BMW Wheel using 4.75" Bolt Pattern Hubs Discussion

[silver69camaro]

Guys,
Lately I've been reading about some people using BMW wheels on their cars which have 4.75" BP hubs & axles. I understand BMW wheels are a cheap, durable alternative to aftermarket wheels and many styles look pretty good.



I was bothered by the mentality that BMW's 4.72" BP was "good enough" for the 4.75" hub. The fact is we're talking about the 5 fasteners that attach wheels to the vehicle; we've seen these studs break from fatigue using wheels with the correct BP. Turns out, this "minor" difference in BP bends the stud at a 0.53 degree angle. Now we have a situation were the stud, designed ONLY to be in tension, is undergoing combined stress (tension and bending) during the assembly procedure. This is not good!

So I did a little FEA simulation to see what kind of difference this is going to make. This analysis only shows what the stud is undergoing after the lug nut is torqued. This does not show us:
1. “Twisting” stress from torque applied to the stud;
2. Stress from driving the vehicle;
3. Fatigue over time.

Please note these three situations will make the results worse. Here we go:


Here is the stud using a 4.75" BP wheel. Note the stress concentrations at the chamfer resulting from 10,000+lbs of tension. These results are typical.



Here is the same stud using a BMW wheel. Note the larger stress concentrations resulting from bending stress, and the stress continues all the way up the shank. This photo shows the stud being bent away from us; the other side is in compression and results in a stress concentration higher up the press-in shank (about where the threads start). Stress has increased by a little over 20%. That's a significant increase. Again, accounting for fatigue and torque-related stress will further reduce the stud's service life.

I'm not trying to rain on anybody's parade about using BMW wheels, but I feel this is something people should know when making a decision on which wheel to use.

[Mr.VENGEANCE]

Thats greeeaat... thats just fuc kin great, MAN..

[Apogee]

Nice comparison Matt. It's remarkable what .012" absolute position difference can do.

On the same subject, we've been asked if upgrading to a larger stud would offset any of the increased stress due to the combined loading scenario created by the wheel pattern mismatch, and the short answer is no, it doesn't. A larger diameter stud is typically torqued to a proportionally higher value than a smaller stud, so the internal stresses should show similar trending regardless of stud diameter.

Tobin
KORE3

[69LT1Nova]

Alright, I'll bite. I'm an engineer... I've done plenty of FEA... 20% stress increase on a stud that has a huge safety factor to begin with isn't that bad. 190,000 PSI tensile strength on standard press-in studs to begin with...

Yeah yeah, tension vs tension and shear loading. I don't think it makes a BFD when the studs are overbuilt to begin with. Fun analysis, and good for people to know, but I wouldn't hesitate to put BMW wheels on a street driven car.

[oestek]

Yeah, but under an autocross situation with a 4000 lb car?

[69LT1Nova]

Yeah, but under an autocross situation with a 4000 lb car?

Every application is different. I said street driven car, not autocross. Like the original post stated, good for people to know, and make their own informed decision.

I stand by what I posted.

[dhutton]

Is there some kind of scale to accompany the colors on those analyses?

How far from the rated capability of the stud is this?

Sort of hard for me to put this into perspective without some kind of absolute numbers to go with it.


Thanks,
Don

[Bow Tie 67]

Damn there goes using my GTO space saver spare to finish a race.

[silver69camaro]

Alright, I'll bite. I'm an engineer... I've done plenty of FEA... 20% stress increase on a stud that has a huge safety factor to begin with isn't that bad. 190,000 PSI tensile strength on standard press-in studs to begin with...

I agree the SF on wheel studs is way high for obvious reasons. But, like I said before, we've all seen studs break before. Granted I've never seen a brand new stud fail, but as you should know fatigue will wear the stud.

New stock-style studs aren't even legal in some classes, which raises some eyebrows about their ability in the first place.

Again, 20% is the minimum increase. Fatigue, corrosion, etc. will increase that.

Notice I did not say that people shouldn't run BMW wheels. Its up to the individual. I'm not saying it would fail, and I'm not saying it wont. It's all about education.

[JJSmitches]

I was just curious under what conditions you have seen the studs break with these wheels?

I was searching and also read a post by someone who had their spacers break, but his was the only one I saw.

[silver69camaro]

Is there some kind of scale to accompany the colors on those analyses?

How far from the rated capability of the bolt is this?

Sort of hard for me to put this into perspective without some kind of absolute numbers to go with it.

I have a feeling this might become a lively thread. Good. Things have been a little quiet here lately.

