Steering spacer tube .

Torontonian

Torontonian

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Recently rebuilt the front end , knocking out the old rusty head bearings , using a 1 1/2 " socket , tapped into place both new ones home . Home being a machined lip in the internal frame headstock , that prevents both bearings from going any deeper into the frame steering head . So why the need for a steering stem tube at all ? Just noticing / thinking .
 
Torontonian said:
So why the need for a steering stem tube at all ? Just noticing / thinking .
Click to expand...

Why is the tube there, because the factory deemed it necessary could be the short answer.

The outer race becomes part of the frame, the inner race part of the stem so by default the inner can be put into axial load upward.
If the tube was removed the bottom bearing would take all the upward load, with the tube in place the upper bearing shares the load due to the interconnecting spacer tube between them. It also minimises (if the correct length) the bearings being pulled together (Preload the inner races towards each other) when the lower nut is tightened.
 
Time Warp said:
Why is the tube there, because the factory deemed it necessary could be the short answer.

The outer race becomes part of the frame, the inner race part of the stem so by default the inner can be put into axial load upward.
If the tube was removed the bottom bearing would take all the upward load, with the tube in place the upper bearing shares the load due to the interconnecting spacer tube between them. It also minimises (if the correct length) the bearings being pulled together (Preload the inner races towards each other) when the lower nut is tightened.
Click to expand...
Hmmm.
 
Stephen Hill said:
The need depends on the type of bearing.
Click to expand...
Steering spacer tube .
 
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Different years used different kinds of bearings. 1968, 69 and 70 used cup, cone, and individuals ball bearings. An additional nut on the stem provided adjustment for end play, so no need for a spacer tube.
1971 on used ball bearings and the stem had no good method of adjusting endplay, so a spacer tube was used.
If you substitute tapered roller bearings on a later set of yokes as per the above photo, good engineering calls for a spacer tube to establish correct endplay. The spacer tube would need to be custom made.
 
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When you tighten that bottom nut up you are torqueing up on the bearing INNER. whereas it’s the bearing OUTER that’s located in the frame.

So without a spacer tube in there, as you tighten that nut, you will squeeze the inner racers towards each other. You’ll squeeze the balls to one side of the race and put them under a high load, one they’re not designed for (this is the axial load TW mentioned). This means they will wear fast, then you will lose the torque that you set the nut too, and the front end will get very sloppy.

With the tube in place, you clamp both inners and the tube together tight, yet they are free from the outer.

In short, that spacer tube is important !
 
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Good engineering is not rocket science when the effects of tolerances, adjustments and loads are considered logically. Motorcycles teach you things. I don't like getting holes in my crankcases, or broken necks.
 
I think only the bottom bearing locates against the the steering stem. You fit that one first and then the top bearing locates against spacer tube when you fit it. I have never measured anything but would not be surprised if the top bearing does not go in as far as the raised edge of the steering stem. As with most steering stems the bottom bearing takes most of the load and the top bearing just centres the stem.

The amazing thing is that from a purely engineering standpoint those are the wrong bearings for the job but the reality is that they work well and last a long time. I am on my second set in 52 years and only changed them as I had it apart for something else.
 
On the frames machined for the later two ball bearing setup the top recess is the depth of the bearing width, the bottom recess is deeper by a few mm. The tube spaces the 2 bearing inners apart with the top bearing setting the position relative to the frame and the bottom bearing floats and its position is set by the length of the tube. The yokes for this later setup have the central tube attached to the top yoke.

Steering spacer tube .
 
kommando said:
On the frames machined for the later two ball bearing setup the top recess is the depth of the bearing width, the bottom recess is deeper by a few mm. The tube spaces the 2 bearing inners apart with the top bearing setting the position relative to the frame and the bottom bearing floats and its position is set by the length of the tube. The yokes for this later setup have the central tube attached to the top yoke.
Click to expand...

