Electronic Evolution Electronic Evolution

Electronic Evolution

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Friday, August 12, 2016

Main photo © BrakeThrough Media

All derailleurs shift, but getting that shift to execute smoothly and feel right is what separates the best components from the rest. Crafting the ideal shift is an undertaking that not only requires engineering acumen, but also a nuanced understanding of what riders want.

One of the key individuals responsible for engineering the shift performance of SRAM RED® eTap® is SRAM Global Director of Drivetrain Development, Scott McLaughlin. We asked Scott to explain how his design team went about this process in this in-depth interview that discusses SRAM RED® eTap’s shift feel, shift speed, ergonomics, and more. 

SRAM Global Director of Drivetrain Development, Scott McLaughlin.

What were the design parameters laid out at the start of the eTap® project and how did you arrive at those parameters? 

From the outset SRAM RED® eTap® was designed to be wireless, light weight, intuitive, easy to set up, and reliable. With the exception of the wireless requirement, these are goals that we have for every SRAM product, so while eTap® is radically different in many ways when compared to a mechanical groupset like SRAM RED®, the main thing we are striving for is an ideal rider experience. That focus comes from feedback we get from our professional athletes, customers, and input from our own engineers. A key strength we have as a company is that SRAM is rider owned and operated. This makes it easy for us to understand what customers want, because as riders and racers ourselves, we want the same things. 

Photo © BrakeThrough Media

RED mechanical is known for its crisp shift performance. How did the design team go about replicating this using a completely different electronic actuation mechanism?

Sharing the same crankset, cassette, and chain goes a long way toward maintaining a similar feel. Getting the feel of the shift buttons right was another big step toward creating that crisp shifting sensation. While we didn’t try to mimic the feel of our mechanical shifters, we knew there had to be a positive tactile experience for the rider to communicate that a shift was executed even when riding with full finger gloves. We actually spent a lot of time getting the button’s throw and click to feel just right. We designed it to have just the right amount of resistance that grows gradually until you feel that ‘click.’ Making the click both audible and tactile was also critical for us because it has to tell the user that they’ve successfully made a shift. We even worked to ensure that the feel of the button release was right as well.Photo © BrakeThrough Media

When it comes to actually moving the chain across the cogs and chainrings, there are fundamental differences in the way that mechanical and electronic groupsets function, and in our view that’s a good thing because it opens up new functional possibilities. For example, on a mechanical system, rear downshifts are made at the speed that the rider moves the shift lever inboard. This allows the rider to move the lever so fast that the chain skips over the teeth on the cog being shifted to; causing a harsh shift that damages the drivetrain. Conversely, if the rider moves the lever slowly, the shift can lag which causes clicking, more drivetrain wear, and of course, slower shifting. So mechanical systems rely on the rider to shift just right to get that perfect shift. Add some vibration from a bumpy road or thick gloves on a winter day and this becomes more difficult. We set out to improve these issues when we designed SRAM RED® eTap®.

Photo © BrakeThrough Media

With eTap® we can replicate that perfect shift every time. This is because the system controls the shift speed. We’ve taken this a step further with eTap’s front shifting. With an eTap® front shift we not only have the ideal shift speed, we can alter the intermediate motion of the front derailleur as it moves the chain from one chainring to the other depending on what gear you’ve selected on the cassette. This solves the problem of chain drops during front shifting. It works so well that we don’t include a chain watcher with our eTap® front derailleurs. You just don’t need it.

Testing shift quality in the lab.

You mentioned drivetrain wear as a factor that influenced eTap’s shift speed. Can you elaborate on that? 

As I said, reliability was a top priority from the outset. For us, that reliability extends to ensuring that SRAM drivetrains not only last as long as possible, but that they also protect the chain from breakage. eTap’s shift speed is designed to move the chain smoothly onto the shift ramps on our cassettes to ensure a crisp shift. If we went any faster the chain could miss these ramps causing damage to the cassette and chain and increase the likelihood of breaking a chain.

Were there any other considerations that influenced the shift speed SRAM decided upon?

Yes. The shift speed we decided on also makes the shifting as consistent as possible, prevents chain drops, and extends the life of the derailleur batteries. Another factor that influenced shift speed was input from our professional riders. They pedal at high cadences so we’ve optimized the system to shift best at higher cadences. 

A Team Katusha mechanic installs a PC-1170 cassette. Photo © BrakeThrough Media

Why do race teams use the PG-1170 (Force) cassette rather than the XG-1190 (RED) cassette?

Our PG and XG cassettes have different manufacturing methods and therefore different design tradeoffs. As a result, the PG-1170 cassettes offer slightly faster outboard shifts that can also feel more harsh than outboard shifts with XG-1190 cassettes, but this is a tradeoff the pros prefer. The PG-1170 cassette is also easier to clean and its added weight helps to keep bikes above the UCI minimum weight of 6.8 kilos (14.99 pounds). 

While the XG-1190 cassette is lighter and offers improved shift performance over the PG-1170 under high loads, teams prefer the PG-1170 cassette to hit that weight limit target and to have slightly faster shifting in most conditions. SRAM engineers collaborating on SRAM RED® eTap® in the research and development lab.

Let’s discuss ergonomics. How did you arrive at the hood and lever shape on the final rim brake and hydro versions? Did Reach Adjust and lever Contact Point Adjustment factor into this?

We did a number of design studies that included using different style buttons in different locations, but found that the shifting layout that we’ve evolved to was the most preferred and most ergonomic. 

For the hood shape we went with smaller diameter hoods. This allowed us to better accommodate riders with smaller hands. The smaller size was made possible by the elimination of the shifting mechanism that is used in our mechanical shift levers. We ultimately went with a shape that worked for a wide variety of lever reach adjustments. Getting the lever pivot in the right location was critical to achieving this. In the end you get a lever that is not only comfortable, but also provides more finger wrap for added security and a lever blade shape with a wide flat area in the middle of the blade that aids braking efforts when hands are on the hoods.

For our eTap® HRD™ levers we kept the diameter as small as possible and the lever reach identical to our cable-braking eTap® shifters. The primary difference is the added material at the top of the hood’s horn that houses the brake’s hydraulic internals and Contact Point Adjustment™. Photo © BrakeThrough Media

You’ve worked on the eTap® project since its inception. If you had to pick a favorite aspect to SRAM RED® eTap®, what would it be? 

The consistency and quality of the shifting performance. You can count on a great shift every time.

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