What to Know About Wheel Studs

[Megatron]

    Not really sure if this fits the general discussion forum, but since it applies to almost every vehicle ever made, I started here.

 

 The question I have about wheel studs relates to the fact it is a bolt. My question is why with bolts like head studs/bolts can you only retighten them a couple times before they are considered bad? Even flywheel bolts only get used once. To me wheel studs are some of the most important assemblies on your vehicle. Yet, if you follow retune maintenance, like tire rotations and replacement, you will re use these bolts/studs a whole bunch of times. Is there something I am missing like they are made of different alloys or hardened to different standards. Or am I missing somewhere that says I should be changing them out after 3rd time I tighten them?  

 

   I have been to a few local tire shops that hammer them on with an impact wrenches and no torque values measured. Been guilty myself of twisting the old 4 way around until I saw little stars in my vision. Yet still they hold up without question. Recently one of my crew trucks did suffer a rear dually failure at the wheel studs. Now, I do know that 2 days prior another crew had rotated the tires, so I went with loose hardware that lead to that failure. After further inspection, all the other lug nuts on the truck were loose lol

 

   So what gives? Anybody have an answer??

[biggman100]

There is actually two parts to the reason why wheel studs last longer than say head bolts, and that is heating and cooling effects, and the way the bolt is tempered, or hardened.

 

I will start with the heating and cooling effect first. Over time, engine bolts are subject to extreme heat, which can cause one of two things to happen. Either the bolt will stretch, or the threads will become brittle and when that bolt is removed, the threads will start to get rounded just a bit on the edge of them, both of which in turn cause that bolt to not be as strong as when it was first machined. Another issue, particularly with head bolts, is that the expanding and contracting of the head and the block, usually at different rates, as the engine heats up and cools down, will cause the bolts themselves to stretch, and change the temper of the bolt over time. I have actually seen some extreme cases where if you take a used and a new head bolt, and compare them with a digital micrometer, there can be as much of an 1/8 of an inch difference in length. I have also seen where people have tried to cut corners and reuse head bolts, and in less than 5000 miles of normal driving, the heads would literally come off of the head bolts.

 

Wheel studs, on the other hand, are tempered to an extreme hardness, to withstand repeated use. Without going into all the particulars of tempering, since i dont understand the entire process myself, a wheel stud, or any other hardened bolt, is done through a heat and chemical process, to allow that bolt to withstand extreme stress for long periods of time. Engine bolts, particularly head bolts, cant be tempered the same way, because the heating and cooling effect of an engine will just cause the bolt to become more brittle over time, and the heating of an engine can interfere with the chemicals used to harden a bolt, so instead of wasting the time and money to harden those bolts, they just say its better to replace them.

 

Flex plate bolts, on the other hand, are made to withstand extreme side to side loads, due the the flex plate spinning, and over time, the flex plate bolts can start to wear in the area right under the head of the bolt, and eventually, even though it might take many years, that area can actually be wore down enough to pop the head off the bolt.

 

I worked at an engine machine shop in college, for about a year, and i learned over time that a lot more science and thought goes into the job of each bolt on the engine than you would expect, especially considering it's just a bolt.

[Megatron]

Thanks for the reply biggman100.

 

     I knew there was more to it than luck lol. I build communication towers and have been familiarized with some of the structural designed bolts like the A325 and A590's. I know the process to make an A590 leaves it unable to be galvanized after it is hardened. A325's however can still be galvanized but isn't as strong. I know each bolt is designed for its intended purpose but when it came to wheel studs nobody ever says much about them. I figured they had to be a good grade bolt given their job and abuse levels.

 

 I guess I will ask another question that I haven't figured out on my own yet and that is about measuring bolt stretch. I know this is the latest craze in the way of measuring  torque value, but this can only be applied to a bolt that can be reached and measured from top to bottom correct? It may sound like a stupid question, but its one I have never seen an answer for yet. I see where they do it on rod bolts but I have never seen anybody measure head bolts this way. How much more important is bolt stretch compared to torque yield?

[Joe Friday]

A simple answer would be, "No." You don't have to replace your OE studs unless you can no longer reach the specified torque without slipping. The stud spec I have in front of me (Jeep CJ) says they start with SAE 1020 steel, roll the threads, and harden to a Brinell Hardness of 248 (very hard). The tensile strength is over 110,000 PSI.

You really don't stretch passenger car/suv studs like the stretch on a head bolt. You need to stretch the head bolt in the elastic yield range to reduce the effects of fatigue as mentioned from heating/cooling cycles. You never want to have the bolt get cold enough to shrink and lose torque.

Wheels are typically designed to flex a little and act like a big lock washer. The exception to this rule is what the wheel balancer companies erroneously refer to as Hub Centric or Lug Centric. If the wheel does pilot on the hub or 'spigot', then the wheel probably does not have tapered lug nuts, but instead has collared lug nuts with washers. These washers are not flat, and they act like a bellville cone to provide torque retention for the nuts. I read somewhere (a Toyota Bulletin I can no longer find) that after three tire changes, or when these washers lose their cone shape, whichever comes first, the washers should be replaced.

[Moses Ludel]

Joe Friday mentions an interesting test for stud fatigue: You can no longer achieve the right torque. So, to apply all of this to Megatron's question about stud/bolt replacement, wheel bolts/studs last because they are rolled threads, have relatively high tensile strength (though 1020 steel in the OEM Jeep case is not extraordinary metal and likely selected for its ductility), and wheel bolts/studs do not go through the heat cycling stresses like a cylinder head bolt, which needs "elasticity". Also important and stated in the discussion, the wheel rims act like spring washers (steel rims, anyway), and some wheels use cone washers and nuts on the studs. 

 

So what about alloy wheels?  We apply torque evenly to nuts securing alloy wheels. Sometimes alloy rims have capped nuts with longer threads, most OEM wheel nuts are not capped. It appears that the only resistance to these nuts loosening (on alloy wheels) is the clamping force of the alloy rim hub against the wheel hub or hub/drum. 

 

This raises a question: When we install alloy wheels, what is the equivalent of the "spring effect" you get with steel rims and conical seat nuts?  With alloy wheels, are we strictly dependent upon the clamping force of the rim against the wheel hub face when the nuts get tightened?  Alloy rims have very little "yield" and certainly do not act "springy" like steel rims with raised, conical nut seats...Other than uniform nut torque and clamping force, what, if anything, keeps the conical nuts from coming loose on alloy wheels?  

 

Moses