The standard brakes on my 86 SRX6 (jap import) are far from perfect. Based on some info sent to me by my friend Cam, I decided to have a go (with his help!) making some changes to improve things.

On the front, the SRX6 comes standard with a 5/8″ (15.8mm) master cylinder, and twin 10.5″ (267mm) discs on an 18″ wheel, with two 1.5″ (38.18mm) sliding type calipers. Brake lines are a one-into-two rubber hose arrangement standard, but I’ve fitted braided stainless lines. As far as I’m aware, I’m running the standard Yamaha brake pads and Dunlop arrowmax 100-80/18 tyres.

Overall, braking isn’t that good.

The SRX6 pulls up quick enough, but requires quite a bit of lever force – 3-4 fingers. Performance didn’t change much after fitting the braided brake lines. The first stage of modifications involves a Master Cylinder change – but first let’s go into the maths!

(right here I have to thank Cam for doing the research which I’m about to summarise here)

There are a number of factors which affect lever force required, and lever feel. Some of these are: The brake pads, brake line flex, wheel to disc size ratio, and probably the least thought about – Master Cylinder (M/C) size to caliper size (in hydraulic terms) ratio.

The M/C to Caliper size ratio dictates the mechanical advantage you will gain through the hydraulic system. push a cylinder in at the lever end x millimeters, move pistons that push brake pads y millimeters. x is large (relatively – say 10) and y is small (say 1). This ration dictates force. If the ratio is larger, you need less force but more travel (pull the lever in further). If it’s smaller, less travel but more force – 4 finger brakes.

According to Cam’s research, there’s an optimum ratio between the caliper piston area and master cylinder area of 27:1. With a ratio of 27:1, you should have one-finger, sharp brakes. Less than this will require more force, with around 23:1 and lower feeling wooden.

Doing the maths on the SRX6′s stock brakes gives a ration of around 23.5:1, which makes sense, given that the brakes feel a bit wooden. To increase the ratio, I need a smaller master cylinder – 14mm and 15mm are the two options. 15mm gives a ration of around 25.9, whereas 14mm gives a ratio of about 29.7. I decided to give a 14mm master cylinder a go, seems as 1) higher than optimum should mean less force, but more lever travel, which i think should be better than the other way around, and 2) Cam had a spare 14mm M/C lying around I could borrow.

So, with that, I swapped the M/C with the new 14mm unit (with much jiggery pokery and subsequent help from Cam, which I won’t go into, but needless to say it would have been oh so much more difficult without Cam there) and took it for a test ride.

Hooray! brakes are now awesome!

It took me a bit to get used to them, but I’ve now got much better lever feel. One finger is plenty to pull it to a stop under normal conditions. Two fingers will easily lock the front wheel up at 60kmh (tyres aren’t sticky enough for stoppies). Cam adjusted the brake lever to give it a bit more adjustment, which probably helps a lot with lever travel. I think that without the mod that he made (cutting a spring to allow more travel in the adjustment screw, or something) then I’d have more of an issue with lever travel.

So, overall, it made a big difference. Turned wooden brakes into nice, responsive, powerful brakes. If your brakes don’t feel so good, I recommend looking into changing the master cylinder to get an area ratio of around 27:1. It makes a big difference. I fitted braided brake lines a while back, which made almost no difference to me. I’d imagine that maybe now if i went back to the old brake lines I might notice a difference, but without the right hydraulic ratios, I don’t think braided brake lines make much of a difference.

So what’s next? Well, next, I’m going to try a different set of calipers. Someone mentioned that they fitted FZR250 calipers (which are dual pot leading/trailing calipers) to their SRX and got really good results. I’m going to track down a set of these and give them a go. Hopefully I’ll be able to find some specs for them so I can do the math and see what the appropriate M/C size is. The guy who recommended them used the stock 15.8mm M/C, so it’ll be interesting to see what the maths works out as… After that, it’ll be new tyres and new brake pads. Stay tuned!