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Where to shim?

Posted: Tue Nov 05, 2019 11:45 am
by matanshapira
Where should I shim if I want to reduce front foot pressure without changing the stability of the foil?

1. Fuselage to mast connection
2. Mast to board connection
3. Rear wing to fuselage connection (stating it just for the record as it will affect stability)

Re: Where to shim?

Posted: Tue Nov 05, 2019 1:28 pm
by Horst Sergio
Hi matanshapira,
tried all your shims, but there is not enough information to give a 100% correct answer as the very often used wording "to much front foot pressure" is not a precise diagnosis, but more an impression, but most likely it is not connected to shimming (1.-2.) but to relative foot placement to foil placement or in the beginning when not yet being used to normal board speeds, so riding to slow.

a) So if the real diagnosis is that you just have more body weight on the front foot as the pressure point of the foil is closer to it, it is easy to see, that you just have to move foot position towards front or foil connection towards back, which is the same.
b)/"1." If you can't do that, another solution would be to pitch up the whole foil at the fuse to strut connection even if this is the most sensible place for static reasons, so you should be sure to do it well, 1,5 - 2° as step should mostly be enough and well to feel.
c)/"3." But the solution I normaly would prefer is to decrease the negative pitch of the stabi and therefore also its drag, but as you said this can also decrease foils pitch stability, but as we know from monofoiling this is more an idea in our heads ... :wink:

And as said there are also bad trimm situations feeling like front foot pressure, but in fact are more connected to bad board angle to foil "2." (when one leg is always bend), which then could more likely solved by pitching between board and foil. But on the end as seen, after the ideal solution, there is always more then one way especially as 80% of the foils I test aren't setuped and balanced well anyway. But it is also about riders weight and your prefered riding speed, which can also be setuped by foil trimming.

Re: Where to shim?

Posted: Tue Nov 05, 2019 3:18 pm
by Peter_Frank
1 And 2 :thumb:

Re: Where to shim?

Posted: Tue Nov 05, 2019 3:20 pm
by Peter_Frank
1 is the only thing that works, but if board is level now, you need 2 also to avoid the board pointing down.

Re: Where to shim?

Posted: Tue Nov 05, 2019 6:13 pm
by grigorib
matanshapira wrote:
Tue Nov 05, 2019 11:45 am
Where should I shim if I want to reduce front foot pressure without changing the stability of the foil?

1. Fuselage to mast connection
2. Mast to board connection
3. Rear wing to fuselage connection (stating it just for the record as it will affect stability)
I don't think it''s possible to shim without affecting stability.

No #1 and #2 make no sense nor difference
Go for #3 pointing stabilizer few degrees up and that will have the effect. That will reduce stability but way less compared to improved stance.

Re: Where to shim?

Posted: Tue Nov 05, 2019 8:18 pm
by Peter_Frank
grigorib wrote:
Tue Nov 05, 2019 6:13 pm
matanshapira wrote:
Tue Nov 05, 2019 11:45 am
Where should I shim if I want to reduce front foot pressure without changing the stability of the foil?

1. Fuselage to mast connection
2. Mast to board connection
3. Rear wing to fuselage connection (stating it just for the record as it will affect stability)
I don't think it''s possible to shim without affecting stability.

No #1 and #2 make no sense nor difference
Go for #3 pointing stabilizer few degrees up and that will have the effect. That will reduce stability but way less compared to improved stance.

It IS possible, #1 and 2 has been proven, in earlier threads and videos.

I made a scetch showing why it works, and without altering stability as the wing aoa's are not altered :thumb:

8) Peter
ShimPrinciple.jpg

Re: Where to shim?

Posted: Tue Nov 05, 2019 9:01 pm
by matanshapira
Horst Sergio wrote:
Tue Nov 05, 2019 1:28 pm
Hi matanshapira,
tried all your shims, but there is not enough information to give a 100% correct answer as the very often used wording "to much front foot pressure" is not a precise diagnosis, but more an impression, but most likely it is not connected to shimming (1.-2.) but to relative foot placement to foil placement or in the beginning when not yet being used to normal board speeds, so riding to slow.

