# Drag brakes, heat sinks, what?



## dir-T (Jan 20, 2004)

Over on RBR's tandem forumn is a good deal of talk about running a rear drum brake as a "drag brake" or "heat sink". I assume that means that the brake is used to hold the bike bike on long decents without the potential to overheat your rim and blow a tire.

Wifey and I are going to get a MTB tandem next summer for our 5th anniversary and I haven't seen any mention of these types of brakes for MTB tandems. Are they necessary or not?

We'll be using the bike (looks like a Fandago Tio or SC-9 at this point) for loaded and unloaded touring on dirt roads and trails as well as pavement and some singletrack.


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## Cary (Dec 29, 2003)

Before the advent of reliable disc brakes, the only way to have adequate braking for long and/or steep descents was to use a Aria Drag Brake. They are called that because they are used for speed control, not stopping, and you run a friction shifter to them so you can set the amount of drag. They provide good speed control and can dissipate a lot of heat.

With the advent of good disc brakes, drag brakes are not necessary. Good discs (many use avid mechanical or Magura's on tandems) using 203 rotors front and rear provide more than enough heat capacity for loaded touring of any sort.


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## aka1972 (Sep 22, 2005)

dir-T said:


> Over on RBR's tandem forumn is a good deal of talk about running a rear drum brake as a "drag brake" or "heat sink". I assume that means that the brake is used to hold the bike bike on long decents without the potential to overheat your rim and blow a tire.
> 
> Wifey and I are going to get a MTB tandem next summer for our 5th anniversary and I haven't seen any mention of these types of brakes for MTB tandems. Are they necessary or not?
> 
> We'll be using the bike (looks like a Fandago Tio or SC-9 at this point) for loaded and unloaded touring on dirt roads and trails as well as pavement and some singletrack.


We (overall weight ~180kg incl. tandem) are using our MTB tandem only "unloaded" and I am more than satisfied with the performance and reliability of our disc brake.
2 Weeks ago we where at Lago di Garda in italy and biked from Riva up to Pso. Tremalzo. 
https://fotos.mtb-news.de/img/photos/3/1/5/0/5/_/large/IMG_4521-klein.JPG
The up- & downhill was around 1.900m, partially trails and quite steep. As it was not possible to release the brake at a long steep section I decided to do a pause, the brake itself was still fine at this point.
So for us an additional Arai drum brake is not necessary. Ah, forgot to mention, the brake we use is a magura gustav m. .


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## TandemNut (Mar 12, 2004)

Oh boy, you've opened the proverbial can 'o worms now! Nothing gets folks on road tandem boards in a tizzy quite as much as a disc brake vs drag brake discussion.
As others have posted, prior to the introduction of appropriately-powered disc brakes to the bicycle market, drag brakes were necessary (and may still be) to keep the speed of heavily loaded tandems in check on long descents. In extreme situations, a drag brake might actually still be appropriate, but those circumstances would be something along the line of a fully loaded (meaning expedition-sort-of-loaded) tandem, with maybe a trailer behind it, in the Alps, etc etc. Sherwood and I actually discussed putting a drag or disc brake on a Bob trailer recently, but we haven't actually done. (there's a project for you Nate!)
OTOH, as stated above, there are disc brake systems available for tandems that are very capable of keeping a speeding tandem in check in virtually all typical off-road tandem situations: The manufacturers that approve their disc brakes for use on tandems are Avid (BB7 brake with 203mm rotors F&R only), Magura (Gustav M with 210/190mm rotors, and Louise Tandem with 203/203mm rotors), and Hope (M4, M6 and Moto, 203/203mm rotors). Each manufacturer's "tandem rating" is contingent on specific minimum rotor sizes being used, and in some cases, different linesets (braided lines vs regular). When spec'd according to the above-specified parameters, there have ben very few (if any) documented cases of brake issues or failures on tandems, at least in the recent past. I do know that Chris Timm managed to melt a line off a Gustav rear brake several years ago, but that was a 160mm rotor, which isn't large enough for tandem use anyway.
However, if one searches among tandems for sale, one can find some entry-level tandems from some name-brand companys that are spec'd with 160mm rotors front and rear, and one will also see all sorts of brake combinations used on tandems by individual folks who either don't know or aren't concerned with the manufacturer's clearance for use on tandems. If there were a disc brake issue with tandems, I suspect it'd be that sort of situation.
That being said, one can use improper braking technique and overheat virtually any brake that's practical to put on a bicycle, so disc brakes don't relieve the operator of the responsibility for proper braking technique. The difference is in the failure mode; some disc brakes will lock up completely, others will get softer levers to the point of being scary, the Avid might melt an adjusting knob off, and I'm sure it's still possible to melt a line off a hydrualic brake if one tried hard enough. But on a rim-brake equipped tandem, the heat buildup on the rim would probably have blown the tire off the rim well before the point of failure for a PROPERLY SPEC'D disc brake.
I can tell that from our own experience of having started riding tandems off-road just prior to disc brakes, that the introduction of disc brakes to off-road tandems has, dare I say it, revolutionized off-road tandeming. Now some place with dry weather and no steep or long descents may argue that point, but in our experience, it made the difference between even being able to ride in less-than-ideal conditions or not. We have personally melted V and hydrualic rim brake pads trying to slow a tandem down, and the performance of any rim brake in less than ideal conditions is virtually always inferior to a properly set up disc brake. 
(here come the flames... )
For road tandems, there are, apparently, different laws of physics at work. If you peruse road tandem forums long enough, it will become apparent that one should never use disc brakes on road forks at all, and never use disc brakes on tandems as the only means of braking, or disaster will surely follow. I've never figured out what's actually different on a road tandem that invokes those other laws of physics, but it must be true, 'cause I read it on the internet! 
FWIW, our next road tandem will be dual disc brakes. There are quite a few road tandems with dual discs running around out there successfully flaunting said laws of phsics.
Personally, I wouldn't mind seeing even larger rotors available for tandem disc brakes (Santana makes a 10" rotor version of their Winzip brake). Hayes had/had a 9" rotor kit available as well. Larger rotors would still provide plenty of clearance for trail obstacles, but increase braking capacity/heatsink capabilities measurably. Seems like a 9" or 10" rotor setup would be ideal.


