Sponsored article presented by König Wheels.
Wheels are a wear item. The trick, though: Knowing when to pull one from service so a little problem doesn’t become a big problem.
“A wheel does have a lifespan, and unfortunately it’s not clocked on a mile basis,” explains Scott Weiss, marketing director at König Wheels. “It can last 10 seconds …
Metals have a fatigue life. Within a certain band of negative/positive stress (compression/tension) it can have infinite fatigue life.
Get out of that zone then you start counting stress cycles.
The greater the stress per cycle the fewer cycles the metal will last before cracking.
We have all probably read about metal fatigue being the root cause of some airplane crashes.
The compromises of weight vs strength for wheels, especially wheels used in racing, put them into the stress zone where fatigue life is used up and they must be changed.
Makes you think about the wisdom of buying used wheels from an upper level racing team.
^ interestingly, aluminum does not have an infinite fatigue life, no matter how low the stress.
From Wikipedia, a chart comparing S-N curves for steel vs. aluminum:
You can see that steel has an endurance limit. Below this amount of stress, you can cycle steel infinitely without a fatigue failure. By comparison, aluminum has no such limit. In theory, if you cycled aluminum with 5 lbs of force for an infinite amount of time, it will eventually fracture. Design also plays a critical role in fatigue life. The point being- those lightweight aluminum racing wheels we all love are more prone to fatigue failure than steel wheels or heavily built wheels, and should definitely be inspected frequently.
I'll be honest, I really take my wheels on my Fit for granted.
I think I'm going to give them a good look-over when I get home today.
In reply to jharry3 :
Yup, metals have a fatigue life. My favorite line in that piece:
“Wheels are metal,” Weiss continues. “They’re not magic.”
I have two sets of wheels for the Datsun, both are 13"x6", the Hotwires are 13lbs and the CSS (Shelby) are 12.5 per wheel. I could save 2-3lbs per wheel by going to something lighter but I know that I pound the kerbs mercilessly so I stick with the wheels I have.
I've seen people bend the lighter wheels by taking a lot of kerb, they only discover it because the car picked up a vibration.
On the used racing wheel front; I once got offered a set of magnesium wheels for super cheap and I politely declined.
Amazingly I've only ever seen one person with wheel failure trackside; that came from the wheel not seating properly on the hub. The wheel cracked and plucked the center right out of the rim. The remaining part of the rim was still firmly attached to the car......scary stuff kiddies.
I know of at least one person who uses dye penetrant testing around the holes in three piece wheels. Its semi cheap cheerful and easy to use.
In reply to ShinnyGroove (Forum Supporter) :
Good info for understanding the trends in fatige life. I hope people reading your post realize that these two curves really only represent one material spec each.
Different alloys, different heat treatments, different levels of cold-working, chemical exposure, heat cycles will have an effect on fatige life.
Not all steels will have a 29 ksi endurance strength, and cycles to failure may be different than the pot given here.
The cycles to failure are not deterministic, but typically given with a probability of failure. Stuff can break earlier or later than the graph might lead you to believe.
Interesting graph.
I raced with 5 year old steel rims for a bit - until I ripped the centre right out of one (centre remained tightly bolted to the hub). Switched to mag alloy wheels and no issues - still running when probably 40 years old (but that graph would make me leery if I still owned the car with the wheels). Now run (stronger) steel wheels (no options - OEM peg drive knock offs with alloy substitutes unavailable).
There was a popular Miata wheel that was cracking earlier this year. The vendor stated they were a wear item. Internet unhappiness followed.
I've been wondering for a while if a lug with a spinning seat (not mentioning brand names) would be easier on the wheels.
Question: So, I have had a bad vibration in the steering wheel from about 55 mph and up since I bought a 1978 Fiat spider 35 years ago. After 3 sets of wheels, numerous alignments, tightening all the bushings and replacing them and tightening the steering box on the right side, at least three sets of tires, Pirreli and Bridgstone, I still have the shimmy. Tightening the steering box helped the most. For the last 25 years I have had a set of wire wheels on my Fiat. The other day a fella jacked up my car and spun the front wheels. He measured the run out at .060 max., not at the edge of the wheel but on the inside of the outer lip. Do any of you fine, knowledgable people know if .060 is good or really bad?
I'm not a wheel expert, but 60 thousands of runout sounds like a lot to me for a heavy rotating object. That's 1.5mm.
