Rear rim brake won't stop bike

twowheels
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Rear rim brake won't stop bike

Postby twowheels » Sat Jan 26, 2008 11:24 pm

I'm resurrecting an older bike. In order to get it moving I had to be pretty liberal with cleaning etc the drivetrain. Once I got it moving I noticed the rear brake won't bring the bike to an emergency stop. So I degreased the rim, washed it off with detergent, "wet and dryed" the rim braking surface (aluminium - Mavic), then rinsed off with water. While doing this I soaked the pads in degreaser and then gave them a deglaze on the wet and dry.
Still the brakes won't perform a complete/emergency stop on the rear brake.
Front brake does. So I swapped over pads front to back. The pads from the back will perform an emergency stop on the front, but the pads that performed well on the front still won't give a complete/emergency stop on the rear brake, ie coasts to an eventual stop over 1.m metres from only about 5kmh.
What is the next thing to try?

stryker84
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Postby stryker84 » Sat Jan 26, 2008 11:30 pm

Um, do you mean that you can't slam on the rear and the bike slams to a stop? That's because most stopping power is from the front brake. Have a read on Sheldon's site as to why this is the case. Rear braking on its own won't usually emergency stop, more often it'll cause a rear wheel lock and skid.

However, if you mean that the rear brake is working less efficiently than it should, then I'm not sure. Check the brake mechanism, that the pads are contacting the rim firmly and smoothly? Sorry, more technical details I don't have.

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sogood
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Postby sogood » Sat Jan 26, 2008 11:40 pm

Some brake calipers are just crapy with poor stopping power.
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twowheels
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Postby twowheels » Sat Jan 26, 2008 11:49 pm

stryker84 wrote:do you mean that you can't slam on the rear and the bike slams to a stop? Rear braking on its own won't usually emergency stop, more often it'll cause a rear wheel lock and skid.
quote]

I'll have another read of the Sheldon Brown, but yeah that is exactly what I mean. I'm comparing the braking efficiency of another rim brake bike I've got that I can emergency stop on the rear brake, ie I can rear wheel lock & skid on most (read all) bikes I've owned before, skid is modulated to controlled stop at brake lever

stryker84 wrote:However, if you mean that the rear brake is working less efficiently than it should, then I'm not sure. Check the brake mechanism, that the pads are contacting the rim firmly and smoothly? Sorry, more technical details I don't have.


Done these checks.

twowheels
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Postby twowheels » Sat Jan 26, 2008 11:50 pm

sogood wrote:Some brake calipers are just crapy with poor stopping power.


Agreed, though in this case Shimano 600, on Nishiki frame, former road racer.

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Postby sogood » Sat Jan 26, 2008 11:56 pm

twowheels wrote:Agreed, though in this case Shimano 600, on Nishiki frame, former road racer.

Poor cable adjustment? Faulty installation (poorly cut cable outer and anchorage)? Old cable that's "stretchy"?

One other thing is, rear brake cable is longer and there will be more stretch.
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Kalgrm
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Postby Kalgrm » Sun Jan 27, 2008 12:37 am

All of Sogood's answers, plus try isopropyl alcohol from Bunnings (on the bike, not in your Coke .... ;)) The alcohol will disintegrate any oily residue on the rims.

You will, in all likelihood, need to replace those pads. Old ones deteriorate and no longer grip as they should.

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Graeme
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Kid_Carbine
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Postby Kid_Carbine » Sun Jan 27, 2008 2:24 am

Since the rear pads are now on the front & working well, & the previously good working front pads are now on the rear, & NOT working well, I am going to rule out CoF between pad & rim & instead, look to the mechanical components.

My suggestion is to drop the rear wheel out & simply apply the brake. The lever should move all the way to the bar both smoothly & with freedom of movement that will tell you that all is well with the lever, cable & caliper.

If the movement of the lever is not free &/or it does not move all the way to the bar, & I suspect it may not, then disconnect the cable from both ends & test each of the three elements again for full & free movement. [lever, caliper & cable]
Personally I would not be surprised if you found a problem with the cable since that's the most common brake problem that I have been finding in my fleet of hire bikes. In my case they are usually dry & with a little corrosion, but sometimes with damage to either the sheath or inner wire.
Sometimes removing the inner wire & fully lubricating bith the inside of the cable outer as well as the wire is enough, sometimes replacement is required but all components must have full & free movement. Any drag or sticking is a sign that something needs attention, & chances are that the pads are probably fine since they worked well when they were on the front.
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Postby thomas_cho » Sun Jan 27, 2008 9:38 am

I am not convinced that getting your rears to lock is a sign of good braking.

