masiman

True the wheel is connected via the hub, however try to think about where the wheel is being grabbed. For calipers, V-brakes and cantilevers, they grab the wheel near the crown. The force of braking is applied and transmitted near the crown. For disks, that force is applied and transmitted near the dropout. If you think of a board that is held on both ends (head tube and ground) and then try to break that board. If you try to break it near the one end (caliper near head tube), You have to apply alot of force, but the one end will slip at some point (tire will break friction). If you try to break that same board near the middle (disk brake), you will have to use less force.

pel

thanks Masiman - maybe this should be separate thread?

Agree with you on where the braking force is applied on the wheel - circumference for rim brakes; close to drop out for discs.

But once that wheel starts slowing does the force ultimately go through the hub to the front fork (front braking)?? Hence disc brakes require a stronger front fork than rim??

Nachoman

I weigh 145 lbs and stoker weighs less. We currently have a rear disk brake, but we've pedaled for years with and without the disk brake. At our weight level, and with good brakes, I wouldn't worry about disk v. non disk.

deanack

I am 230 lbs & 6'5" my 08 Cannondale 2 with discs is the 1st tandem I have rode that stops well for me. I have been riding Tandems since 1981.

Dean

blamp28

The braking force people are concerned about is not exerted where the wheel mounts to the frame. It is exerted where the brake mounts to the frame. As the brake is engaged and friction is applied to the wheel, force is required to counteract the inertia of the bike. On rim brakes, that force is counteracted and absorbed high on the fork near the crown causing less flex and vibration. On disc brakes, that force is absorbed low on the fork near the wheel mounting causing more flex and vibration. The difference may not be that great on a single bike but with the increased size and additional rider of a tandem team, it is multiplied. The most important factor here is that the fork, disc or otherwise is tandem rated.

By the way. I love the BB7s on my mountain bike but the advantage is wet weather riding there. They do NOT stop and better than the SD7 rim brakes they replaced when both are clean and fresh. However, when riding takes me through streams and mud, the discs greatly out perform the rim brakes because of the environment they are forced to operate in.

I would agree with those that have said that it depends on your terrain and more importantly on your individual sense of need. Tandems have worked fine without them for years but they could be of benefit in the wet.

jbyrne

I don't see much discussion here on the merits of disc brakes versus rim to the end user. While some of the discussion is relevant to someone designing a proper tandem-disc-fork, the end user shouldn't care as long as it is designed appropriately.

My experience with a rear disc on a tandem is kind of mixed. For one thing with discs, in order to generate sufficient friction with the metallic pads, a large braking force is required. In order to generate this force with hand levers, the pads move a very short distance. This can make life with a disc brake kind of a pain. If it warps just a little or even gets slightly dinged then it's going to rub. Trying to straighten a bent disc is an exercise in futility.

Although it's nice to not have to worry about blowing the rear tire off, I found the disc brake to fade and exhibit some bad characteristics. I weigh 150 and my wife about 105. In the Pyrenees are brakes were pushed to the max. I had Avid V brakes in front and an Avid 203mm disc in back. The disc would get so hot I'm sure it was glowing. It would make awful sounds and vibrations at those temperatures.

Now, my rim brake pads melted as well. Once melted, they didn't perform too well. But I was able to find replacement pads in local bike shops to keep us going. I think if I had had kool-stops I would have fared better out there. I have kool-stops on there now back in Colorado and haven't had any problems.

I'm not trying to pan disc brakes but they have their limitations as well. The 203mm disc is still a relatively small piece of metal that doesn't have much heat capacity. It has plenty of braking power when cool but if you are doing a lot of braking they will fade badly in my experience. I would guess that the rim could possibly take more heat before brake fade (with good pads) since it is such a big piece of metal.

Also, disc brakes preclude using a standard cargo rack. I think Old Man Mountain makes a disc version but it is kind of ugly in my opinion. Mounting fenders could also be complicated.

