How many bolts to attach a floor on 4x8 trailer?

[ukewarrior]

My daughter and I have started building a "Simple/Rimple" on a HF 4x8 trailer. (Build thread will be started soon)

I have 3/8" bolts which fit the existing holes nicely in the frame pieces for attaching the floor.

How many bolts is sufficient?

My current plan was to use two bolts per cross member (only one set in the middle) which is a total of 10 bolts.
These would have fender washers top and bottom to help distribute the load and a lock washer with the nut.
I also plan to use loctite.

I've seen lots of different techniques for mounting the floors, but not a discussion on how many bolts will do the job right.
The bolts are through a sandwich insulated floor, but the bolts themselves are through nothing but wood.

Thanks in advance for the advise and opinions.

[Jdw2717]

I used 20 on my 5x8 frame which was 4 per cross member. It was overkill but the thought of this thing coming apart on the interstate was too scary. I will say that I have 5 cross members running from front to back though.

[mikeschn]

The heavier your body, the more bolts you need in the floor. For my foamie I'm doing 3 on a side, and 2 in the middle, for a total of 8.

A 4x8 is probably going to be about 800 pounds, so maybe 4 or 5 on a side and 2 in the middle. And they have to be large enough to withstand a shear load. I'm thinking 5/16 or 3/8". Maybe Angib can help me out here...

Mike...

[dales133]

Ive got 2 front,4 down each side and one in the middle at the rear..i plan on gluing it to the trailer with sikaflex prior to final boltage!

[noseoil]

Nucor's shear values for bolts, all grade 2 / hardware store type fasteners / value is per individual bolt in a single shear application:

1/4" = 2179#
5/16 = 3405#
3/8" = 4903#

If you have 10 - 5/16" bolts, you have a shear value of about 34,000 pounds in shear (just figure it's like scissors cutting each individual bolt, all at the same time). In other words, if these bolts shear due to an impact load or stress, you have other things to worry about than the bolts being weak.....

[aggie79]

Remember that you trailer is connected to your vehicle via a single 1/2" or 5/8" hitch pin so there is no need to go overboard. Instead of bolts, I used trailer deck screws at 12 inches on center.

[KCStudly]

What Tom said. I feel it is more important to have a spread of fasteners from front to rear in order to transfer the stiffness of the cabin into the trailer frame. You want them to act like one unit. I have sixteen (16) 3/8 in bolts, 4 at each xmbr.

[TwoFeetShorter]

I have the exact same trailer. I used 3/8 bolts with washers on both sides, drilled through my sub floor framing then used a spade bit to counter sink the bolts flush. In every pre drilled hole in the trailer long sides I put a bolt, plus one dead center on the front and rear, for a total of 10. I also made my floor an inch shorter than the trailer so the weight of my walls and roof are resting on the trailer and not the subfloor. I didn't use loctite on the nuts underneath, but they all got hit with undercoating when I sealed the bottom so I doubt they will work loose!

[noseoil]

I'm using nylon lock-nuts on my build for the bottom side. Figure it's a "one time" assembly, so I'd rather have the peace of mind knowing the nuts can't loosen over bumpy roads & highway miles. Black goo would also be a good way to go, or even paint once the nuts are in place, just so they stay put & can't come loose.

[KCStudly]

Building submarines the specs for nylon insert self locking nuts was one shot use during new construction, but after delivery that got relaxed to five (5) uses (assembly/disassembly) provided that the nut could still not be turned by hand. For most non-nuke and less critical applications no one ever had any practical way of knowing how many times a nut had been assembled, so it defaulted to the hand turn test. If you could turn it by hand, get a new one; if not, okay to use again. Self locking bolts with the pellet insert or patch got renewed every time.

That's the rule of thumb that I use to this day.

[angib]

There's no need to go all he-man on things like bolt diameter and grade as they are attaching a wood structure so will just pull out of the wood if loaded to the point where grade or diameter mattered.

The bodies of most fiberglass trailers like Scamps and Bolers are held to the frame with six to eight self-tapping screws and you never hear of one of those bodies detaching from its frame (excluding ones that have been rolled or crashed). And those are now rusty self-tapping screws on 30-year-old trailers, and still they don't fail.

As I have suggested before four to eight big lumps of well-chewed chewing gum would probably hold most bodies on - and lets not get into a discussion about how long it has to have been chewed.

[ukewarrior]

Thank you for all the feedback.

I went with 10 bolts, 3/8" with locking washers and loctite.
All the bolts on my trailer have or will be getting the black goo or rustoleum paint.

[Franco Novo]

You're good
I just did a quick build....HF folding frame
Just bolted the plywood directly to the frame with the 3/8" bolts with locking nuts that came with the kit

On the cross members I had to rip some 1/8" strips, because they sit a little below the side rails
Pretty sure I used 4 bolts down each side rail....3 in the middle 11 total

After years in the building trade, I've torn apart too many remodels where previous folks had like 50 nails in a 8' length of baseboard.
like where is it going to go.....

best,
Franco


less than a month later:

[rowerwet]

My 6x13 foamie has four 5/16" bolts total, two in each end. Other than when I slam on the brakes as the tear is weightless over a bump, most of the time, friction does a better job of keeping things together, than the bolts.

[KCStudly]

Friction is determined by the surface quality (coefficient of friction) and the force pushing the two surfaces together. The gravity and bolts apply the force. More bolts = more friction = less likely to slide off.

Sure, too many bolts is just overkill, but I like the idea of tying the rigidity of the cabin into the chassis to prevent relative movement (chaffing, slap, breaking of any sealant or welting that may be used) so I like to see at least a couple of bolts along the sides as well as at the ends.

I'm sure I am beating a dead horse here.