If you set your rears to lock and skid, that would throw out your rear and cause you to lose control. In fact ... Campagnolo's current brake design have a single pivot rear design to precisely prevent rear brake lock.

Your main stopping power comes from the front brakes. The rears just slow you down. Sheldon brown explains it very well ...

I am sure you will get an improvement in responsiveness if you change out the inner and outer cables.

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Bnej
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Postby Bnej » Sun Jan 27, 2008 10:35 am

thomas_cho wrote:If you set your rears to lock and skid, that would throw out your rear and cause you to lose control. In fact ... Campagnolo's current brake design have a single pivot rear design to precisely prevent rear brake lock.


It's primarily to save weight, you can still lock the wheel with enough force on the lever. Because you can't stop any harder than that, they reckon you don't need the extra mechanical advantage at the rear.

I'd change the brake pads, because once they're old enough they tend to harden and not work as well. I'd also change the cables and outers, because if the outers have deteriorated then they can stop doing their job properly which will reduce the amount of cable pull you get. Inners too because it's a couple of bucks more and if there's rust or wear you can stop worrying about it!

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europa
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Re: Rear rim brake won't stop bike

Postby europa » Sun Jan 27, 2008 11:03 am

twowheels wrote:What is the next thing to try?


New pads, new cables. Both are dirt cheap and both will make a dramatic difference to the way she brakes. Go for it son.

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Kid_Carbine
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Postby Kid_Carbine » Sun Jan 27, 2008 3:14 pm

thomas_cho wrote:I am not convinced that getting your rears to lock is a sign of good braking.

If you set your rears to lock and skid, that would throw out your rear and cause you to lose control. In fact ... Campagnolo's current brake design have a single pivot rear design to precisely prevent rear brake lock.

Your main stopping power comes from the front brakes. The rears just slow you down. Sheldon brown explains it very well ...

I am sure you will get an improvement in responsiveness if you change out the inner and outer cables.
This is an "older" bike, so I assume the brakes are conventional side pull or center pull from the period up to the 80's. If this is true, then it should have the 'ability' to lock the rear with relative ease.
I have no idea how anyone can "set the rear [brake] to lock & skid" since the performance of the brake is entirely dependant on the rider & if one has enough ability to pedal & stay upright at the same time, then one has the ability to control the brake lever.
If the mechanical design of the brake is so poor that it does not have the 'ability' to lock the rear, then it's a good candidate for landfill.

If you have a conventional caliper brake, then it will do no harm to strip it down, clean it completely, then lubricate the pivot parts, re-assemble & carefully adjust it so that it does not bind, yet has no looseness in this pivot.
You should not have any real difficulty in restoring braking performance to as good as new.
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twowheels
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Postby twowheels » Sun Jan 27, 2008 6:09 pm

Thanks all for your responses.

THe performance of the rear brake was so poor I couldn't feel any brake response while down changing gear with my right hand and braking before a corner with my left hand/rear brake. I realised things could get a bit hairy.

Anyway this morning I gave the rim a good rub down with metho and lubed the cable & outer with Inox, ie slid the outer off enough that the cable remained in the outer, so lubricant gets forced within strands of inner >>> noticed some improvement.

On the way back from the beach picked up some new pads by "Baradine" , $25 was more than I expected to pay and also a new brake cable & outer.

Fitted the Baradine pads, voila, problem solved. They seem to have a ball & socket within the pad so as you apply the bake they tilt in relation to the rim surface and by placing a shim at the rear of the pad you can establish toe in automatically.

MMMMmmmmm, Baradine brake pads

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sogood
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Postby sogood » Sun Jan 27, 2008 10:09 pm

Bnej wrote:It's primarily to save weight, you can still lock the wheel with enough force on the lever. Because you can't stop any harder than that, they reckon you don't need the extra mechanical advantage at the rear.

Yep, single pivot on my Campy rear caliper is certainly strong enough to lock up, as experienced a week and a bit earlier. Coming round a corner and suddenly seeing a car sitting there blocking the lane... Rear locked and skidded sideways. Fortunately was able to bring it back and stayed upright.
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