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Suspension Tech: Closer look at Trek’s new Thru Shaft shocks

Trek re-aktiv thru shaft rear mountain bike shock made by fox
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Trek re-aktiv thru shaft rear mountain bike shock made by fox

Trek’s introduction of a thru shaft shock design for mountain bikes seems like it could solve a real problem with suspension. Have you ever hit a section with so many rapid fire bumps that your rear wheel seems like it just can’t keep track and starts bouncing or skipping over the terrain? The claimed benefit of the Thru Shaft design is that it can react quicker than a traditional IFP-backed shock because it’s not pushing oil against that secondary internal piston. It might help to start with our post about how an IFP works.

We covered much of the new Thru Shaft design tech in the original launch post, but we still had questions. So, to more fully explain how the Thru Shaft works, we got answers from Trek and Fox…

Trek thru shaft mountain bike shock design diagram

Because the piston shaft runs all the way through the damping chamber, oil volume remains constant. So, Trek’s Travis Ott says “no backfill or recirculation is needed as the volume remains the same. In a traditional shock, when the damper shaft enters the oil it changes the volume forcing the IFP to compress. With a thru shaft there amount of damper shaft in the oil remains constant so no need for volume compensation.

Fox’s Mark Jordan adds “In our thru-shaft design, oil flows through the main piston during compression and rebound. A thru-shaft design is not displacing oil like a standard shock – oil just flows through the piston and back. Because it does not have increasing volume from a shaft during compression, a thru-shaft shock doesn’t require an IFP design to compensate for added volume. However, there are other things to consider like internal pressures and heat expansion.”

Trek re-aktiv thru shaft rear mountain bike shock made by rockshox
Trek uses this Rockshox model on their longer travel Remedy and Slash, so it gets a small piggy back overflow reservoir to deal with heat related oil expansion.

OK, so how do you deal with fluid expansion from heat?

“That’s a great question,” says Ott. “On the FOX shock found on the Fuel EX there is less oil than on the larger volume (Rockshox shocks used on the) Remedy and Slash. So thermal compensation on the FOX shock takes place within the damper shaft. On the Rock Shox execution of Thru Shaft there is a small external piggy back unit that deals with oil expansion.”

Indeed, Jordan confirmed there is a tiny spring-backed (as opposed to nitrogen pressure) IFP inside the thru shaft to deal with pressure changes and oil expansion caused by heat.


The fun never ends. Stay tuned for a new post each week that explores one small suspension tech, tuning or product topic. Check out past posts here. Got a question you want answered? Email us. Want your brand or product featured? We can do that, too.

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Tom in MN
Tom in MN
5 years ago

They claim it’s simpler but it now needs shaft seals at both ends of the shock, and still needs to deal with thermal volume change. Longer shaft probably makes it heavier too. All engineering involves trade-offs.

Richard
Richard
5 years ago

With massive reference/credit to Horst Leitner’s 25-year-old AMP shocks…

Chader
Chader
5 years ago
Reply to  Richard

Horst used, but did not “invent” thru-shaft shocks. His were supremely simple and were good/bad because of it.

The Trek/Fox/RockShox versions are conceptually the same, but far more advanced.

jasonmiles31
5 years ago

Any chance we could see a side by side pic of a Thru Shaft shock and a conventional sock with the same stroke length? I am curious how much bigger the Thru Shaft shock is.

Sam Sr
Sam Sr
5 years ago

I always use test tubes when Overhauling a shock. Makes me look super smart

If this is so much better why haven’t Fox or Rock Shox developed it on their own without Trek paying them to do so?

Colin M
Colin M
5 years ago
Reply to  Sam Sr

You don’t know how the supplier process works huh? Trek is a powerhouse in the industry and is responsible for many of the incremental changes we’ve seen in the past few years. They dictate changes to parts suppliers, not the other way around. They have an agreement with Fox to produce certain rear shocks that only can be sold to Trek because they are based on Trek design features. I’m sure Specialized has the same agreement.

Sam Sr.
Sam Sr.
5 years ago
Reply to  Colin M

You don’t know how advertising works huh? You say powerhouse Trek is responsible for many incremental changes? Name one that is currently used today. How about the DCRV? Yeah, that was a keeper. Completely revolutionized the industry. Or how about Specialized Brain? Yep, another keeper. There are exactly zero brands who have adopted either technology and made their own versions of it. This “thru-shaft” is the same as everything else; marketing fluff which will be gone in a few years.

Keith B
Keith B
5 years ago
Reply to  Sam Sr.

Boost Spacing?

Matt Ciancia
5 years ago
Reply to  Sam Sr.

DRCV…..just saying. Not to mention patents expire slower than technology evolves. This one will probably be adapted because there’s only a 2 year embargo on it which tells us that someone Fox, Rockshox, or Penskie thinks that it will actually take hold. Not saying there isn’t fluff but there are some details in this one that seem a little more solid.

Darryl Duck
5 years ago
Reply to  Matt Ciancia

There is only a 2 year embargo because it cannot be patented.

