Yet Another Cycling Forum
General Category => The Knowledge => Topic started by: salar55 on 19 December, 2018, 05:21:51 pm
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Have HS66 brakes on the tandem, been looking for another set for ages but no luck. Going to build up another touring bike, a Cannondale F 400. Headshok with spring instead of air so no issues with seals etc. Ordered 105 disc levers any one done the swap with the levers and HS 33 rim brakes .?
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before you take the plunge I suggest you check the MC dimensions; IIRC the caliper pistons are a similar (ish) size between shimano disc brakes and magura rim brakes, but the pistons move (in a single lever stroke) >1mm in the latter case and <0.5mm in the former case.
This suggests to me that the MC bore sizes/leverage ratios are completely different; if is is as I suspect, you may end up with ferociously powerful brakes, but that offer stuff-all running clearance at the rim if you use HS33 calipers with shimano MCs.
At least the brake fluid is thought to be compatible though.
cheers
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Yes. they use the same fluid, I use magura blood oil (mineral oil) in my shimano mtb stuff. The magura's are adjustable for distance using the holders, that should help with getting the modulation reasonable. If they are still full on when applying the brakes a little air in the system might help.
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if my hunch is correct then air in the system may just further reduce the running clearance in the brake; it all depends how much of the caliper piston retraction is achieved via seal rotation.
Another evil possibility is that if seal rotation causes enough piston retraction, the brake may not come on (anywhere near the top of the stroke) when you pull the brake lever.
I guess you will find out by experimentation; I shall be interested to hear what happens.
cheers
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Seal rotation? Pistons don't turn, I think the seal would need replaced.
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I think Brucey is referring to the fact that as a piston passes over/through and O ring, that ring will distort to a small degree, and at some point will "spring" back to an unstressed position.
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indeed; the 'O' ring (piston seal) is usually more square in section and 'shear deformation' is probably a better description than 'rotation' but the elastic deformation/recovery of the piston seals provides much of the piston retraction in many brake designs.
This illustration
(https://www.jaguarforums.com/forum/attachments/xk-xkr-x150-33/50873d1376410267-brake-fluid-change-necessary-every-two-years-mump_0209_understanding_ford_brakes_11_z_disc_brake_caliper_piston_seal.jpg)
sort of shows what is going on (in a car brake). Modern brake seal/groove designs are different to that but the same applies; in many brakes the seal only slides past the piston when the piston is pushed back (for new pads) or to accommodate wear in the pads. During normal braking the seal may deform but not slide as the brake is applied, and the extent of the recovery is dependant on the seal design.
This means that if you try and use a MC with a smaller piston, you may or may not end up with a different running clearance and you may or may not ever end up with the brake coming on anywhere near the top of the stroke.
cheers