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So....my question then is, in Drawing "or this" you have even MORE than 80" front to rear. Right?
Only if I was able to set it up on the stands with the suspension loaded on all 4 corners. My cribs are 15"Lx15"Wx12.5"H so they are a little more than 10 inches narrower than the wheel/tire. If the wheels/tires were to hang down, I would have less room. The opening would change from basically a nice rectangle to more of a trapezoid if the wheels hang. Overall distance at the floor might increase but the area in the middle and top where my fat *** would slide under would be narrower. Then we would be right back into the sidejacking issue and narrowing the ingress/egress from the front and back of the car.
Of course we could always use junkyard jackstands. I asked the man at the counter if there were any accidents and he said not in 40 years that hes been there. I wouldnt go under a car like that would you?
Im curious to know where the weight of the car is supposed to be resting on these cribs? Should it be on the cross pieces of 2x? or on the solid outer columns? I have seen black belts break 2x4s with their hands so I would say the weight should be on the outter columns not on the cross pieces which rationally should only serve to hold the columns in place. So far thats what Im getting from this discussion. The width of the cribs and where the tire is resting seems crucial to the safety thereof.
The weight of the car is centered in the center of each tire's contact line on the transverse boards. On a properly built crib, no part of the tire will be over the columns formed at the corner intersections of the boards. Take a look at the pic of the Nova (I think it is?) in Post 1. The typical C4 tire contact patch is 10" wide or less, so if you build cribs so narrow that the tread overlaps the columns then you have cribs that are way too narrow to be stable. And the taller your cribs, the wider they need to be to resist falling over sideways. If you look again at the cribs under the Cuda in Post 3, that's an accident waiting to happen for that very reason...and the treads still barely overlap the columns even on those way-too-narrow cribs.
The conundrum with cribs for supporting tires is that the wider the better for lateral stability, but the narrower the better for vertical load capacity. Proper cribs can't be made to carry the tire loads directly on the column, because they will be too narrow for good lateral stability. But yes, it depends on how long the boards are between their supports as to how much weight they can support: the longer the distance between supports, the less it can support. For an example of this, look at ratings for deck boards over joists - the ratings all depend on the joist spacing. Typical deck joist spacing (for 2x6 decking, which carry 50% more load than the 2x4s we're discussing) is 12-18" max, and that is intended to hold maybe one human or a grill between the joists, at most.
In this way, cribs for cars are not at all like putting a house on cribs, because in that case the load is supported directly by the columns. The 6000lb load capacity for 4" timbers that ihatebarkingdogs quoted may be correct for a load that covers the corners of the cribs, but it doesn't apply to using cribs for cars where the load has to be between the vertical supports. I'd suggest that 18" outside dimensions are probably the minimum for stable cribs to support a car, which leaves a 12" unsupported span between the columns. A pine 2x4 on its side is not rated to support 6000lbs in the middle of a 12" span. That's insane! The load rating under such conditions is probably 1/10th that, maybe even less. A typical C4 is going to have corner loads somewhere between 750-850lbs. The distribution of that load between the columns is tricky to calculate because the tires aren't rigid. I have no idea how to figure it. In my estimation, the margin probably isn't high enough to risk it. And I will once again add in the stresses that expansion and contraction of the tires and wood will place on the top lateral boards: that reduces margins as well.
If you're going to build cribs to support a car, my suggestion is to make them at least 18" in outside dimensions for lateral and longitudinal stability. Use at least 4x4s for the top lateral pieces that will actually support the tires between the columns. That will at least double the load capacity between the columns (maybe more than double it?).
I built mine with three 2x4s per layer except on the the top where I have four. So mine have 9 vertical columns as opposed to the 4 you see with a typical two board per layer crib. I have an extremely smooth garage floor which works out excellent because when I drop the car onto the cribs, they spread out as the suspension loads and the car settles back almost exactly into the same position it was when I drove it into the garage except it sits 12" higher.
Matt is correct that the limitation is the span capacity of the top two boards. But he overlooks that the load is not in "the center of the span". The tire is 12" wide. Because the air pressure inside the tire is equal, the weight transferred to the board is equal all the way across that span. So only 1/12th of the load is exactly over the center. Much of the load is close to the corners.
I didn't overlook that: I addressed it directly: because the tire is elastic and has some shape it is very difficult to determine how it spreads the load laterally. I can tell you two things that are incorrect with your statement:
A typical C4 tire is 275/40/17, 255/45/17, or 255/50/16 and none of those have a tread width of even 10", much less 12". If the unsupported span between the columns is 12", the tire cannot possibly spread the load across the entire span.
If you look at an image of a contact patch from a loaded tire, you'll see that it is not shaped like a rectangle as we would expect for an even loading all the way across the tread. It is usually oval shaped, with the most longitude in the middle, and that means that it concentrates more of the load toward the middle. There are a lot of variables that affect this, of course. However, the tire doesn't spread the car's load across the tread width evenly, much less across the total unsupported span of a properly-sized crib.
But he is incorrect that the load is not "over the columns". Yes it is. The top two boards transfer the load into the 4 corners. (Columns).
Yes, of course they do. You're missing the point, though: Garack and I were discussing ability of the top two lateral boards to support the entire load of a tire, which they must do before the columns can support the load. I am not questioning the ability of the columns to support the car's weight. When I wrote that the load is not over the columns, I meant that the tires don't directly load the columns: they load the top two lateral boards first, which then transfer the load to the columns. As you wrote, the top two boards are limiting factor, and their load limits are waaaay lower than that of the columns. If they fail then it doesn't matter how much the columns can hold.
If you are going to build cribs for your car, and you are concerned about the "span capacity", place a third board under the top two boards. Now the span load on the top two boards will be on four span boards. Doubling the "span capacity".
