Twinpot Brake upgrade on 78 Skunk

[hannibal]

So this thread is very large and I haven't read the whole thing. Is there a parts list for this install for a 78 GS1000 factory twin disc bike? Is there a kit anyone knows of? Love my Skunk, stops like ****.

https://www.thegsresources.com/_forum/showthread.php?140253-Twinpot-Brake-upgrade-on-78-Skunk&p=2236887#post2236887

[johnod]

Saltymonk makes a mount kit.

[ChrisM]

So this thread is very large and I haven't read the whole thing. Is there a parts list for this install for a 78 GS1000 factory twin disc bike? Is there a kit anyone knows of? Love my Skunk, stops like ****.

[salty_monk]

Ok, I am taking the kids camping & getting in some riding whilst away so forgive me if I don't respond till next week.

[hannibal]

Thanks! Got the calipers on the way and working on the rotors and master cylinder. I'll PM you for payment details by the end of the week.

[salty_monk]

PM sent back. No problem with that line arrangement, that's what they do OEM. Correct on the Bolts. Yes I believe those rotors work.

[hannibal]

Salty, sent you a PM.

I'm going from a single disc GS750B to the 2 piston dual disc setup. Should have some dual disc forks soon so I'll be starting like a GS750EC.

I've read through this whole thread and I have a few questions. I replaced the hoses with stainless steel when I bought the bike 3 years ago. Currently have one line from the master cylinder to the splitter under the headlight and a second line from splitter to caliper. I'm planning to add a double banjo bolt with a third line from splitter to second caliper. Any issues?

Where do the mounting bolts go? The first existing bolts goes through the upper mount of the fork and second through the adapter bracket, spacer, and caliper mounting plate? And the 40-45mm bolt with nut goes on the lower fork mount? Use blue loctite on all three?

I read that non ABS VTR750 rotors will work. Can anyone confirm? They look just like the 95-96 F3 rotors.

[salty_monk]

Looks very nice! Lets see how it bolts up!

[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]

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]

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]

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).

[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!

[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

[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?