# Bicycle Mechanics - Rear brakes can't stop the bike?

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View Full Version : Rear brakes can't stop the bike?

makeinu
07-27-08, 05:13 PM
I have an older bike that I've only recently started using, got it serviced by the local bike shop, but the rear (disc) brakes still can't stop the bike - They slow it down, at best.

The front brakes seem (overly) powerful in that half a pull of the brake lever will stop the wheel, but the rear brakes slow the bike down slowly even when fully pulled.

Is there a way to adjust the tension required? Also, is this by design, or a problem?

The bike has mid-1990s Shimano Deore XT levers.

DannoXYZ
07-27-08, 06:34 PM
That's physics for you. Momentum (or Newton's 2nd law) causes weight to shift to the front as you decelerate. This increases maximum-grip possible from the front-tyre while decreasing grip from the rear. Depending upon the height of your centre-of-gravity (COG), the wheelbase and deceleration-G, at some point as braking-force increases, ALL of the weight will be on the front wheel and zero on the back:

http://i42.photobucket.com/albums/e346/DannoXYZ/Motorcycles/ElenaMyers2.jpg

This is when you have maximum deceleration force and shortest braking-distances possible. As a test, go out and practice and measure maximum-braking distances between front-only versus rear-only brakes. Measure the distances between the two and see how effective the rear-brakes are for maximum-braking force. Also practice braking with your hands in the drops, arms aimed straight-forward and your belly on the seat.

carpediemracing
07-27-08, 08:09 PM
If you have a car and you think it's in proper operating condition (i.e. it stops normally) you can prove it to yourself.

Go out on a deserted road that is infinitely long so there is nothing to hit in front of you. Accelerate up to something like 30 mph. Pull/push the emergency brake (hold onto the release too so you can release it if your rear brakes are fooked). This is braking just using your rear brakes.

Now get back to the start point and do it again but this time use your brake pedal. This uses front and rear brakes.

On your bike you can do an experiment too. Roll the bike forward. Use the rear brake. It locks up the rear wheel. Use the front brake. It stops the bike. Does this mean the rear brake isn't working well?

Well, to see if the rear brake is working, now roll the bike backwards. Use the front brake. The front tire locks up and skids. Use the rear brake. The bike stops.

The brake that is towards the front of the bike will stop you. The brake towards the rear is not that useful for stopping.

The exception is if you have a lot of weight in the rear - a heavily loaded touring bike or a tandem. On a tandem I can use the rear brake pretty aggressively and it's effective.

cdr

Tabor
07-27-08, 08:37 PM
I think the OP would have mentioned/noticed if he was locking the rear wheel.

lymbzero
07-27-08, 09:28 PM
It is a common misconception that disc brakes are actually better than v-brakes.
However, if they set up properly, both can have amazing stopping power.

You mentioned mid 1990's XT levers. Where those originally made for v-brakes or cantilever?
If they were originally for cantilever brakes then that might be an issue.

But since you said the front brake works great then....

Common problems.
1. Your rear disc brake pads might be worn out or not yet worn in.
3. You might have guck on your disc or disc brake pads.

A little bit of oil will totally mess up a disc brake/pad.

If you have mechanical disc brakes then you can use the barrel adjuster located where the cable exits the brake lever.
If you extend them a bit (unscrew), it should tighten up your brake.

Hope that helps.

DannoXYZ
07-27-08, 11:00 PM
Well... rear discs are kind of overkill eh? Considering you can already fully lock up any rear-brake with any kind of braking-system, the rear discs won't improve braking-power of distance. Its main advantage is heat-dissipation and water/mud build-up. These bonuses would be much more evident on a front-brake.

As a control, the OP should measure braking-distances using rear-brake only on his bike. Then try the same on another bike without disc-brakes. If the braking-distances on his bike is longer than the other one, there's definitely a problem with the disc-brake of some sort. But don't expect to have any shorter stopping distances compared to other braking systems.

On the front wheel, that's another thing altogether. Especially on a downhill bike or tandem when you're going for maximum-speed and are on the brakes at 100% into every corner.

BCRider
07-28-08, 01:56 AM
If you find that the front is locking up semi easily on pavement then you're almost certainly snatching at the lever too suddenly. If you progressively squeeze it over a 1/4 to 1/2 second you'll be amazed at how well many bicycle tires can grip. And a lot of them are quite capable of gripping hard enough to end up with you lifting the back wheel in the air just like that motorcycle pic.

Now if you tell us that your lever is pulling back all the way to the grip or that it's incredably hard to pull the lever well enough to stop at all then that's a different story. But XT levers working with most disc calipers should be easily capable of locking the rear with little effort. If it can't then your rear mounts or rear caliper setup is not right. This may be from bad alignment by the mechanic or it may be due to the frame being distorted and not holding the caliper on the right angle.

makeinu
07-28-08, 06:38 AM
To be sure, I can't lock up the rear wheel. Front brake, fully pulled, the front wheel is not going ANYWHERE. If I were to pull the front brake all the way while going 30 mph I would fly off the bike. The rear brake will only stop the rear wheel when the bike is already stopped. Push gently with the toe of a shoe and the rear wheel moves, albeit with a horrid squealing.

