Twinpot Brake upgrade on 78 Skunk

[salty_monk]

One back at ya... Those Disks & Zephyr Calipers look like the right ones.

I have brackets on the shelf. Powdercoated black as normal. Henry Ford Model!

[swampig]

cool! can ya send me a set please? i'll message you!

[salty_monk]

Yep, one back today. I should be able to ship them out tomorrow if all goes to plan

[Adi]

Hi Dan
The brake bracket kit you sent through is excellent, thank you so much. I intend to use it on a twin disk front end with 296mm CBR disks etc as advised, but I have been playing around with single disk set up on one of my GS750’s. I picked up an Aprilia Tuono 320mm disk and drilled out the disk holes to 9mm to make it fit, though considering remaKing the holes in between the original holes with your spacer as 8mm holes in exactly the right position seem a better idea!

The sticking point at the moment is the disk offset as it is only about 8mm on the Aprilia 320mm disk and, as I measured it, about 21mm on the original disk, but I am thinking I can double up the spacers behind the disk and replace the original bolts for slightly longer ones to compensate.

Would this seem plausible?

I added an bandit 600 master cylinder with stainless hoses and it all seems far more effective and significantly lighter.

This got me thinking about the rear caliper and particularly the weight of it and the fact that it is way over powered.

Does anyone have any suggestions regarding rear brake replacements / upgrades? I have put this out as a separate thread question but thought I would ask here!

Regards
Adi

[Chuck78]

This got me thinking about the rear caliper and particularly the weight of it and the fact that it is way over powered.

Does anyone have any suggestions regarding rear brake replacements / upgrades?

275mm katana front right swap to GS rear:

https://www.ebay.com/itm/263409313238

The holes are 10 mm or so to accommodate the special very large shoulder bolts originally on that year of katana. I just drill a new hole pattern with 8 millimeter holes in between those holes, but I suppose you could figure out a way to use the katana shoulder bolts if the thread size is the same, which I believe it is. Spacers would need to be fabricated though, so you may as well fabricate a top hat spacer to reduce the size of the hole to 8mm, or else just redrill.

The reason your rear "caliper" is so overpowering is because 1977 GS 750 use a single disc 295 mm rotor up front, and also a single disc 295mm rotor in the rear, same rotor... it is the rotor diameter, NOT the caliper, which causes the overpowering rear brake...
In 1978, the Suzuki engineers switched to a 275 mm rear rotor. This was not overpowering, but the rotor was still very heavy as all of that day are. This late 80's/early 90's Katana rotor is the same diameter, same deep 23mm offset, but has floating rivets, and is SIGNIFICANTLY lighter. You can then use a rear disc caliper hanger off of a 1978 or newer GS with 275 mm rotors to upgrade a 77 monster rear brake to a reasonable and useable size, and then bolt a slightly lighter gs650 caliper to it if you wish (not a significant difference in caliper, but DEFINITELY in rotor weight).

I have been playing around with single disk set up on one of my GS750’s. I picked up an Aprilia Tuono 320mm disk and drilled out the disk holes to 9mm to make it fit, though considering remaKing the holes in between the original holes with your spacer as 8mm holes in exactly the right position seem a better idea!

The sticking point at the moment is the disk offset as it is only about 8mm..., but about 21mm on the original disk, but I am thinking I can double up the spacers behind the disk and replace the original bolts for slightly longer ones to compensate.

Would this seem plausible?

Also, 320 mm rotors do not really work with the bracket configuration for calipers that are used for this modification, You won't be utilizing the outer 5mm of the disc friction surface and that area will be extremely close to rubbing the caliper or caliper pins.
BUT...a 310 mm rotor does work for the GS single-disc forks from bikes that used a 295 mm factory disc. I would abandon your 320 mm plans and just find a Honda 310mm rotor. Honda CB1 (20mm offset?), Honda CBR1100XX Blackbird for a couple years in the 90's (17.5mm offset), and 98-99 CBR900RR (23mm offset). All will need the holes very precisely aligned and drilled in a drill press from 6mm to 8mm. This will work pretty darn well.

