Cracked D-Force wheel

[djs325]

I had a couple of D-Force rims crease (like a bend & crack combine) on the inner edge of the barrel last year. I had 2 sets and everyone was well out of balance after half a season.

Switched to Team Dynamics Pro Race 1.3, no issues since.

[tammer]

I have two sets. The odd thing is that the other set has been used with R6s and track only without issues. This set was autocross and street tires. Seems like they would have less stress.

FYI - I have no issues with the wheels and would buy another set too.

I think the street = low stress is a bad assumption. A few street potholes at 40-50 mph are much more damaging to a wheel than an unloaded inside wheel hopping a curb on the track. The curbs can be rough, but most of them are pretty well ramped.

Also, don't count out a wheel that breaks when hit. They're not designed to take lateral impacts, and even a "light hit" is a ton of energy when you're talking car into car or solid object.

Any wheel should be inspected regularly for cracks or deformation. Given how many people are running the D-Force LTW5s, to only hear of 5-10 failures so far is actually quite good. I've had good luck with mine so far, but I don't really expect a $250 wheel to last more than a few seasons. If they do, great, but I'm betting they'll last more than half as long as BBSs that cost more than double as much. I've bent two street-only BBSs in the last year. Does that mean they suck? Nah, it means my streets are horrible (thanks, Philadelphia!).

-tammer

[LukeP]

The guys above have a point. I've damaged more wheels on the street than ever on the track.

[JClark]

oh i know that, i just cant afford to have that happen down the road (college student means small budget), in the end the TDs are cheaper, and so far were indestructible for me. guess im more paranoid than most

That doesnt make any sense, the Team Dynamics wheels are all cast too aren't they? Same issue. The only way you can get around it is a $2000+ set of 4 forged wheels, the same cost as 8 cast wheels.

[Calaway16]

Whatever wheels I run on, i'd rather have them bend instead of break:

http://www.northamericanmotoring.com...to-happen.html

Sounds like Team Dynamics handled the situation pretty well.

[everbruin]

thanks for the pics; handy to know where to inspect for cracks =)

[Volcom8190]

That doesnt make any sense, the Team Dynamics wheels are all cast too aren't they? Same issue. The only way you can get around it is a $2000+ set of 4 forged wheels, the same cost as 8 cast wheels.

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.

[das borgen]

that is honestly the first failure I've heard of for the d-force's

not me



they still under warranty, OP?

[ScotcH]

Been there, done that. 3 broken D-Force, and 1 broken K1 in the same spot. I now use BBS RG-Rs without issues.

[elbert]

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

Can't remember who told me this.

[GotBHP?]

I am curious as to why the cracks are typically happening in that area vs. another area on the spoke with a smaller cross section. Is there any design detail on the back of the spokes in that area (stress riser/concentration)?

I've bent several forged wheels, and I typically never keep the cast wheels long enough for this to happen.

[dradernh]

Been there, done that. 3 broken D-Force, and 1 broken K1 in the same spot.

Any particular circumstances upon which you'd like to comment? Offs? Curbs? Prangs? Plain ol' fatigue?

Did these occur on the track or were they observed in the paddock?

[odortiz]

it looks like a low spoke count stress thing. i've seen IFG wheels, which are forged, break (3 on the same car) while tracking an e36 on ra-1's. they are a 5 spoke design. 4 spokes were broken when the driver saw them. one spoke was holding that wheel on the car. that was the worst wheel.
it looks like stress from "oil canning" the spokes where it goes from full lateral stress at the bottom of the wheel to no stress at the top of every rotation. combined with aluminum's lack of resilience and we could see how that would be possible. aluminum is bad at bending back and forth. apparently the lateral flex is enough to fatigue the spokes.
seen kosei k-1's do the same, and now d-force.

[ScotcH]

Any particular circumstances upon which you'd like to comment? Offs? Curbs? Prangs? Plain ol' fatigue?

Did these occur on the track or were they observed in the paddock?

According to Mr. Clay, we put too much lateral load on these wheels, and they just can't handle to for long. We don't really bounce off curbs too much, but sometimes. Tracks are pretty smooth. The wheels were over 1 year old though.

