What are the strongest joints for a wheeled cart?

[radhak]

My friend wants to build a wheeled cart using 4x4 pine wood he already has. He also has a few 6 x 4, which he might use for the horizontal pieces.

He has made the design below. The overall dimensions will be 68" x 68" x 36" high.

All the horizontal pieces' will get half-laps at their meeting points, and then probable screwed. The vertical pieces will be simply screwed and then supported by steel L-brackets 3" wide and long, two at each joint, as shown in the diagram.

The purpose is to cart around stuff he works with - car parts, rock sculptures etc. He expects the weight could go upto a few hundred pounds, for a maximum of 700 or 800 lbs. He would move it much, just from garage to back-yard, etc, but it still would be a few hundred yards at a time .

Given the weight and size, would these L-brackets be enough to provide the strength required? Or is there any other way he can join these?

[dbhost]

I may be blowing smoke here, but I would think half laps well glued for the cross pieces, and then Mortise and Tenon for the verticals. The corner brackets would be Very low on my list of strong joints...

[Black walnut]

Agree with Dave. Plus pinned tenons for the uprights. Lots of glue.

[LCHIEN]

A quick calculation shows over 150 pounds of wood in the frame at 34 pounds per cu ft (Yellow pine).
Add 700-800 pounds of load this is going to be a tough mother to push around unless the path is very flat (he said a few hundred yards)
Absolutely can't have four steerable wheels, must have two steerable and two fixed.
The cart is too low to push that kind of weight, you will break your back bending over. Or else push the load on top which then risks it sliding off the cart since the pushing force will be quite high.
You need to push from the movable wheel end. This has no means of pushing.
To move that weight, esp. is any slope at all will take major side force, I don't like the lack of diagonal bracing. If on a slope and you have 950 pound rolling weight, you're going to need brakes!!!!

Pushing that thing is going to take considerable side force, the L brackets vary in strength/thickness, but they will be under a big bending moment on one side while the width of the column in compression supports it on the other. In the right illustration below a left direction force applied will bend the steel L-bracket with a 2-3 foot lever arm, lifting the vertical support away from the base you might think its screwed to but its not.


Some 45 degree braces using a 2x4 just 6 or 8 inches long in some of the corners will be much, much stronger.

Putting the verticals right over the lap joint for the horizontals makes it difficult to join the vertical using a mortise/tenon, or dowels or anything else other than those L-brackets. Why not move it over? You have 6 inches hanging off the sides or you can put it inside of the laps.

Overall I don't like this idea very much, it seems impractical. It doesn't seem well thought out. But then I'm not using it.

[LCHIEN]

After some more thought, I still don't like this design.

Here are some more things not to like:

How is the load to be placed on this cart? All on the upper "deck"? If so, how is the load distributed with a large open area in the center and the support only around the periphery. If a 700 pound load is placed on two opposite sides of the square as a two 350 pound point loads the Sagulator says the 4x4 will bend excessively (0.15 inches) but that if the load was distributed 350 pounds uniformly across the 4x4 on each end then the sag would be acceptable (0.7 inches).

However if all the load is placed on the top rails, with unsupported 5 feet in the middle, then the bottom level 4x4s are purely wasted as excessively heavy duty tension rails.

There probably needs to be some means to secure the 700 pound weight to the cart for safety's sake.

[leehljp]

WHEEL/CASTER SIZE: On something that big and especially that much weight, incorporating wheels no smaller than 5", with 6" being better - and will take more stress off the frame. The smaller the wheels, the more stress is put on the screws of the caster/cart joint. The smaller wheels will not roll as easily over small bumps or tiny rocks and throw a "racking" force (twisting of the frame out of alignment - loosening the joints) into the frame, resulting in a frame rocking loose or caster(s) coming off. Larger wheels with a good thick rubber/polyurethane tread. I am not an engineer and do not know the formulas for figuring this, but every inch larger wheel, I would guesstimate that one gets a 25% easier roll and 20% less force in "racking" the frame. 4" wheels "might" be doable but it sure would roll better with 5 and 6" wheels.

For moving a "few" hundred yards with that weight = 6 " minimum and 8" would definitely be better.

SCREWS: Attaching casters: use lag screws, drill the correct size hole out for each screw, make it minimum of 2 1/2 inches, but 3 inches would be better.