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## itsdoable (Jan 6, 2004)

BigNut said:


> ...For road tandems, there are, apparently, different laws of physics at work....


Thats because they live in a different world...

Actually the laws are the same, the _average_ road descent is noticeably faster than the _average_ mtb descent, and heat buildup increases with the square of the velocity. You _can_ get significantly more heat buildup in a road descent. Like everything in this world, there are exceptions.

As BigNut said, there's no replacement for good braking technique, we have a few riders that can cook a 203 rotor (to the point of being brakeless) on a <1000' trail descent. There's no way I'm stoking for them.... just a minute, I already have. Damn!

Personally I have no issues running appropriate discs on road, tandem or other. And the control you get on wet slippery conditions is just so much better.


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## Trails4Two (May 12, 2008)

*Brake test*

My stoker and I will be testing our Maguras on the Monarch Crest trail next week - I'll let you know if we can burn them up. (Alex, we will also break in that new Jr. T!).
We used to ride a Cannondale with hydraulic rim brakes and a drum brake. We never melted or blew anything, but my hands really appreciated the power increase of discs. We had the drum brake hooked to a cheap SRAM grip shifter on the stoker's bars - it worked great, but I still like discs better.


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## Speedub.Nate (Dec 31, 2003)

I've demoed V brakes and cantilevers on rentals, had Magura HS22s on our initial Cannondale setup, and have been using Avid mech w/ 220mm rotors since then.

The Avids are crazy strong and allow me to descend with a great degree of confidence, much higher than previously. They haven't let me down yet, but I can say that without question, we're heating these rotors much hotter than on any single bike, despite their oversized diameter.

On a single bike, I feel relatively comfortable being able to stop and/or manuever if I were to experience a sudden, single brake failure. I'm not so sure about that ability on the tandem, and I'd never want to unnecessarily put my wife in harm's way (who would take care of me, change my diapers and server me cold beer?  ). 

So I opted for a third brake -- not really a drag brake -- in the form of a rear Magura HS33, controlled from the stoker's handlebar. It gives her an opportunity to check our speed if she feels uncomfortable, it's a very effective back-up brake in case we cook one of the Avids, and with a bit of communication, we can take turns switching between the rim and disc on longer descents to manage any heat build-up.

The HS33 as a reserve is probably unnecessary -- some might say overkill -- but at less than 400g and $100, I can't say that it's "weighing us down" nor overly-expensive. Instead, I look at it as cheap insurance in the long run, and wife placating here and now.


