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Reasoning behind hub-centric and lug-centric wheels ?

  1. Aug 1, 2014 #1

    jim hardy

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    What's reasoning for hub-centric vs lug-centric wheels ?
    (I found those terms on a Chevy performance forum... )

    I grew up in the fifties when life was simple.
    Dad's old '46 Plymouth wheels had a big hole in the center to clear the hub and the tapered lug bolts pulled the wheel into final alignment. Lug-Centric?

    My 92 Oldsmobile wheels have a center hole, with five flat spots radially aligned with the lugs. These flats extend maybe 0.020 inside periphery of what would be otherwise a perfectly round hole with clearance for the hub. The flats slide over the hub with a mild friction fit.
    I assume those are to center the wheel perhaps more precisely than the lugs would, particularly were the lug nuts tightened unevenly or one left off. Hub-Centric?

    Well, that's a nice idea for the factory guy
    but after a couple of winters out west in ice & snow country the metal parts rust and those flats grab the hub with a vise- like grip because rust expands
    meaning
    when you get a flat tire you cannot change it because the wheel won't come off the hub without a sledge hammer.
    So it's a terrible idea for the vehicle owner .

    Today i took a wire brush to the hubs,treated the rust with phosphoric acid, applied liberal anti-sieze compound (copper Fel-Pro_)

    then i took a grinder and removed those flats from the wheel's center hole.
    The wheel now slides over the hub with comfortable wobble-room, ~0.030.

    Have i violated some fundamental law of Automotive Engineering ?

    It struck me that the zero clearance hub fit might be to prevent movement should somebody forget to tighten his lugnuts; thereby postponing when the wheel works its way off.


    Thanks,

    old jim
     
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  3. Aug 1, 2014 #2

    Simon Bridge

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    OEM (i.e. factory fitted) wheels are designed so the central bore of the wheel exactly fits the axle it has to sit on. That way the center of the wheel supports the weight of the car, not the lug nuts. The nuts are just supposed to prevent the wheel coming off laterally. If there is a gap between the bore and the axle you get more metal fatigue.

    To get around this, you can fit a hub-centric spacer to fill the gap.

    http://tires.about.com/od/understanding_wheels/a/hub-centric-vs-lug-centric-wheels.htm
     
  4. Aug 2, 2014 #3

    Ranger Mike

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    it Embellish what Simon has stated...The mounting boss is present on the axle flange as well as the spindle nose on the front end. This large diameter boss provides a very tight fit to the wheel center I.D. Makes a real problem if you need to change the wheel out when the vehicle has been run a long time (thermal expansion). This larger diameter spreads the load over more area than the small diameter lugs. Lug nuts do have a taper face to fit into the taper seat of the wheel. I had a 3/4 ton Van I towed with and this design permitted a lot of weight with relatively inexpensive wheels and non floating axles.

    Jim...caution- 0.030" run out axial or radial is at max end of acceptable for wheel/tire run out.
     
  5. Aug 2, 2014 #4

    jim hardy

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    I never had any trouble hot or cold until the rust set in. Then whooo-eeee are they stuck !

    I've only enlarged one wheel.
    The rest i think i'll file just a couple thousandths off the flats so i stand a better chance of being able to change a flat on the roadside..

    and do a better job of keeping after the rust. Annual cleanup and reapply anti-corrosive paste.

    That sounds plausible.
    I had assumed the lug nuts apply sufficient clamping force that rim to hub friction precludes motion, the friction fit at hub being only a backup for loose lugnuts.

    Likewise plausible if there's insufficient clamping force, for that would allow the parts to move .

    What prompted all this is yesterday i had to fix a flat.
    Of course i had rust after last winter in Idaho, and had to sledge hammer the rim off the hub.
    Good thing i was at home not on a roadside someplace.