a) So if the real diagnosis is that you just have more body weight on the front foot as the pressure point of the foil is closer to it, it is easy to see, that you just have to move foot position towards front or foil connection towards back, which is the same.
b)/"1." If you can't do that, another solution would be to pitch up the whole foil at the fuse to strut connection even if this is the most sensible place for static reasons, so you should be sure to do it well, 1,5 - 2° as step should mostly be enough and well to feel.
c)/"3." But the solution I normaly would prefer is to decrease the negative pitch of the stabi and therefore also its drag, but as you said this can also decrease foils pitch stability, but as we know from monofoiling this is more an idea in our heads ... :wink:

And as said there are also bad trimm situations feeling like front foot pressure, but in fact are more connected to bad board angle to foil "2." (when one leg is always bend), which then could more likely solved by pitching between board and foil. But on the end as seen, after the ideal solution, there is always more then one way especially as 80% of the foils I test aren't setuped and balanced well anyway. But it is also about riders weight and your prefered riding speed, which can also be setuped by foil trimming.
Image

I have LF Impulse(1100sqm) with Kanaha Shapes 37" (no tracks) and I'm a pretty competent foiler. When I use a very narrow stance as the above image shows then I get equal pressure on both feet.
"pitch up the whole foil at the fuse to strut connection" - wouldn't it make the board point downwards so it won't be parallel to the foil? would it affect takeoff and touch downs? kill the rocker?
stabi shimming - I'll probably do that because once my stabilizer broke and lost 50% of it's size and it was very fun, but I still want to have the option for high stability.
"more connected to bad board angle to foil" - how can this ever happen? the fuselage is perpendicular to the struct which is perpendicular to the board. What am I missing?
In the Kanaha Shapes the rocker starts right where the foil is mounted, so the front foot will be higher than the back foot by design for optimal touch downs on such a small board.

Thanks for your help.

Re: Where to shim?

Posted: Tue Nov 05, 2019 9:07 pm
by matanshapira
Peter_Frank wrote:
Tue Nov 05, 2019 8:18 pm
grigorib wrote:
Tue Nov 05, 2019 6:13 pm
matanshapira wrote:
Tue Nov 05, 2019 11:45 am
Where should I shim if I want to reduce front foot pressure without changing the stability of the foil?

1. Fuselage to mast connection
2. Mast to board connection
3. Rear wing to fuselage connection (stating it just for the record as it will affect stability)
I don't think it''s possible to shim without affecting stability.

No #1 and #2 make no sense nor difference
Go for #3 pointing stabilizer few degrees up and that will have the effect. That will reduce stability but way less compared to improved stance.

It IS possible, #1 and 2 has been proven, in earlier threads and videos.

I made a scetch showing why it works, and without altering stability as the wing aoa's are not altered :thumb:

8) Peter
ShimPrinciple.jpg
Lets see if I understand correctly. If we do only #1 or only #2 the board will point downwards, so your suggestion is to combine #1 and #2(shim on board and on strut->fuse) to make a swept back mast and keep the foil parallel to the board. Any downside for this? Stiffness?

Re: Where to shim?

Posted: Tue Nov 05, 2019 9:22 pm
by airsail
This helped me get my head around it. I only shim the rear stab if front foot pressure changes with speed.

Re: Where to shim?

Posted: Tue Nov 05, 2019 9:32 pm
by Peter_Frank
matanshapira wrote:
Tue Nov 05, 2019 9:07 pm
Peter_Frank wrote:
Tue Nov 05, 2019 8:18 pm
grigorib wrote:
Tue Nov 05, 2019 6:13 pm


I don't think it''s possible to shim without affecting stability.

No #1 and #2 make no sense nor difference
Go for #3 pointing stabilizer few degrees up and that will have the effect. That will reduce stability but way less compared to improved stance.

It IS possible, #1 and 2 has been proven, in earlier threads and videos.

I made a scetch showing why it works, and without altering stability as the wing aoa's are not altered :thumb:

8) Peter
ShimPrinciple.jpg
Lets see if I understand correctly. If we do only #1 or only #2 the board will point downwards, so your suggestion is to combine #1 and #2(shim on board and on strut->fuse) to make a swept back mast and keep the foil parallel to the board. Any downside for this? Stiffness?

Correct, and not really any downsides, apart from you need to shim so it does not fit as elegant, as perfect tapered carbon will.
But most likely it doesnt matter, compared to having a trim that is off, or a stab that isnt very stable (if using #3 so you shim it less stable to obtain less front foot pressure)

It is by far the best solution IMO, if possible - but depends on the given hydrofoil.

I can not shim mine at all as an example, but doesnt matter as it fits in trim.

8) Peter