Designing the Satchell Link

[911industries]

Here are some pics of my problem and the solution. I've done a couple of drags since I installed the diagonal braces and everything is looking good.

The first shot you can see my crossmember with the break on the left side of center. If you look real hard you can also see how it buckled the floor pan on the left hand side. My idea was that the floor pan would help with the structure. Turns out it helped until the crossmember broke.

Second shot is inside where the rear seat normally is. You can see my diamond gausetts. Also behind the crossmember is a crack in the floor that was also caused from the broken crossmember.

Third shot you can see the diagonal braces. Outside where they connect to the frame rail are connectors to the rocker and roll cage. One thing that isnt pictured is another diamond gausett I put spanning the two center welds. I actually broke it a second time after I installed just the braces.

[sik68]

Sorry, I have nothing to contribute...but everytime I see this thread I think of this:

[911industries]

Okay guys I'm back again. I'm pulling getting ready to pull out the drawing board and do some redesigning. Since I last posted I've moved across the country, over to Savannah, GA. I finally got around to installing a custom sway bar which made a huge difference. Problem is I'm afraid to abuse the car since that crossmember keeps cracking. I installed gaussets from the diagonal bars spanning the link mounts.

Since I have done so much strengthening of the crossmember and it keeps failing I think my design is flawed. An important thing I think is that I back braced the axle which increased stiffness. I think that my design binds the suspension when the body rolls. I have tried to test it with the coilovers off and it seems to roll freely to a certain degree.

I'm just so stumped on why I can't make it work. If anyone knows a suspension guru let me know. Otherwise once I solve this problem I will surely post it.

Suspension numbers: X-left right Y-front rear Z-up down Zero is center vehicle on the ground

X,Y,Z
ULF- 15,22,15
ULA- 15,0,15.5
LLF- 2.5,16,11.5
LLA- 18.5,0,10

[Norm Peterson]

The floor pan buckled because without (more) fore/aft stiffening it can't carry the loads that you are developing during a dragrace launch. Once the floor buckles elastically, the crossmember has to carry essentially everything, which it apparently isn't able to do. Once the crossmember cracks or otherwise gives, the floorpan buckle is likely to become inelastic, or permanent. Like I mentioned in post #39, you need more fore/aft stiffening, and it needs to tie in to more than just the new crossmember.

A Satchell link, like any other triangulated/converging 4-link, does not inherently "bind", at least not over relatively small deflections, as long as all eight of the link pivots are sphericals of some sort. As soon as you make any of the end connections "cylindrical" where the pivoting is forced to be about an axis that is a fixed line in space (the bolt) rather than a point that can rotate freely in any direction (ball & socket), the stiffness of the cylindrical bushing material starts adding stiffness that resists this motion.

Off the wall question - are you pulling the LF tire on your drag race starts? If so, the car starts rolling over much faster in response to the engine torque reaction, and it all piles up out back (since the front end is no longer resisting much roll).


Norm

[Norm Peterson]

The first part is some failure thoughts based on having a little more information - the buckling in the floor tells you that the chassis side attachments for the lower links are moving too much under the acceleration load.

The second part is about four link suspensions not having to "bind" as long as the end connections are sphericals (at least for small movements in bump or roll).


Norm

[911industries]

Okay, I've done some testing and I think I've figured it out this time. Just to clarify things, the floor pan is not part of the structure, it just happens that its in the way. The crossmember is 2x2x.120 tube and where it had to be forced me to put it through the floor. I welded the floor to it to just hold the floor in place. In post #41 you can see the diagonal braces that are welded in to solve all the for/aft forces. Those braces go from the frame(same size as crossmember) to the crossmember.

Now onto what I've found. I did another check to find bind and couldn't find any. With nothing but suspension links attached the axle articulated nice and freely. Bind is out of the question.

I had a helper take a couple pry bars and put a load on the axle to simulate the rotational forces from acceleration and deceleration. Heres where I found the suprise. The fore/aft forces on the four link get transferred into vertical loads on the crossmember. Basically when the frame mounts stop the longitude forces they then transfer into vertical forces. On top of that the crossmember is broke from braking forces and not acceleration. That is probably why I didn't figure this out earlier. It was crazy to watch a braking force(pinion trying to look at the ground) actually push the crossmember down towards the ground. It moved a lot.

Here is my solution. I'm essentially going to create a structural driveshaft loop. Right now the crossmember goes up and over the driveshaft with multiple welds. I want to use 1 3/4 x .120 mandrel bent tube to connect just outside the lower frame links. I have some more testing to do so I can figure out what kinda strength increase I will get. I got some pictures I'll post in a few.

[911industries]

Here are my test subjects, some good ol balsa wood. My plan is to support them from the highlighted areas and load them in the direction of the arrows. I'm trying to locate a scale to measure the breaking points. I will use the straight piece as a constant and then compare against it. The one in the middle is essentially what my crossmember looks like now. The bottom is what it will look like. The black sharpie lines represent where my lower frame links are.

I think this will give me a good idea if I will be able to fix the problem. Let me know what you guys think