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Discussion Starter #1 (Edited)
I know there has been much talk of doing a rear disc brake conversion, but not sure if anyone has done it, so here is my version so far. I think I have spent many more hours doing the customization over the last year than actually riding it! But for me, I just like making things. I think the hardest parts of the conversion are done which would be finding a donor caliper setup and mounting the disc. Since I already did the larger front disc conversion, I thought I would use the left over original front disc for the rear. I went to the local cycle junk yard and found a 2005 Yamaha R1 rear disc complete with line, master cylinder and pedals that looked like it would work. I have a rear wheel, swing arm and frame to do the mock up on.

I first made the disc adapter, lots of chips:








Then a spacer/adapter for the caliper mounting arm (you can see it in the previous picture). The swing arm needed to then be modified to accept a bracket for the caliper boss that is used to keep it in position:











Weld it in place:





The following pictures show the assembly in place. The items that are left to do is mount the master cylinder and connect it to the brake pedal. I haven't decided on the best placement for the master cylinder yet, but have a few ideas.














I'll be adding pictures as I get farther along in the build. Let me know your thoughts and what you think.








;)
 

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Looking good! You should have plenty of stopping power now.

Is that rear disc adapter a sold hunk of aluminum that bolts in where the rear brake drum used to be mounted? Did you also put bearings in that adapter and fab a spacer to mate up with the TW 's smaller axle?

Brian
 

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Hmmmm... I still want to do the fat front tire but dont want to sacrifice the disc. Looks like you machined an adapter? I eventually want to have dot approved atv tires front and rear. :tyrannosaurus:
 

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That my friend looks verrrrrrrrrrrrrrrrrrrrrry good.. I retired from a machine shop doing things like that......... 35 years of fun......... I do miss it from time to time.......... Wait........ Not much !!!!!!!!!!!!!!! OMM.
 

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Nice work as always, Randucci. That thing's gunna be like tossing out an anchor.

FWIW I may have accidently stumbled onto some possible donors that may make things more doable for we mere "mortals with metals". Not absolutely verified but I don't have the time to persue it right now and I'm hoping you, Mekdad or someone else will run with it. If I'm right, in terms of custom parts it could involve as little as a simple, solid 3-hole disc adapter/spacer.

Unfortunately I've had ample opportunity of late to peruse a lot of scooters' disc setups in various hospitals' parking lot "bike shows". There are always a number of commuter bikes and scooters at major hospitals and to kill time I've been taking my calipers and measuring them whenever I can get away with it. In looking at the thumbnails on RisingSun's link most of the parts look like a lot of what I'm seeing on scooters. That looks a lot like the FRONT brake setup from a Zuma et al, but I could be wrong.

Quite a few scooters have discs which share our 3-bolt drum's mounting pattern.

A lot of their caliper mounts would appear as if they would engage the anchor lug inside our swingarms, as well. I can't read Japanese, but are there any details discussed? From what I've seen that whole hot mess may have come from a scooter, but it would be handy to know WHICH scooter. If it did it's just a matter of finding the right combination of stuff, probably boring out the spacer hole for our larger axles, maybe some grinding on the anchor lug in the swingarm, some spacer finnagling to get the caliper mount to line up with the disc and finding a way to actuate the m/c.

found this on a Japanese blog. This guys set up looks much simpler for a rear brake adapter he made. No cutting the swing arm.
TW???????????? - ?????????????? - Yahoo!???

That's no "adapter" on that Japanese bike. lol. He turned his drum brake upside-down and through-bolted the disc to it. Pretty scary setup, guaranteed to twist and sheer the mounting bolts under braking torque. Yamaha did the same thing on their XTTW show bike, but like a lot of concept bikes it was a non-functional styling excercise. That may be where he got the idea in the first place, as did I. Bolts of the length required for that amount of offset need to pass through something solid for their full length rather than through 2 inches of air in order for that to arrangement to be safe. Aside from that I can't find fault with it.

I would however have no issue with using that method for mockup purposes and work out all the details of the swap around it. The offset is as good a starting point as any and you gotta start somewhere. The drum would make a helluva "jig" and provide a fixed dimension. You could work everything else out right down to the last detail using that offset, then have a machinist make a more proper adapter based on the drum's offset. Done.

