Back wheel rim goes concave - why?

[Petrina Quinn]

In getting my bike serviced and ready for the AUDAX - Big Black Crow
ride this weekend... by who has to be one of the best bike mechanics in
the business - and he lives in Wagga Wagga (Place of Many Crows) - an
unusal observation was made ... well unusal to me.

The back wheel rim had developed a concave surface! With calipers the
measurement of the back wheel to the front was substantially less- i.e,
about 5%.

The view was that possibly - as I tend to brake using more back than
front brackes - I had worn the rim enough to then - under 100+ psi
cause the concave metal stress fracture on the aluminium rim. I weight
about 60- 63kgs.

The immediate solution - premised on safety - was to change the wheel -
which we did!

My question though is have others experienced a concave reformation on
their back wheel rims - and if so - what is believed to be the cause?

Petrina Quinn

 

[Donga]

Petrina Quinn wrote:
> My question though is have others experienced a concave reformation on
> their back wheel rims - and if so - what is believed to be the cause?

Yes, I changed a rear MTB rim recently for the same reason, plus a few
tiny hairline cracks around the spoke nipples. The hub and spokes were
still fine, so I built in a new rim, rather than getting a whole new
wheel. Hence it cost me $100 instead of a whole lot more. You might be
able to build yourself a spare rear wheel.

Donga

 

[Bleve]

Petrina Quinn wrote:
> In getting my bike serviced and ready for the AUDAX - Big Black Crow
> ride this weekend... by who has to be one of the best bike mechanics in
> the business - and he lives in Wagga Wagga (Place of Many Crows) - an
> unusal observation was made ... well unusal to me.
>
> The back wheel rim had developed a concave surface! With calipers the
> measurement of the back wheel to the front was substantially less- i.e,
> about 5%.
>
> The view was that possibly - as I tend to brake using more back than
> front brackes - I had worn the rim enough to then - under 100+ psi
> cause the concave metal stress fracture on the aluminium rim. I weight
> about 60- 63kgs.
>
> The immediate solution - premised on safety - was to change the wheel -
> which we did!

Or at least, the rim.


>
> My question though is have others experienced a concave reformation on
> their back wheel rims - and if so - what is believed to be the cause?

Brakes. It's perfectly normal.

 

[Dave Hughes]

On Wed, 30 Aug 2006 02:30:51 -0700, Bleve wrote:

> Brakes. It's perfectly normal.

Retro grouch.

I had the interesting sensation of having a loud squeal from the front
disc brake the other day, cruising down second slight downhill from home
after a nice little pootle. Sure enough, the pads had worn down to the
point that the return spring was contacting the rotor. Nice little warning
system that, though I knew they were getting low anyway.

I wonder what I can get for a razor sharp return spring on ebay? Probably
a trip to camp x-ray for supplying terrorist equipment!

--
Dave Hughes | [email protected]
"My ambition, naturally, is to have a student quote my own words back
to me without attribution in a final paper. That's an office hour I'd
look forward to." -- Kieran Healy, on plagiarism

 

[Peter Signorini]

"Petrina Quinn" wrote:

> The immediate solution - premised on safety - was to change the wheel -
> which we did!

Better solution is to just change the rim and get a new one built onto the
hub. It'd be stronger than a ready-built machine laced wheel.

> My question though is have others experienced a concave reformation on
> their back wheel rims - and if so - what is believed to be the cause?

Simply wear from braking. My rear rim gave out last Easter after 8 years of
riding. I'd guess that, even though most people put more brake load onto the
front rim, the rear rim and pads pick up more raised dust and dirt, causing
greater wear.

--
Cheers
Peter

~~~ ~ _@
~~ ~ _- \,
~~ (*)/ (*)

 

[531Aussie]

I once let a rim wear too far, and one day it EXPLODED, along with the tyre and tube, blowing off a section of rim about 12" long.

Luckily it was a rear wheel and I was going slow