Truss fork

[surlypud]

I am planning a 29+ build based on the Knard. I am considering building a truss fork (pretty much going to copy Jone's fork) but I'm not sure what thickness of tubing I should use. Initially I was thinking 5/8" x 0.035 would do the trick. I don't ride rough or shred the gnar, fire roads and smooth singletrack are what I enjoy, but I weight a hefty 250#. Would 0.049" be a safer choice?

Thanks,
Chris

[Andrew R Stewart]

Of course .049" tubing would be safer. Do your homework. Search for "segmented" forks. Watch where you place vent holes... Andy.

[adarn]

Probably depends on the fork blades you use.

[Chad]

Not trying to be a pot stirrer, just clearing this up in my own mind.

Aren't segmented and truss quite different?

[EricKeller]

yes, a truss fork is quite a bit different than a segmented fork.

[ToddFarr]

Not trying to be a pot stirrer, just clearing this up in my own mind.

Aren't segmented and truss quite different?

Truss


Truss fork 9 by Halekai, on Flickr

Seg


Dang this fork is going to look Rad shredding Singletrack by Bronto Bikes, on Flickr

[drwelby]

Aren't segmented and truss quite different?

I think he's referring to the fact that both have blades welded to struts, and a little googling will turn up interesting failure modes when the blades are too thin. One data point would be the Fat City forks and the addition of the teardrop gusset to the back of the leg.

[Chad]

I think he's referring to the fact that both have blades welded to struts, and a little googling will turn up interesting failure modes when the blades are too thin. One data point would be the Fat City forks and the addition of the teardrop gusset to the back of the leg.

Gotcha, didn't see the connection, seemed like an odd response. Makes sense now.

[Andrew R Stewart]

Yes, this is my concern/point. Fork failures are a big deal. And the OP is a BIG guy. Andy.

[jaredjerome]

Not to mention those tiny blades would look pretty gross on any bike. Just sayin'.

[Will Neide]

Not to mention those tiny blades would look pretty gross on any bike. Just sayin'.

http://www.jonesbikes.com/?option=co...1320&Itemid=58

I would disagree, and I still think there is a bit of confusion on here as to what is a truss fork.

The only similarities I see between segmented and truss are that they are both forks. You're obviously not going to use the same blades or even the same angles. The designs are completely different.

[Andrew R Stewart]

http://www.jonesbikes.com/?option=co...1320&Itemid=58

I would disagree, and I still think there is a bit of confusion on here as to what is a truss fork.

The only similarities I see between segmented and truss are that they are both forks. You're obviously not going to use the same blades or even the same angles. The designs are completely different.

Some of the concerns of forks made completelyseparateperate pieces of tubing is the specs of the tubing. But some of the concerns is also the design/process. Sure, this is a concern with traditional crowned forks too. But with fork crowns there is a vastly greater history that drives their designs as well the liability of the manufacturers at risk. Both tend to drive out of the market those crowns which are not up to the task. With completely self designed forks none of this is present. (Hence the OP's question, "what do I use?") The builder takes on a larger responsibility and, in this case, the OP is seeking help. We are just offering advice.

Many have said before that building a fork is just as critical/hard as building the rest of the frame. I agree to a point. Building a fork is MORE critical then the rest of the frame. It's the one structure where there is only one end of the part connected to other tubes. (Although a strut fork is the sort of exception. Sort of because the top of the strut is typically not welded/brazed to the steerer but clamped instead). Any one part of a frame can (and do) suffer a failure/crack and there is another joint helping to keep the structure together while you slow down. (This is why two tubes joining a head tube is better then one). But with the VAST majority of forks the crown stands alone. Andy.

[surlypud]

I appreciate all of the good information. The advice to look up segmented fork failures was particularly enlightening.

This created some new questions for me. It seems like 0.049 wall thickness isn't the best idea. Is 5/8" diameter 4130 tubing going to be sufficient for my clydesdale+ size, even if the wall is 0.058? I assume that going up to 3/4" would create a stiffer fork, but not necessarily any stronger, correct? At this point I'm thinking 0.058 throughout would be the safest bet. For the top and bottom steer tube clamps, I was planning on using 1.25 x 0.58 as it would be a nice slip fit with 1.125 steerer.

[Rody]

Surlypud,

Andrew has given the most sage advise...building a non traditional fork is for an experienced builder who has an extensive background of experience to draw from. If you have simple questions as to tubing diameters and wall thicknesses and how they perform in this configuration, you should re-consider the venture for your own safety.

That being said, the strength of the truss fork is more about load distribution and fabrication technique than it is about just what tubing to use. Proper triangulation, manipulation of the tubing, tight miters and a mastery of technical joining is paramount in a truss fork. Without having all those dialed in, the fork has a high propensity of failure.

If I were to build this fork, I would create a unicrown blade from .875 x .o49 that sweeps lateral from the 1.125" steerer and then is radius bent on the bottom third toward the hooded dropout. Vertical support legs of .625 x .035" would extend from the dropout to the steerer above the headtube. The two sections of the fork would be tied together with three struts below the headtube at the anterior steerer tube, with the strut over the tire being radiused slightly for clearance for the tire. This triangulation allows the bottom of the fork to flex as a single unit, using the thickest portion of the steerer as it's anchor support with the additional load transferred to the top of the steerer.

