Recently, a few comments have emerged on this blog in regards to Trek’s Full Suspension Technology.
At first, I was a little thrown off by how each poster wasn’t posting in exactly the most friendly manner, but then I remembered that forums and blog postings can be rude. And, when dealing with a large company, it’s easy to forget that there are actual people dealing with things.
So, rather than lash out at each poster and degrade them for poor etiquette, inexperience and just plain rude attitude, I decided to ask Dylan and Jose, our in-house suspension gurus.
Hold tight, this is going to be a long post.
But before you get started, here are a couple book suggestions courtesy of Jose Gonzalez that will help you understand the issues:
And now, here’s a look at the comments that inspired the feedack.
“I just read the article. I can't say I am buying the "engineering" behind the hype. First off, the graph which shows the comparison between 07 and 08, where the first bullet states better small bump compliance while the graph shows lower initial leverage ratio is a clear mistake. Lower leverage ration means the suspension is harder to compress, so no, it is not more supple for small bumps. Your comments on DW-Link are also incorrect. Since the air spring has a progressive rate curve with very little progression in the mid stroke (curve nearly flat), it is wise to reduce compression ratio, which reduces the wallow, whereas at the end of stroke, it is again wise to increase the leverage ratio to fight the highly progressive behavior of the air spring and be able to achieve full travel. These are just a couple of comments I have. Seems like the Full Floater would be best used with a coil sprung shock. A high volume sleeve air shock may work OK as well.
- Michael K”
“The Trek spiel is incomplete and has some really bad mistakes as well. Spring curve changes with starting pressure?! Wrong. The shape of the curve is the same starting with 10 psi or 10000. Not debatable, just wrong. They don't include the shock spring curve and the force at the rear wheel vs travel. You'd see a graph of what the rider feels. It's a level or two deeper in the calculations. Maybe next year. Then the following year, they can start on anti squat. After that they can ask Dave for a license once they understand what's going on!
We would like to start by apologizing to both of you. I can understand your need to argue over this as we’ve obviously made you feel very bad about your own bikes, thus ruining all your rides since reading this.
Rick, you’re absolutely right – shape of the curve doesn’t change with pressure change. However, the higher pressure creates more stiction, and combined with the higher compression damping needed, the rider will feel a firmer overall ride.
Michael K, you’re also right. A lower lever ratio does mean the suspension is hard to compress – at the same pressure or spring rate. But for the same travel and rider weight, a lower initial ratio means less air spring pressure is needed, reducing friction, and allowing a more fluid suspension response.
You’re right too that we didn’t include the “force at the rear wheel vs travel” graph. However, the information from that chart only shows the amount of force required to move the wheel at any given point in the travel. It does not show the effect of the damper and the speed at which the shock shaft is moving -- and that is a more accurate picture of what the rider feels than just spring force. That's the reason that we look at and manipulate instant leverage ratio curves to truly “tune” how the system feels to the rider.
“A level or two deeper in the calculations”? Hardly!
“Force at the wheel vs travel” is one of the first things I look at as a frame designer – it’s what is used to determine loads on the system, and what I use to determine bearing sizes, pivot sizes, tubing wall thicknesses, etc. The “level or two deeper” part is understanding that instantaneous lever ratios are the true variables that affect suspension feel and response.
Michael K, again you’re completely right in referring to how leverage ratios should work with an air spring. Reducing leverage ratio works extremely well in helping to control the flat mid-stroke curve of an air spring, and increasing it at the end allows the system to reach full travel with the progression of an air spring.
However, here is where actual suspension design experience comes in handy. SOME leverage ratio increase at the end helps the system get full travel – the Mojo’s huge regression at the end leads to blowing through the travel at the end if the shock is properly set for sag and small bump compliance. If the shock were set to deal with this huge regression, the overly high levels of compression damping needed would make the bike feel harsh initially and on small bumps. As we stated, it is nearly impossible to tune a shock to work properly with such levels of ratio change.
And then of course there’s “anti squat” -- what the motorcycle guys have understood for years and refer to as “Chain Pull Effect.” Chain pull effect refers to the drive system’s (in our case, the chain and drivetrain) ability to extend the rear suspension. This is generally a good thing – when we accelerate, the weight shift causes the suspension to compress, and extending the suspension helps work against this. Extending the suspension also helps push the tire into the ground, increasing traction (especially helpful in steep climbing situations, for example). The higher the chain pull, the more suspension will want to extend. So more must be better, right?
Now keep in mind that although this sounds good on paper, our “motor” has a pretty low frequency to it, has a very on-off power stroke, and has two heavy, oscillating pistons that move through a large stroke.
When you combine this highly on-off power system with a bicycle that “rides” at sag – that is, about 30% on average into the suspension travel, and given that “chain pull” forces want to pull the suspension all the way to fully extended (there is no magic to make it stop pulling at sag) and you can easily see how too much “chain pull” quickly leads to a very “bobby” ride. Power on, pull chain, extend suspension, power off, suspension sinks back to sag. Power on, pull chain, extend suspension…..
The calculations and diagrams used for determining chain pull have been used by motorcycle designers for years and are exactly the same as used to determine chain pull effect in bicycles. So let’s take a look at how some various designs stack up. Keep in mind, this is shown in the same gear for all bikes. These curves move depending on the gear – no way around that, no magic that changes that.
The Santa Cruz Blur LT actually has more chain pull the deeper you get into the travel. This does actually make the Blur a very efficient pedaler. However, this comes at the expense of large amounts of pedal feedback, and a system that has to have very high bump force to overcome the chain induced suspension extension. In other words, suspension extension is always fighting against bump absorption, meaning efficient pedaling, but less effective suspension.
Due to its low instant center, the Specialized FSR bike has a very low amount of suspension extension under pedaling. While this does make for good bump absorption and a very active suspension, there is not much force to resist suspension compression due to weight shift. This is why FSR bikes are not very efficient pedalers and are reliant on high levels of shock platform or lockouts.
The Iron Horse MkIII, while an efficient pedaler, has higher levels of suspension extension early on in the travel, and like the Blur, this translates to loss of small bump compliance because of the higher mechanical extensive force the system has.
The Ibis Mojo actually has less suspension extension than the Trek Fuel EX at sag. And although it has higher levels deep into the travel, that is worthless as the rider is never going to really be pedaling that deep into the travel. Since full suspension bikes are designed to ride at the sag point, having high levels of chain pull very early in the travel actually works against small bump compliance. That is, by engineering a bike with more chain pull effect early in the travel, the bike is suspension is actually less efficient for pedaling, even working against you rather than for you.
The pivot location of the Trek Fuel EX has been carefully chosen and tweaked over the years to give us exactly the pedaling efficiency we’re looking for – this is not something we simply picked. It has been chosen and modified to give us exactly the balance of pedaling efficiency, minimal pedal feedback, and ability for bump force to overcome pedaling extension force that we were looking for and have honed over many years.
Keep in mind too that these chain pull forces are pretty small and the differences are not huge. You can see from the graph that at sag, all these bikes have similar amounts of chain pull. Although someone might make an argument as to why a higher chain pull force might be desirable, there is no denying that this comes at the expense of lower small bump compliance, more pedal feed back, and a linkage that combines more bearings, parts and complexity in an area subject to very high loads, limited clearances and close proximity to mud and dirt.
Also, simply designing around a particular suspension extension amount means nothing without designing the bike to work as a whole. The bike’s geometry, stiffness, weight, instantaneous lever ratio, chain pull effect, and shock tuning all have to be considered and designed for AS A WHOLE. All those factors work together to give the bike the ride it has.
I hope this helped give a clearer picture of why we've done what we've done.