Oh No, Not Another Fork Upgrade Thread

[Hobot]

Ok Rich, I misunderstood the lack of damper rod in your cartidge set up and therefore your big damper hole function. Should not be much stress on holey damper tube - unless something is striking it.

W/o the damper cap on and no extended top bushes there is 6" bush to bush travel. If you don't have that then something is mis machined and snagging.
Greg that now sells hobot kit thought I was nuts till he found bad fit fault, for 4 months, back and forth like I was being mean liar. Triple check eh.
If you make a top out buffer that bares against the damper tube, better re-do the holes. Its been done by longer tube or rod + spring or same tube-rod + rubber bumper washer. Apparently did not work out well enough to evolve.

I assume you have modified the other fork leg to avoid the damper cap - valve top out clunker.

The spring rate is a big deal. In general I hear and felt myself the factory springs as too harsh for the little stuff yet too soft for the big bumps.
In general I read reports the common universal supplied progressive springs are too soft all around. I luck out to have read an ebay blip that mentioned custom 3 rate spring to avoid the general complaints and think they got it pretty right.
I've though up a cheap simple means to graph the progressive rate as ball park to aim for in next custom batch. But even these custom springs were a tad too hard and benefited with short second very soft to very hard spring spacer, to take up the slack the full 6" travel leaves in factory spring length, which coil binds before full 6".

190 cc is getting close to hydro lock up level, don't error with too much.
I'll be into Peel forks again soon, leaned on living room wall right now, to see how involved to flip cartridge and retain full 6" shock absorbing handling.

 

[Hobot]

The inevitable consequence of more travel ( = allow the slider to slide further down ) is that you need more oil .
Removing the damper rod increases air volume by roughly 30 cc .

Ok good data point 30cc for factory damper rod vol.
Might over fill some time to see what ya can't get away with.
I'm not clear yet on why extra oil is needed nor the actual fit and dynamics of the emulators but will be and have more questions, suggestions or recoil opinions.
At this point on hobot simpleton Roadholders I can't imagine what left to improve or how ride to reveal any weakness, but maybe tighter fit of top bushes on stanchions for visual calmness but no effect I can tell on handling or compensation.

I'm impressed and pleased to see more things to keep my project never ending.

 

[Rich_j]

Rich J ,
I have practically the same setup like yours , but I do not have the topping out problem .
I see you shortened the tube on which the valve sits , , but are you shure that , on full extention , the seal is still above the holes in the stanchion ?
25 mm preload seems a lot . I use only 10 mm , with slighty softer springs than std .
It takes about 190 cc ( also SAE 20 ) fork oil to keep the valve submerged all the time .
Could it be that , on full extention , the complete valve is lifted from its seat (tube ) ?
(unlikely with sufficient preload ..)
Maybe you could try to increase rebound damping by shimming the larger spring , or using a stronger one ? ..
A side remark : drilling the holes in the damper tube at 90 ° is not so good for strength : better is to drill them in line .

Just catching up, been away for a bit of sun and sea .

The seal is clear of the stanchion holes on full extension according to CAD
I think you're right that my preload is too high, I'll reduce it for the next test which should be in the next few days.

Not sure why you believe holes in line to be stronger than staggered, all loads are axial and amount material bearing the the load is the same regardless of the hole position?

Rich

 

[swooshdave]

..Not sure why you believe holes in line to be stronger than staggered, all loads are axial and amount material bearing the the load is the same regardless of the hole position?..

mm..how shall I explain this .. :
Suppose you made the holes bigger and bigger ..till they join .
In case of staggered holes your tube would fall apart in 2 pieces .
If the holes where in line , you would have one elongated hole , but the tube would still be in one piece .

But the holes aren't bigger and bigger. By removing material from each side you've left more material between the holes. But I can't wait for a better explanation.

 

[swooshdave]

No, I understand the concept (didn't even need to ask the kid!) but are we assuming completely vertical loads? Is there any side load on the part?

With the forks fully extended with a side load (assuming the inherent flexibility of the forks) would the stanchion tubes have any chance of distorting?

