Thursday, September 11, 2014

Fairwheel Bikes brake test: mechanical ratios

Fairwheel Bikes, the most technically engaged bike shop I know and one which has made an incredible impact for its size at numerous bike shows including Interbike and NAHBS, earlier this year published the results of a brake test on the WeightWeenies forum. They did a number of tests, including stiffness and braking force. Here I'll look at the brake deflection versus cable pull.

But first, here's the brakes in the test, with mass for the pair measured without brake pads or holders, and a price. No price is listed for the Campagnolo single-pivot, perhaps because it is typically sold paired with a dual-pivot brake (single for rear, dual for front):

brakemasscost
KCNC C7111 grams$330
THM Fibula116 grams$1429
KCNC CB3126 grams$335
EE Cycleworks139 grams$610
KCNC CB4150 grams$200
KCNC C6161 grams$180
Far and Near168 grams$290
Sram Red190 grams$350
Campag Single pivot212 grams 
Shimano 7900216 grams$400
Shimano 9000218 grams$400
Campag Dual Pivot232 grams$355

Brake pads and holders would add around 50 grams.

They attached the brakes, optionally with Edge grey pads, to an apparatus which pulled on the cable by a specific amount. By not using brake levers, they avoided conflunding the results of the brake with the design of a specific lever. For brakes designed to work with a specific lever (for example, Campagnolo, Shimano, and SRAM are presumably all designed to work best with their specific levers), this may provide misleading comparisons. But other, 3rd-party brakes don't have matching levers, so this allows comparison of different brake options. Since Fairwheel is a shop which specializes is less common, typically extra-light parts, this was the best approach. But the results need to be interpreted in the context that the lever will also make a difference, and different levers have different mechanical characteristics.

Jan Heine, in Bicycling Quarterly, has done an incredible review of the history of brakes. One conclusion from that work was that some relatively recent models which have been considered novel were instead based on designs which existed long ago. We sometimes think the history of brakes began with the single-pivot calipers which dominated in the 1980's and into the early 1990's, but this is incorrect. More recently, direct-mount brakes have become increasingly popular, but direct-mount brakes have a long, long history, pre-dating the single-pivot design.

In any case, Fairwheel tested a variety of brakes of different design. Campagnolo has two designs, a dual pivot and a lighter single-pivot. Shimano, which started the long run of dual pivot dominance (at least on road racing bikes) with its 105 brake in the early 1990's, is dual pivot, as is Far & Near and KCNC. SRAM Red is a modified single-pivot design. EE Cycleworks is relatively complex multi-pivot design: Jan did a specific review of that brake in Bicycle Quarterly which I highly recommend, but I don't have access to that article to reference right now (on train from Basel to Zurich: my BQs are sorted on my bookshelf in San Francisco). I wish they'd included the Camillo Zero-Gravity brake, which I use, but that's considered generally inferior: it uses a cam-enhanced single-pivot design similar to the SRAM Red which followed it.

As Jan described in his Bicycle Quarterly article, brake design is a balance between range of motion (allowing the brakes to clear the rims with sufficient margin) and mechanical advantage. If I pull the lever a particular amount the force exerted on the pad is proportional to the ratio of the cable pull to the pad deflection. This is why disc brakes are considered powerful: they sit extremely close to the rotor and so relatively little deflection is required, providing a high ratio of cable pull to pad deflection.

A compromise is to use a variable rate of deflection. As the cable is initially pulled, the brake pad moves relatively rapidly, closing most of the gap to the rim. But as the pad gets close to contact, the rate of deflection with respect to cable motion decreases, providing greater mechanical advantage. This provides the best of both: plenty of clearance for out-of-true rims or removing wheels with tires whose profile extends beyond the rim width, while allowing for plenty of braking force once the brakes contact the rim.

A key here is that the mechanical advantage felt by the hands on the brake lever come from three sources. One is the conversion of lever motion to cable pull at the brake lever. The second is the conversion of pull at the lever to pull at the brake. This is ideally 1:1 unless the cable stretches. The third is the conversion of cable pull at the brake to brake pad deflection. This third stage consists of two parts: one is the unloaded conversion ratio, and the other is a reduction in this ratio due to bending of the brake with increasing load. Stiffer brakes will maintain closer to the unloaded ratio. The important thing is the product of these stages. So I can tune the characteristics of the mechanical advantage either at the brake or at the brake lever.

This test looks only at the brake. Here I plot the inverse of unloaded mechanical advantage, which is the ratio of the brake pad deflection to the cable pull. A high value means the pads are closing rapidly, but the braking force is proportionally low. A low value means the pads aren't moving much, but the ratio of braking force to lever force is higher. I plot some of the brakes tested by Fairwheel, omitting only the KCNC models:

image

One thing to note about this plot: Fairwheel measured the brakes at 3 mm, 6 mm, 9 mm, and 12 mm of cable pull. There is also an implied measurement at 0 mm cable pull when the brake is not in contact with a rim. To measure the marginal mechanical ratio, I'd take the slope between measurement points, for example between 0 and 3 mm, or between 3 mm and 6 mm. This results in a slope estimated at the average pull of the two points. So using 0 mm and 3 mm cable pulls, I get the slope estimated at 1.5 mm cable pull. So from their measurements I get ratios at 1.5 mm, 4.5 mm, 7.5 mm, and 10.5 mm.

