[rp19sti]

Hi all,
I have a 2019 STI stock everything but a Perrin pitch stop mount. I was reading up on forums and this seems as it does more bad than good as my other mounts are not upgraded. Can anyone clarify this? Should I put back to stock? I really do like the improved driving feel but will revert back if it is harming the car.

 

[coolgraysti]

:subscribed:

 

[Leemanfor]

You should upgrade your engine/transmission mounts with a pitch stop mount. There is speculation that since the stock mounts allow more flexing, having an upgraded pitch stop mount can cause issues with your chassis firewall breaking.

If you are concerned, upgrade your mounts to something a bit more DD friendly like Group N.

 

[rp19sti]

You should upgrade your engine/transmission mounts with a pitch stop mount. There is speculation that since the stock mounts allow more flexing, having an upgraded pitch stop mount can cause issues with your chassis firewall breaking.

If you are concerned, upgrade your mounts to something a bit more DD friendly like Group N.

Would using the Perrin pitch BRACE fic this or should I just upgrade all the mounts instead?

 

[mheyman]

There is speculation that since the stock mounts allow more flexing, having an upgraded pitch stop mount can cause issues with your chassis firewall breaking.

The speculation is misguided. Stiffening the other mounts will cause more stress on the firewall at the pitch stop mount not less because stock soft stock mounts absorb more energy under anything near stock power levels than stiff mounts, and stiffening any one sharpens the peak force everywhere, not a one point. I am ignoring direction here . Not going to go into it unless warranted.

If your car squishes bushings to the point of "flattening", then stiffer mounts might be able absorb more energy than a soft one. You are going to deal with more force no matter what.

 

[rp19sti]

So

The speculation is misguided. Stiffening the other mounts will cause more stress on the firewall at the pitch stop mount not less because stock soft stock mounts absorb more energy under anything near stock power levels than stiff mounts, and stiffening any one sharpens the peak force everywhere, not a one point. I am ignoring direction here . Not going to go into it unless warranted.

If your car squishes bushings to the point of "flattening", then stiffer mounts might be able absorb more energy than a soft one. You are going to deal with more force no matter what.

So should I be alright if the pitch stop is the only mod done? Thanks for reply!

 

[drummerboy]

So should I be alright if the pitch stop is the only mod done? Thanks for reply!

If you do one, do all so the stress is spread out. Its a system, don't look at them in isolation.

Doing only one means that one point its having to brace more movement than if all were done. Think of it this way, the soft stock mounts make the engine/tranny rock/twist more right? So if only the pitch stop is stiffer, the engine is still allowed to move with the soft engine mounts, so that force is translated into the pitch stopper joints because its more rigid than the rest. Where soft rubber would've absorbed the force, its now hard so the force continues to move through into the firewall; the firewall takes the brunt of it with only one hard point.

Now if you do the engine, tranny AND pitch stop it doesn't rock/twist as much to begin with -- the movement overall is stopped at multiple, source points of movement (ie. at the engine and tranny) instead of relying on just one rigid point to stop it. The stress is spread to the chassis the engine sits on, the firewall, and the tranny crossmember where the tranny sits on.

 

[Leemanfor]

If you do one, do all so the stress is spread out. Its a system, don't look at them in isolation.

Doing only one means that one point its having to brace more movement than if all were done. Think of it this way, the soft stock mounts make the engine/tranny rock/twist more right? So if only the pitch stop is stiffer, the engine is still allowed to move with the soft engine mounts, so that force is translated into the pitch stopper joints because its more rigid than the rest. Where soft rubber would've absorbed the force, its now hard so the force continues to move through into the firewall; the firewall takes the brunt of it with only one hard point.

Now if you do the engine, tranny AND pitch stop it doesn't rock/twist as much to begin with -- the movement overall is stopped at multiple, source points of movement (ie. at the engine and tranny) instead of relying on just one rigid point to stop it. The stress is spread to the chassis the engine sits on, the firewall, and the tranny crossmember where the tranny sits on.

I agree with this. The mounts are a system, so you should upgrade all of them at once.

will you be ok with just a pitch stop mount? Probably, as Mheyman said theres really no concrete evidence it will cause issues.

 

[mheyman]

Physics works though it may not be understood.
There is a correct and incorrect answer here.

