Tower tow point extension project...

[trayson]

Okay, so some of you might have seen the overhead board racks that I made. In my first version of these, I mounted the racks mostly forward of the tower clamps. This was great for rope clearance, but it caused way too much flex on the aluminum flat bar and I didn't like all that constant flex on my boards up there... (especially the foil board).

Version 1: racks too far forward


So I moved my racks so they were centered over the tower clamps. this was great for load distribution of the boards on the rack, but then with the racks more rearward, it caused some interference with the wakeboard line when I was cutting all the way out. it would rub on the board in the rack, especially with thicker surf boards up there.




So I came up with the idea of adding an extension to my tower tow point. I would have a piece of solid aluminum machined so that it had a hole bored in the bottom that would sleeve the existing tow point's shaft (with the nylon bushing and cap removed). then I would have the top of the extension machined to have a similar shaft, so I could re-use the nylon bushing and cap on top of the extension, with a longer 3/8" bolt that would have 1" thread insertion.

Here is a close up of the existing tow point with the countersink socket cap screw backed out...

[trayson]

Here is a picture of the existing tow point with the cap and nylon bushing removed:


And here is the nylon bushing:


And the cap that's drilled for the countersink socket head bolt.


And here is my drawing of what the extension will look like. The blue is solid aluminum. the yellow is a hole drilled through the center for the 3/8" bolt that will be about 4.5" long with at least 1" thread insertion. (although I found out that anything over 7 threads adds minimal extra strength.).

Basically, I'll take off the nylon bushing and cap, slide the extension on top of the current tow point, then replace the nylon bushing/cap on top of the extension and bolt it all together with the longer screw.


I found a friend that has a couple metalworking lathes at his work and is going to machine this for me for $40, which is obviously a killer deal. I had another engineer friend to the math/physics and used a lot of fancy words like PSI, insertion depth, material thickness, yield strength, lever arm, and tensile strength...

Ultimately he came to the conclusion that while we were about doubling the tow point height from 3.5" to 7" roughly, that the extension would not be the weak point. If anything, the line would get pulled out of my hands, the line would snap, or the welds on the OEM tow point would fail before the extension would fail. So that made be feel better.

I used my digital calipers to again give measurements to my friend (measure twice) on the height and width of the hole he'll be boring into the extension. So that's where I'm at right now. I'll keep you all updated as things progress.

[Stazi]

I made something like that for each of my front forks on my motorbike to lift the front of the bike. They refer to them as fork extensions. The key to success is that you need to really control the tolerance of the ID of the tube where it is going to slip over the old spindle so that it's slightly (10-20 microns) larger, so that it's just larger than a line fit. Make sure you measure the existing OD of the spindle a few places and get a good record of what it is then have your buddy bore the sleeve 10 microns over and test the fit. If it's too tight to get on then have him throw it back on the lathe and take another 5 microns off., etc. also make sure you have sufficient thread in the existing post so that when you bolt the extension on you can snug it down good. Any wiggle or excessive slop on this design will cause you problems.


Sent from my iPhone using Tapatalk

[ian ashton]

< que dustys mom jokes >

[mmandley]

My concern is also with strength. As you lengthen the tow point you are dramatically increasing the pull force on the weld.

You could literally crack the weld on your tower.

I know people pull tubes off that point (they really shouldn't)

You might consider this in your plan

[trayson]

My concern is also with strength. As you lengthen the tow point you are dramatically increasing the pull force on the weld.

You could literally crack the weld on your tower.

I know people pull tubes off that point (they really shouldn't)

You might consider this in your plan

I had another engineer friend to the math/physics and used a lot of fancy words like PSI, insertion depth, material thickness, yield strength, lever arm, and tensile strength...

Ultimately he came to the conclusion that while we were about doubling the tow point height from 3.5" to 7" roughly, that the extension would not be the weak point. If anything, the line would get pulled out of my hands, the line would snap, or the welds on the OEM tow point would fail before the extension would fail. So that made be feel better.

I did consider it. See above.

And I talked to my engineer friend who actually computed the amount of force that would be required to bend the metal, the tensile strength, the yield strength... He compared that with the height of the lever arm OEM vs. the height of the lever arm with the new tow point extension. He examined the cross-sectional area of a close up pic of my welds, took into consideration the diameter of my tubing, the direction of my tubing, the cross bracing of my tubing... We talked about speeds that we'd be pulling a wakeboarder at, the line materials and strength that we'd use...

I think we covered the considerations pretty thorough. If there's something we missed, I'd be happy to have him take that into consideration in his analysis...

Also, I never pull tubes from the tower and always warn others against it. I don't pull skiers from the tower either. So a wakeboard at 21 to 23mph is it. And we even talked about towing doubles and the only time I've ever done that we put the other rider on the lower sun deck tow point.

[moombadaze]

why are you only going up inch's when you can go up feet


https://casanovasadventures.com/cata...m#.WTch0I2YaUk

[trayson]

why are you only going up inch's when you can go up feet


https://casanovasadventures.com/cata...m#.WTch0I2YaUk

I'm fine with 7" (that's what she said!) and don't need to overcompensate with 3 feet! LOLOLOL

[trayson]

so my buddy used some 7075 aluminum and made me this:




(pics were taken by him. I don't own a giant scooby doo)

I had him carve that little channel in it in case I wanted to attach some LED's as an anchor light. I don't think I'll end up doing that, but it doeesn't hurt anything to have the channel there.

Here's what it looks like installed. I just have to go pick up the longer bolt tomorrow and then I'm done.

It slides on with a near perfect fit. We estimate the clearance is 0.001 inch between the OEM shaft and the new one that fits over it.

[zabooda]

Great job! Over time, you would expect galvanic action between the steel and aluminum so this installation could eventually be permanent. The moment arm has now doubled so the maximum force needs to be half of what it would have been before. One more, the nav light should be above the tow point to provide 360 degrees of coverage. Good to have friends like that. Are they some of your riders?