Disc brakes without disc brake braze-ons, plus a wide choice of response curves

[Danneaux]

Very helpful comparison, George.

But the one thing that I really noticed is for hard braking where I needed to decelerate more than typical, the V brake worked better than the disc in dry conditions, the disc appeared to have an upper limit for how much braking power it has.  I did not brake hard enough to break the tire loose from the pavement (skid), thus I think my comments on disc vs rim brake is not due to one being in the front and one being in the rear.

Well, sure...weight transfer has a direct effect on braking limits, so this may seem to amplify the power of the v-brake (it being on the front and less likely to lose ground traction) versus the more lightly loaded rear disc (which will tend to lift under braking). However, I sense you mean the ultimate clamping force by the disc is not as high as the clamping force of the v-brake on the rim-disc? I'm really interested here.

In a slightly different vein, one of my randonneur bikes employs centerpull calipers mounted to brazed-on mounts. There's minimal flex in the arms between pads and mounts and it works well. Three of my bikes use various front and rear editions of the Scott/Pedersen SE and SunTour variants of the self-energizing helix-pivot cantilever. The pivot mounts are fixed and the canti arms rotate forward on a helix as they travel inward. They can be very fussy to setup (Scott/Pedersens) or easy (SunTours), but are really intriguing because the forward motion of the rim moves them inward to a degree once the pad makes contact. I chose them to ease hand pressure when braking with heavy loads and they surely do that, working well on loaded touring singles as well as my tandem. I like them very much, but they are an acquired taste and do require some rider adaptation compared to standard brakes with no helix.

I am about to convert my tandem from these SE brakes to v-brakes for a different reason. I took a spill on the bike last fall and though my hand served as a cushion between the pavement and the lever body, I still managed to crack the (1984 vintage Dia-Compe Aero Gran Compe) lever body. Instead of replacing it with another lever from my backstock, I am going with a Tektro RL520 for better *hand* leverage/geometry...the hood is longer, the lever blade is cranked, and I can grasp it with four fingers from atop the hood. Yes, the same lever is available for cantilevers, but this was a good excuse to compare a v-brake setup to the SEs and will be a fun experiment and far faster to setup. I'm not sure if net braking power at the rim will be similar with the identical Kool-Stop Salmon pads or will simply feel better at my hands.

The tandem uses a massive Arai drum brake on the rear wheel, actuated by a bar-end shifter to serve as a drag brake to limit top speed when coasting downhill. It is very effective in tat role (and can glow red at night after heavy braking!) and limits speeds to a point where the rim brakes can easily retard progress with the weight of two people, a full touring load and a loaded trailer. Without, it is easy to top 102kmh/63mph coasting downhill. With the drag brake set, speeds are limited to about 68kmh/42mph on the same hills. When I got the bike the drum brake was setup as a regular brake, the other lever controlled both front and rear cantilevers; I found this too limiting. The drag brake on the rear did not provide sufficient braking when serving in that role and operating both cantilevers with one lever required careful balancing of cable tension and rarely gave a good result. The present arrangement of a barcon-operated drum drag brake and separate levers for front and rear cantis has been ideal.

Best,

Dan.

[mickeg]

mickeg ... on which bike is that.  IT seems a very different location for the brakes compared to most  mounts and the Thorn way of it. The cable routing seems a bit strained but otherwise seems to be well thought out for rack clearance. Any thoughts with respect to the mounting location on the frame?

Is that Titanium?  If "steel is real" does that mean titanium is unreal? 

I think Dan does not like it to sound like we are pitching a non-Thorn bike so I am not going to name it here, I will send you a private message.  Yes, Titanium.  I think it is real enough.  The brake mount on the chainstay is unusual, that company does it the more conventional way (on the seatstays) on their other models.  But I think they did it this way on this model as it is a touring model so a rack mount that will not interfere with the disc brake unit is more important for touring where a good rack is needed.  The brake mount is bolted to the frame with three bolts, they can also fit a flat mount instead of a post mount this way.  The cable does not seem strained when I strung the cable, but they used to mount the cable clips above the chainstay and more recently they moved it below the chainstay on the one they shipped to me, I suspect that the cable will be more direct if the flat mount brake is used. 