 

[mheyman]

If you do one, do all so the stress is spread out. Its a system, don't look at them in isolation.

Of course and that is the whole point. The system is loaded with engine from the engine. Than Having bushings, soft, hard, or none does not change the amount of energy. Stiffening a mosunt in many cases will mean that it will dissipate less energy. In that case, ALL the mounts and other bushings have to work harder.


So sure, adding a stiffer bushing (any one) will likely mean more force at the mounts. But adding more stiffer bushings will just increase the forves further, not lessen them.

Movement: There is negligible movement in the system! What I mean buy this is that a geometric model were made to analyze the forces of the mounts the changes in angler and lengths/distance of the model would change only a few percent (if that) over the whole range of motion of the bushings. Themovemnt would be negligible.

Doing only one means that one point its having to brace more movement than if all were done.

No. The movement in terms of forces is negligible. But movement will wear stuff out. If your firewall moves/flexes too much or too many times it may fail.

Again, adding stiffess may well dissipate less energy in bushings, and leave more for the mount to deal with. But absorbing less energy in the bushing anywhere, leaves it for the mounts deal with not less.

Think of it this way, the soft stock mounts make the engine/tranny rock/twist more right?

Absolutely not. Mounts do not make the anything move, The forces generated by the engine /tranny make the mount move. This is important because it is the cause of latter errors in this description. They do move, they absorb some energy, they return some energy.

So if only the pitch stop is stiffer, the engine is still allowed to move with the soft engine mounts, so that force is translated into the pitch stopper joints because its more rigid than the rest.

Movement isn’t translated. Forces are. The softs mounts are absorbing energy. Replace them with stiff poly ones and there will be more energy and more force at the pitch stop.

Where soft rubber would've absorbed the force, its now hard so the force continues to move through into the firewall; the firewall takes the brunt of it with only one hard point.

True!!! This is true everywhere. The less energy absorbed by bushing leaves more energy every here!

Now if you do the engine, tranny AND pitch stop it doesn't rock/twist as much to begin with --

True! But the forces are present. The energy not absorbed buy bushings is in the system.

the movement overall is stopped at multiple, source points of movement (ie. at the engine and tranny)

This is always true. Your engine is allways in the same basic geometric position applying forces just as it always does. It’s movement is almost negligible.

instead of relying on just one rigid point to stop it. The stress is spread to the chassis the engine sits on, the firewall, and the tranny crossmember where the tranny sits on.

This is always true regardless of the type mount. If the system didn’t rely on a part you could simply remove it with not affect! This give rise to t a thought! For those really really really super paranoid about your firewall I have a cure! Simply remove your pitch stop! There will be no force at its mount!!!

 

[drummerboy]

@mheyman I'm not saying the forces are gone, I'm saying the forces that one rigid point (pitch stop) sees is less with upgraded engine/tranny mounts because the engine moves less to begin with. Contrast that where soft bushings absorbing the force means the engine will move more to begin with.

Lets rephrase it:
The fact that soft engine mounts absorb means the engine will move as the rubber absorbs. Lets say the soft engine mount squishes 1 inch means the engine moves 1 inch, now the hardened pitch stop has to deal with it. With no softness in the pitch mount to absorb that movement, the force causing that movement keeps going to the firewall. Now change all mounts. The harder engine mount squishes only 1/4 inch or less. Yes the engine mount experiences the same force on that axis, but the hardened pitch stop only has to deal with that smaller movement meaning less force at the pitch stop.

 

[raisingAnarchy]

On the '15+ chassis, certain model years are afflicted by a weak pitch stop mount. They literally thinned out the metal welded to the firewall, which caused higher levels of mount failures. Check out spacemonkeysti's car/profile on this board. Also, Perrin used his car as an example for the '15+ chassis development.

That being said, I sure as hell hope there was an update, and I want to say that occurred for '17+ vehicles. Might be a TSB regarding it and affected model years.

Lastly, lots of good advice in here about upgrading as a system. You can do the complete driveline for about $500-600 using Group N components. If it's a street car, that's highly advisable as poly mounts are most likely going to be too race car for the road.

 

[Leemanfor]

I can vouch for the Group Ns. Not too much NVH, but enough to keep the car solid.

