Lets talk about Torque and wheel stubs

[johnvw]

Hello All,
Two things (amoungst many) that have often puzzled me.

1) I am familiar with torque measurements ( Ft Lb/ Lb Ft),..... O/k so cut me some slack at 63 you can not always remember. I have noticed a torque of eg. 40 Ft/Lb plus 90 degrees. It is this angular addition that has me
curious. Why not an increase on the original torque measurement.

2) Having often had to remove the tyre from other family members cars and nursing the aches and pains from very tight wheel stubs, I for many years have put a smear of grease on the wheel stubs. To date this has proved invaluable when changing tyres in awkward positions, with no painful after effects.
I have seen information in regard to this as BAD PRACTICE and again noted this in the manual of a recently purchased motorcycle.
I assume the lubricated threads may alter the specific torque rating for pull up of the wheel stubs.

I look forward to your replies and the unburdening of these small concerns in my senior years.

Best Regards
John

[zollaf]

where a torque and then an angle is used, it is because the bolt in question is a stretch bolt. the first torque just settles it, then the angle stretch it into place. more often than not these bolts can only be used once. the angle is used rather than a torque due to the bolt stretching where its actual torques doesn't go up in a linear fashion, or something like that. it is this way because its the 21st century and things have to move on ??, or so i am led to believe.
as for grease on wheel studs, big debate ?

[Phil K]

Morning John, I'm afraid I don't know the answer but I can confirm my father has also used grease (copper grease to be exact) for years on wheel bolts. We have never lost a wheel and it does help release the bolt. He also uses a light smear on the hub face which helps avoid the alloy 'sticking' to he hub... He hasn't used a torque wrench either preferring to spin them up flipping tight.

For many years I followed my fathers footsteps adopting this same practice but later learnt that it is advised against - even in the vehicle owners manual. So for the last 2 or 3 cars my wife and I have owned I have not used any form of grease and have still done the bolts flipping tight for lack of said torque wrench.

We recently traded in out B7 Avant and I needed to remove our aftermarket wheels and re-install the originals. No grease had been anywhere near these bolts or hub faces. I strained muscles I didn't know I had just loosening the bolts and managed to get the first wheel off to swap over. The bolts made a horrendous screeching as they loosened and the wheel was stiff to remove from the hub. I swapped the wheel over and the bolts made similarly horrific sounds as I put them back in. I moved on to the rear, same sounds were emitted by the bolts but this time the wheel was stuck fast to the hub. It was tipping down with rain, I was using the 'widow maker' emergency jack and no amount of kicking the wheel to release it was helping - only adding to the fear of the car falling off the jack.

Three thoughts crept in. I flipping hate this B7! Why didn't I use some bloody copper grease? It would be much warmer and easier to pay someone else to do this... £25 and a trip to Guildford tyres later and all wheels were swapped over and the B7 was on its last journey with us as the owners - for many reasons I was delighted to see the back of it but the last hurdle with the wheels left me sore and reluctant to put them on the new B8 before I dig out a bucket of copper grease!

Sorry for such a long winded reply but I have to say I agree with your logic for obvious reasons and have never been quite sure why its considered bad practice. Obviously grease is intended as a lubricant but if the bolts are done up flipping tight then I really can't see them going anywhere and life is much easier when it comes to future maintenance!

[ScottyUK]

I too use a light smear just to stop them getting stuck....but similarly have seen loads of posts since about why it's bad practise. It makes it easy to over torque them so just go steady.

On and I used LM grease rather than copper grease.

[Soot1e]

http://www.precisionenginetech.com/tech-explained/2011/05/20/tool-review-torque-plus-angle-in-one-tool/

the attached gives a full explanation of the reason for torque plus angle tightening.

[johnvw]

Gentlemen,
Tonight, ..... I rest in peace.
Many thanks
John

[ScottyUK]

http://www.precisionenginetech.com/tech-explained/2011/05/20/tool-review-torque-plus-angle-in-one-tool/

the attached gives a full explanation of the reason for torque plus angle tightening.

Interesting read. I didn't realise modern stuff was using torque + degree of rotation.

However what I don't understand from that link is the accuracy thing. It starts by saying that due to the variation in friction due to threads, lubrication etc that torque can vary by a lot. I totally agree with that hence my earlier post about being careful not to overtighten (or actually over stretch the bolt). Since this torque wrench still does the same thing and then after adds in the benefit of measuring the degree of rotation, surely it can still be as far out as any other digital torque wrench i.e. the torch sensor can still be fooled by the friction variation in the threads.

What am I missing here?

