Joe's Corner : Floaters
I was reminded recently by Gerald out at the Bike Doctor in Saskatoon that I had mentioned that I would talk about "floating shocks" in one of these articles, and I never got back to it. I hate disappointing the Canadians, especially when they need something to read on those long dark cold winter nights.
To float or not to float? That is the question.
First the definition: A "floating shock" is a frame configuration where the shock isn't attached to a part that doesn't move. In practical terms, this means the shock is not attached to the front triangle directly. In most permutations of this, one end of the shock is attached to the swing-arm, and the other end of it is attached to a link.
Joe's Corner : Bearings
We work continuously to improve our bikes. There are small changes that occur to our models regularly that many people don't notice, and we don't focus on communicating the specifics. Our philosophy has always been to let the results speak for themselves - jargon isn't as much fun as riding. At the same time, we're proud of the work we do, and since you volunteered to receive more information about Santa Cruz Bikes, we've decided to dedicate a regular article in this newsletter to share with our dealers and customers more of the specific details. Keep in mind that we really spend a lot of time balancing compromises to get the best outcome. This article is intended to be an open and honest discussion of some of the technical aspects of Santa Cruz engineering and R&D efforts. Our hope is that you, our customers, can gain a better understanding of what we're working on, our thoughts on different trends in the industry, and, quite frankly, attempt to cut through some of the industry noise with open discourse all in the interest of making bikes that are fun to ride and easy to own.
Joe's Corner : Instantaneous Center Migration
Migration is defined by the American Heritage Dictionary as: "To change location periodically". For a bicycle suspension system, the Instantaneous Center (IC) is the point that the rear axle is rotating around at any given instant. On a single pivot suspension, the IC is the pivot - and it doesn't migrate. Other mechanisms can change the point that the rear axle is pivoting around as the suspension compresses. Most common in bicycle suspension is a four-bar linkage with the axle located on a link that is not connected directly to the front triangle. It's referred to as "instantaneous" because the pivot point can move, unlike the single pivot mechanism. Therefore, at any given point in the suspension motion the IC can be at a different location. IC Migration is the subject of several patents on bicycle suspension, some with detailed claims and some with more vague claims of the advantage of a particular system. Some patents don't say why its better, but simply protect something about the configuration of the mechanism. The IC can be determined by tracing lines through the first and third bars of the linkage (see figure 1).
Joe's Corner : Axle Path
It's easy to claim that something sucks. And it's easy to claim something else is "better". But it's hard to prove that the claim is true. And it's even harder to define what "better" is. However, you need to do both.
This is the first in a series of articles in which I'll talk about what we at SCB know about bicycle suspension technology and what we're working on for the future. After years of developing and selling suspension bikes, and watching the industry evolve with us, we think it's time to take stock of what is happening right now and what we can all look forward to. We plan to come clean about things we've learned and things we don't completely understand. Along the way we'll debunk some myths, explain some commonly misunderstood concepts, and generally give a sense of where we are and where we're going.
Bicycle suspension is complex. We're not interested in taking years of experimentation and accrued knowledge with our group of dedicated engineers and techs and converting it into a simplified claim that is drowned out by the chorus of "me too" heard annually at Interbike.
Axle Path Don't Matter No Mo': Part 1 of a Series
Here's something that nobody wants to hear: Axle path doesn't matter for bicycle suspension. At least, it doesn't matter nearly as much as some people say. It's true. Axles move up and down, and everyone can imagine that they follow a certain path, so it's an appealing thing to think about. But unfortunately, describing an axle path as "vertical", the classic "near-vertical", "s-shaped", or "rearward", is an over-simplification of the suspension system. It's also dead wrong. Santa Cruz once published a postcard showing the axle path of the original V10 as being "S-shaped". It was misleading and technically incorrect, and we apologize. We even have a US patent that covers that specifically: Patent # 5628524. But we no longer employ it in our designs, because it doesn't really matter.
