Main Motorcycle: T140 cafe NRE900cc
Join Date: Jan 2012
Other Motorcycle: Yes
Extra Motorcycle: Of course
I think everyone with a Triumph rear disc has had that ‘why do I seem to be fishtailing all over the road’ moment.
The problem is not really the power of the brake, but the feedback (lack of it) to the rider.
If the ratio between the area of the face of caliper piston(s) is low compared to the area of the face of the master cylinder piston is too low, then the brakes will feel wooden and give no feedback. The 5/8" bore master cylinder coupled with the calipers twin 41mm pistons give a ratio of app:13:1, this is very low.
If you then couple this with a massive mechanical advantage lever from a very long brake pedal, that imparts a lot of force, but again with very little feedback, you have the perfect recipe for brake that works like an on/off switch.
The cheapest option is to lower the friction co-efficient of the brake pads, using ee (0.25-0.35 cof) ff (0.35 to 0.45)or gg (0.45-0.55) pads will help, do not use hh (>0.55) brake pads.
This will reduce the grip of pad on the disc, raising level where the wheel locks, but you still will get very bad feedback or feel to the brake, it will just be harder to lock the wheel, but the lock up point will still be unpredictable.
I have made several changes and after a lot of mucking about I have got a rear brake that is OK, not brilliant but 100 times better than the stock setup.
First to reduce the huge mechanical advantage by 50% to 70% I fit rear sets, T140 footpegs are too far forwards for my liking anyway.
The unfortunate consequence of this however reduce the rear braking power to pretty much useless.
The second stage is then to reduce the rear master bore diameter from 5/8" to 13mm, this increases the hydraulic advantage from 13:1 to app:20:1. This restores (some of) the brake power lost by the reduced mechanical advantage.
The change in in hydraulic advantage vastly improves the feel of the brake, and this is fed back to your foot much better through the shorter pedal.
You end up with a brake slightly less powerful than the standard setup, but you can brake progressively right up to the wheel lock point.
I have used a new alloy 13mm brake master cylinder from the USA (now available everywhere) and cylinder sleeve kits from RGM (Norton).
DO NOT put a reduced diameter master cylinder on the REAR brake without reducing the mechanical pedal advantage first, it will be lethal.
I am pretty happy with this setup, however I have it in mind a further reduction to 12mm or even 11mm master cylinder diameter might be better for feel and power. The problem with this is there is no ‘off the shelf’ solution and I will have to make my own master cylinder sleeve down kit.
Quick fixes are named after how long they stay fixed, not how long it takes to make them.