Bryan_H

Few custom cages are mandrel bent if any. A mandrel bender is a large expensive piece of equipment that does not reside in many race shops that build cars for amature racers. Manufactures of madrel equipment do not reccommend them for this use. A mandrel bender is used to keep the inside smooth, round and sized properly as in headers. It thins the wall in every bend due to the way it opperates. A mandrel (round object) is pulled through the tube as it is bent and the tubing is streatched over it. I think the only place I have seen that requirement is in the NASA rules. Therefore my question of same.

BH

Bryan Watts

Good point. You're right in that a true mandrel bend using a mandrel bending machine isn't likely what NASA means. With respect to cages, I had always considered a "mandrel" bend to simply be a bend that didn't crush/buckle the tube...that seems to be how it is commonly used around racing referring to everything from cage tubes to exhaust tubes, whether it's technically correct or not. You're right in that very few, if any, cages are built with an actual mandrel bender because the radii aren't tight enough to require a mandrel inside the tube to prevent crushing and no one is worried about flow restriction inside the cage tubing. I've even seen pretty nice "mandrel bent" exhausts that weren't bent with actual mandrel benders either because the bends weren't tight enough to require it...noted, even though it's not crush bent, there's certainly more flow restriction than if the tube had been mandrel bent.

I understand what you are saying now. Should probably be cleared up in the rules.

fatbillybob

I build my own cages and have done it 3 times. I'm not a pro but I can read engineering books and learn much from real pros like Redlineman here on Rennlist. I fabricate my own parts at times. I own plasma cutter, a MIG and a TIG welder and a bunch of other junk.

You can MIG weld cromo but it is better to TIG it mostly because MIG will make a larger heat effected zone. Normalizing and stress relieving crmo has been misused in the purist form on the terms and what you really want to do is temper it. Cromo is inherently more brittle and harder than mild steel. Mild steel is tougher. TIG welding leaves a wacky martensite which when tempered reduces the added embrittlement of an already naturally brittle cromo. You don't want your cage to fracture near the joints. You can temper a weld joint by localized heating to 1200 degrees F for about 1 hour per inch of metal thickness or just a couple mintues in our thin tubes and then let the joint air cool. this has been mitakenly called normalizing it is really tempering. Because our tubes are thin many including Lincoln believe we do not need to do this process but it is easy enough to do if other structures are not in the way to burn.

I can fabricate cromo cages but I will always make DOM 1020 mild steel cages. I think the properties of mild steel are better for fabrication and better for actual crash worthiness. It has all the best properties you want in a crush structure. The only reason to use 4130 cromo is if your rules allow smaller tubing thus lighter overall cage. 4130 is really all about being light not strong. This is different from cromo in a suspension arm which sees a totally different action than a crush structure. Both are strong enough for the application and the argument of which is better is really moot. Its about weight.

I mandrel bend my own tubes with a model 3 JD2 mandrel bender that costs about 500 bucks. If you measure anywhere along a JD2 bent tube you will find that the ovalization of the tube falls within spec for size consistency with SCCA for example. You may be able to do better with a $20k hydralic bender but maybe not. JD2 is just fine. All bends in a proper cage design are supported by supplemental tubes anyway. Bends while weaker than straights are actually stronger in practice becasue these areas are always fortified.

MIG welding a mild steel cage is way easier to fabricate than TIG welding cromo or TIG welding mild steel because access of the MIG spagetti shooter gun can get in tight places one handed where the TIG torch and second hand of filler rod cannot. So unless you plan on removing your window glass and remove your roof to gain proper access for welding and proper tube placement as close to the chassis interior as possible stick to a MIG welder for easier access.

Another important reason for mild steel over cromo is that your fabricator needs to be better. He needs better preweld fitment of tubes with cromo TIG welding. Will your pro take the time needed to do this or will problems be hidden under the weld? Mild steel still needs good fitment. The closer the fitment the better but with mild steel joint fitment is more forgiving. Do you or your pro have the handskills to do that?



Unfortunately many pros are just robots who have done the same cage elements for 20 years and have no clue as to why. There are very few pros out there like our Redlineman who think about this stuff and are constantly learning to do it better. I'll see 20 cars with unsupported NASCAR bars all fabricated by pros. IMO that is the same as medical malpractice! That is just one example of great potential harm done everyday by a pro. So even when you work with pros you still need to do your homework and ask questions. Even if a pro does it, may not make it right. For those not doing your own work choose carefully...do you own homework!