I did notice my piston slap got louder with the Group N pitch stop though

 

[drummerboy]

I did all the Group N mounts back in my 06 WRX and again on my 17 STI. I also did the Group N tranny bushings this time too. I felt more of a difference with the GD than the VA, but maybe it’s because my GD was tuned and put down much more torque than my stock STI so it was more night and day.

I can hear more whining noises now at a certain rpm range though, but not really vibration or harshness. I’m happy with it overall.

 

[mheyman]

Im tired of telling drummer boy hes wrong. I hope he's not an engineer, a scientist, or even a student who's taken and passed any significant physics clssses.

 

[Jay11STI]

What is going on in here lol. I thought this was a simple one about a pitch stop mount. I too have been curious about one just never got around to it. To the OP I think its best to upgrade everything all at once. Engine mounts, trans mounts, and pitch stop all together. Given your car is a 2019 and still so new, i dont think you really needed it. I wont get into the physics/science part of it, its not my style. I prefer to replace things like this in pairs or as complete as possible just to keep the engine and drivestrain all nice and tight. So when ever the time comes for engine mounts on my car, ill just do the whole shabang. I will most likely go with Group N or continue with SuperPro everything.

 

[mheyman]

I would not disagree with that at all

What happened was that the OP was told to do something for reasons that were physically incorrect. I strongly objected. IOK , Ill add a to my previous post.

Ahhh, I think he was saying that limiting movement at the mounts will reduce the force that can be applied at the pitch stop - if it worked like that he'd be correct. Will think about it before i respond. - ie Added - clearly marked ADDED in a previous post.

 

[drummerboy]

Im tired of telling drummer boy hes wrong. I hope he's not an engineer, a scientist, or even a student who's taken and passed any significant physics clssses.

That's ok, I don't mind if you disagree. Though I don't think that last sentence was warranted -- implied put-downs aren't needed to get your points across.

We've said our points, so hopefully the OP got an answer in a round-about way.

 

[mheyman]

You are right. I apologize - (and I was apologizing before I wrote the following)

What you were saying - that making all the other mounts stiffer will reduce motion/travel/ distance of firewall flex is correct. the firewall force is only as much as the firewall pushes back for a given travel. So if the firewall acts like a spring, the forces there will be less. Less flex, less force. This is because a weak firewall flexes moves, it is not resisting until it has flexed. What this means is that the pitchstop is not sharing the load due to the system geometry, but solely to the the amount it flexes.

OP: take note !

 

[Scargo]

Oh boy, I want in on this! A lot of decisions surrounding upgrading to stiffer bushings relates to what you want to use the car for. If you don't race (or don't have the luxury of being able to drive very aggressively on winding roads) then there's really no point in stiffer bushings anywhere. In fact it's a negative for comfortable driving, just like some who modify their exhaust get very tired of the drone while on the highway!
You increase noise, vibration, and harshness (NVH) with stiffer bushings. The extreme is solid bushings where nothing shifts. It's not like soft bushings, which is like having a box in your trunk that slides forward and bangs into the back seat when you stop. Soft bushings slowly absorb the energy of each motion so you virtually don't realize the force that's being exerted. Whereas, with solid bushings, the mass is still there, wanting to move, but it is rigidly integrated into the whole chassis and never has an opportunity to move very far in any direction. The engine/transmission mass can (basically) only move the sheet metal attached to it. Of course they do it in pure race cars and often they have to beef up the mounting points in the chassis or subframes. As mentioned, once you start solidifying these isolators you start hearing and literally feeling everything that's happening inside the engine and transmission. Imagine having straight-cut gears! The torque and energy is still being transmitted in the form of vibrations which create a lot of noise.
BTW, In my pure track car I'm not out to win prizes/trophies. I have done some podium worthy laps, but I have left some things on the table for the sake of reliability. I use urethane everywhere; mostly SuperPro. Nothing is absolutely rigid except for my sway bar end links.

An interesting story: A guy tried to build a small sports racer powered by a snowmobile engine. It had no suspension! It was fast, but scary. It hopped around on the track at speed and vibrated so badly that his vision was blurred!

You don't need to understands physics much to realize that the energies created by moving mass have to go somewhere. That and the fact that a normal passenger car is greatly compromised for the sake of comfort. I chuckle when people make a car race-worthy and loud, but still spend huge sums of money on a high-end sound system.
Drummerboy, I love that double entendre, "don't look at them in isolation."