Monday, September 9, 2013

Suspension Design - Santa Cruz Heckler vs. Ghost Cagua

I've been lucky enough to have some test bikes at my disposal.  And not just a week or whatever...A couple of months with each.  And while each bike is profoundly similar (6x6 travel, 650b, etc.) they are also remarkably different (American vs. German, Single Pivot vs. Pseudo Horst Link, etc.).

As I have had both of these bikes for some time, I've managed to plot out both of their suspension characteristics (somewhat).  I will follow this up with some wheel path diagrams from my Autocad plots.  Feel free to fire away with questions in the comments.

There's one major disclaimer.  All of these graphs are based off of Spreadsheets that I created.  All of the data for those spreadsheets comes from simple pivot diagrams that I created in Autocad.  All of the measurements used to create those diagrams were me with a tape measure.  They're pretty close...and an 1/16th of an inch here and there won't make "that" much difference, but there will be some minor discrepancies.

Chainstay Growth vs. Wheel Travel

We will start with an easy one, chainstay growth vs. wheel travel.  Weagle looooooves to talk about instant centers and....shit...what's his other one?  Oh ya.  Anti-squat.  It goes on and on.  To me, it's a lot simpler than that.  Cyclists push down on the pedals when they pedal.  This is a downward force (you're essentially jumping on your pedal a bit, right?).  The easiest way to counteract that is to build a bit of chain growth into your system (I stress a bit).  That force will counteract the force you are putting into your pedals.  This is important in about the first 33% of travel.  Once beyond 50%, you are most likely not pedaling, and all chain growth does at this point is cause pedal kickback.  See how similar the two bikes are with respect to chain growth.  The Cagua does not bob at all, so I think they have it nailed.  The Heckler has great pedaling characteristics too though.

 Shock Travel vs. Vertical Wheel Travel

This graph doesn't really tell us much.  But we will use it for other things later.

Leverage Ratio

Here's where it gets a bit weird.  Leverage ratio.  This is simply the travel at the wheel, divided by the travel at the shock.  Which, if you look at the above graph, is the inverse slope of the graph.  Simple really.  Except...We talk about rising rate and falling rate bikes.  Rising rate implies that you require more force at the end of travel to compress a similar distance as earlier in travel.  (i.e. 100 pounds compresses an inch of travel early in the travel, but it takes 150 pounds to compress an inch later in travel).  Falling rate implies the opposite.  However, look at the graph:
 The Heckler leverage ratio is increasing as we get further into travel, but this is a falling rate bicycle.  What's going on?  Well, we talk about leverage ratio relative to the rear wheel (i.e. the Heckler has a roughly 2.2 leverage ratio - so the wheel moves 2.2 inches for every inch of the shock).  But when we talk rising rate, we're actually talking about the leverage ratio at the shock.  We'll see that later.

So from this graph we learn that the leverage ratios for these two bikes do the opposite as one another (one goes up, one goes down) and we learn that the Santa Cruz has a slightly lower leverage ratio (which explains why the travel is a bit less than the Cagua with the same travel shock, among other things).  These are both very, very simple leverage ratios.  If you talk to the designers at Santa Cruz, they love to talk about how they change the rate throughout the travel on their VPP bikes.  Falling then rising, or whatever suits the style of bike.

Inverse Leverage Ratio

Here's the graph that really tells us what we need to know about the bike.  The inverse leverage ratio is the slope of the Shock Travel vs. Wheel Travel graph.  I've compressed the axis of this graph so things really show up.
 So this is where a falling rate bike shows that it is falling rate and a rising rate bike shows that it is rising rate.  Santa Cruz counters this by putting a relatively low volume air shock on the bike.  I'm assuming Ghost has a bit more volume in their shock...but couple an air shock with a rising rate and you're not going to bottom this bike very easily.

Other Things

In my opinion, you can tell a lot about a bike from these simple graphs.  Not everything, but a lot.  What we haven't talked about is braking...wheel path...instant centers (Weagle's fave) and many other things.  I hope to have more bikes up and more info to follow.  At some point.

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