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Driving with a split rim

Discussion in 'General Discussion' started by andyjay123, Oct 26, 2016.

  1. andyjay123

    andyjay123 Active Member

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    It has been welded, but do you or don't you?
     
  2. Pontoneer

    Pontoneer MB Club Veteran

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    Think I'd maybe use it as a spare wheel , to be used with caution only when needed . It ought to be OK , but how good was the job ?
     
  3. PaulG

    PaulG Hardcore MB Enthusiast

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    There are all sorts of welds in the car which, if failed could kill you..

    If you're confident that the weld has been carried out with good workmanship, there's no reason to doubt it.

    You could always get it X-rayd if in doubt.
     
  4. daveenty

    daveenty Hardcore MB Enthusiast

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    Have a look at some of the threads on the MIG Welding Forum about this problem. (Just search there for "alloy wheel repair")

    It seems the consensus is a big no, due to there being so many different materials used to make the wheels, and the welder not knowing what said material actually is.

    I'm no expert on metallurgy, but would be reluctant to use a welded rim.
     
  5. zenman63

    zenman63 Hardcore MB Enthusiast

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    A good tig welder will cut the crack out 5mm wide with a angle grinder and tig weld no problem at all with no trace when cleaned up.
    The problem is when its snot welded along the crack, it look ugly and will be out of balance and probly crack each side of the weld.
     
  6. OP
    OP
    andyjay123

    andyjay123 Active Member

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    Ive driven for 500 miles without realising I had a split rim. Car went in for a service and they flagged it. £850 plus vat for a new one. There are no second hand ones out there. Now i know its there i am minded to change it for peace of mind especially with kids
     
  7. PobodY

    PobodY Hardcore MB Enthusiast

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    That's the difference between repaired and dodged in my book. - A skilled welder's work would be invisible as you say.
     
  8. Teflonso

    Teflonso Hardcore MB Enthusiast

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    Weld repairs are great in many applications, especially if subjected to good NDT afterwards - , X-ray, ultrasonic and crack testing. Once upon a time I used to do that sort of stuff, but for applications a lot more valuable than a damaged wheel.

    But there's always the issue of grain size (too large or too small), heat affected zone and other variations from homogeneity, even if the weld is clean, inclusion free and devoid of micro-cracks.

    Add to that the cyclic nature of a car wheel under load, it means the potential for eventual fatigue failure at or near the weld is a prospect that would put me off the idea. It's at least as risky as buying a second hand tyre; it could easily have come from a write-off that gave the side wall a good biffing before someone painted it up with tyre black and told you it was perfect.

    Are you feeling lucky?


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  9. Pontoneer

    Pontoneer MB Club Veteran

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    Insurance ?

    New tyre as well .
     
  10. hotrodder

    hotrodder Hardcore MB Enthusiast

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    The best welder in the known universe cannot weld aluminium without locally softening it. Car wheels are virtually always made from heat treatable flavours and precipitation hardened to a T6 temper whether they're cast, cast and flow formed, forged from one piece or a multi piece wheel with spun rim sections

    Production: the best quality from excellent suppliers - Following strict Mercedes-Benz guidelines - Daimler Global Media Site describes the process for MB wheels, GK-AlSi7 is a partial designation that's probably AlSi7Mg (aka EN-AC42000 / A356.0) or some proprietary variation of it. The heat treatment described is T6, solution treated and artificially aged although obviously exact details no doubt vary a little with the various different suppliers that make wheels for MB and various other car manufaturers... O.Z. for example use AlSi7Mg & a proprietary variation of it for low pressure casting and also AlSi10Cu for gravity casting some smaller wheels IIRC

    Weld a heat treated aluminium and the HAZ will contain partially annealed and overaged areas both of which result in grain coarsening and a drop in mechanical properties, both strength and usually ductility too i.e. it gets weaker and more brittle

    Welding is akin to casting in that grain structure is pretty much random. One of the reasons forgings are "stronger" and less prone to cracking is that they have a grain structure more akin to wood which is the result of squeezing the part into shape [/gross generalisation]. Flow forming and spinning impart the same grain structure into wheel rims by squeezing the material into shape. Unsurprisingly welding a forged/flow formed/spun part locally destroys this grain structure

    Most heat treatable flavours of aluminium can't really be welded with a matching filler wire for various reasons to do with solidification and or liquation cracking so even if it was economically viable to re heat treat a repaired wheel and go through the same amount of NDT that new wheels are put through the weld won't respond to heat treatment

    Obviously it's fairly complicated as the exact flavour of aluminium makes a difference as does the amount of heat input by the weldor but a rough generalisation is that in the 'as welded' state a best case scenario is around 60% of the origional parts UTS and ~ 50% of the origional yield strength for weld metal

    FWIW i'm a welder & fabricator that specialises in aluminium, stainless and exotics (read, i weld more magnesium or titanium etc than i do mild steel) and as a general rule i don't touch wheels... physically repairing them is dead easy, knowing what sort of properties the repair has on the other hand can't be answered without making the repair uneconomic most of the time. Aluminium, unlike steels or Ti, has no endurance limit i.e. anything made from Al and subject to cyclic stress WILL fail from fatigue at some point. https://en.wikipedia.org/wiki/Fatigue_limit goes into a little more detail

