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[Andy B]

I removed a wheel to clean off all the old goo from previous balance weights. When it was all nice & clean I found this ... 

[Doctor Gollum]

What size are they?

[Andy B]

What size are they?

9J x 20

[Sir Tigger KC]

What will you do Andy?  Get it welded or buy a new/newish one?   

[Andy B]

What will you do Andy?  Get it welded or buy a new/newish one?   

I've ordered a used rim off eBay. I think I'll get the cracked rim welded as a spare for when I find another cracked rim 

[Migv6 le Frog Fan]

Big rims, ultra low profile tyres, potholes = cracked rims. 

[Andy B]

Big rims, ultra low profile tyres, potholes = cracked rims. 

They're low profile, but wouldn't say they're ultra low. 265/45 x 20
I can't remember now what profile I had on my Omega .... 235/45 x 17 if so, the wall is actually taller on the ML that it was on the Omega.
I've had it suggested that straddling speed 'pillows' is the problem 

[STEMO]

Big rims, ultra low profile tyres, potholes = cracked rims. 

They're low profile, but wouldn't say they're ultra low. 265/45 x 20
I can't remember now what profile I had on my Omega .... 235/45 x 17 if so, the wall is actually taller on the ML that it was on the Omega.
I've had it suggested that straddling speed 'pillows' is the problem 

Straddling speed pillows (everyone does it, all the time) might be bad for your alignment, by forcing the wheels outwards from the bottom, but I really can't see how it would crack a rim like that. The tyre would deform, but not enough to bust the rim, surely? If it really was the case, I'm sure we'd see a lot more of this.
Just follow any vehicle and you'll see them doing it, it's a natural compensation to stop one side of the vehicle taking a hit.

[STEMO]

You could always slow right down, but nobody does that either 

[Andy B]

You could always slow right down, but nobody does that either 

There are half a dozen or so pairs of speed humps on the main drag through our estate ... I do try not to take the pi$$ but 20mph isn't easy  . I straddle the pillows in the Merc but go between them in the Smart! 

[johnnydog]

Big rims, ultra low profile tyres, potholes = cracked rims. 

They're low profile, but wouldn't say they're ultra low. 265/45 x 20
I can't remember now what profile I had on my Omega .... 235/45 x 17 if so, the wall is actually taller on the ML that it was on the Omega.
I've had it suggested that straddling speed 'pillows' is the problem 

Straddling speed pillows (everyone does it, all the time) might be bad for your alignment, by forcing the wheels outwards from the bottom, but I really can't see how it would crack a rim like that. The tyre would deform, but not enough to bust the rim, surely? If it really was the case, I'm sure we'd see a lot more of this.
Just follow any vehicle and you'll see them doing it, it's a natural compensation to stop one side of the vehicle taking a hit.

The weakest part of an alloy rim is the inner edge of the rim where there is no strength from the spokes. Straddling 'pillows' puts unnecessary strain on the weakest part of the rim. If the profile is such that the tyre absorbs the deforming, no problem, but low profile tyres can result, in the worse case scenario, damage to the rim, which is then usually only noticed when balancing replacement tyres.
I'd rather drive between two of them with one wheel, and drive over the top of the adjacent 'pillow' with the other wheel, which is a pain.
I hate speed humps, 'pillows', chicanes and all forms of traffic calming measures with a passion, as in reality, they are vary rarely well thought out in their planning.

[Sir Tigger KC]

Scuse me for being a bit fick, but what's 'straddling the pillows' ?       

[Doctor Gollum]

Scuse me for being a bit fick, but what's 'straddling the pillows' ?       

That makes it sound pervy 

Basically negotiating those square speed humps with a wheel either side. Which ends up running the inner edge of the wheel over a smaller crown than the whole hump, but still higher than the road surface where the outer edge of the wheel is. Effectively this puts the full weight of the vehicle on the inner edge of wheel as you drive it over the front and rear edge of the speed hump. Doing this at a pace smacks the inner rim into what is usually a straight edge,  which significantly increases the loading on the wheel in a manner that it cannot support.

[STEMO]

Big rims, ultra low profile tyres, potholes = cracked rims. 

They're low profile, but wouldn't say they're ultra low. 265/45 x 20
I can't remember now what profile I had on my Omega .... 235/45 x 17 if so, the wall is actually taller on the ML that it was on the Omega.
I've had it suggested that straddling speed 'pillows' is the problem 

Straddling speed pillows (everyone does it, all the time) might be bad for your alignment, by forcing the wheels outwards from the bottom, but I really can't see how it would crack a rim like that. The tyre would deform, but not enough to bust the rim, surely? If it really was the case, I'm sure we'd see a lot more of this.
Just follow any vehicle and you'll see them doing it, it's a natural compensation to stop one side of the vehicle taking a hit.

The weakest part of an alloy rim is the inner edge of the rim where there is no strength from the spokes. Straddling 'pillows' puts unnecessary strain on the weakest part of the rim. If the profile is such that the tyre absorbs the deforming, no problem, but low profile tyres can result, in the worse case scenario, damage to the rim, which is then usually only noticed when balancing replacement tyres.
I'd rather drive between two of them with one wheel, and drive over the top of the adjacent 'pillow' with the other wheel, which is a pain.
I hate speed humps, 'pillows', chicanes and all forms of traffic calming measures with a passion, as in reality, they are vary rarely well thought out in their planning.

