Page updated:
30th Jan 08

Technical topics.

  1. Tandem disc brakes - Issues, performance and techniques.
  2. Alternative descent techniques - for any tandem braking system.
  3. Riding tips - the joys of riding with SPD's.

Tandem Disc Brakes. Issues, performance and techniques.


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.

Hope Logo

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.

click for larger view

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.

Alternative descent technique for any tandem braking system.


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.

Science & Maths clipart

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.

Riding tips - the joys of riding with SPD's.


W   e were early adopters of SPDís 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 ďsituationď, even one completely removed from our course or passage. The SPDís mean that whilst a quick wobble might result, thatís 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 thatís it. I have mine adjusted for quite high release pressure whereas Janetís 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 60ís I regard indexed gears and SPDís 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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