E34_nut

Hi all.

On page 310-8 of the E34 Bentley Manual are the final steps for reinstalling the front hubs.

13. Install a new collar nut. DO NOT TIGHTEN TO ITS FINAL TORQUE AT THIS TIME.

14. Install the brake rotor and mount the wheel and lug nuts. Lower the car to the ground to gain leverage.

15. Tighten the collar nut and then stake the nut to the axle.
16. Raise the car and remove the wheel.
17. Install the brake caliper as described in 340 Brakes.

I don't understand why step 13 says "DO NOT TIGHTEN TO ITS FINAL TORQUE AT THIS TIME."

The collar is tightened to the stub axle. The stub axle is firmly secured to the car. Surely it's easy to apply 290Nm of torque to the nut. What extra leverage is gained by lowering the car to the ground. I don't understand.

Can someone please explain why the manual says to do this ?

I think this should work equally well.

13. Install a new collar nut and tighten to "finger tightness".
14. Install the brake rotor.
15. Tighten the collar nut and then stake the nut to the axle.
16. Removed.
17. Install the brake caliper as described in 340 Brakes.

Please help me understand why the extra steps are needed in the process described by the manual.



Regards,

Reg.

__________________
from THE Antipodes , the real Land of Aus.

"You're gunna die of something, it might as well be fun !"

'88 Ducati 851 Superbike Strada - the original tricolour
'83 Ducati Mike Hailwood Replica - "900" kick start

Imagine a luxo like an E38 "L" with the current 4.0L Diesel !

IvanDias

I thought it was because the Bentley manual was based for the home mechanic and the introduction states the car must be held aloft by axle stands and not the jack.


290nm is a whopping 213lb/ft and that's a serious and lovely expensive torque wrench to own!

It's also a lot of grunt to apply to a car being held up by axle stands and not over a pit in a garage where it can't fall on you.



Valid theory?



Regards

Ivan

__________________
"Never enter Karussell when on the brakes! I have gone round there on the roof, I know what I'm talking about." ~ Sabine Schmitz

farrell

Hi Reg
Ivan is right.
Make sure the hub assembly is fully seated on the stub axle
The torque wrench I use to tighten this nut is a metre long.
290 Nm should be applied to the hub nut with the wheel fitted & the weight
of the vehicle on the ground.
Make sure you fold the locking tab in.

Cheers

__________________
www.darjac-racing.co.uk

E34_nut

Thanks Gents.

I suspected it was something to do with safety but my analysis of that problem dismissed it. See my analysis below. I just wanted to make sure there wasn't some other issue I hadn't thought of.

An Expensive Torque Wrench
My torque wrench will apply 160Nm so plainly that's useless, but we do have a local equipment rental organisation that will rent a 600Nm torque wrench (about 1.2m long) for UKP15 a 1/2 day or overnight. No need to spend a "million dollars" on a torque wrench.

When Farrell pointed out the required torque on the EDC rebuild thread about 2 months ago, I thought it was a typo !

Why I thought lowering the car wasn't necessary
I agree that 290Nm is a whopping amount of torque to apply to a hub nut, but I don't think the size of the torque, in and of itself, causes a problem. It's the force required to apply the torque that could be a problem.

Please note, I'm not saying there isn't a problem, just saying I can't see it.

To apply 290Nm using a 1.2m torque wrench, is the equivalent of hanging about 25kg off the end of a 1.2m long wrench (25kg x 9.8N/kg x 1.2m). I'm guessing the wheel + inflated tyre weighs about 25kg (or at least a something close to that) and fitting that to the car doesn't cause it to "totter on the edge of stability". If the wrench was say 100mm long (admittedly highly improbable) and I had to put 300kg (300 x 9.8 x 0.1) on the end of that to get 290Nm, then there could be a serious increase in the risk of the car falling off the jacks.

The car is supported on 4 jacks at the moment. I've had wheels on and off; I've had struts on and off. Together, these additions and removals change the weight (effectively force) on either side of the car by 2 or 3 times more than the force required to apply the 290Nm of torque to the hub nut. They don't result in any obvious change in stability of the car on the jacks.

Can either of you see the error of my logic ?

Thanks,

Reg.

PS Thanks for the reminder to lock the hub nut, I'm sure it's on my "I'm all done checklist", but plainly, it's worth checking.

__________________
from THE Antipodes , the real Land of Aus.

"You're gunna die of something, it might as well be fun !"

'88 Ducati 851 Superbike Strada - the original tricolour
'83 Ducati Mike Hailwood Replica - "900" kick start

Imagine a luxo like an E38 "L" with the current 4.0L Diesel !

farrell

Hi Reg
The axle stands are under the vehicle I guess.
If I have this right, you can't see the difference between tightening the hub nut with the wheel on the hub & loaded with the weight
of the vehicle with hub nut static on the ground, compared to trying to tighten a nut on to the end of a free sprung hub that will move the strut
against spring load vertically upward if you pull up or downward if you push as you apply torque with the tool.

As well as a very large nut with very small load bearing lands for size moving about, you will put stress on other potential parts & run the danger
of not getting a fixed torque value againt the hub which could then allow the hub to work against an undertorqued nut.