Jason

rmac

Could you explain, then, where the force is being exerted to counteract the forward inertia of the bike? The brake calipers are being pushed forward at the crown and on the fork. Somehow this has to be translated into a force applied backwards.

Old Army

Brake fade is related to the brake friction materials and the amount of heat they can tolerate. A relatively small disk brake metallic rotor can take an extreme amount of heat, but if the pad can not take the heat, you get brake fade. Manufacturers mitigate this by either making the pad more heat resistant, or improving the cooling of the disk. They also attempt to mitigate the gasses built up between the pad and rotor with slots or holes. So mitigating brake fade with either rim brakes or disks is a matter of choosing the proper friction material. But this comes at a cost. Generally the more resistant to fade a brake material is, the more heat is required to achieve full effectiveness. So you trade hot performance for cold performance. Also to be considered is pad life and rotor/rim life.

In braking, no matter they system, you are dissipating the energy of the moving bicycle by converting the energy to heat and dissipating that heat to the bike materials and the air. So no matter the braking system, you will have to dissipate the same amount of heat for a given amount of speed.

A rim brake has a whole lot of area to act upon (the entire wheel rim), so it ends up heating up more area to a lesser degree than a disk. But it is not as efficient as cooling as a disk is. And the rim dissipates some of its heat to the air within the tire and the brake shoes themselves. This can lead to melted pads and exploding tires.

A disk may have better heat dissipation, but it produces higher temperatures due to the reduced surface area. So boiling brake fluid (hydraulics) and melting plastic brake caliper parts becomes a concern. If you melt parts in the caliper, you are replacing the whole thing, not just the pads. Some people concern themselves with warped rotors, although I've never experienced one.

The bottom line is that neither solution is ideal for a heavy tandem under extreme, prolonged braking. That is why there are drag brakes.

Disk advantages: More heat tolerance (i.e. no blown tires), less risk of contamination, less brake lever force required than rim brakes

Rim Brake Advantages: Light-weight, simplicity, cost, availability of spares

masiman

At the contact point between the moving bike and the motionless surface. Hopefully that is between the tire and ground . If you brake on ice, almost nothing happens wrt to braking. Sand and loose stuff prevent finding equilibrium via braking, the friction between the to the two objects that are in motion relative to each other (bike and earth).

rmac

Sorry, I should have been more specific. I should have said: "Could you explain, then, where on the bike frame is the force being exerted to counteract the forward inertia of the bike?"

justcrankn

While braking there are two main forces acting on the fork. The brake mount will pull/push forward. The axle will push back. This is the same for rim or disc brakes. These forces are transmitted to the headtube, as a torque if you're braking hard enough.

jbyrne

The big difference with disc brakes as it relates to fork design is the torque that is applied at the tip of the left blade. The caliper mounts only a few inches above the dropout so it has a short lever arm in order to apply force to the fork tip.

In fact this torque was so high that when disc brakes first came out there were reports of front wheels being ejected from the QR skewer being pulled from the dropouts. Most forks I see now have the dropouts reversed to counter this torque.

On rim brakes, this torque is applied over the whole length of the fork so the braking force has less mechanical advantage to bend the fork tip. And it is symmetrical across both blades.

Yes, forces are all transmitted through the head tube and this is the same for disc in rim. The real difference as far as fork design is concerned is what happens at the tip of the left blade.

Jason

rmac

If this is true then why do some people say that the spokes for a disc wheel have to be stronger? Isn't the same force translated from the rim to the hub through the spokes?

jbyrne

I think that there are several concepts going on in this thread that are getting confused. There are fork requirements, spoke requirements, etc. My last post was just addressing the fork.

You bring up a good question about the spokes though. I think with the rim brake that as far as the wheel is concerned, the braking force doesn't put a torque on the spokes. I think the braking force is trying to make the rim not be round any more and the spokes have force on them to counter this but it's not a twisting force. With discs it's easy to see if you try to slow the hub down then you will twist the spokes as that force transmits through the spokes to the ground. How does that sound?

