Why's my back brake so rubbish?

For discussions about bikes and equipment.
nirakaro
Posts: 1004
Joined: 22 Dec 2007, 2:01am

Why's my back brake so rubbish?

Postby nirakaro » 18 Feb 2019, 12:14pm

Same brakes, Tektro CR730 cantis
Adjusted to same MA
Same rims, Ryde x-pace, cleaned with same methylated spirit
Same pads, Clarkes
Same levers, Sora.

The front brake is great – feels like it would have me over the bars if I snatched it too hard; the back brake's really feeble – I can actually pedal against it at full squeeze. What's going on here?

Brucey
Posts: 35515
Joined: 4 Jan 2012, 6:25pm

Re: Why's my back brake so rubbish?

Postby Brucey » 18 Feb 2019, 12:28pm

nirakaro wrote:Same brakes, Tektro CR730 cantis
Adjusted to same MA
Same rims, Ryde x-pace, cleaned with same methylated spirit
Same pads, Clarkes
Same levers, Sora.

The front brake is great – feels like it would have me over the bars if I snatched it too hard; the back brake's really feeble – I can actually pedal against it at full squeeze. What's going on here?


is there any such thing as CR730? If you are using fictional brakes you can expect fictional braking performance.... :wink: presumably you mean CR720. These are a low MA brake.

I expect your cables are bit crap. There may also be a small variation in boss height which will affect the brake MA.

But if you have a recent SORA model these levers have a low MA. I wouldn't use CR720 with low MA levers.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

nirakaro
Posts: 1004
Joined: 22 Dec 2007, 2:01am

Re: Why's my back brake so rubbish?

Postby nirakaro » 18 Feb 2019, 12:44pm

Woops - my mistake. i knew they were ten away from the 720 - they're actually CR710s!

Vorpal
Moderator
Posts: 17190
Joined: 19 Jan 2009, 3:34pm
Location: Not there ;)

Re: Why's my back brake so rubbish?

Postby Vorpal » 18 Feb 2019, 1:16pm

The obvious difference between the two brakes is cable length. Poor brake performance can often be explained by a dragging cable, wear in the sheath, or related things.

Try dribbling some teflon lubricant between the cable and housing? Otherwise, replace cable or housing, or both?
“In some ways, it is easier to be a dissident, for then one is without responsibility.”
― Nelson Mandela, Long Walk to Freedom

User avatar
Audax67
Posts: 4493
Joined: 25 Aug 2011, 9:02am
Location: Alsace, France
Contact:

Re: Why's my back brake so rubbish?

Postby Audax67 » 18 Feb 2019, 3:12pm

Front brakes are always more effective than back. IIRC about 2/3 of braking force is supplied by the front brake & only 1/3 by the rear.
Have we got time for another cuppa?

User avatar
Mick F
Spambuster
Posts: 46322
Joined: 7 Jan 2007, 11:24am
Location: Tamar Valley, Cornwall

Re: Why's my back brake so rubbish?

Postby Mick F » 18 Feb 2019, 4:04pm

The only difference between front and rear brakes, is the length of the cable-run.
Eliminate the inner cable friction, and eliminate the compression of the outer cable, and you'll have a great rear brake.

First thing is to buy a compressionless outer cable. Jagwire for instance.
Then, a good quality SS inner cable suitably lubricated.
Make sure there are no sharp bends in the run to the rear.

Do those three things, and you'll be amazed at how good your rear brake will be.
Mick F. Cornwall

User avatar
foxyrider
Posts: 4511
Joined: 29 Aug 2011, 10:25am
Location: Sheffield, South Yorkshire

Re: Why's my back brake so rubbish?

Postby foxyrider » 18 Feb 2019, 4:36pm

Mick F wrote:The only difference between front and rear brakes, is the length of the cable-run.
Eliminate the inner cable friction, and eliminate the compression of the outer cable, and you'll have a great rear brake.

First thing is to buy a compressionless outer cable. Jagwire for instance.
Then, a good quality SS inner cable suitably lubricated.
Make sure there are no sharp bends in the run to the rear.

Do those three things, and you'll be amazed at how good your rear brake will be.


The brakes might be the same f to r but the stopping moment is different. I can lock a back wheel but still be travelling forward, do it on the front and i'll be on the deck!

So whilst you can have the rear brake working fantastically it's effectiveness in halting you is much less than the front.
Convention? what's that then?
Airnimal Chameleon touring, Orbit Pro hack, Orbit Photon audax, Focus Mares AX tour, Peugeot Carbon sportive, Owen Blower vintage race - all running Tulio's finest!

User avatar
Mick F
Spambuster
Posts: 46322
Joined: 7 Jan 2007, 11:24am
Location: Tamar Valley, Cornwall

Re: Why's my back brake so rubbish?

Postby Mick F » 18 Feb 2019, 5:03pm

Yes of course.
The OP was asking about why his brake was rubbish, not the fact that he was skidding when he applied it.

In order stop skidding when applying a good back brake, you need to shift your weight. Not easy or quick enough in an emergency stop situation of course, but when descending a hill you have time to shift your weight rearwards and not skid.
Mick F. Cornwall

fatboy
Posts: 3346
Joined: 5 Jan 2007, 1:32pm
Location: North Hertfordshire

Re: Why's my back brake so rubbish?

Postby fatboy » 18 Feb 2019, 10:05pm

What's the bike? My old tourer was a compact frame and the cable routing was rubbish. Made it better by not using the cable stops and running the cable outer the whole way.
"Marriage is a wonderful invention; but then again so is the bicycle puncture repair kit." - Billy Connolly

gregoryoftours
Posts: 1016
Joined: 22 May 2011, 7:14pm

Re: Why's my back brake so rubbish?