Yeah, this would help a lot as long as the tire doesn't sink low enough between the boards to hit it. You wouldn't need a full third board, actually: you could just put short chunks of board between the top lateral boards and the second-from-top lateral boards to fill in the gaps between them.
Originally Posted by Kristy84
when I drop the car onto the cribs, they spread out as the suspension loads and the car settles back almost exactly into the same position it was when I drove it into the garage except it sits 12" higher.
You actually bring up another important point that I hadn't even thought of: the tire contact patches move laterally quite a bit as the suspension goes from fully unloaded to fully loaded. That's why cars usually sit higher after you drop them off a jack until you roll them around. All of that lateral movement will want to push the top of the crib stack outward. In your case, you are saying the bottom of each crib moves with the top to prevent the stacks from wanting to tip over, but most people's garage floors are bare concrete and will not allow that movement.
Only if I was able to set it up on the stands with the suspension loaded on all 4 corners. My cribs are 15"Lx15"Wx12.5"H so they are a little more than 10 inches narrower than the wheel/tire. If the wheels/tires were to hang down, I would have less room. The opening would change from basically a nice rectangle to more of a trapezoid if the wheels hang. Overall distance at the floor might increase but the area in the middle and top where my fat *** would slide under would be narrower. Then we would be right back into the sidejacking issue and narrowing the ingress/egress from the front and back of the car.
Got it. Thanks. To ME, those differences are so infinitesimal, that it doesn't warrant carpentry projects and the storage thereof. Plus, I can go higher w/my stands than I could w/cribbing so that trade off outweighs the cm's of diff that you've illustrated. Don't forget; you could also stand outside the axle area, too.
The pick-a-part yards around me use these. I'll go under the car, but my enhanced "safety awareness" since the accident I cited earlier, makes me do two things:
A: I push and pull on the car in all directions, and check the stability.
B: If I am alone (I usually am), while I am under it, I continuously scan for people (their legs and feet) getting close to the car I'm under. If they get close enough where I think they may open a door, drop something through the engine bay etc, I get out from under it.
Just think safety.
Again, I totally agree. Pick 'n Pull around here does the same. I go under them....but I try to shake the car off 'em first.
You actually bring up another important point that I hadn't even thought of: the tire contact patches move laterally quite a bit as the suspension goes from fully unloaded to fully loaded. That's why cars usually sit higher after you drop them off a jack until you roll them around. All of that lateral movement will want to push the top of the crib stack outward. In your case, you are saying the bottom of each crib moves with the top to prevent the stacks from wanting to tip over, but most people's garage floors are bare concrete and will not allow that movement.
This. Something that I brought up earlier. This is also a safety issue...although not a "deal breaking" one....just something that needs to be considered when lowering the car on to them.
I'm doing a complete body off frame restomod on my 73 Big Block.
I put it up on jack stands and it made me very nervous sliding under the car and seeing a thin metal pad about 2" X 3" contacting the uneven jacking point on the chassis.
I pushed the side of the body and the entire car wiggled pretty good, which made me even more nervous about crawling under it and doing any work.
I built four wooden cribs using 2" X 4" studs 12" high as shown above.
Each 2" X 4" block is screwed together with #10 3 1/2" wood screws using two screws in each corner in a alternating pattern.
I also added a center 2" X 4" for every cross section for additional support.
Pushing on the side fender, my car doesn't move a bit except through the suspension.
The car is solid, safe and sturdy.
Here is a picture showing a portion of one of my cribs.
These cribs would hold up a house and there was absolutely no movement of the cribs when placing the weight of the car on them.
St. Jude Donor '09-'10-'11-'12-'13-'14-'15-'16-'17-'18-‘19-'20-'21-'22-'23-'24
FWIW, a few years ago somebody in one of the generations on here told of working under his car while it was on jack stands. His wife called him into the house for something. When he came back out a few minutes later, a jack stand had failed and the car would have killed him. I THINK a weld failed but can't recall.
I always use two separate means of support, one actually holding the vehicle and the other as close to contact as I can get it.
Those drums can hold quite a bit of weight. Shipping and storage regs allow stacking loads of around 4500 lbs. They can safely hold 5-6x that though in a flat, non-moving scenario. Takes about 30 tons to crush one. Not suggesting their use but the weight on those drums isn't an issue in the slightest.
Those drums can hold quite a bit of weight. Shipping and storage regs allow stacking loads of around 4500 lbs. They can safely hold 5-6x that though in a flat, non-moving scenario. Takes about 30 tons to crush one. Not suggesting their use but the weight on those drums isn't an issue in the slightest.
They can hold a lot of weight...until they can't. It just takes one dent in the side. Ever done the trick where you stand on a soda can and then tap the side?
St. Jude Donor '09-'10-'11-'12-'13-'14-'15-'16-'17-'18-‘19-'20-'21-'22-'23-'24
Yes, the railroad can be an unforgiving environment. Occasionally when rerailing cars you have to go under, and believe me it's essential that everybody is on the same page and knows what to do and not do. But any time there is an option to do something from outside, that's the way to go.
Among the supports I use are a couple pieces of railroad ties and bridge timber. They're a pain to shove and move around under the vehicle but the degree of safety is reassuring.
I've often "day dreamt" about a device for working on car like a "positioner"....like a rotisserie that you could connect to the car's structure, lift and rotate so you could do all of your under car and corner work while standing next to the car, on your feet, working right in front of your chest. That would be mint...imagine "pulling" (not "dropping") the exhaust, trans/clutch etc, working right in front of yourself? Lighting? Awesome. Safety? You're not under it. Ergo's? Perfect.
Now that tool would be a huge space-suck! But my god...the benefits would be massive, the work would be so easy and pleasurable.
03-24-2022, 02:44 PM
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