Rim Brakes vs. Disc Brakes
 

After being away from tandeming for several years I find myself in possession of a modern tandem with disc brakes and 200mm rotors. I have put several thousand miles on tandems with cantilever and V-brakes so I am excited to finally have braking that will not seem sketchy. While I have a natural inclination to consider a possible wheel upgrade before even riding my new bike, I wonder about the stresses disc brakes will put upon my wheels. My wife and I live in the mountains so we will do some serious climbing and, as our reward, equally serious descending. Applying this kind of braking power to 400ish pounds of mass that prefers to remain in motion seems like a great way to trash wheels, and seems more likely to do so (at least to me) by applying the force at the hub instead of the rim. This is not really a post about brakes but about the stresses different brakes put on wheels. I would love to have your input, theories, and experiences.

I have had a rear disc brake since 03 on my tandem. The first rear wheel went 25,000 miles before I rebuilt the wheel, much farther than I have ever ridden a rear tandem wheel before rebuilding. It is just one wheel but a disc brake and earlier wheel failure have not been my experience.
Sheldon Hall
Greenfield, In

I've also not heard of any issues related to type of brake used and durability of hubs. There are many other factors that the type of brake affects, but not hub durability IME.

I wouldn't be terribly concerned with hub durability either. Disk/drum brake do place more stress on the hub but I would be much more worried about spoke strength and, in the case of the front, fork strength and durability. For rim brakes, I would worry about heat related inner tube failures. For disk brakes, warping rotors and heat related fade.

Since our touring weight is about the same as your team weight, what we have are 48 single butted Sapim spokes and Phil Woods rear hubs on 26" rims. The front brake is a set of cantilever brakes (for fender clearance). The rear is a disk. I am installing one of the Shimao IceTech rotors to see if it does better when it gets really hot. We have a plain steel disc but it would warp and fade when it gets really hot.

If you want something lighter, I am sure there are others with opinions. :-)

Thank you for the input. Hub durability is not really my concern, but I did not make that clear. I can just imagine clamping down a big disc rotor while descending at 50 mph......the hub will be quickly decelerated while the rim desires to keep turning at it's high speed. I would think there would be a tendency for the spokes to wind around the hub and put lots of stress on the rim. I think the stock 40 spoke wheels are sturdy enough but I wonder if something lighter with fewer spokes could handle it.

Or how about clamping down on a rim bake at 50 mph. where the rim wants to stop and the hub wants to keep turning. Either way a strong wheel is a good idea.

Tom, I do agree that is a similar situation. It is just my experience that the disc brakes will put much more stopping force on the rotor than rim brakes can generate which will seem to exaggerate the effect. Thanks for the input!

Tom; I am not disagreeing with you, but I am not sure the hub or the rim 'wants to stop' per say, but when the rim is impeded by friction from the brake shoes, it begins to spin at a lower RPM. The hub is very small in diameter compared to the rim/time and likely has very little inertia in comparision also...nothing that would bother the equation.

If you had a rim brake on your tandem you would find it interesting to feel the rims after varying brake efforts to see how much heating occurs, on our first tandem with a total
weight in the 480 range slowing down from 35mph to zero with ~120' drop over ~1/3 mi the rear rim would be too hot to touch and the front warm. For that reason we speced
a rear disc on the newer tandem. Brake adjusting is a little finicky at times and the pads last ~7-9kmiles or about $0.01/mile but I feel more comfortable with the disk on the rear.
Also don't have to worry about the very occasional wet rim and the rim is not abraded by the brakes, FWIW. Weight penalty is ~8-10 ounces per wheel.

If your so worried about it, set up some drag brakes your stoker can use to slow down enough to run the disc.