Basically, the center of your rear wheel can't move much more than 20mm in distance from the center of the bottom bracket or your pedals feel like they are getting tugged around a lot. It has taken some time, but this is something known and understood to us, and it should be known to everyone else making suspension bikes. This is especially true for multi-ring set-ups, because smaller chain-rings make the cranks move more for a given amount of chain growth. Our V10 has more than 30mm of chain growth, but since it's meant to be ridden with a big ring all the time, pedal feedback isn't such a big issue. Check out this image.
So, the endpoint of your 5-inch travel bike can be anywhere from no chain growth to 20mm of chain growth. When you combine that with the fact that any effect in compression has an opposing effect in rebound, there aren't many legitimate options from the bottom to the top in that range. Not enough that it's really going to mean the difference between a good bike and a crappy bike. Or that something "pedals great" or "is neutral under braking" - whatever that means. That's bogus. It's what marketing guys, get paid to do: Make up a simple, believable reason why a product is superior. And then repeat those reasons over and over until they become accepted as common knowledge despite the lack of real justification. The truth is that axle path should be a result of other parameters, not a goal in itself. Therefore, axle path doesn't matter.
Joe's Corner : Shock Rates
With each of these columns, the areas we are talking about have become more complex. This one, dealing with shock rates, is not gonna be a walk in the park. But hopefully it'll give some of you a little bit more insight into the how and why of suspension workings, and how our designs in particular behave.
We have to start with this. Levers are one of the "simple machines", of which there are five (or six, depending on how nerdy you are). They hold company with the inclined plane, wheel and axle, wedge, pulley and screw. In a nutshell, this is what leverage is: you get a longer lever arm, and through the principal of torque (force x lever arm) you can exert high forces via a small lever arm by exerting less force on a longer lever arm.
With suspension bikes, Wheel Travel divided by Shock Stroke = average Leverage Rate (eg, a 5 inch travel bike with a 2.5" travel shock has an average leverage ratio of 2.0).
Shock rate is the inverse (opposite relationship) of leverage. Shock Stroke divided by Wheel Travel = average Shock Rate. The aforementioned bike would have an average shock rate of 0.5. Why use shock rate instead of leverage rate? The hell if I know, but it's common for bike people to use the term "falling rate" and "rising rate", both of which refer to shock rate. So at some point in the past, I decided to use shock rate so it fits that nomenclature. It's easy to convert - just remember that a high leverage is a low rate, and that rising rate means falling leverage. Ready? I didn't think so.
Rising rate on a bike means that at the beginning of the travel, the rear axle has a higher mechanical advantage than at the end of the travel. This typically implies that the bike won't "bottom out" easily. Falling rate means the opposite; the bike may bottom out easily, as the mechanical advantage increases through the travel - it "uses" the travel easily. A bike with a constant leverage, or shock rate, has the same leverage throughout the entire travel. I usually figure that +/- 3% can be considered constant.
Spring rates are a measure of how much the spring pushes back at you as you push into it. A constant rate spring is assigned a value, typically referred to as "k". The k value of a spring is in units of force per unit length, or "lbs per inch". Practically speaking, if you have a 500 lb. spring, 500 is the k value, meaning 500 lbs per inch compressed. At zero inches compressed, it pushes back zero pounds, which is handy because then it just sits there on your desk holding your pencils. At one inch compressed, it pushes back 500 pounds. At two inches compressed, it pushes back 1000 lbs, and so on. Coil springs on bike shocks aren't perfectly linear, but they are damn close. Air shocks are different, though. Air shocks work by compressing air, which doesn't push back the same way as a metal spring does. The air's force pushing back has to do with volume, which is tough to make a linear. So you set air the pressure (the volume is typically constant, and set by the shock manufacturer), and when the volume is cut in half, the pressure is doubled (remember PV=nRT from chemistry?). So half way through the shock stroke, the force pushing back is doubled. Then, 3/4 of the way thru the stroke, the pressure doubles again. And so on. Hence, that "rampy" feeling, since the spring rate skyrockets toward the end of the stroke as the volume gets cut in half over and over and the force goes way up.
