Mushy brakes

[Holdsworth]

Whatever I do to try and improve the performance of my cantilever brakes (Tektro CR720) nothing seems to work. The pads have been changed, toed-in and set close to the rim and I have centred them as best as I can. The rear brake is especially poor and barely any stopping power and feels really spongy in use.

I really would like to fit plain old V-brakes but the STI levers won't allow it and would mean installing "travel-agents".

Any suggestions are welcome.

 

[HLaB]

I take it you have changed your brake cables (inner and outers) at some point, if so but have you considered changing your brakes brand to something like Avid Shortees?

 

[RecordAceFromNew]

Any suggestions are welcome.

If the front and rear have the same geometry (straddle wire angle etc.), I wonder if the longer rear cable could be a culprit.

Also sponginess is caused by high mechanical advantage. Canti mechanical advantage can be changed by changing the length of the straddle wire. Have you tried that? It is likely, though not necessarily, that shorter straddle/lower yoke means higher mechanical advantage.

 

[snailracer]

Sponginess is partly due to the pad/rim/arms flexing, but if those flex it's a good sign - it means the force is being transmitted through the cable. Canti brakes should feel a bit spongy, if set up correctly for a person with average grip strength.

However, sponginess can also be due to poor cables - inners stretch, outers compress. The outers compressing can be due to poor cutting or loose ferrules being fitted to the cut outers. Compared to cantis, V-brakes are more tolerant of poor cabling, because the cable compression is lower and cable travel longer.

So, check the brake cables as well as adjusting the straddle cable.

 

[PpPete]

These (on the rear only)
with Kool-stop pads

The "SE" design counteracts the mushiness inherent in rear cantis. Fiddly to set up.....make sure you follow each and every line of the (included) instructions to the letter, when done right the rear braking is remarkable. Be prepared to spend more on tyres though, easy to lock the rear wheel.

And I'll buy the redundant CR720 off you for the same price.

 

[Holdsworth]

I take it you have changed your brake cables (inner and outers) at some point?

The only cable that has been changed was the rear, when the original frayed after an attempt at removing the secondary brake lever to make more space on the bars. The outers stayed on as they were only a few months old and in good nick.

If the front and rear have the same geometry (straddle wire angle etc.), I wonder if the longer rear cable could be a culprit.

Also sponginess is caused by high mechanical advantage. Canti mechanical advantage can be changed by changing the length of the straddle wire. Have you tried that? It is likely, though not necessarily, that shorter straddle/lower yoke means higher mechanical advantage.

The rear brake has the yoke closer to the tyre than the front, it has been this way from new as it came like this from EBC, the main cable is shorter at the front so the yoke cannot be made to go lower. I still don't quite understand the principal of mechanical advantage, but should I try to raise the rear yoke and lengthen the straddle wire to lower the MA?

TBH both brakes are pants, I wouldn't trust the front one to perform an emergency stop. It can take quite a few yards to stop fully. Performance is noticeably worse than the single-pivot callipers on my road bike which are 20 years old, with them I can stop quickly with little force while on the hoods.

 

[RecordAceFromNew]

Assuming poor housing/ferrule issues snailracer usefully pointed out aren't the culprits, then if I were you I would try different yoke elevations to see what happens, probably starting with the front (since it is less dependent on cabling issues, and front being far more safety critical). Once optimised you can simply duplicate at the back. It is difficult for me to predict remotely whether you need a higher or lower yoke because it depends on your specific canti geometry. A useful tool that should assist, though perhaps not for the faint hearted, is this calculator:
http://www.circleacycles.com/cantilevers/
where the maths that helps to explain is at:
http://www.circleacycles.com/cantilevers/canti-geometry.pdf

It takes a little time to digest, but works ime.

 

[Holdsworth]

Just had a thought while looking at the CRC site, I have seen a pair of long drop callipers that looks like they may have enough clearance for my 32c tyres and mudguards. The mudguard mount holes on the forks and brake bridge should take them easy enough.

http://www.chainreactioncycles.com/Models.aspx?ModelID=72658

 

[PpPete]

The inevitable flexing of the long arms of very deep drop calipers like that introduces it's owm "mushiness".... and you'd still have the canti studs just sitting there looking ugly.
Seriously - I would try those Self-Energizers I linked to in my previous post. £8.49 inc P&P.
Reminder to self - buy another pair before they all disappear!

Oh yes and your mudguard mount holes on the forks and brake bridge may be drilled for nut mounted brakes ? Those Tektro 559 are allen key mount, so potentially some drilling out of frame to do.