274 replies to this topic

[Johnny D]

I check for play in the hubs after each session, It's when, not if.

Had new hubs on for one weekend, next weekend on Sunday, there's the play.

J~

[Keith Novak]

The stud to hub system stresses follow the "thick-wall" model, as the thickness of the material is >> than 1/10th the radius of the hole.  In the thick wall case, the tangential (parallel to the hole) stresses are much greater than the radial (perpendicular to the hole) stresses.  Hoop stress is the tension stress measured at the inside wall of the hole, and would result in a failure at the hole itself.  If the forces induced by the press fit interference are at play, then the primary failure stress would be tangential stress, not radial or hoop.

My stress analysis is obviously rusty.    My thought was that since fatigue is all about the tension side of the cycle, and the tension in the metal caused by the press fit would add to the tension in bending and contribute to the fatigue.  The moment would be greatest nearer to the bearing and be the primary stress component so rather that would still be the primary failure mode, helped along by the stressed from the press fit.

[Glenn Davis]

I check for play in the hubs after each session, It's when, not if.

Had new hubs on for one weekend, next weekend on Sunday, there's the play.

J~

Play in the hubs is most likely in the bearing/race system if the hub is still intact.  The fracture scenario probably happens very quickly.

 

The ductility of the steel depends on, among other things, the amount of carbon.  If the hubs are high carbon, their ductility is lessened so there ability to handle stress just reaches a certain point and the system fails as opposed to reaching a well-defined yield point and entering a protracted elastic region where the material recovers and then a plastic region where the material deforms in reaction to stress.  I think one would need to be very luck to discover this off the track.  I have not had the failure, but I'm guessing that there is some small amount of time as the hub begins to fracture when it can be felt.  As soon as the fracturing starts, it leaves the rest of the hub to pick up the loads of the now fractured section.  The rest of the failure probably progresses very quickly.

[Johnny D]

No expert again. I wouldn't think it would happen all at once.

One crack, couldn't the rotor/wheel, bridge across for strength and band-aid it ?

2 cracks is where you start to see it fail, correct? And how long does that take?

IDK,

J~

[Glenn Davis]

My stress analysis is obviously rusty.    My thought was that since fatigue is all about the tension side of the cycle, and the tension in the metal caused by the press fit would add to the tension in bending and contribute to the fatigue.  The moment would be greatest nearer to the bearing and be the primary stress component so rather that would still be the primary failure mode, helped along by the stressed from the press fit.

Agree with all of this on the fatigue side.  I have no idea how fatigue might lessen the ultimate strength of the hub over time.  The tangential stresses of the press fit are actually compressive.  In a right hand turn, the inside of the left hub, at the bottom of the rotation, is in compression.  This would add to any tangential stresses created by the press fit.  I'm sure shear stresses, as well as other dynamic forces play a role.

[Keith Novak]

Agree with all of this on the fatigue side.  I have no idea how fatigue might lessen the ultimate strength of the hub over time.  The tangential stresses of the press fit are actually compressive.  In a right hand turn, the inside of the left hub, at the bottom of the rotation, is in compression.  This would add to any tangential stresses created by the press fit.  I'm sure shear stresses, as well as other dynamic forces play a role.

Now that I think more about it, your explanation of the stresses at the holes makes much more sense. 

 

I'm not an expert on fatigue by any means but what little I've picked up goes about like this:  Microscopic cracks begin to form at the surface.  With a rough casting surface it's worse because you have more irregular grain boundaries in the material.  The cracks cause both a stress concentration and effectively reduce the moment of inertia reducing the ultimate strength.  The cracks grow and it gets worse.

[Bench Racer]

 The ductility of the steel depends on, among other things, the amount of carbon.  If the hubs are high carbon, their ductility is lessened so there ability to handle stress just reaches a certain point and the system fails as opposed to reaching a well-defined yield point and entering a protracted elastic region where the material recovers and then a plastic region where the material deforms in reaction to stress. 

From my side of the fence, investigation first it seems would be that those with failed hubs should have a material inspection/hardness test completed. Thoughts???

 

This hub failure seems as tho it is/or will be a major player causing crashes $$$ 

[Glenn Davis]

From my side of the fence, investigation first it seems would be that those with failed hubs should have a material inspection/hardness test completed. Thoughts???

 

This hub failure seems as tho it is/or will be a major player causing crashes $$$ 

A quicker look would be to investigate the dimensional variations in studs and hubs.  I have not seen a true engineering drawing of a hub, nor a stud, with the tolerances.  We may just be dealing with the odd case where a stud diameter on the very high side meets with a hub hole diameter on the very low side.  When you combine maximum interference with the ever-increasing stresses caused by increasing forces involved with grippier tires, more powerful engines, heavier cars, etc., we might just be seeing the edge.

[Keith Novak]

From my side of the fence, investigation first it seems would be that those with failed hubs should have a material inspection/hardness test completed. Thoughts???

 

This hub failure seems as tho it is/or will be a major player causing crashes $$$

Aside from process controls WELL beyond what they use for hubs, castings result in the least controllable material properties.  I would imagine you'll find a wide standard deviation in measurements that will make it difficult to draw valid conclusions from the data.

[Keith Novak]

A quicker look would be to investigate the dimensional variations in studs and hubs.  I have not seen a true engineering drawing of a hub, nor a stud, with the tolerances.  We may just be dealing with the odd case where a stud diameter on the very high side meets with a hub hole diameter on the very low side.  When you combine maximum interference with the ever-increasing stresses caused by increasing forces involved with grippier tires, more powerful engines, heavier cars, etc., we might just be seeing the edge.