DWalker

good comments

Jeff Lamb

I have really enjoyed reading this thread for (a) the debate surrounding the material choice for the tubing and (b) the feedback regarding use of proper fabrication techniques (mandrel bends, pre-weld joint clearances, tempering, etc). However, I noticed there were MANY postings regarding the importance of the proper design of the roll cage. Therefore, to focus on that topic for a little while, I started a new thread titled "Proper Roll Cage Design?" that you can find here:

https://rennlist.com/forums/racing-and-drivers-education-forum/487266-proper-roll-cage-design.html

Jeff

tkerrmd

thanks all again for the input, my cage is being started tomorrow!! interior is gutted!!

kurt M

Roll bar tubing is most often bent using a die and follower bender. The die looks like a 1/4 pie cut with a 1/2 round groove in the curved side. The follower is a rectangle with two 1/2 round areas machined that force the tube into the die as the die is rotated. The tube is held by the die and the slides along the follower as the die moves.
There is a lot of good info but there are no $500 mandrel benders that could bend a roll bar tube that I know of. $5000 well used perhaps. As was said a true mandrel bender has a ball on a long rod that is held in a fixed place inside the tube right at a set bending point while the tube is forced over a die using a follower. The ball supports the tube wall and prevents it from crimping or deforming out of range.
Crush benders are large mass production machines most often used for mass produced items and bending non round stock. I am not well traveled but have never seen a crush bent roll bar tube. I have seen some ugly roller and shoe bent stuff. Perhaps you are thinking of roller and shoe bent rather than the pictured 3 die compression formed tube as shown? The roller and shoe method uses a hydraulic bottle jack to force a die against a tube while two rollers on ether side push against the back of the tube on ether side. Inexpensive machines but tend to deform the roll bar tubing into an oval in the bend zone and leave deformation from the tubing moving under the two rollers. This is a banned method in most venues.
BTW the second tube in the example of crush and mandrel? bent tubes would not pass most rules as the radius is too small for the tube diameter.

Die and follower is a well regarded method for bending the tubing used in cages. Good enough for good results human powered machines run $300 + another $150 per die size. Add $500and up for motor power.

Mandrel benders are very expensive and not well suited for roll bar tubing and are far more in line with thinner wall header type tubing or in computer controlled high number manufacturing.

Shoe and roller benders are pipe and electric conduit type benders and are ruled out my most venues.

Crush benders are not common at all and are normally used in large scale production of non critical parts like lawn chairs and downstream exhaust piping.

Racin44

The bender type which is described above, as well as the type of bender that the JD2 mentioned is, are called rotary draw benders.

Best Regards,
Nick Riefner

Phokaioglaukos

Please describe in a bit more detail.

DWalker

He means that the "NASCAR" door bars will not be attached to the sill or frame of the car, nor have an inner set of bars connecting from main hoop to front hoop down tubes (yes, thats right, a second set of "door bars") allowing them to buckle inward in an impact.

Bryan Watts

The biggest problem I see are NASCAR bars without any sort of load path for the upper bars...they depend on a long straight tube (the lower portion of the B-pillar bar) to support them rather than having additional bars to transfer the load to the rear of the cage structure as you typically see with an X type setup.

fatbillybob

My bad phraseology. If you go to a random HPDE and look at cages you will find NASCAR bars all the rage. Everyone has them. You could easily go to your favorite home track this weekend and see 20 cars with NASCAR bars and none of those bars supported at the sill plate or with a sill tube. That's the point I was trying to make. To put it another way...I'd bet you lunch the next car you see at a random event of open class cars with NASCAR bars does not have the bars supported at the sill plate or supported by a sill tube.

fatbillybob

Well I'd love to have a $20k bender but I'm not a pro. I also have not seen a pro cage better than my own cage. Here is the JD2 bender and it is a mandrel bender not a crush bender and it will keep the tube within the SCCA spec for ovalization and distortion of the tubing. http://www.jd2.com/ I use the model 3 and it works!!!

kurt M

I have just about the same bender. Mine works great but must be missing the internal ball that is part of a true mandrel bender.

This is a small mandrel bender. Note the long bar that sticks out one side. The tube to be bent is slipped down over this bar and bent as it is drawn off of it. It is a bit more expensive than our plenty good enough benders. Sorry for the terminology tangent.

Bryan Watts

The point, as has been discussed, is that while that is referred to as a "mandrel" bender in racing circles with regards to cage building, it's not really a mandrel bender at all. A mandrel bender actually uses a mandrel.