[Def]

it looks like a low spoke count stress thing. i've seen IFG wheels, which are forged, break (3 on the same car) while tracking an e36 on ra-1's. they are a 5 spoke design. 4 spokes were broken when the driver saw them. one spoke was holding that wheel on the car. that was the worst wheel.
it looks like stress from "oil canning" the spokes where it goes from full lateral stress at the bottom of the wheel to no stress at the top of every rotation. combined with aluminum's lack of resilience and we could see how that would be possible. aluminum is bad at bending back and forth. apparently the lateral flex is enough to fatigue the spokes.
seen kosei k-1's do the same, and now d-force.

Exactly - you're just seeing a fatigue failure here. Aluminum loses A LOT of strength with cyclical loading, and this is a FATIGUE FAILURE, not a wheel that was not designed to be strong enough out of the box.



This shows the typical loss in strength for a heat treated aluminum alloy subjected to cyclical loading (like the spokes of a wheel that see a high lateral load everytime they rotate in a corner, or a vertical loading on bumps etc). Think about how many cycles that is on even one race. Figure the wheel is rotating about 700 times a mile, and the stress is pretty high. You can see how you'd get into the millions of cycles pretty quickly even over a season of racing/DE usage.

What you'd see if the spoke cracked in half is a series of "beachhead" lines with a fairly smooth surface from the fatigue crack. Each line was the new depth of the crack as it would get slightly bigger every cycle it was stressed. There'd be a rough crack after the beachhead lines where the remaining material was just stressed beyond its ultimate tensile strength and it let go all of a sudden.


BTW - the reason why the fatigue crack started there (and on pretty much any other wheel) is that's the highest area of bending stress on a spoke. Think of the tire's contact patch as your hand on the end of a wrench. Your greatest leverage will be as your hand is furthest away from the thing you're torquing. In this case the hub. The wheel has a comparatively large amount of material around the hub though, so the stress (force/area) is low. As the spoke starts narrowing right past the hub (area decreasing rapidly, force only falling off slightly), that's where you get the area of maximum stress in the whole wheel (for bending due to lateral loads).


So for everybody checking your wheel for damage, cracks will usually show up there (towards the hub side of the spoke) unless there's a big hit on the rim area that causes a crack to propogate there.

I have seen some really thin spoked wheels crack out towards the rim of the wheel, but that's usually due to a poor design and them trying to leave out too much cross sectional area right by the rim.

[dradernh]

What you'd see if the spoke cracked in half is a series of "beachhead" lines with a fairly smooth surface from the fatigue crack. Each line was the new depth of the crack as it would get slightly bigger every cycle it was stressed. There'd be a rough crack after the beachhead lines where the remaining material was just stressed beyond its ultimate tensile strength and it let go all of a sudden.

Thanks for the detailed explanation!

So, can you say with certainty whether the cracking will start from the front, the rear, or the leading or trailing edge of a spoke?

[nucci]

So, can you say with certainty whether the cracking will start from the front, the rear, or the leading or trailing edge of a spoke?

I would bet that it started at the backside (caliper side) of the spoke.

I did a lot of finite element analysis of FSAE wheels including reverse-engineering the BBS magnesium mesh wheel and analyzing it.

The stresses imposed by cornering are much higher than the stresses from drive/brake. That means it would have started either at the front (visible side) or back (caliper side).

Most often, the stresses in both tension and compression are highest on the back side of the wheel, on the surface of the area where the spokes join the hub. Def has pointed out correctly that this is a fatigue failure. Wheel spokes are a perfect "worst-case" fatigue scenario, since they transition from full compressive loads to full tension loads once every revolution. We would expect the failure to start at the location with the highest magnitude of stress change from max to min.

One other thing to consider is that the properties of aluminum are highly dependent on temperature, with huge reductions starting at only 150F-200F. Radiant and convective heat from the calipers and rotors cannot help the situation at the backside of the spoke.

Also, because of this reason, you will find that most wheels have spokes which are slighly "wider" on the caliper side than on the front side.

[dradernh]

The stresses imposed by cornering are much higher than the stresses from drive/brake. That means it would have started either at the front (visible side) or back (caliper side).

