Astroman

For a 2700 lb 911...

Is the thinner/fatter 1.75" tubing "better" than the thicker/thinner 1.5" tubing?? Is there a weight difference??

I'm trying to get a consensus... Thanks a lot-

M758

I use 1.75x.095 tubing. I think it is stronger and lighter than 1.5 x.120. Some perfer the 1.5 tube since it can be bent aroud a tigher radius and gives a tiny bit more clearnance due the smaller diameter.

Do check the race or rules to make sure you use legal tubing. It is also to cross refernce PCA,POC, SCCA, NASA specs. I believe they are the same, but it is always a good idea to have cage legal for muiltipe sanctioning bodies.

Cory M

The 1.75 is 5% lighter and is stiffer but it is easier to build a tight cage with 1.5 which is very important if you're a taller driver. Most regs require minimum bend radius of 3x tube diameter, so 1.5 tube allows a 4.5 bend and 1.75 tube allows 5.25 bend. You can build a good cage with either tube.

Matt Marks

Depending on sanctioning body - you may need to use chromoly if you use 1.5 inch tubing. Last time I built a PCA car (some years ago) it was 1.75 mild steel or 1.5 chromoly. Chromoly and mild steel weigh essentially the same, so the weight savings was all down to the smaller tubing.

However, chromoly is probably 30% more expensive for the materials, and will cost more to weld as it needs to be TIG'd (or is it MIG'd?) The welding experts on the board will chime in with more detais....

With NASA, the cage requirement was (from memory) 1.75 mild or chromoly for cars weighing up to 3000 lbs - so you might as well run mild steel and save the $$ if you're building a car strictly for NASA.

YMMV if you try to run a PCA legal 1.5" chromoly cage in NASA. I know people do it, but I don't know what the hassles are...

2BWise

From a structural standpoint mild steel and chromoly have the same stiffness, so with chromoly you have to pay more but get the same performance benefit. If I'm not mistaken it is imperative to stress relieve chromoly where as with mild steel you really don't need to, which would increase the labor cost of a chromoly cage.

Matt Marks

Agreed on the stress relief - otherwise the welds can be brittle.- no idea how it's done OR if most cage builders do it...always wondered.

Larry Herman

Interesting question. I know from bicycling that larger thinner tubng is both lighter and stiffer. The downside is that it is more easily dented, and once dented more prone to structural failure. From that it would seem that a thicker walled tube, even if smaller in diameter might be better to resist bending due to impact, but I too would like to hear from someone who has experience in this. Maybe Chris Walrod might know.

kurt M

Under 1/4 inch thick CrMo does not require heat treatment. Road racing roll cage tubing is all under 1/4" thick

Cory M

In this case the "thin walled" tubing is still .095 thick, and it's steel not aluminum so you don't have to worry about dinging it like a Cannondale bike frame. The 1.75x.095 tube is theoretically a little better than 1.5 (5% lighter than 1.5x.120, a little stiffer, a little more weld area), but in practice you may find that you can get a better fit with 1.5, especially if you are a larger driver. You can make a safe cage out of either size, the ability of your fabricator is going to be the biggest factor so I would go with the size he prefers.

Chromoly is a whole differenet discussion...

2BWise

Is that rule dependent on tube OD or is it steadfast for all tube sizes? I spent a lot of time heat treating 1/2" and 5/8" OD .035" wall and will be disappointed if I wasted all that time.

Weight and stiffness between 1.75 and 1.5 is dependent on the cross sectional area. The 1.75 is lighter by a slight amount and will be stiffer in bending.

Bill Lehman

I'm omitting some details but in general when used as a column (the upright portion of a roll bar) the radius of gyration (r) is important, larger is better. When used as a beam (the top cross member) the section modulus (S) is important, again larger is better. Comparing 1-1/2 x .134" wall (1.95#/ft) versus 1-3/4 x .109 wall (1.91#/ft), The 1-3/4 tube has an r of .58 and an S of .21. The 1-1/2 tube has an r of .48 and an S of .18. The 1-3/4 tube is better in all respects.

mark kibort

most all sanctioning bodies want the 1.75x .095 bar tubing if your car is over 2500lbs. some have variations, allowing other tubing for bars attaching to the main hoops, but 1.5x.120 will kill your chances for getting tech'ed with SCCA for sure.

mk

Astroman

Unless I'm reading it wrong (which is definitely possible), PCA, NASA, and SCCA rules all allow either 1.5" or 1.75" for all cars under 3000 lbs...

mark kibort

PCA/POC/PRC states 1.75", 1.5" only for cars of lighter weights.
SCCA has no flexibilty here. Its all 1.75, unless its a factor race car like a Cup car. We are talkng Sports cars.

mk

fatbillybob

You have to read the rules of who you race with. Most allow 1.75 095 up to 3000lb and 1.75 120 wall 3000 and over and use those sizes as minimums. If you read the lincoln welding website they say as redline man said less than 0.250" cromo does not need stress relieving. One big advantage with larger diameter tubing is more weld surface area which is especially good if you are not a pro welder and or if your fit up is not as good as it should be. Similarly thicker tube can be better as some make the error of blending the fishmouth on tube connections and actually thinning the joining metals. A cage is one place where a few extra pounds is more likely to be good rather than bad. Personally I also don't see a problem with up sizing the thickness on say mainhoop and driver doorbars, driver "A" piller and go thinner at legal minimum on other supports.