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The Definitive (But Wildly Inaccurate) Guide to Brake Cable Pull Ratios

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The Definitive (But Wildly Inaccurate) Guide to Brake Cable Pull Ratios

Old 02-14-17, 09:20 AM
  #26  
joejack951
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Resurrecting to add some data points (measured pivot radius and calculated 20 lever travel cable pull)

Campagnolo Chorus 11 (2016)
  • 24mm
  • 8mm
Shimano Ultegra 6600
  • 18mm
  • 6.25mm
Shimano Ultegra 6500
  • 18mm
  • 6.25mm
Shimano 105 5800
  • 36mm
  • 12.5mm
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Old 02-14-17, 10:26 AM
  #27  
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Thanks for updating, @joejack951!

I'm not sure about the 5800 data. I've got a set of those levers, but they're hooked to disc brakes without a simple release that would give me complete access to the relevant points, but as I recall the reasons I didn't include that were (1) the cable attach point was such that the measurement wasn't easy to perform accurately, and (2) the lever is designed such that it wraps the cable around an irregularly shaped surface.

The second point here is the relevant one. Anyone better at measurement than I am (which is almost everyone) can address (1), but (2) is an intrinsic issue with how this lever pulls able. That is, the method I was using depends on measuring the radius of the circle being traversed by the cable, but with the 4700/5800/6800/9000 levers this radius is variable so the lever pulls cable at a different rate at different points in its motion.

Note that this corresponds to what Shimano says about the change in cable pull from the 4600/5700/6700/7900 ("New Super SLR") to the 4700/5800/6800/9000 ("SLR EV"), which is that the cable pull is compatible but that it pulls at a different rate.

So, as a sanity check, our measurements should indicate that the cable pull rate of a 5800 lever is very similar to the cable pull rate of a 5700 lever. I'm not saying that the measurements shouldn't be the final word, but I am saying that if the measurement is significantly at odds with expectations (which yours is) then it should be double-checked.
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Old 02-14-17, 10:56 AM
  #28  
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Originally Posted by Andy_K View Post
So, as a sanity check, our measurements should indicate that the cable pull rate of a 5800 lever is very similar to the cable pull rate of a 5700 lever. I'm not saying that the measurements shouldn't be the final word, but I am saying that if the measurement is significantly at odds with expectations (which yours is) then it should be double-checked.
Interesting. I just did a quick sanity check and my measurements seem to agree with this (using 105 5800 and Ultegra 6600 levers). I need to do a more precise check though because I'm using levers on two different bikes with very different handlebar shapes (one classic curve and the other erg-style). I am not certain that I am pulling the levers equal amounts as a result of this and they very well could be quite different. So don't quote me on the above!

If I get a chance in the near future, I'll swap calipers between the bikes and see what sort of change it makes as well as attempting to figure out a way to accurately compare degrees of lever travel and the resulting cable pull.
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Old 02-14-17, 11:03 AM
  #29  
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Originally Posted by rmfnla View Post
My measurement system is I mount the brakes and levers I want to use and then try them.

By riding.

If they stop to my satisfaction then the measurements are good, if not, they are bad.

Haven't made any mistakes yet...
Not at all scientific, but kind of fun in this context: I have a bike set up with two different handlebar, stem, levers and calipers. A fix gear I use for climbing (with bigger cogs but still usually very high gears making for long stretches out of the saddle on the hoods. One HB set is for flat ground; road bars, regular Tektro levers and SunTour Superbe sidepulls. Very good stopping like a good '80s racing bike. The other is a wide, deep pair of track bars with V-brake Tektros and Shimano dual pivots. I ride this bike in the drops most of the time except climbing. The brakes take a bigger grab to stop but they do stop very well and I skid/lock up the rear far less. I can descend roads I do not know with far more confidence that I can shut down the fix gear I am riding at 200 RPM to make that corner that is steeper and tighter than I expected.

I like the second setup sop much I duplicated it on my good bike. Yes I do pay some penalty braking hard on the hoods but I just see that as a reminder to ride as the coaches taught us 40 years ago - when the riding gets dicey, ride the drops.

