Crush Tubes are a more than simple piece of hardware, but very necessary to building many of the cars we’re involved with. Best part is, they are adaptable to a variety of applications.
There’s probably a better name or a more official name, but Crush Tubes is what I call them and people usually know what I’m talking about when I mention them. They’re easily made on a lathe and if you have a drill press that drills square you can make them with that. All that’s required there is a reasonable amount of care in setting it up. Just so you’ll know what Crush Tubes are, here’s a photo. They’re the two pieces sitting next to the Yuban coffee can.
The ones pictured are 1" OD and drilled for the 9/16" bolts that retain the 31's front hairpins. If you don't have access to a lathe and don’t care to drill them in a press, it shouldn't be too expensive for a machine shop to drill them for you on a lathe.
Either way, a simple procedure and not too exotic. They do look simple and they are, but there’s always a few touches you don’t think about until you’re in the middle of making them.
Even so, no problem.
Kind of a self-explanatory deal as you go along. Here’s how:
- Using 1" cold rolled stock in this case. Other sizes are ok depending on what you’re doing, but 1" works for most all car things I’ve been involved with.
- Cut off the length you want. Flush works ok - flush defined as 2" long for a 2" thick frame.
Better is 1/8" - 3/16" over length. The over length keeps the bushing washer off the frame and protects the paint during installation and use.
- Here’s a photo of the radius rod bushing standing off the frame. About 3/16" in this case.
- Clean up the edges with a file. Knocking the sharp edges off will suffice for right now. Take em in to the machine shop and get em drilled or knock them out in your drill press.
- Make a mandrel out of the bolt size you're planning to use. Said mandrel being simply a NF bolt, 4-5-6" long, whatever it takes to have a reasonable length of bolt that's not threaded. The non-threaded section is clamped in the drill press chuck. You'll also need enough threads to span the distance of the crush tube.
- Hacksaw the bolt head off, clean that up - sharp edges etc. Put on a nut, spin it down to the bottom of the threads or where convenient, add the crush tube, install and tighten the lock nut.
Clamp the loaded mandrel in a drill press, fire it up and round off the edge with a file.
- Reverse the crush tube and do it again. Don't get carried away with side pressures on the drill press. For a couple of reasons.
- They're not really designed for side loads, but a reasonable amount won't hurt.
- The other reason, heavy side loading can flex the drill press arbor/quill taper fit enough that the arbor drops out of the internal taper crush tube, chuck and all. Gets kind of wild and weird when that happens.
You could use common 1/8" wall tubing, but the bolt is not centered as it should be and I believe that such an arrangement would be very prone to working loose during use.
Drill the required frame hole with a 1" hole saw.
The hole saws do a neater and more accurate job than a 1" drill. The bigger drills do ok in thick materials, but drilling the typical .120" frame wall thickness the 1" drill will dance around and make an overly large hole.
The nice part about the hole saws is that the fit is so close, the Crush Tube gets tapped into place by a soft hammer and stays until welded. In fact, it stays in place so well, you can leave it as is during the mock up and once the final decisions are made you can weld it in.
I like the Starrett bi-metal's usually available only at precision tool places or a machinist's supply outfit like MSC. The bi-metal hole saws cut stainless instrument panels just fine. Milwaukee makes an excellent bi-metal hole saw and these are perhaps more easily obtained.
I get mine at Home Depot and Orchard Supply. The Starrett’s and Milwaukee’s use the same arbor pattern. The bi-metals are designed for metal cutting. Use a cutting fluid and reasonable speed and pressure. Don't be too light on the pressure or you'll just be rubbing and not cutting the metal. Which results in quickly wearing the hole saw out.
I've had good luck using the Black & Decker hole saws for wood. These are non bi-metal. I bought two for cutting the frame of the 32 for the upper four bar, used 30 wt for the cutting fluid and one of them cut four holes just fine. I bought two because I thought they'd wear out fast. Even so, I much prefer the bi-metals. When you cut the frame holes make yourself a drill-block so you can accurately drill the pilot holes in both sides of the frame. Said drill block being simply a chunk of perhaps 1" thick aluminum - steel works fine - drilled pilot drill size, clamped in place and used to assure a squarely drilled hole with both sides matching. Pilot drill 3/16" - 1/4" or so. If you don't use a drill block the frame holes will be off line.
Here’s a picture of a drill block.
When dealing with the swoopy frame of the 32's and you’re drilling in the area with the reveal line, or whatever you call the stamped indent that matches the flow of the front fender, do the drilling for the pilot holes from the inside. Using the flat boxing plate and the drill block makes for an accurately registered hole on both sides.
This photo shows the Crush Tube on the 32 exiting right through the reveal line.
You absolutely need a crush tube when you bolt something down across the width of the frame rails. If you don’t use one, the nut will become be shortly after installation, even if it's a nylock, and repeated tightening will crush the frame sides in.
Here’s a picture of the rear axle installation in the 31.
The rear axle, the popular Ford 9" with a SoCal main leaf and several leafs out of a 40 Ford spring pack. What I want to point out here is that Crush Tubes can be used in other applications. In this installation, four of them were used as a mounting point for the bolts that clamp the rear leaf spring to the rear crossmember. All of them set into a ball milled notch in the rear crossmember to gain some room for the rear panhard bar. Not much was gained, but it was enough so the panhard clears the pumpkin housing by 1/8" horizontally at full suspension compression. With the transverse rear spring, rear shocks, panhard bar and sway bar things get a little crowded and it was a bit of a packaging problem.
If you look close you’ll see the location of a Crush Tube - under the bolt on the far left - on the rear crossmember. This one spaces the rear shock toward the front of the car inside the rear crossmember. Granted, you can’t see the 1" or so that the Crush Tube sticks out, but it makes for a convenient spacer. At the bottom of the shock is a removable stainless steel Crush Tube - which in this case probably ought to be called a spacer - that spaces the rear shock bottom toward the back of the car so as to align the shock for proper vertical travel as well as have it in the right place to match the upper offset. There’s one more Crush Tube in this picture, it’s under the bottom of the rear crossmember and sits in a milled notch. That one done to get the frame end of the panhard bar up as high as it would go. It would have been easy to drill the rear crossmember and insert a Crush Tube, but in this case there was a conflict with the sway bar that’s centered on the rear crossmember. On the right hand side, you can see the floating end of the panhard. The panhard bushing is bolted to a Crush Tube welded to an angle cut piece of 1" x 2" x .120" wall tubing that is gusseted. The two overly long 5/16" bolts hanging down clamp the sway bar pillow blocks in place. Last, but not least the lower spring clamps. Made out of a piece of 1/2" x 1" cold rolled with the top edges slightly rounded so they don’t dig into the main leaf.
The nice part about using Crush Tubes for mounting the spring is a simple change of bolt length will allow leafs to be added or subtracted as desired and have enough threads left to use a nylock. No nylocks in the photo, got em, but no use wasting them during the mockup process. As you can see, Crush Tubes are easily made and you’re only limited by your imagination as to where you can use them. Make no mistake though, leave em out, especially on a suspension/frame juncture and you’re going to end up with little popping and shifting noises and find yourself tightening bolts all the time. As well as watching your formerly pristine and flat frame rails develop an interesting dent where the bolts are.
Kind of like the bottom of an oil can.
But not near as useful....