Disclaimer: I have not tried these combinations or products, I have not confirmed these measurements, I do not recommend doing this. These, as always, are personal notes because it is related to an issue I have. Proceed at your own risk.
There are a fair number of older 26″ bikes which would make excellent commuters, tourers, or tackle the gravel paths that are trendy nowadays. However, even big box department stores now come with 650b and 700c wheels, and support for 26″ parts is dwindling. While you can still get cheap slicks and knobbies, the number of high quality tire options have diminished outside of some niches like touring tires. On a tangent, 650c support is being dropped by the industry for 650b road support, which makes little sense as 650b is not that much smaller, and this is why I have always been skeptical of proportional size “no-compromise” small bikes. Using non-standard parts is in fact a huge compromise. Fast slick conversion tires are extremely rare. It is also increasingly difficult to find replacement rims for 26″. My own issues stem from a 26″ tired pavement tandem (26″ wheels are used for the improved bracing angle). 40h/48h rims are rarer than 32h/36h, especially with classics like the CR-18 and Rhyno Lite being discontinued. Affordable, fast pavement tires are limited, seeing as I’m not a huge fan of Gatorskins, Paselas aren’t that fast, and the actually fast tires cost an arm and a leg.
The traditional method for this conversion is an adapter plate that mounts to the old canti bosses allowing the canti bosses to be moved up. These are hard to find.
However for some reason, folding bikes seem to be extremely popular in China, and there are a large number of products to allow conversions from 406 to 451 wheels because 451 tends to have better fast tire availability. There are 4 such relevant products that will be briefly discussed, although the focus will be primarily on linear-pull brakes. These are 1) linear-pull brakes with extended brake pad slots, 2) brake pad slot extenders, 3) 2:1 cable pulleys like travel agents and 4) cantilever boss relocation adapters.
I don’t like the cantilever boss relocation levers. They require a removable stud, the stud needs to have the right thread, and it relies of being tightened and the spring hole to prevent braking forces from rotating the adapter. The old horseshoe shape adapters are better in this regard. I don’t like the pad slot extenders because it adds another point of failure and flex, it adds thickness to the pad, and it’s a roundabout solution. It is effectively the same as the long slot linear-pull brakes. Whether or not these criticisms are justified is another matter, but as I personally dislike them, I will not discuss them further.
The conversion linear pull brake is a ~108mm long brake with an extended slot for extra adjustability. There are cheap versions for ~$20 front and rear, as well as upgraded CNC finished versions with what appear to be ball bearing pivots for ~$50. These are meant to allow conversion from 406 to 451 wheels, which is a 45mm difference in diameter, or 22.5mm difference in brake reach. This is a bit less than the 31.5mm required for a 559 to 622 conversion, but from the pictures, the slot appears to extend well past the middle of the arm. Naturally, this results in a change in leverage. There are also other options which might offer similar adjustment such as the motolite and a number of vintage MTB brakes, but these are excessively expensive for just updating an older bike or out of production.
The other product is a linear-pull brake cable pull pulley adapter. Using a pulley with two diameters, it pulls twice as much cable at the brake than at the lever, with a corresponding loss in force. For those concerned about fatigue, it is in fact possible to make such an adapter that does not rely on pulleys, and such a device exists to adjust the cable pull of the clutch lever on motorcycles. However, these pulleys are readily available, and it is relatively easy to inspect for fraying.
First, it is required to discuss how brake cables actuate brake arms. Cam and linkage brakes will be ignored for this purpose. Otherwise, there generally exists two families of commonly used rim brakes, the direct pull type which directly pulls on brake arms such as sidepulls and linear-pull brakes. The second is centerpull types including traditional cantilevers which utilize a straddle cable. With centerpull types, even if the centerpull arm appears to have high leverage, the cable pull movement is duplicated for both arms. With direct pull types, this is halved because the cable pull movement is split between the two arms.
Second, it is required to discuss linear-pull brakes. Linear-pull brake arms have mechanical advantage of ~4:1 given that the arm is ~108mm and the distance from the canti pivot to the pad is ~27mm. Linear pull brake levers pull about twice the amount of cable (unconfirmed), so the effective mechanical advantage of linear-pull brakes is still quite high at ~2:1, which may be a reason people likely feel that linear-pull brakes feel so powerful. The lever only partially compensates for the change in leverage. Mini-Vs are about 80-84mm and are ~3:1. Lengths of mini-Vs are more about clearing the tire rather than having ideal leverage. These ratios seem very high, like they should be halved again, but I have checked on paper that the previous explanation the preceding paragraph is correct, but I should check this empirically. Unfortunately I don’t keep many flat bar bikes around.