[Partial QUOTE=scycheng;15389595]I wouldn't be terribly concerned with hub durability either. Disk/drum brake do place more stress on the hub but I would be much more worried about spoke strength and, in the case of the front, fork strength and durability. For rim brakes, I would worry about heat related inner tube failures. For disk brakes, warping rotors and heat related fade. ... :-)[/QUOTE]

A few mintues back I posted a posie to the group regarding doing a tandem with both disks and canti's at both ends. If you have thoughts along that line (good, bad, or snickers) would appreciate them tied to that thread. /K

We favor rim brakes.
Been riding as a duo since 1975 with 200,000+ miles under our wheels.
Now on tandem #5.
Live in 'not flat' Arizona and have climbed/descended many a hill/mountain in our day.
Never blew a tire due to overheating a rim, including an 11-mile 7% twisty mountain descent (Kitt Peak) near Tucson, AZ. using old Mafac cantilever brakes (no 3rd brake). Had been told by 'experts' that we could not do that on a tandem without a 3rd (drag) brake..
Did have to stop halfway down Kitt Peak for cramping fingers from doing alternate front/rear braking.
Much is in 'how' you brake and having good working equipment.
While making the short stop checked the rims. They were just warm, not hot.
Continued the descent without any issues.
We are a sub-250 lbs duo.
Have ridden tandems with many kinds of rim brakes: cantis, centerpulls, sidepulls, U-brake, V-brakes and even a tandem with front and rear discs.
Considered the discs excellent stoppers but finicky and prone to early pad wear and in need of fiddling/adjusting, warping and even overheating.
Have seen a stoker pour water on a rear disc to cool it down.
Have had a set of Scott-Matthauser pads (remember those?) on tandem canti brakes last us 50,000 miles (right amount of zeroes) before replacing.
Currently use D/A sidepull on front and Tektro Mini-V brake on rear of our tandem.
Heel/hub/spoke durability have been exceptional.
Quality equipment pays off in the long run.
However, utilize whatever you feel comfortable/safe with as a team.
Just our input/experience.
Pedal on!
Rudy anbd Kay/zonatandem

We've seen low spoke count rear wheel failures on tandems with a rear disc, Rolf wheels specifically. We have not seen this on a disc-braked tandem running a 3-cross traditionally spoked wheel with 32 or more spokes. To my mind the greatest advantage of a disc is extending rim life, particularly if one rides or tours in the rain. We have a rim-braked tandem with a spare touring rear with a drum, but have ridden for a few years with a number of newer tandems with discs. We all give our brakes a good workout in the steepish hills around here. We think rims are particularly important for tandem wheels. Deep-V rims are an almost universal choice around here. Most of us are running 25c or 28c tires. Velocity also makes a wider version of this rim for those who want to run wider tires. I think we are all running 14-15 DT Competition double butted spokes.

We use rim brakes and would not consider disks for our environment. I see the advantages of disks to be better braking and additional rim life in wet conditions plus additional heat capability for big braking efforts. We ride mostly in dry weather and don't have long descents. While we have worn out rims it usually takes at least 10,000-15,000 miles depending on the amount of wet weather riding. As a result rim brakes work well for us. They stop just as well and are lighter. I also agree that they seem to be finicky to me. I keep hearing disk brakes going zing zing zing at tandem rallies.

OK all you engineers! Let's say it takes the same amount of energy dissipation to bring a bike from rolling to a stop whether the brakes are rim or disc. If the bike has rim brakes and the rim slows then the spokes have to transmit the stopping force to the hub. Yes? If the bike has disc brakes and the hub slows then the spokes have to transmit the stopping force to the rim. (yes?)

Either system will generate about the same amount of heat as either system is absorbing about the same amount of energy. In the rim brake the heat goes into the rim. In the disc brake it goes into the disc. Movement through the air dissipates the heat.

As a practical matter we have ridden rim, rim with drum, and disc brakes in mountainous terrain. All were properly adjusted and all stopped our bike safely. (320 lb team) We prefer the discs because:
1. The rim does not get worn down.
2. The pads on the disc have a very short distance of travel which means that the large motion of the brake levers allows much better modulation of the brakes.
3. Almost everyone that I have heard say they would never use discs--have never tried them.

All this aside I think that whichever system is used the best pads should be fitted, the system properly adjusted, and the wheel should be built with quality spokes and be strong. This may be more important than which system is used.

The horizontal force vector for the hub will the the same size, whether the brakes are rim or disc. This vector will be counteracted by increased tension in all the spokes behind the axle center. However and much more importantly, the disc brake acts on the hub, not the rim. Therefore a couple is created between the center of the axle and the tire contact patch. This torque couple winds up the wheel, putting considerable tension on one half of the spokes, assuming a multi-cross wheel lacing. So on a disc or drum braked bike, the spokes are tensioned by two forces when accelerating or decelerating.