I don't so much want the rear brake to lock the wheel - It'd be nice to have the power to do so, but I'm utterly at the mercy of gravity on steep hills. The rear brake can slow me by 5 mph every fifty feet or so.

Going to try unscrewing the cable partially.

Edit: I think something's wrong with the brake itself. Unscrewed the cable for more tension, and the lever is now equally hard to pull as the front brake's - But stopping power has only improved marginally.

The brakes in question are AMP Research D-1s, just wondering if all disc brakes use the same pads or if I'll need to get in touch with AMP.

Szczuldo
07-28-08, 07:14 AM
To be sure, I can't lock up the rear wheel. Front brake, fully pulled, the front wheel is not going ANYWHERE. If I were to pull the front brake all the way while going 30 mph I would fly off the bike. The rear brake will only stop the rear wheel when the bike is already stopped. Push gently with the toe of a shoe and the rear wheel moves, albeit with a horrid squealing.

I don't so much want the rear brake to lock the wheel - It'd be nice to have the power to do so, but I'm utterly at the mercy of gravity on steep hills. The rear brake can slow me by 5 mph every fifty feet or so.

Going to try unscrewing the cable partially.

that seems like it would do the opposite of what you want. From the sound of it it sounds like either your rear pads are worn or there is too much pull and the pads aren't engaging the disk as well as your fronts are. Just tightening the cable to the rear should resolve this problem, but if it doesn't check the pads for wear. Also spin the rear wheel and make sure the disk is going through the caliper fine.

Joshua A.C. New
07-28-08, 08:07 AM
Just tightening the cable to the rear should resolve this problem,

Szczuldo, that's done by unscrewing the cable.

Makeinu, yeah, it sounds like the pads are worn out to me. Did it work at some point and has gotten weaker?

nitropowered
07-28-08, 08:24 AM
Loosen up the front brake. you really dont want to lock up your fronts.

I really think 90% of people like their brakes way too tight. whats the point of having your brakes lock up when you pull the lever a 1/4"? use the whole range of the lever movement so you have more control and modulation. When people ride my bikes they all freak out because the brakes are so loose (and reversed) they are all used to touch and it brakes

Mr. Underbridge
07-28-08, 10:34 AM
Bunch of possible causes:

1) Crappy pads. Stock Tektro pads, for instance, tend to suck. Kool stops are much better.
3) Gunky rims/pads - cleaning the rims and pads helps, many times.
4) Badly aligned pads - if you have a lot of toe-in, or if the angle's off top to bottom, you might not have a lot of brake surface actually touching the rim.
5) Lube on pads/rims - get a little lube on there and good luck trying to stop the bike
6) Not enough travel - it's possible you think it's locked but the lever's actually reached the end of its travel

If it's worth it to you, one test to do is to put the front pads on the rear rims. That should eliminate one variable, anyway.

Ray Dockrey
07-28-08, 02:54 PM
Bunch of possible causes:

1) Crappy pads. Stock Tektro pads, for instance, tend to suck. Kool stops are much better.
3) Gunky rims/pads - cleaning the rims and pads helps, many times.
4) Badly aligned pads - if you have a lot of toe-in, or if the angle's off top to bottom, you might not have a lot of brake surface actually touching the rim.
5) Lube on pads/rims - get a little lube on there and good luck trying to stop the bike
6) Not enough travel - it's possible you think it's locked but the lever's actually reached the end of its travel

If it's worth it to you, one test to do is to put the front pads on the rear rims. That should eliminate one variable, anyway.Most of this doesn't apply because he has disc brakes although switching the front and rear pads isn't a bad idea.

Mistawes
07-28-08, 05:49 PM
I had this problem setting up my racer with discs..

I adjusted the two little bolts which close the pads together and after a bit of fine tuning they were working perfect! It's just like moving the brake blocks closer to the rim on a bike with v-brakes.

On most bikes they tend to be Allen key bolts, (might help you locate them). One's the pivot, one holds the cable, two hold the brakes onto the frame and the other two are the ones you're looking for.

Hope this helps!

mparker326
07-28-08, 06:58 PM
I finally after 2 years fixed the stopping problem with my rear canti's. I had tried everything, new kool stop pads, LBS adjustment, cleaning my rim, and new cable & housing. Nothing worked. I then compared to my dad's old giant Mountain bike and his stopped fine. The only difference was that my cable stop was underneath my frame near the front of the bike. His was near the brake. I got a cable stop from my LBS and attached it near the rear brake and ran a new cable & housing. It stops great now.