New Cannondale bike.. rear disk brake issues!!

rickeverett

My girlfriend got a Cannondale Synapse with mechanical disks. She has had if for 4 months now and the rear disk is terrible. I have spent ages trying to use the adjustment but its so weak that without the aid of the front, you cant safely stop the bike at all.
There is no chance of locking up the rear wheel. It's actually quite dangerous with no stopping power at all. The bike is 105 group but the brakes aren't shimano but promax. The pads are so close to the rotor that there's an ever so slight rub but still no power.

I still don't rate disks on road bikes and compared to my force rim brakes these promax ones are shocking.

Are we missing something or are there any tips on what to do a out it??

kajjal

Promax are relatively cheap cable discs but should work OK. One suggestion is accelerate the bike to a good speed and slam the brake on hard repeatedly for a few runs. This will heat up the discs & pads and burn off most contaminants.

Failing this if you get any oil or similar on the pads or disc then this stops them working properly. If it is only a little disc brake cleaner will get it off, if a fair amount you will need new brake pads and to thoroughly clean the disc with disc brake cleaner.

The sprye cable disc brakes are much better quality and easier to adjust.

marcusjb

IT sounds like the identical bike as my wife's with the Promax Render brakes.

I have to say, I have been impressed with them (and more importantly, Mrs. JB is happy with them). They are easy to adjust and they perform very well. She is still on the stock pads.

Contamination sounds like a real possibility. As above try some cleaner, but you might be as well to also change the pads.

They are plenty good enough brakes I reckon.

crankycrank

Take the pads out and sand off the pad surface with some 150-200 grit sandpaper on a flat surface until the pad surface is all new material. Works better than any liquid cleaners. Use some brake cleaner or alcohol with some Scotchbrite pads on the discs. If this doesn't work, time for new pads.

rickeverett

Many thanks for the replies. Tried down and sanding the pad and there's no real improvement...

Is it possible with disks to push the rear wheel through on full brake lock? On the front, this is REALLY hard to do but her rear disk can be pushed.

meanredspider

There's definitely something wrong - I hardly use my rear disc brakes because they lock so easily. I do nearly all my braking using the front only.

I'd start again with some new pads. Make sure the disc is spotlessly clean. Make sure that there's good movement in the calliper and its is well-aligned.

When all of that is right, bed the pads in. Ideally find a long hill - brake from speed down to almost zero braking as hard as you can WITHOUT locking the rear - you need to do this 6-10 times until the disc is properly hot. Then ride without braking and let everything cool down.

If that doesn't work (I've been riding mech road discs for 6 years) then there's something properly wrong.

Proff

I've got the exact same bike, with the exact same problem. The rear brake is garbage. Struggle to lock up the back wheel. I can pretty much pull the brake lever back to the handlebar. The front is fine. Could the problem be due to the extra length on the cable or maybe how it is routed.
My wife's Giant has the Spyre brakes and they are great. Maybe time for an upgrade.