Tony Posted November 29, 2009 Report Share Posted November 29, 2009 Seen this many times in wim The BMW wheel fitted with a Run-Flat-Tyre (RFT), I've always been convinced they crack thanks to the fact the tyre is unable to absorb road shocks but something has been niggling me and that's the fact the wheels don't flat-spot, like this> Myself not being a structural engineer leaves me unable to explain why the wheels really crack although i'm still convinced it's due to the tyres. Quote Link to comment Share on other sites More sharing options...
SMARTLY Posted November 29, 2009 Report Share Posted November 29, 2009 No doubt you have seen this Tony: http://www.bbc.co.uk/blogs/watchdog/2009/1...loy_wheels.html It's interesting after pressure they no longer sell the 19 inch wheels until it's been investigated. It's not potholes but either tyre or wheel. Presumably runflats have hard side walls hence your idea of the tyres? Quote Link to comment Share on other sites More sharing options...
Tony Posted November 29, 2009 Author Report Share Posted November 29, 2009 I hadn't seen that clip so thanks for that..... The wheel with "seven" cracks is unbelievable, something is very wrong with these rear rims? Quote Link to comment Share on other sites More sharing options...
parthiban Posted November 29, 2009 Report Share Posted November 29, 2009 That's quite an interesting link, I had no idea the 19" wheels were banned at the mo, but you can still buy cars with 19s on? Or are those different wheels? I'm no engineer or anything, but my thinking was that it was the runflats that made the wheel crack rather than bend. With a traditional tyre, the flexing of the tyre absorbs some of the force, and pushes the remainder through to the wheel which gets applied progressively. With a run flat, the stiffness of the tyre absorbs almost none of the force, and just allows it to pass straight through the tyre and effectively "punch" the wheel rim with one sharp full impact force and that's what makes it crack rather than bend. I'm not sure how much sense that makes, but it makes sense in my head Quote Link to comment Share on other sites More sharing options...
Tony Posted November 29, 2009 Author Report Share Posted November 29, 2009 Works for me...... Our machine that fits RFT's runs at 3.5 tons of pressure so these tyres are mighty stiff but i still wonder why it's just the rears and why the rims don't flat-spot. Quote Link to comment Share on other sites More sharing options...
parthiban Posted November 29, 2009 Report Share Posted November 29, 2009 Works for me...... Our machine that fits RFT's runs at 3.5 tons of pressure so these tyres are mighty stiff but i still wonder why it's just the rears and why the rims don't flat-spot. It must be the alloy compound, they'll be trying to balance weight with strength, and a stronger compound might also be more brittle - but they do say in that video that it's a UK only problem, which does point to the potholes, what else could it be? Is it always the rears then? Could it be the increased width? I would've thought it was the front with the additional weight of the engine, when I hit that pothole it was the front that that had a more serious flat spot than the rear.......... Quote Link to comment Share on other sites More sharing options...
SMARTLY Posted November 29, 2009 Report Share Posted November 29, 2009 I wonder if it only a UK problem, and if the identical wheel is fitted to UK and non-UK cars. It's a bit like Lexus wheels only corroding in the UK but that is easily explained by what we use to clean them, if we protect them and what is put on our roads Quote Link to comment Share on other sites More sharing options...
Tony Posted November 29, 2009 Author Report Share Posted November 29, 2009 My gut instinct is that the rear wheel width and the suspension compression radii doesn't work. Basically the distance between the wheel anchor point, the "J" and the compression curve transmits energy to the rim. Geometrically the difference between the wheel J front/ rear and the suspension allows different camber curves and different energy absorption. Think of the suspensions compression curve, the tilt of the camber and where the energy is placed?..... Change the camber angle to suit the curve then the compression would/ could be distributed differently. Quote Link to comment Share on other sites More sharing options...
Rich Posted November 29, 2009 Report Share Posted November 29, 2009 Interesting video. If it is potholes then I would still think it's a problem with the rims as you don't hear about other manufacturers having this problem? Quote Link to comment Share on other sites More sharing options...
Tony Posted November 29, 2009 Author Report Share Posted November 29, 2009 This complaint is solely confined to BMW. Quote Link to comment Share on other sites More sharing options...
hms Posted November 30, 2009 Report Share Posted November 30, 2009 I put it down to run flats and wheel design. the damage was always on the inside of the wheel. the outside of the wheel is supported by the spoked to the hub. The inner of the rim has no support. Imagine the differeence in strength of a biscuit tin, the bottom on the tin with the bottom on will not deflect, the top of the tin without the lid will deform easily. the deformation leading to the rims cracking. The problem is made worse by the run flats not having the compliance of a normal tyre and passing all of the force through to the rim. h Quote Link to comment Share on other sites More sharing options...
Tony Posted December 1, 2009 Author Report Share Posted December 1, 2009 Agreed but they never flat-spot which is strange..... Another consideration is that the wheels are light weight to counter the heavy RFT, maybe this is an issue? Quote Link to comment Share on other sites More sharing options...
