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PostPosted: Mon Sep 02, 2013 5:27 pm 
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Joined: Tue Sep 21, 2010 8:08 pm
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Location: East Lothian
Hello

I have some Magura rim brakes currently fitted and am thinking of using Julie disks on the front.

In order to maintain a sense of symmetry on the bars, can I run the rim brakes on a Julie master Cylinder?


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PostPosted: Mon Sep 02, 2013 8:30 pm 
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Location: Fircombe.
I've seen it done!
Not sure how effective it was .


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PostPosted: Tue Sep 03, 2013 7:17 pm 
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Thanks for replying however from comments elsewhere it seems it will damage the master cylinder.


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PostPosted: Wed Sep 04, 2013 6:37 am 
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Location: Kuala Lumpur
The hydraulic brake system is amplifying the applied force with mechanical advantage via hydraulic actuation. The MASTER cylinder piston has a small cross sectional area, moving a long distance with small force. The SLAVE cylinder (brake caliper) piston has a large cross sectional area, moving a short distance with large force. The slave and master piston sizes and strokes are selected such that both cylinders displace the same volume of hydraulic fluid while delivering the intended stroke and force to the brake pads.
This means that if you fit a different master cylinder with different dimensions, you are altering this relationship. A too-small diameter master piston with same stroke will reduce the required lever force to stop the bike in theory, but in practice will likely not displace enough fluid volume to complete the stroke at the caliper end, or at least will require longer lever travel. By contrast a too-large master piston with same stroke will definately displace enough fluid to give the caliper full stroke but you'd need to input more lever pressure to stop the bike as compared with the correct size master piston.
An untrained person could assemble a mismatched combination of master and slave cylinder and may find that it "works", but in reality some aspect of the assembly will be operating not as intended (master or slave pistons not running full stroke, seals of wrong sized cylinders exposed to higher fluid pressures than intended, levers running shorter or longer travel than intended).
For example, i considered using a 2012 Shimano XT lever/master to operate a Magura HS33 rim brake. The Shimano master piston had much smaller diameter (therefore cross sectional area) than the Magura's master piston, i didn't go as far as to measure stroke or displacement volume because it was already clear that i would be using a home-made and unproven resultant mechanical advantage. While it might've "worked" it was guesswork and that is not a very professional way to proceed on a safety system. In my view if you have a professional approach it's better to stick with correctly designed systems or if wanting to modify then at least determine the specs of the proposed alteration and make an evaluation. However if you're desperate or aren't bothered about expertise then you could try anything - but it isn't advisable. New front teeth are expensive! :wink:


Last edited by RadNomad on Wed Sep 04, 2013 2:19 pm, edited 3 times in total.

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PostPosted: Wed Sep 04, 2013 8:19 am 
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Location: East Lothian
Great explanation thank you.


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PostPosted: Wed Sep 04, 2013 10:30 am 
Gold Trader
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Location: New Forest
2 thoughts;

1. connect an empty syringe to the 2 levers and measure the fluid diplacement at typical lever pull. If they're the same, then you should be fine (depending on the answer to 2).

2. (I only know about Magura rim brakes) HS11/22/33 don't compensate for brake wear, you have to dial this in manually. Most disk systems can account for this, and push more fluid from the master cylinder into the system as needed. If the Magura disc lever compensates, then you're in for a world of pain...


My gut feeling is that a disc lever won't push nearly enough fluid.


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PostPosted: Wed Sep 04, 2013 1:24 pm 
King of the Skip Monkeys
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Location: Moomin Valley
I've been messing with hydraulic brake systems since 1996 which makes me super speshul and faffer extraordinaire

From memory and explained far better above, the magura lever doesnt move enough fluid or it moves far too much fluid (i cant remember which I'm afraid) - either way it doesnt work.

I remember using the ultra rare magura road lever and that also didnt work - it was mated to an early hope sport and it was 'wooden' and ineffective.

Early Hope levers were great with Formula single and twin pots, Grimeca/XT twin pots and a few others. The adjustable master cylinder was great in the cold weather as well as hot days, dialing it in and out to suit. I've mixed up a few others, Hayes, Coda and the like but its been a while now.


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PostPosted: Wed Sep 04, 2013 2:53 pm 
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[quote="legrandefromage"]I remember using the ultra rare magura road lever and that also didnt work - it was mated to an early hope sport and it was 'wooden' and ineffective.[quote]

Yes LGF that makes sense. Magura master pistons for rim brakes are larger than disc master pistons (mine are anyway) so the Maguras would displace more fluid for an equal piston stroke. So when using a Magura rim brake master cylinder with a disc caliper the system is pushing too much fluid and in addition to this the difference in cross sectional area of the master and slave cylinders is less than intended, which means mechanical advantage is reduced, more hand pressure therefore required at the lever to achieve a given braking force. The rider squeezes hard on the lever, it feels solid but the bike doesn't stop as well.
The OP want's to do the opposite (disc master, Magura slave) so in theory should experience the opposite; The too-small master not displacing enough fluid to generate full travel of the Maguras as suggested by M_T_S. It might 'work' if the rear wheel is super true and the blocks are sitting very close to the rim, but even then it may need more lever travel than desired to apply the brake fully, but ultimately should brake fully with less lever pressure. So even if OP got this combination working, the bike would look balanced with it's matching levers but the travel, feel and required lever pressure of the front and rear brakes would not match.
It stands to reason that mixing disc masters and disc slaves of different brands is much more likely to succeed due to the similar design.


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