    Also i encountered another of my pet peeves which is damaged lug nuts from over-torquing.
    All too often your entry level tire technicians just lean on their impact wrench which if it's a big one can torque your lugs to ~400 foot-pounds.
    Then they check with their clicker torque wrench, "Yep it clicks so you're torqued to 95 ft-lbs" when all they've shown is it's torqued to more than that and by who knows how much.
    That overtorque deforms the threads someplace, and i think they intentionally make lug nuts quite a bit softer than the studs so they'll deform first.

    On the way home from town with my repaired tire and twenty new lug nuts, i encountered an almost accident. A Jeep with fancy chrome wheels was stuck in the center of the highway with left rear wheel detached but still in the fenderwell. I got out my jack and lug wrench to help but all five of the studs were snapped off .
    I'm pretty sure those kids had some sort of adapters on their hubs to accept those fancy oversize wheels. I'd wager they had insufficient clamping force so had minute motion, leading to fatigue failure of the lugs.

    That got me thinking about how that wheel to hub interface is engineered to work, hence this thread.
    Thanks to both of you for your input. I'm not a mechanical engineer, but have read a bit about bolts and torquing.

    I always lubricate lugnuts because lubricated threads make more tension(clamping force) per ft-lb torque than do dry ones. And the grease slows down corrosion...

    I'll keep an eye on that one wheel where i've increased clearance. If the lugnuts show signs of loosening i guess i'll have to replace it.

    I found a pretty good article on the subject here:
    http://www.meaforensic.com/wheel-se...etallurgical-expert-mark-bailey-mea-forensic/


    figure-6-large.jpg


    Thanks again

    old jim
     
    Last edited by a moderator: Apr 17, 2017
  6. Aug 2, 2014 #5

    Baluncore

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    Trucks have LH threaded nuts on the LHS to stop them coming undone due to wheel rim distortion counter rotating the nuts.
    The effect is significantly reduced by hub-centric positioning.

    Cars have RH threaded nuts all round, so hub-centric is very important on the LHS.
    I hope it was the RHS you modified.
     
  7. Aug 2, 2014 #6

    jack action

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    Not an expert on the subject, but I have difficulty believing that the difference between the 2 mounting methods is to determine where the vertical load will be held. If it was so, I'm sure we would see some serious warnings against using lug-centric wheels on hub-centric axles. And if the hub was meant to support weight that the lugs can't, I doubt that using a plastic spacer would do the trick as some sites have suggested when using lug-centric wheels on «hub-centric» axles.

    To me, it looks like the difference lies only in the centering method of the wheel on the axle (as the name suggests).

    The most important difference about a hub-centric and lug-centric wheel seems to be in the balancing method of the wheel.

    If there is a tight fit between the hub and the wheel center, than it is automatically centered by the hub; If there is not, then it is centered by the lugs by default. No matter what, the force transmission of the wheel to the axle - in any directions - has to be assumed to be done by the bolts (and the friction they create): Anything else would be a major safety concern knowing how wheel centers and hubs shapes can be alter by wear (i.e. rust) and how popular is replacing OEM wheels with aftermarket ones.

    Just my 2 cents ...
     
  8. Aug 2, 2014 #7

    AlephZero

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    The only sensible way to transmit a load through the hub would be if the hub and wheel were slightly tapered, and the wheel bolts clamped them together. Otherwise, they would only be in contact at one point, and the wheel would have to "roll" around the hub as it turned. That would cause wear, and the "rolling" would tend to fatigue the bolts.

    I suspect the reason is more to do with cost saving and automated assembly. In the UK, older cars usually had studs fixed into the hub and the wheel was held on by nuts. The wheel can "hang" in a stable fashion on the top one or two studs, while you fit the nuts.

    Modern cars often have bolts that screw directly into the hub, which probably saves a few cents by not having to assemble the studs to the wheel bearing, and makes the bearing assembly smaller because it doesn't have studs sticking out of it. The small clearance at the hub is then useful to locate the wheel correctly while fitting the bolts.

