Chassis Stiffening.. Phase 1

By: Bob Goodson
July 19, 2020
Author: Michael Yount
 
The unibody on the Corolla is vintage ’73 design – which is to say, not the stiffest “box” in the world.  One thing that helps a bit is tying certain bits of it together.  A roll-cage helps a lot — but that’s beyond the scope of this course.  Besides – never a fan of a full roll cage in a street car — where soft body parts could come into contact with really, really hard cage parts in the event of an incident.
 
But, I digress — started the mockup of a bar that ties the upper front strut mounts together and into the upper firewall.  Break out the cardboard templates!
 

Used that piece to nail down the dimensions of that upper strut tower.  The next piece simulates this with a bit more metal that I can bend and gusset to support the bar that runs across the top of the motor.  Here’s a mockup with an old piece of chromoly roll cage tubing actually left over from the original yellow “bee”.

I removed the plastic fresh air vents from the cowl of the car.  This allows access to the backside of the firewall without having to come up from inside the car.

I prefer to mock up in wood if I can before I start ordering and cutting steel.  So, lots of careful measuring followed by cutting everything either too small (the holes in the middle of the strut tower plates) or too large (everything else) and then very carefully sneaking up on the correct lengths/angles using the sanding belt/disc.  I think I’ll be using 1/8″ plate and 1.5″ x .75″ rectangular tubing.  It should look like this.

Now for the firewall connection.  This piece will be bolted to both the vertical part of the firewall and to the horizontal “lip” with reinforce on the backside of each connection point.

A bit of geometry hell here – I made this wooden piece 3 times before I got the lengths and angles correct – compound angle on the strut tower side.

Learning curve – only took two tries to get the firewall bracket on the driver’s side correct.

And, I was able to use one of the two ‘failed’ attempts on the passenger side support strut on the driver’s side. Angles matched up perfectly — I simply had to lengthen it a bit.  If you look closely at the picture above, you can see the additional piece I glued on.

Now I can order my 3/16″ plate and the 1.5″ x .75″ rectangular tubing.  Meanwhile, I can disassemble this and take some careful measurements of my angles.  That will give me a good starting point for working in metal.
 
Big thanks to my friend Brad – he had a cool little jig that allowed his plasma cutter to be swung in an arc.  It made short work of the outer radius and the inner hole for the tower plates.
Versa-saw with hack saw blade made short work of the straight cuts.
Then, lots of walking between the attached garage and the detached — measuring, fitting, and hand-filing to fit.  Really happy with how the fit came out.
Driver’s side…..
Found I could use the ‘left-over’ plates after the cuts as a makeshift “brake” to bend the plates.  I plan on heating the tower plate between the “brake plates” (below) and the vice grips (above).  By slightly offsetting the brake plates, I think I can bend pretty close to the 100 degree angle I hope to achieve.
Well…..best laid plans.  Turns out I discovered on a test bend (I’ve learned a bit over the years) that I couldn’t bend nearly as tight a radius as I wanted to using the fab’d brake and heat.  So I resorted to my trusty little 5″ bench vice brake.  It was doing all it could – had to add a lot of leverage to the handle.  But got this 90 degree result on the first plate.

On the second plate, the vice handle gave up the ghost at about 70 degrees….

So I had to resort to clamping it in the vice and breaking out the BFH.  Worked — here are the angles on the car.

On to the firewall tabs.  Using my wooden mockups – I bent up some easy-to-bend aluminum strap to confirm angles.  I then used the aluminum bits as ‘gauges’ to measure the progress of my bend in steel.  It’s a compound bend – about a 96 degree bend coming off the firewall; and then you have to tilt the piece in the brake – 3 and 3.5 degrees – to account for the curve in the firewall.  This allows the two tabs to come ‘straight’ off the firewall – parallel to the longitudinal axis of the car.  It just looks better.  The aluminum gauges – 

Here’s one of the steel pieces mid-bend – you can see how I used the gauge.  Needs just a skosh more.
 

Final bends.

Carefully marked/drilled holes in brackets and firewall for the bolts.  Always a bit tense drilling into perfectly good body parts.  Really happy with the fit.  Sits completely flush in both planes.  I’ll cut gussets for these tomorrow.

Here’s the gusset cuts/fitting for the top plates.  The goal is no gaps because you can’t weld air.

Stay tuned for Phase 2 next week.  Sunday night post. 

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