Home > Other Fun Stuff > Advocacy & Industry News > News

Rock Sled Suspension is Developing a Long-Travel Leading Linkage Fork

rock sled leading linkage mtb fork coil shock damped nicolai g16
18
Support us! Bikerumor may earn a small commission from affiliate links in this article. Learn More

Ashley Kalym, Founder of Rock Sled Suspension, gives us some insight into the what, how, and why of the long-travel fork he has been developing over the last 3-4 years. Ashley believes he has overcome the limitations of a traditional telescopic mountain bike fork design, namely the stiction and binding issues, with a leading-linkage fork design that uses an off-the-shelf-rear shock to damp impacts.

Rock Sled Suspension Leading Linkage Fork

Though uncommon, linkage forks are nothing new in the world of mountain biking. Trust Performance’s Shout and The Message multi-link forks did gain some traction for a short period of time, but the big brand suspension manufacturers have stayed away from the design, choosing instead to refine the damping systems of their traditional telescopic forks, while adding weight to and increasing the stiffness of the chassis.

rock sled leading linkage mtb fork coil shock damped nicolai g16
Ashley has been testing the second prototype of the 180mm travel Rock Sled Linkage Fork on his Nicolai G16

“It never really made sense to me as to why you would tie the chassis, shock, and damper into a single structure. Any force that is put on the structure will in turn have an effect on the damper”, explains Ashley. He’s not the first to have had thoughts along these lines. Motion-France, Adroit and Structure Cycle Works are some more recent examples of brands looking to overcome aforementioned biding issues suffered by the bushings in a telescopic fork under flex, as well as some other characteristics they see as detrimental to performance.

rock sled mtb fork leading linkage 160mm travel prototype coil shock
The Rock Sled Linkage Fork on Ashley Kalym’s prototype enduro frame

While Motion and Trust developed proprietary dampers for their linkage forks, Adroit and Structure’s designs use off-the-shelf air shocks and thus benefit from their wide range of external adjustments. Ashley felt it made a lot of sense to use a design that could use pretty much any rear shock on the market; get the dimensions correct and the leverage curve sensible, and the fork’s behavior could, in many ways, be as tuneable as the variety of rear shocks available.

rock sled leading linkage fork view from cockpit
A view of the Rock Sled Leading Linkage Fork looking down from from the cockpit

All of the aforementioned are significantly more complicated than Ashley’s single-pivot leading linkage fork, utilizing multiple pivots. The first iteration of the Rock Sled (not shown) was single-sided with a one-piece carbon steerer and carbon leg. It had a single link and a shock absorber delivering 160mm of travel. That was a bare-bones, proof-of-concept prototype.

In the V1, the brake caliper was bolted directly onto the link. Ash tells us the anti-dive force was massive. “Riding along, if you braked with any sort of force, the suspension would want to extend a lot, unless you had a lot of weight over the front. It was really quite violent”. Apart from the anti-dive, Ash says the fork felt stiff enough while riding, and that it didn’t really feel any different from a regular fork.

rock sled suspension leading linkage fork floating brake arm
The links are machined from aluminum, as is the brake mount and floating arm

With the second prototype, seen throughout this article, Ash sought to solve the aforementioned braking issue by mounting the brake caliper to a floating brake arm. It takes on a parallelogram arrangement, so when the fork compresses the brake caliper remains equidistant to the fork leg, no matter where in the travel the fork is operating, just as it does in the case of traditional telescopic forks.

So, how does it ride?

“The fork rides like a normal fork in terms of its steering behavior, axle path, and braking behavior. Where it differs is the much greater sensitivity, adjustment, and performance. The use of bearings as the main method of fork movement means that binding and stiction are pretty much non-existent, and being able to swap out the shock for any type that you wish means much greater adjustability. Also, due to there being some very capable shocks available (PUSH, Fast, EXT, etc.) I think the performance in terms of damping can beat any telescopic fork out there. In addition, because the fork has a leverage curve then it can achieve the grail of being sensitive off the top, supportive in the middle, and supportive at bottom out” – Ashley Kalym, Designer of the Rock Sled Fork.