The adapter itself could even be triangular, rather than round, or totally skeletonized to save a ton of weight while still supporting the bolts, full-length. Or it could have two bolt patterns, permitting shorter, stronger bolts like Randucci's does. Except both patterns would be mirror images of the same 3-hole spacing.

Tubular STEEL spacers could even be welded to the interior of the brake drum to guide and stabilize the bolts and give the drum a better base if you wanted to poorboy it with less risk to your anatomy. Nuts could be welded to the backside of the hub flange to lessen the risk of oblonging the holes under brake torque. Lotta ways of doing that part of the conversion more safely.
 

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Discussion Starter #10 (Edited)
Looking good! You should have plenty of stopping power now.

Is that rear disc adapter a sold hunk of aluminum that bolts in where the rear brake drum used to be mounted? Did you also put bearings in that adapter and fab a spacer to mate up with the TW 's smaller axle?

Brian

Yes, the adapter is machined from a solid piece of 6061 aluminum round stock. The is no bearing in it, the adapter/spacer for the caliper mates up to the bearing in the wheel hub. I'll post pictures of all the parts next time I take it all apart.

Hey Russ,

It does look like he made a disc adapter.

View attachment 682

Kind of hard to believe that the three disc mounting holes lined up perfectly with the three drum mounting holes!

Brian
It would be nice to know what that caliper was from to save the machining of the slide I made and welding it to the swing arm.

.
 

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Brian, do I get a pass for being old and blind? Sorry, RisingSun. This is what happens when old folks use cellphones.

I oughta retreat at this point, but is there any indication in the text that I'm at least right about the caliper and disc donors? I've already verified that a lot of them share the same disc bolt spacing. Also found the same pattern on a lot of the Chinese stuff in the parking lots. I haven't actually measured the XT250's rear disc bolt circle but I saw one the other day and it looks ballpark, as well. My local dealer lets me do stuff like that. I'll let you know.

It's the weekend so their are fewer med school student types here. I'll try to narrow things down more carefully and list models and brand names. I've never been a scooter dude, but the majority seem to be Chinese. A lot of them appear to be the same bikes with different brand names slapped on them so they could all just be knockoffs of a particular Japanese model and would be readily identifiable to the scooter crowd.
 

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The spacing between any two bolt holes of the TW pattern is 72mm, on center. That is the number I have been going off of when measuring.

I don't know what that translates to in terms of bolt circle. Bolt circle will be a larger number. Maybe Randucci has that info and you can simply look online for generic rotors by diameter and bolt circle expressed in milimeters and find tons of possibilities. Haven't looked into it.
 

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Yeah, my fuzzy eyeballs mistook the outline of the guy's adaptor for these wear marks on the back of a FRONT drum. I'm a mess. Done too many mods to keep things straight. lol. In fact this is the front brake drum which I modified for use on the rear and that project is stiil on hold. Prolly remain so til I can swipe all this new input from Randucci and start over.

But that's exactly how they mounted the dummy disc setup on the show bike. They just flipped a rear drum over like this and used it as a spacer. Which led me to consider the possibility that Yamaha had rotors elsewhere in their product line which matched the same pattern. You guys might wanna have a good look at what they used for a caliper mount on the XTTW. That may be the case with those parts, as well.

Anyway, a REAR drum might could still be used by someone with normal eyesight and more brain cells in that manner for mockup purposes. Maybe.


Time for my nap.

 

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Randucci,

Do you need a TW with a straight rim to use as a prototype/test model. I have a TW that you can use to experiment with.... :)
 

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Discussion Starter #18
Randucci,

Do you need a TW with a straight rim to use as a prototype/test model. I have a TW that you can use to experiment with.... :)

I bought that rim sight unseen as a good useable rim, looks like the tire was pryed off it and it is pretty rusty, but it works as a holder for the disc adapter. I worked some more on it this weekend and am pretty close to having the master cylinder mounted and working. I'll take some pictures when it is completed.

.
 

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That concept looks great. I am confident when I say there are NUMEROUS members chomping at the bit to follow your lead.
 
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