BlindPirate said:There was a popular Miata wheel that was cracking earlier this year. The vendor stated they were a wear item. Internet unhappiness followed.
Yea verily, with much wailing and gnashing of teeth.
some_guy79 said:In reply to ShinnyGroove (Forum Supporter) :
Good info for understanding the trends in fatige life. I hope people reading your post realize that these two curves really only represent one material spec each.
Different alloys, different heat treatments, different levels of cold-working, chemical exposure, heat cycles will have an effect on fatige life.
Not all steels will have a 29 ksi endurance strength, and cycles to failure may be different than the pot given here.
The cycles to failure are not deterministic, but typically given with a probability of failure. Stuff can break earlier or later than the graph might lead you to believe.
All of this is true. Might be easier just to remove the Y axis and graph them separately to demonstrate the concept.
Also, if just demonstrating the concept, it would probably be better to call that blue line 'Ferrous' and red line 'Non-Ferrous'
john hess said:Question: So, I have had a bad vibration in the steering wheel from about 55 mph and up since I bought a 1978 Fiat spider 35 years ago. After 3 sets of wheels, numerous alignments, tightening all the bushings and replacing them and tightening the steering box on the right side, at least three sets of tires, Pirreli and Bridgstone, I still have the shimmy. Tightening the steering box helped the most. For the last 25 years I have had a set of wire wheels on my Fiat. The other day a fella jacked up my car and spun the front wheels. He measured the run out at .060 max., not at the edge of the wheel but on the inside of the outer lip. Do any of you fine, knowledgable people know if .060 is good or really bad?
That is very excessive runout. The runout on the brake rotor is only a max of .0059, and you have ten times that on the wheel. If you have gone through three sets of wheels, you may have a bent or damaged hub.
Good article, I mean I know metal fatigues but honestly I don't think about it for my wheels, which is a mistake I'm happy to remedy, so thank you
In reply to drock25too :
ThankShinny Groove and drock25too. I have been eyeing a new set of Cromadorawheels, well similar to because they are 15 inch, not 13 inch like the originals The hubs have been repacked several times, so i wouldthinkfolks wouldcheck that out but I havebeen thniking that too.
I have Hypergrams like in the picture above. I hope that's not a bad sign for me. Mine are bronze though, so I'm sure I'll be fine.
I have found more cracked track only wheels than i care to admit to. Generally find the crack on the backside of the wheel where the spokes meet the rim. I check wheels EVERY time they are off. Even the street cars. Never cracked a street car wheel without severe impact though.
john hess said:In reply to drock25too :
ThankShinny Groove and drock25too. I have been eyeing a new set of Cromadorawheels, well similar to because they are 15 inch, not 13 inch like the originals The hubs have been repacked several times, so i wouldthinkfolks wouldcheck that out but I havebeen thniking that too.
If your not looking for it, it would be hard to see. have them pull the tire and brake rotor off and check the runout on just the hub.
I have a dial indicator that resides on my wheel balancer for checking bare wheel run-out/bent wheels and marking high/low spots on the rim to minimize total run-out as measured on the tire.
Most high quality and straight wheels I will measure 0.008-0.015 radial run-out at the most as checked on the bead seats. Had a set of some $$$ 18x10.5 BBS that measured 0.005 on 3 of 4. Last one was bent. I don't typically bother with lateral run-out unless it's noticeable by eye to quantify it. If it has lateral run-out both inside and out the spokes are bent and the wheel is junk.
I check everything for cracks when it will be used for track stuff or has been previously used for track stuff.
A friend bought 5 sets of wheels/tires off a guy getting out of racing, including the BBS wheels mentioned above, some RPF1s, and several Mustang wheels... Every single set of wheels had at least one bend in them over 0.050". A couple had multiple bends in the 0.100-0.110" range.
Personally I will probably never buy used wheels for track purposes unless I can check them prior to purchase. Even then I strongly prefer new.
In reply to BlindPirate :
Yup, we were roundly attacked by the folks that think any lightweight high performance wheel (that's 2lbs lighter and 3" wider than OEM) should last forever even if it sees a dozen or so track days a year. We published an article about wheel industry test standards and the need for regular inspections about a year ago. Looks like Konig saw it and decided to publish something similar. This is a good thing. http://949racing.com/how-strong-is-your-wheel.aspx
Emillio, it is surprising how many people don't understand the trade offs.
Emilio700 said:
Really appreciate the technical info in this article. Thanks.
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