Disc brakes vs rim brakes on front tandem wheel

[321up]

I'm trying to decide if I should insist on a front rim brakes for a new build.  I'm happy about having a disc brake on the tandem back wheel but I have misgivings about having a disc brake on the front wheel of a tandem.  I assume that the rim will be able to absorb and dissipate more energy (heat) than a disc rotor.  When braking hard the front brake has to absorb and dissipate more energy than the rear.  We are not heavy and don't carry a lot of luggage, but we do descend some very steep hills, often quickly, and brake hard to decelerate.  Our front rim brakes work well on both our previous tandems and I'm concerned about a front disc brake potentially overheating and possibly failing.  Does dishing the front wheel for a disc brake have any effect on handling?  Are dynamo hubs with disc brake fittings (e.g. son 28 neu) strong enough for tandem use?  Are my assumptions and concerns correct?  Thanks.

[mrcharly-YHT]

I believe that the answer lies in getting a large and strong rotor.

don't use compound aluminium/steel rotors. There really isn't much metal in a rim so a large rotor should be very comparable in heat dissipation abilities.

[Polar Bear]

And a beefy fork.

[321up]

I believe that the answer lies in getting a large and strong rotor.

don't use compound aluminium/steel rotors. There really isn't much metal in a rim so a large rotor should be very comparable in heat dissipation abilities.

how large is large?

[Chris S]

Braking hard for short periods is not as bad as braking moderately for long periods, so in our experience (we ARE heavy!), heat buildup is only an issue on long descents.

I don't see why a disc on the front should be a problem at all - you're not going to get the back wheel off the road, even with hard braking - but I appreciate there's a lot of force going into the forks; I doubt it would be any more with a disc than with a decent self-energising rim brake.

We tend to do a lot of our drag braking on the rear wheel - I have control of the disc, fboab has two sets of V-brakes that are set to be somewhat weakly effective - and they work really well as drag brakes. Remember - our brakes have to work much harder (even when we're fit and lean - we're still close to a 200Kg team), and we've not had an overheating problem yet.

[321up]

And a beefy fork.

Yes, and beefy fork with large rotors surely means heavier.  For what benefit?

Rim wear on the front wheel has not been a problem for us thus far.

One front & one rear brake has been adequate for us thus far.  If we were travelling with camping gear in mountains a 3rd brake might be a good idea but we have no plans to do that.

[mrcharly-YHT]

I don't see why a disc on the front should be a problem at all - you're not going to get the back wheel off the road, even with hard braking - but I appreciate there's a lot of force going into the forks; I doubt it would be any more with a disc than with a decent self-energising rim brake.

Same torque, shorter lever arm therefore more force.

Benefits of disc vs rim

cleaner rims
Out of true wheel not a serious problem
More reliable braking in the wet
Less force needed on brake lever for good braking

[LittleWheelsandBig]

Santana is a fan of a large (203mm, or larger?) rear disc brake with a front rim brake. The reduced weight transfer and greater weight on the rear wheel means you can do a lot more rear braking compared to a solo.

There is considerably more bending moment (asymmetrical!) put into a disc brake fork than a rim brake fork, requiring a fork with a lot more weight and stiffness.

[Karla]

A hot disc won't make your tyre go pop.

[Chris S]

A hot disc won't make your tyre go pop.

But really hurts if you stand too close to it.
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[Polar Bear]

And a beefy fork.

Yes, and beefy fork with large rotors surely means heavier.  For what benefit?

There is considerably more bending moment (asymmetrical!) put into a disc brake fork than a rim brake fork, requiring a fork with a lot more weight and stiffness.

[rafletcher]

From the CTC forum, while I was googling "tandem front disc fork failure". I didn't find one (though Santana claim "the heat from a tandem front disc will destroy a carbon fork". No examples cited however.)

"We've been riding a disc braked tandem fitted with BB7's and 203mm rotors for about 6 year now, no problems whatsoever.I wouldn't recommend them for very long Alpine descents but for all other riding no worries, best stoppers we've ever had, totally predictable wet or dry."

Though I thought the Spyre was the preferred brake for tandems - mechanical but both pads actuated.

[P Walsh]

A couple of years ago my wife and I took our Thorn tandem over a very hilly circuit around Brecon and Builth Wells. It involved steep twisting descents where letting the speed build up was not an option. It really tested the brakes. Eventually the rear disc brake (an expensive Hope Mono hydraulic six piston set up with a large disc) overheated and stopped working. It was as if that brake was just a lever attached to nothing!

Fortunately Thorn don't trust disc brakes alone, so I had some very nice vee brakes (front and rear) to use. To combat the problem of overheating rims we has tungsten carbide rims. These rims are supposed to be less reliable in the wet, so Thorn now tend to fit one rim like this, and one normal one.

In short, I would avoid discs on their own on a tandem.

[321up]

A couple of years ago my wife and I took our Thorn tandem over a very hilly circuit around Brecon and Builth Wells. It involved steep twisting descents where letting the speed build up was not an option. It really tested the brakes. Eventually the rear disc brake (an expensive Hope Mono hydraulic six piston set up with a large disc) overheated and stopped working. It was as if that brake was just a lever attached to nothing!

Fortunately Thorn don't trust disc brakes alone, so I had some very nice vee brakes (front and rear) to use. To combat the problem of overheating rims we has tungsten carbide rims. These rims are supposed to be less reliable in the wet, so Thorn now tend to fit one rim like this, and one normal one.

