I have the cheaper bolt in ones and I had my muffler guy weld them a little too for added strength. The welds can easily be removed. Most of the Mustang shops actually recommend the bolt in over weld in for a better fit. Then having the bolt in ones welded.

The ones I have fit great, and I can definitely tell a difference in the way the car rides and handles with them and the export brace and monte carlo bar.

 

Seems like these guys know a bit about suspension/chassis stiffening:

http://www.maierracing.com/

http://www.youtube.com/watch?v=5ZJSq1YDDLo

I'll be getting bolt ins from them ...
(I hope to have my 67 Cougar run "just as well" as the car in the video)

 

Get weld in and never have a bolt to check and see if its still tight.http://www.tinmanfabrication.com/index.cfm most of us love his they follow the floor pan better and look oem once you paint them.

 

Why does everyone have such an aversion to bolts? You do realize that virtually all primary load bearing joints in bridges and steel structures are bolted, right? If they're lubed and torqued properly they shouldn't come loose, and if you're really that worried use a nylock nut or a metal locking nut. I've had mine in for years and they've never come loose. Welding certainly won't hurt(if done correctly) but it's not necessary, and bolting will usually distribute the load better.

 

The main reason for welding them in - doing so gives you more total chassis + SFC stiffness than just bolting them in. This is because piggybacking one structure (the SFCs) under another (the existing chassis) ends up giving you more stiffness than the sum of the two stiffnesses taken separately (which is the best that bolt-in can truly guarantee). The closer you get to making it all one structure, the better this gets.

I'm not saying that bolting on SFCs is necessarily bad or inadequate, just that you can do better.

The bridge analogy is a so-so comparison. But it isn't strength we're mainly after here, it's how well the added structure adds stiffness. And bolted end connections will flex/rotate far more readily than weld beads will under the loading that they will be subjected to.


Weld-in SFCs represent a good "stopping point" for most people. But if you're really ambitious and have adequate knowledge and fabrication facilities, you can still do better.


Norm

 

The thing with bolting, is when done properly the force at the point of contact is distributed across the entire surface area of the points of contact(in this case the SFC pocket and SF), whereas with welding it's only transferring force at the area of the weld seam. Yes, welding is less likely to slip(it would need to tear out), but is also more likely to generate flexing since all the force is being distributed across a smaller area. If the holes for the bolts are smaller, to the point of being nearly a machine fit, then the likelihood of slipping is minimal, also when tightened properly the friction between the points of contact also prevents slipping. That's why structural engineers generally call for bolting primary load bearing joints in structures. It's all about load transfer, not what's likely to slip or not.

If you're really paranoid about it, then bolt AND weld, that way the pressure at the joint from bolting still distributes the load better, but when you just weld there's no tension to keep the joint surface area under pressure, so the total load is transferred through the weld seams alone...and the subframe itself is rather thin and flexes easily. It's not just how you connect something, it's what you're connecting.

Welding is good for joints with little surface area contact, since the weld seam is quite strong, the seam takes up most if not all of the contact points between the 2 parts. Bolting is preferred in areas of large surface area contact, since the tension from bolting holds the 2 surface areas under pressure and distributes the load over the entirety of the area and not just at the outside edges where it would be welded. It's not that either one is better than the other, it just depends on the application. In the case of most SFC's they have large surface area contact points relative to their size. Bolting when done properly is plenty strong too, remember that crankshaft main caps, conrod caps and cylinder heads are all bolted components, and when kept under proper tension they don't slip, come loose or walk around, and they're subjected to far more force and vibration than a SFC is.

I'm not opposed to welding, but it seems there's a huge misconception about the strength and usefulness of bolting components, especially with the ready availability of modern high strength fasteners.

 

+1 on the bolt them in than have them welded.

I personally like the Heidt's connectors. They are very high quality and very well built but they are pricier than most.
http://www.heidts.com/index.cfm/page...rod/prd152.htm


Plus they also provide the option for a bolt in chassis stiffener.
http://www.heidts.com/index.cfm/page...rod/prd153.htm

 

When I'm talking about welding something like SFCs in, I'm looking at welding them at much more than just the two end connections. Think SFCs contoured to contact the floorpan at intermediate points or through-the-floor SFCs here. If I'm going to put weight back into a car, I want to get as much function from it as possible.


Norm

 

Welding the SFC to the floor pan as well is certainly a good idea. That way it's tied into the unibody at multiple points, and the load distribution is much better.

 

To your question, bolting is good enough, but proper welding is better. If you weld them, get the ones like Tinman that contact the floor pan the length, not just at the subframe. They are many times stronger.