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Tandem Cycling A bicycle built for two. Want to find out more about this wonderful world of tandems? Check out this forum to talk with other tandem enthusiasts. Captains and stokers welcome!

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Old 01-03-10, 05:34 AM   #1
MaxCady
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Experiences with fixed disc brake tandem forks

One German tandem captain is very incredulous about stability of disc brake forks on tandems. His apprehensions are based on asymmetric forces, which are typical for one site disc brakes. Of course I wrote him about our confidence in our Cannondale Road Tandem...
Are there any bad experiences with broken or damaged disc brake forks? What about official force measurement values for some well known tandem frames with disc brake fork?
Thanks and best regards from Duesseldorf in Germany
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Old 01-03-10, 09:54 AM   #2
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Originally Posted by MaxCady View Post
One German tandem captain is very incredulous about stability of disc brake forks on tandems. His apprehensions are based on asymmetric forces, which are typical for one site disc brakes.
Then I suggest he not consider them for use on his own tandem; confidence in your own equipment is always of the utmost importance in cycling.

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Are there any bad experiences with broken or damaged disc brake forks?
Due to asymmetric forces or other characteristics unique to disc brakes where the fork was actually designed for disc? None that I'm aware of. If you remove the caveats, then there have been a few incidents where forks have been modified to accept a disc and, coupled with some other poor choices, things didn't turn out so good, e.g., a couple of wheel ejections way back before even Cannondale began to spec. Avid dual discs as the standard OEM brakes on it's high-end tandems.

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What about official force measurement values for some well known tandem frames with disc brake fork?
Official measurements, for brake systems? Good luck with that. Most folks who do that type of testing aren't going to share real data. However, for what it's worth, this same subject has come up from time to time in the motorcycle world where single-side disc brakes are also quite common: check out Harley's Sportster and Softtail models along with most early Japanese bikes and Buell motorcycles as a "for instance". Where someone who was knowledgeable weighed in the commentary was along the lines of, "the amount of brake torque measured by instrumented test equipment was below anything a rider could detect". Of course, this makes sense assuming the fork being used was properly designed for a disc brake given how close they are placed to the centerline of the single front wheel. Move the rotor out a foot and, well, then you'll have some brake torque issues, just as you would if you had a two-wheeled vehicle with a single brake on only one wheel.

Remember, the disc brake doesn't operate in isolation: tell your friend to factor in the forces from the wheel acting on the fork as the brake slows the rotation of the wheel. After all, while the brake stops the wheel from rotating, it's the friction between the tire/wheel and the road that actually slows and stops the forward movement of a bike... and all of that energy goes into both fork legs in a direction that's different than the energy path from the rotor being acted on by the caliper.
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Old 01-05-10, 04:56 AM   #3
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After all, while the brake stops the wheel from rotating, it's the friction between the tire/wheel and the road that actually slows and stops the forward movement of a bike... and all of that energy goes into both fork legs in a direction that's different than the energy path from the rotor being acted on by the caliper.[/QUOTE]

It's me who has put the question originally and I try to specify (please excuse for my English):
To transfer the braking force to the street is only half the story. To stop the wheel you need to put in a toque by the caliper and the wheel axis and you have to transfer it to the frame. This is done only by the forks left bracket. The brake torque, which is going on both brackets and the caliper-axis torque have the same size, but that means, that the left bracket has to be three times as stong as the right one. This of course are internal forces within the fork-axis-frame-system and you don't notice anything of this riding a bike. You agree?
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Old 01-05-10, 05:11 AM   #4
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I'm equally nervous about the asymmetric drivetrain forces.

That is to say, not.
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Old 01-05-10, 08:02 AM   #5
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Several times recently I have noticed that my front wheel QR has come loose. This would be noticed only when the rotor would suddenly start rubbing against the brake pads. You know, "zing, zing, zing". VERY scary.

