Frame Disc Mount Design

[Zordon]

In future I'm going to buy custom frame with disc mount. Mod frame. I want to put avid BB7 203mm on the rear. But I heard a lot of bad situations when disc mount/dropout/whole chainstays were torn away. My intention is to make a possibly strongest design capable to survive with rear disc brake at least one year of riding. I've already got few ideas and want to know your opinion.

1. As I wrote above I'm planning to use 203mm disc. Standard frame mounts are for 160mm disc. Putting 203mm with +40mm adaptor would create much more leverage to the disc mount and cause lot of stress there (I think). So my idea is to buy a 160mm FRONT caliper (which on rear is like 140mm, am I right?) and to order +60mm disc mount. This IMO would create much less leverage and stress to the mount. What do you think?

2. As a mod frame, it will have horizontal dropouts. It means the chainstays won't have fixed length all the time because chain during usage streches and to have it stiff enough I will have to put the wheel constantly further from bb. This will change position of caliper to disc and problem may occur (pads will not fully in contact with disc or disc may even rub caliper). My aim there is to make disc mount so the caliper will be perfectly parallel to horizontal dropouts. This IMO should minimize effects of chain streching and therefore changing position of caliper to disc.

I saw the modern disc frames like Monty or Vinco mods have similar solution but I don't know whether they have it because of chain streching or something other.

3. Would it be choosing one piece BB + CNC much better than standard CNC yoke welded to BB shell in terms of using rear disc brake? Did many frames crack there?

If you have any other tips I would appreciate it very much :)

Thanks, Zordon

PS.: Oh, I forgot that I'm gonna have one piece dropout+mount= integrated disc mount, of course.

Edited November 29, 2005 by zordon

[Bigman]

If you plan on using a rear avid, specify the frame with a post mount design, like Manitou use, this will remove the posssiblility of snapping the black adapters that are used on the avid, it will also create a much stiffer mount for the brake.

Adam

[Zordon]

Post mount would be good but I know the factory uses quite soft material not too suitable for threads the post mount require to be made in frame.

[Bigman]

Post mount would be good but I know the factory uses quite soft material not too suitable for threads the post mount require to be made in frame.

If they use a soft material, surely it may not be suitable for a frame?????? Also, you could heli coil them, heli coils are ALOT stronger than a standard thread cut into aluminium.

Adam

[guineasmithpig]

plus, surely using a longer adaptor induces more leverage? as the bigman said, get post pount fittings, and helicoils for the threads if ur worried about it tearing them out. would like to see the positioning of the caliper mount, as i'm strugglin to think o somewhere that would allow use of a disc without damaging to frame, apart from one idea, which would be pretty burely, but quite expensive. but keepin that to myself for now :)

smithy

[Will Arnold]

If they use a soft material, surely it may not be suitable for a frame?????? Also, you could heli coil them, heli coils are ALOT stronger than a standard thread cut into aluminium.

Adam

must say that a thread has to be cut into the aluminium to put the helicoils in, so it isnt really that much stronger.

i would go with a steel frame and post mounts myself :)

[Davetrials]

megamo.

[guineasmithpig]

must say that a thread has to be cut into the aluminium to put the helicoils in, so it isnt really that much stronger.

but the thread on the outside of the helicoil has a lot deeper pitch than that of the metric thread sat inside it. hence why they are used in soft materials to anchor bolts properly

[Will Arnold]

but the thread on the outside of the helicoil has a lot deeper pitch than that of the metric thread sat inside it. hence why they are used in soft materials to anchor bolts properly

are you sure its a different pitch? the helicoil is effectively a coil or wire, i dont see how the pitch can change between the outside and inside of it?

ala : http://www.boellhoff.cz/images/helicoil_vrezu.jpg

but yer, i can see it being slightly stronger due to the diameter of hole it threads into..... :)

Edited November 29, 2005 by WILL ARNOLD

[guineasmithpig]

oo, sorry, i was thinkin o just ones like thesefree running helicoil

yeah, the round ones don't present anythin other than the same thread on the outside, just wider. but diamond sections are availble with much taller outside thread, making them awesomely strong anchor points. best ones for aluminium and such, if u want a stronger link than the standard bolt allows

smithy

[Zordon]

Soft material I mean is weak for threads but generally their frames are strong. Maybe because they just give more material to compensate material quality. Generally I'm against post mount with helicoil 'cause there is still possible to damage the frame. It's easier and cheaper to replace broken adaptor than to replace whole frame or dropout only.

Davetrials: Yeah Megamo Equip Pro was always my dream. I'm gonna steal from its design the additional tubing :]

[Davetrials]

the mount its self is awsome, no snappy happy tabs.