rim wear

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Raph
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rim wear

Postby Raph » 14 Jun 2012, 10:04pm

I've done a search on this but due to my searching un-prowess it's come up with nothing relevant (though I'm sure I've seen threads on this...)

I'm just about to rebuild a wheel after it pinged a spoke last night, I think it took against my re-dishing it a bit carelessly a couple of times recently (long story).

The rim (Mavic open pro sup) seems totally straight, no holes pulled through or bulging even slightly, so I'll re-use it with new spokes all round, but I just want to check - there's about 1.4mm minimum thickness in the rim. It's hard to measure but I've put a caliper over a nail held in the tyre bead cavity, then subtracted the thickness of the nail - cos I couldn't get the caliper into the point where the rim is thinnest.

1 - any better methods of measuring the thickness of the rim at the thinnest point?

2 - any opinions on what's a reasonable thickness for safety?

xpc316e
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Re: rim wear

Postby xpc316e » 14 Jun 2012, 11:04pm

I have seen it recommended that you find a small ball bearing to use instead of the nail. Measure the diameter of the ball bearing with Vernier calipers, then insert it behind the rim and measure the thickness of the rim plus the bearing. All you have to do is subtract the bearing's diameter and you have your rim thickness. A quality ball bearing ought to be perfectly spherical and therefore it doesn't matter where you measure its diameter.
Riding a Dahon Jetstream P9 folder, a Decathlon Fitness 3 flat-barred road bike, a Claud Butler Cape Wrath MTB, a TW 'bents trike, a Moulton-based tandem, a Scott CR1 Comp, and the latest acquisition, a custom-built SWB, above seat steered recumbent.


Raph
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Re: rim wear

Postby Raph » 15 Jun 2012, 12:19am

Yes, I was measuring the rim + nail and subtracting the nail.

The ball bearing idea's good except it would be hard keeping the gauge on the apex of the ball bearing, especially as the ends of the caliper jaws are "knife edged" - but the idea of using a spoke is great, as the head of the nail was the only thing making it hard to use.

Just confirmed it's 1.4mm.

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531colin
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Re: rim wear

Postby 531colin » 15 Jun 2012, 7:31am

CJ bends a spoke so that it lies inside and outside the rim....ummmm...difficult to explain........
but it means the spoke sits in any depression worn by the pads, and also inside the bead hook, and you subtract 4mm from the gauge reading.
1.4mm is a generous thickness, can anybody confirm Open Pros are that thick new?

Ayesha
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Re: rim wear

Postby Ayesha » 15 Jun 2012, 7:35am

Image

Here's a quick photo of the front rim off my Dawes Giro ( Rigida Nova ). Dunno if you can see, there is a visible reduction of the brake track width ( a curved shoulder ). The bike is five years old and has about 15,000 - 16,000 miles behind it.

This is because I trim and set the brake blocks to NOT contact all the brake track.

With a 0 - 25mm micrometer, the worn width is 18.9mm from 19.6 0.35mm of wear each side.

Metal thickness is getting low. Looks like I'll be ordering a new pair of wheels later this year. If not sooner! Why not? A new pair of wheels :wink:

Ayesha
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Re: rim wear

Postby Ayesha » 15 Jun 2012, 7:45am

Here's a quick photo of the front rim off my Dawes Giro ( Rigida Nova ). Dunno if you can see, there is a visible reduction of the brake track width ( a curved shoulder ). The bike is five years old and has about 15,000 - 16,000 miles behind it.

This is because I trim and set the brake blocks to NOT contact all the brake track.

With a 0 - 25mm micrometer, the worn width is 18.9mm from 19.6 0.35mm of wear each side.

Metal thickness is getting low. Looks like I'll be ordering a new pair of wheels later this year. If not sooner! Why not? A new pair of wheels :wink:[/quote]

The tyre went back up to 110 psi without any metallic creaking sounds :D

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531colin
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Re: rim wear

Postby 531colin » 15 Jun 2012, 8:13am

Heres CJs spoke trick from an earlier thread somebody linked....sorry, I got confused, can't remember where it is now.....

Image

But I also saw something about wear indicators on open pros....I think!

Brucey
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Re: rim wear

Postby Brucey » 15 Jun 2012, 8:19am

531colin wrote:CJ bends a spoke so that it lies inside and outside the rim....ummmm...difficult to explain........
but it means the spoke sits in any depression worn by the pads, and also inside the bead hook, and you subtract 4mm from the gauge reading.
1.4mm is a generous thickness, can anybody confirm Open Pros are that thick new?


