cuevélo

Hi,

I'm unable to find a commercially available 54mm reamer. Do people use 2.125" reamers? A boring head? Something else?

I had a titanium frame made with a customized bottom bracket similar to one I have seen in the past, and it's just a bit too tight in the solid section for the eccentric to pass through.

Andrew R Stewart 

And it seems that the builder is of no help? Didn't test fit parts before the delivery? Andy

cuevélo

The builder is Waltly, and the custom design was my choice. I still need to talk to them, but I'd like to start thinking about solutions that don't require shipping back and forth to China.

duanedr 

If a shop/builder has the right reamers, they likely won't want to mix their steel reamers with Ti reamers (or vice versa) . You mention 'the solid section' is that in the center? Pictures would help.

I can imagine a few scenarios that match your brief description and understanding minimum BB shell thicknesses would be critical before making any changes.

In any case, I think your best bet will be a local machine shop who can bore it to the right diameter or modify the eccentric to work.

Now, I'm going to go on a bit of a rant. This is an open forum, you're welcome to ask these questions here. However, if you consult a local shop or builder for this information, realize they are effectively working in the Waltly warrantee support department (for free) to support your savings. Don't expect them to be enthusiastic about the conversation. The money you saved is now being burned up with hassle and a non-standard BB configuration that you will deal with for the rest of the bike's life. It's easy to get seduced by the low price but the outcome is rarely as rosy.

Please post up pictures and we can provide more info/guidance.

dsaul

I would need to see pics of the parts involved, but I would probably opt to modify the eccentric, rather than the frame. Titanium is very hard on cutting tools and requires a lot of leverage to make a hand reamer work.

unterhausen

I was thinking the same, turn down the eccentric. I haven't heard great things about waltly recently. The other big builder whose name I can never remember is the one I would go to if I were getting a cheap ti bike from China.

There are bb's that are bolted in from either side, I think I would explore getting one of those to fit. Or see if something like the bushnell would fit. I'm surprised there are so many options since everyone hates eccentric bb's nowadays.

bulgie 

Ti Cycles mounts the frame in a Bridgeport and uses a boring head. Or did, back when I worked there ~25 years ago. Surprisingly easy and quick job, way easier than reaming.

Prior to working at Ti Cycles, I made a lot of tandems at Santana, R+E (Rodriguez) and Davidson. I "reamed" the shell with a wind-up, a shop-made steel arbor with a slit to hold one end of a 2" wide sanding belt, which is wrapped around until the right diameter is built up. If you make it the absolute biggest that will fit tightly in the shell, it's fairly decent at self-aligning and makes distorted (non-round) holes more round. You have to replace the belt frequently though, because it soon gets smaller in use. It loses the ability to make the hole rounder as soon as it gets even a little smaller. That's why sanding drums are not nearly as good for this, they don't have the ability to make the diameter as big as possible.

Make the arbor as large a diameter as possible, because otherwise the force on the tail of the sanding belt is too high and it will tear where it leaves the slit. Mine was about 2" diameter, with a 1/2" shank to fit in the drill chuck. Make the slit sort of tangential, not radial, so the slit doesn't go into the 1/2" shank, and the angle where the belt leaves the slit is obtuse, more gentle on the belt than a 90° angled slit would be. Make the slit (kerf) as thin as possible, so the arbor is still close to round when the slit closes up to tighten on the belt. I used a hacksaw, but a thin bandsaw would use less elbow grease. I'd show a pic if I had one, but the tool is not in my posession.

This is a tool you can make in under an hour and doesn't need a mill, but it's not nearly as excellent as the boring head method.

Mark B

Kontact 

Another vote for putting the eccentric on a lathe.

bulgie 

Not me, that'll make the eccentric a loose fit in the frame. As I understand it (still waiting on pictures), the problem is the middle of the frame only, and the eccentric currently fits well in the outer ends of the shell. Making the eccentric loose all the way through, to clear the center of the shell, would be a shame. That's asking for squirm and creaking while riding.

A lathe should have been used, but on the shell, to bore a relief in the center where we don't want the eccentric touching. I did that on many of the tandems I made at Davidson and Ti Cycles. Then any post-welding "re-rounding" you need to do, due to warping, is only needed on the outer edges. Also it makes the shell lighter. Partly kidding about the lightening, but I'll admit to doing things sometimes to make the part look lighter. Only someone who looks inside before the eccentric gets inserted will ever see it, but it'll make that person smile.

Unfortunately I don't have a big enough lathe to bore the shell after the frame is built. Imagine a lathe that could do that on a tandem — it would be awesome to see!