Postby gregoryoftours » 21 Feb 2019, 10:21pm

Flexy seat stays can also contribute to a badly performing rear brake, in which case a brake booster brace may help. But yes good quality compressionless cable correctly fitted should be your first port of call.

peetee
Posts: 1355
Joined: 4 May 2010, 10:20pm

Re: Why's my back brake so rubbish?

Postby peetee » 21 Feb 2019, 11:15pm

Yes, quality cable can make a difference but only if it is trimmed to the optimum length and the ends cut with care.
With some concealed cable levers the cable outer can feel like it has seated correctly at the lever but it isn't. once bar tape is applied the outer may be located enough that every brake application pulls it toward the lever and it then springs back when the brake is released. Even a couple of mm movement can make the brakes feel terrible. Most Shimano Sti units are designed to take cable outers without caps.
Another issue with a rear brake cable can come about if the bike frame has cable stops and there is a short length of outer on butting up to the caliper. If this outer is not the optimum length it can introduce an excessive bend that causes friction. if it's too short the caliper arm may not be able to drop enough to allow the pad to contact the rim. The other pad has to move further to create any braking friction and a lot of hand effort is wasted deflecting the rim.
Short cable can also prevent the caliper backing off as, without a slight bend, the cable resists the upward movement of the caliper arm in much the same way as your knee joint can lock out if your saddle is too high.
Current status report:
Latter side of fifty and feeling less than nifty.
Too many bikes on pegs and too few miles in the legs.

MikeF
Posts: 3727
Joined: 11 Nov 2012, 9:24am
Location: On the borders of the four South East Counties

Re: Why's my back brake so rubbish?

Postby MikeF » 22 Feb 2019, 8:55pm

peetee wrote:Yes, quality cable can make a difference but only if it is trimmed to the optimum length and the ends cut with care.
With some concealed cable levers the cable outer can feel like it has seated correctly at the lever but it isn't. once bar tape is applied the outer may be located enough that every brake application pulls it toward the lever and it then springs back when the brake is released. Even a couple of mm movement can make the brakes feel terrible. Most Shimano Sti units are designed to take cable outers without caps.
Another issue with a rear brake cable can come about if the bike frame has cable stops and there is a short length of outer on butting up to the caliper. If this outer is not the optimum length it can introduce an excessive bend that causes friction. if it's too short the caliper arm may not be able to drop enough to allow the pad to contact the rim. The other pad has to move further to create any braking friction and a lot of hand effort is wasted deflecting the rim.
Short cable can also prevent the caliper backing off as, without a slight bend, the cable resists the upward movement of the caliper arm in much the same way as your knee joint can lock out if your saddle is too high.
That length of that piece of cable can be quite critical. The CR 720 is a centre pull canti brake so for that brake the blocks are not affected in the same way as for a caliper. I've deliberately fitted a CR720 on the rear brake of one of my bikes. It's stopping power for a rear brake suits me fine as I don't want to lock the rear wheel as that negates braking effectiveness.
"It takes a genius to spot the obvious" - my old physics master

gregoryoftours
Posts: 1016
Joined: 22 May 2011, 7:14pm

Re: Why's my back brake so rubbish?

Postby gregoryoftours » 22 Feb 2019, 9:47pm

peetee wrote:With some concealed cable levers the cable outer can feel like it has seated correctly at the lever but it isn't. once bar tape is applied the outer may be located enough that every brake application pulls it toward the lever and it then springs back when the brake is released. Even a couple of mm movement can make the brakes feel terrible.


A good way to minimise the possibility of this happening is to completely install the cable and set the brake up before taping. When it comes to taping the top run of the bars that the cables run under, first strongly apply the brake and secure the lever with a toe strap. This pulls the end of the outer firmly into the stop of the lever and into the stop on the other end of that section of outer, and holds it there as you tape the last bit of bar.
Last edited by gregoryoftours on 22 Feb 2019, 9:50pm, edited 1 time in total.

Brucey
Posts: 35515
Joined: 4 Jan 2012, 6:25pm

Re: Why's my back brake so rubbish?

Postby Brucey » 23 Feb 2019, 10:19am

MikeF wrote:….. I've deliberately fitted a CR720 on the rear brake of one of my bikes. It's stopping power for a rear brake suits me fine as I don't want to lock the rear wheel as that negates braking effectiveness.


OP has CR710 brakes but the same applies re cabling. FWIW I have ended up with a low MA canti on the back of my most used bike and a high MA one on the front; this works well enough but it is however by sloth not design; one or other of the brakes has been due to be changed for, ahem, 'some time'..... :oops:

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

MikeF
Posts: 3727
Joined: 11 Nov 2012, 9:24am
Location: On the borders of the four South East Counties

Re: Why's my back brake so rubbish?

Postby MikeF » 23 Feb 2019, 7:22pm

Brucey wrote:
MikeF wrote:….. I've deliberately fitted a CR720 on the rear brake of one of my bikes. It's stopping power for a rear brake suits me fine as I don't want to lock the rear wheel as that negates braking effectiveness.


OP has CR710 brakes but the same applies re cabling. FWIW I have ended up with a low MA canti on the back of my most used bike and a high MA one on the front; this works well enough but it is however by sloth not design; one or other of the brakes has been due to be changed for, ahem, 'some time'..... :oops:

cheers
CR710. I missed that as well as my spelling mistake "it's" :oops:
"It takes a genius to spot the obvious" - my old physics master