Friday, August 29, 2014

Wheel Paths

Talking about wheel path is probably not where we should start.

But I feel good about this topic.  It’s easy enough for anybody to understand, but there’s some technical info in here that even the internet suspension gurus are probably going to be interested in.  And we’re definitely going to start busting some heads.
We start here because wheel path has got to be the biggest topic of bullshit ever created by bicycle suspension marketers.  Talk to some people and you will be convinced you’d be winning world cup DH’s if you could just get the optimum wheel path.  Rearward.  “Vertical”.  I hope the guy who thought of “near vertical” is a millionaire because of it.  Thankfully, this seems to be slowly changing.  Santa Cruz has been setting people straight for years, and even Specialized is coming around to telling people not to worry about it.

Single Pivots

We’re going to start by looking at the simplest bikes to understand, single pivots.  Nobody needs to be told that the wheel path on a single pivot is an arc centered at the pivot point.  You know this.  You learned it in elementary school or college or something, depending on whether or not you were schooled in the British Columbia public education system.
Here we have 8 theoretical 6 inch travel bikes, with 8 different theoretical pivot locations and 8 corresponding theoretical wheel paths.  Let’s zoom in (I’ve also sketched in a 32 tooth middle ring and a 20 tooth cog for perspective).
From here, we can start to see some patterns.  Pivots 1 and 2 are our high pivot locations.  Notice that swingarm length doesn’t have much impact at all.  These pivots offer the most rearward swinging arcs, which is theoretically what everybody wants, and why we get bikes like this:
Pivots 3 and 4 correspond with a medium pivot, roughly in line with a middle chainring.  Once again, swingarm length has little input.  The path looks pretty damn close to "near vertical" to me.

Pivots 5 and 6 are extremely low, 6 being concentrically around the bottom bracket and 5 at some imaginary point directly in front of the bottom bracket.  We’re starting to see some spread due to swingarm length and a some forward arc.  We notice that a shorter swingarm leads to more forward arc.

Taking things to an extreme, we have 7 and 8, way below the bottom bracket.  We get a lot of spread due to swingarm length and start to see some extreme forward arc.

So great, Dave.  You showed us a bunch of single pivot wheel paths, many of them in impossible locations for bikes that don’t exist.  So let’s start to overlay some real bikes over this diagram (shown in red).  We’ll start with the Santa Cruz Bronson.
Which pretty much overlays our single pivot 3 and 4 examples.

Then the Ghost.
Which pretty much overlays our single pivot 3 and 4 examples.

And the Heckler.
Which....ahhh...pretty much overlays our single pivot 3 and 4 examples.

Three totally different bikes.  Three totally similar wheel paths.

Let’s throw them all on one diagram, just for kicks.  I’m not even going to bother labelling them, because there’s no point.

They all look so similar, I should probably come to the conclusion that I’ve doing something wrong.  In fact, they’re way more similar than even I expected, causing me to go back and double check everything yet again.  But there isn’t a mistake.  This is what wheel paths look like.  Even the fanciest bikes with “complex” wheel paths are really only deviating a few millimeters here and there from simple arcs (and there are reasons for this that we will talk about later...oh...nice teaser Dave!).  There’s other things going on...but dramatically different wheel paths are not one of them.  Especially in shorter travel lengths.  Those diagrams they’re showing on their website?  Gross exaggerations of the actual curve.

And you shouldn’t worry about wheel path.

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