How to fix a cracked wheel?

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merc110913

New Member
Joined
Sep 11, 2013
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12
Car
CLS
Hi All

I have a 2005 CLS. I had what appeared to be a slow puncture. I took it to a tyre place and they couldn't find a puncture but on taking the wheel off found a 1 inch crack on the inside edge of the rim and they believe air is thus seeping from the tyre. I assume this is wear and tear or a previous knock as I've not hit a kerb etc that I can remember.

Can anyone tell me who can fix this? Is this a job for an alloy specialist or do I need a welder?

Many thanks
 
Thanks Giantvanman and BlackC55. I'll find a local welder. Any ideas how much this type of repair weld costs? It's a hairline 1 inch long crack running perpendicular from the rim edge.
 
Should be about £40 a wheel to repair.
 
One good reason to avoid potholes especially if your on low profile tyres.
 
Be aware that ally wheels are virtually always heat treated...
Pretty much a copy and paste of something i wrote on the 'other side' a while back...

Dye penetrant* can be used to highlight cracks that aren't visible with the naked eye and can also help guide 'excavation' of the crack. Any decent welder** should be able to deposit a sound weld that is free of inclusions, gross porosity and LOF (lack of fusion) defects however...

Aluminium is a very different animal to steel. Steels have an endurance/fatigue limit, aluminium and it's alloys do not. With basic carbon steels welds are often of equal or even higher strength (hardness) compared to the base metal. Aluminium alloys are annealed (softened/weaked) by heating to temps in the 340 - 410C range i.e. welding aluminium anneals it local to the weld (the HAZ, heat affected zone)

Alloy car wheels are made from various different flavours of aluminium, most of them heat treatable. Heat treatment of ally is typically a 3 step process... a solution treatment where the part is heated close to it's melting point which dissolves the alloying elements (kinda like being able to dissolve more sugar or salt in hot water than you can in cold). This is followed by a quench to 'freeze' them in solution and then a low temp bake (say 8 + hours at upto around 180C) which is the precipitation (aka artificial aging) heat treatment. This final step is what imparts the strength increase- the alloying elements precipitate out of solution in a controlled way as miniscule grains which kinda fill in gaps in the crystalline structure.

Unsuprisingly welding destroys heat treatments in the HAZ i.e. the weld will be softer and weaker than the rest of the wheel. It's not the easiest thing to quantify as the flavour of aluminium and the exact heat treatment effect things on one side and the welder has an influence too- dither about pouring heat into the part and you anneal a larger area. You can locally more than halve the yield strength (force needed to permantly bend it) of a heat treated alloy if you don't know what you're doing and the best case scenario is very generally considered to be around 60% of the origional strength. Obviously the size and location of a crack has a big effect on the result with a wheel... a small crack that's barely grown beyond the lip is much less of a deal than one which has grown into the bead seat area

It's not as bad as it sounds in that everything is designed with a safety factor i.e. made stronger than it needs to be to do it's job + some for good measure. Common sense says that heavy wheels (cheap/style over substance/usually cast) have a bigger safety margin than a lightweight forged wheel that's had most of the weight engineered out. Worth pointing out that forging and spinning/flow forming imparts a 'grain structure' in metal that improves it's mechanical properties. Welding is, in effect, casting and so unsuprisingly locally destroys grain structure
If the welder screws up/the wheel is just near the end of it's fatigue life etc the penalty is usually just a crack that grows back (or fresh ones) and if not spotted at a service/MOT will result in a slow puncture if it grows enough to compromise the bead seat. In the case of say a high speed blowout then you'd think that a repaired wheel is more likely to suffer catastrophic failure than one which hasn't been repaired but (as long as it was well repaired) less than a cracked wheel

Heres an article covering the manufacture of MB wheels that talks about flowforming and heat treatment... http://media.daimler.com/dcmedia/0-9...0-0-0-0-0.html GK-AlSi7 is a partial designation covering a group of aluminium alloys, aluminium silicon itself isn't heat treatable. There'll be other ingredients of which magnesium is almost certainly one as magnesium silicide is a common hardening phase in aluminium alloys. Probably similar to LM25/a356.0 data sheet here http://www.hadleighcastings.com/uplo...y%20Detail.pdf


* VERY basically the area is covered a brightly coloured dye and left for at least 10 mins contact time. The excess is then removed and the area carefully cleaned with a solvent and dried. A developer is then sprayed on (a dryish contrasting colour usually white) which highlights any cracks by drawing the dye out of them

** There are more bad and mediocre welders about than good ones, especially once you get away from basic carbon steels. I'd class myself as above average and specialise in ally and exotics (rarely weld carbon steel). Weld lots of things more critical than car wheels but as a general rule don't touch the later- too many unknowns to make it viable most of the time IMO, pretty much evertime i take on 'a simple little crack' i find additional cracks that the owner was unaware of



Been discussed various times here too, going back a few years i think Number_cruncher that works in NDT and pointed out that they X-rayed a wheel that'd been repaired by a wheel specialist a few years ago and found it was still cracked. The 'specialist' had another go IIRC and they still found cracks? Whether the specialist was a proper welder or a painter with a TIG i don't know- not a dig at painters as i can't paint to save my life but 'proper' welding is a world away from snotting cars back together with a MIG or building gates and railings, especially when you move away from basic mild steels and metallurgy effects things as much as physical skill with a torch

It was the company that Johnsco works for that X-rayed a wheel that'd been welded by a wheel specialist painter and was still cracked http://www.mbclub.co.uk/forums/wheels-tyres-brakes-suspension/72799-cracked-alloy-2.html
Another thread on the same subject http://www.mbclub.co.uk/forums/whee...n/80286-cracked-rear-amg-wheel-what-do-3.html
 

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