Help me do math on wheel cribs.

Cripes... I'm all jumbled up here. 
 

Cribs need to be 15”x20”. 

For 9" tall cribs, I  need 6 pieces of 20” and 6 pieces of 15”.
 

Four wheels, so X4. = 24 pieces of 15”. 24 pieces of 20”. 

Currently have (8)15”. And (5) 20” from spare 2x4's  

I need (16) 15” and (19) 20” additionally  

That’s 20ft of 2x4 for the 15”, you can get 6 pieces out of an 8ft board. So I need (3) 8ft 2x4’s? 

And 32 ft for 20”. I can get (4) 20” out of 8ft board. So I need (5) 8ft 2x4's? 
 

have to be 8footers or less. Can't fit anything longer. And I have a 30mile drive in the snow back to the house. 

The ones you cut the 20s out of will have 16" left over, so you can also get a 15" piece out of each. That gives you six of the 15" pieces you need (one from each of the first four boards, two from the last because you only need three 20" pieces from it, not four). So you should need 7 total 2x4s.

In reply to dps214 :

Thanks I thought my math was near but off. 

OK, I have everything cut now. I guess now the question is whether I should use two screws per corner or four screws per corner. This is a Miata, so a very light vehicle. I know a lot about cars, but I don't know crap about wood. 
 

learn me

Curious what people answer.  To me 2x/corner = 4x total per corner (2 from board into board below, 2 from above into that one).  Its going to be super easy to split one unless you pre-drill just right.

I'd lean toward a single screw and wood glue.  

In reply to ProDarwin :

Wood glue it is! 

I did two per corner on mine. No specific reason other than that it seemed like enough and any more than that was going to be hard to keep from interfering with the screws from the next layer. Had a few minor cracks from holes that weren't predrilled far enough, but nothing that seemed problematic. Haven't used them much, but no signs of any issues yet.

In reply to dps214 :

Also good to know

Also agree that a single screw + wood glue in each corner is plenty.  Glued joints are so strong the surrounding wood will fail before the joint does. Also the joints will all be in compression, so shear is not really a concern

In reply to jerrysarcastic (Forum Supporter) :

And this is why I'm part of this forum. Y'all are engineering geniuseseses

2 screws per corner on mine and offset from the previous layer.

ProDarwin said:

Curious what people answer.  To me 2x/corner = 4x total per corner (2 from board into board below, 2 from above into that one).  Its going to be super easy to split one unless you pre-drill just right.

I'd lean toward a single screw and wood glue.  

I agree that 1 screw plus glue will be fine, but I would build them differently...

You are describing screwing all the frames together into one big cube. I would build them as individual 4-sided frames, and dry stack them as needed. This would mean 3 frames per corner- total 12. 
 

They will store much easier if they are not big cubes, and be easier to carry around the shop. Plus, if you ever need the cribbing to be a little taller than 9" at one end, you could just stack them taller. 
 

They won't fall over. Gravity still works. 

I guess if you did that and you were worried about them falling over, you could just put a piece of 2x4x~4.5-6" long in 2 of the inside corners so they could nest together (except the bottom piece).

In reply to ProDarwin :

That's true, but still overkill. 
 

These things are 15"x20". That's a really big footprint. It's about 6X the footprint of a jackstand. 
 

They aren't gonna fall over. 

We lift entire houses on cribbing that is never attached to each other at all. 

In reply to SVreX (Forum Supporter) :

How tall would you be comfortable going like that? Although with the price of wood on the wasy back up again, I could prolly afford a lift if I had to buy the wood...

In reply to 03Panther :

A long way. 
 

I'm dodging your question intentionally. There would be a whole lot of "it depends" if I were to attempt to answer accurately. 
 

If it is solidly built with strong connections and a solid level floor, it seems perfectly reasonable to go 2' or more.

I've lifted houses 12' in the air on unsecured cribbing set on dirt (these men are professionals... do NOT attempt this at home)

My suggestion.  When using cribing don't put weight of tires down inside the cribing.  That would want to push the cribing apart.  Have something like a piece of 3/4" plywood to place on top of sribing to keep forces going straight down.

pictures or it never happened....

Purple Frog (Forum Supporter) said:

My suggestion.  When using cribing don't put weight of tires down inside the cribing.  That would want to push the cribing apart.  Have something like a piece of 3/4" plywood to place on top of sribing to keep forces going straight down.

I was thinking about this. On one hand I don't want the car to roll about. On the other hand, I do need a flat surface for the tire to rotate on while doing an alignment.

I  made the top layer of mine solid 2x4s. But mine are much smaller footprint so it was less wasteful. In your case I'd probably consider plywood. Although a 14x20 (or space them apart a touch to get 15x20) top surface would only need another two 2x4s which is almost certainly going to be cheaper than a sheet of thick plywood.

In reply to dps214 :

Plus I have extra two by fours now

I sure wish I knew what the heck ya'll are talking about.

In reply to stukndapast :

In reply to SVreX (Forum Supporter) :

I see, said the blind man.  I thought maybe it was something for storing tires, like a rack or shelf.  < Does best Forrest Gump imitation here >. Thanks for the clarification.