Hi guys, Just wonder if any one has bought one of these custom subframes before? http://www.ebay.com/itm/Custom-Cafe...ame-for-BMW-Airhead-Motorcycles-/171231148312 Thanks!
Would not buy. There are so many questionable design choices; curved uprights, single sided top shock mounts, <- mounted on curved uprights, no cross brace... etc
But if the bike isn't a racer, then how important are those issues? I'm after something bolt on that I can attach myself. I've seen the Vonzeti items and those don't look much better. Getting a fabricator to put something together won't be cheap either.
Look how the factory did it. The top shock bolts are supported in two axis and from both sides. That one pictured is supported on one side only and welded to presumably thin wall tubing, in the middle of that tubing, and it's already BENT... :huh What could possibly go wrong...? Racing or not, I see a lot of stress being placed on that brace and welds. Why not modify the stock one?
Understood. But how much stress is going through the rear subframe? Isn't the main stress in the actual main bike frame?
The triangles formed by the stock subframe supports the entire rear end of the bike through the shocks, big heavy final drive and all... True, they basically eliminated the need for it on the Monolever bikes, but added a big cast chunk to hold the shock...
Cheers guys. Appreciate the feedback. My quest for a new rear subframe continues!!! With all the bikes being built these days its a shame no one does a quality item. I'd pay my money on the spot!
Someone posted up a picture of a bike using this setup in this thread. http://www.advrider.com/forums/showthread.php?t=252051&page=456 Sorry I couldn't link directly to it.
You talking about the silver frame with orange snowflakes? That subframe is close, but not the same as the one on ebay.
IMHO, having spent quite a bit of time in both, the daily grind is often harder on chassis than racing. Race tracks are generally smoother and more predictable. Now engines, on the other hand . . .
Yeah Thats what I thought when I first saw it. Whether they may or may not work is debatable however he's made a nice job of that - nice lines. Can't work out how the rear brake works unless there's a disc on the other side.
The only area of structural doubt is that clamped front mount on the tube. I would have welded an elongated tab there to capture the leading mount point. Stock rear disk. You can see the caliper stay and the vented rotor holes if you look through the wheel right below the muffler. You can also see more pictures if you follow the links.
Could be wrong, but that mount looks to be a curved strap (something along the lines of a band cut in half) welded to the downtube, not a clamp. I didn't see the other pics, though.
It sure is. That S-curve really screws it up for me though since it essentially becomes a spring under compression, at least in theory. I think it would have been better, and looked better, if he had come forward in a straight line. Also the front mount looks a lot like the stock horn mount, which is actually overbuilt for holding a horn, but might just work for a frame brace. Having said all of the above, it probabaly still adds to the frame stiffness, just not as much as it could.
I looked again, then at my frame, and it is the horn mount. Which is ironic, since I was thinking of using that mount for a frame stiffener before I saw that picture, but completely forgot about it when I posted. I agree that either a straight tube, or two complimentary arced tubes would be stiffer than the s-profile. Depending on what you are trying to accomplish, this may be adequate, and does not add as substantial a stress concentration point(s) to the frame.