2+2 VS 4 WHEEL STEERING: I have had a somewhat different experience with steering as Loring related. for me, It depends upon the situation. For 1. unlevel areas, and 2. plenty of space for maneuvering, 2 Fixed and 2 steerable work best for me. For smaller areas and tight fit for the table when stored, 4 steerable work better. I have worked with both in many situations and in smaller ares and tight placement spots. I grew up with 2 and 2 and always thought that was always better, but with limited space when I lived in Japan, I found that 4 steerable were much better in that situation. But 4 swivel wheels do NOT work well on unlevel floors!

I have had considerable experience with the above in several tables and have had casters come off because of using the wrong caster size and too short of lag screws.

[LCHIEN]

thinking on it overnight,
Is it really even possible to have a man-pushed cart with Almost 1000 pounds total weight to push hundreds of yards at a time? over less than ideally flat paths?
if you have 700-800 pound payload you want the deck low instead of high to facilitate loading and more importantly make it stable and safe.
with that kind of loads and position, maybe you want axle wheels and a steering carriage instead of casters.
I'm talking about 6" balloon tires on axles in the rear and steering carriage and a yoke to pull it, along the lines of a radio flyer wagon, but scaled up in terms of weight bearing.





You could have a 5-foot long deck 4 feet wide made of say 5 parallel 4x4. Half lap the 4x4 in four or five places and connect with two-by-fours to tie them together horizontally. A couple of blocks in the rear hanging down to pass the rear axle. A steerable axle with a stout bearing in the middle to make a rotatable front steering.
And a pull handle hinged in the up and down direction to pull the cart.
Some stake holders on the outer sides of the outer 4x4 to accommodate stakes to retain or tie the load down.

[d_meister]

I follow Robert Dunn's YouTube channels for the entertaining build-it and automotive content. He's very amusing, but he also made a mobile cart to accommodate a 1,000 pound electric car battery pack. He incorporated a lot of the suggestions made by you folks above:
https://www.youtube.com/watch?v=vqnUvudqhbg

[cwsmith]

Interesting subject and I think Loring and Hank Lee have put in some great suggestions. The only thing I can add (humorously) is perhaps a 'pickup' would better serve the purpose of frequently moving several hundred pounds over several hundred yards.


CWS

[LCHIEN]

Yeah, I was thinking a half-ton pickup truck might be a much better answer here.

[mpc]

As everybody has noted, "racking" forces will likely be quite large. Diagonal braces or "X" braces would help a ton but would make the lower shelf area harder to access. If 2 fixed and 2 caster wheels are used, the bracing need only be on two sides - on the sides sharing 1 fixed and 1 caster wheel as that wheel configuration dictates you push on the cart from one end or the other, but not from the sides. Plywood panels on two sides, instead of bracing, would add a lot of shear strength (=racking resisting strength) at the expense of even worse access to the lower compartment.

Instead of lap joints to form the upper and lower rectangular sections, what about lap joints on the vertical pieces where they join the upper and lower section. Use lap joints on the boards that are on the same side as the 1 fixed + 1 caster so the lap joints help resist the racking force. The remaining boards would be lap-joined offset from the first lap joint. In the picture, imagine the cart has fixed wheels at the left side of the pic and casters at the right side. So you would generally push from the right side. The vertical posts would be lap jointed to the 4 boards that have the 48 inch inner dimension (the ones running left-to-right in the pic). The 4 boards running front-to-back in the pic would be lap jointed to the left-to-right boards in that 6 1/2 inch overhang area. That way the racking forces are resisted by lap joints rather than the metal angle brackets. Adding metal angle brackets would add a little more strength. Diagonal bracing (much like the "Z" bracing used on wooden gates) is still the strongest answer.

Adding a PULL-handle, similar to the wagon idea Loring suggested - ATTACHED TO THE LOWER FRAME - is another way to substantially reduce the racking forces. Only inertia from cargo loaded on the top shelf, when the cart comes to a rapid stop, would create racking forces.

Got a family owned welding shop nearby? They could probably create a very strong frame from square tube material in an hour or so which would be VERY strong. Have them drill holes in the pieces so screws could be run through the metal tubing into plywood shelf panels, have the wheels ready too so they can drill matching holes for the mounting plates.

mpc

[LCHIEN]

I cannot stress enough that with a heavily loaded cart you need to be able to control the steering of the movable wheels, together.
If you use swiveling casters they will swivel to the side when there is a side force because the wheel is behind the center of swiveling.
If you have a heavy car on a sideways sloped path, even just a little, then the cart has a downhill vector (to the side) and the swiveling castors will turn and the load will end up in the ditch even though you are pushing straight ahead.
Most building codes dictate a 2% drainage slope on sildewalk to help them drain. With a 950 pound cart you have almost a 20 pound force trying to turn those casters to the side continuously.