I am not getting rid of my Thorns, I got a great price on this frame so I bought it to see what a Titanium bike is like.  I did a 5 day short tour near home to test it, I like the new bike, so now I have three touring bikes along with the Sherpa and Nomad.

[mickeg]

Very helpful comparison, George.

But the one thing that I really noticed is for hard braking where I needed to decelerate more than typical, the V brake worked better than the disc in dry conditions, the disc appeared to have an upper limit for how much braking power it has.  I did not brake hard enough to break the tire loose from the pavement (skid), thus I think my comments on disc vs rim brake is not due to one being in the front and one being in the rear.

Well, sure...weight transfer has a direct effect on braking limits, so this may seem to amplify the power of the v-brake (it being on the front and less likely to lose ground traction) versus the more lightly loaded rear disc (which will tend to lift under braking). However, I sense you mean the ultimate clamping force by the disc is not as high as the clamping force of the v-brake on the rim-disc? I'm really interested here.
...

I am pretty sure that the TRP Spyre with more and more clamping pressure did not provide that much more stopping power, in other words not really linear.

Without panniers, I could cause a skid in rear, but with a touring load I do not think I could cause a skid.  Maybe the front Salmon pads were that much better?

I used the stock TRP disc pads and disc.  Being my first disc brake, I was careful to run in the brakes as described in the manufacturers literature.

[martinf]

The tandem uses a massive Arai drum brake on the rear wheel, actuated by a bar-end shifter to serve as a drag brake to limit top speed when coasting downhill. It is very effective in tat role (and can glow red at night after heavy braking!) and limits speeds to a point where the rim brakes can easily retard progress with the weight of two people, a full touring load and a loaded trailer. Without, it is easy to top 102kmh/63mph coasting downhill. With the drag brake set, speeds are limited to about 68kmh/42mph on the same hills. When I got the bike the drum brake was setup as a regular brake, the other lever controlled both front and rear cantilevers; I found this too limiting. The drag brake on the rear did not provide sufficient braking when serving in that role and operating both cantilevers with one lever required careful balancing of cable tension and rarely gave a good result. The present arrangement of a barcon-operated drum drag brake and separate levers for front and rear cantis has been ideal.

I had a tandem, bought second hand but nearly unused, which I kept from about 1980 to about 1990. It originally had front and rear Mafac cantilevers operated from a single brake lever, plus an Atom drum brake as used on French mopeds of the same vintage, operated from the other lever.

Like Dan, I disliked the two brakes on one lever arrangement, so changed to seperate brake levers for the cantis, and controlled the drum with a Suntour rachet-type thumbshifter that I fitted under the top part of the drop handlebars. This method of braking reminded my of the Telma electromagnetic retarder system fitted on some of the buses and coaches I drove at that time - the Telma was controlled by a slim rod under the steering wheel. Unlike most brakes, both these systems require you to remove the braking force with a definite action rather than just releasing foot or hand pressure.

[Danneaux]

I am pretty sure that the TRP Spyre with more and more clamping pressure did not provide that much more stopping power, in other words not really linear.

Without panniers, I could cause a skid in rear, but with a touring load I do not think I could cause a skid.  Maybe the front Salmon pads were that much better?

I used the stock TRP disc pads and disc.  Being my first disc brake, I was careful to run in the brakes as described in the manufacturers literature.

