Why isn't my bench stable? advice needed

[Matthew N. Masail]

So I decided to make a new base from my benchtop, this base would allow me to place vises where I want them.

The previous base was rock stable, not a hint of movement, it had legs at the far corners of the bench. the new is base pictured here:
unnamed.jpg

the base is only 40" long, and the bottom bars about 23", which is the same as the benchtop. My issue is that the bench is highly unstable, it rattles, and the far right corner bounces when I use the face vise.

-The legs are flat and all for edges are co-planer.

I found that I would need about 30kg of mass on each side on the legs to keep the bench stable, and while that is kinda doable, I use thinking maybe there is a better solution.

also I'd like to know, did I make this with bad mechanics, or is it just not long enough at 40" to counter balance the cantilever effect from the top hanging over the edges?

I'm open to all Ideas besides building a new base.... I have no wood, or time for that anymore.

[Brian Holcombe]

All the weight is in the top. I'd make the floor stretchers much heavier.

[Matthew N. Masail]

Thanks Brian. I'm not sure how much heavier can they get? I needed 30kg of steel to keep the bench stable... or is stiffness in play here too?

how is what I did different to this? http://www.google.co.il/imgres?imgur...d=0CD0QMygUMBQ

[Chris Hachet]

Thanks Brian. I'm not sure how much heavier can they get? I needed 30kg of steel to keep the bench stable... or is stiffness in play here too?

how is what I did different to this? http://www.google.co.il/imgres?imgur...d=0CD0QMygUMBQ

I think its mostly physics rather than stiffness.

[Brian Holcombe]

I'd make them wide as well, that will help. If it's annoying off the front then I would make them wider toward the back to widen the footprint. That, in addition to making them from heavy stock should improve the stability.

You can beef up the stretchers too. I wouldnt be surprised if the stretchers on my bench weighed 2/3 as much as the top, they are 8/4 by 9" by 40"~, combine that with the heft of the feet and legs and the base is heavier than the top, lowering the center of gravity.

[Maurice Ungaro]

Wider is better, as far as the base is concerned. Short of going BACK to what you had, you could take a tip from the lathe crowd: run a shelf between your two stretchers and add weight to the shelf. My first, and soon to be secondary bench kinda looks like yours, and it's tippy when planing across. I don't know why people enjoy an overhang of their benchtops.

[Tom Vanzant]

Before you reconfigure the base, try shimming under the leg that "bounces". It sounds like your bench feet do not meet the floor evenly. 4"x4"X1/8" pads under the ends of the feet will help bridge floor uneven-ness, but try the shim first.

[Charles Bjorgen]

My bench setup is similar to yours. I'd recommend installing pads under the four corners of the feet. I used pieces of 2x4 for that purpose. You can also make a shelf to span the stretchers for tool storage. Instant weight.

[Pat Barry]

Before you reconfigure the base, try shimming under the leg that "bounces". It sounds like your bench feet do not meet the floor evenly. 4"x4"X1/8" pads under the ends of the feet will help bridge floor uneven-ness, but try the shim first.

This is pretty much what I was thinking as well. I do not think the cantilever is overly large and I do not think it is the reason for your problem

[Dave Anderson NH]

One thing that no one else has mentioned Matt is extending the stretchers and moving the right hand legs to the end of the bench which removes the cantilever from that end. This would have no impact on the location and use of your face vise. From your photo it appears you could pick up almost another 12" and thereby increase the stability. I also agree with Chris Hachet that the front legs should be flush with the front edge of the bench. You could either leave the rear legs recessed to allow full length clamping or make the rear legs flush too at your option. Between moving the right legs and making the front legs flush you would pick up quite a bit of stability.

[Richard Hutchings]

I think the OP mentioned that mounting to a wall was out but I just wanted to add my 2 cents. I love having my benches attached to my walls. I get zero movement and I can make my benches out of lighter/cheaper material. What's a couple of holes in the wall of a shop matter? It's also pretty easy to put lags in your floor. The only issue I see is that you can't rearrange it easily. Choose a good spot, bolt it in and enjoy a rock solid bench.

[Matthew Hills]

My initial thought was the physics -- base too short for the load applied across the top.
Might be able to keep things down with enough weight on a shelf.
Or maybe modify the base to provide a trapezoidal support. (e.g., have the back legs closer to the ends than the front legs)

Matt

[Thomas Schneider]

Thanks Brian. I'm not sure how much heavier can they get? I needed 30kg of steel to keep the bench stable... or is stiffness in play here too?

how is what I did different to this? http://www.google.co.il/imgres?imgur...d=0CD0QMygUMBQ

The bench in the link you provided has toenail feet. On your bench the foot is a flat board. Not sure if that's part of your problem or not but worth a second look.

Tom.

[Dan Clark]

The bench in the link you provided has toenail feet. On your bench the foot is a flat board. Not sure if that's part of your problem or not but worth a second look.

Tom.

I agree about the flat board. From what I can see, it looks like there is limited support for front to rear racking forces. As a test, the OP might want to consider vertically lag-screwing an 8/4 X 8" wide board on the outside edges of the legs to connect the bottom of the front and rear legs. It won't look pretty and it's probably not permanent, but it's a quick and dirty way to see if that is the cause of bench instability.

Regards,

Dan.

[Chris Hachet]

I would push the legs back out to the corners, actually. The face vise could work inside the left front leg rather than outside of it. My MINI Cooper S is a great handling car, the wheels are pushed by the designer to the far outside corners. My bench is about the same size as yours, but the legs are near the edges of the bench...stable as a rock...like yours was....

And here is one area where I agree with Chris Schwartz. The legs really should be even with the front and back faces for clamping options. Much easier to work that way.

I know you said you had no time to fix this....in the mean time, add a shelf on the bottom and dump some sand bags on it perhaps.

Just my two cents.

Chris