CaptainHaddock

This question is not quite a frame building question, and yet, entirely about the why's and wherefores of caliper location on modern frames, so if need be, please delete or move the thread!

So, my wife and I have only recently made the transition onto frames with disk brakes (her commuter, and our/my tandem). On both frames, the caliper location is within the rear-triangle (on the chain stay). However I see "older" bike frames where the caliper location was outside the triangle on the seat stay. So why has this changed? I mean, am I asking a religion question, or is there a structural / mechanical reason to have moved the mounting point?

thanks for all you're combined wisdom!

Andrew R Stewart 

The two reasons I know of are that the chain stay is beefier then ther seat stay and the caliper is not interfeering with rack or fender struts. Andy.

ftwelder

That is an interesting question. Last year people found that mounting the caliper to the seat stay will cause the seat stay to fail. This year they will find mounting the caliper to the chain stay will cause the chain stay to fail also. I have been mounting them like this, with a support tube for about six years now and have had no failures. You need a triangle to prevent the whole mess from twisting in the frame.

Does your heel contact the caliper on your bike?



27 270 by frankthewelder, on Flickr


There have been lot's of conversations about mounting calipers on road forks. You are going to see a lot of broken forks while that is getting sorted out.

unterhausen

the road forks I've seen with disk tabs have made me wish I hadn't seen them. I understand the impulse, but experimenting with fork blades just doesn't seem like a good idea.

I think my first mtb frame is going to have post mount on a slider, but I haven't ordered parts yet

repechage

I have seen a few production frames with the same basic triangle solution.

I have been thinking about how a front fork would be best created for a disc brake. I am leaning toward constant diameter fork legs and trying to get the disc mount to engage the tube adjacent to the inside tangent.
I have ridden a disc brake road bike with mechanical calipers. I must say, modulation is superior.
Makes me want to create one with wood rims and disc brakes.

Shimagnolo

The piece of the frame between the chainstays and the bottom bracket;
Is that a solid piece of machined aluminum?

calstar 

Would you please explain your "post mount on a slider" concept? I can't visualize what you mean.

thanks,

Brian

ftwelder

Yes. The frame weighs 5.5 lbs.

veryredbike

As a bike mechanic, the chain stay mounted designs kind of get my goat.

While you CAN reach what you need to, it's unnecessarily cramped and awkward in a lot of cases. I like the idea of just reinforcing the junction, as you've done, a lot better.

unterhausen

Sorry, I didn't see this before. Post mount on a slider probably doesn't make much sense unless you've seen them. I am thinking of using these dropouts from Paragon

squirtdad

This might give people some ideas.... from Dave Kirk's blog..


https://www.kirkframeworks.com/blog/2...have-a-winner/

unterhausen

I have to admit that is one of the brake tabs that didn't exactly fill me with confidence. People have said that the "Willits style" brake tab is sufficient to keep a road fork from breaking. Paragon sells one, --link-- but I'm not convinced. I have heard of too many road forks breaking to be willing to put a disk brake on one without fatigue testing. Since I'm not going to do fatigue testing, I probably will never put disk tabs on a road fork. I think the fork Alex Wetmore made is likely to hold up, but I don't see how you could be sure of that

ftwelder

This is way wrong. You can't put gussets on the center-line of a tube like that and you can't end it a braze-on.

The caliper needs to me mounted on the other side of the fork, placed in front of the leg and a torque rod attached to the crown or near by or you need a really large diameter very thick fork leg.

PaPa

I disagree. With the caliper mounted on the backside of the blade, the joints are loaded in compression. A structurally questionable fillet won't be an issue.

When mounted on the front side of the blade, the joints would always be loaded in tension - if a crack should develop in either the joint or the HAZ, the mount will eventually separate from the blade because it's always trying to pull itself apart.

fietsbob

Bike Friday puts the disc mount on a U bent .5" tube at the rear,
Fillet-braze/tig weld, [might be either under the paint] is an inch long..
ISO plate for adapter mount, is .125" thick

Of course FTW is building a go out and hammer it
against the rocks and trees MTB of Aluminum ,

these are 20" wheel travel bikes.. I'll check back in a year or so
and report it's continued durability..

7005, My Koga Miyata WTR uses a Sliding Plate Dropout,
It carries the disc ISO mount with the wheel..


