Author Topic: Twin disc conversation master cylinder bore?  (Read 2421 times)

Offline Colonial-Clive (yindi)

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Re: Twin disc conversation master cylinder bore?
« Reply #15 on: September 02, 2018, 06:17:29 PM »
You can get a hose that attaches to the master without using a banjo bolt!

Offline SpannerBrat

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Re: Twin disc conversation master cylinder bore?
« Reply #16 on: September 02, 2018, 11:06:30 PM »
It is the (cast?) main body of the master cylinder unit that’s fouling against the bars so eliminating the banjo bolt arrangement wouldn’t make any difference (but it’s worth knowing for future reference). It seems that the design of the radial brake master cylinder & lever would clear the machines parts of the clip-ons that i’m having issues with with the std unit. Has anyone any experience of any that give good feel / performance?

Offline SpannerBrat

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Re: Twin disc conversation master cylinder bore?
« Reply #17 on: September 03, 2018, 07:19:21 AM »
The clip-ons are 22mm dia & I have a mirror that’s a bar end clamp style so, conveniently, mounting points on the MC, aren’t an issue.
That link looks like the right kind of thing (although I can’t see the bar dia mentioned in the description) but i’m always rather suspicious (rightly or wrongly) about unfamiliar / non-branded parts. I’d prefer a 2ndhand Nissin or similar from an actual bike ideally (i’ve always thought of braking as a part of a bike you want to get “right”) but would consider strong recommendations of reliable alternatives.

Offline SpannerBrat

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Re: Twin disc conversation master cylinder bore?
« Reply #18 on: September 04, 2018, 03:27:06 PM »
Thanks a Oddjob. I have indeed found several MCs on fleabay (from various manufacturers & bikes). I was wondering - for instance - if a Nissin unit from an R1 (correct bar size assumed) would work equally as well as one from a Blade or GSXR1000 (not that I’m specifically looking from a ltr bike)? Or, should I actually looking for a radial MC from any modern 600? There seems so many options available & I can’t imagine they will ALL work as well as each other. I’ve read all sorts of stories of lock-ups to “wooden” feel & it’s not a particularly cheap bit to buy if it turns out not to work.

Offline Johnwebley

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Re: Twin disc conversation master cylinder bore?
« Reply #19 on: September 04, 2018, 07:51:29 PM »


  I am not an expert on brakes ,but surely you need to calculate caliper piston area,and MC area,

 on sohc bike a single caliper has a 12mm bore MC,( I think)

 a Gold wing with 2 calipers ,(same caliper as SOHC ) it has a 16mm bore,again I think,


 basic thought,each of the 2 caliper pistons need to move to squeeze the disc,and will need twice as much fluid to cause that,
hence the dual caliper MC has a larger bore,

   a quick calculation show a 122mm bore has an area of about 113square millimeters,and 16 is just over 200 ,

 nearly twice as much for the same piston stroke .

 hope it helps ,or gives you food for thought
lifelong motorcycle rider,and fan

Offline SpannerBrat

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Re: Twin disc conversation master cylinder bore?
« Reply #20 on: September 12, 2018, 02:15:04 PM »
Hi John (& everyone else),
Thanks for the reply & I believe you're right (regarding needing to calculate what's needed). Unfortunately I've become increasingly confused as I've looked into what I should be looking for in the new MC. From this link - https://www.vintagebrake.com/mastercylinder.htm - as I've got 2 x 38mm pistons (I'd forgotten I was using CX calipers....sorry. Perhaps I should really have started the topic on the CX forum but hopefully it's something some of you can still help with) & based on that page/chart, I should be looking for a MC with an 11mm bore size to give me a "preferred" 23.87 ratio. Taking that as correct (& I've no idea if it is), I'm struggling to find a MC that will be suitable. My hunt is made more difficult because the exiting cylinder - where the fluid is pushed out - of the old style MCs is in line with the bar & as can be seen on the photo of my bike (further up this thread), the mounting end of the clip-on gets in the way of the cylinder.
So, I have 2 questions:
1) Is that ratio figure "correct" in relation to getting the right feel (I appreciate that "feel" is a subjective term but there must be a figure that's widely understood as being inbetween bartouchinly spongy & oak-like wooden)?
2) Taking the ratio as correct, does anyone have a suggested solution to solve the need for an 11mm bore radial MC to clear my clip-on by any chance?
I'll get there in the end but it's resisting me at the moment.
Andy

Offline K2-K6

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Re: Twin disc conversation master cylinder bore?
« Reply #21 on: September 12, 2018, 08:12:50 PM »
I've some specs from similar application,  the SOHC 750 F2 also uses two 38.0mm single piston calipers (possibly the same unit?) but uses a master of 17.4mm diameter piston.

That obviously doesn't come near the preferred ratio on the vintage site,  but those brakes work quite well with decent power, progression and feeling on a 19inch front rim.

Smaller bore master will give you more travel but lighter lever pressure required for same stopping effort.  So softer, longer and more potency.

Larger bore master,  obviously the opposite. So harder shorter and more effort required at the lever to get same braking.

They were quite conservative in that era for master cylinder bore ratio so going somewhere near to 16.0mm I feel would be a good start point. There is a caveat though,  radial operated master can use different mechanical lever ratios placement of the bore relative to the lever fulcrum.  So it depends what the design has done here that can change the effort required.

