Mountain Bike Reviews Forum banner

1, 2, or 3?

1 - 20 of 22 Posts

·
Registered
Joined
·
103 Posts
Discussion Starter · #1 · (Edited)
Posting this to see what the hive mind thinks.
Which fork crown is the best?

I'd prefer answers based off of strength / ease of manufacturing, etc. But looks are important also.

I really like #1, and I really hate #3

#3 would either look terrible having extra weld all around the joint, or would have to be finished flush to make it look nice. I.e, i'd have to remove all of the weld reinforcement for that to work. Definitely not happy with that idea.

1 and 2 seem to be basically the same idea as far as strength go, but maybe I'm missing something. Opinions and/or experience wanted.
(Edit: These are modeled up with 1" 0.035" 4130. Also now that I've thought about it, #2 gives an opportunity to use a larger tube for the crown pieces. Could be interesting.)
Triangle Rectangle Font Cross Parallel
 

·
Most Delicious
Joined
·
1,441 Posts
1" x 0.035" will fail. Do some snooping around the Yo Eddy forks and the need for adding a teardrop gusset. There's also a thread in here somewhere where Walt talks about wall thickness on his forks.
 

·
Registered
Joined
·
3 Posts
1" x 0.035" will fail. Do some snooping around the Yo Eddy forks and the need for adding a teardrop gusset. There's also a thread in here somewhere where Walt talks about wall thickness on his forks.
Yeah Mike Flanigan talks about developing the original Yo Eddy fork (and the gusset) a bit in his interview on Shut Up and Build Bikes. Walt give some details on what he uses for forks here:

 

·
Registered
Joined
·
223 Posts
On One, Sycip and Dekerf have all done version one on their seat stay wishbones. However a quick google image search shows all three versions from various makers (including Sycip and Dekerf), so who knows which is best. Personally I'd go for number 1 every time (see also Wound-Up forks and most cast-crown steel forks).
 

·
Registered
Joined
·
17 Posts
I made a fork out of 1-1/8" .049 and it seems to have held up pretty well so far. I weigh 80kg and don't ride that particular bike terribly hard, but I don't exactly go easy on it either. Was fairly exciting taking it down the local jump trail the first time!
 

·
Registered
Joined
·
103 Posts
Discussion Starter · #8 · (Edited)
Alright, so here's today's episode of "Me using tools I am vastly unqualified to use"
Wanted to share this so the real engineers can yell at me and maybe offer some advice.

But first, some context that I should have shared earlier.
This fork will be for a 'Klunker' type bike. Or in other words, a full-rigid enduro.
Heavily inspired by PVD's series of Klunkers. Go check those out if you haven't seen them yet.
The front tire will be a 29"x3.0". Fork length is nice and short, Somewhere around 430mm.
I'm 200lbs without gear, and my riding style is somewhere in-between "lets see what happens", and "oh ****".

Now I COULD just beef this fork up all to hell and back and never worry about it, but having a little compliance is the goal here.
(Maybe I should just go a little overboard for the first go around? Might be the smartest option.)

The model currently has the legs as 4130 1"x0.035", and the Crown as 1"x0.057"

But anyways, here are the two fork designs I'm choosing between, using F360 'Simulation' tool to try and get an idea of which is stronger.
I added 3mm fillets to all of the joints, to act as my 'weld' stand-in.
I put a simple constraint to the steerer crown race for this test, and added the load to the axle.
The load is perpendicular to the steering axis. As in, under heavy braking, or hitting something is the idea.
I randomly picked 1000N (225lbs-ish) for both. That does actually seem somewhat reasonable, but this was a comparative test more than anything.
Should also note, I have not actually put the brakes on this fork yet. Just trying to get an idea of the crown design for now. Brakes will come later.

My unqualified conclusion? Fork #2 is stronger, with only a marginal decrease in compliance.
Font Slope Parallel Electric blue Triangle
Font Parallel Electric blue Bicycle part Triangle
Tool Font Electric blue Gas Slope
Tool Font Electric blue Slope Graphics
 

·
Most Delicious
Joined
·
1,441 Posts
If you want compliance consider using a straight steerer.

We already know 1" x 0.035" tubing doesn't survive 90s level MTBing so you really should go at least 0.049" if you're going to smash through stuff with an extra foot of front center.
 

·
Registered
Joined
·
3,813 Posts
As one that has a bit of experience with this type of fork,,,,

The last time I built a segmented fork like this was back in the 559/584 era. Riding was a bit different than now as were bike designs.

I used True Temper RC2TT top tubes for the legs. This was a 25.4" tube with 9/6/9 butts. The only issue that I ever had was one fork failing due to "coke canning" at the disc brake mount. I had focused a bit to much energy into one thin spot. This is easy to prevent.

I believe that there is merit to using a quality tube with these dimensions. Will it survive modern use? Maybe. Certainly, we know that 1.125" tubes make for terrible fork legs due to lack of compliance.