Dennis
Dennis
5 years ago
Reply to  Sam Sr.

Probably due to patents Trek and Specialized have on their shock designs that prevents other manufacturers from using the tech.

Greg
Greg
5 years ago
Reply to  Sam Sr

before, shock manufacturers were trying to fit the most stroke in the shortest eye to eye, per manufacturers requests. Most recently, suspension and bike manufacturers talked it over and came out with “metric” shocks, which really was about reducing the stroke/length ratio so they could fit more tech in the damper. I think this is what allowed the possibility of going thru shaft. There’s no way around it – thru shaft shocks are longer overall.

Josh
Josh
5 years ago

Looks like there are a bunch of Haters and keyboard engineers in here already.

duder
duder
5 years ago

So hard to know if shocks work for your bike or not before dropping $500+. I really wish there was some type of try-before-you-buy demo program for shocks. Send me a shock in the mail and let me ride it for a couple days. Let me try a couple before I make my purchase decision.

Michael Myers
Michael Myers
5 years ago
Reply to  duder

What other industry does anything like this? What’s in it for the manufacture? Won’t they just be sitting on a bunch of used shocks at the end of the process, with less sales?

That’s lose/lose.

whobikes
whobikes
5 years ago
Reply to  duder

Rockshox did this for their Ride Experience Sprinter vans one year. Loaded up the vans with all different sizes of shocks and forks and would install them on your bike.

bart
bart
5 years ago

Isn’t a lefty damper Thru shaft?

Greg
Greg
5 years ago
Reply to  bart

Yes

chadquest
5 years ago

STOP TRYING TO MAKE THINGS BETTER AND LEARN FROM IT.
Lets just all ride 1990s tech for the rest of ever.

turok
turok
5 years ago
Reply to  chadquest

Yeah! I don’t want the tech to change because then I have to use my brain to figure out whats different! Plus, learning about new tech makes my existing bike worse!

Please ramp up the marketing and try to sell me the same stuff but in a new way. Why can’t things just stay the same?!

dustytires
5 years ago

to Josh and the other Big Co worshipers on here, do you really think that since Trek et al is huge then the readers/writers here are somehow less intelligent? I would wager that there are many peoples on here every day that are every bit as smart in legal or eng or business as anyone at Trek, Specialized etc. One place that the readers of this fine forum are mere babes in the woods though is marketing, including myself who has typed Treks name 2X whilst trying to make a point! THEY WIN, pretty funny shit huh?

jon
jon
5 years ago
Reply to  dustytires

I went to college to find where all the ‘smart people’ were hiding. What I found out, was that I was already pretty smart, and college only validated that. If you thought college was going to teach you something you didn’t already know, then you probably were as disappointed as I was. I could’ve learned all that shit by reading a book. Although, I will admit, I wouldn’t have taken as many tests if I hadn’t of taken on an Engineering Degree. So you can say, that somebody that graduates has taken a lot of tests. Hopefully, they didn’t cheat, like a lot of my compatriots did (they were usually younger and under pressure from their parents to ‘succeed’.)

Tim
Tim
5 years ago
Reply to  jon

jon wants people talking about thru-shaft shocks (and I suspect, other people, too) to know what he thinks about… universities. Connection? You got me.

jon
jon
5 years ago
Reply to  dustytires

Oh, and some Professors really don’t know what they are talking about. And, some do. But, certainly not all.

Turtlehead
Turtlehead
5 years ago
Reply to  dustytires

I’m sure there are plenty of people reading BR that are smarter than any one single person as personnel at a big bike manufacturer–the size of its reader base almost certainly ensures it. Unfortunately, that is irrelevant. Engineering is fundamentally about applying resources, diligence, and work to solve problems. GB2 worshipping Nikola Tesla.

Josh
Josh
5 years ago
Reply to  dustytires

dustytires never said readers are less intelligent. I just get tired of all the bashing of new tech before you have even rode the product. And this is for all product on bikerumor, not just trek. so don’t assume i am a “big co worshipper” because you know what that makes you.

Lancetigerblood
Lancetigerblood
5 years ago

sick head shock from the 90’s technology

GuyOnMTB
GuyOnMTB
5 years ago

This tech is the best concept in MTB rear suspension in quite some time, though the tech has been around for a while. A couple of people around here have the new shock and say it’s amazing. They are seasoned riders that have been through a lot of gear so I have a trust in their assessments. I’ve been tinkering with 4×4 suspensions on various trucks I’ve owned over the years and since I got into MTB about 18 years ago; seen the progression of FS tech within the last decade and wondered why the IFP was even being used since F1 kinematics are vary similar to MTB FS kinematics and F1 does not use IFP variants. I guess maybe enough people were having the conversation about F1 Kinematics in MTB shocks that someone decided to drop a dime a discover if F1 suspension design benefits MTB shock design. The multi-valve damper with IFP is more of a heavy truck and/or 4×4 concept that works better when scaled up and loads(weight) are dramatically more.

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