Could my QR be worn out? Or do I have gremlins? Is it inherent to disc brakes (BB7's on Co-Motion tandem fork for discs)?
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Old 01-05-10, 08:41 AM   #6
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...I have noticed that my front wheel QR has come loose. ...Is it inherent to disc brakes
As a matter of fact, there is generally accepted evidence that your discs + QRs are causing this. If you'd like to learn more, search "James Annan disc ejection" and you'll come up with some relevant discussions.

The pragmatic advice is to use a good quality QR, verify it is adequately tight, and check it every ride. Shimano (who use an internal cam mechanism and hard serrated teeth) and DT Swiss' RWS are generally accepted as examples of secure, reliable QRs.

If you plug numbers into the equation Annan presents in his theory, you'll find that larger rotors have a reduced tendency to eject (or, rather, less force acting on the axle in a downward direction). But I presume you're already running 203's?
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Old 01-05-10, 02:16 PM   #7
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You agree?
I'm not sure how you get to a factor of 3x (I'd think it's somewhere between 1.5x - 2x), but it's a moot point if the fork is robust enough for long-term use on a tandem and the disc's use doesn't create any brake torque steer effect. Those are the real concerns and, in fact, most well-executed front disc implementations on single and tandem road and off-road bicycles more that address those issues.

Again, there have been exceptions / problems with some poorly designed disc brake fork adaptations relative to drop-out orientation (See aforementioned articles by James Annan). But, excluding those exceptions, I can tell you I've never experienced any brake torque on our off-road tandem which is fitted with a very robust, 4-pot downhill hydraulic disc and 203mm rotor. Now, to be fair, the hub and fork interface is a lot more robust than your average road tandem with a 9mm steel quick release skewer. But, that said, having ridden an Erickson road tandem solo with a front disc and spent time at rallies riding with about a dozen folks who own front disc equipped road tandems, I did not note nor did any of them make note of having any brake torque steering issues or a left-fork failure / fatigue issues.

There are all kinds of examples of asymmetric brake and drive train designs to be found on bicycles and motorcycles, yet few if any problems when the designs are sound and well executed in the manufacturing process. Motorcycles have for years used asymmetric, single front disc brakes as well as single sided rear swing arms for chain and shaft drives. Now, you can certainly take things a bit too far as Cannondale did with their disc-equipped Lefty single leg forks (e.g., Raven with lefty), which came back to bite them... but it was a durability issue not a steering / control problem caused by brake torque steering.

Bottom Line: Front discs work just fine on road and off-road tandems so long as the forks and other critical components are designed for use with a disc brake.
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Old 01-05-10, 03:13 PM   #8
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Could my QR be worn out?
As Nate notes, there are good skewers for disc-equipped bikes, and not-so-good skewers for disc-equipped bikes. So, here are some question:

1. Are you using the original skewer that came on the tandem and/or what is the QR's brand / model / material? How many years / miles have you logged on those skewers?
2. How many years / miles have you logged on the tandem with the front disc and when did you first notice your front wheel was shifting in the drop-outs?
3. Have you inspected the drop-outs to make sure they have not been damaged or deformed?
4. Does the fork still have it's original 'lawyer lips' or recessed skewer cups?
5. Do you always make sure that your skewers are closed with enough force to leave an imprint in your hand?
6. Have you serviced your front hub and/or made sure that it's in good repair, i.e., no bearing or axle slop?
7. Last but not least, have you contacted Co-Motion and asked them for any guidance?


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As a matter of fact, there is generally accepted evidence that your discs + QRs are causing this. If you'd like to learn more, search "James Annan disc ejection" and you'll come up with some relevant discussions.
Hmmmm. James definitely identified some potential issues, but the smoking gun in the initial incident that happened way back in Dec. 2001 was a really awful fork modification. His subsequent research, analysis and efforts did yield some good information that has caused fork designers to look more closely at drop-out orientation, etc., but their poorly designed and executed fork was an accident waiting to happen. This is a link to a web page that James created shortly after the initial incident that has some photos of the the fork after the fact: http://www.ne.jp/asahi/julesandjames/home/fork/

Note the orientation of the drop-outs relative to the caliper (#1 problem) as well as the other issues that James points out in his narrative.

