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Disc brake issue

Mick Tozer2022-02-27 09:41:59

Hi. We have a Dawes tandem that gets used for leisure and occasional triathlons with visually impaired folk,it had been  upgraded to large discs before we bought it and the braking was superb,and one of the reasons we got it.

Just stated riding it again after a long covid break. It was serviced just before lockdown.The back brake isn't anywhere near as powerful now,just slowing us down rather than stopping us.It's safe to ride but I feel I'm over relying on the front brake to pull up.

I' would have thought if the brake was working at all the braking effect would be the same.My question really is,( before I start checking it)is there a fault that can diminish the braking effect?The lever travel is fine.

Many thanks Mick

Alan Morgan2022-02-27 10:44:22

I'm not that familiar with disc brakes but here goes with some basics:

1. Are they cable or hydraulic?

2. If hydraulic, maybe a flush through or bleed is necessary after a long stand up?

3. Have the brake pads or wheel rims become contaminated with oil or grease? Car brake cleaner might help on this one. 

Mick Tozer2022-02-27 12:19:35
Thanks Alan. Ah yes , contamination, i bet that's it I'll check it tomorrow, they are cable operated by the way 
Matthew Bailey2022-02-27 12:50:31

If they are cable and lever travel is fine then I would remove the pads and give them a good going over with sand paper.

I would give the disc a light rough over with the sandpaper too, just very lightly.

Then find a big hill and get some heat into the brake. If no good then its time to try some new pads.

If you are running flat bars an upgrade to even a cheap hydraulic brake will be far superior in my opinion

Mick Tozer2022-02-27 20:26:41
Thanks Matthew. Great advice. The rear brake used to be very reassuring si i hope i can get that safe feeling back! 
David Ebling2022-02-27 20:58:20

It does sound like it could be pad contamination. If you have to replace the pads, clean the rotor and caliper with isopropyl alcohol or brake cleaner at the same time as you don't want to transfer any contamination to the new pads.

If the pads had got severely overheated on a descent, this could cause them to "glaze" which cam reduce brake power and make strange noises. This is easiest to fix with new pads.

Another thing to check is that the cable isn't contaminated with dirt or got rusty. This won't affect the distance that the lever travels but you will probably feel the friction. The force required to squeeze it may be bigger than the force it returns to your hand when releasing.

Mick Tozer2022-03-01 20:20:41

Thanks David I think I'll sort new pads anyway and clean the rotor.

I was interested to hear on a previous post that hydraulic discs are more powerful,so that might be a long term goal.

Chris Bishop2022-03-01 21:20:55

Are hydraulic brakes more powerful or just easier to use, that is require less effort on the brake levers?

What recommendations for cleaning discs please, do you have to keep it off the pads or can you clean them too?

David Ebling2022-03-02 09:16:12

Chris, I think a good hydraulic disc brake is more powerful than a good cable disc brake for the same rotor size. If there is less effort needed at the brake lever for the same deceleration, that means more of the force you apply goes into braking (and there is less friction) which means that with maximum effort you can brake harder. However it isn't quite that simple, and additional factors contribute to how good the brakes are. Firstly, the ability to control the power is every bit as important and the "feel" of the brake affects this. Cable disc brakes can be ok, but good hydraulics are better. I particularly like Shimano hydraulic disc brakes from this point of view. Another important factor is the ability to dissipate heat. This isn't about how hard you can brake but how long you can keep braking hard. There isn't necessarily a difference between mechanical and hydraulic in this area. There is the possibility of boiling hydraulic fluid, but your brakes should not be getting anywhere near that hot whether cable or hydraulic. You're more likely to get fade from overheating pads with any recent brake I suspect.

 

Regarding cleaning recommendations, I use IPA for cleaning discs. There are motorbike disc brake cleaner sprays that should be fine too. It should be fine on the pads - see here (although don't follow the advice on bedding in brakes that is linked at the end of that video, it is the opposite of all manufacturers and Park Tool's advice!)

Matthew Bailey2022-03-02 20:02:06

I would completely agree with David over the benefit of hydraulic brakes. 

As far as cleaning, disc brake cleaner spray on the disc will clean it. 

Although the cleaner won't hurt the pads i tend to avoid spraying anything on them. If a pad is contaminated and sanding the surface doesn't fix it then new pads are required. Once a contaminant has been absorbed its game over. 

Mick Tozer2022-03-02 20:03:28
Thanks David. Lots of very helpful information. Much appreciated.