A CAD render of the Rock Sled Fork at full compression

Where the Rock Sled Fork does differ significantly from a telescopic fork, is in ground clearance. As it compresses, the fork legs are pushed down toward the ground. Ashley says that, with a 29″ front wheel, the ground clearance at bottom-out is 209mm (8.22 inches). Despite that, he says he hasn’t found any issues with it on the trail.

Though the head angle delivered by the Rock Sled seems wildly steep upon first glance, it actually comes in at 63° on Ashley’s Nicolai G16 Enduro Bike. With the linkage design, the axle path differs to that of a telescopic fork. Whereas the latter moves in a straight line, following the angle of the steerer tube and thus head angle, the axle of the Rock Sled Fork will track more of a rearward arc as the fork is pushed into compression.

rock sled linkage fork nicolai g16 nds floating brake arm version 2
Ashley pairs the Nicolai G16’s 155mm rear wheel travel with a 180mm version of the Rock Sled Leading Linkage Fork

The prototype seen above has a 45mm offset at top-out, decreasing to 17mm at bottom-out. A reduction in offset proportionately increases the bike’s trail figure. Ash says this would, at least on paper, increase your steering stability the deeper into the travel the fork is compressed. A third prototype that was recently 3D Printed, designed for 160mm travel with a 571mm ATC is said to have a more standard axle path with a 51mm offset at top- and bottom-out.

A 3D Print of the linkage component of the Rock Sled Fork (V3)

It’s worth pointing out that, even in the Rock Sled Linkage Fork, there will of course still be forces applied to the rear shock outwith the plane in which it provides shock absorption, but Ash has organized the linkage in such a way that, at least in theory, those forces should be greatly reduced in comparison to those experienced on a telescopic fork.

The distance between the pivot and the fork’s 20mm axle is only about 200mm. Ash says the leverage that can be exerted over that distance is quite small. Comparing that to a rear suspension linkage, that distance is considerably less than half the distance between the rear wheel axle and any main pivot on the front triangle.

The latest design of the Rock Sled fork also makes use of a 15mm axle, mainly to more easily serve the market as most trail and enduro riders will have a Boost front wheel (15mm x 110mm). The ambition with this version was to reduce the number of unique parts, simplifying the design from a manufacturing perspective. The left link that does not drive the shock is now the exact same part as the right link that does. This is basically reducing the costs of the CNC machining. Ashley has also reduced the footprint of the fork, bringing the width to <200mm, closer to that of a conventional fork.

What are the next steps for Rock Sled?

If happy with the ride feel of Version 3, Ashley will send the Rock Sled Fork off to EFBE for testing, prior to producing a limited run of forks for sale. He is looking to keep the price as low as possible, in order to enable the greatest number of people to try it out. And, the less money the customer spends on the fork, the more they have left to spend on a high-end shock. Ashley tells us he has deliberately kept things simple in terms of construction, bearings, nuts, and bolts, etc., so as to make the manufacture as cheap as possible, and therefore the retail price as cheap as possible. “I want to aim for under £1,000/$1,000, but how much under this price point it will be I don’t know”.

You can keep up to date with Ash and Rock Sled Suspension on Instagram @rocksled_suspension and @rocksled_bikes

Subscribe
Notify of
guest

This site uses Akismet to reduce spam. Learn how your comment data is processed.

18 Comments
Oldest
Newest Most Voted
Inline Feedbacks
View all comments
Roger Pedacter
Roger Pedacter
1 month ago

Wat? 160mm travel, 51mm offset at extension and compression, and a 200mm arm length? No, it will *not* have anything like the axle path or ride of a telescopic fork or the linkage forks he’s comparing it to. At mid travel (where you actually ride) this will have at least 68mm offset (not sure if he’s giving chord length travel or arc length travel).

I mean, I get it. It’s *super* cool to show off a homebrew suspension fork. Absolute respect for that. But there’s a lot of unavoidable functional failures with that layout. There were a few of these single pivot designs that made the rounds at the races and trade shows back in the Wild West days of mountain biking, and there were three big reasons none of them ever made it to production: Axle path, torsional flex, and pivot wear. There’s no breakthroughs here that address those critical design flaws.

Sorry to dump on this, but it’s a deeply flawed design that 3D printed links won’t overcome. And ignoring the past isn’t going to erase it.