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## DaleTR (Apr 6, 2007)

*Monarch Crest on the Big Bikes!*



Trails4Two said:


> My stoker and I will be testing our Maguras on the Monarch Crest trail next week - I'll let you know if we can burn them up. (Alex, we will also break in that new Jr. T!).
> We used to ride a Cannondale with hydraulic rim brakes and a drum brake. We never melted or blew anything, but my hands really appreciated the power increase of discs. We had the drum brake hooked to a cheap SRAM grip shifter on the stoker's bars - it worked great, but I still like discs better.


Monarch Crest is probably the SINGLE best trail we've ridden on the tandem. you WILL have a great time (we are going down there for our annual crest ride on the 20th....)

You'll have NO issues with the braking there on a pair of Maguras. (really, the descents on the Crest are not too bad... ) We rode it with the Avid Juicy 7/203mm rotor brake set up originally on our bike. I would NOT recommend that setup for any "real" trail riding. We've since moved to the Hope Moto 6, and they are MUCH better for any real descents. The Hopes have been OK on STEEP descents (1000+ feet in about 1.5 miles...). The can get so hot you don't want to touch the CALIPER BODIES, but have yet to go mushy or have any issues. (yep, we usually give them a good cool down after something like that before moving on...)


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## Trails4Two (May 12, 2008)

*Monarch*

Great Pics! We will actually be there the same weekend but don't know if we are riding the 19th, 20th or 21st.


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## twd953 (Aug 21, 2008)

I think Alex is pretty much spot on, but I would (and have already on the RBR post) make a few clarifications. 

First, is that I think a lot of road tandem riders either think of or use disc brakes like do an arai drag brake. As Alex mentioned, braking technique has a lot to do with brake performance and heat buildup, and disc brakes are not drag brakes. Dragging a brake for long periods of time on a long fast descent is going to give any brake a problem except the arai drum. 

I think a lot of road tandem riders think that means that discs aren't effective on road tandems. That isn't the case, it's just that if you use a disc brake as a drag brake, by hooking it up to a shift lever or just by dragging your brakes down the hill, you're going to have problems. 

The other point of clarification between road and mtb tandem use for disc brakes is that MTB tandems with big rotors more often than not are use 20mm thru axle hubs, which are stiffer and more secure than a QR hub. 

The tandem rated Winwood fork I've got on my road tandem has disc tabs but is only rated for 160mm rotors. I haven't seen anything specifically stating that any other tandem rated road forks being approved for use with anything larger than a 160mm rotor. Not that I've asked mind you, but it is a consideration. 

I also have no idea whether the limitation on the Winwood fork is based on the strength of the fork in relation to braking forces, or the security of the QR axle with a large rotor. I've never seen a QR wheel pull out of the dropout on disc brakes, but there certainly has been plenty of talk about it on online forums (urban legend?). I've had problems with wheels shifting in the dropouts under hard braking on several single mtb forks while running 160mm rotors. 

Long story short, I wouldn't be comfortable running an 8" rotor up front on my road tandem, unless I had a fork where the manufacturer expressley approved it for that size rotor. Even then, I still like the security of a 20mm thru axle, which you would have to go to a custom fork to get.

I may still try running the Avids with a 160mm rotor up front, and will still use 
the arai drum in back since I don't have disc tabs on the frame (yet). If/when I do have a disc tab added to my road tandem, I would put a 203mm rotor in back, and I think it would still be an adequate braking setup under the vast majority of road riding conditions including hauling loads.


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## Speedub.Nate (Dec 31, 2003)

twd953 said:


> &#8230;I also have no idea whether the limitation on the Winwood fork is based on the strength of the fork in relation to braking forces, or the security of the QR axle with a large rotor. I've never seen a QR wheel pull out of the dropout on disc brakes, but there certainly has been plenty of talk about it on online forums (urban legend?).


I believe these are separate issues.

Using James Annan's formula for axle ejection forces, a 160mm rotor at max braking force (just prior to wheel lockup) creates a greater ejection vector than a larger diameter rotor. And there are standard MTB 9mm QR forks out there that are manufacturer approved for 203/208mm rotors.

I'm fairly confident any restrictions to rotor size are directly related to the strength of the fork or casting. Running a 203mm rotor moves the caliper further away from the fork, creating a longer lever arm to assist in snaping a fork in two, breaking off the tabs, or crushing the lowers (take your pick).


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## DaleTR (Apr 6, 2007)

*Red Tandem?*



Trails4Two said:


> Great Pics! We will actually be there the same weekend but don't know if we are riding the 19th, 20th or 21st.


