dual disc set up

Bumping this year old thread with my experience in adding dual disks to my SR. I bought the two slotted disks and a caliper (from an XS650 I presume) as a "kit" for $90 on eBay. The bike already stopped better than any bike I've ridden, and I know the purists will point out that the extra unsprung weight will cancel out any stopping improvement with handling penalties.

How this mod came about, and the results (consequences?) of doing it may be helpful for those considering it. It would have to be the easiest and cheapest way to add dual disks and more stopping power to the SR500.

The wife and I were drinking a six pack in the back yard after a quick trip on the SR to the local pub. The bike was gleaming in the late afternoon sunlight as we both were admiring it with beers in hand. Debi noticed the mounting bosses on the right fork and asked "what's supposed to go there, I don't like those holes". Sensing an opening here, I explained the reason for the "holes" and popped the top on another beer for her.

Debi always seems to get her best ideas when she's had a few beers, this is the wife who, after doing a TT tour (Tavern to Tavern) of the biker hang outs, and seeing all the Harleys (they were ALL Harleys) said to me: "Doug, you've got to get a BAD ASS Harley". Most guys would be happy to hear that from their wives, but the ensuing conversation reveled that she thought the SR was "puny". I've since been able to demonstrate to her that the puny Yamaha could run away and hide from those hogs as soon as the first bend in the road appears, but that's another story...

Anyway, three days later the "kit" arrived and even after the beers had long wore off she was pleased that I was able to find something that filled those unsightly holes in my right fork. She was off to play bingo (we are senior citizens) and I was jacking my bike up on my newly acquired motorcycle jack. (a new craftsman model bought from my neighbor for $50).

The kit didn't supply the disk mounting bolts, but my parts bike had them, and a quick wire brushing made them look pretty good so I was in the game. It's pretty hard to manage the weight of the wheel and disks while keeping the speedometer gear in place and guiding the disks into the calipers while fitting the axle bolt, but I managed, and all was in place by the time Debi got home from bingo.

The other thing the kit didn't provide was a dual banjo bolt, so I just tied the brake hose up with a plastic tie and went out for a spin. One of the flaws of doing this mod is of course the extra weight of the disk and caliper, and some here have suggested that it can contribute to head shake because of the two calipers slung behind the forks.

I can confirm this because on the first ride with the new setup I inadvertently hit a significant bump while adjusting the bar end mirror and got a scary head shake that while not quite a tank slapper, was still enough to get my attention. This might have happened without the mod (it was that big of a bump) but I don't think it would have been so severe.

I have not experienced any further head shake but the extra weight does seem to smooth out the little "hop" the bike used to do over highway expansion joints. Subjectively (placebo effect?) I think the bike rides better now.

Another issue brought up here is whether the stock master cylinder can handle the extra brake and lines with the greater volume of brake fluid to move. After finally getting the dual banjo bolt needed to hook the thing up I was concerned about this issue (and was ready to buy a master cylinder from a XS750 or 1100).

The stock brake had a very good feel and the new setup is not as firm as the original but it does work, and work well. Although there is more lever travel to pump all that extra brake fluid, the thing feels like it has power brakes. One finger is all it takes to throw out the anchors, two fingers can skid the wheel.

Right after getting it all hooked up I put it through some serious stopping tests, trying to do a "stoppie" (reverse wheelie) I found that front tire adhesion fails before the back wheel leaves the ground. Has anyone here got their SR to lift the back wheel with braking? In two up riding I can instantly fully compress the forks under any serious braking. I think I'll need to work on that issue. Suggestions here would be appreciated.

A bigger issue is that after all the fun of repeated panic stops, both solo and with a passenger my engine cylinder has started to weep oil at the base! I'm hoping I can cure this by re-torquing the head bolts. Do you guys thing that removing the top engine stay is a good idea? It will allow more frame flex, but I can deal with that. What I can't deal with is frequently replacing my cylinder base gasket.

Bottom line here for me is that I like this addition to my prized SR. It seems to ride better over bumpy pavement, the head shake has not happened again, even after repeated trips over the same bump (two handed) that caused it's first and only (so far) occurrence.

Braking power is certainly there with this setup and could be overkill if you slide the front wheel at the wrong time. The power to lock it up is something to be cautious of.

Lastly it does fill the unsightly holes in the right fork and adds to the "bad ass" look of this little bike.

Heh,heh...well written.

