Tuesday, August 15, 2017

The Missing Linkage

I rode this fork in the 90's. Linkage forks could be a good thing, if not a tad ugly.
So, there was a bit of a hullabaloo in the mtb community about this "new" design which features a linkage fork. Actually, it isn't anything new, or revolutionary. Linkage forks have been attempted and used in mountain biking since suspension became a "thing" back in the 90's.

My friend from So Cal, Grannygear, suggested that the "newest" incarnation of  this idea as shown recently could be miniaturized and employed as a gravel going fork. Actually, even that already exists. It is called the Lauf Grit fork, which doesn't use pivots, but it uses "leaf springs" which link together the two separate structures which make up the drop out and fork legs respectively. Lauf manages to eliminate pivots and a separate spring which a more "typical" linkage fork uses by making the links the springs. It is genius, and it seems to work. The negative here is that there is zero damping. Of course, with the short travel and higher frequency of bumps a gravel going bike would see, this is less of an issue than it is for a mountain bike.

An example of the German Answer "Kilo" linkage fork.
 Let's say we want damping though, well, then you get into pivots and a separate damper unit. Okay, but why? Why would you even want to put one of these ghastly contraptions on a bicycle?

Well, I happen to have some experience riding a linkage fork. I rode an AMP fork for a few years there on a couple of different bikes. In fact, I have two of these unicorns in my basement. Anyway, they did have a very good small bump compliance and steered really well even deep into their travel. There are reasons why that was.

While there have been, and are, many different types of these beasts, what is attractive here are two things. First; You can rid yourself of stiction- the term refers to the energy required to break free from/overcome the friction of bushings and seals on a stanchion sliding in uppers/shock can in a typical cylindrical, telescopic damper unit. (Suspension fork/rear shock) Linkage forks rely solely on pivots which are much freer to move, or as in the case of the Lauf, just any input overcoming the spring force will allow the wheel to move upward.

Secondly, linkage forks can be designed to control the axle path and can be designed to have "anti-dive" characteristics when applying the brake. Think "Split Pivot" for the front wheel, if "anti-dive" doesn't make sense.

Obviously, eight pivots and a damping unit are going to be susceptible to dirt and moisture and the damage those things can incur upon bushings and bearings. That's why most linkage forks put "the business end" of things where the fork crown is. That arrangement, well......actually anywhere you put linkage, is typically not a very attractive solution but it is an effective solution for bump absorption. Much more so than a telescopic suspension fork is, and generally speaking, lighter to boot.

A linkage fork wouldn't necessarily have to look ugly. Weird? Well.....yes.
In terms of gravel riding, a long travel of the wheel isn't a desirable trait, nor is it necessary. Many forks being positioned in this category feature 30mm of travel. That isn't much and wouldn't require much of a damper unit.

So, in my estimation, a fork like the recently shown mountain bike fork, with its frame altering design, is not at all necessary. A short linkage could be designed which would not only give the right amount of travel, but be aesthetically appealing as well. It may even be able to be made to be so compact that the fork could look nearly traditional. 

But then it could be argued that we don't need a suspension device at all. In fact, many would argue that a simple, rigid fork with an adequate amount of compliance will work with a voluminous tire to provide just the right amount of wheel movement without any unnecessary complexity. I happen to be one of those folks, and I think that many companies miss the boat on this when they spec these big section, beefy looking, unforgiving carbon fiber forks.

Many folks feel that carbon forks will damp vibrations. Really? If you own one of these beefy looking carbon forks you should check this out: When your front wheel impacts a road irregularity sometime, watch the fork blades. (Being extremely conscious of where and in what situation you do this, of course.) More often than not you will see the fork blades remain in plane, but the wheel moves backward a bit. What you are witnessing, most times, is a flex of the top and down tubes of your bike, not the fork blades themselves. This is because manufacturers are deathly afraid of fork failures, the industry testing standards are too stringent, and because of the manufacturers reliance on the fairy tale that carbon forks absorb road chatter. Now some do, but most don't on gravel bikes. 

That said, a smartly designed, short travel, tunable, good looking linkage fork with damping is possible, it just hasn't been done yet. If it ever is, I'll definitely want to try it out. It may just be the missing link between rigid forks and full on, traditional style suspension forks for gravel bikes. 
 


7 comments:

phillip Cowan said...

I'm surprised no one has trotted out a bicycle version of the BMW Telelever fork. It should be easy on a bicycle since there is no engine or gas tank to design around. The Telelever also has anti-dive geometry. All these non traditional designs suffer the same problem. They work good but they look bad.

james said...

GT, you stated "What you are witnessing, most times, is a flex of the top and down tubes of your bike, not the fork blades themselves". Really? I can't imagine top and downtubes (together) flexing in a vertical path, the (front) triangle is just too rigid of a structure, at least in a loaded up and down path as you mention.

Guitar Ted said...

@james- You should YouTube some frame testing jigs sometime. ;>) Yes, what is happening is that the fork bends backward, the top tube flexes up towards the rider and the down tube as well. It isn't much, but on some bikes, the result is that the handlebars move forward and back in an ellipse that you can visually see. Think about it- How can the handlebars be moving that much if the frame isn't flexing? It is a forward and back movement, not up and down so much, although that is certainly happening. Try this on a front suspended bike and it doesn't happen. the bars don't move in the same manner. For obvious reasons. (Unless the front triangle is so flexy it allows for this.)

Ari said...

My friend tom t-boned another rider and broke his ti bike down by the bb area. Nothing happened to the carbon enve fork. The frame builder that fixed it said the rigid fork acted like a cantilever to break the frame

OkieBrian said...

Don't take this as gospel, but I believe I read somewhere years ago a similar claim from Brant Richards about frame flex. Only he was talking about the rear triangle. He basically stated that the compliance in the front triangle played a significant part in the actual feeling of a compliant rear end.

Gravelo said...

Do you think that there is any merit in the old "suspend the rider, not the bike" ala Girvin Flexstem, or Allsop, or Breezer Beamer bikes? Not exactly damped in all applications, but certainly minimal travel. BTW, I lusted after an Amp fork for a long time. When I finally took the leap it barely made it out of the box before I said, "No way dude!" and mailed it back. Rode a Girvin fork for many years and loved it though!

R.E. Barkschat said...

First suspension for I rode was an AMP. Had it for a long time on a hardtail until parts started wearing out, the AMP company had moved on to other things so instead of sinking cash into replacement parts I purchased a "traditional" slider/stanchion suspension fork. First ride was very strange. The manner in which the traditional fork moved over bumps and hits of various frequency and amplitude was completely different than the AMP. Even the tire performance and feedback (Fire XC) felt different between the forks. Took a while...and a rubber swap...to get more comfortable on the traditional suspension fork.