Is it important to diagonally tighten wheel nuts?

John Laidlaw

Senior Member
Joined
Nov 15, 2013
Messages
26,373
Reaction score
9,163
Location
Wirral
Your Mercedes
Land Rover Discovery 4
Lubricating your nuts might need a thread of its own!
 

hotrodder

Senior Member
Joined
Oct 18, 2008
Messages
894
Reaction score
26
Your Mercedes
'93 320te, '54 ragtop beetle (in bits)
Lordy, there's been some nonsense stated in this thread! FWIW...

Metals behave both elastically and plastically, anyone that paid attention in school should have a vague recollection of Young's Modulus and Hooks Law... https://en.wikipedia.org/wiki/Young's_modulus and https://en.wikipedia.org/wiki/Deformation_(engineering)

The important bit with threaded fasteners is installed tension aka preload which is inconvenient/tricky to measure directly but is proportional to the torque applied when tightening. There are a couple of caveats though one of the biggies being friction because a LARGE amount of the torque needed to preload the fastener is due to friction as shown by

piechart.gif


which is taken from http://www.boltscience.com/pages/nutorbolttightening.htm
If you don't believe Bolt Science then http://www.gedore-torque.com/linkservid/FD5CA385-16D7-4E99-A889E0D651AC78B2/showMeta/0/ is one of many results from googling 'torque values yield strength'
When a bolt is preloaded properly it stretches elastically making for a reversable slight interference fit. Overtighten it enough and it yields, stretching permantly...

Lubrication has a significant effect on the amount of torque needed to preload a fastener. http://www.spirol.com/library/sub_catalogs/cmpl-Bolt_and_Torque_Specifications_us.pdf is a generic torque chart that provides values for both dry and lubed threads (different K factors)...
M12 x 1.5 class 10.9 is fairly typical of many wheel bolts and the torque figure for MBs with this size of wheel bolt is typically 110Nm. Bigger/heavier/newer MBs typically use M14 x 1.5 torqued to 130Nm or 150Nm in some cases
The torque values given in the link are 131.5Nm dry (k factor of 0.2) or 98.6Nm lubed (k factor of 0.15).
Many manufacturers of anti seize and other thread lubricants publish k factors for their products as the type of lubricant makes a difference for example http://www.na.henkel-adhesives.com/us/content_data/333994_LT6734.pdf

Like pretty much every other car manufacturer MB torque figures are for clean, dry threads unless otherwise stated. Lube things and the standard torque setting is gonna result in higher bolt preload. Not enough to damage things most of the time but if some numpty with a rattle gun overtightens a lubed bolt it WILL be overtightened a chunk more compared to overtightening a dry bolt due the reduction in friction and a bit of copper slip generally doesn't make removing a necked, mangled bolt less painful. Manky threads full of the gunged up remains of what used to be copper grease increase friction and result in a lower bolt preload for any given torque value

Lubed, dry, over or slightly under tightened things can change shortly after tightening for various reasons which is why, as said, it's a good idea to check the torque after a few miles... it's not uncommon to find a few bolts have 'backed' off a nats. This is more important with new wheels and/or new bolts and (used to be at least) stated in the owners handbook- it is mentioned in the handbook for my s124

The number of wheel bolts used (along with their size and torque setting) is down to the desired clamp load... bolts don't like being subjected to bending forces (shear strength is a chunk lower than tensile strength) and their job is simply to clamp things together. The wheel to hub joint then functions kinda like a clutch i.e. it IS friction between the tightly clamped parts that transmits torque from acceleration and braking etc
This is why tiny little light cars can get away with three wheel bolts while lorries and buses have billions of them. This is also why slathering copper grease all over the mating surfaces is a bad idea... people get away with it because virtually everything is over built / made with safety margins etc. For anyone parnoid about wheels sticking to hubs note it's virtually always the centre register that wheels stick to when no one removes them regularly for servicing etc i.e. a tiny smear of grease around the centre register is all that should be needed. It's job is to locate the wheel concentrically because bolts need clearances for assembly and so cannot accuratly locate things
 
Top Bottom