Summer 2003 - Vol. 13 No. 2
A Closer Look at Claps…
Form, Function, and Physics
by
©2001 Barry Publow
Canada
With a half dozen entries in today’s increasingly competitive inline clap frame market, it is difficult for the average consumer to wade through the techno-jargon, sales pitches, claims, and range of design features. In order to present an unbiased viewpoint on these aluminum wonders, what follows is an overview of features which are relevant to engineering and design.
Design
Today’s inline clap frames represent an intriguing blend of form and function. Interestingly enough, this relatively young technology has produced two distinct classes of inline clap frames: free-heel, and split-wheel.
Free-Heel Designs The Mogema M41, Verducci V-Drive, and Raps Heman are free-heel clap frames consisting of two pieces: the lever (which is attached to the boots), and the base (the portion that houses the wheels). A hinged joint located towards the front of the skate connects these two pieces. When the frame opens at the end of the push, the hinge allows the heel of the boot to lift while maintaining all wheels in contact on the road. Springs are strategically placed to “clap” the frame back together during the recovery.
Split-Wheel The Bont Slingshot and Maple Reaction clap entries employ split-wheel designs. These frames are also comprised of two pieces, but instead of having a lever and a base, the frame itself is made up of a hinged front portion and a rear fixed portion. When a skaters reaches full push extension, only the front portion – which contains 2 or 3 wheels – remains grounded to the road. The rearward portion of the frame remains affixed to the heel of the boot as it lifts during extension. A hinged joint accomplishes this movement, and again a spring assembly is responsible for re-joining the two pieces once the skate is lifted off the road for recovery. The Maple model has 2 wheels in the hinged front portion, while the Bont Slingshot keeps 3-wheels grounded during extension of the frame.
Which is Better? The first question everyone asks is which of these two designs is better. I would be an ignorant fool if I conclusively answered this question, although personally I believe the free-heel designs superior. All I can say is that there are two distinct camps that would love to have the opportunity to convince you that their design is best. Proponents of the free-heel design will tell you that having all 5 wheels on the road will give you more stability, traction, and power output. They might also tell you that the split-wheel designs tend to fare poorly on rough pavement, as the hinged front portion can vibrate on rough roads. Those who favor the split-wheel designs will tell you this design is better because there is less mass (only 2-3 wheels) to “clap” back to the boot, reducing the return impact from each stride. They also claim that this design allows for a lighter frame – maybe.*
While each design may offer its own merits and drawbacks, the truth is that the most important functional considerations are likely dependent on this difference. Each frame should be examined for certain qualities that impact the raw performance of the clap mechanism and it’s impact on the efficiency of force output. Without trying to steer readers in any particular direction, below is a general overview of the critical elements of clap frame design, and which considerations are important for determining the value of a given model.
Design Considerations
- Strength & structural rigidity This factor is highly related to both materials and design. While a carbon composite clap frame may well enter the market in the near future, so far all of today’s entries are constructed of high-grade aluminum. With fixed frames, one factor in design is the number, position, and placement of bridging and cross-braces. Like most structures, strength is as much an engineering challenge as it is related to material. When examining an inline clap frame, the same design considerations for building a strong, sturdy fixed frame should be met. But with their hinged design, clap frames experience high levels of torsion and stress. A great deal of research has gone into making these structures strong, and in making the most efficient use of the limited space between the boot and the road.
- Height/Weight One of the early challenges of clap frame development was to build a frame that positioned the boot no higher off the road than a conventional fixed frame. Initially, the bulk of the lever, spring, and hinge assembly made this almost impossible. Today, however, inline clap design has achieved this important requisite. In designing the M41 clap, Mogema engineers used state-of-the-art 3D CAD tools to calculate weight, strength and stiffness of each part. The addition and compatibility of Finite Element Analysis provided them with valuable insight into the torsion (or bending) behavior of the frame under extreme circumstances.
With more parts (lever, spring, hinges, bolts, etc) and the need to stand up to heightened levels of stress, clap frames tend to weigh more than a typical fixed frame. But, it is interesting to note that there are some major differences in weight between some of the contenders. For example, the M41 is a svelte 312 grams (not much heavier that its cousin, the fixed frame M55 at 234 grams). By contrast, Verducci’s V-Drive is estimated to weight in at beefy 490 grams.
Product Comparison All of these clap frames have 5 wheels, are 12.8" in length, and have 6.5" mounting.
Model Design Materials Weight Price (USD) Mogema M41 Free-heel Base: 7219 T6
Levers: 7022 T6312 g $499 Verducci V-Drive Free-heel Base: 6061 T6
Levers: 7075 T6489 g $629 Raps Heman Free-heel Base: 7129 T6
Levers: 8000 T6370 g $460 Bont Slingshot Split-wheel Front: 7075
Rear: 7005390 g $589 Maple Reaction Split-wheel 7000 Series 300 g $325 - The Pivot Point The pivot point refers to the fore-aft position of the hinge joint which attaches the lever to the base. In physics this fixed point is referred to as the fulcrum. The mechanical advantage of a clap skate is affected by the ratio of the length of the lever on the applied force side of the fulcrum to the length of the lever on the resistance force side of the fulcrum. Without getting overly technical, what does this mean? In a nutshell, the design and location of the clap skate’s pivot point has a major impact on the function and “feel” of the frame. The distance relationship between the pivot point and the front the frame is therefore a critical design element, and the position of the hinge in relation to the ball of the skater’s foot is a major factor in determining the efficiency of the push.
