Automotive Kayak Loader Design Help Needed: Spring Placement/Weight

[broncoracer104]

hello, new here but need a hand figuring out a problem that is above my knowledge, i am currently building a kayak loader for myself like the one pictured below, i am trying to figure out where to place the spring and how heavy of a spring i need to help assist the kayak onto my roof, i am sure there is a formula out their to figure it out i just am not sure what it is, i have attached a picture of a rough idea as to what i am trying to do as well as a picture of a actual operational one. if you need any specific dimensions,weights,angles etc. let me know! i hope someone will be able to help, Thanks Josh

 

[berkeman]

hello, new here but need a hand figuring out a problem that is above my knowledge, i am currently building a kayak loader for myself like the one pictured below, i am trying to figure out where to place the spring and how heavy of a spring i need to help assist the kayak onto my roof, i am sure there is a formula out their to figure it out i just am not sure what it is, i have attached a picture of a rough idea as to what i am trying to do as well as a picture of a actual operational one. if you need any specific dimensions,weights,angles etc. let me know! i hope someone will be able to help, Thanks JoshView attachment 112306 View attachment 112307 View attachment 112308

Welcome to the PF.

Sounds like a fun project. So your pics are of a surfboard, not a kayak, right? How much does a kayak weigh? Do you have any videos of loading a kayak?

When you say that you want spring assist, does that mean you pull the vertical rod down to load up the spring tension, and maybe lock it in place in the low position while you attach the kayak?

 

[broncoracer104]

Here is a video of it in action, my kayak is a bit heavier 75lbs maybe

 

[berkeman]

Here is a video of it in action, my kayak is a bit heavier 75lbs maybe

Thanks, that helps. But where do you want to have a spring to assist? Are you going to weld yours up yourself?

 

[broncoracer104]

Yeah the spring is there to help pull it towards the vehicle when you push, i already have mine all welded up just can't figure the spring part out.

 

[berkeman]

Yeah the spring is there to help pull it towards the vehicle when you push, i already have mine all welded up just can't figure the spring part out.

So do you pull the arm down against the spring force and latch it at 45 degrees or something? Then how do you release the latch after you set the kayak on the arm?

As a first approximation, you would size the spring force to be the most you can comfortably pull down with your body weight into the latched position...

 

[broncoracer104]

No you don't lock it, it almost goes past the point of it being able to pull up almost as if the weight of it holds it down then when you push it up the spring helps pull some of the weight. Its greater than a 45°

 

[berkeman]

No you don't lock it, it almost goes past the point of it being able to pull up almost as if the weight of it holds it down then when you push it up the spring helps pull some of the weight. Its greater than a 45°

Could you maybe sketch the steps of putting the kayak on the arm and lifting it up?

If you don't pull the arm down first and lock it before loading the kayak, how does having a spring on the arm help?

 

[broncoracer104]

I will have to sketch it maybe. It helps pull the arm and kayak towards the vehicle, without it pulling up your basically trying to push 100lbs up a hill

 

[berkeman]

I will have to sketch it maybe. It helps pull the arm and kayak towards the vehicle, without it pulling up your basically trying to push 100lbs up a hill

So the arm starts out vertical. Then you lift up the bow of the kayak up to the full height of the arm and set the bow on it. I suppose that would cause the arm to sag down until the spring is extended some, but you don't get much assist in lifting up the kayak then. It would help a lot more to not have to lift the bow all the way up to the full vertical height of the helper bar, no?

It just seems like if you want to make the lift easier and get help from the spring, you would first pull down the arm against the spring force using your body weight and lock it at a low position, maybe 45 degrees. Then your lift of the bow is lower, and the spring helps you lift the kayak back up with a fair amount of force.

 

[broncoracer104]

Here is a great video

 

[broncoracer104]

 

[berkeman]

So it looks like the spring is only a weak spring to help re-position the arm when using it to take the kayak down, -- it has nothing to do with lifting the kayak up.

It also appears that they use a friction lock on the arm when it is pulled down to 45 degrees for loading. Pushing the kayak after the bow is placed on the arm releases the friction lock, and let's you use your body power to finish the lift and push to load the kayak. Does that sound right?

 

[broncoracer104]

Makes sense, not sure how a friction lock works though.

 

[berkeman]

Makes sense, not sure how a friction lock works though.

I can think of several ways to do it. I did a Google Images search on Friction Lock Pivot, and got lots of good hits:

https://www.google.com/search?site=....0.XjTA9DHdtsM#tbm=isch&q=friction+lock+pivot

Including this pivot that looks like it might be similar to your pivot on your arm...

http://www.hellotrade.com/fath-group/pivot-joint-with-adjustable-friction-die-cast-aluminum.html

..

 

[berkeman]

The idea with a friction lock near the end of the travel is just that some rubber/metal or rubber/rubber interface starts to engage there...

 

[berkeman]

Actually, maybe it's not a friction lock, maybe that's just the end of its rotational travel with a hard stop.

 

[broncoracer104]

Ah okay now that you explained it i get the idea