In that case would you agree that distributed holes would be better than ones aligned on opposing sides?

 

[swooshdave]

..are we assuming completely vertical loads? ..
would the stanchion tubes have any chance of distorting?
.. aligned on opposing sides?

the load is stricty vertical .
The stanchion tubes surely flex , but the holes are in the damper tube , bolted in the slider and can't be affected by a distorted stanchion .
(Look at Rich J CAD drawing )
Why opposing sides ? a single line of holes will do .

I think the more existential question might be "does it need more than one set of holes?" Doesn't the stock one just have two?

 

[Rich_j]

..But the holes aren't bigger and bigger. By removing material from each side you've left more material between the holes. But I can't wait for a better explanation.

Swooshdave , sometimes exaggeration helps to get something trough ..

Ok , a second try ..

Imagine you cut the tube open lengthwise and roll it out :

on the left : the tube with the holes at 90°
on the right : the tube with holes in line .


The material coloured red will carry the vertical load directly to the ground ;
the material coloured yellow only helps indirectly .

Now , wich one of the 2 you think can carry the biggest load before collapsing ?
(altough the holes on the left are clearly further apart ..)

If you still don't get it , I give up .. ask a friendly architect ..tell him its a brick wall , and the holes are windows .
or show it to your kid ..I'm shure she will understand ?
( great looking kid , by the way ..)

footnote :
the left drawing is not a correct representation of Rich J 's hole pattern , but I hope you get the idea..

Unfortunately stress doesn't work that way. The load comes from both directions and doesn't create the sort of shadow effect you've illustrated. Basically all that matters in this particular application is the area in each section through the tube as you go up the tube so as long as the holes are on the same pitch in both cases there's no difference in stress level.

 

[Rich_j]

..are we assuming completely vertical loads? ..
would the stanchion tubes have any chance of distorting?
.. aligned on opposing sides?

the load is stricty vertical .
The stanchion tubes surely flex , but the holes are in the damper tube , bolted in the slider and can't be affected by a distorted stanchion .
(Look at Rich J CAD drawing )
Why opposing sides ? a single line of holes will do .

I think the more existential question might be "does it need more than one set of holes?" Doesn't the stock one just have two?

You are trying to get free flow of oil in and out of the damper tube without restriction which isn't the case with the standard set up, 2 holes could be used but they would have to be bigger and could weaken the tube too much.

 

[Rich_j]

..The load comes from both directions and doesn't create the sort of shadow effect you've illustrated..

It shure does ! , but its effect depends largely on the kind of material : strength , elasticity ..
The comparison with a brick wall isn't that far fetched ..
My drawing is of course an oversimplification .

I suggest you make 2 such tubes and put them in a press .

OK, but its a virtual press. Same load applied to two tubes, not a whole lot of difference. An old fashioned simple hand calculation would give the same stress levels for both arrangements, FEA shows a lot more detail but stresses are very similar.

 

[swooshdave]

Pretty sure this is a little more accurate than the sketches. :wink:

 

[Cheesy]

Pretty sure this is a little more accurate than the sketches. :wink:

Until you realise how dangerous a little bit of knowledge is with FEA..... The CAD integrated FEA systems can cause a lot of problems, mainly because they are so easy to use and the people using them dont necessarily know what they are doing (Im not saying that this is the case here though)

 

[Rich_j]

Until you realise how dangerous a little bit of knowledge is with FEA..... The CAD integrated FEA systems can cause a lot of problems, mainly because they are so easy to use and the people using them dont necessarily know what they are doing (Im not saying that this is the case here though)

You're not wrong there, I wish a few of our CAD jockey's would dabble a little less, it's quite scary.

