# how to fix a roofbox / strength of Sikaflex 221??



## 89827 (May 1, 2005)

I've glued some checker plate on to the roof of my motorhome using 221. I roughened both surfaces with wet and dry paper and it's formed a really strong joint. I've no fears the plate will become airborne at speed on motorways. 

However now I'm about to mount a roof box up there and I was wondering whether I simply glue it in the same way. There's an appreciable area of floor for the adhesive to bond to, but then again the increased weight and wind resistance has made me a little nervous. Anyone here glued a roof box on to their roof?? Is it still up there ... 

If so, did you glue it straight to the aluminium skin, or did you reinforce the roof at all? Again I'm tempted to go straight to the ally - the weight should be adequately dissipated through the floor area after all.

TIA,
-Steve Wilson


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## 91929 (May 1, 2005)

No HymerHobo dont think I would like to trust glue to hold a roof rack on

Dont have the formula re air pressure on a flat surface at a velocity ant more

However I do seem to remember that the force on the surface area increases as the square of the velocity and at Mway speeds becomes quite large

We have a van conversion and have fitted a manufacturers roof rack to carry the Box, Plus awning poles, wind breaks etc

Its not so much the speed of the van but also the added effects of a head wind so 60 mph van +30 mph headwind = 90mph totals air speed


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## 93514 (May 1, 2005)

Very interesting subject (to some).  

You need to apply Newton's 1st, 2nd & 3rd laws of motion to this scenario.

You need to consider the effects of speed, acceleration, mass and velocity on all the variables mentioned.

"Its not so much the speed of the van but also the added effects of a head wind so 60 mph van +30 mph headwind = 90mph totals air speed"

Ever so slightly oversimplified perhaps ?

With regard to the specific problem of keeping the roofbox where it was designed to remain, ie. the roof, you need to achieve the effect known as

*zero net force*

If an object is subjected to zero net force, then it will continue to move at a constant velocity, meaning that it will continue to move in a straight line at a constant speed.

which means you'd probably be better attaching the roofbox to a roofbar setup to supplement the sikaflex application.

Hope this helps  

Kev


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## 91929 (May 1, 2005)

Hi kevarts

I had to simplify the subject Its some 15 years+ since I was teaching Mecanics etc
So I have forgotten most of it - Well lets say it has been put into a remote storage area, from whee it is difficult to access

Also as you will probably agree it is a technical subject and we dont want to go too deep
After all we are not teaching just helping out and dont expect our subjects to answer degree level exams


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## Brambles (May 12, 2005)

When I bought my M'home I discussed with the salesman about attaching a roofbox. He said they glue (to weather proof the joint from water collecting underneath) , bolt and strap it as well. He said he has seen too many M'homes with boxes ripped off by gluing only. He also said bolts alone was no use as again he had seen them ripped out. Apparently some roofs are not actually bonded to the insulation and battens because of the dirt collecting darkening the surface and absorbtion of heat causing expansion. If they are glued like the side walls they delaminate eventually so rely on various fitting like roof windows and vents to hold in place from flapping. So gluing alone is not a good idea. Also the materials used for boxes is not an ideal material for bonding.

So bolts and adhesive sealant is what I was recommended. Also I think he meant the strap was for putting over the closed lid incase the locking mechanism fails. 

Lets face it you don't want the thing flying off on the road and as you cannot see it when driving, seems to make sense to take every precaution. 
Jon.


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## 89827 (May 1, 2005)

Many thanks to all who replied. Not sure what I'll do yet though ...


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## 91929 (May 1, 2005)

Hi Brambles I have seen a roof box & rack come off a car it spun round on the road in front of us scattering clothes across the motorway. Luckily it was a quiet time.

My box is fitted to a roof rack secured through the roof with strengthening plates under the roof and a sealing compound between the fixing brackets & van wall

Fitted my own bike rack to the rear doors, when a deales said it couldnt be done

Put strengthening plates behind the sealer behind the outer fixing bracket which sequezed through the holes and spead under the Strengthening plates as the bolts were tightened

roof box we fitted ourselves - Like the idea of a strap over the top just in case


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