Most often, the stresses in both tension and compression are highest on the back side of the wheel, on the surface of the area where the spokes join the hub. Def has pointed out correctly that this is a fatigue failure. Wheel spokes are a perfect "worst-case" fatigue scenario, since they transition from full compressive loads to full tension loads once every revolution. We would expect the failure to start at the location with the highest magnitude of stress change from max to min.

One other thing to consider is that the properties of aluminum are highly dependent on temperature, with huge reductions starting at only 150F-200F. Radiant and convective heat from the calipers and rotors cannot help the situation at the backside of the spoke.

Also, because of this reason, you will find that most wheels have spokes which are slighly "wider" on the caliper side than on the front side.

Good stuff. Thanks!

[Gnarles]

Exactly - you're just seeing a fatigue failure here. Aluminum loses A LOT of strength with cyclical loading, and this is a FATIGUE FAILURE, not a wheel that was not designed to be strong enough out of the box.



This shows the typical loss in strength for a heat treated aluminum alloy subjected to cyclical loading (like the spokes of a wheel that see a high lateral load everytime they rotate in a corner, or a vertical loading on bumps etc). Think about how many cycles that is on even one race. Figure the wheel is rotating about 700 times a mile, and the stress is pretty high. You can see how you'd get into the millions of cycles pretty quickly even over a season of racing/DE usage.

What you'd see if the spoke cracked in half is a series of "beachhead" lines with a fairly smooth surface from the fatigue crack. Each line was the new depth of the crack as it would get slightly bigger every cycle it was stressed. There'd be a rough crack after the beachhead lines where the remaining material was just stressed beyond its ultimate tensile strength and it let go all of a sudden.


BTW - the reason why the fatigue crack started there (and on pretty much any other wheel) is that's the highest area of bending stress on a spoke. Think of the tire's contact patch as your hand on the end of a wrench. Your greatest leverage will be as your hand is furthest away from the thing you're torquing. In this case the hub. The wheel has a comparatively large amount of material around the hub though, so the stress (force/area) is low. As the spoke starts narrowing right past the hub (area decreasing rapidly, force only falling off slightly), that's where you get the area of maximum stress in the whole wheel (for bending due to lateral loads).


So for everybody checking your wheel for damage, cracks will usually show up there (towards the hub side of the spoke) unless there's a big hit on the rim area that causes a crack to propogate there.

I have seen some really thin spoked wheels crack out towards the rim of the wheel, but that's usually due to a poor design and them trying to leave out too much cross sectional area right by the rim.

It sounds like you may be reading the S-N diagram incorrectly. What it is telling you is that a given stress level (alternating or mean) determines your effective fatigue life before crack initation. So you are not losing strength over the lifetime of the part, you effectively have a set fatigue life based on the material selected and stress spectrum (max and min stresses) applied to the wheel. Running a set of sticky tires and subjecting the wheel to extreme lateral and accel/decel forces on a track will obviously shorten the life compared to driving on the street for a given wheel.

Fatigue performance (before crack initiation) is not directly related to material ultimate strength, but is more an issue of ductility and yield strength which can be weak properties in aluminum castings. Additionally, the casting process is known to leave micro-voids in the part which are perfect nucleation sites for a crack to form. Choosing the proper material is usually a compromise between ultimate strength and fatigue performance. Determing the loading of the part (predominately tension, compression, or a mixture) and the desired properties leads to proper material and heat treat selection.

Once the crack has initiated, then you can start to look at the remaining strength of the part (residual strength). Crack growth is again not soley dependent on material strength, but also a property call fracture toughness. This value is largely dependent on the material and geometry at the crack location. Depending on the stress level at the crack initiation point, you may not see any controlled crack growth like you describe. If the stress intensity (stress level * geometric factors * crack length) at the crack tip exceeds this fracture toughness or you run into a net section failure, the crack will grow to failure instantaneously also known as fast fracture.

I'm not saying that these particular wheels were designed incorrectly, these are just inherent weaknesses with cast wheels as others have stated. They are cheaper to produce, but will have a limited life in extreme environments compared to a similar weight forged wheel.

