Steering Stem Swap Question

All,

I have been mulling this over for quite some time now. I have run calculations, talked with fellow engineers at work, etc.... I am going to post this up just to see what your thoughts are. So on my KZ project, I swapped stems. I pressed out the stock ZRX stem out the bottom and ground the weld and removed the KZ stem from the top of the stock KZ triple. The KZ stem has a step at the base and was dropped in from the top vs. from the bottom like the ZRX. The KZ stem base OD was almost EXACTLY the ID of the stem bore in the ZRX triple. When sliding them together, there was very little resistance to seat them. The stem had zero radial play that I could measure.

So I took the KZ stem, welded 3 rings, evenly spaced around the circumference of the stem base. I then chucked the stem in the lathe and turned them until they were .003" larger then the ZRX bore ID. I then added Loctite 609 or 660 (I don't remember which...) pressfit retaining compound to supplement the joint. I then pressed it all together taking 6000+ psig pressure to seat the two together. It seems rock solid.

Now what is nagging me is that the stem has to be pressed in from the top as the stem bearing surface OD is bigger then the ZRX bore ID. It does not have the step like the stock stem that catches when pressing from the bottom. Now the only way I can see this being a problem is if the bike were to slam downward from a wheelie gone wrong. This would cause the frame to try and push the lower triple tree off the stem. There are bearings pressed on, forks clamped on, etc... that would resist this movement. I would also think that the force required to unseat the stem from the lower triple would collapse the forks and probable cause them to slide all the way up in triple trees anyway.

Am I paranoid? Should I just shut up and finish the damn thing?

Cory

prolly just need to quit thinking like an engineer :laugh: !

at this point i would drill through the back of the lower triple and into the stem and tap the hole,then i would carefully drill the threads out of the triple material leaving the threads in the stem and use countersunk allen bolt to act as some piece of mind.

now having said that :whistle: ,
the stem mainly see's shear force being fed into it.
the lower triple transmits the force it see's into the lower bearing and into the neck of the frame (by-passing the stem) thus negating the stem as a main structural piece in the whole works.
the main job of the stem is to keep the upper and lower bearings pre-loaded so that they can transmit the force properly into the races.

that is the reason it is possible to get away with a machined set of top hats and a through bolt for pre-load.



leon

531blackbanshee wrote: prolly just need to quit thinking like an engineer :laugh: !

at this point i would drill through the back of the lower triple and into the stem and tap the hole,then i would carefully drill the threads out of the triple material leaving the threads in the stem and use countersunk allen bolt to act as some piece of mind.

now having said that :whistle: ,
the stem mainly see's shear force being fed into it.
the lower triple transmits the force it see's into the lower bearing and into the neck of the frame (by-passing the stem) thus negating the stem as a main structural piece in the whole works.
the main job of the stem is to keep the upper and lower bearings pre-loaded so that they can transmit the force properly into the races.

that is the reason it is possible to get away with a machined set of top hats and a through bolt for pre-load.

leon

That's what I was thinking. A lock bolt or Allen set screw would work.