hillzofvalp

I have:

-a 40mm 1" spacer that I'm likely to lathe down a little bit for the right height.
-thomson x2 on order with 36mm stack height
-chris king 1" no threadset.
-thomson shim lathed down to 32mm (was supposed to be 36mm to match stack height but I went over on accident). I am able to recover 4-5mm of shim from the scrap I cut off, if needed.
-Old 1" threaded washer that might prove useful between stem and top cap.

ISSUE: I was just putting things together, and I did not fully realize until now that my 1" top cap will not make contact with a 1 1/8inch stem.

Questions:
-Is this a big issue?

Tension in the headset could possibly be maintained with solely the top cap and screw. Also, the top cap is capable of going through the stem because it has an OD of just under 1 1/8". No real stem compression is present unless I use a 1" washer with a wide enough OD and narrow enough ID on top of the stem to adapt the 1" cap.


- will 32 mm of shim effect anything dramatically if stack height of stem is 36mm?

My understanding is that the shim will have to be placed low as because of the need to maintain contact under compression from top cap. Also, the top cap would have to be 4-5mm inside the stem cause I cut my steerer planning on having a flush top cap. OR I could lathe down the 40mm carbon fiber spacer, if it's not going to shred apart. (in cnc lathe)


- What things are there to look for when cutting carbon fiber with a lathe? Will a slow feedrate ensure safety?

I figure I should just buy a 1 1/8" top cap and be done with it.

Motobetird

Why ask questions to which you already know the answer? Of all the expenses on a bicycle the top cap has to be the most negligible.

FastJake

Not really. The stem is what holds everything together. The top cap is just there to set the preload. Once the stem is tightened the top cap can even be removed. So if you can shim it good enough to set your headset preload go for it. It doesn't provide any structural support. If not, just get the right cap.

FBinNY 

It's no issue because the top cap doesn't need to bear on the stem, but you have to take a moment to think it through.

The part that needs to be pressed is the conic centering/compression ring. On a typical 1" headset the compression ring less than 1-1/8" OD so the stem or any 1-1/8" ID spacers won't sit on it and instead will bear directly on the cup. Without compressing the centering cone there won't be a solid fit between the upper bearing and the fork.

Use 1" spacers and the adapter shim to make a stack that reaches above the top of the steerer. Make sure the OD of your spacers is less than 1-1/8" and arrange them so the shim is where you want the stem to be. Then use a 1" top cap to compress the stack against the centering cone in the normal way. Finally slip the stem into position and clamp to hole it all together.

If the OD of your spacers is larger than 1-1/8" you're still OK as long as the shim is taller than the stem. But you'll have to assemble the stem onto the stack before adding any spacers above it.

There is another way, but you'll need spacers with 1" ID and and OD of about 1-1/4" or more (matching the OD of typical 1-1/8" ID spacers. This will allow you to make a stack more similar to the standard 1-1/8" system, (with the shim tucked flush within the stem. If you end with a spacer on top, use a 1" top cap. If the stem is the top element finish with a 1-1/8" top cap so you can compress via the stem.

If you have trouble following this, Draw yourself a quick sketch, keeping in mind that you need to press down the centering cone and work your way back to the top cap.

hillzofvalp

I think I understand what you're saying. BTW, my 1" spacer OD is 1 1/4". I like the idea of using just a 1 1/8" top cap rather than using the 1" one I've got. It would look cleaner and match the OD of the spacer. Regarding the shim, is it absolutely paramount that it be as long as the stack height of the stem or more? I mean, don't ask how, but I ended up with 32mm instead of 36mm (the stack height of stem). Can I use the 32mm one pictured and also the remainder from when I first cut it? I would face it for sure because I can... otherwise I'd have to wait for another shim to be shipped to me. I can wait, but I rather not if I can just use a small 4mm shim and 32mm. The easiest case for me would be to just use the 32mmm alone.

The thomson x2 (on order) has from what I can see 6-7 mm at the narrowest at the top . I would get over 90% of normal clamping contact in my estimation.

One more thing: The shim is currently sitting 4-5mm above steerer tube.

hillzofvalp

Because I'm unsure and inexperienced? I had a good idea of the answer but was befuddled from my bike shop having steered me in the wrong direction.

FBinNY 

No matter how you slice it the total stack has to be longer than the fork by roughly 3mm or 1/8" so the top cap can compress the stack without beaching on the top of the steerer. Since you're compressing by the stem, the Thomson shim length must be shorter than the stem so it can hide inside, so you're OK with the one you have. Just be sure that the shim and stem slot line up.

The top cap diameter depends on the ID of the top element. If a spacer, use the 1" if the stem use 1-1/8".

FBinNY 

Sometimes people only think they know the answer, and want to confirm.

There's a difference between sort of knowing the answer and knowing you know the answer. Then again some people think they know, but they're wrong.

hillzofvalp

Thanks for your input. You said 3mm between top of stack and steerer... I have around 7-8mm. My carbon fiber spacer is one long 40mm. Is it okay to throw it in a mill or lathe and take it down a few mm? or is 3mm a bare minimum?

FBinNY 

If the top element is a spacer, it doesn't matter how sub-flush the steerer is.

If it's the stem it's more complex, because the issue isn't the gap but the length of engagement of stem to steerer. In this case it's analogous to inserting a seatpost in a frame. Not enough engagement stresses the tip and raises the chance of it camming out. Your situation is the most complex, because the holding power is set by the length of the shim not the stem engagement, so you have to use your own judgment.

