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As you will have read we have previously
tandemed in Cambridgeshire where almost any type of brake seemed more
than adequate; few hills you see. That said, we did have XTR V-brakes and they performed
faultlessly, but now that we live in the North we need more robust performance.
But why is that, are tandems not equipped with working brakes? Well yes they are
but they are really solo-bike brakes pressed into tandem service without acknowledging
that tandems are likely to be twice as heavy and capable of higher descent speeds.
Brakes, therefore, might see quadruple the loads of solo installations.
Our Cannondale MT800 had disc brakes as standard, 200mm
on the front and 180mm on the rear and they were upgraded from new to
Hope Mono M4's with sintered pads, the stopping power seemed awesome.
However, when quizzing the guys at the Tandem Club we learned that simply stopping
is less of an issue than slowing down on fast descents because, here, brakes can
be heated to a level where stopping performance wanes and maybe even disappears.
Tests near home showed, when using both brakes as a combined drag brake, that on
steep descents ( i.e. 20% for 0.5 miles) the rear disc turned blue (click on the
picture below to enlarge it); so somewhere near 300 degrees C. Subsequent stops
were never a problem but surely we were operating beyond the upper level of guaranteed
performance. After all, when it's new and dry Dot 5.1 fluid boils at 275 degrees
C and we could not be sure how long it would be before the disc heat began to heat
the brake fluid.
Rear discs.
180mm before and 203mm after
Realising this was dangerous we embarked on a number of changes; we stopped using
the brakes as drag devices (more on this later) and we upgraded the discs.
Discussing our situation at JD Cycles caused them to ring
Hope, who we found did not actively recommend disc brakes
for tandems, but when they were quizzed further it transpired that they would make
a pair of new discs to my specification. So I specified both discs as 203mm and
both to have almost double the original swept braking area, recognising that the
original disc were very skinny affairs with both low thermal mass and low dissipating
surface areas. Again they were tested near home but this time the discs remained
shinny and the rear seemed much more powerful. On then to the more demanding hills
of Derbyshire.
We had a super ride in Derbyshire on Boxing Day 2008. Only about 25 miles but as
the temperature was below zero the wind chill on the descents was merciless. 25
miles was all we could manage. We chose a hilly route because the steep ascents
would warm us and the steep descents would be a good brake testing ground. And so,
at the foot of each descent we dismounted and, whilst jumping up and down trying
to create some body heat, we peered at the discs. They took some abuse and only
on the last descent, where numerous hairpins caused repeated bouts of heavy braking,
did the rear disc turn browny-blue. That's about 250 degrees C.
I drew a number of conclusions from the tests.
- We needed a new technique for descending long hills as the brakes, when used as
drag devices, were operating very near to max performance; i.e. no margins remained.
- Loss of one brake would create a very dangerous situation such that stopping might
be impossible.
- The insurance offered by a third brake, perhaps a V-brake, was therefore essential.
One is currently being fitted.
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W hy is a different technique
of any value? The hill remains the same, the tandem weight needing to be stopped
remains the same so surely any apparent improvement is merely subjective or speculative?
Well no, 2 obvious techniques offer improved performance. A first might be to chop
the steep 0.5 mile hill into many 100 yd sections, stop at the end of each section
and allow the brakes to cool before moving onto the next section. A second needs
a bit of science to understand how it might work so read on cautiously.
At the top of a hill the all-up-weigh of the tandem and the height of the hill offers
a fixed amount of Potential Energy and it is this PE which needs dissipating or
converting to other forms of energy during the descent. As the tandem moves down
the hill it gains Kinetic Energy because the PE is being converted to KE by virtue
of the tandems increased speed and its loss of height on the hill, and it is this
KE which need to be absorbed, dissipated or last of all converted to heat by the
brakes. Travelling moderately slowly would mean all of the KE would need converting
to heat energy by the brakes so being the worst hill descent solution. Travelling
quickly, with no brakes applied, would see the tandem rise to a terminal velocity
resulting in all further gains in KE being offset by losses to air turbulence so
a fixed amount of KE would finally need converting by the brakes, and that would
irrespective of the hill length.
Sensibly some middle ground solution offers the best compromise for most situations
and that solution is to allow the speed to rise whilst offering maximum wind resistance
onboard, so scrubbing off some speed and hence KE, and to periodically bring the
speed down to some low/cautious level by heavy braking. As the brakes are applied
the system will be cold due to recent lack of use and wind cooling and so offer
maximised performance. Then, releasing the brakes will allow for a further cycle
of acceleration and braking. Experience will guide the skipper into what sensible
speeds can be achieved between each braking session. Also rotating around the 3
brakes so that only two are used on any braking cycle will give maximum brake cooling
time.
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W e were early adopters
of SPD’s and we still find them fantastic. It means I can have a more relaxed style
to pedalling and cadence (cavalier maybe) as none of our feet will accidentally
disengage no matter how erratic a situation becomes. Janet has always had a tendency
to freeze her legs when she sees a “situation“, even one completely removed from
our course or passage. The SPD’s mean that whilst a quick wobble might result, that’s
as bad as it gets.
It makes starting easier as well, because as we move off, we do not need the pause
to engage my second foot; I simply click it in on the move. And of course, disengaging
is so simple just needing a quick twist of the foot and that’s it. I have mine adjusted
for quite high release pressure whereas Janet’s are adjusted for easy release and
even hers are never a problem. I sometimes forget to simultaneously drop 2 sprockets
as I drop off the big ring thus causing our cadence to rocket but, ear ache apart,
we continue smoothly.
Since starting cycling in the 60’s I regard indexed gears and SPD’s as the top 2
innovations in cycling. Sadly both Japanese; unless you know differently.
I cannot imagine riding a tandem without them.
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