If you increase the volume a lot, the spring rate doesn't do a linear progression like a coil, and has what is sometimes called a "cavitation" or a place where the spring rate drops below a constant k value. The beginning of the stroke on an air shock also doesn't start at zero force, due to negative springs and all sorts of trickiness that air shock designers could write a book on. Preload doesn't change your k value, it just makes the start point go higher since you start compressing the spring, but you haven't compressed your bike yet.
Wheel rate could be defined as the sum of Bike Rate and Spring Rate. In practice, it's really how your bike is going to behave. I don't typically use wheel rate, since it means picking only one shock, and I like having different options for shocks. Typically we're going for a bike shock rate that behaves well with various spring rates that the bike will be used with. So let's talk about bike rates.
I don't know of another current design that has a mechanism that allows for a change in shock rate that can be manipulated like VPP can. Typically a linkage moves in a linear fashion, and the shock can be positioned to have a falling or rising rate, but not both. The VPP system has two links that rotate in opposite directions. The top link moves counter clockwise, the bottom link clockwise (from the drive side viewpoint). This is unique to VPP bikes, and is specifically protected by US Patent 6488301.
As the suspension compresses, the two links rotate, but not at constant rates. The upper link starts rotating quickly, then slows down mid-stroke, and then speeds up again. The lower link does the opposite, starting slowly, then faster, and then slows down later in the travel. Exactly how much they speed up and slow down can be manipulated by changing pivot points, link angles and lengths. Attaching a shock to one of the links makes it compress at different rates through the travel. On a VPP bike with the shock attached to the upper link, the shock starts w/ a high rate, decreases through the middle of the travel, and then increases again. This gives the rider a feeling of great bump absorption on small and medium size bumps, but then ramps up so the suspension doesn't bottom out on larger impacts.
The V10 shock rate starts very low (it means a very high leverage at the beginning). This allows the rear wheel to move very easily when it drops away into a hole, and then gets hit hard and fast by an obstacle. Instead of "kicking" the bike, the wheel easily moves back to mid stroke which is where it should be when a rider is aboard.
As always, there is a balance here. Too much "falling" in the middle in the shock rate can yield a bike that feels like it wallows in the mid-stroke, or mushes down in corners, without a nice snappy feeling on the rebound stroke. Too much "rising" from mid-stroke to bottom out can give a bike that doesn't use full travel under normal circumstances. Complicating the matter is that different shocks have different spring rates, and can change the way a bike feels. A Fox DHX Air shock, for instance, has a different spring rate than a Fox Float. In fact, different Floats have different air volume canisters to manipulate this. Santa Cruz worked with Fox during the development of the first Blur to offer adjustable air volume (AVA was the Fox acronym) and later Fox fixed the larger air volume to save cost and weight calling it XV (extra volume).
Finally, A Note On Super Low Leverage
Low leverage rates also get big play these days, and we've tried to figure out why. Our latest V10 uses 2.75" of stroke to get 10 inches of vertical wheel travel. Blind back to back testing with longer stroke shocks on prototypes wasn't enough to convince anyone it was worth the weight penalty. Shock durability hasn't been a problem for our bikes, even for Nathan Rennie (although, one potential problem with high leverage ratios is for riders that are up in the well over 200 lb. range, and can't get springs that are large enough weight to fit in there). Our latest Heckler redesign changed from a 200 i2i, 57mm stroke shock to a 216 i2i, 63.5mm stroke shock because it really made a substantial difference in bump feel. The additional half inch of shock length apparently lets the shock designers get stroke over two inches that feels better for a 145mm travel package. It can make tuning of damping characteristics more difficult for lightweight riders (under 140 lbs) however, as the spring rates drop very low, and the damping adjustment range available with the shock may not be sufficient. We continue to try different shock lengths on different bikes, but won't use something just because it's the cool thing to do.
No matter what Roskopp says.
MORE SCB NEWS
Joe talks floating shocks
Lifetime Bearing Guarantee, And The World According To Joe.
Clever Suspension Stuff from Joe the SCB Engineer
Axle path don't matter no mo'
Joe takes us deep into the subject of Standards
We take a lot of pride in our wheel building, here we talk you though our process.
We get a lot of questions about bike geometry