Cracked D-Force wheel

[TeamDFL]

THIS is why I run multispoke wheels and not 5-spoke ones....Koseis, C6 Corvette wheels, D-forces, Rotas etc - all the ones that break - seem to be 5 spoke wheels...never had my Enkei NT03Ms break on me with a lot of AXing and some decent offs at grand sport speedway

Even multiple radial spokes can be susceptible to this sort of failure which I imagine is due to bending loads induced during acceleration or braking. Take a look at the bulk of BBS wheel designs. The spokes com off the hub tangentially which means the spokes are loaded in compression and tension during acceleration and braking.

For those of you who wish to do some research, read about prestressed wheel designs such as spoked bicycle wheels. You will find out why radially spoked wheels and hubs transmitting torque (rear wheel, disc brakes) don't mix.

Can you design a radially spoked wheel strong enough to handle typical track or street use? Sure you can, but I'm pretty sure it is going to have to be heavier than a wheel with tangential spokes.

[ame]http://www.google.com/search?hl=en&q=sheldon+brown+bicycle+wheel[/ame]

[ame]http://www.google.com/search?hl=en&q=jobst+brandt+bicycle+wheel[/ame]

[Mad Dog 20/20]

Even multiple radial spokes can be susceptible to this sort of failure which I imagine is due to bending loads induced during acceleration or braking.

I'm sure multi/mesh wheels are susceptible to the same failure, it just sure seems to almost always be a 5-7 spoke that you see fail like this though.

And I think it is pretty well-accepted that the forces at play are those created by high lateral G's essentially pushing the hub out through the wheel center.

[Def]

I'm sure multi/mesh wheels are susceptible to the same failure, it just sure seems to almost always be a 5-7 spoke that you see fail like this though.

And I think it is pretty well-accepted that the forces at play are those created by high lateral G's essentially pushing the hub out through the wheel center.

The forces on the front wheels during braking are also very large. They'll typically be the largest force you'll see on a FR car's front hubs/suspension.

[Mad Dog 20/20]

The forces on the front wheels during braking are also very large. They'll typically be the largest force you'll see on a FR car's front hubs/suspension.

I don't doubt it. But wouldn't those forces (accel/decel) start cracks on the leading/trailing edges of the wheel spokes instead of the sides?

[Fair]

I'm sure multi/mesh wheels are susceptible to the same failure, it just sure seems to almost always be a 5-7 spoke that you see fail like this though.

Yep, you are correct. Many would say that five spoke wheels are as good or better than mesh wheels, but there seems to be a LOT more of the 5-7 spoke wheels on most racetracks, so we probably notice failures on those. I've seen all manner of wheel designs break, and its usually in the same place (hub/spoke junction), no matter how many spokes or if the "mesh" is involved. And failures usually stem from a hard vertical or lateral impact of some sort. Endurance/fatigue wheel failures are actually somewhat rare in the real world with drivers that are pushing their cars around track. Not many of us are so perfect, so clean that we never have an off or touch the FIA curbing.

In fact "spoked" wheels are preferred by some race engineers over "mesh" designs....

( link) - I believe the "spoked" design meets these design criteria (stiffness) better than the popular "grid" or "Web" design originated by Jim Hall and currently featured by BBS and its imitators. The grid does not usually have the section depth to achieve this stiffness AND it is too complex to achieve consistently good sand castings (talks about stress risers).

...I believe the minimum acceptable number of spokes is four and six is a considerably better better number...

Last November at an AST track test day I was at Eagles Canyon. I was doing a little lead/follow with a friend (me following in my E36 M3 and he in his S197 Mustang GT). He was pushing a bit wide coming out of turn 4 and went off and on. Being 2 car lengths behind him I had a great view of the off. There was a low curb he went over and a small dip in the runoff. Like I said, quick off and on, didn't look bad. I was pushing him so I felt partially responsible, but he was a racing friend and expected nothing less. He ran for the rest of that session and the next. Somebody noticed in the pits later that day that he had mesh spoke failures on the right front and left rear wheels (CCW mesh Classics in 18x10). These weren't lightweight wheels at 21 lbs each, nor were they cheap at $2500 set, and it wasn't "that bad of an off". And I've seen these failures happen any number of times. Like I said, it doesn't take much when you snag a lip or a curb just at the right angle.



Any wheels' spokes should be inspected after every "off" before you make another lap, unless you are willing to risk the continued use (and in a points race where every lap is crucial, you might). Paintings and coatings make the early signs of cracks harder to see, which is why (again I'm quoting from the same book, same page) Carrol Smith always wrote that "paint is flexible enough to both fill and hide small casting voids and to hide the development of small fatigue cracks in the metal underneath the intact paint." I'm not advocating to run bare wheels without any coating, just that a stress crack could form under the paint and propagate into a failure later.