TandemGeek

Sort of, but not exactly... that is unless you're referring to some other early reports that I've missed along the way.

For reference and context, there was an incident involving an off-road tandem with a non-disc fork that was poorly modified for use with a disc. It was the combination of the fork's drop-out orientation, the lack of any axle retention features (QR lips), and the near-parallel orientation of a 203mm 4-pot downhill disc brake's caliper and rotor with the direction of the drop-outs that created a situation where all of the brake energy was transmitted towards the opening in the drop-out.

This configuration resulted in a wheel ejection and the revelation that you can't simply stick disc brake mounts on a fork without also addressing the drop-out orientation relative to the disc caliper, brake energy and axle retention systems. It turned out that this was not an isolated event and after many years of controversay and thrashing about on the Web, fork designers have quietly incorporatated better designs into disc forks that mitigate brake-force-direction issues with drop-out orientation.

As another data point, and although it wasn't a source of major controvery like the first example, a similar problem with axle ejection would occur on Ventana's older frames with their rearward facing drop-outs: drop-outs designed for use with rim brakes in mind, not discs. Once discs became the brake of choice for off-road tandems and were mounted in the "normal location" (above and slightly behind the rear axle on the back of the rear seat stay / brake bridge), it caused the all of the brake energy -- from the disc as well as the wheel -- to push the axle out of the frame: been there and did that many times with our first Ventana off-road tandem.

In other words, it wasn't the amount of the brake energy or the disc, per say, that created the problem. It was the orientation of the disc caliper relative to the fork drop-outs. For example, one of our builder friends recognized the disc brake / drop-out orientation problem early on and simply mounted disc calipers on the front of the right fork legs which, in essense, reversed the direction of the brake energy so that it forced the axle deeper into a standard fork drop-out. Now, you still needed a fork with beefed up fork ends to deal with the added brake energy generated by the disc caliper, but that's a different part of the equation.

jbyrne

TandemGeek,

Yeah, I am remembering back to all those stories that went around the net about the wheel ejection problem.

I agree that orientation of the forces is key to the ejection problem. But, I also think the greater torque at the dropout is another piece of the equation.

Imagine a 2" disc as compared to a 40" disk. The tiny disc will produce more torque at the dropout. I think you could also think about it in terms of the length of the reaction arm of the drum brake. A short arm (analogous to a small disc) would put more force on the chainstay. A long reaction arm would produce less force as measured at the chainstay attachment point. If you had a really short arm it could more easily rip out of the mounting point.

So wouldn't you agree that this was also a contributing factor to the wheel ejection?

Jason

TandemGeek

Using that as a premise, getting on the bike and riding it would also be a contributing factor...

The point being, if you orient the drop-out opening and brake installation correctly so that the brake energy of the brake device (be it a disc brake or a rim brake) acts on the front axle to drive it into the drop-out instead of out, it doesn't matter how much torque you apply, the wheel won't eject.

Just to put this to bed, imagine if you will the lowly front drop-out on a parked bicycle with a wheel stuck in there without having the QR fastened. How much weight would you need to pile on top of the bike to make the wheel pop out of the drop out? The answer is no amount of loading will make that axle pop out because the more loading you apply the more secure the axle is held in the drop-out. In fact, you could probably use a drop-out raked fore or aft by just about anything short of 80° without any risk of the wheel coming out. Now, how much effort would it take to get the axle out by lifting the front end of the bike: not much.

jbyrne

Sure, I agree that the dropout orientation is the predominant factor in ejection. By orienting it properly with respect to the braking force you can make the probability of ejection zero.

Just saying that an improperly oriented dropout with respect to rim brakes would still be difficult to eject because there is so much less torque applied at the dropout. The QR would overwhelm any torque a rim brake could apply.

This was my idea in even mentioning this but it's not really a good example.