    Two comments on wheel balancing: first, "standard" vehicle wheel balancing is not a very high precision operation anyway, and second, if you really want to do it right, you can balance the wheels after fitting to the vehicle - but the average cut-price tire shop might not have the equipment to do that.

    As a check for loose wheel nuts, large vehicles in the UK and Europe are often fitted with tags under each wheel nut as visual indicators both for loose nuts (the tags centrifuge outwards) and overheating wheel bearings or brakes (the tags change color or even melt).

    wheelcheck2.jpg
     
    Last edited: Aug 2, 2014
  9. Aug 2, 2014 #8

    jim hardy

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    Thanks Jack for that link
    it contained this reassuring line:
    The center hole is grabbed by a cone for balancing - so it makes sense that GM could use those flat tabs to define the center hole down to last few thousandths once the rim center and outside pieces are welded together. So i might have adversely affected the ability to balance this one - depending on how uneven i left the hole.

    I'll see how this one balances compared to the other three in a few years when i need tires.. ...probably it'll be ready for the crusher by then - '92 Custom Cruiser stationwagon - unless they become collectible.
    It's not so pristine as this one...
    http://www.edmunds.com/oldsmobile/custom-cruiser/1992/
    1991_oldsmobile_custom-cruiser_wagon_base_fq_oem_1_500.jpg

    RE lug bolts versus studs and nuts:

    Everything i've owned that was newer than 1949 had studs and nuts like Aleph describes for older cars.
    Dad's '46 Plymouth had bolts that screwed into the hubs, as Aleph described for newer UK cars.
    A pin on the hub fit into a hole in the rim so you could hang the tire/rim while inserting the bolts.
    Left side was left handed as best i recall.


    I'm thinking the hub-centric friction fit onto center boss is there for a backup, to reduce motion between wheel and hub in the event of loose lug nuts , and perhaps to limit travel in event of a catastrophic lugnut failure from some single cause like Beavis&Butthead torquing them to 500 ft-lbs . I put on new nuts and torqued them to 80 ft-lbs lubricated, will recheck after a few miles. The studs did not appear elongated, just the nuts had been yielded. I think i remember that tire shop, i grimaced as i watched the guys lay on that impact wrench then check with a clicker exactly like i described in earlier post...

    thanks guys i've learned a lot.

    old jim
     
    Last edited: Aug 2, 2014
  10. Aug 2, 2014 #9

    jim hardy

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    uh-oh i may have messed up that wheel

    http://www.croberts.com/Automotive-wheel-detachment.htm

    Fig 1A:
    image001.jpg



    Fig 1B , note ~3/8 inch gap around hub :
    image002.jpg

    Scary article, that one .
    Just what Simon and Ranger Mike said earlier.

    Live and learn.

    Looks like the right thing is to keep after the corrosion. Annual cleanup.

    Thanks to all -

    old jim
     
  11. Aug 2, 2014 #10

    AlephZero

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    The "lug-centric failure" pictures in that link don't seem to show the feature on the last lug-centric design I had, namely that the wheel bolts mated with countersunk holes in the wheel, to give positive location of the wheel position.

    I'm not convinced by the statements that a "loose push fit" central hub will carry any significant loads, but it's a nice location feature, both when attaching the wheel and for balancing.

    shopping?q=tbn:ANd9GcQhhXFQTAVXQ0XDZoeSMWqTWbLipYE4koFc-Vb-x_cTcAmlKltY&usqp=CAE.jpg
     
  12. Aug 2, 2014 #11

    jim hardy

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    Thanks Aleph.

    I'll replace that wheel anyhow.

    Now i'm contemplating how the center of the rim works in the mechanical design. It's far from a flat plate, a complex curved structure.

    As i said, those flats in the centerbore that contact the hub are aligned radially with the studs.
    Why ?
    Now - to mechanical engineers everything is a spring.
    I think that tightening the lugs deforms the center of the rim ever so slightly causing those flats to move inward a few thousandths to grip the hub more tightly as you approach the final torque .
    So it's a loose fit that tightens up when the lug nuts flatten out the center part of the rim.
    So long as the deformation is elastic they'll release when the lugs are loosened, allowing a tire change.

