Ok, a very noobish question about front disk brakes

I have an 81 xj550 seca. Just got new tires and fork seals. Putting everything back together. I get the front forks and the front wheel on. make sure the forks looked perfectly level, and put 76lbs of torque on the castle nut. Got the brakes on. Now here comes the question. I spin the front tire and although it doesn't seem to be at all hindered I can hear the disk against the brake pad. is this something normal that will go away or did I do something wrong? Thanks for any help you can give me, and like I said, noob question I'm sure.

 

if your front wheel spins reasonbly freely then the rubbing noise you hear will the (new?) pads. once you had put a few miles on the bike the brakes will settle down and the rubbing should albut dissapear. just be a little cautious with the brake application until the pads wear in

 

Well the only thing is that they aren't new pads. The wheel spins very freely so I don't know. Thanks.

 

If the wheel spins freely and all you hear is the rotor "singing" a little, it's fine. The distance travelled by the pads in and out during normal use is *tiny*. A little contact between the pads and the rotor is normal.

 

Yep there is very little clearance between pad and disc. After applying the brakes, the only that pulls them back apart is the springiness of the rubber piston seal and a tiny amount of runout (warpage) in the disc. It's designed that way so you don't have to pull the brake lever in very far before braking starts to occur.

If you have problems with the brakes being noisy when you aren't using them, you can get some "brake quiet" which is basically a thick rubbery glue that you apply to the backside of the non-piston pad. The glue helps hold the pad off the disc. Don't use it on the piston-side pad, though.

 

How freely sould a front tire spin ? With mine, I almost have to force it around. Is this normal ?

 

Nope... it should definitely coast for a bit after you give it a spin. Assuming your brake disc isn't just rusty from sitting for a long time (which will solve itself after your first ride), you can temporarily remove the brake caliper and suspend it with a coat hanger (to avoid stressing the hose). If the wheel is still stiff, either your axle nut is too tight, your wheel bearings are shot, or your fork is bent. If removing the caliper solves the problem, then either your caliper piston or master cylinder is probably sticky and needs rebuilt.

 

Thanks Tylernt...I'll give it a look tomorrow morning.

 

One time (at band camp..ha ha), I took my xj700 for a short spin just up the road. I thought something was not right so I stopped to check things out. When I touched my front brake disc, I nearly burned my fingers off. They were incredibly hot. I loosened the caliper and limped home and readjusted the caliper. I knew this was not normal but this time the wheel will turn but not really freely. Maybe this will account for my SLIGHT lack of power. It seems that my bike is a little REVVY , for lack of a better word at highway speeds. Maybe the front brake is actually a little bit "on" at all times. The front discs do not get as hot as on that earlier day though.

 

It may be time to flush out the master cylinder and clean up the brake piston. I cleaned mine up last year and it made a big difference. The brakes would squeal a lot of the time and when I put new pads in, the brakes were dragging and became very hot. After cleaning the MC and piston, everything is 100%.

 

SWaint.......MAKE SURE THE TINY, MINISCULE PRESSURE RELIEF PORT IN THE MASTER CYLINDER IS FREE AND CLEAR OR YOUR BRAKES WILL NEVER FULLY RELEASE AND WILL NOT ONLY WEAR QUICKER, BUT THE EXTRA HEAT BUILD-UP WILL:
> warp your rotors
> wear out your pads
> dramatically increase your stopping distance. Brakes work by turning motion into heat, and if your brakes are already heated (due to constant engagement), well........you get the picture!



This pressure relief hole is normally right in line (along the centerline of the floor of the master cylinder) with the larger "fluid transfer hole" (about 1-2mm diameter) where fluid from master cylinder reservoir is drawn into the m/c piston bore below the reservoir. This probably-clogged "pressure relief port" hole would be directly "behind" (on the centerline) the fluid transfer hole....."behind" meaning in the direction of the travel of the m/c piston as it is activated. When you squeeze the brake lever, you send the m/c piston heading towards the m/c brake hose fluid outlet port. The piston will "pass" the pressure relief port first, then the larger fluid tranfer hole. When the brake lever is released, the piston "returns" to it's closed position both via spring pressure acting upon it, and via fluid pressure created by the caliper pistons trying to retract into their bores, thus pushing the system fluid back up through the brake lines, against the m/c piston. This fluid needs a "pressure relief" port to compensate for changes in fluid volume as the brakes and the fluid heats up, etc.

Without this pressure being relieved, the expanded fluid has no where else to go once the m/c piston reaches the end of it's travel in its bore (brake lever fully released). So the expanded fluid pushes on the only other thing it can: the caliper pistons, keeping excess pressure on them, and thus "brake drag".