Front disc on rear...

[tkent02]

This on a GS 550/675, mostly 1978.

Replacing chain and sprockets to slightly alter the final drive ratio, while I'm in there noticed the brake pads are almost gone, might as well swap in a nice smooth disc while I'm at it. Looking at my stockpile of discs, I have a lot of the later slotted front discs in nice shape. They are the same diameter, same offset, same hole pattern, quite bit thinner, and less than half of the weight of the original rear disc which is off a '79 550L, as the USA '78 550 came with drum brakes.

Anyone foresee any problems with using the front disc on the rear?

Originality is not an issue, as nothing else on the whole bike is.

[Brendan W]

Piston travel, pad travel. Can they get in at the thinner disk when worn down without falling off the edge of something ?
Will the rattle springs be compatible

[850GT_Rider]

EBC lists the same rotor (MD3014RS) as a replacement for both the right front and the rear, for both your bike and mine.

When I redid the rear brakes on my bike late last year, I bought one. It is the correct thickness for the front, which make it thinner than the stock Suzuki rotor in the rear. I tried it anyway. Bedded the new pads properly and used the bike for daily commuting. After a few trips, I did notice a pulsation in the rear brake & wasn't sure of the cause, although the braking was still effective. But then within a couple hundred miles over a month or so, and fortunately at a slow speed around town and within a mile of the house, the rear brake locked up. Not to the point of non-rotation, but definitely the pads were dragging. Could smell it overheating, then rear braking went away completely (Probably boiled the fluid) Nursed it home on front brake alone. Rotor was discolored from the heat, pads were glazed. Once everything cooled off, brake test in the garage, on the center stand, and it appeared it was working. But didn't want to risk taking it out again.

Brendan's questions are legitimate, and I share them, then and now. I do not know what went wrong. Brake caliper was fully rebuilt, new seals, new pads, all was good. If the rotor thickness is OK for a front application, but below minimum for a stock rear rotor, did the pistons over-extend and cock or tilt in the bores, creating a drag and overheat early on that I didn't notice? Since I had the pulsation, did the rotor warp? With a thinner rotor, is the displacement of the master-cylinder piston correct?

I've now replaced it with a stock Suzuki rotor and new pads. Haven't ridden it since, and with all other schedule conflicts, probably won't be back on it until spring. EBC rotor is tossed over to one side for now. Haven't decided what to do with it yet.

That's my rear brake rotor story - yours may be different.

[salty_monk]

The newer slotted rear rotors are still thicker & heavier than the fronts from memory... I don't see why it should matter.

The extra rotor weight would help it shift heat & the pad travel argument is a valid one. Depends how much you use it & how low you let the pads get... If a pad dropped out it could well jam things up.

[tkent02]

The newer slotted rear rotors are still thicker & heavier than the fronts from memory... I don't see why it should matter.

The extra rotor weight would help it shift heat & the pad travel argument is a valid one. Depends how much you use it & how low you let the pads get... If a pad dropped out it could well jam things up.

The pistons come out far enough, they will clamp on the disc even with no pads. Pads can't drop out anyway, there are pins through them. I don't use the rear brake all that much and probably wouldn't ever heat it up anyway, especially since the front brake works so well.
I put it all together today, had to get shorter bolts with a shorter shoulder to tighten up on the thinner disc, one of the spare front wheels in the attic donated it's disc bolts.
Other than that all is well.
Haven't tried it out yet, didn't get done with the sprocket change... A friend came over so we went up in the moon tails for a nice chilly ride.

[850GT_Rider]

I will measure the EBC rotor again when I get the chance, but if memory serves me correctly, it measured at or below the minimum thickness allowed for the stock rear rotor. I had a question about it in my head, but ass-u-me-d that EBC knows what they're doing. Offset matches the stock rotor.

Upon disassembly, I found nothing wrong in the caliper, or in the setup of the pedal travel, especially free-play. Master cylinder was new (replacement of the master cylinder is what started this whole thing), as were all the caliper seals. The only departure from the norm is the thickness of the rotor.

In the end, I may never know why the rear brakes dragged and overheated, or even what caused the pulsations prior to that. Didn't feel like repeating the experience. Found another stock rotor in great shape, so installed that one. Will know more in the spring I guess.

EDIT: If the right front rotor will work in the rear, why did Suzuki create a different one for the rear, and why is it thicker to begin with? Just a question for the collective.....