    But that's only a suspicion at this point.

    Anyhow - if it's right i have removed that feature from one wheel so will replace it.
    Or it could be that Beavis&Butthead with their "Terminator" impact wrenches yielded my rim resulting in too tight a grip by the flats.

    Either way ---- it's
    Off to the salvage yard Monday.

    old jim
     
  13. Aug 2, 2014 #12

    Baluncore

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    One day last year I noticed that one of my truck's lug nuts was a few turns loose. The wheel was still centred by the hub as the lug nuts stayed on, but most with between ¼ and ½ turn of slack. Long experience tells me that if one is loose, then they will all be loose. Without the hub-centric design, all my studs would have been damaged. I might have noticed the problem earlier. I put the loosness down to flexing of the rim while driving heavy loads on 200 km of winding sealed road. I do not lubricate lug nuts, it is just to risky.

    There is a simple solution to the corrosion problem. Rotate your wheels on the same day every year, lube the hub centre, NOT the lug nut threads.
     
  14. Aug 2, 2014 #13

    jack action

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    I have difficulty believing it is that easy to move a wheel that is correctly tightened on a hub. Let's throw some numbers:

    5 bolts, grade 8, ½" dia.
    Bolt tensile strength (T): 150 000 psi
    Bolt tensile stress area (A): 0.160 in²

    Bolt shear stress: 0.6TA = 14 400 lb (5 bolts: 72 000 lb)
    Bolt preload: 0.75TA = 18 000 lb (5 bolts: 90 000 lb)
    Wheel/Hub friction coefficient: steel/steel 0.5-0.8; Aluminum/steel 0.61 --> say 0.6
    Friction force Wheel/Hub: 0.6 * 90 000 = 54 000 lb

    You need 54 000 lb of force to move the wheel on the hub and then you need another 72 000 lb to shear the bolts (well, they are already in tension, so it will be less) - and that's per wheel ! For a 4500 lb vehicle, that's a hefty safety factor.
     
  15. Aug 2, 2014 #14

    Simon Bridge

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    Not a safe assumption ;)

    [edit] Aside @ jack-action: believe it!
    Those figures involve some unsafe assumptions - like that the damage is all in one go.
    Even accepting your figures - you need a comparison to decide if 54000lbs force is large. It's large compared with the weight of the truck - but that just means that the studs won't shear from just sitting there ... but what about lateral loading when the vehicle is in motion? What about just deformation sufficient to cause a vibration?
    I'd be interested in seeing figures for that.
    Then again - how good is the opening assumption in real life?

    ... quite as expected - after-market wheels are usually lug-centric so they can fit a range of shafts. The alternative is to make the customer get their new wheels machined to fit (or make a different wheel for every shaft size - expensive).

    Mind you, none of the discussions come with figures and there are other factors to consider.

    See link post #2:
    There are some people who will say that driving on lug-centric wheels doesn't really matter as long as the lugnuts are the self-centering cone type, as they will adequately center the wheel. These people are wrong. Driving on lug-centric wheels means that any impact will apply shear force to the lug studs, forces at 90 degrees to those the studs are designed to handle. This can cause the lug studs to bend, leading to a vibration in the car as the wheel slips around on the mounting plate, and possibly damaging the wheel's center bore if it has enough play to contact the axle.
    ... mind you, in practice this would just mean a bit more maintenance for the mechanically aware. I suspect most people are not that mechanically aware...

    The article goes on to point out that spacers are available - the focus there is on "regular" motor vehicles and people who want to customize them without much mechanics knowhow, so the bottom line is: all other things being equal, fit hub-centric.

    @Banucore: just vibration will loosen nuts, and you can get deformation in the studs even when the nuts are tight - depending on load etc. Nice example though. I'm unfamiliar with trucks - I had to learn this stuff when I was fixing-up a small performance car.
     
