Hydraulic brakes for road... mixing components.

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xnavalav8r
Posts: 2594
Joined: Mon Jan 21, 2008 12:09 pm

by xnavalav8r

Would it be possible to pair Shimano R785 di2 hyadraulic brake levers with Magura RT8 TT brakes? No plans to try. Just curious. It seems like it should be possible. But I have no idea if it would work. Incompatible hardware. Different types of brake fluids required. Varying hose diameters. It would be a cool concept for a junior TT bike where drop bars and standard road shifters are required.

TheKaiser
Posts: 653
Joined: Thu Sep 05, 2013 3:29 pm

by TheKaiser

Cool idea!

I can't say for certain on the hardware/hoses topic, but I can tell you that both Shimano and Magura use a mineral oil based fluid, rather than the DOT type fluid that many other manufacturers use. This makes them compatible in terms of the materials used, as it means that you shouldn't run into issues with seals, bladders, etc...swelling or degrading due to exposure to the wrong type of fluid. Now, both companies will of course tell you that you must only use their fluid with their hardware, but the fact that you are considering this setup suggests to me that you are comfortable stretching a bit beyond the official party line. I have seen them both used interchangeably, without issue, in a pinch, and if given the choice, I would probably use the Shimano, both because they claim a higher heat tolerance (not that it should be an issue in a rim brake application), and because the viscosity should best match the lever hardware, which is where the critical fine workings are located.

I think I have actually seen people talking about hybrid Shimano/Magura disc setups on MTBs in the various forums, so I would be surprised if some combo of hoses and hardware couldn't be found to make it work.

One or two other issues that would need to be explored though are that, if memory serves, the RT8 TT brake is a closed system, meaning it doesn't self adjust for pad wear. I can't recall on the RT8 but most other similar brakes have an adjustment at the lever that you screw in to push more fluid into the system, therby moving the pistons resting position inward, and compensating for pad wear. Shimano disc levers on the other hand, are an open system, that will self adjust for pad wear when hooked up to a disc caliper, and have no provision to push extra fluid into the system to adjust lever feel or compensate for pad wear. You could probably work around this stuff with very careful setup, but it would be something to put some careful thought into before buying all of the parts.

Also, you would want to see if you can figure out how much fluid the Shimano master cylinders push vs. the Maguras. I am actually curious now, as I am wondering if they use a similar size cylinder and stroke or not. Intuitively, I want to say that the rim models tend to push more fluid, which would mean that your setup would have awesome power but lack pad clearance. I could be off base with that one though. Out of curiosity, does anyone know if the SRAM hydraulic rim and disc brakes use the exact same lever/master cylinder? Obviously a totally different setup than we are discussing here, but it would still be interesting to see.

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efeballi
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by efeballi

TheKaiser, SRAM's hydraulic rim and hydraulic disc brakes use the exact same lever.


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OzDamo
Posts: 1
Joined: Sat Feb 13, 2021 7:05 am

by OzDamo

xnavalav8r wrote:
Mon Jun 01, 2015 1:11 am
Would it be possible to pair Shimano R785 di2 hyadraulic brake levers with Magura RT8 TT brakes? No plans to try. Just curious. It seems like it should be possible. But I have no idea if it would work. Incompatible hardware. Different types of brake fluids required. Varying hose diameters. It would be a cool concept for a junior TT bike where drop bars and standard road shifters are required.
Yes, this is possible. It works great!

In fact I just paired the Magura RT8 with the Shimano Dura Ace ST-R9120 Hydraullic brake levers. This combo has excellent modulation offers superb braking power. The brake actuation is effortless and extremely smooth. This solution is far superior to the Magura official RT8C solution. I am using SwissStop BlackPrince brakepads on a set of ENVE 7.8 carbon wheels.

There are a few technical challenges but all can be resolved with trial and patience.

The first challenge is with the hose, olive and barb fittings. On the R9120, a special Shimano BR-R9170 Flange Connecting Bolt unit must be used. The Shimano olive is fine for use, but the silver barb insert would be too small for the Magura hydrallic hose, so I used the larger BH59 brass barb instead of the silver BH90. And even with the BH59 it wasn't a tight fit. So I applied a few rounds of the pink PTFE gas tape over the BH59 to achieve a perfect tight fit before assembly. And a perfect tight seal has been acheived.

The second challenge is how to achieve a quick-release mechanism to allow the wheels to be removed for servicing, say for a tyre puncture repair. For this purpose, we enlisted the original Magura RT8C converter. But we are not using it to convert cable pull into hydraullic pressure, instead we utilize 2 very useful functionality that the RT8C offers: First the ability to perform a quick-release of the brake calliper with a push of a button. Second, we utilize the RT8C's ability to perform fine-tuning on the brake pad clearance over the braking surface and hence adjusting the engagement point of the braking.

To allow the RT8C to be integrated into the hydraullic system between the RT8 brake calliper and the Shimano R9120 brake lever, we utilize a 3-way hydrallic Y-splitter - TRAILTECH SPORT HOSE SPLITTER V2 (https://www.tartybikes.co.uk/hydro_rim_ ... 13572.html)
Magura barbs were used with the 3-way splitter. To ensure a tight fit without leak, pink PTFE gas tape has been applied on the screw ends of the Magura barbs before they were fitted to the Y-splitter.

