Overinflating tyre to make rim explode

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Blackredgold1964
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Overinflating tyre to make rim explode

Postby Blackredgold1964 » 19 Jun 2019, 10:29am

Hi All,
I want to check rim wear on a bike without rim wear indicator. The advice was to grossly overinflate, but by how much? Is it twice the pressure that's mentioned on the tyre, and then for a short while?
Another version was to 'slightly' overinflate, and then leave over night. Again, what pressure is 'slightly'?
Any advice welcome.

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RickH
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Re: Overinflating tyre to make rim explode

Postby RickH » 19 Jun 2019, 11:30am

A problem I can see is that you may not destroy the rim but may weaken it so that it then fails in use!

A better, & definitely more benign, solution is to buy an "iwanson gauge" (used for fine measurement by dentist & jewellers). EBay has numerous offers, amazon too, for less than £5. I got mine from an ebay seller based in Stockport.

You can then measure actual rim thickness round the rim (do it in several places as rim thickness may well not be even). You don't even have to remove the tyre, just deflate it & push the bead away from the rim so you can get the gauge in straight. A good rule of thumb is that the rim should be replaced before the thickness gets below 1mm.

Brucey
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Re: Overinflating tyre to make rim explode

Postby Brucey » 19 Jun 2019, 11:45am

if you don't take the tyre off and measure the rim thickness you can't be 100% sure of the wear state of the rim. In lieu of this a pretty good test is (with the tyre at low pressure) to measure the rim width, using accurate Vernier calipers. Compare the width with the specifications for the rim. Then inflate the tyre to the maximum pressure and re-measure.

If in either case the rim measures appreciably wider (at the OD) than the specifications then this should flag warning bells. So should an increase in rim width when the tyre is inflated.

Simply overpressuring the tyre is a good test but it doesn't prove that the rim is good beyond doubt. The reason is that using the brakes imposes a rather different load on the rim; essentially it imposes a bending fatigue load on the braking surface. Once things start to go bad they go bad very quickly indeed, so you could have a rim that passes a tyre pressure test, but then use the brakes for a few hundred yards only to find that the rim suddenly fails.

BTW the reason measuring the braking surface thickness (with the tyre out of the way) is such a revealing test is simple; the strength of the rim goes as the cube of the wall thickness. This means that if (say) the rim is OK at 1.0mm thickness, once worn to 0.8mm or 0.7mm thickness the stresses in the rim are (in round numbers) doubled or tripled respectively.

cheers
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andrew_s
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Re: Overinflating tyre to make rim explode

Postby andrew_s » 19 Jun 2019, 5:42pm

I tried a pressure test once, out of curiosity after I'd noticed that the concavity of the braking surface was more pronounced than would have been reasonable from pure wear - i.e. the outer part of the braking surface had started to bend outwards.
As Brucey noted, once that starts to happen, the brake blocks pushing it back in with each wheel revolution fatigue it fairly quickly.

It failed at about 160 psi, very loudly indeed, whilst I was still pumping (at the far end of the track pump hose, in case of flailing strips of broken off rim).
A post-failure thickness measurement gave about 0.6 to 0.7 mm.
If you do pressure test, it is useful to leave the tyre at high pressure overnight. A couple of clubmates have reported the rims failing several hours after the bike had been parked (after a normal ride, rather than pressure testing).

It's much better to get an Iwanson gauge. They only cost a little more than the inner tube that burst during the pressure test.
You should be able to measure just by letting out all the air and pushing the tyre back from the rim.

