Cracked Triple Clamp

Welding

I've seen triple clamps welded in the past and considered it a possibility for this repair.  Unfortunately, I have lost contact with two very talented weldors from my younger days.  I'm sure they have long since retired. 

I have used a couple of local guys for small welding projects, but their skills are more along the lines of structural steel and farm-implement repair.  However, I recently met a gentleman who builds custom bicycle frames.  He works in titanium, chromoly, and aluminum so I thought his welding skills would be up to the task (plus, he rode trials in the distant past).  

This seemed like the most appealing option since I would get a reinforced part that was guaranteed to fit, and I would not have to deal with the bearing.  Although I had been warned that aftermarket triple clamps use 7075 for the stem and 2024 for the clamp - both of which are essentially unweldable - I figured I would see what the weldor had to say.  After an initial inspection, he agreed to do the job for $100. 

I told him I had machine tools and could remove any excess weld metal, but I was concerned about distortion and made a slug that I felt might keep the bores from collapsing.  He was free to use it or not at his discretion.   

Upon further investigation, the weldor decided he did not want the liability.  I completely understood but said I would not be using the cracked part and encouraged him to experiment, if for no other reason than to satisfy both of our curiosities.

Neither of us felt the result was satisfactory.  No charge.  I don't know if someone with different skills could have accomplished the feat, but it no longer matters.  It's not a solution that would be generally applicable.  

GasGas Triple Clamp  

My next idea was to try the triple clamp from a 2021 GasGas TXT (which also uses 39mm Tech forks).  It is part number BT21000GG-CSV-1 and only cost me $137.04 (delivered) from an online retailer.  Moreover, it was returnable. 

The GG triple clamp is a cast part that does not come with the steering stem. (Some riders prefer the feel of a cast triple clamp over a machined-from-billet part, so I thought that might make for an interesting comparison too.)

The other thing I liked about the GasGas part is that it looked more robust in the areas that had cracked.   Although in hindsight I probably should have also purchased the GG steering stem (about a $41 adder) I assumed the original EM stem was more likely to fit without issues. 

EM Stem, GG Clamp

After getting the old EM stem out, I discovered that the GG clamp's bore for the steering stem is about a half-millimeter larger than the EM's stem.  Had it been the other way around, I could have turned the stem in the lathe. 

For future reference, the ID of the GG triple clamp measured 21.496mm.  There is also a recess on the underside of the clamp that is used as a stop when the stem is being pressed into the clamp.   This recess measures  24.98mm in diameter and is approximately 4mm deep (estimated from memory).

The EM triple clamp has an ID of about 21.08mm with an approximately 24.28mm recess.

By this time I had decided not to mess with it anymore and just ordered an original EM part while there was one still available in the US.

Unfortunately, I did not think to take a photo of the GG triple clamp before returning it.

Bearing Fit

The new EM triple clamp does not come with the tapered steering bearing.  It is a standard 32004 (20 x 42 x 15) part.  I think it cost me $10 via eBay.

The bearing ID is closely controlled to be just under 20mm by maybe 10 microns.  Because the shaft (steering stem) turns, an interference fit on the bearing ID is expected. 

But I was surprised at how large the bearing diameter was on the new steering stem.  It measured 20.039mm.  The old one was only 20.019mm, and that one was obviously very tight.

I decided to polish the steering stem down to 20.02mm at the bearing seating surface.  This is awkward to spin in a lathe, so I did it by hand with 400-grit utility cloth.   It did not end up perfectly round but is fairly close.

Coefficient of Thermal Expansion

In production, bearings are often installed by using induction heating.  I use an electric skillet with a bit of spray cooking oil to improve heat transfer.

My CTE calculations indicated that putting the triple clamp in the freezer overnight would shrink the aluminum stem by about 0.012mm.

Heating the steel bearing on my electric skillet to about 250° F would expand the bore by about 0.048mm.

Even though this should give a generous clearance, I always prepare the press just in case.   The temperature differential quickly equalizes and the last thing you want is to get the bearing less than fully seated.

Luckily, the bearing slid into place easily (the stem is a long guide) and I did not need to press anything.

Measuring the Angle

There was about a 0.095-inch difference in gap over a length of about 4.5 inches.   A bit of trigonometry says this is 1.2 degrees.  So does my angle gauge.

Summary

In the end, I did what almost everyone else would have done initially - replace the broken part with one from the original manufacturer.  But I did learn a few things along the way:

1. Trials triple clamps are not weldable.

2. The EM's steering stem is not parallel to the fork tubes. 

3. The manufacturing tolerances for Spanish bearing fits are pretty wide.