Thanks for this, George; just what I was hoping for (not necessarily results, but the information).
=====
One thing lacking in the braking discussion is rim composition. I've touched on it before, but rims meeting the definition of "6061 aluminum" have some variation in composition, the key ingredient being silicon (sand) content. Rims with little will be "softer" and tend to load pads with worn aluminum; aluminum on aluminum stops poorly. I went through this with some older Araya 16A(3), 16(A)5, and 20A rims. Started out well, but once the surface anodizing wore off, they were deadly with even Kool-Stop Salmon pads. I could clean the sidewalls with steel wool, rinse, and then more aluminum would transfer to my thumb when I rubbed the sidewalls. There seemed to be no end to it except for swapping to a different brand rim with a slightly different composition. Those with more silicon content are harder and tend to be more long wearing and stop better 'cos they don't load the pads with aluminum. Of course I am talking about the base extrusion after the surface treatment -- whether anodized, hard-anodized (there are different degrees of surface hardening depending on the anodizing process) or CSS or ceramic treated -- wears off. If the rims are only polished or soft (cosmetic) anodized, one can get into the base metal pretty quickly. If the rim itself is fairly soft, it won't stop so well as a harder rim with rim brakes, regardless of brake design or pad compound.

All the best,

Dan.

[mickeg]

...
One thing lacking in the braking discussion is rim composition. I've touched on it before, but rims meeting the definition of "6061 aluminum" have some variation in composition, the key ingredient being silicon (sand) content. Rims with little will be "softer" and tend to load pads with worn aluminum; aluminum on aluminum stops poorly. I went through this with some older Araya 16A(3), 16(A)5, and 20A rims. Started out well, but once the surface anodizing wore off, they were deadly with even Kool-Stop Salmon pads. I could clean the sidewalls with steel wool, rinse, and then more aluminum would transfer to my thumb when I rubbed the sidewalls. There seemed to be no end to it except for swapping to a different brand rim with a slightly different composition. Those with more silicon content are harder and tend to be more long wearing and stop better 'cos they don't load the pads with aluminum. Of course I am talking about the base extrusion after the surface treatment -- whether anodized, hard-anodized (there are different degrees of surface hardening depending on the anodizing process) or CSS or ceramic treated -- wears off. If the rims are only polished or soft (cosmetic) anodized, one can get into the base metal pretty quickly. If the rim itself is fairly soft, it won't stop so well as a harder rim with rim brakes, regardless of brake design or pad compound.

All the best,

Dan.

I am largely ignorant of the alloys.  But I can say that Aluminum will readily oxidize when a fresh surface is exposed to oxygen, so you will always have some gray Aluminum oxide dust on your hands when you handle a used rim.

Aluminum oxide is much harder than non-oxidized Aluminum.  Aluminum oxide makes great grinding tools for cutting steel, etc.  I occasionally get out an emory board (a stiff piece of cardboard with sand paper like sides) and try to clean off the Aluminum oxide from my brake pads, my thinking is that brake pads are cheaper than rims.  I do not know if this extends the life of a rim or not, but I continue to do it occasionally.  And because pads are cheaper than rims, I do not clean any metal off my rims, only off my pads.

I am always amazed that I can handle my CSS rims and not get any Aluminum oxide on my hands like I would with regular rims.

[Andre Jute]

Not very fond of the roller brake, but mine is a fairly old model, newer ones may be better.

Heh-heh. Newer ones will do better, all right: they'll throw you over the handlebar for the slightest lapse in attention.

I have two Shimano types, an IM-41 on the rear of the Gazelle Toulouse which has a hub-mounted front disc, and IM-70 rollers front and rear on my Trek Smover. They're chalk and cheese.

Shimano's IM-41 roller brake is basically useless, but on slippery roads acts as a sort of anti-skid brake with the nasty, cheap, unprogressive Shimano front disc Gazelle fitted; this disc brake progresses from semi-useless to a dead lock face plant in about a eighth of an inch of movement.

Shimano's IM-70, by contrast, leaves any hydraulic or rim brake I know for dead for clamping force. Whatever friction there is between your tyres and the road surface, which is what really determines braking capability, the IM-70 will find. The problem is that the IM-70 is like riding a tiger: it wants you to fall off so that it can eat you. You have to pay constant close attention to gentling the brakes just so, or a nasty rider-error incident awaits you; the faster you go -- and some of my downhills are very fast indeed -- the more acute the problem becomes. I don't pay out good money to be bullied by components. As soon as I could, I switched over to Magura's HS-11 rim hydraulics, which are very progressive.