Then in this plot I show the KCNC brakes. I separated them because there are four of them and including them would have cluttered the preceding plot:

image

You can see that most of the brakes have a relatively uniform rate of cable pull to brake pad deflection, while a few have more variable relationships. The deflection axis here is logarithmic, so a given distance on the axis corresponds to a different ratio, as opposed to a different absolute difference, in deflection ratio.

The more variable relationships come from the Campagnolo single-pivot an the Shimano 9000 brakes. The Shimano 9000 has a relatively broad range of relatively uniform ratio between 3 and 9 mm of pull. The Campagnolo brake is more variable, however, with no real uniform range.

It may seem like a good idea to engineer the variable deflection into the brake. However, a disadvantage of doing this is that if the brakes are set up so the sweet spot of optimal mechanical advantage comes with the pads just touching the rim, when the brake pads wear the brakes will need to be deflected more to initiate braking. Either the "sweet spot" needs to be extended to accommodate a range of rim widths and brake pad thicknesses, which reduces the available clearance, or the braking quality will vary from optimal.

If instead you put the mechanical tuning in the brake lever, then when brake pads wear or a wheel with a different rim width (measured at the braking surface) is installed then all you need to do is rotate the barrel adjuster and you've restored the lever range of motion. The mechanical characteristics are thus in tune again.

So a uniform mechanical response is the most resiliant approach.

To analyze this, I took ratios as plotted (incremental ratios). The mean mechanical advantage is considered by looking at the average such ratio. The variability in the mechanical advantage I evaluated using the statistical standard deviation of the natural logarithm of the slopes. I took the natural logarithm because I'm interested in the fractional change of the mechanical ratio, not the absolute change.

Here's the sorted result:

brakevariabilityavg. ratio
Thm Fibula0.03100.5800
Sram Red0.03600.6075
Shimano 79000.04990.5267
Far and Near0.05360.6550
EE0.06300.6092
KCNC C60.06590.7117
KCNC CB40.06720.7900
KCNC CB30.06950.7900
Campag Super Record Dual0.07200.7050
KCNC C70.08260.6333
Campag Super Record Single0.10550.9150
Shimano 90000.13010.6167

So the Fibula, which is also the 2nd lightest of the brakes tested (116 grams compared to 111 grams for the KCNC C7), also has the most uniform mechanical ratio by a fairly good margin. Only Sram Red comes within 20%. The Shimano 9000 and the Campagnolo Super Record single pivot are the most variable, although as I noted the Shimano at least has a relatively uniform range mid-deflection, while the Campagnolo brake is variable through its range to the resolution of the test.

The EE brake is an interesting case. It scores only mid-range in ratio variability. But notice that the variability is due to the first 3 mm of cable pull. The brake has excellent linearity from 3 mm to 12 mm, as seen in the plot. This provides additional clearance when the brake is open relative to, for example, the THM Fibula or Shimano 9000 brakes. So the brake is fine, arguably better with this design.

The average ratio is is mathematically the same as the ratio of deflection with 12 mm of cable pull to that 12 mm cable pull. I show the same data ranked by this:

brakevariabilityavg. ratio
Shimano 79000.04990.5267
Thm Fibula0.03100.5800
Sram Red0.03600.6075
EE0.06300.6092
Shimano 90000.13010.6167
KCNC C70.08260.6333
Far and Near0.05360.6550
Campag Super Record Dual0.07200.7050
KCNC C60.06590.7117
KCNC CB30.06950.7900
KCNC CB40.06720.7900
Campag Super Record Single0.10550.9150

The Shimano 7900 has the least pad motion followed by the THM Fibula, Sram Red, EE, and Shimano 9000 all fairly tightly bunched behind. All of these brakes have an excellent reputation for stopping power, so it's clear the mechanical ratio is significant here. The Campagnolo Super Record single-pivot is a solid last place in this ranking, with KCNC brakes taking the 3 spots immediately behind (the CB4, CB3, and C6).

One note: these are unloaded deflection tests, and this translates to force only if the brakes are perfectly stiff, which they are not. Fairwheel also did several force-based tests but since I don't fully understand the results I won't comment too much on them here. But basically the actual brake performance under loaded braking may be different.

So what do I conclude from this? It seems that if you want a lot of stopping power, the Shimano 7900 is the top choice, but without much margin to clear the rim. The THM Fibula, EE, and Sram Red have a bit less mechanical advantage but more clearance. The EE gains a bit extra clearance by having an increased pad-to-cable motion ratio during the initial stage of motion. Since my rims aren't always perfectly true I like that, and so if I had to pick one of these curves, I'd probably go with the EE. But the super-low mass combined with a very uniform ratio through its range of motion make the Fibula also an attractive choice.

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