Give Me A Brake
I have a 2004 GSX-R600 with 7850 miles on the odometer-I live in Alaska, the bike and I hibernate for half the year. I started to have a handling problem about a year ago. When the bike is coming to a quick stop under fairly hard braking at speeds under 30mph, the handlebars will start to bounce up and down like you're using a jackhammer to break concrete. You hear a really nasty sound kind of like Godzilla's roar and your fillings literally fall out of your teeth. Now at that point you decide to let go of the brake lever and the vibrations instantly stop. A gentle application of the brake lever causes no vibration or noise all the way to a complete stop. And above 30mph even squeezing the brakes hard enough to lift the rear wheel causes no problems.
I'm a professional automotive mechanic and like to play with motorcycles, and I used the excuse of this problem to convince my wife to let me upgrade. The bike now has stainless steel lines, Galfer HH sintered metal pads, Galfer wave rotors, and a fresh set of Pilot Power tires. The bike has never been crashed, I don't stunt it, and it's only wheelied a handful of times, most by accident-honestly! After the upgrade the problem was still present and even felt slightly worse, but the brakes feel amazing other than that. I am a member of a Suzuki-specific forum and had a newer member post about having a similar problem. This led to me chiming in and finding out that there were quite a few others with the same symptoms. Other members had replaced similar brake components and bearings, all to no avail.
I contacted Galfer about this problem. I told them I had already made sure that the rotors had no runout as measured with a dial indicator, the steering head bearings and wheel bearings are tight, and all fasteners are torqued to spec. They said that it had something to do with the fluid in the forks starting to cavitate and that was the vibration I was feeling. They know brakes very well but admitted they weren't suspension gurus and recommended I talk to one. I called Race Tech and they recommended that I change the fork springs to ones that can handle my 250-pound weight and redo the internals. What is actually going on inside the forks, and what do you do to correct it?
You've done a great job of going through all the things that could be causing the jackhammering, but there is one more mechanical issue that is worth checking out. First, though, put a zip tie on your front fork and see how much travel you're using under braking. If the fork is bottoming, that is most likely the cause of the bouncing. In that case, you can add preload or more fork oil to use less travel, and stiffer springs would definitely help too. According to Race Tech, the stock springs in your bike are 8.5kg/mm, and 9.5kg/mm is recommended for your weight and street riding.
The other possibility is that the bushings between the outer and inner fork tubes are worn. If that's the case, the tubes can move around and that sometimes causes chatter. Use a steering stem stand or a floor jack to carefully lift the front end of your bike. Grab the bottoms of the fork tubes and try to move them forward and back-just as you would to check steering head bearings. If the inner fork tubes move around in the outer tubes, that's an indication that the bushings are worn and need to be replaced.
Because I have the "can't leave well enough alone" illness, I mounted a 2001 Yamaha R6 caliper in place of the stock unit on my 1987 Yamaha TZR250. It was a straight swap as far as mounting dimensions. (Other things I haven't left alone include the engine, which is now 340cc) The feel at the lever is completely dead; there is no progression to the brake. The bike stops well enough but why do the brakes feel so "wooden?" What would improve this? Going back to the stock caliper, sure, but what about an R6 master cylinder or a Brembo radial master cylinder or deglazing the pads?
If you look at the piston areas of the master cylinder and caliper that you're mixing, you'll see what's going on. Your TZR originally has a 14mm master cylinder and a single, 4-piston caliper with 34mm pistons. The total piston area for the caliper is about 3600 mm2. The 14mm master cylinder has an area of approximately 154 mm2, so the overall leverage ratio is about 23:1. Because we're dealing with a hydraulic system, the leverage ratio means the force on the caliper pistons is 23 times the force at the master cylinder. Additionally, the distance the master cylinder piston moves is 23 times the distance the caliper pistons move.
The R6 caliper has two 30mm pistons and two 27mm pistons, with a total area for a single caliper of about 2500 mm2. Using your 14mm master cylinder with that caliper gives a 16:1 leverage ratio. Because you have less leverage now, you have to pull the lever harder, and over less of a distance, to have the same force and piston movement at the caliper. This is what gives your brakes a wooden feel.
To fix that you need a smaller master cylinder for a leverage ratio closer to your stock setup or what a stock R6 has. For a 23:1 leverage ratio you'd need a master cylinder with a piston area of roughly 110 mm2, giving a diameter of 12mm. The stock '01 R6 has a 14mm master cylinder as well, for a 32:1 leverage ratio on two calipers. To match that ratio with just one caliper you'd need a 10mm master cylinder.
I would recommend a 12mm master cylinder for a start, as it would give you a similar feel to stock. If the brakes are still too wooden, you can go even smaller, to 11mm, splitting the difference between the R6 and TZR setups. Yoyodyne (www.yoyodyneti.com) has a selection of conventional master cylinders in both those diameters.
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