Again, the answers to my questions should go a long way towards narrowing down the cause / effect or, in the case of #7, bring to bear the insights of the guys who actually designed and support the Co-Motion disc compatible forks.
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Old 01-05-10, 05:02 PM   #9
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As Nate notes, there are good skewers for disc-equipped bikes, and not-so-good skewers for disc-equipped bikes. So, here are some question:

1. Are you using the original skewer that came on the tandem and/or what is the QR's brand / model / material? How many years / miles have you logged on those skewers? Original Salsa skewers, 22k miles
2. How many years / miles have you logged on the tandem with the front disc and when did you first notice your front wheel was shifting in the drop-outs?Recently
3. Have you inspected the drop-outs to make sure they have not been damaged or deformed? Just had bike painted, epoxy basecoat under polyurethane. Very glossy & tough: just noticed that "serations" in dropouts have been obliterated - now is smooth. Ohhh....
4. Does the fork still have it's original 'lawyer lips' or recessed skewer cups?Yes
5. Do you always make sure that your skewers are closed with enough force to leave an imprint in your hand? Yes
6. Have you serviced your front hub and/or made sure that it's in good repair, i.e., no bearing or axle slop?No issue there.
7. Last but not least, have you contacted Co-Motion and asked them for any guidance? No. It hadn't occurred to me. I get more candid answers from those who aren't concerned about product liability litigation
TandemGeek, thank you for the checklist. With the new paint job, I think I might have an explanation.
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Old 01-05-10, 05:55 PM   #10
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Just had bike painted, epoxy basecoat under polyurethane. Very glossy & tough: just noticed that "serations" in dropouts have been obliterated - now is smooth. Ohhh....
Well, fingers crossed, perhaps a bit of scuffing on the drop-outs and/or a new front skewer will give you a more secure wheel? In regard to the skewers, with 22k miles I'd also give those a close look to make sure those end nuts as well as the knurling on the hub's axle end caps are in good shape and cleaned of any embedded crud. Given how often we remove and reinstall from wheels, a front wheel skewer can definitely lose it's bite.

It's amazing how important it is for the hub axle ends & skewer's knurled end-nuts to bite into the drop-outs. Just to underscore how important they can be, here's a little anecdote taken from Update #9 of our Calfee Journal regarding some front wheel hub slippage that we experienced on simply out-of-the-saddle climbing because of a poor interface between a hub axle and fork drop-outs:

Now, I must note we did have one small issue with the Topolinos that initially appeared to be serious but was, in fact, just a minor design issue. More specifically, during hard out of the saddle steep and aggressive climbs we experienced an alarming creaking sound coming from the front of the tandem. Initially it sounded similar to a bearing creak but following the ride everything appeared to be nice and tidy at the hub. On a hunch, I wondered if the titanium skewers that came with the Topolino's might be flexing so I installed a spare set of Salsa skewers with steel axles for our next ride. The creaking was not as pronounced, but was still clearly in evidence and somewhat disconcerting. Once back at home I inspected the hubs more closely and noticed the axle end caps did not have any knurling and were, instead, absolutely smooth. I reinstalled the front wheel in the fork and applied some side loading with my hands and low-and-behold there was the sound: it was a bad case of grip-slip between the front axle and the fork dropouts. Before the next ride I scuffed up the axle end and the fork dropouts and when coupled with the use of the steel skewer the noise was gone. We've since passed along this discovery to the folks at Topolino via our dealer with a recommendation to incorporate knurling for their tandem wheel axle ends as others will no doubt experience this under similar conditions as we're certainly not the strongest, hardest climbing team that will use these wheels.

Update: The response from Topolino regarding the non-knurled end caps has been exceptional. Two new axle sets and very detailed instructions for removal and re-installation arrived just days after a short telephone call to discuss the problem. The next end caps were installed which solved the creaking noise at the fork drop-out.



