[crackerjack]

had enough of this dodging the bullet from BMW DEALERS see below the provisonal report ....which is now being finalised now that BMW have had strong words with MIRA....SO MUCH FOR AN INDEPENDANT ASSESEMENT,that the numpty service manager at Scothall told me to get as i didnt belive their bullshit findings .what upsets me is that i paid for three wheels ,and MIRA have said there is bugger all wrong with the third wheel . they must have spotted me a mile off .

gloves are off .....round 1






SUMMARY ************************requested MIRA to investigate the failures of several BMW alloy wheels from the same vehicle (a 2007 BMW 335i CAB SPORT PETROL). The failures were noticed after the tyres exhibited slow punctures on three of the four wheels. Close examination identified that the rims had cracked through the section and under the tyre mounting bead leading to a leak path for the pressurised air. The wheels and tyres (Bridgestone runflats) were factory fitted and had approximately 12,500 miles usage. The dealer, ************, part of BMW (UK) stated the failures were due to road damage and were therefore a non-warranty item. *********** was not satisfied with this explanation and therefore contacted MIRA to independently identify potential root cause(s). The analyses undertaken identified multiple cracking on each wheel with no evidence of kerb-strilke or similar on the wheel. A Coordinate Measuring Machine (CMM) identified one of the wheels to be only 0.17mm out of round on a diameter of 522mm. Microscopic analysis of the cracks identified the presence of voids and other stress raisers from the manufacturing process. Given the multiple cracking and manufacturing defects and the absence of potential crack initators (eg kerb-strike damage), the author can only conclude that the wheels have failed due to poor manufacturing. All the wheels examined were made within ten days and hence the supposition that this may be a batch problem in the manufacture of the wheels.
CONTENTS Page

1 Introduction .................................................. .................................................. .............. 1
2 Cursory Examination .................................................. ................................................. 1
3 Microscopy. .................................................. .................................................. .............. 6
4 Discussion & Conclusions .................................................. ........................................ 7
5 Further Work .................................................. .................................................. ............ 8
Appendix 1 .................................................. .................................................. ...................... 1
1 Introduction
************************************* requested MIRA to investigate the failures of several BMW alloy wheels from the same vehicle (a 2007 BMW 335i CAB SPORT PETROL). The failures were noticed after the tyres exhibited slow punctures on three of the four wheels. Close examination identified that the rims had cracked through the section and under the tyre mounting bead leading to a leak path for the pressurised air. The wheels and tyres (Bridgestone runflats) were factory fitted and had approximately 12,500 miles usage. The dealer, *********, part of BMW (UK) stated the failures were due to road damage and were therefore a non-warranty item. *************** was not satisfied with this explanation and therefore contacted MIRA to independently identify potential root cause(s).
2 Cursory Examination
Visual Observation of the wheels identified multiple cracking around the inside of the wheel rim (unobservable from the outside of the vehicle) shown in Figures 1 & 2, with no visible damage to the inside rim. The wheels were all made by the same manufacturer in a short space of time between the 21st and 29th March 2007. Although the 29/03/07 wheel showed some signs of exterior rim damage the 21/03/07 was near perfect except for some lacquer chipping on the exterior (Figure 3). With respect to the failure of the 29/03/07 wheel; although it did have some exterior damage that may have been associated with an incidence of kerb-strike, it would be expected (from experience) that the interior crack would have been in evidence on the opposite interior face. This was not the case and the 29th wheel had four such cracks and “ghosting” (See figure 4) of a defect present in a similar position (Figures 5-8). Cracks labelled as #3 and #4 had not propagated enough to cause deflation of the tyre at time of receipt, however #1 and #2 were. Inner rim cracking of aluminium alloys is a common occurrence where a kerb-strike initiates damage.The crack initiates at the shoulder of the indentation and then propagates through the inner rim bead due to aluminium’s poor fatigue properties. However, no such evidence of “kerb-strike” was apparent.




Figure 1: Cracking of 21/03/07 alloy wheel on interior rim. Note the crack opening displacement of approximately 1mm. Figure 2: Cracking of 29/03/07 wheel on interior rim.






Figure 2: Cracking of 29/03/07 wheel on interior rim


Figure 3: 21/03/07 wheel showing no external evidence of damage.






























Figure 4: 29th Wheel showing “ghosting” across the interior rim (Circled).









Figure 5: Crack #1 in 29th Wheel. Figure 6: Crack #2 in 29th Wheel.




Figure 6: Crack #2 in 29th Wheel



Figure 7: Crack #3 in 29th Wheel.






Figure 8: Crack #4 in 29th Wheel.






3 Microscopy.
Electron microscopy identified voids in the fracture surface. These voids would act as stress raisers and support both initiation and initial crack propagation. Figure 9: Electron micrograph of fracture surface showing voiding. X 30 Mag.






Figure 10: 0.4mm defect in fracture surface.






4 Discussion & Conclusions
From the work undertaken, it is apparent that the cause of failure cannot be attributed to kerb-strikes or other damage to the wheel. It must therefore be concluded that the failure is due to the manufacturing process or the material quality. The presence of multiple cracks on both wheels without any commensurate damage on either face of the rim point to the fact that wheel has not been “abused” by the driver. The fact that there are four almost equidistant cracks and additional ghost marks on the 29th March Wheel suggest manufacturing defects. It must be noted that aluminium castings and forgings invariably contain defects of enfolded oxides, voids and inclusions; however, if these defects are large enough or of sufficient quantity they will produce cracks under normal driving conditions. It is the manufacturer’s responsibility to minimise these defects in order that the product lasts longer than the Manufacturer’s or Seller’s Warranty period (Six years under UK Sales of Goods Act unless exempt).
5 Further Work
Given the failures observed, there is little or no further work required in terms of the determination of the cause with respect to ******************. However, determination of the quality of the aluminium used, the magnitude of residual stresses from the manufacturing process and the quality of the process could all be considered by the manufacturer to identify root cause. Techniques such as OES (Optical Emission Spectroscopy) would identify any differences in the composition from specification and CFD (Computational Fluid Dynamics) such as MagmaSoft would assist in obtaining the optimum manufacturing conditions.