Tandem emergency stops: three common limitations.
Limitation one: The brakes themselves. You should be able to instantly lock either or both wheels of your speeding loaded tandem, even when your hands are cold and tired. If you can't readily skid both wheels, it's time to take a good hard look at your brakes, pads, levers, cables and housings, and even rims. We know from our testing that when a tandem is fitted with "tandem specific" brakes and levers it is possible for the average male (strong hands) with a same-weight stoker to lock both brakes using one hand (and dual-cable lever). John Schubert will remember this experiment.
Limitation two: Your ability to continue to apply the brakes even while your brain is screaming NO, NO, NO. Every cyclist will have a tough time overcoming premature modulation. Your single bike experience has taught you to let up on your rear and then your front brake as certain rates of deceleration are perceived. Applying a tandem's front brake hard enough to attain a front skid will, at first, seem suicidal. Actually, the real danger is riding a tandem without knowing how to do this.
Limitation three: In an emergency stop, your tandem's front brake provides 80% of your stopping power---probably 3-5 times more power than can be applied by the front brake of a single. Assuming your tandem has adequate brakes and you have the skill to use them---limitation number three is control. Because I know Mark Bulgier, I can tell you he's a bigger-than-average guy. And any bigger-than-average guy who has ridden one will confirm Mark's report: a Vitus (single bike) with its stock fork can be really scary for fast stops. It shudders! A Vitus rider who weighs less than 150# will deny the problem. A brake application severe enough to induce fork shudder will send a lightweight rider over the handlebars.
Because most tandem riders (including tandem builders) have never tested the limits of their tandem's stopping ability, they can safely continue to insist that their brakes are "really powerful" and their fork is "plenty beefy."
And my point here is NOT to make fun of tandem riders or their bikes. It IS to say that fast deceleration is a primary safety issue---just like wearing a helmet. Actually, now that I think about it, braking ability is arguably MORE important than wearing a helmet. Personally speaking, in thirty years of enthusiastic cycling I've only hit my head once (a solo mountain biking endo that shattered my Giro)---in the same span of years I've successfully used maximum braking to avoid a number of serious accidents (mostly cars turning across my path). And while not wearing a helmet may increase your odds of visiting the hospital, if you can't decelerate quickly enough to avoid an accident while riding a tandem you'll put yourself AND YOUR STOKER in the emergency room.
So how do you learn effective tandem emergency braking?
As others have already surmised (from learning to effectively stop a motorcycle or even a car with ABS braking), if you want to stop short of having an accident, you'll need adequate "tandem specific" practice.
Materials: Your tandem, a pair of tires you want to replace anyhow, a good floor pump, tire irons, a big empty parking lot (one with a 2-3% incline is best), a stoker that is as heavy, energetic and fearless as you are [insert Tim Allen grunting sounds here], and two hours when your regular stoker would prefer to be elsewhere. Actually, your best training partner will be another tandem captain willing to share the cost of a new pair of tires. Why the floor pump and tire irons? Because of rim irregularities, most skids will occur along one section of the tire. During your practice session you'll need to occasionally mark and rotate the tire (on the same rim) to keep from wearing a hole through it.
First lesson---mastering the rear wheel skid. Use the downslope to reach 25 mph before using the rear brake only to lay down a long skid mark. Note that when the rear tire skids, the tandem's wheelbase (and lack of front braking) keeps the bike from sluing sideways.
The rear wheel should skid readily---it doesn't count if the tandem is already halfway stopped before the rear brake develops enough grip to slide the tire. Further, because it's easier to skid the rear tire when you apply both brakes together, using the rear brake by itself is the only way to properly judge its effectivenees.
Next is to learn to modulate your braking at the limit of rear tire adhesion. Keep practicing until you can leave three successive yard-long skid marks.
Lesson two---mastering the front wheel skid. Psyche yourself up a bit: this next part is scary. Remember, a tandem with an adult stoker will skid instead of flip. In years of playing this game I've never seen a captain skid their front tire on a first attempt. Actually, this is just as well. At a degree of front brake application below the amount that will cause a skid, the front fork becomes very active. As you learn to apply the front brake harder, tandems with inadequate forks can buck like a bronco. How far can a fork flex? On cheap tandems (and tandems with suspension forks) this type of testing can produce skid marks on a tandem's down tube.
Through successive laps, build your skill (and courage) to where you can either skid the front wheel OR to where you have explored the limitation imposed by your tandem's fork (the heavier the team, the greater the fork-imposed limitation). A good fork can easily withstand the front wheel skids of a 400 pound team. A tandem with an insufficient fork will shudder and buck enough to make a tandem uncontrollable.
Assuming your brakes and forks are up to muster, you will get to where you can readily skid the front tire. Because you can't really steer a skidding tire, a sustained front tire skid should be avoided. As with the rear wheel, keep practicing until you can modulate your front brake well enough to produce three successive yard-long skid marks.
Lesson three---using both brakes. Because you want to learn braking one wheel at a time, don't use both brakes together until you've mastered lessons one and two. By now, you may think you're a real hot-shot [more grunting noises]. Ready for a test? From 30mph try producing four successive skid marks by alternating between your front and rear brakes. When you can modulate your tandem's brakes with this degree of skill, you truly deserve your stoker's confidence.
Lesson four---if you haven't fully exhausted your time, energy or treadlife---have a contest to see who can stop shortest. Because this type of practice session expends essential carbohydrates, the loser can buy the beer.
In an earlier rejoinder someone remarked because he wouldn't be so careless as to ride a tandem faster than he would ride his single, he really didn't need better brakes. I not only don't want to be this guy's stoker, he's wrong. (Even if he does ride his tandem at single-bike speeds, it won't prevent cars from turning in front of him). Because of its weight, wheelbase and (potential) braking ability, tandeming can be superbly safe and stable, especially at high speeds and/or in an emergency situation. If you've got good skills, adequate equipment and a clear road, why would any tandem enthusiast want to limit their downhill speed to that of a single bike? Jan and I have used the left lane on downhill freeways to pass plenty of cars traveling at the old 55mph speed limit---it's a blast.
By Bill McCready, President
Santana Cycles, Inc.
Sales and Customer Service
(909)596-7570