Bárócz Álmos
Bárócz Álmos
1 month ago
Reply to  Roger Pedacter

This fork has a backwards axle path, so the offset actually decreases when it’s compressed. Theoretically the steering remains fairly constant under compression because as the head angle steepens the trail grows.

Roger Pedacter
Roger Pedacter
1 month ago

Nope. Read the article. He claims to have changed it so it’s 51mm at extension AND compression. That means the axle travels in a forwards arc.

Destroyer666
Destroyer666
1 month ago
Reply to  Roger Pedacter

Can’t really say anything about axle path and torsional flex problems, but is pivot wear really a big issue here? I mean, if I understood correctly, the pivot points have bearings, so when wear occurs you could just change them – similar to rear triangle pivots in many full.susp.mtb’s, right? And telescopic forks have wear issues as well. So would these problems be more significant here than in those other cases or is this just another of case of having to accept that your bike components need maintenance?

Jaap
Jaap
1 month ago

That double-link brake mount is really smart.

Naero
Naero
1 month ago

Cool fork. Was the weight left out on purpose?

Grillis
Grillis
1 month ago
Reply to  Naero

Probably because this is a prototype, nowhere near production yet.

solarider
solarider
1 month ago

Can I be the first to commiserate the poor ugly tree which is now bare of branches as this fork appears to have taken every one out on its way down. Looks like one of those badly put together supermarket bikes where the forks are on backwards which has been ridden into a wall. Even Trust wasn’t that ugly!

John
1 month ago
Reply to  solarider

Does it matter?

solarider
solarider
1 month ago
Reply to  John

Hell yes! The bicycle is one of the simplest and most beautiful machines on the planet. This is an abomination.

Frrealsthough
Frrealsthough
1 month ago
Reply to  John

yes, it does, in the real world it matters more than performance. Having that fork on your bike makes it look like you crashed into a wall and had a frame failure and no one other than a very select few people who care more about mechanics than aesthetics will buy it and the product will fail because of that.

Craig
Craig
1 month ago

I’ve spent many hours on CAD with various iterations of linkage forks and never bothered to make one for the simple reason that no matter if it’s single legged or double, air shock or custom shock, no matter what it will be appreciably heavier than an equivalent telescopic fork. And the linkage fork will have with it it’s own performance and durability downsides.

Linkage forks went through a popularity phase with motorcycles, and then with mountain bikes in the early 90s, so any attempt at successfully marketing a linkage fork is nothing more than a hobby. It’s funny to still see them pop up every few years at a trade show though.

The only way a linkage fork can be competitive on weight is in a short travel design (ie, 50-80mm) that forgoes a hydraulic damper for a simple elastomer layout, and, well, we know how that’ll go. Or what Lauf do where there’s no mechanical moving parts, but of course that has travel and performance limitations.

Eggs Benedict
Eggs Benedict
1 month ago

The side view of the bike looks like he ran it into a parked car.

Marcus
Marcus
1 month ago

This looks very familiar to the Ribi linkage fork from the 1970’s. Certainly not a new idea. It did not catch on in the dirt bike world either.
https://motocrossactionmag.com/forgotten-motocross-tech-ribi-quadrilateral-linkage-forks/

Jose
Jose
1 month ago

I know BikeRumor makes no effort to critique on what they report on. Everything is great, will be, could be great, maybe… Yet they did mention Trust forks. If one knows about those, it should be enough to know why another linkage fork is unlikely to succeed. And there is really good reason why trail motos don’t have them. The video is pretty amusing.

Jrags
Jrags
1 month ago
Reply to  Jose

You don’t think they have ever talked about trust forks? And if you know they have, you should also know they aren’t obligated to talk about every other form design that strays away from tradition, even if those forks are similar to this.

JRags
JRags
1 month ago

There are some very smart and knowledgeable people in this comment section…

Jeremiah Gardner
Jeremiah Gardner
1 month ago

Come on. Ashley got far to concerned with if he could, that he never asked if he should….It’s ugly, it’s heavy and in no way am I going to trust that thing through a rock garden of even marginal gnarliness.

It’s cool that you tried to create something, kudos there….but you should put that time and energy somewhere other than this fork.

Subscribe Now

Sign up to receive BikeRumor content direct to your inbox.

Subscribe Now

Sign up to receive BikeRumor content direct to your inbox.