Quote:
Originally Posted by Norville Rogers
Won't these just transfer forces from the strut mount to the master cylinder?
Something you'd really want to avoid?
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Quote:
Originally Posted by RJasonKlein
That's an intereting question. My understanding is that the strut mounts deflect very little under normal loads, but that the firewall itself deflects much more so due to the driver's leg pressure on the brake pedal under heavy braking. I had assumed the stability of the strut mounts is what actually shores up the master cylinder, but that same stability prevents load from being transferred to the master cylinder in the other direction. Can anyone confirm my hunch?
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That really is an interesting question, and the simple answer of course is yes it will. That's just basic physics, every action will have an equal and opposite reaction.
However, how much force is really going into the master cylinder, and where is that force being going?
First, the fore and aft motion of the strut towers is very small, mainly because the strut towers are very strong in this direction. The large round shape of the strut towers makes them inherently strong, but more importantly they have a large anchor point to the frame on both the top and the bottom of the strut tower.. If you think about it, how the strut towers are mounted to the frame in the fore and aft direction is substantially wider than how they are mounted to the frame in the side to side direction. And to further reduce movement in the fore and aft direction they are braced at the top of the strut towers to the top of the firewall near the cowl (nice and sturdy there compared to lower on the firewall).
Now, what kind of forces are really acting on the strut towers at the point where the master cylinder brace is mounted? Well, you've got the springs and shocks reacting to what the wheel is doing mainly. And since there is a certain kingpin inclination to the shock (and caster) there is a component of force that can be resolved into two forces, one that could transfer to the master cylinder through the brace, and one that would be resolved into a completely vertical force up through the strut tower. However, since the angle of the strut towers relative to vertical is pretty small, the majority of the force is not going in the fore and aft direction, so you're looking at a very tiny bit transferring into the master cylinder via the brace.
In fact, you'll most likely see some of the largest forces in the situation where you are braking (forces going into the strut tower towards the master cylinder), and you're stomping on the brakes which is pushing the master cylinder right back into the brace. But once again the anchor point to the frame on the top of the strut tower and on the bottom of the strut tower, as well as the brace to the firewall is taking the majority of this force to the point that the force transferred to the master cylinder from the strut tower should be pretty much negligible.
That's just the super long winded way of saying that the strut towers are very strong in the fore and aft direction. Because RJasonKlein's answer was probably more than adequate. The firewall is very weak compared to the strut towers. And even if there was a substantial load coming through the strut towers and acting on the master cylinder brace the firewall would just deflect to absorb the force just like it does every time you step on the clutch or the brakes (sans MCB).
We have thousands of master cylinder braces out there in the wild across many many different vehicles. Not a single one has ever resulted in a failure of the master cylinder, or strut tower, or firewall. The transferred force is non-destructive even when it is at it's maximum.
Make sense?
Chase
Engineering