Mark B

Kontact 

I thought we were talking about a shell that is a solid O in the middle and split rings on both sides with pinch bolts. In that case, a minor reduction in the eccentric's diameter so it just fits the middle should fit the split sections just as well when tightened - they all started as the same metal tube with the same inside diameter.

Like this:

duanedr 

This is what I would expect a machine shop to do. I did this on a steel Paragon headtube before I got my 44mm reamers. It's a pain but took less than an hour on my little mill. On a full sized mill, it would be easier - especially if you had tools made up for it like I'm sure Ti Cycles did. The other challenge is machining Ti...ugh...

If the eccentric rings don't enter from both sides and have to slide all the way through, relieving the center section might be the only approach - aside from changing to an eccentric that is inserted from both sides and joined in the middle.

bulgie 

Sure, but the slit at the pinchbolts will narrow down, and the circle doesn't reduce evenly all around, so it's not a circle anymore. It tightens near the pinchbolts, but gaps open up elsewhere.
Likely not enough to cause a big problem, so I'll admit your method is probably good enough. I'm just a bit of a perfectionist. Parts that fit perfectly with no compromises make my heart soar like an eagle — but that's too expensive!

Especially with a no-slit type shell and a Bushnell eccentric, the insert needs to fit the shell pretty well. Boring it on the mill is the best way IMHO.

BTW have you thought through how you'd lathe-turn the eccentric? No turning between centers since there is no center, it's "air" there. You'll probably have to chuck each end to turn the other end, and lose concentricity when you flip the part. OK, I know, it's just a bicycle...

Kontact 

The reason the ends fit looser is that they are flared out at the cut, not because they are bigger than the center. Tighten the bolts and they become exactly the same diameter as the center, then get smaller as they are further tightened. The eccentric and the shell where always mismatched diameters.

A good machinist can flip the shaft without losing center, but they could also mount something in the BB hole and chuck that in off center to turn the eccentric body in one pass. Either way, we're talking about shaving a few thousandths off. It could probably be accomplished precisely with oxide paper in a V-block if you were careful.

cuevélo

Here is a picture of the shell. The goal is to have more frame strength (with the solid middle section) but better clamping (as a pinch-type supposedly has).




bulgie is right, the problem is in the middle section only, which seems to have an interference of around 0.1mm (maybe 0.2mm max) on the diameter. I think I could turn down the eccentric and make it work, but the fit would be better leaving the pinch areas where they are, as it is a tight slip now. The wall thickness is 3.0mm, so I'm not worried about removing the amount needed.

Kontact 

The middle section is only different because it doesn't have a slot cut in it. The whole shell is made from the same tube, and the ends are no different. They're just allowed to be looser because of the cut. Which means that they will clamp down just fine on an eccentric that fits the center.

Andrew R Stewart 

Perhaps just turn down the eccentric's central section only? The eccentric would be a sort of press fit to have one side (at the OEM diameter) slide past the shell's "tight" center section but become loose enough to rotate once the shell is fully inserted. I think this was mentioned earlier. Andy

Nessism

You might want to measure the center of the shell to see if it's round. Maybe there is distortion where a tube attaches. If so, a little localized profiling with a sanding drum may solve the problem.

unterhausen

I suspect the bb shell is pretty oval. If there is penetration into the bb from the welds, I assume OP would have mentioned it. I think distortion normally makes the shell flatter on the weld side. I'm sticking with my recommendation for a two part eccentric. If you can afford to have a machine shop bore the bb, they might not be good enough to do it right. Although you could get them to bore the center section, so unless they are butchers it's not very critical. Just don't tell them that.

guy153

This is exactly the tool I have made a few of in different sizes for this kind of thing. I made a CF/steel hybrid frame where I wanted the fit into the "lugs" to be just right for a good glue joint. I have also used this for head-tubes as I don't own the larger size of reamer. In a HT or BB shell all that really matters is getting the bearing shell to fit in with the right amount of tightness and parallel. An ST requires a much longer stretch of continuous diameter so a proper reamer is likely more important there.

If I was OP I would make an aluminium insert just the right size to fit in by hand to use as a gauge. Then make the sanding drum thing and sand it down until the gauge just fits nicely. It takes patience but not as much as sending it back to China would. I've never tried Ti but it must be possible to sand the stuff.

Andrew R Stewart 

On further thinking no lathe would be needed for reducing the eccentric's central section. It really doesn't need any "precision" of dimension. just not hang up on the shell's matching section. A bastard file could remove material easily. I would want to hold the eccentric in a bench vise if possible and do due diligence in marking out and following where the removal was needed. Andy