Options
1. You could get very hurt trying to apply the torque to a sprung hub in motion.
2. You could have a knackered wheel bearing in short order.
3 You could have a wheel fall off the car which is very inconvenient.

Perhaps I am missing something that pics would high light more clearly.
Having injured myself a couple of times in recent months, I would suggest the path of least resistance
is also the safe path, even if it takes longer.

__________________
www.darjac-racing.co.uk

E34_nut

Hi Farrell.

Thanks very much for taking the time to follow this up with me. The reason why I'm trying to avoid having to lower the car and "reraise it" is as follows. Right now the rear suspension is also out of the car. I cannot refit it, because the company (not BMAparts BTW) supplying the parts required to fix the rears, originally committed to ship them in the last week of May, then the first week of June, then finally did so in the second week of June. I spent every second day sending emails and making phone calls requesting a tracking number but it was never given to me. On Tuesday, the parts were returned to the supplier saying they couldn't find that address in Austria. Since I live in Australia, that's no surprise really ! The parts are supposed to have been reshipped to Australia today (I hope). I'm still waiting for the tracking number.

Why do I mention the rears when this discussion is about the fronts ! When I lower the car, the front wheels will touch the ground and position themselves with a narrower track than would occur if the car was driven to a stop. This means the wheels will be abnormally loaded unless I can move the car a small amount backwards and forwards to unload the stresses normally present when a car is lowered off a jack. Without rear wheels I can't do this.

I'm trying to at least finish the fronts so that when the parts for the rears arrive I will have less to do !

Correct. The stands are supporting the vehicle body, not the axles.

If the phrase "free sprung hub" means a strut, properly attached to the vehicle with hub fitted to the stub axle and secured with a hub nut to about 30 or 40Nm and no wheel on it then, correct.

I agree.

Not sure that I agree, since the suspension is at full extension right now and can't be extended any further when the force required to torque the nut is applied downward.

I think the size of load bearing faces on the nut are a problem with either process. With the wheel missing, there is no requirement to have an extension between the torque wrench and the socket, which reduces the likelihood of the socket slipping off the nut.

Stressing other parts is something I hadn't thought of.
My analysis is as follows.
I think there are 6 parts that will suffer the extra load when I torque the nut by applying a downward force on the torque wrench.

1. the 3 nuts securing the strut to strut towers
2. the nut securing the piston rod to the top strut mount bearing
3. the bearing in the top strut mount
4. the "pinch overs" that hold the bearing in the top strut mount
5. the top cap of the shock absorber case
6. the nut and threaded end of the piston rod securing the piston to the piston rod in the shock absorber

Assuming a wheel + inflated tyre, does weigh about 25kg, these 6 parts all have to be capable of dealing with the force associated with that weight. Funnily enough that's very close to the same force I'm going to apply to the stub axle when I put 290Nm of torque on the bearing nut using a 1.2m long torque wrench. Since all these parts cope with the force associated with that weight, they should be well and truly capable of dealing with the force required to torque up the nut.

I agree. This is possible if you pull up on the torque wrench. Some of the force applied will compress the suspension spring. NOTE TO SELF, DONT DO THIS IF I PROCEED.

There will be no motion if I'm pushing down on the torque wrench. The suspension is at full extension.

I have already "nipped" the hub nut up with the socket whilst ensuring the hub is properly seated on the inner race. I have fitted the disc and caliper to the "lightly" secured hub and the disc spins correctly and is aligned with the caliper. I think this check removes this as a potential problem.

Inconvenient. There's nothing quite like the understatedness of the English !

Once again, thanks for your reply. The content of your responses has made me further analyse the risks and that is exactly what I was hoping would occur. This has been valuable for me.

Since I won't be able to do any more on the car until the day after tomorrow, I thought it worth while investing some time with the wealth of experience that the participants on this board can focus on a problem to try to save the time. If nobody could show me a sound mechanical or safety reason why my proposal wouldn't work, I could use that time to complete a few other bibs and bobs that need doing, before the rear suspension bits arrive.

Thanks and regards,

Reg.

__________________
from THE Antipodes , the real Land of Aus.

"You're gunna die of something, it might as well be fun !"

'88 Ducati 851 Superbike Strada - the original tricolour
'83 Ducati Mike Hailwood Replica - "900" kick start

Imagine a luxo like an E38 "L" with the current 4.0L Diesel !

farrell

Hi Reg
My reference to pushing down on the breaker bar / torque wrench was in reference to unneccessarily loading suspension & steering gear joints.
When the road wheel is attached to the hub & the vehicle sits on the road or ramp normally, you are not applying a loading force to suspension bump stops & suspension components joints that are hanging in a full rebound position.

I hope that you do wait to load the vehicle properly before final tighten of the hub nut.

Take care & take time.
Farrell

__________________
www.darjac-racing.co.uk

E34_nut

The haste / waste thing comes quickly to mind and that's gotta , then .

Thanks for your persistence.

Reg.

__________________
from THE Antipodes , the real Land of Aus.

"You're gunna die of something, it might as well be fun !"

'88 Ducati 851 Superbike Strada - the original tricolour
'83 Ducati Mike Hailwood Replica - "900" kick start

Imagine a luxo like an E38 "L" with the current 4.0L Diesel !