13/16 - 26TPI die for Domi steering stem

elefantrider

elefantrider

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It makes little sense but lower yokes for early fastback Commando and late Domi of the same years use a different thread. 28 vs 26 TPI. All else being the same with the yoke.

The lower nut on these always have a tight spot on the shaft when removing, due to apparently the lack of a bearing spacer in the design.

This was fixed on the later Commando yokes by the addition of a spacer.

Anyone have a 26 TPI die they'd loan out? They are out of stock and hard to find or so it appears.
 
The genuine cup and cone bearing is effectively an angular bearing. A pair of opposed angular bearings don't need a bearing spacer - in fact it would upset the bearings.
Later Commandos used twin radial ball bearings - a step back in my view. Radial ball bearings can take small to medium axial loads only, and the spacer is needed to prevent bearings to bind.

28 tpi UNF appears to be a non-standardized thread for this stem size.

- Knut
 
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mdt-son said:
The genuine cup and cone bearing is effectively an angular bearing. A pair of opposed angular bearings don't need a bearing spacer - in fact it would upset the bearings.
Later Commandos used twin radial ball bearings - a step back in my view. Radial ball bearings can take small to medium axial loads only, and the spacer is needed to prevent bearings to bind.

28 tpi UNF appears to be a non-standardized thread for this size.

- Knut
Click to expand...
It does seem a strange choice of bearing on the later commandos
I remember being very surprised when I saw them in a commando frame
But they do seem to work well!
 
How do you explain the thread tightness about 1/4 to 1/2 inch above the lower nut on every bike I've disassembled which has this design?

Still looking for a 26 TPI die to repair this thread.
 
I think you will have more luck finding a thread file with a 60 deg angle and 26 TPI. It might be a better option than a die as you are only dressing up a tight spot.
Failing that Tracy tools in England has 13/16x26 dies. £12 for carbon steel, £36 for HSS
 
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elefantrider said:
How do you explain the thread tightness about 1/4 to 1/2 inch above the lower nut on every bike I've disassembled which has this design?
Click to expand...
Hard to say. One PO overtightened the top nut in an attempt to prevent it from getting loose? If there is no visible damage to the threads, the stem must have been tensioned, which alters the pitch slightly. This can only happen by torquing the nut(s) - top nut in this case.

If this is the case, chasing the threads could actually strip them at this location. I suggest you assess your treads under a microscope using a thread gauge and decide whether damage is by some kind of local impact load or stem tension.

- Knut
 
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You could be onto something. I think the shaft or top nut may be bending/flexing under transverse loading, and this is amplified the rougher the road.

Hence I would think a custom spacer in between the bearings would help in this area, like on the later Commandos. Or a longer top nut?

Highly doubt the top nut was overtightened. The tight spot was a bit below the top nut thread engagement each time.
And these were highly original, low- mile bikes.

It might be better to live with the tightness rather than to try to fix with a die.
 
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elefantrider said:
You could be onto something. I think the shaft or top nut may be bending/flexing under transverse loading, and this is amplified the rougher the road.

Hence I would think a custom spacer in between the bearings would help in this area, like on the later Commandos. Or a longer top nut?

Highly doubt the top nut was overtightened. The tight spot was a bit below the top nut thread engagement each time.
Click to expand...
Exactly! Stretching, whether from pretension or axial road loads, will stretch the stem _below_ the_top_nut. Axial loads are acting at the downstroke of sliders and wheel. (At the upstroke, all loads enters into the lower bearing, no load passes along the stem.)

The spacer fitted to the Commando is there to (a) prevent excessive axial load on the radial bearings during pretension of stem, and (b) distribute upwards axial loads onto both bearings . The spacer does NOT stiffen the steering shaft.

A spacer between angular bearings of the early design will upset them and it will not prevent misalignment due to bearing tolerances and rotaton of bearing races at their seatings.

Yes, the shaft of the early lower yoke is a bit thinwalled, but ... since single track bearings are effectively simply supported (i.e., unable to pass a bending moment to the headstock), and there is no load at the stem between them, we have to consider displacement (i.e., misalignment) rather than bending. This will affect the steering geometry only, and hardly in a manner noticed by the rider.

If you mistrust the steering stem, I would fit the early Commando yokes as a set, including deep groove radial (or angular) bearings. Then there will be no chance of misalignment at least.

- Knut


Edit: When braking hard, the friction load is divided into an axial component along the fork tubes, and a perpendicular component acting at the wheel spindle. The former is absorbed by the lower bearing (we assume the lower yoke to be infinitely stiff), the latter produces a bending moment which can be seen as acting at a point midway between the yokes. This bending moment is counteracted by a force couple acting at/through the yokes - each force being counteracted by the bearing radial load. So, the only possible cause of bending moments affecting the stem is deformation (in-plane bending) of the lower yoke. A tiny deformation is possible of course, but hardly one which will produce plastic deformation in the stem. -K
 
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I suspect the rougher rural roads here, especially in the 70's, are likely partly responsible.
Both bikes came from rural areas, near Sacramento, California.

I sent an email to A E to see if their replacement shaft can be made stronger, out of harder material or thicker wall.
Do prefer the look of the early yokes and shrouds on the Domi.

The factory's shaft was supposedly .0015 interference fit in the yoke and then brazed togther.
 
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Sorry, meant to write A G. See video above.

Edit: Decided to pass on A G.
 
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Looking to speak with anyone else who has made and installed a new stem in a bottom yoke.
 
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The stem is joined here and brazed.
12FE2C92-03B7-42CC-AE1A-9B236CA4D5FF.jpeg


Need to check if it goes all the way down to touch the flat base of the yoke.
The old shaft should be able to be removed with heat and flux.


If Andover can sell me just the shaft, would likely go that route. Having a local machine shop cut one up to size, thread in BSC and gun drill will be far more pricey than someone who has these on the shelf.

Or may pull one out of another bottom yoke, clean up on a lathe and give to our aerospace-certified welder guy who can braze it cleanly and strong. Once out, the hole will need cleanup on a mill. The ideal recommended joint tolorance for brazing is .0015. Unsure what the factory used but will try to find out.
 
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I am also curious about what material that was used to braze the stem into the casting. We often refer to this type of joint as brazed, but to get the filler rod to flow into such close tolerances, I wonder if the filler rod is some kind of silver/bronze alloy?
 
The custom bicycle people will still build you a brazed-up frame. I believe AMC frames were made this way prior to G85/P11 welded frames. So it is or was a well-developed technique. Ten years ago I watched this being done on a jig so as to get the angles right. I have no idea what the clearance between the two parts is but the result is quite strong. And perhaps best of all the frame can be easily repaired in the event of a bent or broken tube. So I should think the steering-stem could be replaced by similar method.

I once bought some rod I had assumed was brass only to find that the welding supply shops supply bronze. That has a higher melting point. Too high, I believed. I feared weakening the tube with the heat. The frame shop fellow told me they use silver solder for brazing lugs and fittings onto the tube rather than bronze for that reason. Apparently, the double-butted frame tube ends and cast lugs can withstand the heat.
 
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