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grannyknot

Addressing firewall flex

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In my on going efforts to build the perfect Z (for me) I thought I would have a go at firewall flex or that brake pedal that never really feels solid. Nothing new here just copying the principal of supporting the master cylinder - brake booster - firewall - brake pedal box.

Hope this is of use to others.

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Edited by grannyknot

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Very cool. Now draw it up in CAD so we can all have one ;)

HaHa, that's good one, that will never happen if it is left up to me.

 

As previously mentioned, looks solid!

 

Almost seems like you would need flats on the cap so you can add some preload.

 

Really looking forward to your reports on how it feels.

Flats on the cap? Oh I get it, the cap touching the end of the M/C spins freely, the flats are on the threaded rod right next to the cap for pre-load adjustment and the nut is for locking it in place.

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Its too bad our chassis needs such a bulky solution, but well done.

This isn't the only way to address the problem. I think I recall John Coffey saying that he reinforced this area with a piece of 1/8" plate to stop flex. You could probably also brace that to the underside of the cowl if need be.

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I know more modern cars do this all the time, but the Z chassis is SO flexy that I'm wondering if it's a good idea. When I put jackstands under the TC rod buckets on my Z I could WATCH the front end droop 3/8 or 1/2 inch as I lowered the front end off the jack (jacked up on the xmember). If you don't take care of that weakness first and you just use the strut to the master as a brace, I'm just concerned that the pressure on the master could cause problems.

 

Fabrication looks very nice.

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I know more modern cars do this all the time, but the Z chassis is SO flexy that I'm wondering if it's a good idea. When I put jackstands under the TC rod buckets on my Z I could WATCH the front end droop 3/8 or 1/2 inch as I lowered the front end off the jack (jacked up on the xmember). If you don't take care of that weakness first and you just use the strut to the master as a brace, I'm just concerned that the pressure on the master could cause problems.

 

Fabrication looks very nice.

Jon, I'm also going to be stitch welding all the major seams when the car comes back from the acid dippers so I'm hoping that will fix some of the flex. What kind of problems do you for see with the M/C?

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Jon, I'm also going to be stitch welding all the major seams when the car comes back from the acid dippers so I'm hoping that will fix some of the flex. What kind of problems do you for see with the M/C?

I don't honestly know. All I can tell you is that if you can watch the frame rails flex, presumably the strut towers are moving too. I guess the question is: what happens when you push really hard on the end of the master?

 

What would be really interesting would be to take some measurements on how far the strut towers move back towards the firewall under heavy braking. I have the feeling that they move quite a bit.

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Maybe we need to employ a spring setup to set a limit where excess pressure would just be used into the spring. 

 

Surely it can't be too much movement though right? I mean if the shock towers moved that much you would imagine body lines that are mounted to sheet metal would come in contact with each other like the hood/cowl or the fender/door no?

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What would be really interesting would be to take some measurements on how far the strut towers move back towards the firewall under heavy braking. I have the feeling that they move quite a bit.

Wouldn't there be force on the tower forward under braking?  Levering around the TC rod?  Just trying to wrap my head around it.  Straight line pushing the wheel backward, but rotational pushing the top forward.  You could imagine if the top of the strut was disconnected and you hit the brakes it would rotate around as the bottom of the tire is pushed backward.

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Would a thin bushing between the brace and the MC be of any benefit? Seems like movement may wear away metal if flex is bad in other directions.

That is why you pre load the brace against to M/C with the threaded rod so there is no movement to begin with,  the shock tower, M/C and firewall sort of become one so when you push the brake pedal that force is spread out to the front suspension structure and the firewall resulting in less movement and hopefully a better brake pedal. Once that cap is tightened against the M/C there should be no movement between them.

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That is why you pre load the brace against to M/C with the threaded rod so there is no movement to begin with,  the shock tower, M/C and firewall sort of become one so when you push the brake pedal that force is spread out to the front suspension structure and the firewall resulting in less movement and hopefully a better brake pedal. Once that cap is tightened against the M/C there should be no movement between them.

I see what you mean yes, but i was thinking more up/down side/side motions from the strut tower itself flexing in compairson to the position to the MC under hard cornering and big bumps. Just seems to be there would still be other flex and stress forces that could cause rubbing in a chassis as flexy as these cars are.

Edited by Evlevo

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Wouldn't there be force on the tower forward under braking?  Levering around the TC rod?  Just trying to wrap my head around it.  Straight line pushing the wheel backward, but rotational pushing the top forward.  You could imagine if the top of the strut was disconnected and you hit the brakes it would rotate around as the bottom of the tire is pushed backward.

Yes. That is absolutely right. But if the frame rails flex towards the firewall more than the rails flex the opposite way... My ***guess*** is that the beam loading of the whole chassis under braking has more effect than the strut towers trying to fold forward under the brake torque.  

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Strut tower braces to the firewall might do the trick.

No, the deflection is local to where the pedals mount which is too far from a typical strut tower brace. Also in that first pic I can't imagine why one would want to add the cost / complexity / and undesired degrees of freedom by putting rod ends on the two legs to the firewall. There are also lighter, more compact solutions to the firewall flex under the brake pedal but that requires modding the pedal box which is a decent amount of work to get to removing the dash etc.

Edited by heavy85

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I see what you mean yes, but i was thinking more up/down side/side motions from the strut tower itself flexing in compairson to the position to the MC under hard cornering and big bumps. Just seems to be there would still be other flex and stress forces that could cause rubbing in a chassis as flexy as these cars are.

 

 

Strut tower braces to the firewall might do the trick.

 

I noticed that going from a 240Z booster to a 280Z booster reduced firewall flex somewhat.

I agree so I hope I have addressed that with theses braces,

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I`m afraid, those does´t help with firewall flexing.  Here is my solution, didn´t think firewall when did this but maybe helps a bit with it.

 

Not for firewall flex. For strut tower movement. Anything attached to the strut tower (such as a device to arrest master cylinder movement) will also move.

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