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Can I improve BB7 rear disc braking on Co-Motion Speedster?

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Can I improve BB7 rear disc braking on Co-Motion Speedster?

Old 01-21-19, 06:48 AM
  #51  
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Originally Posted by Alcanbrad View Post

aluminum
Interesting, I have seen either all steel, all titanium, or 3 ti and 3 aluminum, but never 6 aluminum disc retaining bolts on the same disc. The bikes that were mixed bolts were all singles too.

Have you run them a lot miles.
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Old 01-21-19, 07:07 AM
  #52  
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Originally Posted by PMK View Post

Interesting, I have seen either all steel, all titanium, or 3 ti and 3 aluminum, but never 6 aluminum disc retaining bolts on the same disc. The bikes that were mixed bolts were all singles too.

Have you run them a lot miles.
Probably around 1500 miles or so. I have never read any guidance or warnings on Aluminum screws, but now that you mention it, aluminum is softer and has a lower shear force. I'll pull them out and inspect them.

I am an EE by training but I am currently employed as an ME (long story, don't ask, but I am having fun) and my limited knowledge of using aluminum screws is you have to be careful about striping the threads due to over torquing. Are there any guidelines or warnings regarding aluminum screws in disc rotor applications?.
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Old 01-21-19, 11:20 AM
  #53  
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Originally Posted by sapporoguy View Post
Rear only. Front is V-brake, although there are posts for rear V-brakes, too. Judging from your post, perhaps our rear disc is optimized and I'm expecting too much of it. For some reason, I though a disk should brake better than a V, but I've never had disc brakes before. I do use the front brake for the big share of braking and am happy with the V up there. Maybe I'll add V-brakes to the back and give the stoker the lever. That would make her happy when I'm getting too enthusiastic for her downhill!
​​​​​​​Wait a minute!? You also have rim brake attatchments on the frame at the rear?

Then what I see here is someone connected the "stopping" brake cable to the disc "drag" brake thinking full disk was the way to go.

Naw, if that's the case, it's no wonder you're here. Hook the rear brake back up, and get a thumb or bar-end shifter to operate the drag brake as a drag brake. Now you have a means of dissipating energy in the way tandems are supposed to.

Last edited by base2; 01-21-19 at 11:24 AM.
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Old 01-21-19, 01:25 PM
  #54  
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Wait, I thought one wasnt supposed to use a disc as a drag brake?
this bike was clearly made in the years when tandems had bosses for both rim and disc. Same with our 2000 Santana Sovereign onto which I added an Arai drag brake but has attachments for disc
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Old 01-21-19, 01:28 PM
  #55  
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Originally Posted by base2 View Post
​​​​​​​Wait a minute!? You also have rim brake attatchments on the frame at the rear?

Then what I see here is someone connected the "stopping" brake cable to the disc "drag" brake thinking full disk was the way to go.

Naw, if that's the case, it's no wonder you're here. Hook the rear brake back up, and get a thumb or bar-end shifter to operate the drag brake as a drag brake. Now you have a means of dissipating energy in the way tandems are supposed to.
I doubt that is the case. The picture below is from the Co-Mo 2005 catalog (which you can find this here: https://www.precisiontandems.com/cat...atalog2005.pdf ) Co-Mo states that if you order the disc option, they include the stand-offs.


Cable actuated disc calipers are not as efficient as hydraulic and even more-so for the rear brake due to cable slack weight. I found new cable housings, pads, and augmenting with a boost return spring, as discussed above, yielded more than satisfactory performance.



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Old 01-22-19, 07:08 AM
  #56  
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Quote:
Originally Posted by PMK

Interesting, I have seen either all steel, all titanium, or 3 ti and 3 aluminum, but never 6 aluminum disc retaining bolts on the same disc. The bikes that were mixed bolts were all singles too.

Have you run them a lot miles.

Originally Posted by Alcanbrad View Post
Probably around 1500 miles or so. I have never read any guidance or warnings on Aluminum screws, but now that you mention it, aluminum is softer and has a lower shear force. I'll pull them out and inspect them.

