Revised: Stone doesn't eat brake caliper. Rim eats caliper.

Robzimbo

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I am in kenya
Dec 4, 2016
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Not sure where to post this - 'Brakes', 'Trip reports', 'pub chat'... but it happened to my 105 so I'll put it here, though I guess it could have happened to any vehicle.

I was up in Meru National Park a couple of weeks ago. I crossed a river, and soon after there was a scraping sound from the front left wheel. I stopped and had a look. Couldn't see anything. Didn't take the wheel off though. The scraping noise continued, on and off, for an hour or three, moving slowly through the Park, then out onto the road, and for a hundred kms or so after that. It didn't sound serious. Silly me.

After a while it stopped, and I pressed on for Nairobi. A hundred kms or so further down the road, the brake warning light came on. Then the brakes got softer. Then they stopped working altogether. No brake fluid.

I pulled into a garage, found a 'fundi' (local for roadside mechanic) who took the wheel off, and this is what he found ...

IMG_7791.JPG

IMG_7792.JPG


Somehow, a stone had got wedged into the wheel rim. Every time the wheel went round, it rubbed against the caliper. Just before it wore through the stone, or the stone got ejected, it rubbed through to the cavity where the cylinder sits, and sprang a leak. A tiny, slow leak, but every time I pressed the brake pedal some fluid leaked out. That much is clear.

But, what I don't understand is this; how did the stone get wedged in the rim? why didn't it get displaced the first time it hit the caliper? how could it move, while remaining wedged in place, so that it scraped away the metal of the caliper?

The tyre is a 16 x 285 - so almost exactly a meter in diameter. So circumference is 3.14 meters. So thats approx 320 rotations per kilometer. So over (approx) 200 kms, that's 64,000 rotations. Say it was only scraping half the time, that's 30,000 impacts on the caliper... Yes, I know. I really should have taken the wheel off to see what was going on.

But... how could it have stayed in place? What kind of stone would resist 30,000 impacts against metal, yet not be obvious on a cursory inspection? Did I have a bloody great diamond wedged in my wheel for 200 kms?

Hmmmm.

Anyway. A second hand caliper cost a hundred quid, more or less. New brake pads, fluid, labour... not a cheap mistake. But a very thorough teaching of a lesson that I should have learned long ago...

'if it's making a funny noise, don't keep driving and hope it goes away. Stop and find out what's going on.'

All the best,

Rob
Nairobi
 

Robzimbo

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Dec 4, 2016
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And to add to the strangeness - I reversed several times during the process. You'd have thought that would have dislodged what it was in the rim, when it hit the other end of the caliper going the other way.

My guess is, an enormous diamond... :)
 
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Towpack

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My guess is, an enormous diamond... :)

Right, get that calliper fixed and retrace your steps!!

Seriously though, never seen anything like that before. Disc damage from a lodged stone yes but not on that scale. Glad you got away with it:thumbup:
 

clivehorridge

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Incredible. I can only imagine that the slope of the rim edged the stone across, like a cutting tool in a lathe traveler.

My steel rims, (the part the tyre sits in) slope very slightly, from a smaller dia at the mounting "flange" to a slightly larger dia at the outside edges.

I'm not explaining myself very well here, but in cross section, a straight line from inner to outer, would show the rim to be smaller dia in the middle, compared with the edges. There's only a mm or less in it, but they do slope.

JMHO...
 

Robzimbo

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Dec 4, 2016
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That, Clive, is the most plausible explanation yet.

So the stone (which was therefore very small, and very hard... my diamond theory again!) initially was trapped vertically between the point where the rim goes from nearly vertical to nearly horizontal, and the protrusion on the inner edge of the rim where the bead sits... as you say, as it eroded the caliper, it moved inward (towards the disc), cutting through the metal in 30,000 strokes, driven by the angle of the 'horizontal' plane of the rim (and through centripetal force? Seems unlikely, that bit, but something must have kept it pressed against the caliper with enough force to keep cutting)

I'm in SA at the moment, so I can't see if the caliper sits closer to the rim at the lower end than at the upper end, but it must do, as the 'cutting tool' cleared the upper end of the caliper as it rotated.