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iow
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Re: Overinflating tyre to make rim explode

Postby iow » 20 Jun 2019, 10:01am

Depending on the geometry of the rim, an iwanson gauge may not always show the true picture.
The rim below showed c. 0.75mm (edit) when measured, but the reality was a lot less. Unbelievably it hadn't failed despite being paper thin, but I certainly got my money's worth (H Plus Son TB14)
Image Attachments
rim.jpg
Last edited by iow on 20 Jun 2019, 10:33am, edited 2 times in total.
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Bonefishblues
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Re: Overinflating tyre to make rim explode

Postby Bonefishblues » 20 Jun 2019, 10:06am

That would have failed Brucey's test in spades, I think!

reohn2
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Re: Overinflating tyre to make rim explode

Postby reohn2 » 20 Jun 2019, 10:16am

iow wrote:Depending on the geometry of the rim, an iwanson gauge may not always show the true picture.
The rim below showed c. 7.5mm when measured, but the reality was a lot less. Unbelievably it hadn't failed despite being paper thin, but I certainly got my money's worth (H Plus Son TB14)

I take your point,but even at an inaccurately 0.75mm* the rim would've been binned,0.9mm is my minimum for rim wear.

*I'm assuming the decimal point is an error :wink: .
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Brucey
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Re: Overinflating tyre to make rim explode

Postby Brucey » 20 Jun 2019, 10:22am

TB14 looks like this when new

Image

FWIW some rims are designed to wear through into a hollow section below the rim tape before the rim fails wholesale. Its not obvious that the TB14 is one such but it might have failed in this way before it split higher up.

Image

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RickH
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Re: Overinflating tyre to make rim explode

Postby RickH » 20 Jun 2019, 10:44am

iow wrote:Depending on the geometry of the rim, an iwanson gauge may not always show the true picture.
The rim below showed c. 7.5mm when measured, but the reality was a lot less. Unbelievably it hadn't failed despite being paper thin, but I certainly got my money's worth (H Plus Son TB14)

As others have said, I presume you mean 0.75mm? I've yet to come across a rim with a 7.5mm thick rim wall (tough but rather heavy)!

Although a failure caused by wearing into one of the cavities is less of a problem. It will affect braking but should still be structurally sound enough to ride carefully to get somewhere sensible (home/ bike shop).

A number of years ago (probably '96), I was about halfway round a 100 mile ride, at about my furthest point from home, when I noticed the back brake starting to catch. Closer inspection revealed a small split/ hole starting to appear in the rim. I rode the 50 miles home trying not to use the back brake.

Not so much of a problem now I ride, pretty much exclusively, disc brake bikes. The iwanson gauge is still useful for checking disc rotor wear.

Bmblbzzz
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Re: Overinflating tyre to make rim explode

Postby Bmblbzzz » 20 Jun 2019, 12:10pm

Pretty obvious, but if do carry out a destructive test, make sure you're wearing eye protection (and ear protection preferably too). And if you're going to leave it overnight, leave it where no one will be walking past.

Blackredgold1964
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Re: Overinflating tyre to make rim explode

Postby Blackredgold1964 » 20 Jun 2019, 10:02pm

Dear All, Bmblbzzz, brucey, RickH, reohn2, Bonefishblues, iow, andrew_s,
Thanks for excellent advice and I've learned a lot now. I'll be getting an iwanson gauge soon.

Regards

LollyKat
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Re: Overinflating tyre to make rim explode

Postby LollyKat » 24 Jun 2019, 9:10pm

How do you avoid the lip on the rim when using the Iwanson gauge? Add a ball bearing? Wouldn't one with arms/legs like an upside down question mark be easier (but I can't find one with a gauge)?

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RickH
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Re: Overinflating tyre to make rim explode

Postby RickH » 24 Jun 2019, 9:18pm

LollyKat wrote:How do you avoid the lip on the rim when using the Iwanson gauge? Add a ball bearing? Wouldn't one with arms/legs like an upside down question mark be easier (but I can't find one with a gauge)?

The "jaws" that you measure with are curved so they will fit round the fatter part of the sidewall.

Here is an image on one in action (obviously not on a rim)
Image

LollyKat
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Re: Overinflating tyre to make rim explode

Postby LollyKat » 24 Jun 2019, 9:29pm

Of course - I wasn't looking at it properly. :oops: (I have an appointment with the opthalmologist next month.)

Thanks.