Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=tandem.9711.0546.eml
From: SANTANAINC(AT)aol.com
Date: Mon, 24 Nov 1997 18:57:38 -0500 (EST)
Subject: Rim vs. Disc brakes

Rim vs. Disc brakes

Santana's brake tests: are rim brakes safe enough?

A fourth and final installment from Bill at Santana:

We recently used Claraboya (a 7/10ths-of-a-mile hill with a constant 18-21%
grade) to evaluate a rim's contribution to downhill safety. If you missed the
preceding 3 installments, they cover the evolution of Santana's brake testing
program and reveal that most disc brakes are failure-prone when installed on
a tandem.

In the case of our recent rim tests, we wanted to know if a deep-section
(semi-aero) rim with large hollow chamber and increased surface area would be
safer than a generic box-section aluminum rim. Unfortunately, the test of the
deep section rim was cut short when (just before the second "stop and spin")
the tire suddenly went flat. A subsequent inspection of our
temperature-recording tapes show the "heat-resistant" rim-strip failed when
the aero rim reached 365 degrees (Fahrenheit). The flat occurred when forty
portions of tube herniated into the rim's deeply recessed spoke ferrules.

A second test with a standard single-wall aluminum rim used an identical
tire, tube, rimstrip and heat-recording tapes. The standard rim surprised
everyone by actually proving itself safer. The tandem rolled a quarter mile
past the aero-rim failure-point before the tire finally blew off the rim. The
subsequent inspection showed the tire itself was still OK. But a portion of
the rim was permanently deformed from where the tire had pushed aside the
heat-softened aluminum. The recorded heat of 425 degrees also affected the
inner tube---the inner face of which had revulcanized itself into a shiny
glaze with forty thinned spots corresponding to the protruding spoke nipples
(this tube developed forty small "innies"---the tube from the aero-rim test
attained forty much-larger "outies"). After reaching 425 degrees the "heat
resistant" rim-strip was fused to the rim and looked like fried bacon.

Is Santana's test on Claraboya too extreme?

Four months ago Jan and I (less than 325 combined pounds) were riding our
unladen tandem down a gently curving alpine highway when we got stuck behind
a farm tractor pulling a wagon full of freshly cut timber. I used my tandem's
rear disc to maintain the tractor's speed. A mile-and-a-half further down the
hill we found a break in oncoming traffic and passed. Towards the end of that
distance, however, my disc brake had faded enough to force me to rely on my
front rim brake. I had already decided it would be prudent to stop to let
things cool down when the chance to pass appeared. When we did stop for a
Coke four miles later (and two miles past the bottom of the hill) the rear
disc was still far too hot to touch, and I couldn't yet hold my fingers
against the front rim.

Answer: I don't think Santana's test is any worse than situations that will
present themselves during normal tandem rides over mountain roads. In the
above situation teams who are: (a) heavier than 325 pounds, or  (b) have
touring packs and/or trailers, or (c) might be too timid to pass a farm
tractor on a busy roadway, would have needed to stop or risk a front blowout.
On the other hand, if the tractor wasn't on that road, rim brakes alone would
have sufficed (the high-speed curves were banked and the gradient was only
10-12%).

Bill McCready
909/ 596-7570

To be tested this winter: the latest crop of disc brakes and a ceramic rim.