With the Radio flyer like front axle directly under the pivot and that attached to the pull bar you can direct the direction of the steering wheels and keep control of where the cart is going.

[radhak]

↵Thanks everyone! Each response above has been interesting and informative. I sat down with my friend to go over all points, and to begin with, he is delighted to have so many inputs from people who "have the experience and know what you all are talking about"! He wanted to examine each point closely and has a comment or context for each:

1. To begin with, some clarifications - the 'load' will be mostly metal sculptures, made by adding bits and pieces of metal at a time till it reaches the shape/size envisioned. This adds weight but there's no hammering or other forces applied.
2. The height of this cart is carefully thought of, and cannot be reduced further: it's the working platform too, and any other configuration takes away from the usefulness.
3. I examined the largest piece he has ever done, and it is definitely not more than 400 lbs. So his net estimate of 800lbs would very rare, if at all, and must only be after including all the stuff that goes into building the cart, like the 4x4 and the plywood.
4. Also, the sculptures are generally squat: more broad/wide than tall. Sometimes the object could be tall but with some light material. Think a flagpole with large, heavy base. Sometimes this makes it taller than the garage door, hence he needs to take it out to the backyard to finish.
5. All load is very firmly secured to the cart with bolts and such. There's absolutely no sliding or slipping allowed, till it's time to hand over to the customer.
6. The path to the backyard is nicely paved, and visually seems pretty much level.
7. The lower deck will carry less weight, but not negligible - there's some tools and materials. So while the lower 4x4 might be an overkill, for the overall purpose of the cart, we can't change the design to that of the wagon in your picture, Loring. But the suggestion of axle/wheels is intriguing and he wants to look into it further, particularly front-axle with a pivot if he can find one to buy.
8. On the upper deck, the weight is generally well spread out, so he plans to laminate two sheets of 3/4 ply as the weight-bearing 'floor' . He had already included the weight of these plywood pieces in his total estimate.
9. The idea of multiple parallel 4x4s for better weight spread is also striking and he's mulling it; definitely a third 4x4 in the middle is needed, as the width is more than 48" and will need two pieces side-by-side.
10. Yes, mortise/tenons is a fantastic idea. He will probably go with loose tenons to make it slightly simpler. I suggested 1" mortises on each piece to accommodate 2" tenons.
11. Loring, great point about positioning the verticals. He wanted them at the intersection to allow both 4x4 'arms' to take the load evenly. Since the half-lap will allow 1.75 inches thick wood for a 1" mortise, will this still be of concern?
    Note that he even envisaged the wheels directly under the verticals. Of course that was before mortise/tenons entered the picture.
12. And 45 degree braces make sense too. He's wondering if he needs braces on both sides of the vertical, or - since he already has the steel L-brackets, if he should use them on the other side of the wood braces.
13. He has also shared another drawing with the stretchers (3/4" ply) he wants to put on the lower part. That should provide some rigidity too, right? (diagram attached below)
14. Yes, the wheels being considered are these 8" wheels. The question he had was - do the fixed wheels go in the rear or the front? (The cart will have a definitive front/rear parts assigned)
15. Since each sculpture takes weeks to complete, a pickup is not suitable. But he thanks y'all for bolstering his argument for one with his 'boss'
16. That youtube video is nice and to the point, thanks
17. Mpc, we'll go over the suggestion of half-laps on the vertical carefully to see how it would lay out. The metal frame is also a nice idea to keep in mind; for now he wants to really use up the dozen or so 4x4s he has.

This has been a very useful thread for him (and me), thanks all!

[LCHIEN]

"The question he had was - do the fixed wheels go in the rear or the front? (The cart will have a definitive front/rear parts assigned)"

Generally the accepted wisdom for push carts is if the load is light then the steering wheels in the front/fixed in the rear (ala shopping cart at the grocery) but if the load is heavy then the fixed wheels in front/steering wheels in the back. Its also easier for tight maneuvering to have the steering wheels in the back (i.e. like a forklift.)

[leehljp]

14. Yes, the wheels being considered are these 8" wheels. The question he had was - do the fixed wheels go in the rear or the front? (The cart will have a definitive front/rear parts assigned)

IF parking in a specific place against a wall, REAR swivel would be best - push cart front to where you want it to be - and then swing the rear to the wall. Otherwise swivel in front.