Custom Rack Mounting on Fiberglass Cap?

[scottg]

I have a jeraco cap on my explorer, and I'm wanting to make a custom rack that holds a spare, and some lights. I already have a prefab "light holder" for a jeep bumper, and its the perfect shape for an endlink on my rack, but it would require mounting it on the very top section of the cap, where the fiberglass is thicker (compared to the very edge of the sides)


So my question is, it is already to have my rack mount up on that, or do i need the mounts closer to the top edge?

thats the light bar, i planned to have 2 of those, and 2 tubes connecting them, with a spare tire holder in the middle, It only needs to support a 33 and 4 lights.

heres a horrible MSpaint sketch of the shape

so, any thoughts?


Thanks

ScottG

 

[adrenaline503]

Should work, I was wondering about doing a similiar mod if I get a fiberglass shell. I would attach it using some large fender washers to help spread the forces.

 

[pete.wilson]

Hey

Which brand is that light bar?

You might want to use a rubber type gasket under the feet on the outside and seal with silicone and a reinforcing plate/ or large fender type washers on the inside of the shell to help cut down on vibration to the mounting holes when you have the lights installed. This will also help prevent damage to the top should you hit low branches out on the trail, otherwise the bolts could just pull through the fiberglass. Just a suggestion.

 

[scottg]

Hey

Which brand is that light bar?

http://cgi.ebay.com/ebaymotors/JEEP...280178292848QQrdZ1QQsspagenameZWDVWQQtcZphoto

thats it, I bought it over a year ago, and its been sitting in my garage

 

[ntsqd]

Composites do not like point loads. They want the loads spread out over a large area. So the spreader plates & fender washers are a step in the right direction, but I would suggest those combined with more attachment points. Say 6 or even 8 "landing feet" instead of four. By the same token you can then use 8mm or even 6mm bolts instead of 10mm or 12mm bolts.
More & smaller fasteners is the way to approach this.

As to where, I would be looking to put the loads at the corners - even if the bolt holes are not directly in the corners. The reasoning is that if you put the load in the middle of a flat span that span now becomes a spring. Putting the load in at the corner removes that possibility.
However, you don't really want holes in those corners as that creates stress risers in an already more stressed area. So make the "landing feet" such that they extend out onto the flat area(s) a short distance and put the bolts there.

 

[Martinjmpr]

It looks like you're wanting to mount the weight bearing portion of the rack near the middle of the shell. I think that's a big no-no. The fiberglass will flex and weaken.

IMO the only way to mount a load to a fiberglass shell is to have rails put on the side near the top. That way the weight is spread all along the side portion of the shell. Also, I would strictly adhere to the 150 pound load limit, or even less if possible. Remember that fiberglass is not steel.

This is how mine is done (it was like this when I got the shell, but I've owned another truck with a fiberglass shell and they did it the same way, leading me to believe it's the industry standard.)


Mounting Gutters:

Rack Mounted:

 

[casfz1]

Scott

I would call the manufacture of the shell and ask them. I know Zack called Leer before they mounted the roof rack on the shell I got from Grant and they told him it would be fine mounting it the way they did.

 

[Nullifier]

Well actually mounting it on top of the foam core allows you alot more freedom. The core material changes the way that stress is distributed throught the laminate. If you were mounting through a solid part of the topper then I would be concerend about it. However the foam core section disperses the load energy over a larger area there for reducing the stress at any given point. A properly laminated composite can support a heavy load no problem even on a vertical plane. the lay out you have should have no issue supporting that wieght. I would only be concerned if you were putting over 250lbs up there.

I would use some custom aluminum "washers" inside the topper. If you rack will have 2 bolts close together at ech "tower" then I would use an aluminum flat stock and cut and drill it to match the bolt pattern. This will help protect the core material from the thin cover layer on the inside of the topper from excess compression. You should not have that issue from the outside since the bulk of the material is toward the outside.

Oh and finally if you are running 4 clamps mounting the topper tot he truck you might consider adding 1 more pair to accout for the heavy load up top.

 

[BCJC]

i had a local fab shop make me a cargo basket, which i mounted to my ARE fibreglass canopy. it has 6 feet, with 4 bolts per, and all feet are mounted to the "edges and corners" of the roof of the canopy. the rack was overbuilt, and weighs more than i would like (around 100 lbs) but i guess i wasn't specific enough. anyways, i've had the rack, 33x10.50 spare, and a set of lights up there for almost a year, and so far so good. i've been on 500 km trips on rutted old logging roads with a 67 lbs canoe up there, and no worries. the bolts have large fender washers inside, and instead of a gasket, i used a tube of sycaflex (sp?) and zero leaks. hope this helps.

 

[Northern Explorer]

I just ordered a Leer 100XL for my truck. I didn't order a rack for it because I already have the mounts and crossbars for over the cab when I need to carry my kayaks. Here is a picture from the Leer website showing how they do the mounting.

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