And I think it is pretty well-accepted that the forces at play are those created by high lateral G's essentially pushing the hub out through the wheel center.

That's the failure I have seen many times - much more often than a barrel failure. High g impacts laterally on the wheel, and then the hub wants to push out from there. Even on street tires the impact loads from a lateral hit or hooking of a curb are incredibly high. The same goes for vertical impact loads from potholes on the street. Those are orders of magnitude higher loads than the standard lap after lap lateral grip loads from the tires. Without aero/downforce the most grip a tire can produce is about 1.4g laterally, and that's simply nothing compared to the 5-10g+ impact loads you will see from hitting a FIA style curb or pothole.

D-Force has a damn fine wheel for the BMW market. Its not simply a cast wheel, so comparisons to Team Dynamics and the like are not exactly accurate. There are 4 wheel makers in the world with patents on some sort of flow-formed casting/spun forged barrel wheel process, which has many of the benefits of a cast wheel and a forged wheel. D-Force wheels are built at one of these factories (read more about the process here). There are many names, but most call it a semi-forged wheel, which applies especially to the barrel. This is why an SSR or a D-Force wheel tends to be lighter than many other cast wheels yet meets JWL standards and has the same or higher load ratings. The D-Force 5-spoke "LTW5" is the perfect compromise in weight, cost, design type, durability and selection. Its suitable for street or race track use, but like any wheel you can buy of any type, it still has a finite lifespan and impact load limits. That lifespan, like that of any wheel, depends highly on the roads you drive on as well as your driving style and "nontraditional lines" chosen both on and "off" track.

Yes, I sell these wheels, but its the only wheel we sell - and we have the opportunity (and constant calls from vendors) to sell any number of wheels. We use D-Force wheels whenever we can on our own BMWs because they offer the best compromise of those properties listed above. We have sold many hundreds since 2007 and have helped D-Force in small ways to come up with new wheel sizes, to do fitment/spacer testing, etc. The guys there (namely Tom the owner) are true car guys and racers. They developed the market for true bolt-on, BMW fit wheels that simply did non exist before, which many have since tried to copy (note the exact same offsets/sizes from other Johnny Come Lately wheel vendors).



I have some of the very first 18x9" and 18x10" D-Force wheel sets they offered, and my first 18x10" set has been on 3 cars, made hundreds of autocross laps, and dozens of track events - plus years of street miles - all without a hitch after 3 seasons. We remove, hand wash and inspect all race wheels after every event and look for any cracks or signs of stress, esp. at the spokes. But that set hasn't been in many offs (1 or 2). With the way I drive, that's just a matter of time, no matter the wheel brand.

And yes, we sold these wheels to Ken - as well as 1-2 other sets. Ken has explained fairly well the possible cause of his failure (an off), and I have driven on track with Ken a number of times in both NASA TT and HPDE settings. No doubt about it, Ken is damn fast in whatever he drives (Porsche, Lotus, Mini, BMW) and he doesn't hold back much. He has many NASA TT lap records to show for it. He has the practiced and fine-tuned car control of a very experienced autocrosser, and takes that level of 10/10ths driving on track, like other autocrossers-turned-track guys I know have done. I've seen a few offs that made me cringe (as his suspension and wheel supplier!) but I can say I've done as bad or worse myself, on the same tracks and sometimes on the same corners.

The CCW wheel failure I described above... well, I had a stupendous off that same day two turns further down the track, and I went off sideways in front of 3 other drivers following closely behind me (on that original set of 18x10s). I had a total of two offs on track last year (M3 at ECR and EVO at HHR), and I'm proud of neither, but luckily neither resulted in any damage. A lot of us old autocrossers don't blink and will push 10/10ths on track - and sometimes 11/10ths - which can make for some "agricultural driving lines". As such, we can be hard on equipment.

Of the many hundreds of D-Force wheels we've sold here at Vorshlag we have had a total of 3 customers have an issue, including a fellow referenced earlier in this thread (who also had a off at that same ECR event). Talking to Tom at D-Force, the failure percentage is extremely low, especially given how many HPDE guys are running these wheels. Some HPDE guys are pounding 10-25+ weekends a year on their D-Force wheels, as well as thousands of street miles. Hanchey at AST was mentioning lately that he's seen HPDE-only customers that literally log more on-track miles on their street cars each year than Club Racers or GRAND AM race teams... then drive to-from the track and to-from work as well, often on the same equipment. HPDE guys just might not take as much time to inspect their wheels, suspension, nuts/bolts like a GRAND AM team does between every event.

So long story short... D-Force wheels are still the best options out there for any price. I've paid a LOT more for heavier wheels with similar strengths and weaknesses. And we should all be vigilant about inspecting our wheels, no matter the brand, especially after an "off".