Thanks,
Don

Don, I chose to leave out the scale on purpose. First, it was somewhat difficult to read. Second, the number isn't really imporant (to a degree) because it's the difference that I was interested in.

Rated capability of the bolt is somewhat difficult to definitively say. There are several methods to determine what a wheel stud load capacity is, and the use of the car will have a great effect. Say, a stud for a track car will have a lower rating than the same stud on a street car...it increases the safety factor.

Yes, the safety factor is high to begin with...but these SFs are based on stock vehicles driving on the road. Many of us no longer fit that profile.

Again, I'm not saying the stud will break for sure. It may never happen. But if you have wide tires that will see high corner loads, the risk is there. And if one stud breaks, there is an increased chance the others will follow soon.

[69LT1Nova]

Notice I did not say that people shouldn't run BMW wheels. Its up to the individual. I'm not saying it would fail, and I'm not saying it wont. It's all about education.

Agreed! Good for people to know this info, especially if they push their cars hard on the track - such as autocrossing or going for max G's on a skidpad.

[silver69camaro]

I was just curious under what conditions you have seen the studs break with these wheels?

I've seen studs break just about everywhere and usually the reason is unknown. Many situations could cause it (over torqueing, fatigued, over stressed, etc) and all this situations could apply to us as well. I don't know anybody who runs BMW wheels, if that's what you're referring to.

Here's a guy at CC.com that had an issue with his former Autocross car:
http://www.corner-carvers.com/forums...ht=broken+stud

A simple search on Yahoo:
http://search.yahoo.com/search?p=bro...p=mss&ei=UTF-8

Yahoo Images:
http://images.search.yahoo.com/searc...b&fr=yfp-t-501

[TnBlkC230WZ]

I'll be the first to admit that there is an extra load because the .5 degree flex, But I'd argue that the lug is actually flexing that much. At minimum, you are torquing down a grade 8 fastener with a hardened steal lug into a conical seat cast aluminum wheel. I would think the wheel would take up at least half the flex. I'd be currious to see what the load difference is at a .25 degree flex.

I'm also curious as to the manufacturing tolerance of most Chinese wheels that people are using today. I may get at least as good of fit with a set of quality BMW wheels

[68Formula]

I'm not sure I would say the importance is higher on a track vehicle. A track vehicle will be pushed to the limits putting higher stress loadings, but tracks tend to have smooth surfaces. Streets have potholes, railroad crossings, and speed bumps to deal with, which will impart impact loads in addition to dynamic loads. More than 20% is enough to make me consider it not worth the risk.

[Boesch]

I do understand that this is only stress from torquing the nuts, but I would beg to differ that stress level does make a difference. What are your stress levels like compared to the yield stress? That's the critical factor. I can design parts all day long that increase stress by 12% and there's no concern that they are going to break. Percentage is all relative the how big of numbers you're working with.

Let's use some round numbers to illustrate my point. If your yield stress is 100ksi and you have 1ksi of stress in your part, then a 12% increase is very negligible and not something to be concerned about. On the other hand, if you have 90ksi of stress in your part, then 12% increase would definitely be of interest and would probably be the straw that broke the camel's back.

Dustin

[RYU]

my small brain would like to ask what I thought was an obvious question. Why can't someone just machine a set of offset conical bushings? Would the bushing be too thin and rendered basically useless?

i'm not an engineer just a dumb finance guy

[86Cutlass383SR]

I'm wondering how many OEM wheel manufacturers hold their tolerances to .03" anyway. I know not many of us are running OEM wheels but there are a LOT of cars on the roads that are. I agree we do put more stress on our cars but...

If you measured enough wheels, I wouldn't be surprised that you'd find some wheels with a 4.70" to 4.80" tolerances.

Just tossing out some idle thinking...

[68Formula]

I'm wondering how many OEM wheel manufacturers hold their tolerances to .03" anyway. I know not many of us are running OEM wheels but there are a LOT of cars on the roads that are. I agree we do put more stress on our cars but...

If you measured enough wheels, I wouldn't be surprised that you'd find some wheels with a 4.70" to 4.80" tolerances.

Just tossing out some idle thinking...

Yes, but if you want to talk tolerances they would apply to both bolt circle-based wheels, so you've added a nominal shift to the basic dimension. Which in some cases they could overlap, but in others, they could be quite a bit more off.

[TnBlkC230WZ]

I would think most factory wheels are withing .01 inch, certainly newer BMW wheels are. Its the Chinese wheels that make me wonder. All these TTIIs and other popular American named Chinese wheels are probably hit and miss with .05 tolerances.

Again the .03 difference is spread across the pattern, so no one lug is more than .015 off.