 

[norbsa48503]

Think about its purpose and how it works. All the real stress is pulling up on the tube not down.
So a 3/8 dia. rod 21 7/8 long has a jam nut on the top bolt with 21 inches hanging down shuttle valve has a nut holding it on the end. Dampener cap has at least a .005 slip fit at this length not really any alignment issues. It is easy to deflect a 3 /8 rod anyway. The valve has somewhat more clearance than this. When the front end is at rest the springs assert themselves and the sliders are down all the way they can go. Depending on what rod length you use.
Now when the fork sliders are compressed by a bump the shuttle valve is slack to the fluid flow and offers little resistance. No real force pushing down. When the springs reassert themselves the shuttle valve goes to work resisting this movement. This is pulling up on the tube but with fluid pressure. The pressure is tiring to pull the rod from the cap bolt and the tube from its base.

 

[Cheesy]

You're not wrong there, I wish a few of our CAD jockey's would dabble a little less, it's quite scary.

I just did a little bit of dabbling myself, as a disclaimer its actually the first time I have used it this tool and I completly guessed dimensions and loads. I managed to produce results that would validate the other hypothesis on here.... my conclusion is that the devil is in the detail of the hole size and vertical spacing. In this particular case as long as the stress is low enough the perpendicular holes are a better solution to minimise the flow restriction from the bottom of the fork travel.

 

[Rich_j]

Think about its purpose and how it works. All the real stress is pulling up on the tube not down.
So a 3/8 dia. rod 21 7/8 long has a jam nut on the top bolt with 21 inches hanging down shuttle valve has a nut holding it on the end. Dampener cap has at least a .005 slip fit at this length not really any alignment issues. It is easy to deflect a 3 /8 rod anyway. The valve has somewhat more clearance than this. When the front end is at rest the springs assert themselves and the sliders are down all the way they can go. Depending on what rod length you use.
Now when the fork sliders are compressed by a bump the shuttle valve is slack to the fluid flow and offers little resistance. No real force pushing down. When the springs reassert themselves the shuttle valve goes to work resisting this movement. This is pulling up on the tube but with fluid pressure. The pressure is tiring to pull the rod from the cap bolt and the tube from its base.

Thread is on cartridge emulators - no damper rod. With this setup there can be no tensile load on the damper tube, its impossible.

With standard forks you can only definitely get a tensile load when the valve hits the cap during topping out but I think you are over estimating the pressures generated damping valve. I doubt very much if the tube can go into tension.

 

[Rich_j]

You're not wrong there, I wish a few of our CAD jockey's would dabble a little less, it's quite scary.

I just did a little bit of dabbling myself, as a disclaimer its actually the first time I have used it this tool and I completly guessed dimensions and loads. I managed to produce results that would validate the other hypothesis on here.... my conclusion is that the devil is in the detail of the hole size and vertical spacing. In this particular case as long as the stress is low enough the perpendicular holes are a better solution to minimise the flow restriction from the bottom of the fork travel.

It's a very simple load case so I don't doubt your results. As you say, depends on size and pitch, as the holes get larger and closer there seems to be a definite advantage to keeping them in line, once they are far enough apart that the holes don't interact then there's nothing in it.
I haven't tried putting anything truly representative in analysis yet in terms of sizes and loads might try that next.

 

[Hobot]

I've done the drill out experiments and found cross drilling weaker than inline row of holes. But in case of damper tube, what big forces does it have to deal with that any sort of hole pattern that don't cut in in half, could bother it?

hobot

 

[Rich_j]

Very nice test (nearly), you're a fast worker.

Would have been a slightly fairer test if you'd used through holes in both pieces and the same total number.
Bit flawed but the the point's conceded.

 

[Rich_j]

..Would have been a slightly fairer test if you'd used through holes in both pieces and the same total number..

I never talked about through holes .
Both tubes have the same number of holes : 4 , and the same diameter .

But I admit I made the holes fairly large to prove my point .

Looks like 6 in one 4 in the other from the photos? maybe just the angle?

 

[swooshdave]

But I'll leave it to swooshdave trying to proove me wrong ..

I'll leave that to someone else.

Two notes though, in your example of the front axle, aren't you dealing with torsional loads? And you test tubes you drilled are larger that the actual examples. If you make the holes large enough you'll prove your case.

Apples/oranges and all that crap.

How do you mount a kill switch on that tube? Which hole would you use? :mrgreen: