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Wheel lug torque

Discussion in 'Wheels & Tires' started by az3579, Jun 1, 2008.

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    az3579

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    I'm sort of confused on this one as I hear conflicting information on the issue.
    My Bentley manual says to torque the wheel lugs to a specific torque, specifically 74 ft. lb. +- 7 ft. lb. So, in my quest to dig for a torque wrench in the garage, my brother comes up to me and tells me not to bother, to just torque it as much as you can with your regular 4-way wrench thingie that you can get in any auto store. That's fine and dandy, but I don't possess the strength to even loosen the nuts nevermind torque them properly. Then he suggested the impact gun, which is fine, but only when I'm home.

    If I plan on attending track events, I definitely won't have access to an impact gun and all I will have is my trusty 4-way wrench and the option of a torque wrench.


    Now the conflict here is that my Bentley manual tells me overtorquing can cause problems and not just undertorquing. It says it could cause driveability problems like vibrations and whatnot. This is what is making me skeptical on my brother's advice; his advice is pretty much "as tight as you can get it". Also, when I used the impact gun, the torque seemed to be fine as I went for a test drive and found everything to be A-OK, but I don't know if that was because I torque-wrenched it AND afterwards torqued the wheels with the impact gun.


    What I'm wondering is, if I was to ONLY use the torque-wrench set to the specified torque, would it be just as effective as using the impact gun?
    • Member

    MGarrison

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    There's a couple reasons to torque the lug bolts to a specific amount. The bolts can only take a certain amount of torque before becoming over-stretched. If you don't use a torque wrench, you run the risk of breaking the lug bolts (I've had older lug bolts just snap being torqued to spec), plus you have no idea how much torque you're applying.

    It's difficult to gauge how much torque you're applying with any degree of accuracy. It's also important that the wheels, brake rotors, and hubs are evenly clamped by having the bolts evenly torqued. If the bolts are unevenly torqued, the different pressure being applied can allow for a brake rotor to warp as it cycles through thermal expansion/cooling.

    I've always used 85 ft-lbs as a stock torque spec. I find that my lug bolts will be loose after a 1/2 hr. track session using that spec, so for track events I torque them to 105 ft-lbs, at which point they don't loosen (however, I think that's above the spec for the bolt). I've found 85 to generally be fine for street driving, and probably fine for auto-x as well.

    Your lug bolts shouldn't be so tight that you can't loosen them using the oem lug wrench (what would you do to change a flat?). Torqued to 85 ft-lb's, you should be able to break them free using the oem lug-wrench; if they seem a little tight, place the lug wrench on the lug bolt, positioned parallel to the ground, and standing on it should break the bolt free (remember righty-tighty, lefty-loosey; except of course for the fan clutch nut).

    For changing a spare tire, presumably you would only be running on it until you get your primary tire replaced or repaired, so not being able to apply exact torque for a little while shouldn't hurt anything, but I don't think it's a good idea to just let it go for the long term.

    Never use an impact gun for final tightening, or torqueing to spec. I put the wheel on, hand thread in at least 1 or two bolts to hold the wheel to the hub, then hand thread the rest of the bolts enough turns to know I don't have any of them cross-threaded, and then, with the impact gun set to its lowest torque setting, zip the bolts in. My air gun has enough torque even at it's minimum setting that it will occasionally torque the bolt a little more than 85 ft-lb's, so if I find that's the case, I have to loosen that bolt and then torque it.

    With all the wheels on securely, flush to the hubs, and reasonably secured by the bolts, then I lower the car, and torque all 4 wheels w/ the torque-wrench. The torque-wrench should be your final step. If you use an impact after the torque wrench, then you've blown the steps you just did to torque the lug bolts, because now you no longer know that they are both tight, and evenly torqued, but only that they're tight.
    • Member

    az3579

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    Drat. I final torqued with the impact gun after using the torque wrench. I don't think the impact gun tightened the nuts any further, but looks can be deceiving.