    All that said the chances of a repaired wheel, even a badly repaired wheel, failing catastrophically are gonna be pretty damn low. What usually happens is the wheel cracks again (either a fresh one or through a bad weld, through the HAZ of a better weld etc) and the crack gets found when looking for the cause of the flat tyre if it wasn't spotted at a service or MOT before the crack had grown big enough to let air escape
     
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  11. Tim203

    Tim203 Hardcore MB Enthusiast

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    Top explanation Hotrodder. I'm very curious if the outer rim is harder than the inner on wheels as my wife managed to dent the inside of the rim with no abrasion to the alloy or damage to the tyre. It was only discovered as the tyre was nearly flat. After beating it straight with a dead blow nylon hammer I was surprised how malleable and soft it was and it stopped leaking!
     
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  12. PobodY

    PobodY Hardcore MB Enthusiast

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    I stand (well, sit) corrected. - A cracked rim needs to be replaced and can only be cosmetically repaired.
     
  13. Doodle

    Doodle Hardcore MB Enthusiast

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    Better structural support.

    As width and offset increase, the inside edge moves further away from the spokes that not only join it to the hub, but also provide support to the shape of the wheel.

    Think of it as trying to push a door open with someone leaning against on the other side of it, vs not.
     
  14. Benzowner

    Benzowner Hardcore MB Enthusiast

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    As I have posted on this forum before, I am driving on repaired wheels and have been doing so for a couple of years. I appreciate the technicalities of repairing alloy wheels, but had a chat with the guy I used to repair my wheels, and the most damaged wheels he encounters are Mercs followed by BMW. Are German road better than our, probably, so are the wheels d capable of taking the hammering our potholes provide?
     
  15. Doodle

    Doodle Hardcore MB Enthusiast

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    On the basis of my recent trip to Bremen and Berlin, they are considerably better. They also don't seem to believe in speed bumps, which pepper our roads like an acne-plagued teenager.
     
  16. Teflonso

    Teflonso Hardcore MB Enthusiast

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    I hear there's a guy down the chip shop swears he's Elvis. Strangers are so dependable.

    Also, as they say in the financial adverts, "Past performance is no guarantee of future returns". The fact you've been doing it for a couple of years simply means you are 24 months closer to the fatigue failure that's going to catapult you into a hedge. If you're lucky it will be a hedge, not a concrete bridge.

    By way of information from wheel repairers, I had a bit of kerb rash buffed out and repainted lately. The guy told me that he's getting a lot of BMW wheels in for straightening after they started having their wheels made in Italy.

    Minor repairs and polishing - good. Straightening - acceptable if modest. Welding - rather you than me.

    I do wonder what the liability position would be if one had a wheel weld repaired, sold the car on and then it failed with catastrophic results. "Sold as seen" is not a legally valid opt-out of being liable for killing or maiming someone.



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  17. hotrodder

    hotrodder Hardcore MB Enthusiast

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    Cheers. As said it's not so much that the inner rim is softer, more that it has much less support so isn't as stiff

    While differential cooling rates might be used when casting stuff like wheels to help with porosity AFAIK there's no easy or reliable way to differentially harden ally... with cast irons and carbon steels cooling rate directly effects hardness so it's fairly easy to harden some areas of a part more than others. Katanas are good example, the rough sword blade is covered in clay before final heat treatment, lots of clay around the back of the blade insulates that area so it cools more slowly than the bare edge when quenched. This is also how katanas end up curved, martensite ('kin hard but brittle microstructure) is less dense than softer steel microstructures so the insulated areas that cool more slowly shrink more bending the blade

    Aluminium alloys are precipitation hardening which is kinda backwards to carbon steels and to do with changes in solubility with temperature... just like you can dissolve more salt in hot water than is possible in cold, metals can dissolve more alloying elements when hot. If they're allowed to cool naturally the excess escapes as they cool and does nothing useful. Quench them from a temp fairly close to their melting point and there isn't time for this to happen so the result is a super saturated solid solution. The excess still wants to escape but now can only do so as miniscule, fine particles which kinda fill in gaps in the crystal structure making the metal harder and stronger. This can take a looooonnng time so a precipitation heat treatment (aka artifical aging) is used to speed things up i.e. warm the thing up to 150 - 180ish °C for 8 - 12 hours. Because ally is an excellent conductor of heat it's not really practical, if even possible(?), to age some areas of a part more or less than others
     
  18. Doodle

    Doodle Hardcore MB Enthusiast

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    It would be impossible to prove liability, there is no requirements of quality or fitness in a private sale.
     
  19. John

    John MB Club Veteran

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    I imagine it is incredibly painful.
     
  20. Teflonso

    Teflonso Hardcore MB Enthusiast

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    Liability is irrepective of mode of exchange in UK law. Just as you can't "sign your rights away" if someone is going to put you at risk, someone who introduced a latent defect into an automobile is every bit as liable if they are a motor manufacturer or a back street bodger.

    You're right, proving it might be challenging. Getting, then recovering a big enough quantum to cover the massive legal costs would also be a non-trivial task. It certainly wouldn't be a job for the ambulance chasing lightweights who advertise on TV or cold call you.


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