Can anybody tell me what the wheelnut torque should be for a Subaru Outback 2.5 '56 reg. ? Alloy wheels, of course.
Two dealers have given me settings, one said 92 ld ft and the other 110 Nm. Ani fule no that these are different.
Advice, anybody ?
Edited by Pugugly {P} on 28/12/2007 at 19:52
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Autodata gives it as 90Nm.
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Owners manual for an '06 Outback gives 100 to 120Nm (74 to 89 lbf.ft)
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On a slight tangent, really how important is torqueing wheel nuts? In over 30 years of looking after cars I've never yet torqued a wheel nut. I tighten them with the tool you'd have to use to remove them if you had a puncture and just tighten until they feel 'right'. I've never had a problem yet.
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It can't be important - in 40 years no dealer or tyre fitter has torqued them properly !!
After warranty service or new tyres, I always loosen and re-torque the wheel bolts/nuts to the correct torque as I usually have to use my full 95kg bouncing on the extended wheel wrench to undo them - not something my wife could do if she had a puncture.
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Hello, Nick.
I've been driving since 1964, and I've had steel wheels until recently. Steel deforms slightly when you tighten up the nut, and then recovers to some extent (Youngs modulus). I've always booted the nuts free and then tightened them by putting my weight on the end of the wrench, say about 75 ld ft. Alloy is different and I imagine isn't as springy as steel. You are supposed to tighten the nuts to the specified torque and again after a set mileage to allow for the nuts to bed in.
When I had my tyres replaced at ATS Macclesfield the fitter did use a torque wrench to tighten the nuts.
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Alloy is different and I imagine isn't as springy as steel.
Can't vouch for other alloy wheels, but all the Vauxhall alloys I've had, they've had some kind of other material where the bolts make contact with the wheel. It's a brass colour, but not sure if it's actually brass or some kind of cadmium plated metal. One thing is for sure though, the bolts don't do up against the alluminium.
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Thanks DD. I'll check that.
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>>Steel deforms slightly when you tighten up the nut
All materials deform under load - the perfectly rigid material doesn't (can't?) exist.
Under an equivalent axial load, aluminium actually deforms 3 times more than steel - the Young's modulus for steel is about 210GPa, and that for Aluminium, typically about 70 GPa.
This is elastic deformation - there's no recovery involved. It's only when you go beyond the yield point of the material that odd things like recovery begin to happen in any significan way.
Alloy wheels usually have a steel insert, which bears against the steel nut. This is because the alloy contacting the wheel nut would deform in a plastic way. This is the so-called bearing stress. To obtain a proper bearing stress** is in IMO just about the only reason to ever use a washer, and so, the washer should always be of better material properties than the joint material you use it with. (And don't get me going about awful lock washers!)
Once you get through the contact area, the sress develops in the thickness of the alloy wheel in an approximate cone shape, spreading out below the hard washer - typically at an angle betwee 30 and 45 degrees. As the area under stress increases, the stress level decreases, so, after a short depth, the alloy is sufficient to bear the load - you don't need a cylindrical steel insert going all the way to the hub.
** by proper, I mean to make sure that bearing stress isn't the stress that limits the end-load developed in the joint - this limiting factor should always be the shank of the bolt.
Number_Cruncher
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I've had cars with alloy and steel wheels (not on the same car, before some pedant steps in :-)). Not had a wheel fall off yet despite doing it by feel. Perhaps I've been lucky or more likely it doesn't matter. Maybe I could sell my services as 'the human torque wrench'.
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Thank you backroomers
Thank you Ruperts Trooper. I looked again at the owners manual. I was expecting to find the data under specifications, but after much searching found the data on page 9.8 under maintenance. Where else, indeed !!
The recommended torque is indeed between 74 to 89 ldf ft, as it states, ??equivalent to applying approximately 40 to 50 kg (88 to 110 lds) at the top of the wheelnut wrench?. I presume they mean the end or it could be painful.
Nick, if you weigh 95 kg and you bounce on the wrench are you overdoing things a bit ? I weigh 77 kg fully dressed. I tried it out by putting the bathroom scales at about wheelcentre height and leaning on my fists. A comfortable steady pressure was 40 to 45 kg, but I could reach 50kg if I took my heels off the ground.
I had a quite delightful experience once when I was working in Rome. Late one evening by the Collesium I came across a young lady in a very fetching miniskirt standing on the wheelwrench and bouncing. She couldn?t shift the nuts at all. Being the gentleman that I was at the time I offered to help and booted them free with my size 10 veldtscoon. I tightened them again by leaning on the wrench in the approved manner.
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95kg bouncing on the end of an extended wrench was necessary to UNDO the bolts/nuts - that's far tighter than it should have been.
When correctly torqued I can undo wheel bolts/nuts without resorting to standing on the wrench.
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A few sums show that there's some margin in wheel bolt design - as indicated by practical experience, your car doesn't fall apart if you torque up the wheel bolts without a torque wrench.
bolt_dia=12e-3; % 12mm bolts
stress_area=70.5; % Stress area of M12 bolts
bolt_torque=100; % 100 Nm torque
bolt_yield=[0.8*800 0.9*1000 0.9*1200];
% Rough estimate of clamp load (N)
clamp_load=bolt_torque/(0.2*bolt_dia)
% Bolt Stress (MPa)
bolt_stress=clamp_load/stress_area
% Proportion of yield for 8.8, 10.9, and 12.9 bolts
prop_yield=bolt_stress./bolt_yield
clamp_load =
4.1667e+004
bolt_stress =
591.0165
prop_yield =
0.9235 0.6567 0.5472
The clamp load shows that each bolt develops about 40KN - that's equivalent to sustaining a weight of 4 tonnes!
The prop_yield result shows how much of the elastic range of the bolt is being used. For an 8.8 bolt, it would be 92% - which is why, you would hope that the wheel bolts are better than 8.8. (Actually, there are some cases where you might use such a large proportion of yield in tightening - but wheel bolts which are on and off regularly and under poor control isn't one of them)
For 10.9 bolts, tightening uses 66% of yield - there's plenty of scope for people overtightening without causing any serious problem.
As a hint, you can tell when a bolted joint isn't *super* critical. That's when you are given only a torque value.
The amount of variation you get in the pre-load of torqued joints is huge. Note, this isn't because the torque wrenches are poorly calibrated, or poorly used, it's because the relationship between torque and load in the bolt is dominated by friction - frction in the threads, and friction under the head of the turning bolt. Relatively small changes in friction can lead to large changes in bolt pre-load.
If a bolted joint really needs to be correctly assembled, you'll be given a torque plus angle spec.
It doesn't get much more involved on cars, but on more expensive equipment, you find there are greater levels of accuracy, with hydraulically pre-tensioned bolts, bolts with integral strain gauge force measurement, bolts with ground faces to enable micrometers to be used to measure bolt stretch, and bolts with ground faces to enable ultrasonic length measurement.
Number_Cruncher
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Thank you number Cruncher.
Give me time to think about that !!
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