Reply With QuoteYes, the ends are at least partially constrained, depending on the amount of preload between the bearings. Tough situation to analyze.
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That's WAY above my pay but have you considered how it's all compressed together against the bearings? It may be weaker but it has no where to go. In other words the force to bend it couldn't be applied.Originally Posted by Brian P
Last edited by inreb; 05-17-2010 at 10:31 PM.
soaked and upset
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Yes, the ends are at least partially constrained, depending on the amount of preload between the bearings. Tough situation to analyze.
can you put a long cylindrical spacer between the two bolts on the threaded rod? tighten it down somewhat, and get the whole thing TIG welded?
that should improve rigidity, I'm not sure by how much though. I'm still only a mech eng in training.
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What he said ^^^
The bearings are there to take the load both radially and axially. There is no way that rod could bend in the stem. Especially since when you tighten the castle nuts on the bearing you are putting tension into the rod/stem. Can't really bend if it's in tension.
I could do that but there would be no point. There is no way for a bending force to be applied to the centre of the stem because the bearing races are rigid with the frame. The only way the stem could see forces like that is if the race was loose and even then you would need it to be so loose that the bearing is falling out in order for there to be any appreciable forces in there. I have utmost confidence that the 5/8" threaded rod will be fine.can you put a long cylindrical spacer between the two bolts on the threaded rod? tighten it down somewhat, and get the whole thing TIG welded?
that should improve rigidity, I'm not sure by how much though. I'm still only a mech eng in training.
I'm with Brian P on this one. OP, I understand your logic about why it shouldn't be an issue but still........ I guess it's one of those things that will work although you think it's odd looking (not saying it's odd, just generalising). Couldn't you get the same machinist to make a new stem for you from some appropriate thick walled steel tube or something, that press fits into the bottom triple and has the diameter and length for your bearings on the upper portion?
I guess the important thing here is that it works and works safely.
Love what you're doing by the way, awesome stuff.
The threaded rod is not in a whole lot of bending. Yes, it is much weaker and less stiff in bending than the original setup...but...it doesn't carry much bending loads. The loading condition is really more of an enforced strain. If it's less stiff, it soaks up less load.
The bending moment from the rake of the front wheel comes up the fork tubes, and is "beamed out" into the upper and lower triple clamps.
The triple clamps exert a force (single shear) on the remaining portions of the stem, and the points of action are slightly above and below the bearings. This forms a couple, causing a ("bending") moment which will be reacted by each bearing. There will be some small bending strains in the rod as a result, as the bearing/housing/triple clamp deflects.
(It's not really "bending" moment though in the beam sense of the word, it's just a moment, or torque if you want to call it that.)
The strain will by cyclical in the threaded rod, which is tension. This is a fatigue issue, not a static strength issue. Cyclically loading threads in tension is a bad idea, generally speaking.
It would have been better to choose a rolled threaded rod in alloy steel, 4130 - 4340 or similar, in the annealed or mildly hardened condition (say <120 ksi) compared to stainless steel, which is much more subject to fatigue, and stress corrosion cracking.
That said...I'd ride it.
But if preload on the bearings starts to loosen up, you gotta pull it apart and make sure that rod isn't cracking. It won't go all at once, but it won't give you much warning either.
I am not an engineer, and I didn't stay at a Holiday Inn last night, but I have studied engineering(Senaca, Ryerson and UBC) (I used to do a job that I called "Structural Draftsman" now they it called "Civil Engineering Technologist"). I used to design very large meccano sets.
I have designed thousands of metal connections.
I wouldn't ride that. Not in a million years.
You have created a new shear plane... in EXACTLY the WRONG place.
The upright tube MUST be rigid to support the bearings.
It is not rigid.
Im no engineer either, but I break alot of stuff they design. And I have to agree, I would not be able to ride that bike comfortably, without constantly thinking about when Im going to snap that threaded rod....Way too much room for flexing with that setup you got there...Its a disaster waiting to happen...I have some threaded rod on my bike that is about that size, and they sometimes snap just from the constant vibration...Not even under any real stress like that will be when your on the brakes....
You should really get a custom stem made IMO.
Last edited by JohnnyP636; 05-18-2010 at 12:30 PM.
When was the last time you Bent or snapped a steering stEm in the middle Johnny ?
Never because Ive never had one made out of threaded rod....The frame will bend before the stem, not in your case thou!
Take an OEM aluminum or steel stem and try to flex it, now take that threaded rod and try to flex it. Youll see that a proper stem wont flex, but your rod will....
If you think its good thats up to you, I just think its a really bad idea...Temporary fix maybe...
And you dont have a true 5/8" of steel there, minus the thread depth your only about 1/2" of steel rod....Im scared for you..LOL
Last edited by JohnnyP636; 05-18-2010 at 01:01 PM.
Interesting discussion.
