I've been looking at roll cage design, including the MSA handbook. All of the roll cages I've seen fasten only to the floor. Why is that? I'd have thought there would be some value in fastening the cage to the B pillars and perhaps the top of the A pillars. It would stiffen the car up no end.

I'm not planning to use my car competitively. It will go to a track day or two but will be for use on the road. Is there a reason to not fasten the cage to the top half of the car (if you don't need to meet MSA rules)?

Not sure on the msa technicality's but a fair few cars are tied in to the windscreen pillar. If I'm not mistaken the wrc cars are done like this.

http://i953.photobucket.com/albums/ae15/Rally78/Rally%20Build/47-July2010.jpg

Mark.

Yup, lots of cars attach to the A pillar, they are close to the drive and take the brunt of forward moving rollovers.

Regarding tieing to B pillars, good question. The high end roll cages that immediately come to my mind triangulate back from top of B pillar to rear floor. Maybe this non vertical angle reinforces the cage instead of letting it fold.

Rally cars and major circuit cars will have it. For extra safety. But it adds a lot of stiffness to the chassis so in most lower classes and formulas its considered the extra safety and cost is less important than the performance increase it gives.

Some sanctioning bodies try to prevent the cage from adding any significant amount of chassis stiffness, therefore they limit the number and location of attachment points.