Originally Posted by
tomtomtom123
Is there any risk of cracking the head tube from the concentrated loading from the cable wrapped around it?
Should I use a single loop or two loops?
A single loop is one continuous cable with the ends connected by the turnbuckle by the cable end looped back onto itself to form an eyelet, so there are two strands running from the head tube to the bottom bracket. Two separate loops means that there's one closed loop around the head tube and another closed loop around the bottom bracket and they're joined together by the turnbuckle. The two separate loops are joined by their own ends aligned in parallel so there are no eyelets or bends.
Single-loop:
Load: Turnbuckle sees half the total system tension.
Angles: Sharper bends at cotter pin and clamp → slightly more slack.
Pros: Safer for turnbuckle, lower stress.
Cons: Slightly more slack, sharper angles.
Two separate loops:
Load: Turnbuckle sees full system tension.
Angles: Gentler, straighter into clamps → less slack.
Pros: Less slack, easier clamp grip.
Cons: Higher stress on turnbuckle and pin
And should I use a thimble in the loop at the ends of the cable? It has a curvature to it And also the diameter of the eyelet of the thimble is twice the diameter of the pin at the end of the turnbuckle which is a cylinder so the thimble contacts the pin at a single point. I'm wondering if the thimble will deform and act as a spring causing slack whenever the frame is loaded which would reduce its ability to remove the play in the hinge. If I don't use a thimble, the bend in the bare cable where it forms the eyelet around the turnbuckle pin will gradually flatten or deform or fray But it will be permanent and won't spring back so there won't be slack.
Good questions.
If using steel cable, it should be a "single" straight cable like Dahon does, no wrap around the head tube or bottom bracket shell.
If using soft high-strength/stiffness line like I did, I like the single loop like mine, as you said, the turnbuckle sees half the tension due to double strands, but more importantly, there should only be half the stretch (strain) in the line, and you want maximum rigidity; Too much stretch, and you get bending loads into the hinge instead of just pure compression shear.
I think the stresses due to line wrap around the head tube and bottom bracket, are small. You have a large line area due to large radius. I have zero concerns on my steel frame. Aluminum... perhaps a bit more, but still less concerns than the small tabs Dahon welds on for Deltech; I had seen Deltech disappear from aluminum frames but not steel, and theorized they might be having fatigue problems on the welded aluminum tabs (too short in front, and loaded in bending at the BB), and then I saw the following for the Japan market, eliminating the forward welded tab, which may confirm my suspicions:
The "Add-On" Deltech (top middle pic in my post #10 above) which Dahon talked about but seems to be vaporware, has steel ring clamps which go around the head tube and bottom bracket, so there; I think low-stretch line would put less stress there than thin steel clamps.
Thimbles: Those are critical for steel cable, which requires a minimum bend radius; Without them, a loop in the steel cable would have a ton of spring to it so not stiff enough, and rapidly fatigue and strands crack. (With steel cable, a better strategy is a single straight cable with swaged end fittings, like the Dahon Deltech.) With very flexible high strength line, it is much more tolerant. Tiny thimbles at the turnbuckle attachments might be good, but I think instead just show careful attention to:
- holes that line are attached to are well chamfered, no sharp edge or shape,
- careful choice of knots and exactly how they are tied; line stress can vary greatly by knot type, some reducing line rated strength significantly. I chose an 8-bend, but even there, it's important that the tension ends (versus free ends) are correct, often taking the route of larger radius instead of smaller, where the line is against another layer in the knot; look online for instructions. And also, use a knot that holds well with slippery line; The downside is that those knots are often very hard to undo, so get it right the first time. Another possible option might be a double-bowline, tied just like a standard bowline, but two loops at the beginning of the process, it holds twice the tension, but I think easier to untie than a tight 8-bend. For application where the loop may take a lot of wear, I like a bowline-on-a-bight, because it's a bowline that ends up with two layers of attachment loop instead of one, I'll often use that as the loop part of a trucker's hitch, because it takes abrasion of the line rubbing hard against it, when cinching down.
Oh yes, if using a straight steel cable with swaged end fittings, you don't have welded tabs on the frame like Dahon, so yes, just closed loops around the head tube and bottom bracket to attach to, will be fine. If the holes on your swage fittings are large enough, the closed loops can be double, two wraps and one knot, if two thicknesses of line will go through the holes on the swage fittings. If not, don't worry about it; The skinny line you see on my bike? That'll lift my car. Strong stuff. But I would say, the V extending from the loop(s) to attach to, have the included angle there be no more than about 70 degrees (35 degrees each side); too big an angle there will increase the line tension in the loop a lot. A longer V (smaller included angle) reduces the tension.