Custom roof rack - need feedback!

[nateh]

I’m working with a local fabricator to design and build a prinsu-style roof rack as they don’t make one for my vehicle. My fabricator now wants me to consider mounting directly to the extruded aluminum cross bars instead of to the side rails like prinsu racks do. The reason being that the roof isn’t square and therefore there’s a bit of lateral “reaching” by the mounts towards the end and he’s worried about weight on the rack warping them under pressure. The attached photo of the design is a bit exaggerated but you see the point.

For those with a rack like this, what would the negatives or positives of mounting directly to the cross bars instead of dealing with these mounts and instead just having the side rails “floating” attached on the ends of the cross bars? I don’t love the idea of not being able to shift the cross bars where you need them based on what you’re carrying.

 

[genocache]

I can't tell what's going on in your drawing. How much weight can your bars hold? How much weight do you intend on putting on them?

I made a custom rack, fully adjustable, low profile, from Super strut and gutter mounts from; Roof Racks — Gamiviti

First part of the build; Land Rover 109 roof rack

 

[nateh]

The bottom of the side rail is the contour of the roof. Each bracket becomes increasingly offset to cope with the non-square roof, pushing this main side rail further out towards the edge of the roof

 

[grubworm]

i have a cargo basket that i keep on the vehicle all the time and it clamps to the cross bars and does fine. i dont intend to use the crossbars without the basket so it works for me. if i was in your situation, i would mount the rack to the bars and permanently attach it and then just get a second set of bars that you can use when you remove the rack and want to do something else. yeah, its a little extra money, but mount the rack first and wait on the second set of bars and see if you will really need them.

fabricating gets expensive and a couple hours of labor fitting and welding would equal a second set of cross bars.

i would also think that mounting to the cross bars would be stronger and better since the cross bars and hardware are engineered for what they are doing and the cross bars automatically give you a flat and level position to attach to. i'm sure your fabricator could make it work, but i do see where there would be different forces on the mounts and that is always a point of concern.

 

[Boort]

I’m working with a local fabricator to design and build a prinsu-style roof rack as they don’t make one for my vehicle. My fabricator now wants me to consider mounting directly to the extruded aluminum cross bars instead of to the side rails like prinsu racks do. The reason being that the roof isn’t square and therefore there’s a bit of lateral “reaching” by the mounts towards the end and he’s worried about weight on the rack warping them under pressure. The attached photo of the design is a bit exaggerated but you see the point.

For those with a rack like this, what would the negatives or positives of mounting directly to the cross bars instead of dealing with these mounts and instead just having the side rails “floating” attached on the ends of the cross bars? I don’t love the idea of not being able to shift the cross bars where you need them based on what you’re carrying.

The Sherpa rack that I have on my 4Runner mounts like you describe with the roof mounts connecting to the cross bars and not to the side rails. I beleive that this was done for the same reason your fabricator is recommending this method. The front and back mounting positions were not level and this allowed for less of an outboard offset reducing torque applied the rack and roof mounting. I can still shift all but the 2 main bars to meet my needs. and can even shift the rack ~1" fore and aft if needed. (could get more if I had the 03-05 style rear spoiler)
( Install video showing this assembly )

Benefits that I've found over the last 2 years:

• Weight of rack is loaded more straight up and down rather than outward.
• Very easy to seal compared to what I've seen on the Prinsu and similar installs
• If needed I can quickly remove the sides and extra cross bars (8 screws) leaving just the 2 mounted cross bars for weight reduction.

Boort

 

[nateh]

The Sherpa rack that I have on my 4Runner mounts like you describe with the roof mounts connecting to the cross bars and not to the side rails. I beleive that this was done for the same reason your fabricator is recommending this method. The front and back mounting positions were not level and this allowed for less of an outboard offset reducing torque applied the rack and roof mounting. I can still shift all but the 2 main bars to meet my needs. and can even shift the rack ~1" fore and aft if needed. (could get more if I had the 03-05 style rear spoiler)
( Install video showing this assembly )

Benefits that I've found over the last 2 years:

• Weight of rack is loaded more straight up and down rather than outward.
• Very easy to seal compared to what I've seen on the Prinsu and similar installs
• If needed I can quickly remove the sides and extra cross bars (8 screws) leaving just the 2 mounted cross bars for weight reduction.