[vwcabriolet1971]

The angle method of bolt tightening uses the pitch of the bolt thread as a means of accurately predicting the amount the bolt is stretched. Thus if the angle is 90deg. then the bolt is stretched 1/4 of the bolt thread pitch. ( i.e. 90deg divided by 360 deg). If the material for the bolt is accurately controlled then the axial load on the bolt can be accurately predicted thus the clamp load is accurately controlled. This the reason why only dealer stretch bolts should be used as pattern parts cannot be guaranteed to match the correct material specification. This angle stretching of the bolt is not influenced by any variation in the torque applied, just sufficient torque to achieve the designed angle rotation and hence the amount of stretch.
The design objective of tightening fasteners is to achieve a certain clamp load - using the torque applied or a certain amount of stretch is an indirect way of trying to achieve the desired clamp load.
The initial torque spec for stretch bolts is to ensure that bolt is tight in the threads before the angle stretch is applied.
Idealy the bolt should be tighted down and measured against a clamp load sensor. This can be done in a laboratory but there are many practical difficulties in doing this in a workshop/DIY environment.

[vwcabriolet1971]

Just a few further point to my previous post . The use of stretch bolts are generally only used in critical applications .e.g. big end bolts.The normal wheel bolts are carefully designed so that there is a large safety margin in the bolt strength . If you have ever tried to remove a rusty wheel bolt you will know that you have to apply a far higher torque than when removing a non-rusty bolt but the bolt doesn't break.
I once used a metre long brake bar on a stranded motorist's rusty wheel bolts . I nearly bent my bar before the bolts were released with a mighty "crack".None of the bolts were sheared off and the threads were not damaged . A good wire brushing removed the rust from the bolts and they then sweetly tightened using the normal wheel bolt torque.

I also have used a smigen of copper anti sieze paste ( it's not a lubricant in the accepted sense of a "lubricant") for many years using a normal torque wrench without any problems. I often wonder how often DIY torque wrenches are calibrated- I'm very lucky that I can get my torque wrenches checked periodically and use the amended readings when using the torque wrenches. I'm sure that the slight overtorque caused by the paste is far far less than a lot of over exuberant tightening used by many DIY car owners.

[ScottyUK]

The angle method of bolt tightening uses the pitch of the bolt thread as a means of accurately predicting the amount the bolt is stretched. Thus if the angle is 90deg. then the bolt is stretched 1/4 of the bolt thread pitch. ( i.e. 90deg divided by 360 deg). If the material for the bolt is accurately controlled then the axial load on the bolt can be accurately predicted thus the clamp load is accurately controlled. This the reason why only dealer stretch bolts should be used as pattern parts cannot be guaranteed to match the correct material specification. This angle stretching of the bolt is not influenced by any variation in the torque applied, just sufficient torque to achieve the designed angle rotation and hence the amount of stretch.
The design objective of tightening fasteners is to achieve a certain clamp load - using the torque applied or a certain amount of stretch is an indirect way of trying to achieve the desired clamp load.
The initial torque spec for stretch bolts is to ensure that bolt is tight in the threads before the angle stretch is applied.
Idealy the bolt should be tighted down and measured against a clamp load sensor. This can be done in a laboratory but there are many practical difficulties in doing this in a workshop/DIY environment.

I appreciate the idea of angle stretch but I don't get the start point. It could either loose, tight or overtight before you apply the angle stretch. How do you get to the correct start point without doing it via torque with can be compromised by lubricant, wear, etc?

[OddBallBoy]

I'd been really struggling undoing wheel bolts of recent, even though I knew I hadn't put them on really tight, I was having to use ridiculous amounts of strength to get them to move and when they did, they went with an almighty crack. So the last time I did a wheel change, I put a very thin layer of copper grease on just the taper but was very careful not to get any on the threads. Worked perfectly, all bolts still tight when checked yet when I swapped onto my winter tyres, they all came undone without any trouble.

[vwcabriolet1971]

As I said in my previous post the initial torque applied before the angle stretch is to make sure that all the "slack" has been taken up before the angle tightening is applied . All stretch bolts are intially tightened to a specified torque value before the angle tightening is applied. This initial torque also streches the bolt but is only a small amount of the total stretch applied ,so that any frictional errors are only a small part of the overall stretch.
It's not a DIY exercise to define what should be the torque figure or the angle amount . These figures are accurately determined by an by a skilled engineer who is very familar with working out the detailed stresses and strains in bolt steels steels to achieve a certain clamp load. The detailed steel charcteristics/properties ( and the properties of the alloy wheel) must be accurately known in advance of these calculations.

[ScottyUK]

This initial torque also streches the bolt but is only a small amount of the total stretch applied ,so that any frictional errors are only a small part of the overall stretch.
Maybe that's the bit I'm missing. Maybe I shouldn't try to understand hehehe

Cheers