SA XL-FD90

[Bonefishblues]

I was reading a thread in another place where the braking power of these was being praised. It set me to wondering if such a front hub might be tandem-compatible/have merit on a tandem.

Now if only we had someone with SA expertise hereabouts

Linky: http://www.sturmey-archer.com/en/products/detail/xl-fd

[bgnukem]

I'm sure Brucey will be in touch

Possible downsides I can see are that the brake shoes seem to take a long time to bed in on these and you might have 'exciting' braking for a while, maybe not ideal on a tandem, unless you live somewhere flat.

Other question would be if the bearings and solid axle are rated for tandem loads, and the risk of overheating on descents with the extra heat generated. Having said that I've seen drum brakes used as rear drag brakes on tandems, though they have usually had finned bodies to assist cooling.

[Bonefishblues]

Am disappoint. I had him down as a kind of genie, summoned not by rubbing with an oily rag, but by the mere mention of the S-word

...but seriously, am interested in an informed view.

[Brucey]

these hubs are (for a drum brake) extremely powerful. If you fit one to a solo you will have enough power to do stoppies practically from the start. Is is it a good idea? No; the chances are excellent that the fork won't be built with one-finger stoppies in mind.

The hub is basically strong enough for tandem use ( the axle is 9mm threaded, where most good tandem front hubs use 3/8" threaded axles). However you can't expect to chuck brake heat into one indefinitely without causing problems.

The logic on a traditional tandem is that you have normal (rim) brakes front and rear, and a third 'drag brake' (usually a drum). The drag brake is favoured on long descents and won't (on steepish descents) be strong enough to stop you by itself but it takes the load off the rim brakes and allows them to run cooler than they would otherwise. When the drag brake gets too hot it lets you know by smoke coming out of it, at which point a prudent crew stops (using the rim brakes) and lets the drag brake cool down before continuing. Good drag brakes will tolerate being overheated in this way.
If you get rim brakes slightly too hot too then you will usually hear/feel the brake blocks wearing at a much higher rate than normal. On a solo you will normally have enough brake left to stop with, but on a tandem the loads are about doubled per rim brake and you may not be able to stop safely (using the rim brakes alone) when rim brakes get this hot, not without overheating the rim and the tyre. So the noise/feel of the brake is your early warning system.

IMHO this approach is in key respects better than using two disc brakes, with or without a third rim brake. This is because you shouldn't use the rim brake as a drag brake, and if you use a (especially hydraulic) disc brake as a drag brake it can easily get too hot at which point it can 'go away' with little or no warning. If both disc brakes 'go away' then you are not going to be stopping, or even slowing down noticeably.

In the world of HPVs they use SA drum brakes (stub axle version) simply because the brake is inside the wheel and this allows a narrower track in tadpole trikes and better aerodynamics of the wheel itself. However this does nothing for the brake cooling and (often) the designs both increase the load on the brakes by making the bodyshell aerodynamic and by fairing in the brakes so that there is not adequate airflow over them. Nearly all the potential energy of a HPV needs to be dissipated within the brakes on a long descent; (without the brakes the terminal velocity on a 10% gradient is often something like >80mph). The results of this are predictable; the brakes get too hot and this causes problems.

There is a subtle difference between a SA drum brake and (say) an Arai, in that the spokes are attached to the outside of an SA drum; there are additional loads on the hubshell and if the drum overheats enough this can corrupt the integrity of the wheel itself. HPV enthusiasts report that spoke holes can ovalise badly when the brakes gets too hot on long alpine descents. They seem resistant to the idea that brake ducts (perhaps with controllable openings so that there isn't induced drag the rest of the time) might solve their problem, or that water cooling might too.

Anyway this is a long-winded way of saying that using SA 90mm drum brakes on a tandem wouldn't be my first choice, not unless it was an additional (fourth) brake or you don't ever drag the brakes for very long. I'd use a SA brake in the rear as a drag brake, with the caveat that I would avoid the longest descents and expect to have to inspect the wheel itself for damage should the brake ever overheat really badly.

How big a hill need you worry about? Well, on a solo a 'big hill' would be about 1000' (and that is enough to cause some hydro discs to boil up to the point of total brake loss). On a tandem the brake loading is much higher (about three times higher; twice because the weight and more again because of the aerodynamics) and you need to be exercising caution anytime there is a 500' descent or more.

Noted tandem manufacturers Santana test their brakes on a hill that has a 500' drop and a 15% gradient. It isn't even 3/4mile long, yet this is enough to reduce brakes that are not adequate to smouldering useless messes. They will fit a (non loaded touring) tandem with just two brakes, and use a massive 10" disc at the rear and a rim brake at the front. They are up to version four of their disc brake system, and have stuck with an all-metal mechanical (cable operated) disc brake caliper, because hydros boil up too easily and plastic caliper parts just melt.

So in a nutshell the SA 90mm drum brake is strong enough for a tandem, powerful enough for a tandem, but may not have the same tolerance for overheating as a good (rear, drag type) tandem drum brake and may not have a benign failure mode when used as a main (front) brake.

cheers

[Bonefishblues]

I am more than obliged, that's an interesting read, thanks.

[Grldtnr]

I am more than obliged, that's an interesting read, thanks.

Yes,quite informative

I will add I specified 90 mm SA drums on my unfaired recumbent trike, and have supreme confidence in the stopping power, previous brakes on the old trike were 70mm ,they just didn't cut it.
They work do well you can brake steer with them , which is fun!