Manufacturers have done a great deal of research to try and determine optimal positioning of the pivot point. What complicates the issue of pivot point locationis that the ideal location of the hinge joint will vary slightly from skater to skater. There is an ideal range, but thanks to technique it is virtually impossible to dictate a universal position on the skate. Skaters exhibit small but significant differences in push mechanics, body weight distribution (center of gravity), joint angles, and ranges of motion, etc. And then there are personal preferences to factor into the equation. So what manufacturers have done is place the pivot point in the approximate ideal position, and allow the skater to make minor adjustments to the hinge in relation to their foot size and boot. The Mogema 41 frame uses small rectangular pads which can be rotated in 90° increments to give more options for both lateral and fore-aft positioning.
Interestingly enough, the clap frames on today’s market each place the pivot point in a different location on the frame. Mogema’s M41 has the hingeat the 2nd wheel, while the Verducci V-Drive positions the hinge more rearward between the 2nd and 3rd wheel. Even if the skater adjusts the frame to position the hinge at a specific point under the foot, the absolute position of the pivot point will affect the functional characteristics of the frame. This is because the handling and mechanical properties of the frame are influenced by the distance between the pivot point (hinge) and the center of the front wheel, and by the distance between the pivot point and the center of the rear wheel. Also, the position of the frame on the boot will affect the stability and handling of the frame. In scenario #1, there is a small distance between the ball of the foot and hinge joint, and there is approximately half a wheel protruding from the front of the skate. The steering of the frame is enhanced but the skater may interpret this as a reduction in control. The push off will be shorter, and accelerating will made easier. Because the distance from the ball of the foot to the ankle joint is very small, a more powerful push can result. In scenario #2, the distance between the ball of the foot and the hinge joint is larger, and there is roughly one wheel of length in front of the skate. The push will be slower because the flexion of the foot/ankle is delayed, and the opening of frame is at it’s smallest. This frame position typically feels more stable for skaters with an inefficient sideways push, or those who are skating on their toes with body weight focused too far forward.
- Frame Adjustment I have been preaching for years that fore-aft frame positioning is just as important as lateral adjustment. And now with inline clap frames, the ability to adjust the frame front to back is absolutely critical. Unfortunately, most boot manufacturers continue to build their skates with little regard for the biomechanical requirements of boot-frame integration. Mogema has addressed the issue by creating a unique frame-mounting system which uses multi-direction square pads with offset slots. The pads can be rotated in 90 degree increments to give the skater added flexibility in terms of both fore-aft and lateral adjustment. Mogema has gone one step further to pair their frame with a boot that uses front-back slots instead of the traditional pre-drilled aluminum blocks. My question is this. when are boot makers going to wake up and start building skates with front-back slots in stead of pre-drilled blocks embedded in the shell? The only speed boot i’ve seen with “intelligent” slots like this is the Rollerblade Equipe (no longer produced). Listen up boot makers! Leave the lateral adjustment to the frame manufacturers. Give us longitudinal slots in the boot!
- Spring tension The orientation and tension of the spring is very important for the clap frame to function properly under all possible circumstances. The four important details related to the springs are:
- Range of Opening & Return Speed The tension of the spring must be low enough to allow the frame to open sufficiently during extension of the foot/ankle, but stiff enough to provide the elastic energy necessary to retract the base quickly during an all-out sprint. The orientation and angle of pull of the springs is also important in this regard.
- Strength The springs must be durable enough to maintain strength over tens of thousands of repeated cycles of expansion and recoil.
- Redundancy The free heel designs all have two springs per frame. This provides added strength and insurance against breakage. That is, if one spring happens to fail, the frame will still function. One drawback for the split-wheel designs is that there is only 1 (laterally-mounted) spring. If the spring breaks, your race is over.
- Spring Maintenance The springs will occasionally need to be changed. How easy is it to access the springs for maintenance. Does the manufacturer offer springs of varying tension for skaters of different body weight?
Try to Try before you Buy
One of the problems with the inline equipment market is it is often impossible to test a given product before investing the money to buy it. At the very least, try to find a shop that keeps the frame in stock. This way you can at least have a look at it. I suggest you bring your boots along and evaluate how the frame will integrate in terms of positioning. The most ideal option is to take a clap frame for a test skate. Your best bet is to find someone who is willing to let you take their (clap) skates for a spin. The problem is that claps require a period of adjustment and adaptation. Just keep this in mind when you try them for the first time.