All the same, at least theoretically, roller brakes have it over all other brakes, supposedly offering all the advantages of both drums and discs, and none of the disadvantages of drums, discs or rim brakes -- except that Shimano has failed in its efforts to make them anything but either limp or overbearing. Oh, and one more thing, they whine (literally in the case of the noisy IM-75) for service, which consists of squirting in a sticky, staining, viscous black fluid through a little nipple for which the twee rubber plug falls out on the road and then is difficult to source for replacement, and expensive in postage for a thruppenny component.

[Andre Jute]

SOME FUNDAMENTAL THOUGHTS ON TANDEM BRAKES

The friction between any particular bicycle's tyres and the road surface determines braking capability, no exceptions.  Beyond a certain effective level, the clamping force the brakes, whatever kind they are, can exert becomes irrelevant because the coefficient of retardation is determined at the tyre patch on the road, not at the brake interface. At bicycle speeds aerodynamic resistance aiding braking is rarely a significant factor. A particular bicycle's loaded mass and velocity are the other important factors in determining braking requirements, which is why this subject arises so often in discussion of tandem brakes.

Heat dissipation is another problem altogether. Heat dissipation won't be improved by increasing brake clamping force.

It seems to me likely that overheating brakes on tandems are caused by trying to make components designed to be lightweight (which is almost every bicycle component) do heavyweight duty.

I've wondered every time I saw a tandem, why people trying to fix a tandem's inherent problems don't start work much more fundamentally, at making sure the fork of the tandem will take a very fat tyre of very grippy material, because if you don't, nothing, but nothing you do will help until you fix the braking coefficient to match the greater weight of the fully loaded tandem. Anything else will be undermined by the fact that most tandems are just long-wheelbase, double- or triple-loaded road bikes given a light dusting of slightly more capable components which do virtually nothing to fix the fundamental problems caused by greater mass and greater velocity-potential on inadequate tyres. But a more effective tyre-road interface will give you better braking, in turn requiring lower clamping forces, less energy uselessly turned into heat and decreasing braking efficiency and requiring dissipation.

Another way of looking at it is that whatever you do to improve tandem braking will add weight to the bike, and the most effective weight, in adding comfort and stopping power, will be wider tyres and the wider rims you fit them to.

[Danneaux]

Not a counterpoint but some thoughts on your above, Andre...

I don't really have a problem braking my tandem with the self-energizing rim brakes and Kool-Stop salmon pads I am using...until speeds top ~72kmh/42mph. The barcon-actuated big Arai drum-drag brake simply keeps speeds to 72kmh and below, where the rim brakes are most effective.

Even so, I have hauled the bike down in amazingly short distances from 100kmh -- once, because overheating prevents a second go till everything cools down again. Jack (Dad) and I were booming down Chambers Hill here in Eugene, topping 101kmh when the traffic light at the bottom turned red shortly before we approached it. Despite seeing then 87 year-old stoker Jack in my rearview mirror, arm waving "go faster", I didn't figure we could make it through the intersection against the light before the cars waiting at it turned. Being pilot/captain, I could see better and was right. To be honest, I didn't even think of the drag brake and simply squeezed the blood out of my knuckles to actuate the rim cantis. Don't forget the magic ingredient of adrenaline in the braking mix! I do recall Jack yelping when the backs of his legs caught some melted brake pad, but the pads were far from gone after that one panic-fueled stop.

Some factors often overlooked by non-tandemists in braking discussions are a) the long wheelbase, which allows front braking at levels that would cause an endo on single bikes and b) the weight on the rear, which more heavily loads the rear tire and ups the level where it would break away in a skid. Of course, all this assumes reasonably fresh, grippy tires on good tarmac to maximize the friction coefficient between tire and pavement. All bets are off when it comes to downhill braking on gravel logging roads, but I can assure you it is possible to drift a fully loaded tandem with well-built wheels on soft surfaces and so scrub off speed on poor road surfaces. My stokers might disagree in principle -- and certainly in advance -- but have been grateful After.