Boort

So if you were me you’d mount directly to the cross bars?

 

[Boort]

So if you were me you’d mount directly to the cross bars?

Yes, I'd make mounts the needed heights and have any bend go inwards (to center of roof line rather outwards as in the mockup pic above.) Then mount the extruded aluminum cross bars to the custom mounts, hang the side plates from the ends of the crossbars and attach the wind fairing + any other cross bars needed between the side plates.
The pics of the Sherpa load bars give an idea for the mounting system in use on my rack:
Roof Track Load Bar System (Pair)

Boort

 

[smritte]

I have a Prinsu rack and run it on the sides. The only reason I bought mine and didn't copy it was because of what I payed for it, was close to my building it. The only mod I did was run a moulding along the edge that sits in the gutter, so it doesn't wear the paint/metal. I am also getting ready to build one for my trailer. This will mount directly to the roof.

My fabricator now wants me to consider mounting directly to the extruded aluminum cross bars instead of to the side rails like Prinsu racks do.

My first question is, Are you talking the factory cross bars and how are they mounted? Next, how much weight are you talking?

For those with a rack like this, what would the negatives or positives of mounting directly to the cross bars instead of dealing with these mounts and instead just having the side rails “floating” attached on the ends of the cross bars?

"IF" your planning on walking on it, the extruded aluminum bars will flex alot. If your planning on putting a tent on it, regardless of how your mount it, I would keep the weight as far to the sides as possible. Another thing I will call into question is, how are your stock cross bars mounted now? If your using the factory nut serts, are they secure? I have seen some loosen with time and leak. The factory ones in my Cruiser are well known for that. When I use them for mounting (nut serts), I add in enough to spread the load and give enough strength incase I crash. I don't want them ripping out.
Is it possible to mount to the factory holes as well as the sides?

I don't carry anything heavy up there. Mostly its lighter things in boxes. I hate having to lift up and down. I would have no issue holding several water/fuel cans up there, I just don't want to. If its a road shower, traction boards, shovel and some storage, it wont matter how you do it. 20 gals of water/fuel, spare tire, RTT and an Elephant (never know when you need one) then I would plan it carefully.

You also made a comment about moving the cross bars. In your picture it appears the bars are screwed in place on the sides. Don't move them, add more in if you want versatility. The aluminum tends to seize the screws. I want/need two more bars on my cruiser. When I buy the metal for my trailer, i will add in some more there.

 

[nateh]

I have a Prinsu rack and run it on the sides. The only reason I bought mine and didn't copy it was because of what I payed for it, was close to my building it. The only mod I did was run a moulding along the edge that sits in the gutter, so it doesn't wear the paint/metal. I am also getting ready to build one for my trailer. This will mount directly to the roof.



My first question is, Are you talking the factory cross bars and how are they mounted? Next, how much weight are you talking?



"IF" your planning on walking on it, the extruded aluminum bars will flex alot. If your planning on putting a tent on it, regardless of how your mount it, I would keep the weight as far to the sides as possible. Another thing I will call into question is, how are your stock cross bars mounted now? If your using the factory nut serts, are they secure? I have seen some loosen with time and leak. The factory ones in my Cruiser are well known for that. When I use them for mounting (nut serts), I add in enough to spread the load and give enough strength incase I crash. I don't want them ripping out.
Is it possible to mount to the factory holes as well as the sides?

I don't carry anything heavy up there. Mostly its lighter things in boxes. I hate having to lift up and down. I would have no issue holding several water/fuel cans up there, I just don't want to. If its a road shower, traction boards, shovel and some storage, it wont matter how you do it. 20 gals of water/fuel, spare tire, RTT and an Elephant (never know when you need one) then I would plan it carefully.

