[fedebz67]

I am in the process to upgrade my CBR to underslung rear bracket with 84mm P2 32 / 34 calipers. I have easy access to a company who produces this part in Italy (Styleandperformance) for SP1 but I need to clarify the compatibility with my application. Both are fitting 220mm disk and this is a big help for what concerns the location of caliper from a radial position and they share the same caliper, so a part of the game is already done. The most important dims to clarify are what is defining the offsets of the datum surfaces in the vertical plane. For this reason I have measured my OEM bracket and would appreciate if someone who has easy access to this info could share to understand if I can go ahead with the project or better "kill" (options are : 1) GO; 2) REDIRECT; 3) KILL) So here you find my "reverse engineering" sketches for my CBR bracket sorry I did not have much accurate gauging in my garage. We do not need to chase microns considering the above conditions and we can accept some kind of misalignment for the mid plane of the disk in regard to the caliper. ( fyi: Brembo calipers have 16mm offset). Surely the shaft's hole diam is different (20mm CBR vs 25 mm SP1) but this can be solved easily with press fit of 25x20x25 commercial bushing).
Geometrical compatibilty with acceptable accuracy is the first goal, then the next point is to understand how these Brembo calipers with twin pistons ( P32 mm or P34 mm can work with the Honda OEM 15.8 mm m/c which was designed to work with 1 x 38 mm cylinder. Has anybody gone through this step, what is the result and is 2x 32 mm probably a better option than 2x34mm for the 15.8mm m/c ? ?thanks
(DISCLAIMER : dimensions may have some accuracy measurement error )

 

[fedebz67]

forgot to add that original brembo m/c for P32/34 bracket is 12mm and this is expected to provide better "feel" rather than 15.8 mm ..so would like to hear proven feedback from those who already have gone through this "upgrade"..

 

[innovativems]

Curious, why not just buy the underslung bracket for the rc51?

 

[fedebz67]

Because then I should buy the entire bike as well 😅 Maybe was not clear, I own a 97 CBR and for this model there is no underslung available.. this is why I made reverse engineering of my cbr bracket, hoping someone may confirm SP1 dims are compatible (one group in fb has made a 10 pcs batch in Australia for 97 cbr some time ago but no parts available at the moment and custom clearance with tax duty fees are quite annoying for italy..)

 

[VFR500R]

I am in the process to upgrade my CBR to underslung rear bracket with 84mm P2 32 / 34 calipers. I have easy access to a company who produces this part in Italy (Styleandperformance) for SP1 but I need to clarify the compatibility with my application. Both are fitting 220mm disk and this is a big help for what concerns the location of caliper from a radial position and they share the same caliper, so a part of the game is already done. The most important dims to clarify are what is defining the offsets of the datum surfaces in the vertical plane. For this reason I have measured my OEM bracket and would appreciate if someone who has easy access to this info could share to understand if I can go ahead with the project or better "kill" (options are : 1) GO; 2) REDIRECT; 3) KILL) So here you find my "reverse engineering" sketches for my CBR bracket sorry I did not have much accurate gauging in my garage. We do not need to chase microns considering the above conditions and we can accept some kind of misalignment for the mid plane of the disk in regard to the caliper. ( fyi: Brembo calipers have 16mm offset). Surely the shaft's hole diam is different (20mm CBR vs 25 mm SP1) but this can be solved easily with press fit of 25x20x25 commercial bushing).
Geometrical compatibilty with acceptable accuracy is the first goal, then the next point is to understand how these Brembo calipers with twin pistons ( P32 mm or P34 mm can work with the Honda OEM 15.8 mm m/c which was designed to work with 1 x 38 mm cylinder. Has anybody gone through this step, what is the result and is 2x 32 mm probably a better option than 2x34mm for the 15.8mm m/c ? ?thanks
(DISCLAIMER : dimensions may have some accuracy measurement error )
View attachment 139510 View attachment 139511

Interesting question. I own a 2004 RC51 and will be installing underslung rear bracket with an 84mm caliper as soon a my garage warms up. I'm in the north central US. I checked the OEM part numbers for both OEM brackets hoping to find a match. Sadly, I did not and I assumed you did this as well. None of the spacers are a match either. I was going off a hunch since the wheel assembly for the CBR954RR interchanges with the RC51. Looking at the parts diagram for the 1997 CBR919RR is see that the drive sprocket and associated hardware are different (5 bolt versus 6 bolt). Axle diameter, offset (controlled by spacers), swingarm width and position within the chassis, along with many other factors all play into the alignment of the caliper. Years ago (back in the 80s) full floating underslung rear brake calipers were the way to go. Custom made hangers were easy to make using bar stock, plate aluminum and some minor welding to put together. I made quit a few back then. Yes, I am old. I am 65, but I ride an RC51. Unfortunately, the current and most likely future trend is to CNC machine them out of solid stock. They are cool looking, but unique to the application.

I did a quick search on the Internet and as you stated, there appear to be none out there. I did see some current and interesting underslung floaters out there using heim joints and tubing. That's how I used to make them and they are very strong. Links like these are used in four-link drag racing suspensions under a compression applications where as they are only under tension in a motorcycle's rear brake application. An added plus is they look cool and where still in use on super bikes in the 1990s, which is the subject of your question. I found that often I could find a front mounting point up near the swingarm pivot that would work quit nicely. If the rod angles come out right, it improves braking performance and the balance of the bike. I still have a 1984 Interceptor in my garage with one of these installed.