    Last edited: Aug 2, 2014
  16. Aug 2, 2014 #15

    Baluncore

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    But there are hidden effects.

    The lug nuts are effectively only patches of contact. The rim is usually pressed from 3.125mm steel plate that repeatedly flexes harmonically, so the nuts must be treated as independent contacts. They can therefore walk independently at an impact of less than 54k/5 = 10,800 pound = 4.9 tonne.

    It is not static steel on steel friction. Paint, oil from the threads or water, between the steel surfaces is a lubricant.

    The stud is perpendicular to any sliding movement so the tension will be enormously magnified by the sideways force once it does move. That is eliminated by hub-centric design.
     
  17. Aug 3, 2014 #16

    Ranger Mike

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    Jim..from a safety point of view..you are probably ok..from a vibration and wobble side of it..you better replace the wheel...i assume you have a mini spare tire..and can not rotate the " modified' wheel for the spare ...
    live and learn...when we are too old to learn something new...its time for the dirt nap!
     
  18. Aug 3, 2014 #17

    jim hardy

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    Guys i'm replacing the wheel for two reasons.
    First is vanity - i'm chagrined that i don't understand the mechanics going on when lugs are tightened and that wheel's center flexes in compression and torsion combined. It's a complex shape and you can tell it's deforming elastically by watching the torque wrench on last couple of turns.
    I looked at my Ford Ranger and saw similarities in the centerbore design - also a five sided hole having flats aligned with studs, but small differences from GM. I guess they work around one another's patents..

    So i'm embarassed to admit this, but i don't know how that deceptively simple rim/hub interface works for stamped steel wheels. It's clearly more than just two flat surfaces in contact. So i'm not going to "fix" something i dont understand.

    I admit defeat. Temporarily.
    SAE has some interesting looking publications on wheels but they want seventy bucks just to read them.

    Second reason is "What if?" .
    I'm aware from working around ME's that a reversing load is far worse for fatigue than a varying unidirectional one. That's why i wasn't worried provided there's sufficient clamping force to prevent motion so the bolts see only tension not bending .
    But now i understand that the hub-centric contact severely limits movement should that clamping force be lost for whatever reason. That would cause the bolts to see a bending load that reverses with every rotation of the wheel.. When one starts giving away his designed in backup mechanisms he is being reckless.

    God Forbid, should that wheel detach like the one on those kids' jeep did(post #4), liability would be on me for the modified centerbore. I'm getting old and risk averse. So to the spare compartment that wheel goes, permanently marked "Spare Only - Bad Center"

    Thanks Ranger Mike for the kind words - indeed , everything i learn makes me more aware of my abysmal ignorance. But we all started out knowing nothing at all, didn't we ? Improvement not perfection.

    Sigh. My neighbor up the hill just this morning put aftermarket wheels on his Chevy truck, twenty inch ultra wide chrome jobs. With 5/16" spacers between them and the hubs........ I'll emphasize to him to keep a close eye on his lugnuts...

    Thanks to all for your interest in this humble thread. If anyone knows of a scholarly article about why stamped wheel centers have the shape they do, i'd sure appreciate a pointer. It looks to me like a subtle mechanical design.


    old jim
     
    Last edited: Aug 3, 2014
  19. Aug 3, 2014 #18

    AlephZero

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    I agree there is a lot of unsubstantiated opinion on this - and most other auto topics of course.

    But engineering common sense says the loads go through the stiff load paths not the flexible ones, so if you can sense the 5 bits of sheet steel around the hub bending when you tighten the studs or bolts, I rest my case.

    As for the "54000 lbf dynamic wheel loads on a 4500 lb vehicle", are we talking about wheels fitted with tires and mounted on a spring-and-damper suspension system, or something else? :smile:
     
  20. Aug 3, 2014 #19

    jack action

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    I'm amazed that I'm the only one impressed by the 54 000 lb figure.