It also helps to have minimal slop in the braking system which includes the frame. My tandem has massive tubing cross-sections, and I brazed a tubular cromoly brake booster for the front and tied it into the crown of the massive unicrown fork. I've ridden the bike solo on any number of 200km day rides, and though it feels a bit stiff up front, the ride is remarkably good due to being suspended well inside the long wheelbase.

All pretense of "good" braking goes right out the window when ridden solo and that is exactly the situation where I find myself shy on tire cross-section. Under heavy braking that would endo a single bike, the tandem ridden solo simply locks up the front tire and plows resolutely forward. I really can't use the drag brake at the rear without causing a skid due to the easy breakaway of the lightly loaded rear tire and the same for the rim-mounted brakes which also cause easy breakaway of the tire contact patch.

It was just this situation that resulted in a sad loss of life several years ago when a husband/captain rode a couple's tandem solo down a local pass to get the car for his exhausted wife/stoker. He "turned up missing" as the odd phrase goes, and a search was convened. He was found at the bottom of a ravine. Analysis of the skid marks determined the crash was caused by excess speed and poor handling...weight distribution was off without the stoker, speed was too high, and tire-pavement friction coefficient was insufficient for good braking/cornering and off he flew, with tragic results.

Most of my tandem day-touring is done on 26x1.5in road slicks. Oddly enough (especially given the overall weight of tandem with riders), they work fine and are remarkably comfortable and durable. When my Dutch pal came over for a tour, we took the bike with these tires up into the lower reaches of June Mountain on logging roads, fully loaded and towing a trailer. All-up weight with both of us and the very heavy Dutch Army tank driver's boots he brought for hiking was right at 272kg/600lb. My Danneauxbuilt trailer alone weighed  57kg/125lb fully loaded with food and lots of water (heavy in itself) for our dry camps. No real problem braking on 10% downgrades provided we kept the speed limited with the drag brake...it also proved up to the job as we descended past the Layng Creek Fossil Beds. On last Fall's tour through the Coast Range, I swapped in a 26x2.0 Schwalbe Dureme at the rear. The bike has clearance for *much* wider tires, but so far these have had no real shortcomings.

I have to admit, compared to my single bikes all expectations wrt to braking and tire cross section on a tandem carrying two people with or without luggage seem to be thrown out the window in practice and have gone counter to all I would have predicted. Biggest reason for the impending swap from self-energizing cantis to v-brakes is simply to try something new and to have a deeper perch for my hands and better leverage/comfort when riding atop the brake hoods. This change is a chance to play and compare, never a bad thing. If results are worse, I'll simply switch back.

You may find these links interesting:
http://santanatandems.com/Techno/UnderstandingBraking.html
http://www.precisiontandems.com/arai.htm
https://www.sheldonbrown.com/tandem-brakes.html
For those pining for a replacement Arai drum brake (no longer manufactured), a machinist local to me is offering a replacement in the Tom Maddox Machine "Mad Dog Drag Brake", described here: https://tandemgeek.wordpress.com/2013/07/10/next-generation-drag-brake-now-available/

I'll end my tandem ramblings here, or the topic may veer enough to be split and moved to the Tandem Talk board:
http://thorncyclesforum.co.uk/index.php?board=24.0

All the best,

Dan.

[martinf]

With Dan on tandem braking. The extra weight seems to help braking. I had reasonably wide 650B x 44 tyres on mine. After I had replaced the fairly useless Mafac brake pads that came with the bike with decent ones (Scott-Mathauser) the Mafac cantilevers were quite effective enough to stop the bike quickly in an emergency and I never experienced any skidding or loss of traction.

The drum brake was useful to limit speed on long descents, without having to worry about rim heating and possible tyre blowoff.

[mickeg]

In a previous post I had a couple photos of my rear disc brake unit mounted on the chainstay.  A question was asked about the cable routing, I have cropped another photo to show the cable rounting better.  The bends in the cable are not a problem, it works just fine.  And here I get to show off a 20 year old derailleur that was in good enough shape to go on my new build.

[Danneaux]

All lovely, George!

Best,

Dan.