In exchange, the huge plus - wonderful handles when out of the saddle!

Ben
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Old 02-14-17, 11:12 AM
  #30  
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I just did a quick check with my 5800 levers, and I think I know what's going on. The cable attaches at a point near the front of the lever (the red arrow below) which is indeed 36mm from the pivot, but it is suspended above the plastic for a length over which it can change its angle, so the effective attachment point for purposes of this calculation is the point at which the cable's movement becomes constrained by the inner structure of the lever (the green arrow below) and that's closer to 21mm from the pivot.

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Old 02-14-17, 01:16 PM
  #31  
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Here's a slightly more visual explanation (motivated by my own efforts to convince myself that what I said in my previous post was correct).

The model I was using to calculate cable pull based on the pivot radius is assuming that the amount of cable pulled is approximately the same as the distance traveled by the anchor point. I mentioned in post 18 where I introduced this method that the cable pulled was actually the chord cut across the virtual circle traveled by the pivot point but for most levers that's a pretty good approximation. This assumes that the pivot point is near the top of the virtual circle. (See Figure 1 on the attached image.)

If the cable is anchored significantly left of the top (relative to the cable exit) of the virtual circle, the approximation would break down, as seen in Figure 2. Here the amount of cable pulled is actually the difference in the distance from the original anchor position to the exit point and the distance from the final anchor position to the exit point, assuming nothing restricts the cable's freedom of movement within the virtual circle.

However, the geometry of Shimano levers since the 5700/6700/7900 series is more complicated. These levers have the cable anchored toward the front of the virtual circle, but the cable rests on a "shelf" that is near the top of the virtual circle. The result is that the cable pull is actually determined by the travel of the resting point, not the travel of the anchor point. Figure 3 attempts to illustrate this idea. You can see there how the inner circle interferes with the movement of the cable and makes the original approximation good, if based on the inner circle rather than the outer circle.

The fact that the cable rests on a shelf rather than a single point complicates this a little bit more. If you imagine a flat shelf extending along the blue line in Figure 3 and then rotating with the circle, you'll see that the effective diameter of the inner circle (i.e. where it pushes against the cable) will increase as the lever rotates, meaning that it would pull more cable later in the motion of the lever. And if that's not enough to confuse you, I believe the Shimano levers don't use a flat "shelf" but rather a curved one so that the rate at which the effective inner circle radius changes is reduced.

In any event, this is a fine t***ng effect and the simple approximation based on the inner circle tells you enough to understand approximately how the brake will respond.
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Old 02-14-17, 01:27 PM
  #32  
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Originally Posted by rmfnla View Post
My measurement system is I mount the brakes and levers I want to use and then try them.

By riding.

If they stop to my satisfaction then the measurements are good, if not, they are bad.

Haven't made any mistakes yet...
I had a calculus teacher in college who once told us a story about an engineer, a physicist and a mathematician who were each asked to prove that you could boil a pot of water.

The engineer wanted to know how many BTUs the stove produced, what the expected ranges of air temperature and barometric pressures were, how pure the water to be boiled was going to be, how much water was to be boiled, how much time was available to boil the water and so on.

The physicist made some assumptions, looked up the specific heat of water and proceeded to perform some calculations.

The mathematician poured a pot of water, put it on the stove and without so much as turning on the stove declared, "QED!"
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Old 06-20-17, 11:40 AM
  #33  
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Having just measured one of my Rival levers (2011), the distance between the centers of the pivots is 23 mm, same as Shimano 4600/5700/6700.
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Old 07-07-17, 08:04 AM
  #34  
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I posted earlier that campag has a 24mm pivot for all 10 speed but I just measured an older 2nd gen 10 spd lever (with the rounded hood top) and it measures about 19mm. This is curious as it implies that campag changed the ratio between 2nd and 3rd (current shape) levers without publicizing it or changing the calipers -or perhaps the skeleton calipers are a slightly different ratio?
Note both levers are Ultra shift veloce but I'm almost certain all the levers of the same shape share geometry. Of the the current 11 spd, I'd say they are the same as the 3rd gen 10 spd as the skeleton calipers have remained similar since inception.
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