As I said earlier, I don't like to put more than 1" of spacers below the stem on a 1" bar. That may be conservative, but I consider it an absolute limit (personal opinion). In your shoes, I'd shorten the long spacer and try to get as close to ideal as I could while still meeting my needs. If you need more stem height, consider one with more rise, so you can bring it closer to the headset.

If you really want to do it right, a number of framebuilders build custom stems. You could have the perfect 1" stem built so you could eliminate the long spacer under it or the need for an adapter shim.

hillzofvalp

If there was an ideal way to go about this, I would just get a different set of bars (mine are 31.8) and use a true 1" stem. FEel free to refer me to someone who will make one without breaking the wallet.

Also, why do they make carbon fiber spacers in 40mm if it's not "accepted" or "safe". the thickness of the walls is around 2.8-3mm, which seems pretty stiff. I would think that having one spacer could be somewhat safer, but I'm no technical expert on bikes.

My shop doesn't have any diamond carbide facing or turning bits, so I'm not sure I'll be able to shorten the one I got, but once I find that it is too high with my 10 degree stem I'll go smaller..

FBinNY 

Your problem isn't that you're using a 1-1/8" stem and shim, but that it doesn't have enough rise, forcing you to use a tall spacer beneath it. I don't know what's accepted or how they determined it, but can tell you that when Aheadset first came out they strongly recommended no more 8mm in spacers between the stem and headset. And that was with steel steerers. Since then steerers have gotten weaker, and the amount of spacers used has gone up progressively.

In a cantilevered structure the bending stress at the base is proportional to the unsupported length, so adding spacers markedly increases the risk of breakage. I'm sure fork makers consider how much space folks will add under the stem, but I'm not privy to their thinking. I do know that steerers have broken above the headset, something unheard of years ago. I tend to be extremely conservative about failure risk at the front of the bike, so my preference would be to have a minimum amount of spacers under the stem. But that's just me.

Ironically, now that sloping top tubes are SOP, and rising stems are available, there's no need to have excess fork extension above the upper bearing, but I see more and more of this and they're getting higher and higher. Just not on my bikes.

BTW, you can cut carbon with a high speed bit, but edge life will be pitiful, though it should last long enough for a cut or two. Or you can buy a carbide parting tool for very little dough depending on the lathe's tool holder.

If you want to consider a custom stem, try Chris Igleheart who made these routinely until his frame order book got healthier. Tell him Francis from Chain-L sent you.

hillzofvalp

Thanks for the info. It's actually an older frame.. no steep mtb-reminiscent top tube. I really want to know more on spacer height theory. I have seen bikes with 1" 40mm carbon fiber spacers before and they didn't seem to be having issues.

Is a failure of a spacer an extreme risk if it actually happened? If it happened I feel like loss of control would only be if it occurs on a sharp turn or heavy pedaling up a hill standing up.

Do you have particular experience with 1" carbon spacers with 2.5mm walls?

I also wonder if I could use an end mill facing tool on a very very low feed rate.

FBinNY 

You missed the point, it isn't spacers that fail. It's the steerer tube that snaps off because it wasn't built to be extended that far. The fork end is heavily reinforced, but not so at the top.

I'm sure that fork makers are beefing them up these days since tall spacer stacks have gotten common, but not sure enough to do it myself. This is a personal risk assessment, but yes, steerer breakage isn't that rare, and high stack height is a risk factor.

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I wouldn't try to shorten a spacer more than one or two millimeters by milling the end. You want to cut it down from the side with a parting tool.

hillzofvalp

I noticed you mentioned that the risk increases with increased spacer height, but isn't the risk also lessened by increased headtube height? I'm building up a 60cm frame with a headtube + headset stack measurement of 205mm. Head tube itself is 180mm. I think this would be much different than putting 40mm of spacers on a 56cm frame, but I am (once again) no expert. stress on the joint just above the headset is going to be much lower as the proportion of head tube to stack stack height varies from an accepted norm.

x= head tube height not considering headset
y= risk assessment, lower being less risk
plotted spacers of 5, 10, 30, and 40mm


a 56cm frame with a 140mm head tube would have around 20% greater risk with a 40mm spacer than a 60cm head tube of 180mm (just from eyeballing it), I would assume the myth occurs more frequently for the general population of riders less than 6 feet tall, or those who are being hard on their bikes either through sheer strength or going off road with no suspension.

FBinNY 

Head tube height is an indirect factor in that it provides an area for flex. But it's the top bearing that forms the fulcrum.

Maybe I can explain by way of a diving board analogy. Typically a diving board is anchored at one end, then there's a central support which acts as a fulcrum and finally the tip you jump from. As you extend the tip farther out from the central support ( analogous to the top bearing) the bending moment there increases, and it's capacity for a heavy diver jumping on it lessens.

But the board is a flexible beam and as you move the central support out you create a longer area for flex behind it in reaction to the load, which makes it springier, but not actually actually stronger. It just flexes farther before cracking. BTW- diving boards do crack from time to time, and it's always at the central support, just like steerers.

Likewise tall riders have more issues with fork flex, regardless of how many spacers, but longer stems and more spacers below increase the risk of fracture.

Understand that I'm not trying to talk you out of anything, and as I've said numerous times, I'm ultra conservative about fork strength. You might be perfectly fine with 40mm spacing above the headset, especially if you don't have a 130mm extension. There's no magic line below which it's all OK, and above which it isn't. It's a curve, and you, and only you, can decide on where along it you're comfortable.

hillzofvalp

Well that's something to consider, cause I'm using a 90mm stem. I didn't choose to include headset height because they can vary so much. Moving the fulcrum in on longer steerer will have lesser effect than moving it in the same amount on a shorter steerer.

hillzofvalp

Well that's something to consider, cause I'm using a 90mm stem.