Rear brake grinding - warped disk or caliper mount?

[Sparker]

Hello! New GS500 owner here and a new biker.

Unfortunately I've had a little bike drop. The bike landed on the right side on the grass, was a bit flipped over and was resting mostly on fuel tank, engine and muffler, and also the rear brake lever was pushed into the ground.

I was quite happy to drive home from this, but when I came home (maybe 40km of driving or so) I've realized that the rear brake is making slight periodic scratching noise, and there are now serious grooves in the disc. There is a dent in the muffler which means that it has pushed into the caliper.

Obviously the disc and the brake pads need replacement now. But my question is, what do you think could have been bent there? Do you think the disc is warped? Do you think the V-shaped rear caliper mount could be bent too, or is it too strong and impossible to bend? Or maybe someone has had a similar issue after a drop on the right side?

Thanks for any help!

[dropbear]

Anything can bend, but it might not be as bad as you think. Consider putting the bike up on the centre stand and in neutral and turning the rear wheel to see how it turns and what is contacting what. Also, take the pads out and check condition. That wear looks to me like something is wrong with the pad or something other than the pad is contacting the disk. I saw a car disk like that after it went metal to metal and wonder whether you could have damaged or fouled the pad, taking out the rotor and that is all that needs replacing.

That said, brakes are safety critical, so as a new rider, unless you're sure the brakes are operating correctly, you should probably have a mechanic look at it.

[Sparker]

Hi, thanks for your reply.

I've put the bike on center stand. The grinding noise is happening when the wheel is in a specific position, but even with that seems to rotate quite freely. The brake seems to produce braking OK despite the problem.

I will try to remove the pads later, I think it's good to know how to maintain brakes. Most likely I will let the motorcycle service diagnose it afterwards, unless I find some obvious fault there like dirt.

[dropbear]

When checking how it turns, it's not just about whether it turns freely, but also whether everything is properly aligned and stays that way through a full circle of rotation. If you sit directly behind the bike as you turn the wheel, this should be clearer.

I'm wondering if you've knocked your rear wheel out of alignment. There are various YouTube clips showing how to check and adjust this, the simplest of which is probably just measuring distance to centre of rear axle on each side. You can also buy a simple and inexpensive tool which mounts to the rear sprocket and indicates alignment, say when you adjust your chain.

[Sparker]

I'm wondering if you've knocked your rear wheel out of alignment.

I also thought of this, but then the rear brake caliper mount is aligned with the axis regardless how the axis is aligned. So in this case the caliper and the wheel axis are aligned anyway. Or does the caliper mount have some play which allows it to be misaligned a bit? By caliper mount I mean this part:
https://i.ebayimg.com/images/g/K34AAOSwjbpfYnry/s-l1600.jpg

[dropbear]

No. The brake caliper is mounted to the swing arm. The wheel is mounted to the axle, which has an adjustable point of contact with the swing arm at each end of the axle and if those aren't aligned properly, the wheel will rotate on a different plane to the swing arm (and rest of the bike). Whether this has happened should be apparent from the inspection I suggested, but you can confirm by measurement, just like you would do adjusting a chain.

[Sparker]

Good evening,

I haven't been able to remove the whole rear caliper yet, but I've removed the pads. The photo is attached. The right pad is on the right.

The pads are worn and there is a groove from outer edge of the disc, it's bigger on the right pad. On the left pad the groove is deeper at the front edge of the pad.

Does it give you a hint about what's going on there? Should I just buy a new pair of pads and try if it eats through them again or would it be just a waste of pads?

[herennow]

Dude those pads are toast, you should replace them at a couple of MM of pad left.

The grinding is the lip of the disk (that the pads did not war down, contacting the pads.

Good lesson for folks to check their pads. In my experience you get about 3 pads to a disk. maybe check your disk also?

[Sparker]

Hi,
Yeah of course they are done, point is that they were ok before the ride, and after the bike fell on the right they started producing some scratching sound. After I returned home I discovered the brake state, plus the damage on the disc - it's shown in the first message picture.
And what I am trying to find out is if the bike falling on the right side could have bent something in the rear brake.

[Sparker]

Sorry for not keeping you updated. I drove the bike to a repair shop, the mechanic has replaced the disk and the pads. I've done 150km of occasional traveling after that, no signs of excessive disk wear so far.
So I guess the disk was bent after all.