My only point is that disc brakes apply more torque on the fork tip (on one side) and require beefier legs. So, enough about wheel ejection!

Thanks,
Jason

rmac

True, about 188 grams or about 0.4 lbs. And that includes a fairly beefy disc brake mount.

TandemGeek

I think you're under estimating just how much brake energy a good rim brake with good pads can generate on that ~622mm rotor that pulls double-duty as a bicycle rim.

If I had a better grasp of math I'd explain this with a proper model but, in short, if you built a fixture that positioned a rim brake behind the front wheel -- with roughly the same orientation between the rim brake pads / rim as you find on disc brake pads / rotors relative to conventional fork drop-outs -- the rim brake would also eject the wheel lacking a secure skewer or other retention device(s). The physics involved aren't that much different and the real-world braking forces at the axle aren't all that different either.

I'll leave y'all to hash on about fork legs, spokes, disc are great/suck and the like as the archives are already bursting at the seams with similar threads and postings.

rmac

Of course, that could be said of most of the topics posted here. Is it just me not remembering correctly or have $300 tandems been discussed before?

stapfam

When we first got our tandem- Dale MT 2000- it had "V" brakes fitted. They did not cause a problem and stopped us well enough on the fast descents offroad and were completely adequate. What was not adequate though- was the pilots hands. We were into long distance riding then and after about 60 miles offroad on a tough route- the braking effect was depreciating due to the pilots hands giving out.

Changed to disc brakes and the pilot was happy.

But these were top rate Hope Mono M4's with 200 mm discs front and rear and cost a bomb. And then there was the wheels that had to be changed and the forks that would take the extra force these brakes were giving them.

Disc brakes work- But so do "V" Brakes.

pel

rmac I think you have posed the crux question here - and have not got a straight answer to it. 'where is the force being exerted that stops the forward inertia of the bike?'

As I understand it there are a range of forces involved - rim or disc, when applied both result in the mounts being pulled forward and a slow down in the wheel relative to the bike and riders (action and reaction). The slowing wheel (let us just say front brake for now) eventually stops the moving bike and riders - I think we can agree on that. The slowing wheel force is excerted on the bike through the hub and front fork. Yes?? The comments wheel ejection and drop outs appear to confirm this.

It is my experience that disc brakes can, if needs be, stop a given bike faster (more fiercely) than rim brakes (and are better in the wet). Yes??

oldacura

Do a "free body diagram" on the front wheel with brakes applied - one with a disc and one with a rim brake. The 3 places that the wheel is acted on by external forces are the road, the axle and the point where the brake pads contact. The force to stop the bike is delivered through the fork tips. The tangential force applied by the road creates a positive torque (force x distance) attempting to rotate the wheel about the axle. The tangential force applied by the brake pads creates a negative torque that opposes and balances the positive torque created by the road contact. If this is a rim brake, since the radius that the force is acting at is nearly the same as the road contact (rim radius -vs- tire radius), the ******ing force is about the same as the driving force. For a disc brake, the radius is much smaller so the braking force is proportionately higher. If you estimate that the rim radius is about 350mm and the disc radius is about 100mm, the disc braking force (not necessarily the clamping force) is about 3.5 higher than the rim braking force.

As long as the braking torque is below the driving torque, you have a controlled braking condition. If the braking torque exceeds the driving torque, the wheel locks up and you have a skidding situation (not good).

Though I have never tried it on our tandem, I have heard reports that rim or disc brakes in good condition can lock the front wheel. Once that happens, all of the energy transfer (from kinetic to heat) moves to the skidding surfaces (i.e. the tire/road interface).

Ultimately, disc -vs- rim brakes is a choice of compromises. Either one is "better" only as percieved by the rider.

In both cases you are converting kinetic energy to heat. While a rim has a lot more metal to absorb the heat, a disc can get much hotter than a rim and still function properly.

pel

Thanks oldacura - that is the explanation we have been waiting for - eloquent!