mikemelbrooks

When we bought our tandem it came with 180mm basic steel rotors and cable Avid bb7 cable discs with a rim drag brake operated by the stoker. We ride Audax through the winter and even using the drag brake sparingly found that the rim was wearing quite quickly. I switched up to a Hope 225mm front disc and a 200mm rear with hydraulic calipers. The feed back of the brakes is much better. And my hands are much less fatigued at the end of a 100mile ride. I am sure that if we kept with rim brakes I would be looking to change the wheels far more often. And the supply of suitable tandem rim brake wheels will only get smaller as the majority of bikes go to Disc.

TobyGadd

The failure on Mt. Ventoux seems to be credited to the leader telling people to drag their rear brakes. If that's what actually what happened, then that clearly demonstrates why that's terrible advice!

Here's what I did to test how quickly rotors cool down:
Test 1: I dragged one brake until it chattered--and then let it up and coasted for about 30-45 seconds before stopping with the other brake. The dragged rotor was still somewhat hot, but cool enough to touch! The other rotor was too hot to touch.
Test 2: I dragged one brake until it chattered, and then I quickly stopped with the other brake. Both were too hot to touch, and they both took a long time to cool down while we were stopped.
Conclusion: Based on these tests, I believe that spinning rotors cool down surprisingly quickly as long as friction isn't being applied, and that stationary rotors take a long time to cool down. My real-world riding experience seems to correlate nicely with these findings as well. I'm guessing that all of the cutouts on the rotors are designed to increase airflow and cooling.
Limitations: This was a very small sample size, and the tests weren't done on exactly the same stretch of road. I used my fingers intermittently, not a real-time thermometer, to evaluate temperature. It was done on a cool day, probably around 50F. I didn't use a stopwatch, but rather did ballpark estimations of time. "Chattering" isn't a terribly objective way of evaluating temperature or speed. Many other variables weren't accounted for. Basically, a limited test that could be used as a starting hypothesis for a much more rigorous study. Anyone want to write a grant proposal?

One other thought: I've descended some long and steep hills on a MTB tandem. Wow, do the brakes get hot! On technical terrain, the "stab and release" technique isn't terribly feasible. Nor is aerodynamic braking. While I've never melted anything, or lost the brakes, I have certainly had massive chattering and a bit of fade before deciding to stop for a bit to let things cool down. My Magura MT4 MTB tandem rotors look like they've been hit with a blowtorch--but they still work!

PromptCritical 

That seems like a pretty good test to me. Yeah, I’ve had more brake challenges on downhill MTB stuff than tandem rides, but I’m way more aggressive on a MTB than a tandem.

5fietsers

Hi, I'm new to this forum and thought that this might be a good place reach out with a tandem brake mounting question. We have a steel frame tandem that has a rear IS brake mount and a post mount TRP Spyre with an adapter and a 203 mm rotor which needs to be replaced. I'd like to upgrade to a 220 mm rotor, but I haven't been able to find an IS rear to 220 post mount adapter. Has anyone done this upgrade and/or found an adapter for this application?

mikemelbrooks

I switched to a 220 disc on the front of a Cannondale tandem and just made up some additional spacers out of aluminium. You could just use some washers and longer bolts if needed.

Duragrouch

I've thought often about an aero drag brake on a bike and car. I also sail.

The ocean is far more consistent in texture than highways. Towing a rope or sea anchor or series drogue off the stern with a chain weighing it down (the latest thinking) is far more predictable than trailing a rope on a highway that can get snagged with catastrophic results. Even a heavy chain (which you would not want to carry otherwise) has poor friction against the road. The best thing I could think of would be an electric generator driving a foward facing jet thrust-reverser, but the fan size and weight would be extreme. On a car, I had a small wagon with a big roof, I envisioned a roof flap not unlike the drag brake on an F-15, that would be simple and probably effective with, not only increased frontal area, but all the turbulent flat-plate-drag wake it created behind it.

In the end, what I did on my wagon was leave it in gear (manual tranmission) but turn the ignition off; The engine continued to turn so I had power brakes, power steering, and engine lubrication, but fuel shut off, so more engine drag, and air going through the engine so excellent but gentle cooling from the climb up (this strategy was used by GM first on the Northstar aluminum engine in case of coolant loss, it would constantly selectively shut off fuel to different cylinders to air cool, saying for use "up to 50 miles", heh, GM drove a Northstar across the USA on no coolant, no damage, just reduced engine output). Plus this involved no additional starter wear, at the bottom of the hill I would just turn the key back on. With electronic speedometer, this also stopped miles being recorded, fine as engine is off, but fuel usage claculations did not reflect true (greater) fuel mileage and lower fuel consumption. However I only know now, that this move disabled the airbags with the ignition off.

