How hot is too hot?

Through a combination of a friend recollecting how he ended up being
air-lifted to hospital after his front tube went bang[1], and discovering
after ~989m of descending[2] that my front wheel rim was too hot to touch
for more than a few seconds, I've been wondering how close to a "Bang!"
I've been getting.

Or to put it another way, should I go out and purchase some discs
instead? :-) And if I was to go for Avid BB7s, for instance, would
there be benefits to considering a 185 or 205mm front disc over the
165mm? (I'd assume that the braking could be better, and the heat
capacity larger but actually worth it?)

[1] Though I think his descending technique is poor, as it seems to
involve riding the brakes most of the way down. Or at least, to a
much greater extent than I do.
[2] Over ~7.93km.
--
These opinions might not even be mine ...
Let alone connected with my employer ...

In article ,
Alistair Gunn wrote:
Through a combination of a friend recollecting how he ended up being
air-lifted to hospital after his front tube went bang[1], and discovering
after ~989m of descending[2] that my front wheel rim was too hot to touch
for more than a few seconds, I've been wondering how close to a "Bang!"
I've been getting.

Surely you would mainly use the rear brake for descending ? That way
(a) if you do get a blowout you can probably still keep control and
(b) if you get bad brake fade you've got the front to stop with so you
can wait and let things cool down.

--
Ian Jackson personal email:
These opinions are my own. http://www.chiark.greenend.org.uk/~ijackson/
PGP2 key 1024R/0x23f5addb, fingerprint 5906F687 BD03ACAD 0D8E602E FCF37657

Ian Jackson twisted the electrons to say:
Surely you would mainly use the rear brake for descending ? That way
(a) if you do get a blowout you can probably still keep control and
(b) if you get bad brake fade you've got the front to stop with so you
can wait and let things cool down.

I tend to go for the "brakes off on the straights, then cram both anchors
on for the corners" approach.
--
These opinions might not even be mine ...
Let alone connected with my employer ...

"Alistair Gunn" wrote in message
...
Ian Jackson twisted the electrons to say:
Surely you would mainly use the rear brake for descending ? That way
(a) if you do get a blowout you can probably still keep control and
(b) if you get bad brake fade you've got the front to stop with so you
can wait and let things cool down.

I tend to go for the "brakes off on the straights, then cram both anchors
on for the corners" approach.

It's a classic tandem problem, because they can go rather faster downhills.

Brakes off on the straights, slam on for corners can be best - the faster
you go, the more energy you lose to air resistance, and hence the less there
is to be dissipated into the rims. Plus the cooling effect.

However it's not always that easy...

Dragging them all the way down is likely to be worst though, unless you're
going really very slowly indeed.

So, I do a combination of both - let air resistance do as much as possible,
and favour the back brake for slowing down, leaving capacity in the front to
stop if something tricky happens. Even losing the back tyre can be a little
exciting (Boscastle this Easter - sidewall gave way, not through heat,
loaded tandem, was quite hard to control), so I really don't want to lose
the front. And on the tandem I've also got a drum brake to take care of the
heat.

Clive George twisted the electrons to say:
Brakes off on the straights, slam on for corners can be best - the faster
you go, the more energy you lose to air resistance, and hence the less
there is to be dissipated into the rims. Plus the cooling effect.

However it's not always that easy...

nods I think avoiding the truly prodigious number of cowpats on the
upper sections may have resulted in more dragging than is usual for me!
--
These opinions might not even be mine ...
Let alone connected with my employer ...

In article ,
Alistair Gunn wrote:
Through a combination of a friend recollecting how he ended up being
air-lifted to hospital after his front tube went bang[1], and discovering
after ~989m of descending[2] that my front wheel rim was too hot to touch
for more than a few seconds, I've been wondering how close to a "Bang!"
I've been getting.

That depends on whether you use natural rubber or butyl :-)

More seriously, it will depend a lot on the tyre materials and
construction. It's very hard to guess.

[1] Though I think his descending technique is poor, as it seems to
involve riding the brakes most of the way down. Or at least, to a
much greater extent than I do.

Alternatively, he is less interested in claiming a Darwin award
than you are :-)

Again, more seriously, it will depend on your relative weights and
abilities to control bicycles at high speed (assuming that the road
is safe for those). And the latter will depend a lot on the bicycle
as well as the rider.


Regards,
Nick Maclaren.

On Thu, 24 Sep 2009 16:01:25 +0000 (UTC)
Alistair Gunn wrote:

Through a combination of a friend recollecting how he ended up being
air-lifted to hospital after his front tube went bang[1], and
discovering after ~989m of descending[2] that my front wheel rim was
too hot to touch for more than a few seconds, I've been wondering how
close to a "Bang!" I've been getting.

Or to put it another way, should I go out and purchase some discs
instead?

You could just give the rim an occasional squirt from your bidon.

On Thu, 24 Sep 2009 21:03:19 +0100
Rob Morley wrote:

You could just give the rim an occasional squirt from your bidon.

Careful! You'll get those who didn't understand the word "peleton" all
hot and bothered....


Mike
--
Mike Causer

Thu, 24 Sep 2009 21:03:19 20090924210319.6878947d@bluemoon
uk.rec.cycling Rob Morley

On Thu, 24 Sep 2009 16:01:25 +0000 (UTC)
Alistair Gunn wrote:

Through a combination of a friend recollecting how he ended up being
air-lifted to hospital after his front tube went bang[1], and
discovering after ~989m of descending[2] that my front wheel rim was
too hot to touch for more than a few seconds, I've been wondering how
close to a "Bang!" I've been getting.

Or to put it another way, should I go out and purchase some discs
instead?

You could just give the rim an occasional squirt from your bidon.

Ummm, presuming we are both thinking a bidon is a can full of easily
ignitable fluid and someone has a very hot wheel are you seriously
suggesting the two should meet?

petrol, in addition to its well known burning properties, is also a
lubricant, so it isn't going to aid braking (which requires friction)
much.

Yours confused,
--
Wm...
Reply-To: address valid for at least 7 days

"Wm..." writes:


Ummm, presuming we are both thinking a bidon is a can full of easily
ignitable fluid and someone has a very hot wheel are you seriously
suggesting the two should meet?

petrol, in addition to its well known burning properties, is also a
lubricant, so it isn't going to aid braking (which requires friction)
much.

Yours confused,

For cyclists (especially French ones) a bidon is a water bottle.