In short, I would avoid discs on their own on a tandem.

Do you know if that was a hydraulic failure, or due to the pads/rotors?

[P Walsh]

I don't know and would welcome suggestions. After cooling down for an hour or so the disc brake seemed to get back close to its normal confidence-inspiring performance. I changed the pads as a precaution when we got home.

I like the disc brake and still use it as option 1, but I trust the rear vee brake more and rely on it for occasions when the disc might let me down. I should add that I do alternate between all 3 brakes (vees front and rear, and a disc on the rear) to avoid overheating on difficult descents, and I let the tandem roll when I can.

[The French Tandem]

For safety reasons, I would insist on having two different kinds of brakes based on two completely different technologies.  That's a basic lesson of reliability engineering. If one brake fails for any reason, you want to make sure that your other brake(s) won't fail for the exact same reason. On our tandem, we have two rim brakes and one disc on the rear wheel, but never experienced any serious brake failure. If we had to ride a tandem with discs on both wheels, we would feel safer with at least one extra rim brake, just in case the two discs fail for some reason.

[P Walsh]

I agree with that. I now use the disc brake that failed with the knowledge that my life does not depend on it. I should add that the terrain it failed on is just about the worst you could imagine: long steep descent with gravel and tight bends, and a very warm day. On more normal terrain that brake has been faultless, and its use means we have very little rim wear.

[Kim]

For safety reasons, I would insist on having two different kinds of brakes based on two completely different technologies.  That's a basic lesson of reliability engineering. If one brake fails for any reason, you want to make sure that your other brake(s) won't fail for the exact same reason.

The corollary to that is that if they're based on the same technology, you can if necessary use parts from one to fix the other.

Academic, I'd have thought, as you really want three brakes on a tandem, and only two can use the same tech.

[The French Tandem]

Yes, using only one model of brake will reduce the number of spare parts that you need, and will make maintenance easier by letting you swap parts. However, if you ever have a brake failure while descending the Tourmalet, and I do not wish this to anyone, all you can do is to pray for the other brake do not fail the same way!
 
My previous post was motivated by the photo that 321up posted on another thread:

https://yacf.co.uk/forum/index.php?topic=85516.msg1802419#msg1802419

Only 2 discs, no backup brake! Don't buy this, unless you can fit a rim brake as a backup, or you are sure you will never ride on very hilly ground.

[321up]

I suspect the failure that P Walsh described may have been due to the hydraulic brake fluid vaporising.  I think that cable actuated disc brakes have an advantage in this respect.

Three or four brakes on a tandem might be desirable or necessary to dissipate heat in some circumstances depending on the combination of weight, gradient, length of decent, and riding style.  In our experience two brakes (one front & one rear) have been effective for all the hilly rides we have done thus far.  I think that having a second rear brake purely as a 'backup' would be of limited use to us.  In the event that the front brake fails completely the ability to slow/stop on a steep decent will probably be limited by the traction of the rear tyre more than the limitations of a single rear brake.  The front wheel usually has better traction than the rear when braking on a decent as the weight is thrown forward so a rear brake failure would be less of a problem unless the road is very slippery.  I think that it highly unlikely that two independent brakes will fail simultaneously.  I doubt that it will be necessary but I'll enquire if the rear bridge can accommodate an additional brake so I have the option to fit one in future.

Front disc brakes are outside my experience, hence my concern about having one as our main (and only) front brake, irrespective of how many rear brakes we might have.  Also the stiff forks required for the front disc may have implications for comfort.

[Kim]

I think that it highly unlikely that two independent brakes will fail simultaneously.

Except through overheating.  And for "in rapid succession" values of 'simultaneously'.

Front disc brakes are outside my experience, hence my concern about having one as our main (and only) front brake, irrespective of how many rear brakes we might have.  Also the stiff forks required for the front disc may have implications for comfort.

They work extremely well on small-wheeled touring recumbents (somewhat less all-up weight on a solo, but dynamically quite similar to tandems).  Those tend to have sturdy forks/kingposts, of course, and not an awful lot of rim to dissipate heat into.

Personally, I'd want one purely on the basis of consistent performance in the wet.

[P Walsh]

Kim, I expect you are right about disc brakes on solos. But the issue on tandems is of course the extra work the brakes do because of the extra weight. I have seen pics of discs made of two layers of steel with an aluminium middle layer, and the ally has become molten and started to ooze out because of the heat generated by tandem braking! If I had been using discs and nothing else in the Brecon Beacons when my rear disc overheated and stopped working I would not have trusted the remaining brake and we would have walked down the remaining hills.

[Kim]

Yes, certainly not a thing to be a weight weenie about.

[LittleWheelsandBig]

I've been on a MTB tandem that melted the plastic parts on the hydraulic calliper. There was too much walking that day.

[321up]

I think that it highly unlikely that two independent brakes will fail simultaneously.

Except through overheating.  And for "in rapid succession" values of 'simultaneously'.

That's a fair point if you assume that overheating will lead to complete failure of the brakes as can happen when overheating hydraulic callipers and perhaps composite rotors.  Please correct me if I'm mistaken but I think that a good mechanical calliper with a steel rotor will fade when overheated, but not usually fail completely (but I have no experience as to what degree the brake will fade & loose power when overheated).