I ... Are there any guidelines or warnings regarding aluminum screws in disc rotor applications?.
I went a googling and found this warning on a online vendor hawking aluminum disc bolts:

important note: because of the lower fatigue of aluminium compared with steel or titanium please consider the following weight limits (rider & bike) if you will use the bolts for disc brake fastening:
  • 80kg with a 203mm rotor on your front wheel
  • 90kg with a 180mm rotor on your front wheel
  • 100kg with a 160mm rotor on your front wheel
  • on your rear wheel, there is no limit of the disc rotor size but on the total weight of rider & bike. It's 110kg.
We are "a bit north" of the weight guidance's so I looked up my original order for the red bolts and it turns out that they are steel, so all is good. I had thought they were aluminum, it's been a while since I purchased them.

I would think that with the guidelines above that a vendor, even with the warnings, would steer clear of aluminum bolts due to the liability.

Thanks for raising the awareness.

Last edited by Alcanbrad; 01-22-19 at 09:12 AM.
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Old 01-22-19, 01:59 PM
  #57  
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Aluminum brake bolts = rubber crutch. (as in, "Funny as a ...")
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Old 01-22-19, 02:06 PM
  #58  
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Originally Posted by sapporoguy View Post
Rear only. Front is V-brake, although there are posts for rear V-brakes, too.
Judging from your post, perhaps our rear disc is optimized and I'm expecting too much of it. For some reason, I though a disk should brake better than a V, but I've never had disc brakes before.
I do use the front brake for the big share of braking and am happy with the V up there.
Maybe I'll add V-brakes to the back and give the stoker the lever. That would make her happy when I'm getting too enthusiastic for her downhill!
The other recent poster is correct. A rear brake is never as important as a front brake. Not to be obvious about it but mechanical disc brakes these days are often cheaper than really good v-brakes. That would not inform an opinion in me anyway that mechanical discs are better. They do have advantages however.
They do not heat innertubes. They are impervious to rim warping. They are more consistent in changing weather and mud, snow, etc. A front brake is often the only brake you NEED. I have entrusted both our lives and $150.00 worth of Thanksgiving Eve shopping to just one off-brand v-brake on a downhill bomb to a stale green light (that we didn't make!). Would I dare do that with only an off-brand v-brake at the rear??!!

A disc brake CAN work as a drag brake if it is big enough. That means 203mm. Some rear ends will not accomodate rotors that big. Our first disc brake equipped tandem had 160mm rotors f/r. Avid BB7. Stopped fine. Even so we upgraded to 180mm f/r. An earlier post asserting that all mechanical disc calipers are single action is incorrect. TRP Spyre/Spyke disc calipers are mechanical discs but also dual action. Both pucks are active. Obviously there are no 'rules' about any of this but there are 'conventions'. Conventional tandems have front and rear brakes of the same technology v-brake or disc controlled by the captain as per regular single bike practice. To this may be added a 'drag' brake of a different technology. Front and rear 203mm discs usually do not need a drag brake! In 10 years of riding in a large tandem club with plenty of well heeled (wealthy) tandem teams that could run any kind of braking setup wanted I haven't seen a team with 203mm f/r that used supplemental drag braking. Full disclosure: our routes do not take us through the Alps with any kind of regulairity. I don't know where or when the convention of giving the drag brake function over to the stoker originated but I personally don't care for it. On the tandem we had with a drag brake I didn't find it that hard to deal with the third lever since it is never actively used to slow the bike. It is only used to establish a steady state descent speed on a long downhill. A rim brake should not be used for that. The stoker should not IMO be given control of one of the two main brakes stopping the tandem. FWIW.
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Old 01-22-19, 02:15 PM
  #59  
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Originally Posted by Alcanbrad View Post
I doubt that is the case. The picture below is from the Co-Mo 2005 catalog (which you can find this here: https://www.precisiontandems.com/cat...atalog2005.pdf ) Co-Mo states that if you order the disc option, they include the stand-offs.


Cable actuated disc calipers are not as efficient as hydraulic and even more-so for the rear brake due to cable slack weight. I found new cable housings, pads, and augmenting with a boost return spring, as discussed above, yielded more than satisfactory performance.



Ok, no worries then. It just seemed odd to me, but appearantly not. I guess I learn something new every day.
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Old 01-22-19, 02:39 PM
  #60  
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Originally Posted by Leisesturm View Post
I don't know where or when the convention of giving the drag brake function over to the stoker originated but I personally don't care for it. On the tandem we had with a drag brake I didn't find it that hard to deal with the third lever.
I was really saying that tongue-in-cheek about giving power to the stoker. On our Santana, the captain has all three brakes. The Arai-drum lever is an old Diore friction thumbie that works great. I have it situated so I can pull on it without taking all fingers off the front or rear V brakes.
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