Based on the preceding premises, below is a chart of relative mechanical advantages:
|New Shimano Sidepull||n/a||n/a||~1.4-1.5|
|Dual Pivot Sidepull||n/a||n/a||~1.3|
|Single Pivot Sidepull||n/a||n/a||~1.0-1.2|
Notes on the above: Linear-pull brakes are calculated from specified arm lengths (108-110mm for long, 80-84mm for mini) and the standard pivot to pad spacing of ~27mm. In terms of design the primary determinant of length is the need to clear above tires. New Shimano sidepull is often listed as 1.4:1, but when I checked the newest ones with symmetrical pivots, it seemed to be closer to 1.5:1. Older Shimano dual pivots and other brands are often listed at around 1.3:1, which is about right as one half is a single pivot and the other half is half of the symmetrical dual pivot. Symmetrical dual pivots actually predate the half-and-half, but Shimano’s half-and-half popularized the design. Single pivot ratios are taken from Berto. Cantilevers can vary based on design and setup, but it can be assumed to be stronger than single pivot and weaker than halved linear-pull, so around dual pivot strength. Centerpulls vary based on design and straddle length. A 750 is about 1:1, a 610 is about 1.3:1, a Racer is somewhere in between.
Note that it would appear that mini-V plus a travel agent should give much closer to a dual pivot feel, but it seems most people running linear-pulls with standard road levers have 3:1 for mini-Vs or 2:1 for linear-pull w/ travel agent. Halving these values would put them more in line with standard road brake options, such that it feels I have made a calculation mistake somewhere, but I have checked multiple times.
The conclusion is that 1:1 is weak like vintage “speed modulators,” 4:1 is borderline non-functional, 3:1 is within the range of barely tolerable with high potential for rubbing and clogging with mud, 2:1 is strong but retrogrouches dislike it, and 1.5:1 is strong and most people have no complaints. Reasonably, this means the target range for brake mechanical advantage should be in the range of 1.3-2, after adjusting for lever and pulleys for cable pull.
Below is the table for leverage with the brake with extended slots:
|26″ to 650b||108||40||2.7||1.35|
|26″ to 700c||108||59||1.8||0.9|
As expected, using the linear-pull brake with 26″ wheels on 26″ canti bosses functions exactly the same as any other linear-pull brake and requires a linear-pull brake lever or cable pull adapter. Using it for 650b puts it in the range of a mini-V, but halving it puts it closer to a dual-pivot, meaning either may be viable depending on preference. Using it for 700c results in a cable pull between a strong dual-pivot and cable pull adjusted linear-pull brakes. Doubling cable pull for 700c would result in a weak brake. Keep in mind that tire clearance for larger rims will be limited and there will be considerably less clearance than a mini-V with 700c.
Also note that force applied to the brake lever does not translate directly to clamping force based on leverage alone due to return springs and cable friction. Even with the same mechanical advantage, a brake with a strong return spring would require more hand strength for the same clamping force, and it is noteworthy that cantis and linear-pull brakes have adjustable return springs. A lighter spring can give more tactile feedback when feathering because it changes the ratio of force acting against the spring versus force being translated into clamping force. Further note that clamping force is not the same as braking power. Friction is based on the force and the coefficient of friction, and the coefficient of friction varies based on brake compound, as anyone who has replaced 30 year old hard rubber pads with fresh salmon pads can attest to. Pad choice can also affect the feel of initial bite. In addition, note that flex in the system can affect if the lever bottoms out, and that a flexible sidepull with 1:1 leverage can bottom out due to flex in the system and excessive pad clearance. Even though a higher mechanical advantage requires more cable pulled and therefore more lever movement, stiff components, well seated cables, and compresionless housing mean less cable/lever pull wasted on flex and allow greater pad clearance to reduce rubbing and clogging. Lastly, note that disc brakes vary in strength and few (there are some) complain about the excessive strength of disc brakes and the reduced lever effort. User complaints often have to do with familiarity to with an expected response or lack of feel.
It appears possible to breathe new life into 26″ canti frames with 650b and 700c wheels using these folding bike adapter linear-pull brakes. The major caveat is tire clearance. Tire diameter can not deviate far from a wide 26″ tire. The exact leverage may not be ideal depending on the rider, but the mechanical advantage seems to fall in the range of other commercially available options for rim brakes. There also exist other options to tune them closer to personal preference, although there is no guarantee that an ideal setup can be achieved.