Impact force of the board landing

Just wondering if anyone knows or has thought about what load the board (and your knees) withstand on your standard w2w jump. Of course it varies depending on hitting the transition ect, but lets assume worst scenario and you case it or end up in the flats.

If anyone knows how to work it out that'd be cool.

Rider: ......................200lbs
Board (142cm): ..........55"long, 16" wide (mid), 12" wide (ends), .75" thick

Avg Height of jump:.......6ft
Avg distance accross wake: 15ft(?)
Boat speed:.................24mph

Whats this measured in? Ft/lbs per inch? I really dont know.

Just curious what the breaking point of a board is mathmatically.

Well the pressure distributed over the bottom of the board would be in psi but it's not going to be an evenly distributed load. If you're looking for the force on your knees it would just be in pounds. Why do you want to know? this isn't exactly an easy problem because there are a lot of things you have to take into consideration.

ok...more curious about the board, so psi.

Im just curious what the load capacity of a board is and at what stress level they break at...Im curious about what the materials have to hold up against.

I guess its more of an engineering question.

The thing about it is that its very hard to simulate in a lab setting, slow systematic pressure would break at a different time than what a real wakeboard landing would. Also, each board uses different density foam, different glass, different pressure in the layup no two are identical.

Got everything to do with how fast the deceleration is. You can calculate pretty easily the downward velocity a rider has at impact from their COG height. It's then a matter of how quickly their COG stops moving from the time the board hit's the water and starts producing an upward force. This is why stiff legging is bad and results in broken bones or torn up knees, you need to absorb the shock. Also why landing in the flats sucks so much, you get instant full impact instead of that partial on the downslope of the wake over a period of time.

Longer time for velocity loss is less impact force. F=ma right?? So big air to flat is instant schplako and big deceleration which is more impact force etc. Nice that boards are made with those center spines to help out with a cushion, or a little bit longer time over which you lose velocity.

Then you have casing the back of the wake. My favorite thing for sure. This one is quite awesome because you actually take some or your lateral speed and turn it into impact force.

So what have we learned?? Go little, land on the downslope of the wake, and rock the Tindy always.

(Message edited by Flux on July 17, 2008)

I could strap on an accelerometer from work and figure it out rather quickly. They are kind of expensive to have out on the water though. ;)

I think it was lyle who had a post like this a while ago. He was testing different materials for a wakeboard and was testing to see what loads they could support. Try e-mailing him

I doubt the board companies know that. I'd bet they put prototypes on a bench and press them til they break. They probably know from past experience where a marketable board needs to be at to not end up with a huge return ratio.

I've seen both extremes, my son has never broken a board in 15 years of riding (he broke his femur though).

I watched Jason Buffalow break 2 boards in one session!

Will the rider be landing flat or on edge?

I like the input so far. I bet the board companies probably dont know either and just go by trial and error.

I think if we put some constants/assumptions in place you could figure it out.

Lets say the landing surface is half the board and not on edge to make it "easier" as we ussually land tail heavy.

Would you have to account for lateral direction as well as boat speed? How fast is the ussual cut accross the wake? 10mph?

What other constants would make this equation workable?

lyle @ Chrome Dome Industries has done a lot of testing on breaking points of his boards. I would bet he has some idea of what production model will do under similar tests.

I think you're landing area is more likely the entire base of the board. Spines and channels DO slow the board down and skews things a bit, but we're not working on Nobel here, and the entire board could contact the water on the hard landings. maybe isolate the flat spot?

Tindy is rad. Go soft!

As Flux said...

What you're interested in is vertical speed and the rate of deceleration from your descent speed to a stop. Boat speed/cutting speed doesn't matter.

If you can time how long it takes you to change your vert speed from max to zero then you can use this to do your calculations. And without an accelerometer or some other accurate way of measuring it, you not going to do it.

You really can not accurately calculate it without instrumentation. There are way too many variables to account for by making assumptions.

200lb rider

Dumb down the board area so its basically one long rectangle and 4 triangles (on the edges) and you get an area of:
6*55 = 330 in^2 long rectangle
2*(27.5*2)= 110 in^2 triangles made into two squares on each side
Add em up you get 440 in^2

go halfzies for this case and its 220in^2

so:
200lbs/220in^2 = 0.909090909 PSI

Granted this is very cut and dry to the point where you aren't considering the kinetic interaction with the water, the bouyancy force of the water "pushing back" what angle you land at etc.

If you wanna get crazy with it:

200lb=90.718474kg
F=ma 9.81*90.718474kg = 889.94823Newtons*1.8288 meters = 1627.53732 joules of energy from that height.

Also gotta consider the bouyancy force of the water you are riding on so you need the density.
Water about 80Degrees warm (water right at the surface maybe) has a density of .9718g/cm^3

You need to know the amount of water displaced by the board right when you land, at that exact instant, in order to figure out the bouyancy force which would be added to the forces exerted from just you landing on your board. Kind of like your weight and the water squeezing your board.

Some other things:
Speed in the Y(up and down) direction leaving the wake and landing:
32.9396325 ft / s

Time in air: 1.48 secs

Speed moving across the wake parallel to the boat, lets say you take it to the flats if your wake is 15ft wide so lets go to 20ft:
13.4973753 ft / s

This is where it gets gnarly:
LIke it was said earlier, you probably need to figure out how you fast you are slowing down and this would seem pretty easy once you got the kinetic force of the water on your board but you aren't traveling in a constant radius to the boat because you are moving at 24mph and once you land you are essential sliding forward while trying to progress through that radius. Each bit of area that you are moving away from the center of the boat the forces on you and the direction you are going change. I think you would have to have some sore of differential equation modeling for this. Or if someone just strapped on a GPS with a data logger and did exactly what the problem called for, same speeds, height, length etc... so yeah, Get on that everyone


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Hahahaha I want to see this get figured out, but I'm not sure I want to know how much force my knees are taking on hard landings...

If you are talking about the strength of the board, then you are interested in the "stress" it's exposed to which is measured in psi, but won't be uniformly distributed as proho mentioned above.

I can guarantee you, there is no way to calculate it theoretically and be accurate. Maybe you could bond strain gauges to the board and take readings while someone is riding. Probably have different riders try it multiple times and capture peak values in different places of the board. It would be a lot of data to sort through, but not too difficult to set up.

It's not nearly s cut and dry as i even appears, and it doesn't appear cut and dry at all. Boat speed does matter, as it affect how deep the board will sink on impact, and spines, although cushioning the landings, can add to the stress on your legs due to the fact that they allow the board to punch a deeper hole. Flat bottomed boards, although they land abruptly, don't strain your legs as much because they land and stay higher in the water. That's the theory behind Ronix's line. Ever ride a Parks? 3DS? both have pronounced spines, neither land soft.

A note on the boat speed issue... Raley landings can bruise your heels, but rarely put a huge strain on my knees because the speed at which you land keeps your board higher in the water. However, boards break easier on abrupt landings due to the shock, but knees break easier on landings where the board "cushions" into the water

cont'd....
due to the drag

The strain guages would probably be the best way to determine the stresses upon landing. I know very little of kinesiology so I don't know how to find the stresses on your knees but I'm sure it would be easier than figuring out the stresses on the board.

Again, you have to look at why you want to know this. If you just want to know relative strengths of boards than you can just put them in a hydraulic press and see when they break.