DIY/cheap tunnel stiffener

steepski

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Nov 20, 2018
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(not sure if this is the right forum, feel free to move it or tell me where to move it).

I've got 08 arctic cat that I'm mostly using for mountain sledding and sled-skiing, but occasionally want to be able to put a bit of cargo on the back and tow.

I'm thinking of making a super simple/cheap tunnel stiffener for my sled, that will also double as a minimalist rack, and a mounting point for a larger rack and ski rack. Its similar to a simplified version of the one in this post (and the red powdercoated one further down) Here's the plan:

Get a two sections of 1/8 in x 3 in aluminium bar (like this), that are as long as the tunnel from the end of the running board to the back of the tunnel. bolt these bars onto the tunnel along the entire length, and so they "stick up" about 1in above the tunnel behind the seat. I'll drill some holes for bungees and mounting ski racks onto this bar. Then, for bigger cargo I can bolt another piece of 3 in aluminium bar to this, to create a larger cargo rack.


Here's a photo that illustrates what I'm talking about. The red outline is the 1/8 in x 3 in aluminium bar that serves as the tunnel stiffener and minimalist rack. The blue outline is the larger cargo rack that is bolted on so it's removable. The long tunnel stiffener (red outline) could have some big holes drilled in it to lighten it up a bit, but it'll only add a few pounds or so to the sled at the most.

Any thoughts on if/how this would work? Will drilling holes in the tunnel to mount/bolt this on weaken the tunnel more than this helps? Would that piece of aluminium be strong enough to strengthen the tunnel?

Thanks!
 

jim

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Nov 26, 2007
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In short, no, you won't strengthen the tunnel because the Aluminum you would use will be much softer and more flexible than the actual tunnel...so, as the tunnel flexes a certain amount, that strip will do almost nothing to add strength.

And, yes, you can weaken an tunnel by adding too many holes in the wrong area(s). But, you would have to drill holes in critical stress points. Anywhere there is already hardware or brackets riveted into the tunnel, avoid. And use rivets made from passivated steel (don't use aluminum rivets unless you enjoy seeing them come lose). Passivated won't corrode.

But the good news is, that tunnel is already nice and strong. I had the same tunnel on a M1000 and did full-on cornice drops will full 2.5 gallon gas cans (20 lbs about). No issues after 4000 miles of hard backcountry riding. When installing, check for clearance from track (don't use too long of rivets).

And one last bit of advice, dull the edges and round the corners of the aluminum that's exposed to the outside where your legs will rub...you don't want that cutting your gear open. And the bracket on the back of the tunnel...round the corners so that if you fall back onto the bracket (or your passenger more likely), it is just a bad bruise and not a laceration.

Have fun!
 

steepski

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Nov 20, 2018
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Thanks for the advice! Good point about the 6061 not being as stiff as the tunnel. Sled tunnels must be 7075 aluminium?
 

jim

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Nov 26, 2007
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Thanks for the advice! Good point about the 6061 not being as stiff as the tunnel. Sled tunnels must be 7075 aluminium?
The sled tunnel will be more about geometry based stiffness, in combination with cold working/bending, than the material selection. You are definitely on-track with using a stiff aluminum like 6 or 7 series. But, again, I don't think it will match the stiffness of the tunnel being a single strip.

I honestly don't think you need the long strip along the length unless you are using that to mount ski carrying hardware. You could pretty easily just mount the rear brackets to the back of the tunnel.