Bolting through or bonding to a fiberglass top?

[boardrider247]

I'd like to hear some opinions, discussion, pro/con regarding the best way to attach a roof rack to a fiberglass top.
I would like to build a low profile roof rack/platform which could hold let's say 400# while parked and 200# while in motion.
Please discuss the best way to attach this rack to my fiberglass top.
Through bolts or Adhesive? If adhesive then which one and why? If bolts why and how to prevent the fiberglass from cracking?

Thanks in advance for your input

 

[spencyg]

I'm going thru this exercise right now with a new roof rack on my ancient fiberglass topper. If you have the option (meaning, clear and open access to the underside of the fiberglass top) than you might consider an exterior and interior aluminum plate sandwiching the fiberglass at each of your mounting points. I don't have access to the under side of my topper due to a full interior, so I've gone the route of bonding aluminum rails to the top with 3M 5200 elastomer sealant/adhesive. I have significantly more surface area making contact with the fiberglass than I likely need, but since the bond of an adhesive joint is all about Pounds of force per square inch, I opted to maximize my square inches.



The rack will attach to the tabs, and only 5200 is used to bond the black rail to the topper.

I wouldn't just thru-bolt with a fender washer providing support on the inside of the fiberglass, it it would likely lead to stress fractures before long. Spread out your load on to as much fiberglass as possible, and try and keep a majority of the load near the vertical walls instead of in the middle of more flexible horizontal surfaces.

Good luck.

SG

 

[luthj]

Use backing plates if you bolt through. They need to be sufficient thickness to spread the load evenly.

Bonding is an option if you have good quality gel coat or paint to work with.

Sikaflex 252 (or any 200 series) is very strong, and used to structurally bond the bodies of trailers, Semi-cabs and similar composite body structures. See the attached data sheets and primer guide. If you use sikaflex, you must follow the primer and activator instructions. They will ensure maximum bond strength. As little as 100 square inches can be enough bond area to support several hundred pounds.

It is also important to get the right adhesive thickness. Using spacers or foam tape to ensure it is correct is also desirable. Of course if you are able, doubling the bond are is a option as well.

 

[Corneilius]

I through bolted because I could easily get to the backside. One thing to consider which i didn't learn until after I drilled is that my top has a honeycomb internal reenforcement on the horizontal roof. The vertical sections are just a few layers of fiberglass. Took a few trips to Fastenall to get the right length bolts. I made sure to use several large "feet" with matching plates inside.

 

[calicamper]

If a glass structure has a core in it. Drill a larger hole than needed. Put epoxy in it. Let it cure then redrill the hole to the right size. This keeps the core material sealed and protects it against water intrusion if your sealer fails.

Probably the largest issue is just structural strength of the glass shell vs the loads being added. If sizable weight is being added, its no a bad idea to consider a simple foam/glassed beam under the load area. Its added work but in cases where your building up an old rig with some $ invested, adding a simple foam/glass beam to avoid roof failure may be worth it.

 

[Corneilius]

If a glass structure has a core in it. Drill a larger hole than needed. Put epoxy in it. Let it cure then redrill the hole to the right size. This keeps the core material sealed and protects it against water intrusion if your sealer fails..

Sweet Pro-tip. Wish I had thought of that. Ill keep an eye for stress cracks/ upgrade if I every pull the rack off.

 

[boardrider247]

Thanks for the input. Keep it coming.
I don't have any sort of solid plan yet so I'd like to hear any and all thoughts.
Even if they are crazy :sombrero:

 

[luthj]

If a glass structure has a core in it. Drill a larger hole than needed. Put epoxy in it. Let it cure then redrill the hole to the right size. This keeps the core material sealed and protects it against water intrusion if your sealer fails.

Probably the largest issue is just structural strength of the glass shell vs the loads being added. If sizable weight is being added, its no a bad idea to consider a simple foam/glassed beam under the load area. Its added work but in cases where your building up an old rig with some $ invested, adding a simple foam/glass beam to avoid roof failure may be worth it.

A modification to this approach is to bond a metal bushing of the correct length into the hole. This will not only protect against water intrusion, but it will provide crush and tear out protection. This is the approach I use on many aircraft parts.

 

[derjack]

I wouldn't Drill through the Fiberglas also. It's just too weak!
My roof is more than 20 years old now, hasn't seen any off-road yet, just Norway streets. The roof shows cracks at the a pillar and windshield already.

The sika idea is good. Probably glue a plate on the roof with where you can bold onto.

I personally wouldn't even go that road. I would build a support to the rain rails.

 

[boardrider247]

I would build a support to the rain rails.

This^^^ is another option I am exploring.
Again I'm open to opinions and have no firm plans

 

[simple]

If the rack has sufficient bonding surface, clean fiberglass with acetone, bond with 5200 and thru bolt with 1/4" fasteners. should be bomb proof. Don't over tighten. Fasteners only keep bonded surface from peeling, 5200 should handle all sheer strength and weather sealing.

 

[Bbasso]

I bolted through, large backing plates and filled the holes with grey rtv. Over a year ago and still good.