Author Topic: Twin disc conversation master cylinder bore?  (Read 969 times)

Offline Johnwebley

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Re: Twin disc conversation master cylinder bore?
« Reply #30 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 #31 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 #32 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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