upgrading 77 GS750 to dual discs - will GS650G forks & brakes work??

[Chuck78]

I am looking to upgrade the single disc front brake on my 1977 GS750 to duals, and found a GS650G in the junkyard that has the dual disc setup and drilled rotors. I believe from reading on here that they both share 35mm fork tubes, but that the 650 is about 3/8" shorter height. I was thinking maybe I could swap just the lowers????

I also read just one mention of the wire wheel rotors possibly being different from the mag wheel rotors (spacing/offset?). I will be wanting to swap the mag wheel GS650G drilled rotors onto my 750 spoked wheels, is this possible or are there offset spacing issues?

Are the hub width spacings or triple tree widths different to accomodate the dual discs? I was hoping to grab just the lowers and the entire brake setup, calipers, rotors, levers, hoses, and maybe rear rotor if the same diameter and spacing as my 750's.

My bike may get some serious horsepower upgrades to the tune of 920cc, so more brakes are definitely very very good. Will a GS750 or GS1000 dual disc have larger diameter rotors??? The junkyard may very well have some 750/1000/1100 forks on the shelves if I ask, but I will already be tearing the top half of the engine off of the shafty GS650G to make a 674cc 9.4:1 GS550 kickstart chain drive engine, fun winter projects!

 

[salty_monk]

I *think* you should be fine. Take the fork legs, discs & calipers & bolt it on. Might not hurt to measure the triple trees first just to make sure you're not going to have any offset issues.

I wouldn't worry about the 3/8". You can probably lose 3/16" - 1/4" of that by running the fork leg low in the triples. The rest might just quicken the steering a bit (quite a few of us run longer rear shocks to achieve the same thing so I wouldn't worry about it too much!)

What's the reason for changing? If it's power you are after then I can tell you that you won't notice that much difference. The single rotor is bigger in diameter than the dual rotors & so has a bigger leverage ratio which means the twin setup isn't "double".

To really get power you could fit the Twinpot Kawasaki calipers to the 650's fork legs. Lots of us doing that...

 

[Chuck78]

That's some food for thought... Well, either way then I will need fork legs with dual disc mounts. I'll look into your Twinpot mods, I recall reading your username quite often in reference to the twinpot mod... Maybe I will research that more before I drop a chunk of change on the 650's brake setup.

 

[GSScoobie]

More friction material with dual discs equals much more braking effectiveness.
Brake sweep area is doubled.

 

[tkent02]

Not doubled, like Monk said, the discs are smaller. I bet it is about 75% to 80% more area.

They do stop better. A single brake if everything is perfect can brake adequately, maybe not one finger braking but at least two.

 

[salty_monk]

That's some food for thought... Well, either way then I will need fork legs with dual disc mounts. I'll look into your Twinpot mods, I recall reading your username quite often in reference to the twinpot mod... Maybe I will research that more before I drop a chunk of change on the 650's brake setup.

Yes. I had a 1000G (rebuilt setup with SS brake lines) that I used to ride 2 up with my wife. Neither of us are big but on the steep gradients around here braking was marginal at best.
Another guy on here did some research & I basically finished it off to a "polished" solution.

You'll still need the 650's fork legs...

I have brackets if you did want some (actually I also have a full set of parts as well).

No pressure, I just wouldn't like to see you go through all that to not get the result you're looking for.

 

[Chuck78]

Yes I am very interested in your brackets and interested to hear whatever parts you have. Please send me an email or pm, as I don't have the tappatalk app for my phone and my phone's browser won't let me pm you for some reason

 

[salty_monk]

PM sent

 

[Chuck78]

Another big advantage to dual discs is that you have a much greater rotor & pad surface area, which will give you much better cooling of the brake surfaces, which in turn will give you far less brake fade under hard continuous braking, such as on the race track or riding hard through the hills in the twisties. Brake fade is when your pads and rotors get hot enough to diminish breaking performance substantially

 

[Chuck78]

Can I just use the GS650 rotors with the Kawi Ninja twinpot calipers? Or is there some spacing issue? salty_monk, I saw in another one of your posts that you had GS550 rotors on the "Skunk" GS1000, but noted that they are much heavier. Same diameter as the CBR F3's? I am just researching your posts now to get the lowdown on this mod. May see how much they want for the OEM GS 650 brakes & base my decision on that.

Thanks for the help, got your PM, I'll be getting back to you on that asap!

 

[tkent02]

The 650 brakes are probably the best of the old GSes, they are the larger dual disc brakes as used on the later model 1100 and 850, on the smaller and lighter 650 they are really adequate. My 650G stops very well indeed.

SaltyMonk's swap is far better. I have that on my 1100G, it is excellent.

 

[salty_monk]

You Can use 550 Rotors.
They are the same size as the CBR ones but about twice the weight of the lightest CBR ones.
The offset is different so you don't need a spacer but they don't "centre" on the calipers 100%.
The rotor material is not 100% compatible with the stock Tokico / Kawi pad compound, it chews them up fairly quick but then they are cheap to replace (and braking performance is not affected).
I suspect the drilled rotors of the CBR add a performance enhancement in the rain (doesn't rain much here)...

Not sure what else I can tell you really