Leisesturm

Do you even know that you can fit a 220mm rotor in the rear of your bike? Plenty of bikes are limited in how big a rear rotor they can have. Just because 220mm rotors exist doesn't really mean that anyone needs them. I'm not gatekeeping, I'm trying to reality check you into reassessing the situation. Maybe save you some $$$. Never a bad thing.

5fietsers

Thanks for the idea, I would assume that you haven't had any problems with this set-up? I'd prefer to use a single piece solution, if it was available, to increase the rigidity of the attachment.

5fietsers

Thanks for the sanity check! Although the rotor would be close to the chain stay it would fit.

Duragrouch

There may be some issues with this. The boss/stud for the caliper take load in tension holding the caliper in place, but also bending in the direction of rotor rotation and pads resisting that motion. A stack of washers will not have the same rigidity if there is any sliding between the washers or spacers and the (original) caliper mounting boss/stud. Even if no sliding, it could also move elastically due to the higher load, small enough you won't see it but enough to cause fatigue failure over time. The total shear load on the bolts would be the same, but since spaced further out, would result in greater bending load on the bolts, with maximum moment in the threaded area. Depending on the height of spacers or washers, bending moment could increase by a multiple. Damage might also happen at the female threaded boss/stud, if not designed for the greater lateral bending load. Check, perhaps due some basic calculations on how much the bending moment is increasing for a given lateral force at the pads. This calculation would not be based on the increase in rotor diameter, but increase in caliper bolt attachment length, specifically, the length between the threaded attachment boss and the underside of the bolt head. If that's increasing by 100% (doubling) or more, or even 50%, caution. You are threading into aluminum, which is notoriously sensitive to loading for fatigue life/failure. Longer bolts is not the same loading condition as longer bosses with same length bolts.

mikemelbrooks

Thanks for your reply, and your concern. I did consider the extra load before going ahead with this. As the vast majority of the load is in compression rather than in shear I believed I was good to go. Originally the bike was fitted with bb7 brakes and conical washers. I have fitted hydraulic post mount brakes which no longer need the conical washer so the increase in bolt length is minimal. The maximum braking force is increased but the main reason we I went in rotor size was to have a disc that was more resistant to over heating. The reason we went to hydraulic to reduce the hand effort need to stop us especially on the rear brake as it has such a long cable run. The bonus is much less brake adjustment (none). We have done 4 to 5 thousand miles with the larger disc and I periodically check the brake mount.

mikemelbrooks

I see velo orange amongst others are selling post mount spacers I would think they would if they thought it unsafe. https://velo-orange.com/products/pos...-brake-spacers

Alan_F

Another option to consider to upgrade disc performance is switching to Shimano Freeza rotors which have the aluminum cooling rims extending towards the center of the rotor. They used to only make them in center lock, but they've been available in 6-bolt for a while.

We had been using Shimano Ice-Tech rotors for years, and they're supposed to work better because they have aluminum sandwiched between the steel outer surfaces of the rotor. But those don't have the extended aluminum fins . I didn't think the Ice-Tech rotors were any better than plain steel rotors I've used in the past.

After switching to the Freeza rotors, I've noticed a clear difference in performance. I can push these rotors further and I've yet to get them to fade at all even on long descents where I've had to drag them a bit.

We're a reasonably light team at about 310 lbs, so YMMV.

merlinextraligh

wasn’t at the Ventoux ride, but I can add that the same leader gave that advice regarding a 10,000 foot volcano descent in Hawaii. ( which we ignored and were fine)

merlinextraligh

I’ve posted this elsewhere, but for completeness, I’ll put it here. Since my post, we melted our ENVY rims on a 15% descent. Those rims had likely 20,000 miles on them and dated to tech around 2012. Amazingly ENVY will full knowledge they were used on a tandem replaced them under warranty. And their position was improved tech in the interim would make it ok to use the replacement rims.

That said, we decided, with discretion being the better part of valor switch to disc brakes for big mountains.

PromptCritical 

Guess he doesn't want any repeat buyers.....That's felony stupid!

PromptCritical 

So, what happened to the rims? Blowout or just warping?

We are having a custom fork built for our Santana so we can have front and rear disc brakes. Part of me wants to keep a rim brake as an emergency brake. One doesn't think of San Diego as particularly hilly, but there are some significant hills; the ride down from Mt. Palomar being one of them.

merlinextraligh

The front deformed to the point it blew out. I felt it happening, so we were going pretty slow when it blew,and came to a stop without incident.

203 mm discs, and some prudent braking technique, we’ve had no issue with discs, and have done some long and technical descents

Duragrouch

I've thought if I did a world tour with long stretches through non-advanced-bike areas like Mongolia or such, I'd want discs but with V-brake posts at least on the front fork, and capable rims, just in case. This might also influence decision for hydraulics on the discs or not, as with not, I could just reroute the cable, whereas with hydro, I'd need to have separate cables for the V-brake. I know they briefly make hydraulic rim calipers, but I'd prefer to not do hydraulic line open-heart-bypass in the middle of nowhere.