Shock Tower Brace

[grannyknot]

This is the rear shock tower brace I made for my Z,  I'm now making up a bunch of other sets for guys in my local Z club and started wondering if there might be an advantage to fixing the bottom of the diagonal brace to one side over the other.

Can anyone see an advantage one way or the other?

Thanks,

Chris

[rossiz]

looks really nice - i've been thinking about a similar brace that was a pair of diagonals, or a horizontal "X" which would put a lower point in the center. this would make it easier to access/use the storage space between the brace and back of the seats (which is a perfect place for a couple bags of groceries).

 

not to hijack, but maybe a broader design discussion? any thoughts on this?

[John Coffey]

Regarding the direction of the diagonal brace:  it doesn't matter.  A straight bar is plenty given the factory diagonal braces already installed. 

Edited February 27, 2015 by John Coffey

[Carl Beck]

I wonder - without the rear strut tower brace - who has actually measured the amount of flex or distortion that is taking place at the tops of the shock towers to begin with.  Is the brace supposed to hold them apart - or hold them down? {especially on a street car}.

 

As far as body rigidity, or suspension movement etc goes - you might be better off bracing something else on a street car - without going to a full roll cage.

 

As to your question - I don't see any difference between bracing one side or the other - but then I'm not an M.E... I could see the suggestion of using an "X" between the two sides - to help keep the floor or that section of the body from flexing. Up front you have a strong cross member holding the engine - and a strut tower brace above. With the rear you don't really have a lower structure as strong. At least no one else is offering anything like that {effective or not - }.

 

Carl

[Carl Beck]

How about something like this...?

 

 

[Mark Maras]

 Many years ago during a Conference driver training course at PIR, I found the rear end in my stock 71 Z wanted to hop rather than slide in an oversteer situation. I installed a roll bar, for novice racing, mounted to the shock towers & the front side of the wheel wells (no x bracing). The change was quite remarkable. The rear slid, instead of hop, in a very controllable manner. I've always wondered how much deflection, in all directions, that simple addition eliminated.

  Many people shy away from roll-bars in a street machine but they are a great way to reinforce a uni-bdy car.

Mark

[Jarvo2]

Chris -- How much are you asking for the rear strut brace that you've been making?  I'm in the market for one and would prefer a 3-point rather than the cheapo adjustable ones that we see pop up too often.

[Zed Head]

I wonder - without the rear strut tower brace - who has actually measured the amount of flex or distortion that is taking place at the tops of the shock towers to begin with.  Is the brace supposed to hold them apart - or hold them down? {especially on a street car}.

This gets discussed quite a bit on various forums.  To John C's point, the towers are already braced for lateral motion,under the vinyl.  To Mark M's point, maybe bracing forward and backward (what a roll cage would do in addition to lateral) would be more helpful.  Also, the 240Z is known to be flimsier than the 280Z so what helps on a 240 may not help so much on a 280.

 

The use as a cargo barrier/holder though has obvious value.  Maybe that's enough.  It may also help in an accident, basically inert metal until needed.

 

From what I've picked up, if I remember right, a triangular brace from the firewall to the front towers is the simplest bolt-on brace that you can add.  It addresses fore and aft, and lateral, motion.  Here's an example - http://www.racetep.com/zfront.html

[rossiz]

while i've never tracked my 280 and am certainly not qualified in any way to evaluate body/frame flex and handling (no sports car experience other than this one) the symptom that brought the issue to my attention is how much the interior of the car squeaks and creaks over bumps and when thrown into turns. this leads me to believe that the car is flexing enough to cause the interior plastics to twist and rack. the sound is definitely coming from the rear of my passenger compartment, and since i've upgraded my sway bars, replaced struts & springs and cleaned/lubed all of the suspension except for the rear control arm spindle pins, i don't believe what i'm hearing is related to suspension components.

 

my first reaction to the factory installed diagonal sheet metal bracing on the sides of the rear towers is that it appears to be too steep an angle and it braces into the sheet metal subfloor deck, which appears to be a pretty flexible component. that's why a simple pair of diagonals making an "X" seems to make sense to me on a gut level.

 

i don't know how the towers would flex in any other direction than laterally, as it would seem that since the struts are angled to the center of the car, the towers would want to lean towards the center of the car when suspension load is placed on them. this would be solved with a lateral brace between the left and right towers.  the control arm design is triangulated to resist front-to-rear loads, and even if there were front-to-rear flex in the control arms, they are only connected to the tower through the strut, which has ball pivots at both ends so the only forces going to the tower would be in line with the strut.

[Zed Head]

Figuring out how the loads get distributed can be hard to do.  Under lateral loads, like cornering, you could imagine that the highest loads are at the two inner attachment points of the control arm (transverse link in Datsun-speak),  Which transfers in to the links that drop down from the body, and the diff mount/crossmember.  The top of the strut only sees the weight of the car, essentially, as you describe, in-line.  At first glance, it looks like the tops of the towers are important load-transfer points of a triangular structure.  But because of the shocks and springs, and the flexible hinge at the hub, they really only see changes in "weight" of the car.  It's interesting to look at because you can imagine a much weaker strut tube than is used.  The double wall nature of the strut tube with a shock tube inside is really much stronger than it needs to be.

 

So it leaves up-and-down as the force at the top of the tower, coming from the suspension.  Which should really only be unusually high if you bottom out the shocks.  So, in travelling over uneven bumps like a driveway entrance, you're probably really combating flex from twisting of the whole metal box/body.  Which seems to me like it would a blend of fore and aft, and lateral, a diagonal flex.  My 280 creaks the most when a front and rear wheel on opposite sides (diagonal) are unloaded at the same time.  Like a driveway entrance.

 

That's just one pass through the brain.  Something new pops up every time I think about it.

[grannyknot]

Thanks for all the replies, some good ideas bouncing around.

I built the black single bar rear brace in the foreground a couple of years back,

 

It seemed to me to really tighten up the rear end, but then read somewhere that the triangulated rear bar was even better so I put together the grey one behind it last fall. I haven't had a chance to wring the car out since I put it on BUT I noticed right away that most of all the squeaks from the  interior panels had disappeared. I hadn't really noticed how loud they were until they were gone.

 

This is the new version of the triangulated front shock tower brace,

I just smoothed it out from the first one I built.

 

Well that answers my question, also another good thing about having the

diagonal bar attach to the lower passenger side shock tower is it's a great place to secure a fire extinguisher that is easily reached from the drivers seat.

Chris

[Mark Maras]

Geoff

 The X bracing is the best & cheapest way to go. Also the possibility to make it multi-purpose as Carl suggested. Go for it. As a fabricator, I'll bet you could knock it out & have it installed in a couple of hours max. It would be great if you could do an accurate before & after either audibly or dimensionally. Maybe with the brace installed, one could belt a passenger to it to take measurements, tower to tower while cornering.