[Adi]

Hey Chuck78, really appreciate the advise and I definitely think losing the rear disk for weight alone would be a huge benefit and worth the effort but if I can also get a more mellow rear brake by doing this and avoiding swapping out the caliper that is a bonus. I will post some feedback on results once I have finished making a mess.
I did consider the 310mm Blackbird disk but I was trying to max out the front brake strength with the largest disk possible, you live and learn I guess.
Thank you again for the guidance.
A.

[limeex2]

I used a 84-86 GS550E rear brake disc. They bolt right up. I did use different caliper and spacers though. The bolt pattern was the same, not sure about offset.

[johnod]

275mm katana front right swap to GS rear:

https://www.ebay.com/itm/263409313238

The holes are 10 mm or so to accommodate the special very large shoulder bolts originally on that year of katana. I just drill a new hole pattern with 8 millimeter holes in between those holes, but I suppose you could figure out a way to use the katana shoulder bolts if the thread size is the same, which I believe it is. Spacers would need to be fabricated though, so you may as well fabricate a top hat spacer to reduce the size of the hole to 8mm, or else just redrill.

The reason your rear "caliper" is so overpowering is because 1977 GS 750 use a single disc 295 mm rotor up front, and also a single disc 295mm rotor in the rear, same rotor... it is the rotor diameter, NOT the caliper, which causes the overpowering rear brake...
In 1978, the Suzuki engineers switched to a 275 mm rear rotor. This was not overpowering, but the rotor was still very heavy as all of that day are. This late 80's/early 90's Katana rotor is the same diameter, same deep 23mm offset, but has floating rivets, and is SIGNIFICANTLY lighter. You can then use a rear disc caliper hanger off of a 1978 or newer GS with 275 mm rotors to upgrade a 77 monster rear brake to a reasonable and useable size, and then bolt a slightly lighter gs650 caliper to it if you wish (not a significant difference in caliper, but DEFINITELY in rotor weight).




Also, 320 mm rotors do not really work with the bracket configuration for calipers that are used for this modification, You won't be utilizing the outer 5mm of the disc friction surface and that area will be extremely close to rubbing the caliper or caliper pins.
BUT...a 310 mm rotor does work for the GS single-disc forks from bikes that used a 295 mm factory disc. I would abandon your 320 mm plans and just find a Honda 310mm rotor. Honda CB1 (20mm offset?), Honda CBR1100XX Blackbird for a couple years in the 90's (17.5mm offset), and 98-99 CBR900RR (23mm offset). All will need the holes very precisely aligned and drilled in a drill press from 6mm to 8mm. This will work pretty darn well.

Why or how does the rotor dia. produce over powering braking?

[Chuck78]

Why or how does the rotor dia. produce over powering braking?

Bigger diameter rear rotor = more mechanical leverage against the forward motion of the bike.
Due to immediate weight shift forward upon brake application, the rear brake is only about 30% of a vehicle's stopping power, as once the brake is applied, all of the weight shifts off of the rear and onto the front, therefore a very large rear rotor will tend to have little modulation in braking force application, and go from no braking to locked up rear wheel very immediately.

To make this even more noticeable, all of the rear brakes on bikes like ours are opposed piston calipers, to do pistons acting on the same pad area, whereas the front brakes on our bikes are single piston sliding calipers, so the rear definitely can overpower everything very easily. 275 mm is the absolute maximum, but 1977 GS 750 used a 295 mm rear rotor and opposed piston caliper. This ended up with a bike that had subpar front braking and very touchy rear brake that was almost unusable and could end up in the rear end sliding out if not careful. And also being a single disc front without a fork brace or significant bracing in the fender, the front fork would twist the axle and twist the whole fork lower that the caliper is attached to. A fork brace is a very nice addition for braking also!

What diameter is that GS550ES rear rotor?