Monarch was GREAT yesterday.. Upper 40's and crisp fall air. aspens on the Poncha Creek Road were just getting going too. Only have half the pics so far, If there are more good ones, will post them after I get the other half.

Heard "reports" that we were not the only 2 big bikes seen that day (20th). Was that you on a red bike?


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## Trails4Two (May 12, 2008)

*Red bike rampage*

Yep, that was us. I think we were just ahead of you. The Ventana/Jr T combo did great! We were really nailing the downhill on Silver creek until two consecutive pinch flats made me be more cautious. Must find new rear tire...
Here's pics


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## DaleTR (Apr 6, 2007)

*Figured*

Figured it had to be you we heard about... Can't be TOO many big rigs up there on one day. Looks like you got in the Silver Creek/Rainbow finish. Jealous. 2 times up there, and we've not had enough stoker interest in the climb up to Silver Creek, so we bail down Marshall Pass or Poncha Creek Road. :sad: One of these days.....

Posted a few other pics on the FR forum:

http://forums.mtbr.com/showthread.php?t=454258


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## grawbass (Aug 23, 2004)

Speedub.Nate said:


> Running a 203mm rotor moves the caliper further away from the fork, creating a longer lever arm to assist in snaping a fork in two, breaking off the tabs, or crushing the lowers (take your pick).


The lever arm is longer, but the being applied to the end of it is lower. It probably equals out in the end.


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## Speedub.Nate (Dec 31, 2003)

grawbass said:


> The lever arm is longer, but the (force?) being applied to the end of it is lower. It probably equals out in the end.


It's lower if you're riding exactly the same as you did with a smaller rotor.

The problem comes if you begin riding at higher speeds or are a heavier weight or are just plain braking more aggressively. It may only be an increase of 10% or 20% in rotor diameter, but I've measured some setups where the "lever arm" (measuring from the outside of the fork's stanchion) is approximately doubled, as compared to a 160mm setup. The potential for breakage is definitely there.


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## itsdoable (Jan 6, 2004)

grawbass said:


> The lever arm is longer, but the being applied to the end of it is lower. It probably equals out in the end.


The smaller the rotor, the larger the force at the dropout, it just a lever arm between the wheel and the rotor - the ratio between the wheel and rotor diameter gives you the multiplication factor for the force at the dropout. Larger rotors put less force at the dropout, which helps if your wheel is shifting in the dropouts. Take it to the extreme, a rim brake is just a rotor that is slightly smaller than the wheel, and exerts the least force at the dropout (about equal to the inertial braking force).

The issue with larger rotors is due to the fact that the caliper mount does not move out with increased rotor size, and sees increased peak forces with increased rotor size.


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## TandemNut (Mar 12, 2004)

itsdoable said:


> The smaller the rotor, the larger the force at the dropout, it just a lever arm between the wheel and the rotor - the ratio between the wheel and rotor diameter gives you the multiplication factor for the force at the dropout. Larger rotors put less force at the dropout, which helps if your wheel is shifting in the dropouts. Take it to the extreme, a rim brake is just a rotor that is slightly smaller than the wheel, and exerts the least force at the dropout (about equal to the inertial braking force).
> 
> The issue with larger rotors is due to the fact that the caliper mount does not move out with increased rotor size, and sees increased peak forces with increased rotor size.


I would think it depends on how the caliper is extended from the fork leg to reach the larger diameter rotor. If the caliper mounts are simply farther out from the same point on the leg, such as with some IS type mounts, then the leverage on the leg would be increased more with a larger rotor than if the caliper mount was further up the leg, but still roughly the same distance from the leg itself, such as some post-mount setups. Which may be some of the reason for the newer 203mm standard post-mount setup on some DH type forks.
I don't know if I articulated this correctly, but it makes sense in my little mind...


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## grawbass (Aug 23, 2004)

Speedub.Nate said:


> It's lower if you're riding exactly the same as you did with a smaller rotor.
> 
> The problem comes if you begin riding at higher speeds or are a heavier weight or are just plain braking more aggressively. It may only be an increase of 10% or 20% in rotor diameter, but I've measured some setups where the "lever arm" (measuring from the outside of the fork's stanchion) is approximately doubled, as compared to a 160mm setup. The potential for breakage is definitely there.


Yes "force" sorry. 

Good point. I guess if the mounting distance doubles, it's not quite a wash.


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