[Mad Dog 20/20]

it looks like a low spoke count stress thing. apparently the lateral flex is enough to fatigue the spokes.
seen kosei k-1's do the same, and now d-force.

Yeah, I've seen pics of many 5 spoke wheels that do this - some cheap gravity cast designs and some higher-end forged designs. Seems you never see the "basket-weave"/mesh type of wheel do this.

Apparently, the concavity of a wheel also plays a part in its lateral strength. The high offsets used on e36 fitments does not lend itself to a lot of inherent concavity/strength. You then factor-in a relatively small, flat and convex spoke design (and only 5 of them), and you have the recipe for a fairly weak wheel.

[Def]

It sounds like you may be reading the S-N diagram incorrectly. What it is telling you is that a given stress level (alternating or mean) determines your effective fatigue life before crack initation. So you are not losing strength over the lifetime of the part, you effectively have a set fatigue life based on the material selected and stress spectrum (max and min stresses) applied to the wheel. Running a set of sticky tires and subjecting the wheel to extreme lateral and accel/decel forces on a track will obviously shorten the life compared to driving on the street for a given wheel.

Fatigue performance (before crack initiation) is not directly related to material ultimate strength, but is more an issue of ductility and yield strength which can be weak properties in aluminum castings. Additionally, the casting process is known to leave micro-voids in the part which are perfect nucleation sites for a crack to form. Choosing the proper material is usually a compromise between ultimate strength and fatigue performance. Determing the loading of the part (predominately tension, compression, or a mixture) and the desired properties leads to proper material and heat treat selection.

Once the crack has initiated, then you can start to look at the remaining strength of the part (residual strength). Crack growth is again not soley dependent on material strength, but also a property call fracture toughness. This value is largely dependent on the material and geometry at the crack location. Depending on the stress level at the crack initiation point, you may not see any controlled crack growth like you describe. If the stress intensity (stress level * geometric factors * crack length) at the crack tip exceeds this fracture toughness or you run into a net section failure, the crack will grow to failure instantaneously also known as fast fracture.

I'm not saying that these particular wheels were designed incorrectly, these are just inherent weaknesses with cast wheels as others have stated. They are cheaper to produce, but will have a limited life in extreme environments compared to a similar weight forged wheel.

I know the material does not get weaker in the true sense of the word. I took a little bit of liberty with the explanation by simplifying it and just saying "it gets weaker" instead of saying that at even a low level of cyclical loading compared to its initial strength the material will eventually fail. Most people have a hard time wrapping their heads around that (even many engineers), so the concept that the part gets "inherently weaker" the more its loaded for resisting a fatigue failure is a little easier to keep in mind. Not 100% true, but not far from the apparent reality, and it gets you to the same conclusion of having to life parts based on the loading they've undergone and are expected to experience in the future.

But yes, you are 100% correct in your explanation.

[NeilM]

I know that this is tougher for you guys trying to manage a busy race season schedule, but for my track car I make it a point to dismount and thoroughly clean the wheels between events. This isn't just to indulge my inner OCD — it forces me to look carefully at the wheels.

Over the years I've found two wheels that were beginning to crack and retired them (and eventually the entire two identical sets) before getting to the track.

Neil

[SZwain]

I am not sure how isolated the issue is. I have had four wheels crack in the same area. It may be a flaw in the design.

[dradernh]

I know that this is tougher for you guys trying to manage a busy race season schedule, but for my track car I make it a point to dismount and thoroughly clean the wheels between events. This isn't just to indulge my inner OCD — it forces me to look carefully at the wheels.

A professional race team owner made it very clear to me that there are good reasons he has the wheels removed & everything in sight carefully inspected every time the car returns to the paddock.

Pain in the ***, right? Which is why my air jacks are going back in the car this winter. With no crew at the events I attend, I need all the mechanical assistance I can afford.

At one event last year, I had a situation where I should have had the wheels off for a thorough inspection & didn't do it. I found this out once the car was on the lift back at the shop. Given that the issue was brake-related, and the event was an open track day at the Glen, where 5 out of 6 braking zones are at 100 MPH+, I knew I'd been lucky. Lesson learned.

[vdshenoy]

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