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Old 01-05-10, 06:31 PM   #11
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James definitely identified some potential issues, but the smoking gun in the initial incident that happened way back in Dec. 2001 was a really awful fork modification.
The orientation of the dropouts is Chapter 1 of James' theory, but his Chapter 2 attempts to explain the loosening of the QR:

http://www.ne.jp/asahi/julesandjames...tml#unscrewing

Cyclesafe, I don't have any personal experience with Salsa QRs, but others in some threads related to this subject point fingers at Salsa QRs specifically, and their external cam in general, as inadequate QRs.
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Old 01-05-10, 07:05 PM   #12
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I don't have any personal experience with Salsa QRs, but others in some threads related to this subject point fingers at Salsa QRs specifically, and their external cam in general, as inadequate QRs.
And what threads might these be?
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Old 01-06-10, 05:51 AM   #13
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It is generally acknowledged that the average external cam QR is not as strong as the average internal cam QR. External cams are now more fashionable for some reason, but Shimano still uses the internal cam, even on their fanciest wheelsets and hubs. I therefore only use internal cam QRs on all our bikes. With a front disc, I think it would be very unwise to use an external cam QR - so go get yourself a good quality Shimano QR!
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Old 01-06-10, 10:24 AM   #14
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It is generally acknowledged that the average external cam QR is not as strong as the average internal cam QR.
This is a better characterization than "inadequate". The external cams work, but they demand more attention to make sure they are clamped down tightly, take more effort to get the same level of 'bite' that an internal cam model does, and typically use softer materials for the end nuts and axles which, while it reduces their weight, reduces their clamping force and 'bite' on the drop-outs.

In general, an internal cam would be a better choice for a disc. But, that said, if the fork drop-out has lawyer lips or some other type of axle retention system and the person using the QR follows proper practices for installing them there shouldn't be any problem that could result in a catastrophic failure or wheel ejection. Again, going back to the original event with James & Jules, they had the worst of all things: external cam QR, no lawyer lips, bad design, etc...

So, yes... If it was me I'd be using a Shimano or Campy skewer with an internal cam on any tandem with front disc. But, as already noted by our friend, he's had 25k miles of problem-free performance out of his external cam, Salsa skewers. I also use Salsa skewers and they are clearly good skewers, as far as external cam models go.... but then again, I don't have any road tandems with front discs.
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Old 01-06-10, 01:57 PM   #15
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I'm not sure how you get to a factor of 3x (I'd think it's somewhere between 1.5x - 2x), but it's a moot point if the fork is robust enough for long-term use on a tandem and the disc's use doesn't create any brake torque steer effect.
Factor 3?, I have thougt about it again and found out that you are completely right: The factor is between 1.5 and 2. My fault and recognicing this it makes me feel much better!

Second: I have never noticed any brake torque steering issues. The asymmetric forces are kept within the fork-axis-system and the only effect by these forces I noticed has been the discs moving out of the caliper center. This effect we already discussed at the German tandem forum (tandem-fahren.de) and we found out, that I have to use a stong quick release skewer with high tension. And it works.
The main reason to put my question to you was my error about this factor and the fact, that there are many more tandems with disc brakes within the U. S. than in Germany and that you should know about it, if there are special problems with this. So thank you very much for your patience and your elaborated answers!!!
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Old 01-07-10, 10:12 AM   #16
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Second: I have never noticed any brake torque steering issues. The asymmetric forces are kept within the fork-axis-system and the only effect by these forces I noticed has been the discs moving out of the caliper center. This effect we already discussed at the German tandem forum (tandem-fahren.de) and we found out, that I have to use a stong quick release skewer with high tension. And it works.
My apologies for focusing on the stability / brake torque steering.... that's what our friend MaxCady mentioned in his original post that created this thread and where my head kind of got stuck.

That said, I think the wheel ejection risk associated with a front disc has been covered elsewhere in the thread and, as you also discovered in your tandem-fahren.de discussions, a stout fork designed for discs + a robust skewer installed and used correctly are critical to mitigating the potential risk of a disc brake being pulled askew or out of the drop-outs.

Happy trails
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