Another roof bars and lifting roof question

[turbodave]

Hi, I have cheap roof bars on my AFT and was going to change the set up completely and put in runners so that I may put my cross bars further back so that my roof box can sit further back,as recommended on the fact sheet here. My question is...this positioning at the back of the roof goes against my engineering instincts, in that an aero dynamic roofbox that follows the contours of the Bongo's front, and applies pressure to the front of the roof would surely be beneficial no?...or have I got this wrong? Cheers Dave

[g8dhe]

The problem surely is about reducing the lifted mass ? If you put the weight at the rear of the roof then you will reduce the mass to be lifted as its close to the pivot point, if you put it at the front then you will be increasing the mass as the lifting mechanism runs from the front towards the back, thus causing the amount of leverage to increase as the roof is raised, there was a case recently where an AFT broke in half due to the mass at the front - in this case snow! http://www.igmaynard.co.uk/bongo/forum/ ... 27#p547427
The Bongo is a brick in aero dynamic terms, I don't think having a shaped box will help a lot!

[turbodave]

The problem surely is about reducing the lifted mass ? If you put the weight at the rear of the roof then you will reduce the mass to be lifted as its close to the pivot point, if you put it at the front then you will be increasing the mass as the lifting mechanism runs from the front towards the back, thus causing the amount of leverage to increase as the roof is raised, there was a case recently where an AFT broke in half due to the mass at the front - in this case snow! http://www.igmaynard.co.uk/bongo/forum/ ... 27#p547427
The Bongo is a brick in aero dynamic terms, I don't think having a shaped box will help a lot!

Sorry, we may be at cross purposes here...I am referring to the potential for the roof to spontaneously lift due to high wind when driving. When the Bongo is stationary I will be taking off the roofbox so it isn't an issue. Unless I have misunderstood you, in which case I will lick your Bongo boots! Dave

[g8dhe]

Does a two tonne vehicle need any more pressure being exerted downwards ? What would be the likely down-force from the box be I wonder ?

[turbodave]

Does a two tonne vehicle need any more pressure being exerted downwards ? What would be the likely down-force from the box be I wonder ?

No, Again, maybe there is a misunderstanding here.....is the AFT 2 tonnes? and is there no reference to the roof lifting when the vehicle is moving? Is it not a valid question to ask if there is a major issue with putting the box at the front? Cheers Dave

[haydn callow]

The roof will not lift of it's own accord.....it will be locked down

[Ian]

As I see it, the problem is that the fixing bolts for the roof rack are quite shallow on the AFT. If you fit a box there will be quite a big gap between the AFT and the box. In a head wind there may be airflow that exerts upward pressure on the box, with the possibility of uplift, and ripping the bolts out of the AFT. Which is why we always recommend fitting the box as far back as possible.

[turbodave]

Cheers chaps, So if I can reduce the gap between the bottom of the box and the top of the AFT, then you think I will have reduced the issue? Dave

[Ian]

Yes, that sounds like a good idea. Some kind of skirt should do the trick.

[turbodave]

Thanks Ian but I refuse to wear a skirt...I would rather lose the AFT!

[New S Wales 2010]

Personally I gave up on my roof-box and now use a small trailer (far more convenient - try getting something out of your roof box in the middle of the night when its raining and the roof is already up!)

Also the AFT occasionally struggles to reach the top anyway. (Have to back up and try again, or even give a small helping push from inside). Adding more weight to the top just seems like tempting fate!

On a related topic has anyone tried replacing the AFT gas struts or sending them away to be refurbished, and if so how much does it cost? I read that gas struts loose 10-15% of their power every year, so after 15 years they are probably down to almost nothing. Are the struts needed to help lift the weight of the roof, or are they just there to damp and smooth the motion ?

[helen&tony]

Hi
Here's the people to contact:
http://www.sgs-engineering.com/gas-stru ... ng-service
I believe it was DBO or Dabs who pointed me in the direction of these folk. I have been chewing over the idea of putting the spare wheel on the back hatch for a couple of years, and getting a bracket made over here...should be cheap, and can get powder coating done!
Cheers
Helen