With the way the lower and upper clamps hold both fork legs the stem would move in unison. With the bearings properly adjusted would there be enough movement to set up a fatigue cycle? I'm not qualified to answer that but certainly worth considering especially using threaded SS.
Unlike JohnnyP636 I haven't brocken a lot of chassis components (can't think of any atm) despite seeing some questionable designs.
Coming at it from the back side....if it was your job to break that stem by riding the bike you'd think you were given an impossible mission.
Bottom line for me: the previous posts are legitimate red flags.
PS, just thought of one. GPZ550 shock link snapped.
soaked and upset
You guys are barking up the wrong tree. It's not a beam structure, it doesn't carry bending loads. If you think that rod's going to break in bending, you're simply wrong. It doesn't have anywhere to "bend to"...it's constrained at each end by the bearings.
The questionable issue is of torsional stiffness in the threaded rod, not bending stiffness.
The only way to bend that threaded rod is to physically rotate both the upper and lower bearings in a way that they are very stiff.
Torsion of that threaded rod is another story. Headshake, excessive deflection while steering, and perhaps fatigue life are the issues, not static bending strength.
From what I've read I think everybody understands it's not a bending load.
We're at the fatigue stage now. Is there enough deflection and/or torsion to set up a fatigue cycle? Are they cut threads or stamped threads? It matters.
soaked and upset
Really so why do almost all OEM shop manuals have section to measure the stem for deflection? And clearly state that if the stem is bent to replace it?
You really think that a "loose" ball bearing on each end will completely eliminate any flexing of the stem... Think about that!
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It may carry a bending load if the bearings are not adequately preloaded against each other. I know, that's not how it's supposed to work by design, but it may be a situation that develops over time without the rider realizing it.
Torsional loads between the upper and lower clamps are usually absorbed by the clamps to the fork tubes, not necessarily by the stem, since both the upper and lower are clamped to each fork tube - and a tube is a very torsionally rigid shape.
Still, there are enough different red flags being raised by enough different people here ...
"Would I ride it" -> As a "get you home in an emergency", slowly and carefully while avoiding bumps and certainly not doing any wheelies, yes. Bikes that were a lot more compromised than that, have gotten me homeBut I wouldn't trust it for 100,000 km of normal riding and I wouldn't trust it on the racetrack.
Well I dunno, FS72 asks JP636 when was the last time he bent or snapped a steering stem in the middle, JP636 replies that you can bend a threaded rod but not a steering stem...they're still hung up on bending it seems.
No mention was ever made of torsion or twisting so I thought that was worth bringing up...
The threads are rolled fwiw. If they were machined it would be a definite no go.
Fatigue life of steels is a really difficult subject. If it was aluminum, I could calculate the fatigue life of it, based on some big assumptions of course...but I could chew on it for a few days and come up with a conservative life span. I can't do that for steel. Mild, Alloy, Stainless, doesn't matter, I can't do it. Can't even guess.
My materials bible is military handbook MIL-HDBK-5J. It has everything and anything. If it's not there, it's not there for a reason. Fatigue (SN) curves are simply not published, for any steel. The reason, as I understand it, is that simple fatigue loading is not what causes fatigue failure in steel.
Eventual failure is determined by impurities, SCC, grain structure, residual machining stresses, etc etc. Evaluating those things is beyond my ability. I dunno, I'm beat too.
But yes, from my experience with breaking a few driveshafts and CV joints, which were under very low stress but underwent cycling, fatigue would definitely be an issue to worry about.
I meant the triple clamps twisting, so that the fork tubes are not parallel.
Normally, that doesn't happen...ie they can't become non-parallel if the steering stem is rigid in torsion. (Which is probably why steering stems are beefy..) But if the steering stem is flexible in torsion because it's a threaded rod...
ie holding the front wheel rigid and giving a steering effort to the right, the top triple clamp will rotate clockwise as viewed from the top, relative to the bottom triple clamp. This makes the fork tubes non-parallel, and that small deflection angle they make is taken up by twisting of both the triple clamps.
But yeah, thinking about it more, not sure it's really track worthy, which is the general idea of the build.
As I said before I like your project and do realise what it's like to think something out, do the hard work and people start shooting it down. But also realise it's only because most that have commented like the project and want it to succeed safely.
That being said I have one more thing to ask. Have you considered the shear forces on the lower extremity of the threaded rod where it is screwed into the lower triple. Now the mere fact thet it's screwed in there generates very small high frequency movement between the threads, no matter how you tighten them, and even if this doesn't have issues the high stress risers of the threads at this point may be. Food for thought, again we're not picking on you.
As I mentioned in my earlier post, a custom shaft should be seriously considered.
Would it be better for the road with it's less than ideal surface? Potholes, expansion joints, grating, speed bumps are all abundant.
No. It has a jam nut. It doesn't move at all.
Because you're not flexing the s*** out of everything 98% of the time on the street, and because the threaded rod sees no cyclic loading whatsoever when riding in a straight line, Yes.
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