Improving your front brake power

[Fred the Zed]

Hi fellas

I've just had to send an email to an associate who wanted to improve the brakes on his S****i Bandit, but thought as being I'd gone to the effort of sharing my experience with him, I might as well share it with those of you who aren't adverse to a bit of modification... I will almost guarantee that someone will tell me I'm wrong and that putting a bigger bore master cylinder on is what should be done, but I would refer them to the late great John Robinson's book ' Motorcycle Cycle Tuning - Chassis' and my racing experience.

So here goes.....

Anyway.... to brakes. I will assume that your brakes are well serviced, clean and functioning properly. If not, make sure they are before any modification!

Always think of brakes as the most powerful part of the bike. (Except 1970's Kawasakis LOL) A mid capacity UJM may accelerate from 0-100mph is around 11 - 12 seconds but will decelerate from 100-0mph in about half that time. I've already said that necessity is the mother of invention and when I raced in the Hornet Cup, we weren't allowed to modify the brakes beyond braided lines and a free choice of pad material. Being slightly larger than the average jockey sized racer and possibly slightly smarter than the average bear I realised that, to keep up on the track, I had to be able to outbrake every other bike on the grid going into the corners (its where most racing overtakes are done) because there was no way I could out gun them coming out of the turns. The Hornet Cup rules prohibited any caliper or master cylinder changes so I had to cheat a little....(sorry, I've said my Hail Mary's since) so I did some research and found with a bit of maths and a bit of research I could actually increase braking power significantly for about £25. Imagine the cost of tuning the engine for the same gains for the same outlay.... bleedin impossible!

The answer lies in the hydraulic ratio between the master cylinder (handlebars) and the slave cylinders (caliper pistons). If you have a Haynes manual you will almost certainly be able to find the bore of the front brake master cylinder and the bore of all of the pistons in the calipers (some calipers have different sized pistons). If you don't have a Haynes manual, then either get one off Fleabay or make friends on the relevant forum.

You then need to work out the surface area of the master cylinder piston ( Radius squared x Pi). So say for example that the master cylinder bore was 14mm, then the calculation would be 7(radius) x 7 = 49... 49 x 3.14 (Pi) = 153.86

You then need to work out the combined surface area of the pistons in both of the front calipers, so say they are dual 2 piston calipers with 30mm diameter pistons...then using the formula.. 15 (radius) x 15 = 225...... 225 x 3.14 = 706.5 x 4 (total number of pistons in the calipers) = 2826

The ratio is calculated by dividing 2826 by 153.86... = 18.37. This ratio is your datum starting point.

To simplify how hydraulics work, you need to accept that a smaller bore master cylinder piston provides a higher pressure for a given amount of lever travel than a larger bore piston. I can explain it and provide the formulas if you like but I'm guessing you are bored enough already!

The key to improving braking performance is to increase this ratio. So say you can find a 13mm bore master cylinder on ebay... then by using the formula the ratio increases to 21.3. It look like a small gain but when calculated as a %, its about 17%. A very noticeable difference indeed.

As an example.... I replaced my Hornet's 14mm master cylinder with one off a VFR 800 / Blackbird which was 12.7mm. It cost me £15 plus a tenner for a service kit!! I could stand the bike on its nose from about 100mph afterwards and it looked identical so the scrutineers had no idea what I'd done.

As with anything in life, there's compromises to consider. Increasing the hydraulic ratio will lead to an increase in lever travel. Select a too high a ratio and the lever will come back perilously close to the bars (either that or throw you over the bars if you grab a big handful) but you will get a lot more 'feel' from the brakes. For some reason, ill educated bikers seem to think that the more solid the lever feels, the better the brake are. I say 'Bollocks'. As long as there's no air in the system these idiots are either insensitive Neathanderals or just not trying hard enough. For road riding, I'd aim for a ratio increase of about 10-15% maximum, and no more.

When looking for a replacement master cylinder, work out what size bore you need to be looking for and work from there. Other considerations will include such things as whether theres a mirror mount or switchgear built into the existing master cylinder as this may dictate your choice of replacement. When buying master cylinder second hand, I'd heartily recommend getting a service kit too. Piece of piss to service a master cylinder and you'll have peace of mind. Bleeding the brakes after a m/c change is always a pain in the arse, but remember that patience is a great virtue.

I'd take it easy to start with after the change too. It would be easy to get carried away. I'd also recommend learning how to brake properly too. For the front brake, apply lightly first but enough to get the forks to dip slightly - this is an indicator that the weight has transferred to the front tyre and it is now gripping better and then apply the front brake harder but not so hard as it locks. Once braking is completed then gently release the brake to reduce fork rebound from unsettling the bike. Think of braking force as being poo-shaped.

Hope this helps

Fred the Zed

[jimmock]

As an example.... I replaced my Hornet's 14mm master cylinder with one off a VFR 800 / Blackbird which was 12.7mm. It cost me £15 plus a tenner for a service kit!! I could stand the bike on its nose from about 100mph afterwards and it looked identical so the scrutineers had no idea what I'd done.

The scrutineers do now!!!!

[Fred the Zed]

.... I don't race any more, so don't really care

[Pigford]

Trouble is - the increased power-ratio will exert more force than the calipers and seals were designed to cope with

[Fred the Zed]

Hi
You make a point but this modification will be well within the design tolerances of the manufacturers. I never once had a problem with this modification.

AP Racing's variable ratio master cylinders exploit the advantages of increasing hydraulic ratios with far higher pressures than I'm suggesting so I'm sure that if your calipers are in decent condition and have had new seals in living memory, all will be fine.

Fred