Can Incorrect Wheel Nuts Torque Really Warp Disks

[xr6tForMe]

A while back when we had issues with brake shudder one of the possible reasons I read was "incorrectly tightened wheel nuts".

I have seen discussions about wheel nut torque and brake problems on the forum.

I just cannot see how this could contribute to the problem.

The nuts are pressing on a thick metal plate which is then separated from the actual braking surface by a thick metal tube. I cannot see how wheel nuts could deform either of these areas at all, let alone the braking surface.

Even if you over tightened 4 nuts and left one off altogether I cannot see how you could affect the braking surface.

Is it really possible that incorrectly tightened wheel nuts could cause issues, or is this just a cop out excuse for poorly manufactured brake disks?

[Dagabond]

Yes it can.

Tightening Sequence

That last question is where the class instruction started. In the old days, star-pattern tightening using torque wrenches was the only way lug nuts were tightened. Not any more. Today, it is throw the tire and wheel over the lugs, hand start the five lug nuts, then use an air wrench in a once-around circle pattern, and you are done.

Doing this, the first few wheel lugs lock the rotor into location while the last few wheel lugs pre-load the rotor, which is like slightly bending a spring. Even worse, even if perfectly tightened to 100 ft-lbs, the last few lugs are false readings. Some of the torque is absorbed in flexing the rotor, not tightening the lug. This is almost guaranteed to cause uneven rotor wear (incorrectly called warped rotors), which results in the tell-tale pedal pulsations after just a few thousand miles.

The unevenly torqued rotor, even with the correct amount of torque, will not be bent when the rotor is cold. However, as the rotor heats up in normal use, it will expand unevenly. The most uneven area will, of course, be near the first tightened and last tightened lugs. As the rotor heats up and expands, a runout will be caused, i.e., a high spot on one side and a high spot on the other side. These high spots will come into intermittent contact with the retracted pads during normal driving, i.e., without brake pedal pressure.

As this happens, the semi-metallic pad used with police cars will grind away the high spots on either side. On the other hand, the ceramic pad used with retail cars will transfer material to the high spots. In just a few thousand miles, the rotor will have a significant thickness variation, either from worn away rotor or material transfer from the pad. Rotor thickness variation causes brake pedal pulsation and steering wheel vibration!

Air Wrench Blues

The wrong tightening sequence, even with the right amount of torque, can cause a rotor problem. So can the right tightening sequence with the wrong amount of torque. (No, your air impact wrenches are not calibrated!) Here is what happens.

The air wrench increases or decreases in torque as it is being used. The line pressure falls, the air compressor kicks in, the line pressure increases. The result is that an air wrench nominally set at 100 ft-lbs may actually torque the lug nuts to 85, 110, 100, 90, 110, 90. Any variation above 10 percent is bad. The result is exactly the same in the situation where warming causes uneven expansion, as in the situation with the incorrect circular tightening sequence situation.

These two causes of premature rotor wear can be easily fixed. First, use a star pattern instead of a circle pattern to tighten the lugs. Second, for those using a torque wrench, first pre-tighten the lugs with the socket wrench, then tighten them afterwards with the torque wrench. Don’t overdo it with the socket wrench. The torque wrench must be allowed to tighten the lug at least a little. If it clicks-out without tightening the nut at all, the nut could be way over-torqued. That is what happens if you use a breaker bar for the initial tightening.

Third, for those using an air wrench, use torque sticks. Or set the air wrenches under the necessary torque and finish them off with a torque wrench. Rule one: star pattern. Rule two: torque sticks or torque wrench. Follow these rules every time you replace a tire.

[arronm]

A brake disc only needs a few thou runout to cause vibration.

[BRK]

It is a fair question and the answers are spot on. Yep, it can. Ask an engineer who is involved in any sort of big time work - like ship building - and they will tell you about tightening sequences and torque.

I have posted before but the guy I get my tyres from insists on the torque wrench. I won't argue with 40 years in the business.

[ratter]

there are torsion bars available to go onto an impact gun, the air pressure needs to remain consistent and when the gun is adjusted they can tension correctly

[masda74]

Brk, where do you get your work done in Perth.?

I have called a few places in the area and they all want to sell me new rotors and pads. They do not want to skim my DBA T3s for some reason.

My shudder is only noticeable from 80km/h and higher.

[PhilMeUp]

Brk, where do you get your work done in Perth.?

I have called a few places in the area and they all want to sell me new rotors and pads. They do not want to skim my DBA T3s for some reason.

For rotor machining, take them to Nick at Marfleet in East Perth.

He's good at taking off the minimum amount required.

http://www.whitepages.com.au/searchBus.action?name=Marfleet%20Brakes&location=Perth%20WA

[BRK]

I think as above for the brake skim. Tyres - a small place in Bibra lake / Spearwood - Classic Tyre and Battery. Have used Brakes West in Myaree but I am not familiar with their approach to the T3 rotor.

[discostig]

The slots could scare a few off, think it can damage the machine.

[stretch4660]

Interested to hear anybody's thoughts on having rotors machined more than once during their lifetime.?
8 years in the auto trades and personally I choose to only machine rotors once, before replacement next time the brakes need doing (yes rotors have minimum thickness' and you may be able to get a second cut out of the rotor but I chose to advise people to go for replacement on the second instance, which most of the time they agree)