Oct 1, 2007

Rear disc brake: how to stay unlocked

Someone asked about the rear disc brake and how to use it best. This post presumes that you've already gotten over the misconception that using the front brake will throw you over the handlebar and the rear brake is the real lifesaver...

Sometime ago, I got lucky and was handed a gorgeously orange phantom generation Pulsar 180 to ride at the Bajaj track in Chakan. The bike had normal brakes, of course, with a drum at the rear.

Over the first few laps, I found the rear wheel skipping about annoyingly under braking for corners. Then, I remembered that the same thing had actually happened before. So I responded with the same solution – I took my foot off the rear brake entirely. This halted the skipping and only created a feeling of instability that faded (either that, or I got used to it) after a couple more laps. What it brought home is what I've known since my RD350 riding days: the faster you are going, the more useless the rear brakes are as a means of stopping.

Yes, I mean that.

To understand the rest of this, you need to know (without me explaining the full physics of it) that when you use the rear brake, it tends to slow the expansion of the rear shock absorber under hard braking.

Now, if you remember the taper braking tip, you use the rear brake to begin your weight transfer (notice, how no one ever gives the rear brake any credit for slowing the bike down). Then you progressively brake harder with the front brake, scrubbing off the biggest chunk of speed with it and then just before you come to a complete stop, you bleed the pressure off the front brake (because it becomes grabby at low speeds and tends to cause the front suspension to pogo up and down to a stop... ungainly) and re-engage the rear brake to produce a smooth stop.

When you add a rear disc brake into this, especially a sensitive one, it only serves to complicate matters – one of the reasons I've never thought that anything under 40-odd bhp deserves or needs anything more than a good drum brake at the rear. Like I told someone who was asking about the optional rear disc on the RTR FI, I'd save my money and stick with a drum brake instead.

Anyway, so now you have a P220 and the first time you needed a panic stop, you ended up with a sliding rear end, eh? First, practice. There's no way around it. Second, the next time you get your bike serviced, ask the service chap to try and either increase the lever travel (may not be possible) or set the lever so that it sits much lower (should be possible).

Essentially the problem is that the lever effort to braking effort ratio changes quite drastically when you move from a lever-actuated drum brake to a hydraulic disc brake. What is happening is that you muscle memory is causing you to depress the lever too hard and causing the lock up. You can teach yourself to be more gentle in time, but in the short term, you will get more benefit by simply moving the lever further away from your foot (adjust lever position) or by allowing yourself more room by increasing the distance between the normal lever position and the fully depressed lever position (harder to do with a hydraulic unit).

Ideally, get on the bike and do a couple of slow speed stops with the rear brake only, noting the angle of the pedal at which the wheel locks. And then, get him to set the lever just below that. In an emergency, if you stand on the pedal, you still should not be able to lock the rear wheel. A locked rear wheel is as useless as onion-flavoured ice cream. On a side note, while my more mechanically inclined colleague tells me this does not work, the last time I rode a Bajaj Pulsar 220 DTS-Fi I ran into the no travel in the rear brake lever problem. In my case, ejecting the pads, grinding them up a bit created the extra travel I wanted.

Then you practice. Practice hard stops where you do no more that feather the rear brake. The sole purpose is to achieve a light pressure between the pads and disc, just enough to stop the rear suspension from extending too much and making the bike go all nose heavy. Maximum braking must come from the front brake, and with practice, you should be able to produce hard stops where both wheels leave a trace skid, also called a threshold skid mark (grey coloured, not black like a locked wheel).

Related links:

3 comments:

Gopalakrishnan said...

thanks :)

muscle memory : last ride was Al' Green with about 3 feet of lever travel :))

will practice, am ofc no where near leaving grey mark proficiency

will post again after a practice session or two - after about three weeks of self imposed time out from bikes (fed thumb into sprocket ...)

was also upset with my starting the sequence by locking eyes on cyclist and ending sequence with eyes locked with cyclist :( mebbe I'll get blinkers like they put on hosses :)

thanks again
gr

Anonymous said...

in the end a one line explanation would have worked too..."practice makes perfect"...lol

Anonymous said...

Good to hear that rear brakes should be used minumum.I am already doing that.. Reason- My rear brake light isn't working, so I am forced to use front brake as much as possible.