You also made a comment about moving the cross bars. In your picture it appears the bars are screwed in place on the sides. Don't move them, add more in if you want versatility. The aluminum tends to seize the screws. I want/need two more bars on my cruiser. When I buy the metal for my trailer, i will add in some more there.

So my vehicle (Land Rover LR3) has four single bolt factory mounting points on each side. So, 8 in total. They're all waterproofed from the factory (I've tested to make sure :))

I hope to have a RTT up there soon. Maybe a full sized spare and some water/gas/pelican cases when no RTT. Probably no elephant...

I believe I have 9 cross bars planned. 4 would be fixed to the roof in this scenario, leaving 5 with more "wiggle" room. My original design was to be like Prinsu and mount the side rails so all 9 would be adjustable.

 

[smritte]

With the four cross bars close to the roof, you should be fine. Mine sit about an inch over the roof. If I put weight in the center or try to stand on it, it will flex enough to touch the roof. That would be mounted on the sides with 40 inch(ish) 1X2 cross bars. Its not enough of a concern to tie into the existing roof mounts, even though it would add more rigidity. I like the design of the bars, I thought they would have been more ridged. If I strapped fuel/water cans or a tire to it, they would be to the side not near center unless I made a mount plate to distribute the weight better. This is one of the reasons I want to add in more bars.

If you haven't planned it yet, add in loops/holes in the side plate and or mounted to the rails for tiedowns. I use 1/4" stainless eye bolts with a T-nut in the rails. They work but I wont secure anything heavy. The design I have is 3" angle. It would mount under the cross bar with 2-4 T-Nuts. It will stick up 2" and be radiused for a 3/4"(H) X 1"(W) hole. This will be movable and create a stop to keep things from sliding around. On long excursions over rough ground, everything up there likes to shift some.

 

[Boort]

So my vehicle (Land Rover LR3) has four single bolt factory mounting points on each side. So, 8 in total. They're all waterproofed from the factory (I've tested to make sure :))

I hope to have a RTT up there soon. Maybe a full sized spare and some water/gas/pelican cases when no RTT. Probably no elephant...

I believe I have 9 cross bars planned. 4 would be fixed to the roof in this scenario, leaving 5 with more "wiggle" room. My original design was to be like Prinsu and mount the side rails so all 9 would be adjustable.

When talking about walking/standing on the Aluminium cross bars. I did some napkin math on this for a question in another forum. The Jist of that post was:
As long as you distribute the load across multiple cross members. I would not be really concerned about the cross members carrying the weight.
The cross member profiles of my Sherpa rack are 1x2" extruded aluminum about 47" long installed with the wide side parallel to the ground. If you weigh 200 lbs and stand in the center of the member it would be expected to deflect a bit more than 1/3" down. Not accounting for twisting or deflection of the side panels that are supporting the ends.

Each cross member is connected by 2 Stainless Steel 1/4x20 bolts at each end. The shoulder of the cross member aluminum profile would take out much of the leverage by the 1/3" deflection caused by the center load. So a 200 lbs load focused on just one cross member would be focused primarily on the shank and heads of these bolts. A grade 5, 1/4x20 bolt reports a clamping force of 1117lbs over a clamping area of 0.0318 Sq Inches. 200lbs spread evenly across the 4 bolts works out to each bolt supporting ~50lbs + 1/4 the ~1 lb weight of the cross member itself.

The next item down is the mounting points on you're rig's roof which I don't have specific details for other than the original rack's load rating of 150 lbs. But that was likely due to all of the fiberglass/GRP in the stock rack rather than the mounting point failure load.

TLDR: you probably don't want to focus all of your weight on just 1 or 2 cross members but otherwise I think the math holds up that a Prinsu/Sherpa style rack can take a static point load of a 200lb person standing on top.
If I need to get up there I grab the plywood of my sleeping platform and use it to spread my weight across at least 4 cross members. I know of multiple photographers who use their racks as photography platforms.

Boort