Project Pure Performance E46 Time Attack

by Mike Kojima

In the pages of MotoIQ, we have partnered with Pure Performance in Laguna Hills California for many of our BMW projects. Why?  Because Pure Performance, eats sleeps and breaths BMW performance both on the street and through racing with customer cars and their own shop race car.

We will be collaborating with Pure Performance on the building of a RWD limited class car for Superlap Battle.  The car of our choice is a E46.  We choose the E46 because, well, we have always loved the styling of the car! The fact that the E46 chassis is coming down in price on the used market and Pure Performance just happened to have a straight stripped shell lying around sealed the deal.  

In the first stage of the cars construction we will be approaching the building of the backbone of any car, a nice all inclusive roll cage. For any race car the cage is the heart of the chassis.  For proper suspension tuning, the chassis must be as rigid as possible. This is for two reasons, the natural frequency of the body structure must be higher than any natural periodic vibration that can be induced into the structure in operation. If the induced vibration is the same as the natural frequency or a low order harmonic of it, the vibration can build and become a force that can cause tire shock. The body of the car is a huge undamped spring that if allowed to flex freely, can add uncontrolled energy into the suspension itself causing tire shock and loss of potential grip. 

The other reason for a good cage is to protect the meat sack behind the wheel in the event of a crash.  A good cage can keep the drivers compartment intact in a severe crash, yet have some controlled compliance to help spread out the force impulse to protect the driver.  A good cage should also be able to absorb multiple impacts and rolls.

Take a look and see how our cage came out as our car progresses from a bare shell to a hopefully competitive Time Attack Machine.


Our cage ties the entire unibody together from front to rear.  It attaches to the chassis in many points and ties all of the suspension pick up points into its main structure. Our cage is mostly made of 1.75" 1020 DOM tubing with a 0.095" wall thickness. NASA mandates the use of DOM vs the cheaper ERW (Electrical Resistance Welded) tubing.  DOM stands for drawn over mandrel and is a manufacturing process where a steel strip if rolled into tube shape and the seam electrically welded.  If you were to use ERW tubing, the process of making the tube would stop here.  After this the tube is drawn over a mandrel to cold work it into it's final dimensions. The cold working refines the steels grain structure, aligning the metals crystal lattice internal structure improving it's mechanical properties. It also gives the tube more exact internal and external dimensions, important when the minimum wall thickness is mandated by a sanctioning body. 
1020 denotes a mild steel, high carbon steel alloy. This is considered to be a high strength steel with a tensile strength of about 85,000 psi.  We prefer 1020 to super high strength alloyed steels like 4130 chrome molly because it is less sensitive to embrittlement from welding heat, does not require post welding heat treatment for full strength and is more ductile, allowing it to absorb more crash energy and take multiple hits without failing as easily. Some sanctioning bodies like NASA and NASCAR mandate the use of  1018,1020 and 1026 DOM for these reasons.  For our cage starting at the front you can see that the front legs have base plates that go to the floor as far forward in the chassis as possible that also wrap around the frame rails.  A strong dash bar ties the legs together.
Page 1 of 4 Next Page
Bookmark and Share
Wednesday, January 27, 2016 4:54 AM
Starting with a road-car chassis that is known for its inherent stiffness is another good reason to have chosen the E46. I’m looking forward to seeing more of this build.

Good use of the factory opening that was originally for the HVAC inlet at the upper portion of the firewall. It seems to me that the structure could have been better if the dash bar connections were “crossed” to the strut towers: left-side dash bar connection to the right-side strut tower, right-side dash bar connection to the left-side strut tower. Of course, they couldn’t be entirely straight, since they would interfere with the rear of the engine (inline 6-cylinder I assume) but the bends would be far less severe than the current bends as seen in the first picture on Page 2. Also, since the two bars would cross in the middle, that would introduce yet another point for triangulation. Not pulling from the quality of craftsmanship or expertise of the build; just seeing a possible point of improvement.
Mike Kojima
Mike Kojimalink
Wednesday, January 27, 2016 7:31 AM
The reason the bars are like that is because of a possible engine choice!
Wednesday, January 27, 2016 8:07 AM
This cage looks better built than one of your last projects, but you still have a ton of dead nodes in this build. Especially on your main B hoop, many tube connections with nothing to directly support it. I don't understand why you allow this.
Mike Kojima
Mike Kojimalink
Wednesday, January 27, 2016 10:00 AM
What would you propose to do differently?
Wednesday, January 27, 2016 10:29 AM
Have all of the tube joints meet at a central point instead of being spread all over the surface of the tube. Spreading them out creates shear points, or will just bend the adjoining tube when impacted. Every joint should be supported on the opposite direction to the exact same location (as much as possible). Look at where your shoulder bar meets the B hoop, every tube that meets is spread all over that area. They should all meet at a common point to truly transfer and share the force of an impact.

