Joe's Corner

Axle path don't matter no mo'

August 16 — 2012

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.

Words by: Joe Graney, SCB Engineering

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.

Bike Suspension Axle Path Diagram

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.

More in the next installment:
Instant Center Migration.