|   What makes a frame accept a dual crown? | P-burg
Oct 3, 2001 10:16 AM | | Why can't all frames accept dual crowns. |
| re: What makes a frame accept a dual crown? | Inclag
Oct 3, 2001 10:42 AM | | The reason that not all frames can't accept dual crown forks is because the amount of stress that they exert on the headtube. This is because they are taller and therefore creat a larger torque. If the frame isn't strong enough then the headtube will either snap off or peel off. This is why most frames that accept dual crown forks either have top and downtube gussets or are made out of oversized or heavier gauged tubing. |
| well almost got it... | zedro
Oct 3, 2001 10:59 AM | | its not so much that the forks are taller (because some DC forks are 4" to 5" just like some single crowns), but that the extra torque is exerted by the top crown, transfering more stress into the head tube. On single crown forks, the lower crown and steer tube can flex to some extent, thereby reducing the overall stress transfered to the headtube area. The taller fork only magnifies the problem. |
| Can you build a brace. | P-burg
Oct 3, 2001 1:09 PM | | Has anyone constructe a brace that would fit around the head tube and clamped around the top tube, and down tube to alleviate the stress. |
| Sure, wrap it in carbon fiber.... | ssaddict
Oct 3, 2001 1:57 PM | | I'm not saying it wouldn't break still (as there is a way to break anything) but it would take more stress to snap it. If you were to put about 3-4 layers of carbon around the headtube, and the first 6" or so of both the top and down tube it would greatly increase the strength of the headtube. |
| Sure, wrap it in carbon fiber.... | Atomic
Oct 3, 2001 2:03 PM | | How can you do that?? Where can you buy carbon fiber, and how would you fix it?? Thx |
| Um, it didnt work so well for GT, but | Jm
Oct 3, 2001 4:20 PM | | my guess is you would have to sand down the surface of the headtube area to get rid of any paint and coatings, then you start wrapping carbon fiber and resin around it, you have to layer it and it has to "cure" to harden, this sometimes involves heat, and heat might mess up the alu underneath.
This is NOT a good idea.
You want a good single crown, like a Shiver SC, Marzocchi DJ1, Dorado SC(not quite sure about this one based on its bigger brother), Risse had their Roxxxy sc fork at interbike-it's friggin MASSIVE, there might be a few other contenders in 2002 as well, in any case its smarter in the long run. |
| plus... | zedro
Oct 3, 2001 6:57 PM | | you pretty much have to know some principals of carbon fiber mechanics and stresses in frames to actually benefit your frame. The direction of the fibers and how the sheets distribute the stresses is critical. Just randomly slapping down sheets wont do much good. Plus you may just end up transfering stresses to somewhere else on the tube set that cant handle the stresses either.
Bad Idea Jeans |
| HAHAHA!!! | MMike
Oct 3, 2001 8:28 PM | | "Ordinarily I use a condom. But I figured, hey! When am I ever going to be in Haiti again?"
"Now that we have the children, I feel so much safer with a gun in the house."
Classic.... |
| she was a cocain junkie, but hey, she needed a place to stay | Jm
Oct 3, 2001 8:37 PM | | heh, yeah those were great. |
| When did GT ever use Carbon? | ssaddict
Oct 4, 2001 8:05 AM | | No, its not the greatest idea but either is sticking a fork on a bike not designed for it. Doing as I said would distribute the forces across a wider area but it could create stress risers in other areas that could lead to failure. |
| um, STS carbon, Lobo carbon | Jm
Oct 4, 2001 1:23 PM | | both had carbon wrapped headtubes, both had problems with them breaking off. |
| Yeah, great idea! | Atomic
Oct 3, 2001 2:02 PM | | Yeah, that would be real great, even if you dont use double crown. I would make the frame way stronger. Just think of the riser bars that have bolt-on cross brace. |
| They're too rigid. | Dougal
Oct 3, 2001 4:09 PM | | DC forks are too rigid. More rigidity means you get higher impact forces for the same obstacle. Translation is more stress on the frame which usually rips the headtube off.
Some frames have plate or butterfly gussets, some use a monocoque front section, others just use really thick and heavy tubes/butting.
Don't try to weld something to your frame yourself. I've seen an aluminium frame that wasn't heat treated after welding plate gussets on. It sheared cleanly at the edge of the heat affected zone. The bike was weaker than it would have been without the plates.
Dougal.8m.com |
| This does beg an interesting question.. | Fooman
Oct 3, 2001 5:11 PM | | As I understand it, frames break for two reasons because of dual crowns.
1) They are taller, therefore excert more leverage to the
2) headtube, which is doubly stressed because the dual crown forks will not just flex at their steerer tube because of the top crown.
Now the interesting question is whether travel is the accurate determining factor to figure out if a fork is ok for your frame.
The reason being that inverted forks are shorter than dual crown traditional forks. IE, a Marzocchi Shiver is actually shorter than a Marzocchi Monster-T, even though it gets an inch more of travel. (this is actually heresay, but pictures I see of the two side by side confirm it, I'd love to hear confirmations either way).
So.. If you frame can only take a 7" fork, is an 8" shiver acceptable as well, since it will actually excert less stress on the head tube? Or is the geometry going to be so screwed that you wouldn't want to bother?
-FooMan |
| dont think theres any frame out there... | zedro
Oct 3, 2001 6:52 PM | | that dictates you can use a 7" but not an 8". These frames are built to take abuse in the first place, an inch difference shouldnt cause it any harm. Geometry would be more of a concern, but thats up to your current frame and ride style to determine that. |
| Leverage changes bugger all. | Dougal
Oct 4, 2001 1:08 AM | | The percentage change in total height is bugger all between say a 5 and 7 inch fork and can be easily circumvented by installing a longer headtube to keep the stresses the same.
It's not like we're riding planes around, Play bikes are on the overbuilt side anyway.
Dougal.8m.com |
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