That might be the case but seems like a very odd (polite) design to have the bottom bearing floating (around 1.25mm iirc) when the load is on that bearing from the suspension and weight of the bike, it has the possibility to be knocked up with only interference (x 2) stopping it from doing so and even odder, no real support tipping (I have seen two frames with obvious distress in that lower cup)

I have no practical experience with that stock set up though as I got rid of it and used taper rollers with seated outers, that took simple spacer modifications and attention also to the altered head light bracket height dimension. (Machined lower triple boss to reset)

If someone was inclined, they could alter the spacer length on the deep groove ball bearing set up, so the lower bearing seated fully and the upper bearing had a few thousand's float then a shim to reset the headlight bracket dimension. (Perhaps 10 minutes work)
For me it is about maximising what is there while retaining the stock look.
 
Time Warp said:
That might be the case but seems like a very odd (polite) design to have the bottom bearing floating (around 1.25mm iirc) when the load is on that bearing from the suspension and weight of the bike, it has the possibility to be knocked up with only interference (x 2) stopping it from doing so and even odder, no real support tipping (I have seen two frames with obvious distress in that lower cup)

I have no practical experience with that stock set up though as I got rid of it and used taper rollers with seated outers, that took simple spacer modifications and attention also to the altered head light bracket height dimension. (Machined lower triple boss to reset)

If someone was inclined, they could alter the spacer length on the deep groove ball bearing set up, so the lower bearing seated fully and the upper bearing had a few thousand's float then a shim to reset the headlight bracket dimension. (Perhaps 10 minutes work)
For me it is about maximising what is there while retaining the stock look.
Click to expand...
It only 'floats' until its pushed hard against the spacer tube by torquing the bottom nut then its fixed but it has to be able to initially move so the spacer tube then can act against both top and bottom inners. Your suggestion is good practise for fitters working off a production line and is better but when you have a line of bikes and a semi skilled workforce then the 'Float' works.
 
Assuming we are talking about the later yokes, and the ball bearings are swapped for tapered roller bearings,.
Will the spacer tube slide through the outer bearing race from the bottom?
This would make minute adjustments to the spacer tube a lot easier.
 
kommando said:
On the frames machined for the later two ball bearing setup the top recess is the depth of the bearing width, the bottom recess is deeper by a few mm. The tube spaces the 2 bearing inners apart with the top bearing setting the position relative to the frame and the bottom bearing floats and its position is set by the length of the tube. The yokes for this later setup have the central tube attached to the top yoke.
Click to expand...
I'm probably misreading/misthinking but it seems to me that the weight of the bike is equally on the top and bottom bearings and with the bearing inners locked together, the top one must move upward until the bottom one seats.

In truth, when both are seated, the spacer is not pinched in most bikes. So, it seems like the spacer either does not lock them together or the inners are pulled against the outers. I've often thought that the spacer should be a tiny bit longer to do the intended job.
 
Time Warp said:
Yes, the spacer tube will drop out with the outer in place.
I think initially I used Moto Guzzi final drive shims but, in the end, machined a heavy 4140 shim washer that was lapped the final 0.0005" or so to set the bearings.

An aluminium boss/spacer fit over the stem to push on the bearing inner with the nut tightened, the spacer tube could then be dropped out with the shim washer for lapping. I think it might have been a 20-minute job.

Obviously, it is not for everyone but for me it was a tick for engineering.


View attachment 116708 View attachment 116707
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What is the number for the tapered bearing? My original bearings need renewing, I want to replace with tapered roller bearings.
 
I've always assumed that the probability of the machining of the bearing bores in the frame being of the correct depth is unlikely. I carefully tap the bearing home until it just snugs on the spacer then check after tightening that the yolks turn freely
 
Commercial vs engineering.

The stock bearings require no adjustment (indeed none is possible). Making the assembly process MUCH quicker. Also making the service schedule / process easier.

They also eliminated the possibility of poor owner adjustment causing issues with handling, premature wear, etc.

So whilst they may not be exactly the correct bearing for the job from a pure engineering perspective, they do do the intended job well and solve / eliminate a number of issues / factors.

They are an easy to fit item requiring little skill and no adjustment, they’ll work well if replaced at the specified intervals.

So I can easily see why they were deemed a good commercial solution in a volume production item.
 
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