It's probably easier to control the dimensions than the material values.  Fasteners usually hold very tight tolerances.  The machined surfaces in the hubs are the most controllable dimensions.  Typical tolerances that I see for machined parts are a lot better than B basis vs. A basis material values and cast parts don't fit into either of those buckets.

[Bench Racer]

Don't disagree guys. There are many contributors to this failure.

Source of hubs, OEM or OEM salvage yard, aftermarket?

Newish or oldish hubs?

Hardness of flange failed hubs versus hardness of outer bering race failed hubs. 

Also agree not much we can do about hub manufacturing except maybe learn which hubs are better.

 

Can't cost a fortune to have a steel quality control/analytical house do a hardness check.

[Ron Alan]

Agreed.  Whether it was 3 lugs or 8, IF and once you made it fail at this part of the joint, it would break along straight lines between studs, similar to above.  Do you agree that excess compressive/"hoop" stresses around and caused by too much intereference in the stud holes could be contributing?

 

The original poster's pictures look to me like the breakage at "5 oclock" was breakage that started with a crack (and post-breakage "beach marks" right at the stress riser, versus the other 3 locations appearing as more "ductile" failures.

 

The details of the joint (the spacer, the rotor, the wheel profiles) obviously matter. 

 

Has anyone experiencing the failure had wheels come in with mysteriously low lugnut torque?  That is often a sign that something in the joint is wonky (e.g. the rotor holes needing chamfered, powdercoat or paint chipping away under the lugnuts on new wheels, etc.), and it CAN start the localized fatigue - this seems to be what happens when the rear hubs fail (it is often preceded by mysterious need to retorque one or more lugnuts after a session). 

Cant speak engineer...but from my basic blue collar knowledge, is it a mistake to re-torque right off the track when everything is hot? Did this once when there was a driver complaint of vibration(flat spots...rookie!)...every lug nut seemed loose. Later that day when i went to change wheels...my impact could hardly take off the lug nuts! What i thought was 80ftlb was more like 100+???

[Alberto]

Lug studs were mentioned a few times in this thread with the word safety attached to them.  Can someone explain to me why longer lug studs offer safety benefits?  

 

Lug nuts thread into the stock studs the same amount on both stock, OEM 14" wheels as they do on my 15" x 7" +25 D-Force and Team Dynamic wheels so I don't understand how stud length plays a role in safety.  If it did, wouldn't whatever "un-safe" issue exist in the OEM configuration?

 

Thanks.

[ChrisA]

Also agree not much we can do about hub manufacturing except maybe learn which hubs are better.

Unless we can source a higher performance unit and get a rules allowance due to safety factors.

[Keith Novak]

Lug studs were mentioned a few times in this thread with the word safety attached to them.  Can someone explain to me why longer lug studs offer safety benefits?  

 

Lug nuts thread into the stock studs the same amount on both stock, OEM 14" wheels as they do on my 15" x 7" +25 D-Force and Team Dynamic wheels so I don't understand how stud length plays a role in safety.  If it did, wouldn't whatever "un-safe" issue exist in the OEM configuration?

 

Thanks.

The ARP studs which is what people are generally talking about are significantly stronger than stock studs although I don't actually know what the strength rating is on the stock ones off hand.  They also allow complete thread engagement of the lug nuts when using spacers.  The rule of thumb is you always want at least 2 threads sticking out of the nut to ensure you're getting all of the available strength.  It's been a while since I've had stock studs, but I remember when I put 5mm spacers on for +30 wheels I wasn't getting full thread engagement and wouldn't run the spacers.

[Glenn Davis]

It's probably easier to control the dimensions than the material values.  Fasteners usually hold very tight tolerances.  The machined surfaces in the hubs are the most controllable dimensions.  Typical tolerances that I see for machined parts are a lot better than B basis vs. A basis material values and cast parts don't fit into either of those buckets.

I'm thinking that parts from the same manufacturer are very similar and have little variation.  Parts from different manufacturers may not be as similar.  So some combination of hubs and studs might be worse for interference stress.

[Parity]

 

 

Can't cost a fortune to have a steel quality control/analytical house do a hardness check.

I've got a rockwell hardness tester. Getting a proper reading on a failed part maybe a bit of a challenge if the hub is distorted. It needs to be able to rest flat and fairly balance or the readings will be too soft. Also, that is a superficial inspection as it really doesn't look at the substrate material. A proper failure analysis would entail cutting a test section of the ruptured area and examining under magnification (400%).

[Blake Thompson]

We need to start retaining the failed parts, Hubs. Maybe we can talk Bennett (saul) into doing some metal fatigue/ testing,

Hub failures with out warning is scary stuff and could end up getting someone hurt

 

Note to Racers, save your failed parts lets look into it further rather that guessing.

 

Mark are you interested in looking at some failed hubs??

 

There is a train track that runs by my shop.  I should just load all this stuff in a train car or should I buy an Intermodal box?

[Johnny D]

I wonder if any annealing is going on ?

J~

[Bench Racer]

I've got a rockwell hardness tester. Getting a proper reading on a failed part maybe a bit of a challenge if the hub is distorted. It needs to be able to rest flat and fairly balance or the readings will be too soft. Also, that is a superficial inspection as it really doesn't look at the substrate material. A proper failure analysis would entail cutting a test section of the ruptured area and examining under magnification (400%).

Somewhat understand the intricacys of testing steel. The fractured cross section of the second picture posted had suspect looking material color/density. If the material is suspect many things being discussed become secondary. Blueskying is occuring and I am not saying the things being talked about are not relevant.

Saul has a video http://www.youtube.c...h?v=ls9rR1yCwrs of improved front hubs. He has considerable more knowledge of these hubs. He has hardness tested the outer bearing races.  Don't know if he hardness tested the hub/bolt flanges.