I use an old spoke on the inside of the rim and accept the possibility of a small error if the calipers must be angled to reach the bottom of a depression in the braking surface.

Here;

http://forum.ctc.org.uk/viewtopic.php?f=5&t=55007

new Open Pro rim thickness is reported as 1.3 -1.5mm.

Image

from

http://forums.mtbr.com/cyclocross/clincher-rim-cx-tires-748869.html
and
http://www.starbike.com/p/Mavic-Open-Pro-301-en

FWIW people often say that the rim braking surface 'goes concave' when they are worn out. IME this is as much because the rim is bulging outwards as much as anything else. If the brakes start 'pulsing' in use it is often because the rim is bulging.

As you can see an Open Pro is 19.6mm wide when new. If the brake surface gets concave looking and the verniers start reporting an overall width significantly more than 19.6mm then you know the rim is worn. The beauty of this test is that the tyre need not be removed, so it is very fast. You just need to know how wide your rims should be.

cheers
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RickH
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Re: rim wear

Postby RickH » 15 Jun 2012, 9:00am

531colin wrote:But I also saw something about wear indicators on open pros....I think!

Open Sport have a groove running all the way round the brake track, Open Pros don't appear to have anything (& there's no mention of any wear indicator on the website)

Brucey wrote:As you can see an Open Pro is 19.6mm wide when new.

My nearly new Open Pros (wheels re-rimmed in April, a couple of hundred miles of riding on them) both measure near enough exactly 2.0mm, admittedly with some cheap plastic verniers of unknown provenance (the battery in my digital one is flat). What would you class as "significantly more"? I'll have to measure them with the tyres flat to see if that makes a measurable difference (they have 25mm Conti Gatorskins running at c100psi).

Rick.

Brucey
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Re: rim wear

Postby Brucey » 15 Jun 2012, 9:06am

well I think you have answered your own question there; you need to take an accurate measurement. You won't have worn out your rims yet....But if the rim width is genuinely up more than 0.5mm at the lips then something has happened, which would then trigger me to measure the thickness in other ways.
Note my suggestion of measuring the overall width is a quick check, not a complete substitute for other measurements.

cheers
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RickH
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Re: rim wear

Postby RickH » 15 Jun 2012, 9:35am

Letting down the front tyre appears to reduce the total width by about 0.1mm (I was getting 19.9mm with negligible pressure) & back to 20.0 again with the tyre re-inflated to 95psi. The verniers I was using look like a magazine freebie as they have "Practical Motorist" moulded into them! :? I'm not worried about the rims, the grey on the brake track isn't all worn off yet. The measurement is most likely to be inaccurate but seems to be precise enough as it is repeatable. I'll take 19.9/20.0mm as "normal" for my rims with those verniers.

Rick.

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531colin
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Re: rim wear

Postby 531colin » 15 Jun 2012, 10:45am

Here..http://forum.ctc.org.uk/viewtopic.php?f=5&t=18238&p=147656&hilit=rim+wear+rim+failure+pump

TC seems to think open pros have a wear tell-tale

EDIT....proper description and everything lower down this thread

Raph
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Re: rim wear

Postby Raph » 15 Jun 2012, 2:33pm

I used the spoke method which takes into account the very slight concaveness in the braking surface, which is roughly a 0.1mm dip in the middle compared to the edges, and the result is 1.22mm wall thickness at the thinnest point.

Probably worth re-building then.

Thanks for all the replies!

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andrew_s
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Re: rim wear

Postby andrew_s » 16 Jun 2012, 5:35am

The easiest way to measure rim wall thickness is to get a tenth gauge (£5.25 here)

When I've checked sections of failed or almost failed rims, the remaining thickness has been about 0.7mm. The usual recommendation is to arrange to replace the rim if it's below 1mm.

Brucey wrote:If the brakes start 'pulsing' in use it is often because the rim is bulging.

Pulsing brakes are due to dented flanges from hitting a pothole or rock, a patchy rim coating, or the rim bulging because the rim has already started to fail. If the degree of pulsing changes when you change the tyre pressure, it's definitely a failing rim and you should reduce the tyre pressure and go straight home.
If you notice pulsing and haven't just hit a pothole, check straight away. A rim can progress from a "pulsing" crack 2 -3cm long to "bang" very quickly.