I recall Surly's Troll puts the caliper mount and the axle
into slotted adjustments to do similar , IGH chaintension.
But the caliper location on the disc, and the axle
are separate re adjustments, as you pull the axle back
the caliper has to chase it there, with a bit more tweaking.



I do wonder why Calipers cannot go on the front of the right fork-blade.
seen pix of some custom tandems doing that..

It would resolve the axle walking out of the dropout situation.
of rear of left side mounts, but most forks are left mounted discs.

unterhausen

the failures I have seen are a tension failure that starts on the backside of the fork at the stress riser caused by the disk tab. If you put the caliper on the front as Frank proposes, then the brazing and gusseting is on the compression side. The other problem that people have is that the fork gets un-raked.

ftwelder

If you were to weld a caliper bracket to the front of the leg this would be true. The method I suggest has the brake force directed to the crown, not to the leg. This method would resemble the "floating brake" found on the rear of some older motorcycles. The caliper attaches to a link that can rotate around the axle (or to some degree) and to that, the stay rod is attached. I hope I am explaining this clearly.

calstar 

When you guys are talking about structural failures, what % of them are on production line bikes(typical bike store brands) and what % are custom builds? Also, is a uni-crown with fat blades better to handle the stress than a standard crown with beefy blades? How about using tandem specific fork crown and blades on a single, less likely to fail? I have a custom hardtail (Curtlo) that I am going to change to a non-suspension fork and keep using discs, nice to have this collective opinion thread going at this time to better help me choose a fork design.

thanks,

Brian

PaPa

I'm not seeing how, exactly, you can avoid loading the fork blade when the caliper is mounted only 2-4 from the axle.

If reducing the bending loads on a single fork blade is your goal, then halve the bending loads and install two calipers - one on each blade. Problem then is the unnecessary added weight, and you'd need to reduce the diameter of the rotors.

fietsbob

More gussets ! the tube across the triangle is one type of gusset.

Werkin

I'll add it offers a little bit of protection for the caliper arm.


It's difficult to access at least one of the CPS bolts (Avid) using a torque wrench with the caliper mounted in the inner stay triangle.


If the IS mount bracket is stiff enough with a tie-in to seat and chain stays, there should be no potential harm mounting the caliper on the back of the seat stay. My integrated IS mount & drop-out is extremely stiff. I could not bend it at the axle notch, and a tech with greater mass than me could only get about .5mm movement. That's just on the horizontal plane (bending), vertically it is many times stiffer. A caliper mount like this will distribute the load between both stays.

I agree with Dave Kirk's assessment a straight fork is down on comfort. Mine is particularly abusive with skinny high pressure tires, not so bad with wide low pressure ones. What I get instead of compliance, at least with the fork on my bike, is good responsiveness and no flop. Note the tubes on the black fork are not oversize and maintain some taper. I apply great braking forces against this fork on steep descents and do not feel twist or odd behavior. Also note the dent in the lower leg. I know it's not there for rotor clearance, which causes me to speculate it may serve a stiffening function. I would like to know if applying a dimple would increase resistance to tube bend in one direction?

ftwelder

I will produce a drawing or sample so we can discuss it later..

fietsbob

German Tout Terrain's use of a bigger diameter fork blade
on the side where the brake is .. seems well done..
they fit the Dropout, made to open forward , rather than down.

Werkin

Keep in mind adding a second rotor will reduce spoke bracing angle significantly on a 100mm axle. BTW, I can see where a composite wheel would have advantages in an application like this. Also, side specific calipers are not in large scale production (dual disc hubs either), so the calipers would need to be mounted on different sides of the fork in most cases. Plus, there is the issue of synchronizing the two calipers; differences between each will be magnified by relatively low bicycle/rider mass & velocity. I know the dual rotor approach is an attempt to divide the load with the goal of maintaining a safe margin of fork elasticity, but I think it is less of an issue than speculation would make it seem; that's why I showed the photo of my fork. I also know it's only a matter of time before someone posts the photo of the bent fork designed for a rim brake, where IS caliper mounting tabs were welded to it and it failed in use; it's easy to see what's wrong with that picture.

bellweatherman

Mounting a disc brake to the front side of the fork leg is a bad idea. Under constant force, if a failure occurs then the brake is ripped off the front of the fork. Think about it. Would you mount a disc brake on the bottom-side of the chainstay? That would be the equivalent of mounting the disc brake caliper on the front of the fork leg.