the prop shaft .thanks Ed

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Re: diff nose hights

#6 Post by Clifford Pope » Wed Apr 25, 2018 5:38 pm

There's a lot of discussion here about the inter-relationship between nose piece type, sub-frame mounts, diff rear mounts, suspension sag, suspension leg bolts to nose piece, and propshaft alignment, and the effects on camber angle and vibration. Also the different mounting points for the trailing arm shackles on the suspension legs.


viewtopic.php?f=13&t=1931&p=15297&hilit ... gle#p15297

Also more threads if you search for "prop shaft angle".


I've read somewhere, possibly in connection with prop shafts on motorboats, that to minimise wear and vibration the two end flanges of a prop shaft should either be parallel or have the same angle to the prop, no more than 5 degrees.
Not the same thing as being exactly in line. With a fixed installation, the spider joints need to walk backwards and forwards to spread the grease and the wear points. Obviously not relevant on a car with suspension, but the parallel thing is or the two sets of spider joints do not cancel each other out properly and vibration can result.

Surely if you move the nose piece height you are tilting the diff and hence the prop flange? I'd have thought the thickness of the rear mounts would be relevant too, either to maintain flange angle or to correct it.

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Re: diff nose hights

#7 Post by bigestate » Wed May 02, 2018 10:18 am

Very interesting read through the different heights and vibration status, I to have this annoying issue albeit have changed everything in the car apart from the body shell and still I have the vibs which start at around 55 mph and disappear after 65 mph. New tyres, hubs, diff, nose piece, rims, re-balances, 6 props, new gearbox mount, I even ended up fitting a complete used rear end from a smooth running PI saloon and my car still vibrates to the extent that it feels like a prop plane with a blade missing off the prop :roll: not to mention the drumming noise. I've kind of got used to this strange phenomenon over the years but it's most off putting for other passengers or drivers :twisted:
This morning I removed the prop and will send it away to have it checked and re balanced if necessary just to eliminate this part. However while I was under the car I noticed, well I have always known that the gearbox sits really low and the gearbox cross member has occasionally made contact with the speed humps and in one case bent the floor pan and bolts and mount backwards which had to be removed and straightened. The mounts are the right length for MoD, as are the spacers. The box can't go any higher as the top of the box fowls the floor pan.
My query is, is the box too low causing this pain in the butt vibes in my Estate?
DSC03053 (Small).JPG
Also has the later diff nose piece, I have tried both different types to no avail :(
DSC03046 (Small).JPG
DSC03054 (Small).JPG
Many thanks for any suggestions on my vibey old estate :)

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Re: diff nose hights

#8 Post by Charles H » Wed May 02, 2018 1:10 pm

All very familiar! The rally car also has this issue, and I suspect one of two things now having gone through similar swops and replacements. I even changed the gearbox, but used the same overdrive. So, overdrive unit? or engine balance, possibly the front pulley damper? Clutching at straws. I too have learnt to just live with it.
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Re: diff nose hights

#9 Post by bigestate » Wed May 02, 2018 8:05 pm

My next plan of attack is to alter the engine mount height and experiment with these. I tried another manual overdrive gearbox thinking it was that, still no change. :?
I just try to avoid driving between 55 and 65 :lol:

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Re: diff nose hights

#10 Post by Clifford Pope » Thu May 03, 2018 5:58 pm

Somewhere I remember seeing some years ago a video of a prop shaft viewed under stroboscopic light so that it appeared stationary. Except that it wasn't quite still. You could clearly see the centre section speeding up and down rythmically, which is correct of course because of the effect of the spider joints, but they cancel out.
But also you could see other vibration and regular movements at one flange compared with the other. Altering the angle between the two flanges and and the degree of misalignment clearly pinpointed what was going on.

Is this diagnostic technique available anywhere?

Also, it occurs to me, how are prop shafts balanced? Is it simply rotational, or are they balanced along their length? If they only have a single balance weight stuck on, then it doesn't follow that it has been fully balanced - you would surely expect to see several weights in different positions along the length? In the same way that a wheel and tyre can be balanced rotationally, but also properly need balancing as between the two rims. Full balancing, as used to be carried out, was done in situ on the hub, after which it was important always to keep the wheel on the same studs. Along several feet of shaft you'd have thought this was very important for perfect balance.

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