    OK, let's put some load on the wheel.

    Vertical force: Let's put 5000 lb on the wheel. That is the equivalent of the entire car sitting on a single wheel (all other three are in the air). The suspension is actually bottomed out at that point; a situation that [STRIKE]is not[/STRIKE] should not be typical driving. This is for sure a lot worst than the peak that you could get from any typical dynamic situation.

    Lateral force: A good hi-performance tire can have a friction coefficient of 1.0. (Most typical vehicles would have at most 0.9). So the max lateral force before slipping occurs would be 5000 lb.

    Longitudinal force: For simplicity's sake, let's assume the wheel is locked under braking and that there is also the maximal friction force in that direction, i.e. 5000 lb. Note that it is impossible to have the maximum friction force in both lateral an longitudinal directions. But let's say it does.

    Sooooooo ...

    If I have a 5000 lb lateral load on my «inside» wheel (let's assume it is pulling the wheel out and not pushing it in: Even though this is almost impossible to do in a real situation), the net effect will be that I will have 85 000 lb instead of 90 000 lb of preload on my bolts. That drops my 54 000 lb of friction force down to 51 000 lb.

    You know what? The 0.6 friction coefficient for the wheel/hub might be too optimistic (although possible), let's use the lowest I've seen up until now for steel-on-steel: 0.2. So the 51 000 lb goes down to 17 000 lb of friction force between the wheel and the hub.

    So the forces left in the wheel/hub plane (vertical and longitudinal) are both 5000 lb; Which give us a 7000 lb force when added up as vectors.

    17 000 / 7 000 = 2.4

    With all the crazy assumptions I've made - not even possible to repeat in real life - I still get a safety factor of 2.4. I'm still missing 10 000 lb of force to be able to slide one plate over the other. For this to occur, I would have to redo my calculations by assuming that there is a vertical force that is equivalent of two cars sitting on a single wheel ...

    Vibrations? They even use slip-critical joints for bridges BECAUSE there is stress reversal, heavy impact and large vibrations:

    If bridges can't create enough force and vibration to move a friction joint, it must be possible to do the same for a tiny wheel, don't you think?

    You can play it safe all you want, but I can't believe that there are other load bearing components between the wheel and hub apart from friction. It is certainly not a «loose-fit» hole stamped in a steel sheet over a 3" hub protuberance that will do the trick.

    It is most likely that hub-centric/lug-centric only defines the point of reference for the center of the wheel; an important detail when you want to balance the wheel separately from the hub.
     
  21. Aug 3, 2014 #20

    jim hardy

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    Thanks Jack.
    From your wiki link:
    That bit of civil engineering rather reinforces my initial thought that friction is the intended shear load bearing mechanism for automobile wheels.
    But it requires that the lugs provide sufficient clamping force.
    Personally i've never had lug nuts come loose, so can't add personal experience.
    I would think that were clamping force too light because of loose lugs, one would have fretting as the wheel moved minutely about its normal position, making iron dust .
    Photos of egged out holes suggest that motion can go on for a while.
    Photos of fatigue fractured studs support that too.

    How long until fatigue fracture set in i guess would be related to how far the wheel could move, ie how much bending strain it applies to the studs. In that scenario it would seem that limiting its travel by interference at hub would add a measure of safety against fatigue failure caused by loose lugs.

    This article is quite readable and explains several mechanisms that lead to loss of clamping force.
    http://www.meaforensic.com/wheel-se...etallurgical-expert-mark-bailey-mea-forensic/
    it can be as simple as not cleaning off the rust before mounting a wheel.
    figure-10.jpg


    Do you think it possible that while the sheet steel wheel bends as the lugs are tensioned, that loose fit might become a tight one? I am frequently awed by the ingenuity of mechanical engineers.

    Anyhow - seeing those kids the other day with their jeep wheel detached and all five lugs snapped off set me to thinking about this.

    old jim
     
    Last edited by a moderator: Apr 17, 2017
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