You can still get these. In fact, I have them on the front of my tandem (because of the design of R+E's cantilevers, they don't work on the back.)

Braking slows the bike through frictional forces between the tires and the road. Rim brakes transfer this force from the tire through the rim to the frame. The spokes have little involvement in braking forces. You can use a radially laced wheel for the front if you desire with rim brakes. Hub or disc brakes must transfer the braking force from the tire through the rim, then through the spokes to the hub, and finally to the frame, so the spokes must be configured to transfer this force when using a disc or drum brake.

The energy dissipated is the same, but rims tend to have much more thermal mass and surface area for convection cooling than discs, so the peak temperature will be lower on a rim dissipating the same energy, although being in close contact with the tube and tire, a hot rim can cause blowouts in extreme circumstances. On the other hand, fading is less of an issue at the lower temperatures experienced with rim brakes.

Hit the Brakes, particularly the front, and the rest of the bike, still in motion [Newton, inertia],
pushes against the slowed wheel,
flexing the fork, back, .. more or less, depending on fork construction..


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you can get the same stuff in their many types offering the Salmon* compound..

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Looking at this like a lever a (stopping) force applied to the rim acts on the tire patch which stops the bike throught friction. But isn't the hub the fulcrum? And if the hub is the fulcrum then the spokes have quite a role to play.

No, the hub is not a fulcrum with rim brakes. The braking force at the brake shoes exactly equals the braking force at the road. It can't be any other way, since the two surfaces are connected by the rim. However with a disc, the hub forms one end of a torque couple as I described in my previous post. The hub, and therefore the disc, is only connected to the road through the spokes, which therefore windup due to the torque couple.

It is true that if there were no spokes, the bike would move forward over the wheel when braking and the wheel would drag behind the brakes, being held to the bike only by the brake pads in a vice grip on the rim. So yes, the spokes play a role, but only the same radial role they play in holding the bike up off the ground. There is no rotational force on the spokes as there is with a disc or hub brake, at least to a first order approximation.

OK The rim brake transfers the stopping energy to the road through the rim. But this doesn't slow the bike down until the rim transmits the force through the spokes to the hub and thence to the fork which is connected to the bike. I just can't visualize the spokes transmitting stopping force with a disc and not with a rim brake.

I believe that the force transmitted thru the spoke when rim brakes are used is simply forward inertia. There is not wind up force because that is absorbed by the brake shoes at rim and transmitted through the brake to the fork and then the frame. This is not usually a concern because bicycle wheels are designed to resist in line downward (or forward) force. Simply running a bike into a wall deals with this force and even a radial spoked wheel can handle it pretty well.

When a disk brake is used the forward braking force still exists but in addition to that force the unbraked rim and tire continue to be forced by the pavement moving under the tire to wind the spokes around the rim. The spokes must deal with this new force and that would be a problem for example with a radially spoked wheel.

In both cases friction stops the bike so another way to look at it is that a disk brake wheel transmits force through tire then rim then spokes then hub then disk where pad friction acts on the disk. A rim brake wheel has force though the tire and rim where friction with the brake shoe acts on the rim. The hub is just coasting in this case.

Look at it this way...

When driving or braking loads are applied at the hub of a spoked wheel, there is an uneven amount of torque applied to only 1/2 of the spokes, i.e., the "pulling spokes" if you will.

When rim brakes are applied there is an equal amount of loading applied to both the pulling and trailing spokes that, in effect, cancel each other out, i.e.,t there is no torque on the spokes.

The easiest way to visualize this is to think about a radially spoked front wheel where there are no pulling or trailing spokes. Rim brakes work just fine with a radially spoked front wheel because of the cancelling effect of the forces coming from the stopping action of the rim brake at one edge of the rim against the forces coming from the tire contact patch with the road, i.e, the rim carries the load.

Now imagine if you will what would happen to a radially spoked front wheel if a disc brake were used or, if it would help, imagine what would happen if you tried to use a radially spoke rear wheel when power is applied to the rear wheel via a chain driven sprocket at the hub. All of the spokes are going to be "pulled" at the hub as they work against the rim/tire contact patch with the road.