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Post by svaero on Mar 25, 2016 15:48:42 GMT -5
I have a 1981 XS650SH that has a 19" spoke front wheel with dual disc brake and an 18" spoke rear wheel with drum brake. I want to convert to a rear disc set-up that uses a 19" front wheel, but I'm not sure what year wheel to look for. I believe there's a change in the hub around 1977 which uses a different offset and requires a different disc than post 1977. Can anyone clarify this for me? Also, does anyone have a wheel/hub that they want to sell?
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Post by grizld1 on Mar 25, 2016 20:41:04 GMT -5
In 1977 Yamaha made several changes to the front end. Fork tube OD went from 34 mm. to 35 mm. The early dual piston rigid caliper was changed for a single piston floating caliper, and the mounting points on the fork sliders were changed to fit the new caliper. And finally, the disc itself was changed from a 2-piece design to a 1-piece design that was more deeply dished. There was no change to the hub; spoked hubs from 1974 forward were the same, with the exception of the Heritage Special hub with its crazy high spoke count.
I wouldn't recommend using a stock front rim in your conversion; it's too narrow for use with any rear tire that you'd want to run. And yes, I know the style mavens run big fat 120/19 tires on those little skinny 1.85" rims, but it's a serious mistake. Another factor to consider is the rim type. The late Special II's used steel rims, anything from 1974-1979 used H-pattern alloy rims (AKA "dog pee catchers"). Locating decent 19" rear tires will be a problem.
The drum brake on the XS650 rear wheel is entirely adequate, and IMO the rear disc brake did little to nothing to improve performance. If you must have a 19" rear, you'll avoid more headaches than I'm in the mood to list if you install a 19" rim on your current rear wheel.
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Post by svaero on Mar 26, 2016 20:25:09 GMT -5
Thanks, your information is helpful. The reason I'd like to convert to a rear disc is purely for the "cool factor". My bike has the alloy shouldered wheels. If I could find a suitable rear disc type rear hub that I could lace to my existing 36 spoke 18" wheel, that would be awesome. However the only rear disc conversion I'm aware of uses a front wheel.
Do you know of anyone who's adapted a rotor to work with the rear drum hub? If the sprocket bolts went through the drum hub and attached to an adapter that fit inside the drum of the hub, the adapter could facilitate the attachment of a rotor. A spacer might be needed to replace the spacer used on the drum design, but it could conceivably work. I'm not a machinist or I'd take a stab at it myself.
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Post by grizld1 on Mar 27, 2016 9:57:39 GMT -5
Re. converting drum hub to disc, my guess is that you'd wind up with something ugly, very heavy,and probably dangerous, since braking loads would be applied out on the ends of some pretty long studs. You'd also need a disc large enough to allow the caliper to clear the edges of the drum; again, heavy and rather ugly.
If you just have to have a 19" rim and a disc brake in back, the easiest way to achieve it would be to lace a 2.15 or 2.50 x 19 rim to a front hub. I think I have a hub that I can part with; I'll send you a PM when I've looked.
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Post by svaero on Mar 28, 2016 12:37:16 GMT -5
I've read some blogs claiming that using a front hub on the rear is questionable. I'm not sure I agree with that. Braking loads are significantly greater for the front wheel. The rear wheel has to deal with the engine torque, the payload (including rider) and some braking. My current rear wheel is an 18" alloy "H" type. I may be able to lace a front hub to the existing rim which would suit me fine. It may require custom spokes to use the 18" rim so a 19" rim might be a better choice since I could use stock spokes. If you have a front hub you're willing to part with, I'd like to see it and talk to you about price. Hopefully, the hub you have is from a later (late '77 and newer) model because the offset and rotor attachment are different on the earlier models.
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Post by svaero on Mar 28, 2016 12:54:45 GMT -5
I know about the Omar's kit to make the front wheel work on the rear. Someone on this forum mentioned a guy by the name of Jim Henry who also does the adapted kits, but at a more reasonable price. The blog included his email so I sent him an note asking for price and availability, but never received a reply. Do you know this person and could you provide his current contact information?
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Post by grizld1 on Mar 28, 2016 15:55:15 GMT -5
Sorry, I don't know him; you might try a Google search. BTW, spoke hubs are exactly the same from 1974 forward; only the rotor itself changed, as a fast check of old part numbers will show you. However, the hub in question is from a 77.
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Post by svaero on Mar 29, 2016 17:54:03 GMT -5
Any luck finding that hub you were talking about?
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Post by grizld1 on Mar 29, 2016 19:53:17 GMT -5
Sure did, rim's off and it's ready to send if you want it. I sent you a personal message two days ago. Check and see if it's there, if not let me know and we'll try again.
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Post by svaero on Mar 30, 2016 16:02:59 GMT -5
Sorry, I wasn't aware you sent a message.
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