Fair enough, but I wouldn't say 120mm, as on Andy's car, is low profile.

[amba]

Even with the Omega I tend to drive full wheel over the top of 1 and let the other go between on flat ground.Yes a real PITA as requires compliance from oncoming traffic and at the speed I go over them tends to really p*** over anything behind but with the weight I carry in the boot exhaust bottoms out if I try and straddle them.

I hate any form of traffic calming as it just encourages most to use them like a slalom course.

[johnnydog]

It's difficult to say what has caused that crack - it's not the usual buckle on an alloy that you would expect from a pothole / kerb impact. All I can think of if the wheel has had some form of impact on another part of the rim which in turn has caused the rim to flex somewhat in the process, and not being  strong enough to absorb the flexing, has caused the edge to crack at another place, rather than buckle near the point of impact? Can't think of any other cause...?

[Andy B]

It's difficult to say what has caused that crack - it's not the usual buckle on an alloy that you would expect from a pothole / kerb impact. All I can think of if the wheel has had some form of impact on another part of the rim which in turn has caused the rim to flex somewhat in the process, and not being  strong enough to absorb the flexing, has caused the edge to crack at another place, rather than buckle near the point of impact? Can't think of any other cause...?

Though it's probably hit a pothole or 2, there's no other obvious impact damage. 

[Field Marshal Dr. Opti]

I removed a wheel to clean off all the old goo from previous balance weights. When it was all nice & clean I found this ... 

?

Is this a common well known fault of the breed, or unusual?

[Andy B]

....
Is this a common well known fault of the breed, ...

Not that I know of 

[Doctor Gollum]

....
Is this a common well known fault of the breed, ...

Not that I know of 

If by breed, you mean large diameter alloys, then yes.

Had to replace two wheels on the second Insignificant due to similar damage.

[Field Marshal Dr. Opti]

....
Is this a common well known fault of the breed, ...

Not that I know of 

If by breed, you mean large diameter alloys, then yes.

Had to replace two wheels on the second Insignificant due to similar damage.

I believe Mr Beanz to be  the most wealthy man in Bury, so replacement should be financially painless.

[Andy B]

.....
I believe Mr Beanz to be  the most wealthy man in Bury,

I wish! 

so replacement should be financially painless.

A USED replacement is on its way 

[Sir Tigger KC]

Scuse me for being a bit fick, but what's 'straddling the pillows' ?       

That makes it sound pervy 

Basically negotiating those square speed humps with a wheel either side. Which ends up running the inner edge of the wheel over a smaller crown than the whole hump, but still higher than the road surface where the outer edge of the wheel is. Effectively this puts the full weight of the vehicle on the inner edge of wheel as you drive it over the front and rear edge of the speed hump. Doing this at a pace smacks the inner rim into what is usually a straight edge,  which significantly increases the loading on the wheel in a manner that it cannot support.

Ah I see. Thanks for the explanation. 

The places I tend to hang out, folk are more likely to straddle a sheep than a pillow. 

Isn't that right Rae? 

[BazaJT]

Some years ago a cracked rim[or more than one]was a common occurrence on BMWs.

[Lizzie Zoom]

I must have been lucky, but my front near side wheel has had two large direct ‘hits’, once with a large pothole, the other, my fault, when I collided with a speed reducing, sticking out in the road, kerb. Although I lost the low profile tyres in both cases, the alloy received no damage at all, which was confirmed by the specialist tyre shop I use.

Near miss obviously

[Doctor Gollum]

The 235/45/17 tyres on your Omega aren't exactly low profile, nor the wheels, although the early MV6 wheels were known as Toffee wheels for a reason. Later MV6 and Elite wheels are as strong as the 16" ones,  but weigh twice ss much as the original MV6 ones.

The 225/55/16s on the CD/CDX wheels fitted to plods were basically bombproof.

The wheels that suffer are those that are not only greater diameter, but also wider. To keep the weight down,  these types of wheels can be lacking in material thickness, which makes them prone to buckling, cracking and other stress related damage.

Anything much over 18"/7J is potentially susceptible, and the larger those two numbers,  the greater the liklihood.

[Lizzie Zoom]

The 235/45/17 tyres on your Omega aren't exactly low profile, nor the wheels, although the early MV6 wheels were known as Toffee wheels for a reason. Later MV6 and Elite wheels are as strong as the 16" ones,  but weigh twice ss much as the original MV6 ones.

The 225/55/16s on the CD/CDX wheels fitted to plods were basically bombproof.

The wheels that suffer are those that are not only greater diameter, but also wider. To keep the weight down,  these types of wheels can be lacking in material thickness, which makes them prone to buckling, cracking and other stress related damage.

Anything much over 18"/7J is potentially susceptible, and the larger those two numbers,  the greater the liklihood.

Thanks DG, very interesting