I recently came across a blogger post for upgrading the single piston front brake caliper for a Honda CX500 using the dual piston brake caliper from a 1982 cb900. I had been researching bolt-on upgrades to improve the braking ability as well as the brake feel for my old CB750c.
I wanted improved braking. The cb750 brake lever has a wooden feel and requires a fair amount of pressure for a decent stop. My suspicion is that disk brake technology was so new that the engineers didn't really have time to figure out all the details they needed for effective brakes. These poor disk brakes were already a big improvement over the original drum front brakes.
My experience on a new Vulcan 500 convinced me it could be done because it has great brakes with good feel and the configuration is single disk with a single piston caliper just like my 750.
My next hurdle was deciding what caliper I would use in my upgrade since I had 2 left calipers. With only 2 auctions, I inadvertently purchased 1981 cb750f, 1982 cb900c, and 1985 vf500 calipers. One left caliper with 30mm pistons, and one left caliper with 32mm pistons. The right 30mm piston caliper was either for parts or to put up for sale at my first opportunity since I didn't think I would be going to a dual disk dual caliper setup this year (or ever with this bike).
I found this brake ratio chart (now defunct) which explained a great deal about why I needed so much lever effort to stop my bike (4 finger squeeze vs the 1 or 2 finger squeeze on the Vulcan 500). My cb750c has a single piston caliper that measures 43mm and the master cylinder piston measures 14mm. These numbers give me a brake ratio of 9.43 using the table in the brake ratio chart and some handy junior high school math to calculate the area of a circle since my 43mm piston is not calculated in the chart. (The brake ratio chart page also discusses what the preferred ratios should be. )
I calculated that upgrading the single 43mm caliper to a dual piston 30mm caliper from the cb900 (or cb750f) is actually a downgrade because the brake leverage ratio falls to 9.18 from 9.43 (target ratio is 12-14). This would cause a more abrupt braking behavior and require slightly more hand effort in pulling the brake lever. The VF500 Interceptor brakes are 32mm and bolt on to the cb750f caliper bracket (Hooray for eBay mishaps). This caliper would give me a ratio of 10.42, still not good, but an improvement from the original setup.
here is a similar table to the brake ratio chart linked above with additional calculations:
My final hurdle was dealing with a very slight rubbing with the caliper bracket since the cb750f must have a thinner and lighter rotor. I removed a little of the bracket material to give the rotor some clearance and will wait for some pads and time to bed them in for a test ride
Another reason for upgrading to a dual piston caliper is because the caliper has more pad choices. I'm installing EBC fa69/HH pads, I don't believe the high performance pads are available for the old single piston calipers. I also plan on upgrading the master cylinder to a smaller 12.7mm (or 1/2") piston bore unit currently available on eBay for a very reasonable $50 including shipping. This should bring my brake ratio between 12 and 13.
here's the formula, since the website no longer exists:
I wanted improved braking. The cb750 brake lever has a wooden feel and requires a fair amount of pressure for a decent stop. My suspicion is that disk brake technology was so new that the engineers didn't really have time to figure out all the details they needed for effective brakes. These poor disk brakes were already a big improvement over the original drum front brakes.
My experience on a new Vulcan 500 convinced me it could be done because it has great brakes with good feel and the configuration is single disk with a single piston caliper just like my 750.
My next hurdle was deciding what caliper I would use in my upgrade since I had 2 left calipers. With only 2 auctions, I inadvertently purchased 1981 cb750f, 1982 cb900c, and 1985 vf500 calipers. One left caliper with 30mm pistons, and one left caliper with 32mm pistons. The right 30mm piston caliper was either for parts or to put up for sale at my first opportunity since I didn't think I would be going to a dual disk dual caliper setup this year (or ever with this bike).
I found this brake ratio chart (now defunct) which explained a great deal about why I needed so much lever effort to stop my bike (4 finger squeeze vs the 1 or 2 finger squeeze on the Vulcan 500). My cb750c has a single piston caliper that measures 43mm and the master cylinder piston measures 14mm. These numbers give me a brake ratio of 9.43 using the table in the brake ratio chart and some handy junior high school math to calculate the area of a circle since my 43mm piston is not calculated in the chart. (The brake ratio chart page also discusses what the preferred ratios should be. )
I calculated that upgrading the single 43mm caliper to a dual piston 30mm caliper from the cb900 (or cb750f) is actually a downgrade because the brake leverage ratio falls to 9.18 from 9.43 (target ratio is 12-14). This would cause a more abrupt braking behavior and require slightly more hand effort in pulling the brake lever. The VF500 Interceptor brakes are 32mm and bolt on to the cb750f caliper bracket (Hooray for eBay mishaps). This caliper would give me a ratio of 10.42, still not good, but an improvement from the original setup.
A smaller piston master cylinder is in store but thinking about the disk brake to wheel ratio also needs to be considered. If you have a larger rotor to tire ratio, that rotor will have a higher leverage (example: 19" wheel with 15" rotor or roughly 80%) you would choose a master cylinder at the lower end of the leverage ratio. If you have a smaller rotor to tire ratio (example: 19" wheel with a 10" rotor or roughly 53%) you would choose a master cylinder at the higher end of the leverage ratio.
NOTE: higher leverage will produce more lever travel. Steel braided lines are advised.
NOTE: higher leverage will produce more lever travel. Steel braided lines are advised.
Target leverage ratio (choose the lower target number for larger rotors and the higher target for smaller rotors):
- single caliper: 12-14
- dual caliper: 24-28
here is a similar table to the brake ratio chart linked above with additional calculations:
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| Click for larger image |
My final hurdle was dealing with a very slight rubbing with the caliper bracket since the cb750f must have a thinner and lighter rotor. I removed a little of the bracket material to give the rotor some clearance and will wait for some pads and time to bed them in for a test ride
Another reason for upgrading to a dual piston caliper is because the caliper has more pad choices. I'm installing EBC fa69/HH pads, I don't believe the high performance pads are available for the old single piston calipers. I also plan on upgrading the master cylinder to a smaller 12.7mm (or 1/2") piston bore unit currently available on eBay for a very reasonable $50 including shipping. This should bring my brake ratio between 12 and 13.
here's the formula, since the website no longer exists:
remember the formula from high school math? area=pi * r^2
x= caliper piston area (3.14*(piston diameter/2)^2) * ( # of pistons)
y= piston area of master (3.14*(piston diameter/2)^2)
leverage ratio = x/y
coming up:
cb900 Cam drop in upgrade
cb900 Cam drop in upgrade
Have a 81 cb900c. Thinking an upgrade will this work?
ReplyDeleteit depends on the mounting holes on the forks. Honda used 2 types of mounts, a wider spaced mount and a shorter spaced mount like on my bike. If you look at a bunch of the early 80s bikes you'll begin to see a difference in the mounting hole distance and then you can buy according to what looks like it will fit.
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