[Brendan W]

At a guess the back isn't thicker, the fronts are thinner. Being out in the air more they can get rid of heat better and they are sharing the load.
In my experience twin disks aren't noticeably quicker stopping than the single disk version of the same bike.

[tkent02]

EBC lists the same rotor (MD3014RS) as a replacement for both the right front and the rear, for both your bike and mine.

When I redid the rear brakes on my bike late last year, I bought one. It is the correct thickness for the front, which make it thinner than the stock Suzuki rotor in the rear. I tried it anyway. Bedded the new pads properly and used the bike for daily commuting. After a few trips, I did notice a pulsation in the rear brake & wasn't sure of the cause, although the braking was still effective. But then within a couple hundred miles over a month or so, and fortunately at a slow speed around town and within a mile of the house, the rear brake locked up. Not to the point of non-rotation, but definitely the pads were dragging. Could smell it overheating, then rear braking went away completely (Probably boiled the fluid) Nursed it home on front brake alone. Rotor was discolored from the heat, pads were glazed. Once everything cooled off, brake test in the garage, on the center stand, and it appeared it was working. But didn't want to risk taking it out again.

Brendan's questions are legitimate, and I share them, then and now. I do not know what went wrong. Brake caliper was fully rebuilt, new seals, new pads, all was good. If the rotor thickness is OK for a front application, but below minimum for a stock rear rotor, did the pistons over-extend and cock or tilt in the bores, creating a drag and overheat early on that I didn't notice? Since I had the pulsation, did the rotor warp? With a thinner rotor, is the displacement of the master-cylinder piston correct?

I've now replaced it with a stock Suzuki rotor and new pads. Haven't ridden it since, and with all other schedule conflicts, probably won't be back on it until spring. EBC rotor is tossed over to one side for now. Haven't decided what to do with it yet.

That's my rear brake rotor story - yours may be different.

Interesting, I'd like to know why this happened. I will definitely be checking for heat when I start riding it.
Thanks for the story.

[tkent02]

At a guess the back isn't thicker, the fronts are thinner. Being out in the air more they can get rid of heat better and they are sharing the load.
In my experience twin disks aren't noticeably quicker stopping than the single disk version of the same bike.

With perfect brakes and strong fingers, both will stop at the limit of what the tires can do.
It's more about easy stopping than quicker stopping. Light pressure with one or two fingers is a lot easier to control than pulling as hard as you can with all four.
I also have some newer bikes with excellent brakes, having one or two old clunkers which require twenty times the lever effort to slow the bike doesn't work well for me.

[Brendan W]

The line I got sold about twin front was more to do with fork twist than anything else.
Like an over engineered 70hp road bike is going to do that at my chicken speeds.
Am I dreaming or do I recall Honda being successfully sued in California in the 80s for having front brakes that were too powerful and for a while some 750/900models went back to single fronts only.

[tkent02]

The line I got sold about twin front was more to do with fork twist than anything else.
Like an over engineered 70hp road bike is going to do that at my chicken speeds.
Am I dreaming or do I recall Honda being successfully sued in California in the 80s for having front brakes that were too powerful and for a while some 750/900models went back to single fronts only.

Fork twist is BS, at least on these bikes. Like you said, it's all over engineered.

[tkent02]

Well it seems to work, got hot when I used the brakes, stayed cool when I didn't. Went out on an empty highway and heated it up good, braked fairly hard a few times to bed the pads a little. Once that was done it seemed to slow the bike a tiny bit better than the old brakes. After that I rode it several miles without using the rear brake, the disc cooled off again. I will definitely keep an eye on it but for now it seems fine.

[850GT_Rider]

Well it seems to work, got hot when I used the brakes, stayed cool when I didn't. Went out on an empty highway and heated it up good, braked fairly hard a few times to bed the pads a little. Once that was done it seemed to slow the bike a tiny bit better than the old brakes. After that I rode it several miles without using the rear brake, the disc cooled off again. I will definitely keep an eye on it but for now it seems fine.

Glad to hear it's going ok!

[postman_pat]

...Once that was done it seemed to slow the bike a tiny bit better than the old brakes.....

Actually, I noticed that too. I recently replaced the rear brake disc on my GS750 with the front right disc from a 1983 GS(X)550 - just as an experiment. Even though the disc has a smaller diameter, it seems to have better "feel" and performance than the original. I've done about 3000km (1800 miles) on this setup - no problems to speak of.

[tkent02]

Huh, the '83 550 has a smaller disc than the '78 rear disc? Mine was the same diameter. Took a longer faster ride last night, still working fine.