You have noted pretty well all the things that change on the SR when you do this mod. The brakes are much stronger for sure and you can regulate that with your right hand so that's no real issue. The brakes end up far stronger than the one disc...from memory they use the same size discs etc. (different thickness) to bring the XS1100 down to a halt!

The 'feel' of the master cylinder is another thing that is a personal thing. I rode one for a fair while with the standard master cylinder...it felt like a clutch lever at standstill...but worked fine on the road. It feels much more 'normal' with a twin-disc mastercylinder though...as for brake hoses I have run both one into two and two lines all the way to the calipers...take your pick. I would recommend braided SS lines though. That will of course make the brakes even stonger...that applies to the single disc as well.

It does make the SR's front dive much harder as it immediately exposes the fact that the fork springs are fairly soft. That's a matter of personal preference, but if you do make it a little firmer, and I'd say with two up riding that you probably should anyway, you also need to do the rear suspension as well. They should be evenly matched front and back. Perhaps Ikon shocks or similar. They give you an adjustable damping ability as well and they aren't real expensive. There are several ways to go with the fork legs and there are a few posts on that topic here. Even heavier fork oil may assist a bit...

In the right (wrong) situations the mod will exacerbate the front end tank slappers for sure; as will running a heavier drum type brake such as a Suzy 750 4 leader, etc...I've had them lock to lock a large number of times over the years...one time is solidly etched in my brain!...a steering damper will asssist here. Harder fork springs with no change in damping will probably get it going as well...

Personally I'd leave the head steady there...it's up to you. I know that Wizz954 has now gone to great pains to put it back again for racing...he may have a different slant on it for you.

This is where I'm going with the current cafe racer I'm building...this is still being set-up for the caliper bracket position to stop the wider caliper meshing with the front spokes...not a good thing...(although two zip ties would be cheap...heh, heh):

autos.ph.groups.yahoo.com/group/YamahaSR500CafeRacers/photos/view/d5d5?b=27&m=f&o=0

I had considered twin discs with dual 4 piston calipers...from an R6...and custom made discs...but that would probably be too much. Right now I think that I'll stay with the one 330mm disc and one R6 caliper...may change when it eventually gets on the road.

SR

Removing the top engine mount will cause you more problems than it solves. I ran my bike without because that's how it was when I bought it. The extra vibration is the main problem, causing all other things to break due to the extra stress and you'll find the handlebars really hard to keep hold of due to the vibration, it's that bad!
My advice is to keep the top engine mount and fix the base gasket leak properly. I had an oil leak that was caused by a missing O ring and a poor quality after market paper base gasket. All head bolts were torqued up and even under racing conditions, I couldn't find any evidence of stretching of the head studs and consequential loss of torque on the head bolts. Yamaha now spec a metal base gasket which should stop leaks. Also make sure the O ring under the base of the cylinder is there and in good condition.
I've heard someone say before that the top frame tube flex causes the base gasket leak, but I can't see it myself, there are so many bikes out there using the engine as a stressed member.

Raising compression with thinner head gaskets, now you're talking my language!
While you are replacing the base gasket, this is an ideal time to do something. I tried a thinner copper head gasket and ended up running 2 of them because of clearance problems between the valves and piston. I have a high compression piston and a mildish camshaft.
Not an easy one to answer, as everyone is running different pistons, cams, gaskets. To get it right, you really need to build the engine carefully and use plasticine to measure your valve to piston clearances when fitting the gaskets. Get it wrong and you will end up with bent valves, not nice.
Even without using copper gaskets, you might find the new base gasket thinner than the one you are currently using and this will effectively raise your compression too.
Roll on the winter, strip it down and work out a plan!

I have a brand new genuine Yamaha head gasket in my box of spares and the uncompressed thickness of the head gasket is 1.06mm. I don't know what the compressed thickness would be.
The copper head gaskets I am using are 0.56mm thick but I'm using 2 of them, so together they're thicker than the Yamaha gasket.
This is probably because I have a high compression piston and using the new metal base gasket and higher lift cam.
If you're running standard Yamaha piston and cam, you can probably make a calculated guess and get away with it.

Hi,

I prefer to fit a HC piston and leave the surface of the head alone as much as possible...you can always undo what you've done in that case...except for an increase in bore. In all cases you'd need to check the clearances, valve to piston, piston the head etc. using plasticene or similar just to make sure.

I have always meant to play around with the shape of the 'squish' between the head and the piston...but so far haven't bothered, read a bit about it though...and twin plugs as well...all in the aid of more efficient combustion.

Perhaps I need a Daytona head...heh, heh...

SR