Bicycle Mechanics - Drilling out rear brake boss for countersunk calipers

Is this nuts?

I know it's covered on Sheldon Brown's site. The problem is you can't get a drill between the seat post and the underside of the brake boss.

I do have a hand boring tool. I beleive I can get it in there and might be able to hack away enough steel to stuff a new DP caliper in ( I currentky have cheap single pivot sidepulls; after switching the front to a DP I am really hating the lousy non-centering rear sp).

fwiw it is an 86 Trek road bike, Tange straight gauge cro mo seat stays.


My main concerns are will this weaken the stability and stiffness of the rear brake and will it make a difference if the hole I bore out is a bit out of true round?

Some folks advocate using the front brake on the rear and using the rear in front by simply sticking the recessed nut inside the bottom fork opening (tighten it inside the steerer tube by putting the allen wrench through the back of the fork). Is this a safer bet?

If these are both ill-advised can someone PLEASE help me center this %#*%^() SP brake?????

Merci

Do you already have the new recessed mount rear? If not, just buy another front and put a nut on it. If you do, see if you can swap it somehow, or sell your rear on eBay and buy a front.

It won't weaken the bridge too much, so that shouldn't be a worry if you do decide to drill. Keep in mind the rear doesn't do much braking, anyhow

I do have a rear already.

The trouble is it takes long reach calipers and I don't want to shell out any more big Bucks on this bike to buy a new front long reach, especially for the rear, since as you say the front matters most. I just hate the centering/rubbing issues with the SPs. I've only owned one set of sps (Modolo speedy) that worked well.

If I sold my old set of SPs and the new DP rear I might get a little bread to cover it..... but they are not big $ on ebay alas.....Sheldon Brown has nice Ultegras in long reach for $90 and Tiagra for $40. Any thoughts on the quality difference?

http://sheldonbrown.com/harris/brake-calipers.html#47-57

My big question is - doesn't anyone (Black and decker? Makita? DeWalt?) make a small drill with a right angle head for tight spots like this?

I do have a rear already.

The trouble is it takes long reach calipers and I don't want to shell out any more big Bucks on this bike. Sheldon has nice Ultegras in long reach.

My big question is - doesn't anyone (Black and decker? Makita? DeWalt?) make a small drill with a right angle head for tight spots like this?

Drill from the backside (not the seat tube side), enlarge both holes in the seatstay bridge.

right-angle drill-bit attachment

Drill from the backside (not the seat tube side), enlarge both holes in the seatstay bridge.


I was thinking that too......

assuming I only need a drill bit just large enough to allow the nut to get in from the back, and that i may need a washer in front (between the caliper and the boss) if the hole i dril is too big.

Have u actually done this?

right-angle drill-bit attachment

I apologize I am a non-handyman:o . Can u pls post a link or pic showing exactly what you mean? Thanks!

I drilled mine from the backside as mentioned above. I had a washer laying around from another bike that was cut to fit the curved brake bridge. This provides plenty of support for the brake against the brake bridge.

I think I would just drill thru from the outside. That little extra hole diameter's not going to hurt on the rear, especially if you have a radius washer against the bridge.

have u actually done this?

Yes.

Thanks folks

Unless I can find this right angle drill attachment I will probably go with the drill from the outside (rear of bike).

The part of the boss to which the caliper actually rests on is actually squared off - like a little steel box in the center of the round crossbar connecting the seat stays. So even a regular flat washer should work. I am pretty sure I have some of those concave washers laying around just in case (I imagine I am like most on this board who keep all these little parts from many many old bikes ago - never know when they will save the day!;) )

Thanks folks

Unless I can find this right angle drill attachment I will probably go with the drill from the outside (rear of bike).

The part of the boss to which the caliper actually rests on is actually squared off - like a little steel box in the center of the round crossbar connecting the seat stays. So even a regular flat washer should work. I am pretty sure I have some of those concave washers laying around just in case (I imagine I am like most on this board who keep all these little parts from many many old bikes ago - never know when they will save the day!;) )
Look for one of your old knurled or serrated washers to place next to the frame

I will - thanks!

Here's what I'm talking about, a right-angle drill attachment (http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92188). Might need to chop down a drill-bit as well.

http://www.harborfreight.com/cpi/photos/92100-92199/92188.gif (http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92188)

Although if you have a flat surface where the brake-bolt goes, then you shouldn't have any problems with drilling it straight through from the outside and enlarging the outer hole as well. Just make sure it's the exact diameter as the shank of the allen-nut for the brake-bolt. The tight fit with the nut will provide the centering function.

Thanks that sounds good.

I also found this http://www.makita.com/menu.php?pg=product_det&tag=DA312DWD but not sure of cost - it'd prolly be cheaper to buy an ultegra front from Harris!!

You need to use a 5/16" or 8 mm drill bit. Do NOT drill completely through the brake bridge.

I did the same job by taking a short, sharp 5/16" drill bit and clamping it at 90° into a pair of Vise-Grips. I was able to turn it about 1/4 turn at a time using the Vise-Grips as a handle and kept at it until I opened the front side of the brake bridge hole to the drill bit's diameter. It was a little slow but not too bad and it worked well.

There was no way to get a drill motor, even with a 90° attachment, between the brake bridge and the seat tube so my manual approach was the only alternative.

oy vey...........

First I only had a drill bit one size too small.

then I found an extra box of bits in the bottom of my old tool box. One was the right size but turned out to be a wood bit.

The next morning I grumpily reattached my old sp sidepull to go get breakfast, and - in a moment of mad desperation, after reading every repair manual I had one more time, I actually centered the old brake!!

Now why couldn't I have just figured it out before I drilled? :rolleyes:

I guess I was motivated by hunger.....or maybe my mech skills improve when I'm not awake yet.

The brake seems a bit more flexy than before but still stops fine. I will eventually pick up an 8mm steel bit and slap on the DP caliper. But nice to know my brake is finally centered....

Or you can just install the brake reversed.

U mean on the underside of the seatstays? (between the seat tube and seat stays?)

I still need to widen the hols from either side to get the allen nut in there - it's long enough where it would need to clear the front (outer) side of the brake boss too.

U mean on the underside of the seatstays? (between the seat tube and seat stays?)

I still need to widen the hols from either side to get the allen nut in there - it's long enough where it would need to clear the front (outer) side of the brake boss too.
Then you need a shorter recessed nut in any case

Then you need a shorter recessed nut in any case


I was thinking that too. Third hand/loose spokes carry those??? Or is it a LBS thing?

I was thinking that too. Third hand/loose spokes carry those??? Or is it a LBS thing?
Either should have it.

Thanks!!