This spec is based on my level of fabrication attainable within my shop. I do not know your skill or experience level, so if you DO choose to pursue this project, you will need to take that into account.

My advise would be to continue to work with other less critical fabrication until you achieve the necessary level of craftsmanship to complete this with confidence.

cheers,

rody

[surlypud]

Thanks for your input Rody. I don't necessarily want to admit it, but I think that a truss fork is beyond my capabilities at the moment.

[halekai]

ToddFarr,

You had to pick THAT fork out of all of them! HA! Well for all of those wondering just what the hell is going on with that noodle looking truss fork Ill have to explain...

That fork pictured above was one of my early experiments. I wanted to see how light I could make a truss fork and at the same time try and build in some passive suspension that Id been reading about with the Jones forks. Well...The fork in question certainly has some passive suspension. So much that it never made it to a frame.

It was however an awesome experiment because despite appearences it is actually quite strong with forces in line of travel. Where it fails is in lateral stiffness.

So I learned a hell of a lot from building it and it sits on the back of my bench laughing at me and taunting me to build more truss forks.

Not including the above mentioned aborted attempt I have built 5 truss forks that are currently on the road. All 5 are incredibly strong. 4 of them incorporate mechanical disc brakes and one has Paul retro cantis and an integrated rando rack. 3 of them have the truss portion totally removable using grommets and bolts the other two are brazed fast.

My advise to anyone building a truss fork for the first time and esp to a relatively inexperienced builder would be to...

1. Ask yourself why you are building it. Trusses are not passive suspension they are triangulated stiff systems. They are not light weight. But they do have some significant advantages that have to do with spreading out the load over a greater distance and making for one tough front end capable of hard braking and punishment.

2. If you are not sure then overbuild it. Worst that can happen is you end up with a cool looking heavy ass fork. My first roadworthy fork was 2.2 pounds. The lightest one so far is 1.8 pounds. That is as light as you can go in steel as far as I am concerned. I used Henry James 24mmround chainstays as forkblades in a beveled unicrown and 3/8" x .028" 4130 for all of the truss work.

3. Be prepared for a sh*t ton of work. It is waaay more work than building two forks especially the first time when you are figuring stuff out. If you want to see what that looks like check out this 7 part time laps of me puting one of these things together...Its so much work I havent had time to post parts 6 and 7! He he.

Truss Fork Sequence Part One | Flickr - Photo Sharing!

@ Surlypud...dont get discouraged. If you can build a fork that you trust then you can build a truss fork. It just takes longer. Give it a whack!

Bottom line for me is that I love the look and feel of the truss especially with a disc brake involved. I will be building many more of these in the future. h
truss11.jpgtruss33.jpgtruss22.jpgtruss44.jpg

[halekai]

Surlypud,

You will probably want to use a steerer tube clamp on the top only. Weld or braze the bottom in. Jones uses a clamp down low which allows hom to use Ti, Carbon, Al2+ or whatever he wants as a steerer. But it is complicated getting the crown race seat to work. So use steel and fix it. Its just easier. The upper truss bolts into the upper clamp and can be manipulated out of the way for installation.

For my clamps I use the upper fork tube from a 1969 Honda CB350. Nice thick wall and inside diameter is nearly a press fit. Here I am giving away my secrets!

Another secret for you. One of the reasons these segmented forks can fail is that the caps are not built or attached stoutly enough. if the cap weld is compromised then the structural integrity of the whole system goes to hell. I had one of my caps come loose when the brass melted out of the seem during attachment of the blade to the crown. I tested the fork by squeesing the blades together and I had the damnest time figuring out why there was so much flex...turned out the cap was to blame. Now I bevel the blade all the way around before brazing in the cap to allow the brass a place to sit and it provides a nice visual check after everything goes together...its a nice wide band of brass instead of a thin seem...if that makes sense.

[blasdelf]

try and build in some passive suspension that Id been reading about with the Jones forks

I thought his whole point was that he designed them to have no suspension whatsoever

for ultimate rigidity and precision, using a 135mm front hub to get them laterally rigid too

with a fat ass tire for the comfort

[halekai]

Yes, Turns out that is the point entirely. Goes to show that you should not take what you read on the internets as gospel. My idea was to try and build a magic fork that was suitable for cross type riding. I went too small on the first attempt and came up with something rediculous...live and learn.

Passive suspension is sort of a myth. it would mean deformation to a high degree followed quickly by collapse.

On the other hand when we are talking vibration dampening I think there is is something there...ie a super stiff fork that is elastic and compliant within a tiny tiny range...food for thought.

[Andrew R Stewart]

Yes, Turns out that is the point entirely. Goes to show that you should not take what you read on the internets as gospel. My idea was to try and build a magic fork that was suitable for cross type riding. I went too small on the first attempt and came up with something rediculous...live and learn.

Passive suspension is sort of a myth. it would mean deformation to a high degree followed quickly by collapse.

On the other hand when we are talking vibration dampening I think there is is something there...ie a super stiff fork that is elastic and compliant within a tiny tiny range...food for thought.

Not to through water on your dampening thought (Sorry, bad pun, couldn't help myself)- Steel isn't a material that most think of as offering damping. And a super stiff fork of any material will have less damping then a more flexy one. Andy.