Most riders are familiar with how different mountain bike frame sizes relate to changes in geometry and how each of those sizes will fit a rider as a result. Throwing geometry aside, we wanted to learn about some of the less apparent changes manufacturers make between their smallest and largest frames. Wouldn't shorter tubes create an overly stiff frame for smaller riders and a frame with the support of a wet noodle as tubes get longer for taller riders? The short answer is yes. That's why Pivot Cycles approaches the topic of frame sizing with an emphasis on creating a consistent ride experience and ride characteristics across all frame sizes for mountain bikers of any stature.
Photos by: Jeremy Ray
Pivot allowed us to experience this for ourselves by providing two of the same frame in different sizes. To test around the same parameters their bikes are designed within, we went out for a ride in Pivot's backyard at South Mountain Park in Phoenix, AZ, aboard two Switchblades at opposite ends of the sizing chart. While it was a short ride, we could easily repeat sections of trail and put a lot of side loading and square edge hits through each bike to properly analyze trail input from both riders.
We wanted to see if each rider noticed any unfavorable characteristics of either frame size, such as an unwanted level of flex out of the rear triangle, overly stiff feeling across off-camber terrain, or flexing while sprinting under load. Our afternoon of rock smashing and skipping our way through the technical terrain on South Mountain revealed a consistent experience across the board. Both size bikes performed as well as we had hoped, with a predictably playful feeling, excellent rear wheel tracking, comfortable compliance, and no flexibility or stiffness issues to report from either size rider. Aside from geometry and frame compliance, both riders were most impressed by the pedaling efficiency of the DW Link suspension platform for its fully active feeling under pedaling forces while retaining excellent small bump absorption.
While our ride experience was great, and we were left without any adverse qualities to report about either size bike, we wanted to better understand how our findings were made possible with two bikes on opposite ends of sizing. To gain some insight into what steps are taken to achieve consistency across a full-size run of bikes, we spoke with the head of Engineering at Pivot Cycles, Kevin Tisue, and President/Founder, Chris Cocalis.
What is the focus of the tuning, and what aspect do you try to keep consistent between frame sizes?
KT: The main focus is to provide the proper flex in each frame. We have specific tests that we perform for each size frame and size-specific stiffness goals for each of those tests. Using the same size tubes and the same carbon layup schedule for each size will actually make the smaller frames considerably stiffer than the larger frames. By changing the flex characteristics, we can better tune the flex for a more consistent feel across the size range.
How are the bikes tuned for each rider's size?
KT: Primarily the bikes are tuned for each size using different cross sections for the tubes as well as different carbon layup schedules per size. Changing these two things allows us to modify the flex of each size frame to better match the size/weight of the rider for each size. Smaller cross sections allow for more flex in a frame while larger ones result in less flex. As far as the layup schedule goes, we can change the flex characteristics by either adding or removing carbon material or by changing the angle of the carbon fiber layers.
How do you decide on the flex/stiffness profile of a frame?
CC: From years of experience and feedback from our own testing and our riders. A good example would be where we are at with the Firebird today. With the 27.5” Firebird we developed stiffness numbers based on Pro rider feedback. My personal feeling was that it was a bit too stiff. Pro rider feedback was that they liked just how aggressive they could get throwing the bike into corners. However, we could see in the results that this might be great at the beginning of the day, but not at the end when you’re getting tired. We started to refine the way the chassis felt and where it flexed resulting in a feel that still gives that immediate feedback but is a lot more forgiving when the terrain is more punishing and/or the rider is getting tired.
Do you design a size medium first since it’s in the middle, and then match XS and XL to those tolerances?
CC: Actually, we have specific numbers for every model and every size frame before we even start the project. We need to design this from the start when we are creating the profiles of the front triangles and also at the beginning of the layup development for each size. Oftentimes, the large and x-small frames are being developed at the same time because a large or medium is what most of us need to test, but the smallest size often creates the most challenges.
Are there any changes to the rear suspension kinematic between sizes?
KT: This is one that we change sometimes and don’t change other times. This really depends on the type of bike we are designing and what we are trying to achieve. On bikes where chainstay length changes with size, there is a natural alteration of suspension kinematics, but we account for that by shifting our pivot points around to get to more optimized curves per size. We do sometimes change suspension behavior with size on other bikes as well, but for most bikes, shock air pressure and tuning work to account for different rider sizes and weights using relatively consistent suspension kinematics across all sizes.
What are things you don’t change across frame sizes?
KT: There isn’t much that we don’t change per size. Some geometry numbers stay the same, seat post/seat tube diameters, pivot bearing sizes, headtube diameters, bottom bracket dimensions, general frame features, and cable routing rarely change with size. Water bottle options are always done on a size-by-size basis because the available room inside and outside of the frame changes with size.
While the pedaling efficiency out of the suspension platform is the most obvious standout trait on the trail, the less apparent characteristics are equally important in creating the most enjoyable riding experience. Being able to ride without thinking about what the bike is doing is one of the best attributes a bike can have, and the approach taken to achieving that requires more attention to detail than meets the eye. For our testers to hop on opposite-size bikes and attack trails in the same way without compensating for frame flex or adjusting our riding styles is a testament to how those details stack up in the final product. As Kevin revealed, these finer details are at the foundation of every Pivot frame and a large focus of their design process. The ability to test both size bikes simultaneously solidified the expectation that all frame sizes should offer the same ride experience for all size riders. However, seeing how that expectation is actually achieved was eye-opening.
For more information, please visit pivotcycles.com