Also FYI these "twinpot" calipers are such a big big braking upgrade because they have two smaller pistons that are stacked one in front of the other instead of one large piston, as one large piston would be taller, and half of the large piston would be clamping down on a significantly smaller diameter area of the rotor, much closer to the axle, which is an area of less mechanical leverage. Two pistons one in front of the other acting on the very outer edge of the rotor will be grabbing the most leverage against the forward motion of the bike.
These are a 2 piston ("twin pot") sliding caliper. The caliper body slides onto caliper pins, so that when the brake is applied, the piston pushes the pads against the rotor, and then pulls the other side of the caliper over until those pads contact the other side of the rotor. On an opposed piston caliper, pistons come in from both sides from a fixed position caliper that does not move aside from flexing, so when the brakes are released, the pistons just retract slightly and the caliper remains stationary

[Chuck78]

Floating rotor maintenance?

One question for you all, has anyone experienced any sort of warped brake rotor type of feeling with these floating rotors? I know that the floating rivet buttons should be free and able to spin if you try and rotate them with some force with your fingers, and I know that it has been stated that you do need to clean them periodically. I get a pulsating lever feel and a shaking up front upon brake application, and I was not sure if I need to be looking for new rotors or if this is a common thing if you go many years without cleaning the floating rivet buttons. I do grab a big handful of brakes sometimes!

[salty_monk]

Sorry will try to answer a bunch of points....

Blackbird 310mm rotor - It works with the brackets I have but the caliper overhangs the edge by about 5mm so you have to chamfer the back of the pad (friction material) to stop it wearing an edge. Reportedly it works very well though. A few have done it.

320mm Rotor - Should be the perfect size but I haven't been able (despite multiple ebay purchases on possibles) to find something with enough offset in that size. The problem with that rotor is that spacing it out would get in the way of the speedo drive so I guess the answer is that you could make it work if you moved to an electronic type aftermarket speedo.

The twinpiston thing to google is "swept volume" if you want to understand why longer pads & more pistons give more power. Rotor size gives more leverage for a given caliper "power".

For pulsing - check the back of you rotor, any spacer etc are flat (wet n dry on a piece of glass). Other than that it could be related to the rotor, either they are warped or not moving right. The floating function doesn't open up until they have some heat in them in my experience - I've only ever had one set you could spin by hand & they rattled like crazy (a tinkling sound at low speed).

[jabcb]

Sorry will try to answer a bunch of points....

Blackbird 310mm rotor - It works with the brackets I have but the caliper overhangs the edge by about 5mm so you have to chamfer the back of the pad (friction material) to stop it wearing an edge. Reportedly it works very well though. A few have done it.

320mm Rotor - Should be the perfect size but I haven't been able (despite multiple ebay purchases on possibles) to find something with enough offset in that size. The problem with that rotor is that spacing it out would get in the way of the speedo drive so I guess the answer is that you could make it work if you moved to an electronic type aftermarket speedo.

The twinpiston thing to google is "swept volume" if you want to understand why longer pads & more pistons give more power. Rotor size gives more leverage for a given caliper "power".

For pulsing - check the back of you rotor, any spacer etc are flat (wet n dry on a piece of glass). Other than that it could be related to the rotor, either they are warped or not moving right. The floating function doesn't open up until they have some heat in them in my experience - I've only ever had one set you could spin by hand & they rattled like crazy (a tinkling sound at low speed).

A while ago I bought an EBC MD1124LS, which is 320mm. Its for a T500 cafe build that will have a GT500 triple tree, 81 GS550T forks & your twin-piston brake upgrade kit.
Am waiting to get the frame back from the powder coater. I have the rotor but don’t have the triple tree, so I can’t bolt things together to check the offset.


Are you now of the opinion that this combo won’t work?


More info is in this old topic: https://www.thegsresources.com/_foru...light=md1124ls

[salty_monk]

No that's about the best one I've seen but I can't find the offset numbers for it either. Also very hard to get hold of so I've not tried one.

If you put the rotor face down on a flat plate what does it measure to the back side through the centre hole?

[jabcb]

Looks like an excellent match.
The MD1124LS compared to a GS rear rotor that we used to have on the front of a GT250. That GT250 now has an EBC VMD rotor & your twin-piston adapter kit.
cafe-T500-0836 by jabcb, on Flickr


Don’t know what variations EBC has. The center section of my rotor is gold & nicely matches the gold Suzuki winged “S” badge from about 1970.
cafe-T500-0808 by jabcb, on Flickr

[salty_monk]

Looks very nice! Lets see how it bolts up!