    But it is possible to ONLY use the torque wrench to properly install the nuts, right? It should be "enough" and not require an impact gun as well?

    Also, I torqued my (lug) nuts to about 74 ft. lb. as per the Bentley manual, but you're saying you use 85. What's different on your car; what justifies the different torque?
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    johnh

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    Do NOT use an impact gun to tighten your lug bolts. Use a torque wrench. BMW for many years used the 74 ft lb spec but in recent years changed it to 88 ft lb, plus or minus a few. (SAVs are higher - don't have spec handy).
    The key is to torque them, tightening bolts opposite each other, not in a circle as you go around the wheel, and torque them all the same.
    • Member

    az3579

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    I'm aware of the torque pattern, but was unsure of the torquing tools to get the job done. It seems that the torque wrench is adequate to do so.

    That 88 ft. lb. I keep seeing; could that apply to all the passenger vehicles they have except the SUV's and the like? Any reason for the large jump from 74 to 88? Does that apply to mine?

    agranner guest

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    I wonder if you know what the torque spec is for.

    The specification is a measure of how much the fastener is stretched. Up to a point, more stretch means more squeeze holding your wheels on; just like a spring. Beyond a maximum, the fastener just stretches and doesn't pull back; not like a spring. This means that if you over-torque a fastener, you could actually make it looser. Of course you could also break it (I would prefer this). You need the fasteners to "pull back" enough to keep the threads locked and evenly so the wheel doesn't wobble.

    Any lever (socket wrench, lug wrench, breaker bar, torque wrench, impact gun, long piece of pipe) can apply torque to a fastener, but only a torque wrench can MEASURE it. Use anything you want to apply torque but you must measure it before you're done.

    PS. The great thing about an impact gun is you don't need the wheels on the ground to use it. That's because the lever is very short and a little hammer (basically) applies the force. You can get "torque sticks" which are sockets that are calibrated to twist a certain amount. Used with an impact gun, they limit the torque to a certain amount but only with an impact gun, not a wrench.
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    MGarrison

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    Ditto's on the above - you can use only the torque wrench if you want.

    It's important to make sure the bolts aren't cross threaded as you start them -
    After that, you want to snug them up to the hubs - use an electric drill, twirl a lug wrench, or whatever (an impact gun, particularly set to max force to be applied, as mentioned, is not a good idea - mine has 4 levels, and for putting on wheels, on the least setting, I lightly press the trigger just enough to spin in the bolts, and snug them to the rotors and hubs). Even the stock lug wrench will work for snugging them up.

    If the wheels are tight to the hubs, and you've applied enough force to the bolts that they're threaded all the way in and none are loose, but not yet torqued, then you can put the car back on the ground, and do the final tightening w/ the torque wrench.
    • Member

    az3579

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    I put the wheels back on a few days ago. The torquing was done with a torque wrench first and the impact gun as the final measure. I did all this before starting this thread.

    I used the car for a few days and there seems to be no issues with the way I put my wheels back on. If it's not an issue, can I just leave it or should I retorque the wheels anyway? Or could it still become an issue if I leave the wheels on like this for a while longer?

    agranner guest

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    The issue may be what I mentioned before: they might be too loose because you over-tightened them. (I know you over-tightened because the torque was correct, then you added more). Congrats on no problems so far. The other POSSIBLE issue is if you drive really hard for some time and heat the things up: i.e. track day. The problem is the stretch characteristic changes with heat.

    Here's an easy way to make sure you have the right torque. This will take less than 5-minutes, you won't have to lift the car, and you won't get dirty at all. Take the lug wrench (generic cross or the one in the trunk); pick a wheel; "break loose" the lugs (1/4 to 1/2 turn); put the 17mm (I think) socket on your click-type torque wrench and tighten each one until it clicks; look at the impact gun but don't touch it ;) repeat on each of the other three wheels. I do this before every autocross. It's REALLY EASY. There's no reason not to do this today.

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