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PostPosted: Tue Sep 06, 2011 9:04 am 
retrobike rider / Gold Trader
retrobike rider / Gold Trader

Joined: Mon Oct 15, 2007 9:05 pm
Posts: 9245
cyfa2809 wrote:
Or just get a dual/split banjo. Hydraulics see 8)


Well no, not really. Hydraulics see. :lol:

Levers and calipers have to compliment each other. Running two slave pistons (caliper end) from a single master cylinder (lever end) would leave you with little braking power. There is simply not enough capacity in the master to run both brakes without the lever bottoming out (its the same feeling you get if the pads are really worn). Then there is the problem of bias adjustment. Your natural instinct is to pull one lever harder than the other to get the braking effect you need depending on conditions. Having both brakes apply the same braking force is rarely desirable. On dry tarmac you brake with a bias to the front, on slippy wet mud you tend to go harder at the rear. Therefore you need to be able adjust braking bias to suit conditions.

If you want something to work well, you have tomput the work into the system. Nothing is ever simple.

Si


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PostPosted: Tue Sep 06, 2011 9:40 am 
Retro Guru

Joined: Mon Jun 02, 2008 2:52 pm
Posts: 347
It would be a lot cheaper and easier to use a brake bias valve like they have been using in (road) cars for ever. A dual master cylinder with balance bar is a bit crazy :D That Pikebike dual Hope master cylinder is awesome though.

http://www.compbrake.co.uk/BRAKE%20ACCE ... VALVE.html

For a lever just mess around till you hit the right one, a motorbike/motocross/trials master cylinder would probably push enough fluid for both.


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PostPosted: Wed Sep 07, 2011 10:38 am 
Old School Grand Master
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Posts: 4433
Location: Herts UK
cyfa2809 wrote:

Getting varying powers from different pistons could just mean using different calipers. Single pot on rear and 2pot on the front or whatever.


yes, exactly. I have Shimano XT brake levers on my commuter but being an early GF, it has the rear calliper within the frame so the 4 pot or any other Shimano IS calliper does not fit ..... but a Magura SL does.

... and it works well but the Magrua has larger piston so the LH lever has a bit more travel that if it was using Shimano calliper.

So as you say, knowing the calliper piston diameter of different brakes, a system could be devised.


Ofcourse it is not ideal as different amount f braking is needed in different condtions, usually front takes the brunt but on steep hills, the rear is used more, even to the point of controlled sliding of the locked rear tyre as locking the front would send you over the bars. .... but from the sort of riding the OP is talking about, that should not be an issue.


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PostPosted: Thu Sep 08, 2011 12:28 am 
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Joined: Mon Jun 14, 2010 11:27 pm
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Location: mid cheshire
Nick C wrote:
Maybe with a bit of tinkering it could be fettled for F&R use and avoid any over the handlebar shenanigans with twin pot Hopes?! :shock: :wink:


I had a shimano sti with a thumb/thumb arrangement a fair few years back, possibly early deore lx (early LX for trigger shifters at least)

for the braking, could you knock up a system where the brake cable was split to send braking power backwards off the same lever? if it was done cleverly with hose pressures it could potentially be done with a positive front brake bias so as not to lock up the rear all the time, another option would be to look into twin cable discs and adapt a bmx giro to split the braking (if only having a front one is an issue.)


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PostPosted: Wed Sep 14, 2011 8:39 am 
Old School Grand Master
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Location: Herts UK
.... actually the two callipers connected to one brake master cylinder may not be as problematic as I first thought. :)

as mentioned earlier, I am running mismatched brake and calliper (simano XT and magura Marta) on the rear and on the weekend I bled that brake.

The amount of lever travel is the same or less than for the front whwich is full Shimano set up.

So given it some thought, what I think happens is when the lever goes back to its resting position by means of the internal spring, the pads are pulled back by the brake fluid. If there is a mismatch, i.e. the calliper piston diameter is not as per manufacturer's, then the pistons and subsequently the pads will be draw out more or drawn out less than intended.

If they are drawn out less (pistons are larger diameter than intended), providing there is still clearance between pad and disc, there is no problem and lever travel is the same for braking. 8)

This will still be the case running two callipers off one master, but the clearance between the pads and disc will be less but as long the pads do not rub, all will be well.


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PostPosted: Thu Sep 15, 2011 10:11 pm 
MacRetro rider
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Location: DUNDEE
you get levers with a split line, big eck used them on his dual front disk set up.


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