The Shimano Mineral Oil has been choosen over the Magura Royal Blood as it has a higher viscosity to allow proper functioning of the Shimano Hydraullic brake levers. Shimano Mineral Oil also has a higher boiling point.

The most technical part is bleeding this 3-way system. It will test your patience when trying to extract the last bit of air bubble from this system. On the brake lever, the R9120 requires a special larger diameter Shimano Dura-Ace ST-R9120 Funnels Adapter and O-Ring to be used in conjuction with the standard Shimano Bleed Funnel BL-M575. On the RT8 brake callipers, it requires a bolt connector with the syringe connection from the Magura Pro Bleed kit; on the RT8C converter bleed port, it requires the special Magura bleedtube and bottle attachment from the Magura Pro Bleed kit.

After much trial-n-error, the most effective way i found, is to clear out most air bubbles between the Magura RT8 brake levers and the RT8C converter first. Repeat the push-pull action of the syringe and try to engage and disenage the RT8C quick-release lock. Each time this quick-release function is utilized, small amounts of air bubbles would appear in the hydraullic line. Repeat the purging process until no air bubbles can be seen. Flush out most air bubles first. Once this is done, close the bleed valve on the RT8C end.

Now pull out the plunger stopper from the bleed funnel on the R9120 lever. Ensure that the funnel is full of Shimano mineral oil. Draw the syringe until much of the air bubbles have been extracted. Make sure the plunger of the syringe is facing upwards so no air can re-enter the system once extracted. Now push all the mineral oil back into the bleed funnel. Engage the brake lever in rapid successions, then pull the plunger of the syringe. You should see small air bubbles coming thru. Repeat this process as many times as necessary. It may take around 20 mins to completely purge all the air from the system.

Even when all air bubbles have been purges, this setup still needs a small amount of preload from extra fluid to make the brake pads engaged properly. To achieve this, draw the extra fluid into the system with Quick-Release on RT8C engaged, then close the bleed port on the R9120 brake lever and use a quick-grip clamp to hold the RT8 brake callipers together into full closed position. This would expose the hydrallic piston. Remove the black plastic ball-bearing rail guide on the top of the piston to expose the nylon piston ball head. Whilst applying pressure on the syringe to push more fluid into the system, pull the nylon piston ball head in an upwards direction. Now unlock the bleed screw on the RT8 brake lever and remove the syringe. Reinstall the bleed screw. Reinstall the black plastic ball-bearing rail guide and release the quick-grip whlist ensuring the ball bearing lands on the correction position on the guide rail.

The final step is to ensure that 1. With the Quick-release engaged that the seperation distance between the brake pad is greater than or equial to the width of the tyre/rim combo; 2. With the Quick-release disengaged and the brake lever depressed that the seperation distance between the brake pad is less than width of the braking surface...so ensure sufficent brake travel is achieved. Of course the RT8C also offers some degree of adjustment to this clearance, but it is best to have this dialed in to the approx distance during first setup. Use a digital calliper to measure these distances. And if needed, purge a small amount of brake fluid from the system to achieve the optimal setup. Always use Quick Grip to clamp the brake callipers into closed position first and use the RT8C bleed port to complete the bleeding process. Depress the brake piston slightly down to purge the amount of brake fluid required until the optimal brake engagement point has been achieved.

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Lelandjt
Posts: 817
Joined: Tue Jan 19, 2016 7:10 am

by Lelandjt

On the Ridemonkey DH forum there's a "Frankenbrakes" thread of guys doing combos like this to pair their favorite lever with favorite caliper, or try a different ratio of piston sizes than the manufacturer offered to change the lever throw/power.

mathemagician
Posts: 62
Joined: Tue May 19, 2020 11:44 pm

by mathemagician

This seems really interesting, I've recently been thinking about doing the same thing...I have some spare Shimano hydro shifters, I have found Magura calipers online. Would you mind answering a couple of questions?

Is the RT8C converter put in solely to create a quick release mechanism? I'm thinking if the wheel can be removed by deflating the tyre that's something I can live with, so could I get away without installing the converter?

Does the RT8C converter need to remain fixed into the system, and if so where have you fixed it on the bike?

What would you say the max wheel width and tyre clearance is? I'm looking to use with 28mm wide carbon rims, and 28c tyres if possible.

I read a review from a couple of years back on Cycling Tips of the Rotor Uno rim brake system, which I believe uses the RT8 calipers. They made a point of saying the calipers lacked power...would you agree? How do they compare to, say, a Dura ace rim brake set up?

I really appreciated your detailed post by the way, super interesting!

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mathemagician
Posts: 62
Joined: Tue May 19, 2020 11:44 pm

by mathemagician

Oh, one more thing. I don't think the calipers have cables included. Would you say it was better to use Magura cables, or could I get away with BH59 cable and use the Magura barb / olive at the caliper end? BH90 sounds like it might be too narrow?

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