Using a front master cylinder for dual discs with 1 disc

[mnferwerda]

Hey,
A couple of years ago I added a second front disc to my '83 GR650 (very similar to GS 650G except 2 cylinders). I did this by using a right fork from a 650G. I had to do some spacing to get the wheel, disc, and caliper to line up correctly. However I'm not getting a good fork oil seal and fluid is leaking. I've replaced the fork seal once and it still continues to leak. I'm assuming I didn't get it quite lined up correctly. So I've decided to put the original fork back on and revert back to 1 front disc. However what is the effect if I still use the same front brake master cylinder that I have on it now? I wouldn't mind if it was bit more powerful. I'm already running steel braided lines.

 

[Nessism]

Single disc GS's use a 14mm master and doubles use 5/8". A single disc system with a 5/8" bore master will have a firmer and shorter travel lever pull for the same amount of braking force compared to using a 14mm master.

 

[bitzz]

No doubt it will stop the bike, but it could be better.
For the right "feel" you want a ratio of master cylinder piston size to caliper piston size of about 27 to 1. You are no where near that.
Check this:
http://vintagebrake.com/mastercylinder.htm

 

[mnferwerda]

OK,
Does anyone know the size of the piston in the caliper? I know it is the same as the '83 GS650G. I've looked through my factory manual and I can't find that spec...

 

[mnferwerda]

bitzz,
Not sure what "nowhere near that" is based on since you didn't provide any figures. I did a rudimentary check on my piston size and got 43mm. Based on the link above, a single 43mm piston with a 14mm master gives me an approx ratio of 10:1, With my double MC: 7.7:1
In order to get the 14:1 -12:1 ratio recommend for single disc, it looks like I need to go down to 1/2" bore.

Know of any MC brake levers that have a 1/2' bore?

 

[Steve]

No doubt it will stop the bike, but it could be better.
For the right "feel" you want a ratio of master cylinder piston size to caliper piston size of about 27 to 1. You are no where near that.
Check this:
http://vintagebrake.com/mastercylinder.htm

That is not quite the whole story. :-k

I read the information in the link and also saw this:

[FONT=&quot]For 2 piston opposed calipers, I like ratios in the 27:1 range, feeling some line and caliper flex. For a firmer lever, use 23:1. I think ratios lower than 23:1 produce a lever feel so "wooden" as to have little, if any feel. Combine "low" leverage ratios with sticky pads, and unpredictable lockup is the result. The high effort required at the lever also results in undesired input to the bars. Single piston calipers are much happier in the 14:1 to 12:1 range.

[/FONT]

Your 27:1 ratio would work fine for the REAR brake, but the front calipers are of a single-piston design.

.

 

[mnferwerda]

Agree, I should have said single piston vice single disc...

 

[Nessism]

As mentioned earlier, Suzuki speced a 14mm master cylinder stock. Why question what size when Suzuki already did the engineering?

 

[mnferwerda]

Because Suzuki engineering always gets it right? I hope the engineers that did the brake design are better then their electrical engineers.

Anyways, I'm going to stick with the stock size. Thanks for all your help!

 

[GregT]

As mentioned earlier, Suzuki speced a 14mm master cylinder stock. Why question what size when Suzuki already did the engineering?

Because that was back in the 1980's....Pad materials and friction coefficients have moved along since then.

As an example, two twin-piston Brembos as used on a multitude of bikes late 80's used a 17mm mastercylinder in most applications.
What you'll find people using now is either a 15mm master for road use or a 14mm master for racing.
Times change.

 

[Nessism]

Because that was back in the 1980's....Pad materials and friction coefficients have moved along since then.

As an example, two twin-piston Brembos as used on a multitude of bikes late 80's used a 17mm mastercylinder in most applications.
What you'll find people using now is either a 15mm master for road use or a 14mm master for racing.
Times change.

Not really. The piston diameters work together and that doesn't change. For example, put a smaller master than a 14mm on a GS single disc system and the lever will feel really spongy. Who wants that?

 

[limeex2]

bitzz,
Not sure what "nowhere near that" is based on since you didn't provide any figures. I did a rudimentary check on my piston size and got 43mm. Based on the link above, a single 43mm piston with a 14mm master gives me an approx ratio of 10:1, With my double MC: 7.7:1
In order to get the 14:1 -12:1 ratio recommend for single disc, it looks like I need to go down to 1/2" bore.

Know of any MC brake levers that have a 1/2' bore?

Kawasaki EX500's have a 1/2 inch bore. It's a common mod on ZRX's. Its been argued to death, but the smaller bore gives better feel and more pressure. I will not debate it. It works. Here's a link. https://www.dsportmag.com/the-tech/...physics-and-science-of-the-motion-management/ Go to the paragraph called "Mastering the cylinder" As far as a leaky fork seal goes, it is almost impossible to put it in so crooked that it leaks. When new seals leak, it is from using "never leak" seals(use oem), scratched or pitted tubes(clean up nicely with Scotchbrite) or most common is wore out fork bushings. Bushings get worn to the point were the fork leg moves due to excess clearance allowing oil the seep past seal. In the dealership we did bushings on bikes with high miles or good looking tubes.