Rear end squirms under braking.

[Erin_S]

Hi,

I've recently reinstalled my rear brake (thing fell off on the road...previous owners o.o) and now when I push down, I feel my bike squirming about an inch to the right when I push down. It seems abnormal... any idea what would cause it? Thank you!

[Alex]

If you are making no other control inputs, and just pressing on the rear brake causes the bike to squirm to the right, I can only think of two options:

- you're locking up the rear wheel and not realizing it, and the bike is sliding the rear wheel
- your rear-wheel alignment is *way* off, and it is tweaked so much that rear braking force is pulling the bike from a straight path.

I think option 1 is more likely than option 2, but I'd also think that you'd realize the wheel was locked, so . Good luck!

[Erin_S]

Only if the rear locks up with the lightest of inputs...

Hmm. How do I check the alignment on a bike?

[Alex]

Originally Posted by Erin_S

Only if the rear locks up with the lightest of inputs...

It certainly might. Rear brakes on some bikes often don't have the best feel, and it takes some practice to figure out the right amount of pressure to provide appropriate braking force without locking them up. Personally, I'd recommend that new riders completely forget about the rear brake, and concentrate on learning how to use their front brake properly before adding the rear to the equation. On any modern short-wheelbase bike, maximum front brake pressure will lift the rear wheel off the ground, where it can provide just about zero force to slow the bike down. Look at any motorcycle racing event, and you'll see bike after bike lifting the rear an inch or two in the hard braking areas.

If you're not providing that much force to the front brake, you're not using as much of it as you should be when you really need to stop quickly. People don't realize how quickly they actually can stop when the front brake is applied correctly (progressively, smoothly, quickly but not grabbing at it), and as the front end dips as the weight shifts forward, you can apply more and more front brake.

The extreme other side of the coin is someone who uses rear brake only and no front. These riders can be identified by the 200+ foot skid marks where the rear wheel locked up and continued to provide very little slowing force to the bike.

(There are shades of grey between these, of course, and if a bike is truthfully putting all of its braking power it can to the front without breaking traction, and the rear wheel is still on the ground, then yes, a little rear brake can help a little bit. And when we're not in the middle of an emergency stop, applying both brakes can make for a smoother ride when we're braking with light to moderate force. And on wet or otherwise slippery roads, combination braking can come in handy as well when you don't want to apply maximum pressure to the front tire)

Originally Posted by Erin_S

Hmm. How do I check the alignment on a bike?

Measure from the center of the swingarm pivot to the center of the rear axle on both sides. Also, look at the axle adjustment screws at the back of each arm of the swingarm, and confirm that the same number of grooves are showing on each exposed screw (assuming that the bike is still using identical axle adjusters on both sides).

[Erin_S]

I'm beginning to get a good feel of threshold braking the front end, thank you for the concern though.

But I know how to use the rear brakes. They make the rear pull to the right for a moment, then provide progressive clamping force as they should. This is massively wrong. I'm thinking I might need to rebuild the caliper.

I'll also check the alignment though, thank you for that idea.

[kkim]

check your alignment and check your rear tire too for any signs of distress or misalignment

[Alex]

Originally Posted by Erin_S

I'm beginning to get a good feel of threshold braking the front end, thank you for the concern though.

Can you put the rear wheel in the air under braking? If not, no worries, it's something that will come with time. But it should bring you to a realization that you're not at the threshold for front braking right now. Like anything else, it's not magic, just takes time and practice.

There's nothing in the caliper that would make a bike pull in one direction or another. There's only one wheel back there in contact with the ground, right? If the caliper is actually locking the wheel and then releasing, I guess that would sound like your bike's symptoms, but that's hard to understand why it would behave like that, as once it's locked it takes even less pressure to keep it locked, and I wouldn't know why it would then release. Again, a big from me.

[Erin_S]

Originally Posted by Alex

Can you put the rear wheel in the air under braking? If not, no worries, it's something that will come with time. But it should bring you to a realization that you're not at the threshold for front braking right now. Like anything else, it's not magic, just takes time and practice.

I understand, I was just trying to steer the thread back to the question. :P I know enough about brakes, tires, vehicles, and riding other motorcycles to know that the rear shouldn't lock up THEN apply light force... that's the problem...

I'll have the alignment checked/fixed when I get new tires in a few days.

[Alex]

Sounds good, I hope you get it figured out soon.

I only mentioned the front brake, because more often than not, it actually is the problem. When a newer rider (maybe not you, but let's say other newer riders) posts up "my rear brake is behaving funny", whether it's locking, doesn't feel right, isn't slowing me enough, whatever; the correct guidance isn't "you're not using the rear brake correctly", it needs to be "you're not braking correctly".

[cbass78]

Hey Erin_S,

did you ever find out what the problem was?

Was it the alignment?