22 February, 2010

Saving frames with 73mm BBs


We get lots of e-mails thanking us for importing the VO threadless BB. It's saved many a bike with stripped BB threads from the scrapheap. This BB is made for standard 68mm wide BB shells, which is what 95% of bikes have. But what if you have an old mountain bike frame, or one of those odd Raleighs with a wider shell.  John-Paul has found a solution which he describes below. And he's posted a photo tutorial. Thanks John-Paul!
I thought I should send in a report about using the Grand Cru threadless bottom bracket. I own a commuter that is built up around an early-1990s Specialized Rockhopper frame, one that has some damaged threads in its bottom-bracket shell. It had become impossible to turn the pedals with any force without producing a grating creak, no matter how much grease or teflon tape I used when screwing in the bottom bracket. I therefore ordered one of the threadless BBs and tried to install it earlier this week.

The first night, I despaired. No matter what I did, I couldn't get the non-drive-side cup to turn onto the cartridge's threads. Eventually I realized that the problem was that the Grand Cru is a 68mm bottom bracket but my model of Rockhopper has an over-sized, 73mm shell. I had decided that that was the end of the road and started pricing new frames.

The next day, though, while I was talking to another volunteer mechanic about my problem, he asked me why I didn't just shave a few millimeters off of the bottom-bracket shell. My automatic response was, "Because that would trash the threa...oh."

On Saturday therefore I did just that: scribed and ground off 2mm from each side of the BB shell, re-tapped and -faced the shell, and then slipped in the threadless BB. Success! A bike that I was almost certain was a lost cause is now riding smoothly and quietly for the first time in about six months. And, if someone has the appropriate tools, they can add those with over-sized BB shells to the list of bikes that they can rescue with this type of bottom bracket. Truly, a great design.

I put together some photos of my process, more because I wanted to show other amateurs how I got it to work:

http://www.flickr.com/photos/the-macnab/sets/72157623481976138/

17 comments:

Gunnar Berg said...

This is valuable world stuff, and nicely, clearly illustrated.

Steve said...

Brilliant!

philcycles said...

A couple of comments. My bona fides are that I'm a frame builder.
The scribe setup is nice but it is simpler to use a small carpenter's adjustable square to scribe the line. Set the rule in the square to the depth you want and scribe away. It's what I use to cut, for instance, fork blades to length.
Also, please use safety glasses when using a grinder that way. the discs have an alarming tendency to fly apart.
While I haven't used the VO bracket it seems that it doesn't require the threads be chased. I would simply put a light chamfer on the shell. Also it doesn't seem to require perfect squareness or flatness on the faces. Again, two squares can be used to check squareness and parallelism.
Those of you who have actually used one of the brackets are free to call BS on me.
Phil Brown

philcycles said...

Also, while it isn't spelled out this operation should be carried out on the non drive side for obvious reasons.
Phil Brown

Rex said...

This is such a cool product. I don't currently need one but I'm thinking about all the great old bikes that could be rescued with this.

Chris, if there is ever a risk that this product might go away, would you give a nice long warning period? I would try to find the coin for one or two just to have on hand.

Preston said...

Wow! A wonderful solution, but
I'm sure I would just have succumbed
to temptation and purchased a new
VO Polyvalent for commuting.
Preston

Casey said...

That scribe tool is golden. I agree that it's overcomplicated, but my inner tool-nerd and my DIY complex just high-fived anyway.

John-Paul said...

Phil: No BS, don't worry. I agree that a carpenter's adjustable scribe would have been simpler (If you check the other photosets on Flickr, you'll see me building furniture) but we didn't have one in the shop. I am completely with you on the face-mask point, which is why everyone who held that grinder had a face-mask on and why, since we only had one, I wasn't eager to let someone else brace the bike stand.

From what I've read, it isn't strictly necessary to have perfect squareness or flatness on the faces, but you can understand the impulse to make things true.

philcycles said...

I understand the impulse to have everything square and true. I'm a carpenter as well and I am frequently guilty-like today-of applying metal working standards to wood. Tough to get oak true to 5 thou.
Phil Brown

Anonymous said...

I still don't understand why you would use a last chance frame saving device like this if you were able to properly clean the threads up?

Anonymous said...

I don't understand why you wouldn't just get a good new 73 mm BB (like an SKF unit, for example?) and fix or re-tap the shell to Italian?

Le Cagot said...

You can't just re-tap stripped threads. If they are stripped there is no material left to tap. This BB is probably just as good and just as secure as any threaded BB. Why spend more on a SKF? Why tap any BB shell now that this design is available?

Anonymous said...

Actually, you *can* re-tap stripped BSC or French/Swiss threads to Italian. Bringheli even makes an inexpensive brass sleeve that will reduce the now-larger threads back to 'normal.' Also, just how 'bad' were his threads to start with? You can also get cups with slightly-oversized threads (0.5 mm, typically) if the threads are simply well-worn. You can use thread-locking compound. There are just oodles of less-extreme solutions than hacking up the frame.

Le Cagot said...

Anon, Why would you re-tap a 68mm BB shell frame to Italian (which is 71mm)? A better solution is to simply slide in this BB tighten it and be done. No harm or modifications to the frame are required. Using Locktite or .5mm oversized cups or a shim is just delaying the next failure until you're 100km from home.

This post is about 73mm frames and, as it explains, those can't be re-tapped to a larger size.

redvic said...

If you went ahead and cut down the shell and tapped to accept a 68mm BB, couldn't you just have used a regular 68mm BB at that stage, since you have cut out the worst 2mm of threads?

Anonymous said...

Italian is 70 mm wide, not 71, but that doesn't matter in this case. Once re-tapped, a 68/73 cart. in Italian threads would have installed sans probleme.

Anonymous said...

...and there is an EDCO sealed-bearing BB that slips into an Italian shell, and locks in place via tightening the eccentric cups. It is available in (at least) a 116 and a 121 mm length (designed for Stronglight/TA doubles and triples, respectively). Those could also be used in other types of BB shells that are damaged or stripped.