A more than million to one chance - it would have been hidden on the inside of the rim when I looked. Exactly the right size stone, in exactly the right position, with exactly the right hardness to cut through cast steel.

And your explanation would also explain why it was actually a very mild noise, the scraping sound - it didn't sound like something serious, just the kind of scrape-scrape that you'd hear if, say, one of the mud deflectors was rubbing against the tyre. Nothing more dramatic than that.

Wow. We always joke, down here, that Africa eats cars. But I've never seen it actually happen....
 

clivehorridge

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It is rather odd though, you'd expect the stone to jam into the caliper and score the rim if anything, not the other way around, or am I tripping again on this cola I'm drinking?

It's an amazing piece of machining though, just like it's been milled.
 

frank rabbets

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That caliper was lose with only one bolt attached and therefore resting on the rim which has made a profile of itself on the caliper. Seen it before.
 

Shayne

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I'm looking at that and thinking a bolt fell out and the caliper was in contact with the wheel itself ?
 

Robzimbo

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And I'm thinking Frank and Shayne are right. Thanks for that. It's a weight off my mind to know that there isn't a bloody great diamond in a ditch somewhere up beyond Embu.

But I'm kicking myself for not actually doing the work myself - too many phone calls to make to change plans because of being stuck by the side of the road. The fundi by the roadside didn't mention a missing bolt - but he might not have noticed, and he just blocked off the brake hose to that wheel, refilled and bled the system, and took a few quid for his pains. The lad who put the new caliper on didn't remark on the lack of a bolt, but then, I wouldn't necessarily expect him to.

Lesson learned number 2 - 'If you want it done properly, do it yourself.' And number 3 - 'the simplest explanation is the most likely explanation'.
 

Shayne

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Have a long hard look at the wheel it was sand/dust/grit doing the cutting and that should have cut both surfaces !
 

frank rabbets

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If you look at the picture you can see where the wheel has cut through the bolt threads !! It looks like it's worn through the fluid gallery which joins the 2 halves of the caliper together exactly between the 2 bolt threads. Really interesting.
 

Robzimbo

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Frank, Shayne, thanks a lot for this. I now have a new (second hand but good) replacement caliper fitted. I'm away on business in South Africa for a couple of weeks, but will give it all a good check when I get home to Kenya.

Here's a picture I took after limping back home a couple of weeks ago. You can see why I was confused. It's really bizarre - maybe the lower bolt broke, and the upper one was loose, so the caliper bounced against the wheel? There's definitely some wear on the wheel rim, but it's nothing compared to the wear on the caliper.

I'll talk to the mechanic who fitted the caliper when I'm home - but he certainly didn't remark on a broken bolt or a 'flying caliper' when I picked the car up. He was also sure it was a stone, but also couldn't work out the physics of it...

IMG_7779.JPG


Thanks for your thoughts, all.

Rob
Pretoria
 

frank rabbets

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2 things can happen.

1. The caliper can lean permanently on the rim.

2. The caliper is in the normal position until you brake then the caliper is forced into the rim.
 

Robzimbo

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Or possibly, in this case, both 1 and 2. Caliper forced up to the rim, where it rest and abraded until there was no point of contact (end of noise); then brake applied, caliper forced against the rim, a bit more abrasion...
 

StarCruiser

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It depends on which bolt comes out. I had one drop out (I didn't tighten it for sure) and the calliper would dig into the rim when going in reverse. Quite a jolt.
 

clivehorridge

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The missing bolt theory is much more plausible than my "traversing stone" musings.

I write claims for a living, so I can explain anything, even complete and utter nonsense :laughing-rolling:
 
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