Yes, a true 100% forged wheel would be ideal, but the cost usually is 4-8X more per wheel, and they aren't always lighter - I've seen forged "race wheels" lately that still out-weighed the D-Force wheels by 2-3 pounds per corner for the same size/offset. Luckily for all of us, D-Force is working on an all new line of forged wheels coming in 2011. They have been sharing some of these early design pictures to the D-Force dealer network this month and I believe they are showing these at the PRI show this week. If you are at the show make sure to stop by and take a look. I can't wait to show these or talk about the unbelievably low pricing structure they have planned.

More soon,

[lowflyer]

Good post. Thanks for taking the time to expand on this Terry.

[ken o]

Thanks Terry. I agree. I think D-Force makes a great wheel and look forward to the new forged wheel. Let me know if you need someone to test them out!

[xsport652]

Very excited to see this new D-force Design.

Thanks for the thorough info Terry!

[Jeremy Lucas]

mesh spoke failures on the right front and left rear wheels (CCW mesh Classics in 18x10).

FWIW, John at CCW stands behind his wheels even when raced. Back when I had my Z06 track car, I cracked a 2nd hand CCW wheel (not even the original owner!) that had been used for many years on a SCCA T1 race car. John knew full well the history and sent me the stronger centers at no cost.
wheel pictures

[speedblind]

Nothing to add other than this type of level-headed, information-based discussion is why I come to this forum (and the Track sub-forum in particular). On so many "other" forums for "other" cars this would have degraded into name calling and Internet hearsay.

Thanks to all the engineers that weighed in with layman's-term explanations of the forces acting on a wheel at speed - great info.

[dradernh]

Thanks to all the engineers that weighed in with layman's-term explanations of the forces acting on a wheel at speed - great info.

Second that.

[MZ3PO]

Excellent thread about wheels and how they react. Thanks Terry for an great post!

I have been a D-Force LTW user for years now. From autocross to DE's to time trials to SCCA racing, BMWCCA and NASA. Have NEVER had a broken LTW. (knock on wood). They have been excellent wheels, plus the customer service at their shop is great.

I am currently waiting for the boys at D-Force to come up with the offset i need for the E36. CANNOT wait! I have been using CCW's and honestly, love/miss the LTWs...plus cant beat the price. A++ for LTW's

[ScotcH]

I am currently waiting for the boys at D-Force to come up with the offset i need for the E36. CANNOT wait!

What's wrong with the offset? The fit e36 fine. We also ran them with a 25mm spacer if you need a lower offset.

[MZ3PO]

What's wrong with the offset? The fit e36 fine. We also ran them with a 25mm spacer if you need a lower offset.

LOL. Nothing wrong with offset. Have you seen my race car?
Very wide.

[raopoo]

Hey, Good post.. actually the air can leak out of the crack,...

[D. Hitchcock]

What a tremendous thread. I really appreciate the discussion among the engineering heads and explanation from Terry F.

I'm on my way to the basement to inspect my autocross wheels now: Kosei K-1s.

Thanks and cheers,
d.hitchcock

[jamesclay]

The best bang-for-the-buck that I've found in forged in the 17X9 I run are Fikse Aros at $2720/set - which is why I went with disposables at D-Force's prices.

We have Volk TE-37 that we use for our CTC program that are around $600 each and we will have the GS-spec wheel on the shelf shortly, as well as alternate fitments. Didn't BEND a one in a full season of racing, which is unheard of for us - we are good for about 8-10 bent expensive forged wheels per year normally.

And to a good degree, this will depend on the driver. I will typically bend almost any set of cast wheels in a few hours of driving if I am trying to squeeze all the time out of a car - that is why I buy forged

They are cast, but they are cheaper and I've seen and had sets that have gone thru utter hell that came out completely unscathed.

See below

When buying wheels, you can pick only two of the following:
- cheap
- lightweight
- durable

Can't remember who told me this.

Hmm, yes who... This is absolute fact. The TD are stronger, but their 17x9 for instance is 22# - not all that light...

[M3 Adjuster]

The CCW wheel failure I described above... well, I had a stupendous off that same day two turns further down the track, and I went off sideways in front of 3 other drivers following closely behind me (on that original set of 18x10s). I had a total of two offs on track last year (M3 at ECR and EVO at HHR), and I'm proud of neither, but luckily neither resulted in any damage. A lot of us old autocrossers don't blink and will push 10/10ths on track - and sometimes 11/10ths - which can make for some "agricultural driving lines". As such, we can be hard on equipment. ,

Hi Terry! (I was one of the three cars...)

Anything non-forged has a short service life. Wheels are consumables for track/race cars. That's why they're half the price of forged wheels, buy a set of spares to have with you.

Another reason to have spares is in the event the wheel gets discontinued and you bend or break one... I h currently have 12 of the same wheel in my garage (Kosei K-1)

Sorry to hear about the failure Ken..