HighlandPete Posted December 1, 2009 Report Share Posted December 1, 2009 Agreed but they never flat-spot which is strange..... Another consideration is that the wheels are light weight to counter the heavy RFT, maybe this is an issue? Tony, the wheel example you show in the second picture... isn't it a samller diameter? 16" isn't it? Could be a different alloy, than the larger 18/19" rims that typically fail with the cracking. I've seen pictures of the larger wheels that have had impacts, none so far have flat-spotted. They more or less 'shatter' and the grain structure shows they are a harder material, IMO. Fits with the look of stress cracking, which we see on the examples in the Watchdog video. I recorded the Watchdog prog' at the time, so that I could freeze frame on a large screen. The sample with the long irregular fracture, shouts stress/fatigue fracture to me. HighlandPete Quote Link to comment Share on other sites More sharing options...
djwozza Posted December 1, 2009 Report Share Posted December 1, 2009 My guess (& it is a guess) they are simply not a strong enough alloy to withstand the forces applied by the RFT, either the compound itself is at fault, or just needs to be thicker in certain areas to enhance the strength ? I really do think RFT's are not the pinnacle of Tyre development we are being led to believe....... Quote Link to comment Share on other sites More sharing options...
Tony Posted December 2, 2009 Author Report Share Posted December 2, 2009 Agreed but they never flat-spot which is strange..... Another consideration is that the wheels are light weight to counter the heavy RFT, maybe this is an issue? Tony, the wheel example you show in the second picture... isn't it a samller diameter? 16" isn't it? Could be a different alloy, than the larger 18/19" rims that typically fail with the cracking. I've seen pictures of the larger wheels that have had impacts, none so far have flat-spotted. They more or less 'shatter' and the grain structure shows they are a harder material, IMO. Fits with the look of stress cracking, which we see on the examples in the Watchdog video. I recorded the Watchdog prog' at the time, so that I could freeze frame on a large screen. The sample with the long irregular fracture, shouts stress/fatigue fracture to me. HighlandPete The other wheel was 15" or 16" and used as an example for a flat spotted wheel (not BMW) Your "fatigue fracture" seems very plausible even logical in the absence of any other explanation. Quote Link to comment Share on other sites More sharing options...
CIH Posted December 2, 2009 Report Share Posted December 2, 2009 so this issue has made it on to Watchdog ? Quote Link to comment Share on other sites More sharing options...
Tony Posted December 2, 2009 Author Report Share Posted December 2, 2009 Yep and BMW deny a problem Quote Link to comment Share on other sites More sharing options...
Waz Posted January 10, 2010 Report Share Posted January 10, 2010 No doubt you have seen this Tony: http://www.bbc.co.uk/blogs/watchdog/2009/1...loy_wheels.html It's interesting after pressure they no longer sell the 19 inch wheels until it's been investigated. It's not potholes but either tyre or wheel. Presumably runflats have hard side walls hence your idea of the tyres? apart from not being able to tolerate the ginger witch, (so i missed that bit) where did it say that BMW stopped issuing 19" rims? it wasn't in the text of the article. I work for BMW Group and our scheme cars still get delivered with 19's so we 'inside' the business are not aware of it. Maybe they are just using the fix on fail method to gather more info inside the group, as our wheels and tyres are included in wear and tear as part of the scheme? Quote Link to comment Share on other sites More sharing options...
SMARTLY Posted January 10, 2010 Report Share Posted January 10, 2010 It said on one of the follow up BBC Watchdog programs that they were not using 19 inch wheels. Quote Link to comment Share on other sites More sharing options...
Waz Posted January 10, 2010 Report Share Posted January 10, 2010 It said on one of the follow up BBC Watchdog programs that they were not using 19 inch wheels. Interesting. I've got a 4 week old 5 series with 19's on. Will be interesting to see how the availability of the M-sport packages are for wheels when colleagues changeover their cars in the next few months. Could be that internal customers will be getting the stocks and externals offered only the 18s until it's resolved. Predict new range of wheel offerings quite soon. Quote Link to comment Share on other sites More sharing options...
Tony Posted January 10, 2010 Author Report Share Posted January 10, 2010 However read there's something wrong, wim has displayed this topic for years, and then recently Watchdog so we're not "urinating" in the wind. I think the lightweight wheel and the RFT tyre has some transfer issues meaning the tyre is unable to transmit perturbation's to the suspension. Quote Link to comment Share on other sites More sharing options...
HighlandPete Posted January 16, 2010 Report Share Posted January 16, 2010 It said on one of the follow up BBC Watchdog programs that they were not using 19 inch wheels. Interesting. I've got a 4 week old 5 series with 19's on. Will be interesting to see how the availability of the M-sport packages are for wheels when colleagues changeover their cars in the next few months. Could be that internal customers will be getting the stocks and externals offered only the 18s until it's resolved. Predict new range of wheel offerings quite soon. It appears it is just on the 3-series that the 19 "rims have been deleted from the options. Also there is a different attitude to replacement, even though BMW are not yet admitting liability. Seems to be confusion at some dealers, some appear more enlightened than others. HighlandPete Quote Link to comment Share on other sites More sharing options...
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