Obviously, I feel that there are far better fork designs out there but this type has value for folks in Ben's situation.
 

·
Registered
Joined
·
310 Posts
The load is 600n full reversal for fatigue on a fork to pass the iso test there is also a drop test which is available in the Iso testing standard that simulates a mass falling , for example from a curb, in simple terms it will cause the fork to bend forwards under compressive load a modern MTB coming off a dropoff for example will still be a relatively easy calculation. You can use any arbitrary figure for a front impact test it can be variable

https://web.wpi.edu/Pubs/E-project/...ricted/Fatigue_Analysis_of_a_Bicycle_Fork.pdf you may be able to enhance your knowledge on the matter by reading papers and arriving at your own conclusions via real world testing and cross referencing other validated works , you may prefer however to produce pretty coloured pictures and hope
 

·
Registered
Joined
·
103 Posts
Discussion Starter · #12 ·
The load is 600n full reversal for fatigue on a fork to pass the iso test there is also a drop test which is available in the Iso testing standard that simulates a mass falling , for example from a curb, in simple terms it will cause the fork to bend forwards under compressive load a modern MTB coming off a dropoff for example will still be a relatively easy calculation.

https://web.wpi.edu/Pubs/E-project/...ricted/Fatigue_Analysis_of_a_Bicycle_Fork.pdf you may be able to enhance your knowledge on the matter by reading papers and arriving at your own conclusions via real world testing and cross referencing other validated works , you may prefer however to produce pretty coloured pictures and hope
I gave a quick glance at that paper, definitely going to read it. Thanks.
And yes, I do like pretty colored pictures. They're free (minus my time) and I'm learning way more than I thought I would.
I can make a hundred stupid mistakes on the computer and learn a ton before I even begin the process in the 'real world'.
 

·
Registered
Joined
·
103 Posts
Discussion Starter · #15 ·
Time for more pretty colored pictures!
I updated the model, figured out how to make the fillets in F360 much more like real welds. (Setting "Radius Type" to "Chord Length" was the ticket, for those following along at home)

To summarize what I tested, increasing the leg wall thickness from 0.035" up to 0.057" yielded a %44 increase in "strength", and a %29 decrease in "compliance".
Is this the real world? Nah. Am I having fun? Yeah.
Haven't done this for Fork #2 yet. The classic looks of fork #1 are slowly swaying me away from #2...

Product Organism Font Circle Graphics


We already know 1" x 0.035" tubing doesn't survive 90s level MTBing so you really should go at least 0.049" if you're going to smash through stuff with an extra foot of front center.
I've been brainstorming this a bit, trying to quantify what differences there really are between 90's riding and today's riding (and equipment) in this context.
The first big that comes to my mind is the front tire I'm going to run. 29"x3.0", with a cushcore in there. maybe something like 18-22-ish psi.
That's a far cry softer than what was around in the 90's.
But of course our riding style and trails are also vastly more aggressive. Lots of factors to consider. Mainly just thinking 'out loud'

At any rate, I've got a while before I even think about building this thing, so plenty of time to play around on the computer and see what I can learn.
 

·
Registered
Joined
·
256 Posts
The first big that comes to my mind is the front tire I'm going to run. 29"x3.0", with a cushcore in there. maybe something like 18-22-ish psi.
That's a far cry softer than what was around in the 90's.
But of course our riding style and trails are also vastly more aggressive. Lots of factors to consider. Mainly just thinking 'out loud'

At any rate, I've got a while before I even think about building this thing, so plenty of time to play around on the computer and see what I can learn.
Front tire pressures on 29x3.0 are more like 12ish psi, maybe less with Cushcore if it offers some sidewall support/squirm prevention. 18-22 would be bounce city.

It is a shame Walt was run off, I would imagine he has pretty easy answers to these questions with real-world use to back up those answers.
 

·
Registered
Joined
·
103 Posts
Discussion Starter · #20 ·
There's another student paper floating around where they looked at the effects of the angle of the strut tubes in segmented forks. Might even be linked somewhere in here.
I'll definitely see if I can find that. That other paper certainly had some good stuff in it.

Front tire pressures on 29x3.0 are more like 12ish psi, maybe less with Cushcore if it offers some sidewall support/squirm prevention. 18-22 would be bounce city.
Yeah, that was kind of just a wild guess. I've never actually ridden a 'plus-size' bike of this sort. Will take some experimenting to see what PSI I need.

Walt sells the fork legs he had drawn for his custom forks:


The old round True Temper track blades at 1" x 1.4mm seem to have good survivability if you want another data point to consider.
That looks like not too bad of an option. in any case, going to 1-1/8" would certainly take all of the worry out of it breaking.
I may end up going that route and see what happens. Then one day make another fork with 1" legs and see the difference for myself.
Still lots of homework I need to do.
 
1 - 20 of 22 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top