Stolen from someone who sounds smart: "Nodes are where tube meet, you want your tubes to meet in as few places as possible so the force is concentrated in a strong area instead of spread out among several weak areas."

Should look like this: http://www.pirate4x4.com/forum/attachment.php?attachmentid=434702&stc=1&d=1239247705

Not like this: http://s1280.photobucket.com/user/Tyler_Knauer/media/Mobile%20Uploads/2015-03/3DB3D433-2B2C-435A-8F07-BD375A87CEE3_zpsdhsayylc.jpg.html
Wednesday, January 27, 2016 10:53 AM
Thanks for going into metallurgy once again Mike! As a welding engineer that has built tube chassis and unibody cages proper materials selection makes a big difference.

A few questions as I am always trying to learn. It looks like the photos are of two different chassis based upon the a-pillar downtube angles and where they hit the floor. Is one car going to be a pedal box/extended steering column car?

How did the fabricator manage to get to the backside on some of those joints? Specifically the lower door bar and FIA bar? (second photo down on page 3) I had a similar challenge in my WRX built to Rally America/FIA 253J specs and I had to push the forward lateral in towards the transmission tunnel enough to get a 360 weld on it. It's the only part of my cage I dislike fitment on.
Mike Kojima
Mike Kojimalink
Wednesday, January 27, 2016 11:16 AM
Thank you for the detailed answer!
Wednesday, January 27, 2016 12:00 PM
Are these two different cars or did you make a decision to change the door bars?

Why did you go with the new design?

Just curious as I've always seen the cross bars done as they are in the last picture and when the taco gussets are mentioned. But in all the other pictures, it shows the downward cross bar terminating in to a length running bar.
Wednesday, January 27, 2016 12:13 PM
More good Nodes:


Here is another bad Node:

Wednesday, January 27, 2016 1:53 PM
That's a lot of triangulation. How much does the cage, by itself, weigh?
Wednesday, January 27, 2016 2:30 PM
Is the two door E46 still heavier than the four door version? Because the E36 two door uses parts from the convertible which make it heavier than the four door. Or have they changed this with the E46?
Wednesday, January 27, 2016 10:31 PM
Nice to see a close conforming cage directly seamed to A pillars. I have always done this and believe it is the strongest method (although I have no scientific results or proof apart from many alive drivers after very serious offs) So often though customers want gusseting because 'it looks cooler'.
Wednesday, January 27, 2016 10:37 PM
Although I'll point out, My own method involves the A pillar bars conforming VERY close to the unibody A and B pillar and has substantial amount of seam welds the entire length, not just one or two.
Mike Kojima
Mike Kojimalink
Wednesday, January 27, 2016 10:44 PM
Cage weighs between 140 and 150 lbs
Thursday, January 28, 2016 8:16 PM
More blasphemous engine swaps please!
Friday, January 29, 2016 12:46 AM
@Mike Kojima That's not nearly as heavy as I thought it would be. The extra triangulation and supports make sense. Looking good Sir. Can't wait for more updates!
Micah McMahan
Micah McMahanlink
Friday, January 29, 2016 8:06 AM
Speaking of dead nodes...

I understand the want to gusset the corner of the windshield 'box', but that bar beetween A-pillar and door bar...At the least, they should have carried that back down to the node at the floor. Then at least it would be loading something other than putting the door bar into bending. That longer bar would also help with vertical loading.

However that bar usually runs to the top of the A-pillar.

I always liked how this cage distributes loads across the A-pillar and halo via a triangle. Actually from your own site. It was based off of the later GD chassis WRC cars (don't recall specific gen).
Saturday, January 30, 2016 10:49 PM
I believe in general you should slice the chassis into X number of lateral planes. You make these planes slice through critical structural areas, for example front and rear shock towers. You build a stiff, triangulated structure along these individual planes. After that, you begin to tie the lateral planes together with longitudinal triangles. Every cell of the structure should be a triangle. From there you can build gussets, etc

But, if you really want to get some real understanding of how to build a cage, you have to do some simulations. So you really have to build a simplified model in a FEA program like ANSYS.

Your Image
Miguel Yetman
Miguel Yetmanlink
Sunday, January 31, 2016 7:53 AM
@Htpzt Sedan E46s are 50% more stiff than coupes and weight the same. BMW has outright said this
Post Comment Login or register to post a comment.

MotoIQ Proudly Presents Our Partners:

© 2018 MotoIQ.com