I hope I didn't bore you or anyone else with this answer. I just wanted to suggest an option you may not have thought of.

Good luck and Best regards,
Tom

 

[fedebz67]

Tom, thanks for your reply. I am getting 56 but still playing with the cbr too. Assuming you have the underslung available, and a caliper (mm) would you please take the basic measurements concerning the location of the disc's mid plane to the mating surface of the swingarm (35?) and the distance from the latter to the mating surface with the caliper (19?) and the distance btw mating surface of the caliper with swingarm to the outer surface of the square slot that slides into the swingarm sliding guide (7?).

Brembo offset is 16mm for the 84 mm caliper. See the sketch below to clarify the concept:

 

[VFR500R]

Tom, thanks for your reply. I am getting 56 but still playing with the cbr too. Assuming you have the underslung available, and a caliper (mm) would you please take the basic measurements concerning the location of the disc's mid plane to the mating surface of the swingarm (35?) and the distance from the latter to the mating surface with the caliper (19?) and the distance btw mating surface of the caliper with swingarm to the outer surface of the square slot that slides into the swingarm sliding guide (7?). View attachment 139523 View attachment 139524

Brembo offset is 16mm for the 84 mm caliper. See the sketch below to clarify the concept:

Tom, thanks for your reply. I am getting 56 but still playing with the cbr too. Assuming you have the underslung available, and a caliper (mm) would you please take the basic measurements concerning the location of the disc's mid plane to the mating surface of the swingarm (35?) and the distance from the latter to the mating surface with the caliper (19?) and the distance btw mating surface of the caliper with swingarm to the outer surface of the square slot that slides into the swingarm sliding guide (7?). View attachment 139523 View attachment 139524

Brembo offset is 16mm for the 84 mm caliper. See the sketch below to clarify the concept:

Give me a couple of days and I will get you those measurements. It was - 7 C this AM and pretty cold in my shop. I need to pull out these pieces anyway since I am going to install them as soon as it warms up and is fun to work in my shop again. I need to order the Brembo caliper since there was no rush to get it until I was ready to install. I have the bracket in hand and can easily provide you with all of its measurements. I can provide additional measurements once I have the caliper and start the install.

Question: Do you know if the 1997 CBR919RRs use the same diameter axle as the RC51s and CBR954RRs? If the CBR919RR is smaller, you will need to have a reducer bushing machined for the hanger if you end up using one for an RC51. This bushing could incorporate the spacer and solve an offset problem at the same time. I love doing these projects, but there is always a lot of frustration and dead ends before you are rewarded with the finished project.

Tom

 

[fedebz67]

Thanks Tom, you are right about shaft holes 20mm CBR vs 25 mm SP1. This should be easy manageable with the press fit of commercial bush 25x20x25 (Dxdxh), but indeed I need to know the accurate diameter of the bracket's hole, so thanks for measuring this dimension. Should be any offset issue, good to incorporate this amount into the bushing but I am still hoping the option is not needed. As both application share 220 mm disc and same caliper, I am rather thinking there is no issue with all the dims defining the location of the caliper to disc. Below the bush that I am targeting to fit, in case no offset is required. I love these kind of projects that require these kind of investigation more than the real potential benefits that are expected.. the next point we have to solve is the hydraulic ratio of the original brake system (15.9 mm m/c with 38mm single floating caliper for CBR is very different than Brembo system that makes use of 11mm m/c with 2x32 or 2x34 pistons). Directionally I was targeting some old VFR Nissin pump which should be at 12.5 mm m/c and are expected to fit for holes spacing.. After we have clarified this, we can think about brake lines length and hose geometry.

 

[fedebz67]

with all the limitations that a superimposition of pics has, the result is supporting the initial idea that CBR 92/99 and RC51 SP1 is likely to be compatible in the XY plane (where Z axis is vertical to disc's plane), see the result

 

[VFR500R]

View attachment 139528 View attachment 139528
Thanks Tom, you are right about shaft holes 20mm CBR vs 25 mm SP1. This should be easy manageable with the press fit of commercial bush 25x20x25 (Dxdxh), but indeed I need to know the accurate diameter of the bracket's hole, so thanks for measuring this dimension. Should be any offset issue, good to incorporate this amount into the bushing but I am still hoping the option is not needed. As both application share 220 mm disc and same caliper, I am rather thinking there is no issue with all the dims defining the location of the caliper to disc. Below the bush that I am targeting to fit, in case no offset is required. I love these kind of projects that require these kind of investigation more than the real potential benefits that are expected.. the next point we have to solve is the hydraulic ratio of the original brake system (15.9 mm m/c with 38mm single floating caliper for CBR is very different than Brembo system that makes use of 11mm m/c with 2x32 or 2x34 pistons). Directionally I was targeting some old VFR Nissin pump which should be at 12.5 mm m/c and are expected to fit for holes spacing.. After we have clarified this, we can think about brake lines length and hose geometry.

I still have to investigate the master cylinder issue. I assume it is pretty easy to tackle since a Brembo underslung caliper conversion is quite common on RC51s. The answer should be just an Internet search away. Whatever the answer is for my RC will be the same for your CBR. If I have to come up with an answer, it will come down to the fluid volume moved based on the master cylinder's piston stroke - volume geometry.

It is 1700 hours CST here in the US - I am out of here for the weekend. I will get back to you with some numbers.
Ciao