flying_rhino

Hello,


Last fall I picked up a new bike - let's call it Bike B - on clearance, and after a few rides realized that the no-name brakeset was not very good. Bike A is one I have had for several years and after much trial & error is dialed in perfectly for me. Since Bike A has FSA SL-K brakes, I bought another set for Bike B. After one ride on Bike B upgraded with FSA brakes, it was clear that the braking performance was terrible and no where near what I was used to. So, on went a set of Kool Stop pads; no real improvement there. I tried switching the wheelset from Bike A onto Bike B, thinking that maybe their was a difference in the way that the sidewalls were machined. That test didn't change a thing.


On the old Bike A, I can brake hard enough to nose wheelie; on new Bike B that is not even close to being possible. I am running the same type of Jagwire cables on both bikes. The only other thing that I can think of that might be making the difference is is that Bike A has older Ultegra 10-speed levers, while the new Bike A has Tiagra 10-speed levers. In searching for similar problems to mine, I found mention of Shimano changing the brake pull ratio for their newer levers. Upon further searching though, I found that any levers with external derailleur routing (like the ones on new Bike B) still used the "normal" pull ratio, so the FSA brakes should work fine. I am at a dead-end and hope that someone out there is smarter than me and has some idea of what I can do to get the brakes working correctly. It is to the point where I do not like to go faster than 25 on the new bike because it takes so long to stop; on downhills, the rims heat up to the point where I can't touch them.


Thank you,


John

Andrew R Stewart 

Shimano has changed their lever VS caliper leverage specs a few times since STI came about. I suppose you could swap over the calipers between the two bikes to help confirm that the levers are the source of the difference. Andy

Rim heating- rims heat up by transferring the energy of motion into heat via friction. If the bikes have the same weight, same sweep area of the pads/rims and same initial speed then the same energy is exchanged, or the rims have the same amount of energy heating them up as the bike comes to a stop. Now how a rider senses this can be rather different from day to day or hill to hill.

AnkleWork

People hate measurements -- they get too close to the truth.

RoCou

Wilson, in Bicycling Science, demonstrated there is a descending speed at which the rim tempreature peaks; his test consisted of maintaining a descending bike at constant speed with constant brake application. Below the "peak" speed, the amount of energy absorbed by the brake + rim system is lower; above it, the aerodynamic cooling increases faster than the energy absorbed.

Perhaps you are demonstrating the same?

All kidding and theory aside, in general a weaker brake would not heat the rims more. At the same speed, you should get the same temperature, regardless of brake efficiency. This said, most probably you have a pull ratio issue, assuming the brakes are not set too far from the rim; you did not specify if you are running out of lever travel, or if you are running out of lever force?

AeroGut

This is a remote possibility but worth checking - make sure all of your brake cables have proper brake cable housing and not more compressible shift cable housing. Also that the housing is long enough in all places. Basically you’re looking for any potential “squish” in the system where you could be losing force.

flying_rhino

Running out of lever force - the same input as the old bike does not yield the same braking force. The pads are set moderately close to the rim since I ride the hoods most of the time and am only using 2 or 3 fingers to brake.

flying_rhino

Hi, are you suggesting their is a way to measure cable pull vs. lever movement? How would a guy do that and get reasonably accurate measurements?
Thanks,
John

Andrew R Stewart 

1- AeroGut- You have it backwards. Current SIS spec cable casing is considered by most all (manufactures included) as the compressionless type. It's more linear spiral of the casing strands are more able to resist compression. The common brake casing uses a spiral winding with more flexibility at the expense of more compression.

2- There have been a number of published attempts to measure lever and caliper mechanical leverage, cable travel and total system "power". John Schubert was the first in my biking like to do this in the late 1970s/early 1980s for bicycling Mag (and was it also for Bike World before too?). It's not hard to do so, just a lot of work and no one wants to pay for it. Andy

AeroGut

Yes, “compression” wasn’t the right word choice. I was referring to the way that the strands of shift housing can squeeze out the end and push past ferrules and housing stops under braking forces. The effect is the same as if the housing compressed- a squishy feeling and loss of braking force.

AnkleWork

A good start:
1. Disconnect the cable in question from the brake.
2. Mark the bare cable an inch+ or so from the last housing stop with a e.g. Sharpie while holding tension on the cable sufficient to remove the slack.
3. Hold a ruler next to the marked cable while someone pulls the lever fully. Observe.
4. Compare with the other bike, etc.

flying_rhino

Hello,
I thought about what you said and I do believe the weaker brake is heating the rim far more. On a downhill with my old bike I can grab a handful of brake and scrub off speed relatively quickly. On the new bike I'm riding the brakes almost constantly and that causes the rims to heat up more. Or I could be totally off-base; what say you?
Thank you,
John

flying_rhino

Hello,
Excellent suggestion, I will try it when I get some time. Thank you very much!
John