Dual front disc/rotor question

[tundratiger67]

I apologize ahead of time if this topic has been covered. I did a search and couldn't find an answer to my specific question.

I'm rebuilding the front end of a 81 Spec II and wanted to install a left side caliper and rotor. I picked up a couple of new MikesXS calipers and a pair of the narrower slotted rotors from eBay. Yesterday I installed the right side caliper and it appears the rotor-to-pad space on the inside is too wide. I assume this is attributed to the narrower than stock rotor. If so, do I need to add a shim or two to the inside pad to reduce the space? If so, will the shims available through MikesXS work? They look designed for the earlier round calipers and after a little searching it appears the later shims are obsolete and NLA.

Thanks for any help!

 

[scabber]

Welcome to the site. The slotted rotors are not from a 650. A picture of the rotor and the caliper and the space you are calling too wide would help. If the rotor is the same outside diameter and the caliper has a space on both sides of the rotor to allow the brake pads to make contact before the caliper hits the rotor. You will not have to use any spacers. It sounds like you have enough space.

 

[grizld1]

Wrong. For best performance when a thinner rotor is used with a single piston floating caliper, the caliper should be shimmed at the mounting points on the fork sliders so that there's just enough clearance between rotor and inside (non-driven) pad to allow the wheel to spin freely. This is particularly important if floating or semi-floating rotors are used; if there's too much of a gap between the non-driven pad and the rotor, the driven pad will stress the rotor buttons before the non-driven pad makes contact and loosen them past the limit of play.

 

[scabber]

The slotted Yamaha rotors are slightly thinner than the non slotted. I would think the floating caliper would self adjust the gap on the non-driven pad. If it did not then you would have to re-shim when the pad wears. If the wheel spins freely now it should be ok my opinion. I have a floating rotor that has a different offset from stock. Spacers were required to make up difference.

 

[grizld1]

You may think the caliper would self-adjust, but you'd be wrong. I used a thin slotted non-floating rotor for a few years. It took awhile to figure out that the "clunk" heard when giving the lever a hard squeeze was caused by the non-driven pad smacking the rotor; due to the gap, the floating caliper built up momentum before contact.

 

[tundratiger67]

Thank you for the interesting dialogue about the set up of the caliper with the slimmer rotors.

Grizld1, you say "the caliper should be shimmed at the mounting points on the fork sliders" but if I were to do that, in my way of thinking, this would increase the space on the non-driven side (moving the caliper further inside) and exacerbate the problem.

 

[5twins]

Yes, it would. You would need to do some filing on the caliper or fork leg but I wouldn't do that just yet. I run a 5mm thick SR500 disc with no shimming, filing, whatever. It's been on there for several years and I have no problems with it. I did some research and measuring before mounting it just so I wouldn't encounter any problems like the ones you think you may have. I compared the part numbers for the SR500 and XS650 fork lowers and found them to be the same. From that, I assumed the caliper mounts should be spaced in-out the same. I measured the offset of both discs. From the mounting surface to the centerline of the disc thickness, they are the same. It seems that to make the 5mm (2mm thinner than the 650) SR disc, Yamaha removed 1mm from the front and back. That left the offset the same.

I would check the offset on your slotted rotors but I'm thinking it's the same, done in the same manner as the SR disc. If that's the case then the caliper should just self-center itself once in use. Those slotted rotors are a pretty common upgrade and I've never heard of anyone encountering problems or shimming them. They just bolt them on and go.

 

[grizld1]

Right, sorry for the goof re. spacers; I had the drill for floating rotors (different dish) in mind. IIRC the disc I was using was from a Virago--no way to be sure, it came from an unmarked stash of used discs at the local indy shop. The caliper didn't self-center; a hard, fast pull would produce an audible "clunk" when the driven pad yanked the inside pad into contact. I ran it that way for awhile, then installed a floating disc and never looked back. The issue wasn't a caliper defect--it's still in use. Anyway, I'm glad to hear that the problem hasn't affected everyone who's swapped out the disc. I agree with 5twins re. filing--not a good solution.

 

[XSLeo]

I have a pair of the 5 mm slotted discs on the front of my 75. On the right side the disc centers up in thn caliper just fine. As you pull the lever the pads will move out to contact the disc. now when you release the lever the piston pulss back just a bit. The seal around the piston flexes as the piston slides through it when released the stretch of the seal is what pulls the piston back. If you hear a clunk everytime you use the brakes I would check the cliper.
Now on the left side the caliper may not be as well centered. If off by a mm or so loosen the cap nuts on the bottom of the for and move the fork in or out a bit to center up the disc in the caliper. Tighten the cap nuts back up.
I checked a lot of part numbers on bikes. The ones that use the 7 mm disc use the same caliper as the 5 mm disc. No need to shim anything. All bolt right up.
Those shims you see on Mike's and used to come with the brake pads are not to shim the pads for thinner disc but to help prevent rattles and squealing.
Leo

 

[XSLeo]

Just reread your first post. If the disc isn't centered in the caliper. It sounds like you might have not tightened the axle correctly.
When you install the wheel you need to tighten the axle nut properly first, this pulls the hub tight to the fork, this sets the caliper to disc clearance. then tighten the cap nuts on the left.
If the hub and fork are both right the caliper will be centered over the disc. If not the hub or fork might be worn. A while back someone wrote about this. Apparently some where along the line the axle was not properly tightend and as the wheel turned it wore the side of the fork. This let the axle pull the wheel to far and the disc hit the caliper.
Pull your wheel and check the right fork for wear. If it looks like it is roughed up, then yours may be worn too. A thin washer between the wheel and fork to compensate for the wear on the fork will set the caliper back to center.
Leo

 

[tundratiger67]

Thanks XSLeo, 5twins, grizld1, et al, for the responses.

I guess it was mostly a case of "newbie" anxiety. Having looked it over again, it appears to line up well on the right side and, thus, should self-adjust after use. I had some rubbing of the rotor and caliper on the lower end but a little filing of the caliper casting cleared this up. The left side is another story but, as XSLeo pointed out, I have to do a better job centering the rotor by adjusting the fork alignment on the axle. If I can move it in a couple mm's I should be fine. I'm relieved to know that shimming isn't required.

Randy

 

[Maxx650]

5twins, do you run SR500 discs front and back?

 

[5twins]

I have a Standard model so only have a front disc, drum in the rear. I don't know if the SR rear disc is a direct swap with the 650 rear, I've never looked into it. Honestly, I don't know if I'm sold on this 5mm disc swap anyway. It's supposed to be lighter and hence, maybe improve handling slightly, but I can't say I notice. My front tire is about done so I will be swapping my spare wheel in shortly which has an original 7mm thick 650 disc on it. It will be interesting to see what difference (if any) it makes. One thing I do recall after switching to the 5mm disc was that it seemed to heat up a bit more.

 

[Maxx650]

Thanks for the info! Let us know if you figure out what your preference is after swapping back in your spare wheel.

 

[Royboy]

Rear rotors on XS650 and 78-79 SR500 are identical except for the 2mm difference in thickness.

roy

 

[scabber]

The slotted rotor I run on the rear brake is a front brake rotor off a XS750 dual disc. Not Shure about the SR 500.