Offline SpannerBrat

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Re: Twin disc conversation master cylinder bore?
« Reply #22 on: September 12, 2018, 09:10:07 PM »
Hi K2-K6,
Thanks. it seems like a bit of a minefield when it comes to messing about changing / mixing braking system components (& not a comfortable field at that). It sounds like i’m going to have to take a bit of a punt on a 16mm radial MC (Nissin seems a good solid option), hook up some new braided hoses & test that when it comes to the bike moving under its own steam. It’s not running yet so I was hoping sorting the front end out would be a relatively simple & inexpensive job before getting round to the pricier jobs like  sending the carbs off to be rebuilt (that’s a task that’s well outside my comfort zone / pay grade). It seems a shame not to act on all this useful info i’ve gathered so will scour eBay for a suitable MC (& matching clutch lever if possible).

Offline K2-K6

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Re: Twin disc conversation master cylinder bore?
« Reply #23 on: September 12, 2018, 10:01:36 PM »
No,  correct as stated.

As the bore size increases it moves more fluid and puts you (your hand) progressively at more of a mechanical disadvantage. 

It's like using shorter spanner/ratchet to turn the sale size bolt,  longer lever makes it easier to exert force on bolt but moves further.  Shorter lever,  hand moves less but can't make the same force.

That's why you can spin a loose nut up quickly with your fingers but need a lever to tighten and loosen it when under load.

Offline Trigger

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Re: Twin disc conversation master cylinder bore?
« Reply #24 on: September 12, 2018, 10:26:38 PM »
Nice way of putting it Nigel.
I have always said, bigger the bore, sharper the brakes and you don't want sharp brakes.
The more up to date bikes with 8 pistons on the front use a small bore and the reservoirs are getting smaller all the time.

Offline K2-K6

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Re: Twin disc conversation master cylinder bore?
« Reply #25 on: September 13, 2018, 09:52:20 AM »
Yes Graham,  they are making them on modern stuff obviously potent but extending the that initial phase of application to get some feel into the brake.  Most of them are one or two finger use now though, so quite low effort on input.

Going back to that vintage chart, they are focusing on race stuff with twin piston calipers generally for the desired ratios. He puts a condition in his text for single piston calipers near the end saying ratio of 12/14 to 1 is most desirable. 

16mm takes you very close to that range and in reality you'll probably end up somewhere from that down to 13mm as a workable range if you where able to test them individually.

With your calipers,  16mm will make a good baseline and probably won't need to be altered.

Offline Johnwebley

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Re: Twin disc conversation master cylinder bore?
« Reply #26 on: September 13, 2018, 10:37:48 PM »
So are you saying that because the lever stays further out and it travels less because it's pushing more fluid that your hand is at a disadvantage because the fingers are extended and you get more force as the hand contracts ?

I did this mod many many moons ago and used a Yamaha MC, bore size was bigger but the lever had an adjustable screw where the lever meets the end of the piston, so you could adjust the lever to sit closer to the bars, TBH it felt quite hard which was a big change from the Honda MC which was always a bit spongy and had loads of lever travel, I did change the lines to Braided at the same time to stop the infamous rubber line expansion the early Honda fours were known for. I raced that setup for a few years, had plenty of feel without much lever travel, it could lock up the front wheel if pulled really hard but it didn't feel even remotely dangerous, in fact it felt safer as the braking improved tremendously.





 its more to do with finger pressure and strength,the bigger the bore will increase the amount of power needed by the fingers,
a smaller bore will give long travel and lighter pressure needed to lock the wheel,

 its all to do with mechanical advantage
lifelong motorcycle rider,and fan

Offline K2-K6

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Re: Twin disc conversation master cylinder bore?
« Reply #27 on: September 14, 2018, 08:58:28 AM »
I liked those Yamaha brakes around the 250/400 DX time.  They seemed to be less conservative with the ratios they used compered to the other makes and had a mix of light touch with power and feel that was a really good balance.

Going back to the leverage.  If you first strip out the brake lever and just look at the hydraulic section it makes it easier to see what's happening.  If you extended the piston to the same area of the calipers (extreme for illustration purposes) then you have no ratio advantage at all,  and probably wouldn't be able to get any appreciable force on the brake pads, this would be a 1 to 1 ratio.  Then you leave the calipers alone and keep reducing the bore of the mastercylinder to a point at which you have the ratio advantage you want.  This shows that by reducing the mastercylinder bore you create a ratio that trades movement against leverage,  so better able to pressure the pads but you now have to move the mastercylinder piston further to achieve it.  In other words a mechanical advantage.

Then you add the lever back into the mix.  If you measure the distance from lever pivot point to centre of piston and compare it too the lever length,  it will give you another ratio that will act as a multiplier on top of the hydraulic one.

So depending on the area of the caliper's pistons,  if you use a mastercylinder of (obvious used range) between 10/16mm to get a ratio you want then manipulate that total with a different lever ratio,  you end up being able to produce a similar brake line pressure but with different "feel" to the lever.

The newer radial type mastercylinder seems to go for lower end bore size (as Trigger mentioned) to get the most potent hydraulic advantage but then use a variable/ adjustable lever fulcrum ratio to alter feel for rider preference.

The screw on those old Yamaha levers just moved the lever to accommodate differing hand sizes and not the ratio. If you move your hand out to the end of the lever you do increase the leverage though,  compared to using your two principle fingers nearest to the pivot point.

Offline Yoshi823

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Re: Twin disc conversation master cylinder bore?
« Reply #28 on: November 12, 2018, 11:07:26 PM »
I used a standard 550F master cylinder with the double disc installation on my commuter 550. I had Ferodo Supersport pads in one caliper & Vesrah pads in the other. This gave me double disc braking in the dry & single disc braking in the wet, as the Vesrah pads in those days weren't sintered at all  & absolutely useless in the wet. I also installed Goodrich Aeroquip braided brakelines, as these were the only ones available back in those days. A double length banjo bolts held the double lines into place on the brake switch junction.



A few years later I swapped the double discs over to Mrs. Yoshi's 550F2

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