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Thoughts on Apex Engineered 8.8 subframe

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1 hour ago, Ben280 said:

I'd be interested to hear a more in depth discussion of why mounting the diff and rear control arms to the two largest pieces of hardware in the rear of the car (that go through the rear frame rails) isn't such a good idea? Maybe something that integrates the locating holes. 

 

On my car, the factory dropdowns were woefully inadequate. Pretty flexy in lateral loading, the only real thing preventing shift was pressure from the opposite wheel. It's even been suggested by several members on my build thread to connect the factory mustache bolts to the rear control arm pickup points for added stability in cornering. I've boxed the factory dropdowns, but potentially moving to an Apex style rear subframe for additional stiffness. 

 

I'm also intrigued by the rear control arm style this affords, using inner heim joints to free up bind, rather than the traditional bushed mounts. That lower mounting hole is probably too low for most cars, but if you went up with the car for rally use, might be helpful!

 

Here are some issues I have with this:

 

- the LCA's are mounted to a frame which is hard mounted on one side and soft mounted on the other side (!)

     - this will cause a toe-out scenario under cornering which can make the car snappy and unpredictable
- they coupled diff torque reaction to the LCA pickup

- the diff is potentially mounted on isolators at the snout but hard-mounted at the case (hard to tell based on all the photos floating around)

- what's the mass save (or add)?

- other stuff, see screenshot

 

image.png.fb9c901c26a4586fe01ccf773aa770d9.png

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s744029745302559756_p31_i15_w640.jpeg

 

Seems like the "soft" mounting in the back of the diff would only affect it in the fore and aft direction. 

 

Having no isolation on the front of the cradle (where the front rear suspension bar bolts to), and having isolation on the rear of the cradle (where the mustache bar usually bolts to) could affect things.

 

How much will it affect the overall dynamics of the vehicle? How will that affect NVH? How will that affect the life of the cradle/isolators/suspenion links/alignment/etc?

 

I don't know. 

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Posted (edited)

Well I would assume since the mustache bar is now connected to the control arm mounts it would flex and affect many things.  Cant really predict exactly how. 

 

I would personally just drill out the tabs and weld in a tube where normal bushings could be inserted.  However for something that costs that much should be designed in a way where the bushings actually are correctly implemented.

Edited by MonocleDinosaur

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4 hours ago, Ben280 said:

I'd be interested to hear a more in depth discussion of why mounting the diff and rear control arms to the two largest pieces of hardware in the rear of the car (that go through the rear frame rails) isn't such a good idea? Maybe something that integrates the locating holes. 

 

On my car, the factory dropdowns were woefully inadequate. Pretty flexy in lateral loading, the only real thing preventing shift was pressure from the opposite wheel.

 

I'm not an expert in chassis design or even an engineer.  So I try to keep things simple.  My first thought would be "what do the parts have to do in their original design?"  and what will they be doing in the new application?  So those two "big" studs hanging down, in their original purpose, only have to stop an 80 lb chunk of metal from moving sideways or fore and aft due to inertia, and also direct loads up in to the frame rail, or resist a tensile load downward, from the torque moment of driving the axles.  And they have the front mount to help the fore and aft loads. In the new application they have to resist the weight of the whole back half of the car during cornering.  It's much more mass putting a higher side load on the studs.  Consider the length of the studs and you can imagine a pretty high leverage on the stud ends as they bend.  Then imagine fatiguing over time and it's easy to imagine cracking at the frame rail.  And, since it's rubber mounted at the studs the front solid mount will do all of the work until flexing causes the rubber at the studs to compress.  More fatiguing at the front end.   The studs just just weren't designed for the new loads, no matter how big they are.  One view.  Fun to think about.

 

Curious, how do you know the factory drop-downs were flexing?  In the factory configuration everything is floating in rubber anyway.

 

If I was Apex I'd have some sort of blog where people could point these things out.  Then they could explain their thinking, for reassurance.  They could even join the forum and respond to this thread.

 

image.png.89ba549931877485efc0959105a67a35.png

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39 minutes ago, NewZed said:

Curious, how do you know the factory drop-downs were flexing?  In the factory configuration everything is floating in rubber anyway. 

 

Stuck a gopro back there, saw em move. No rubber left in the bottom of my car, closest thing is the LCA pickup bushings which are delrin.

 

I do think these studs are enough to hang the rear frame on as they've done. Most "modern" cars from Nissan (s13, s14, 300zx 350z) have similar methods for mounting their subframes. The only difference would be if the upper plate is thinner than on these newer cars. They also use 4x of these, with the rear frame connecting the points. It looks to me like the Apex piece attempts to solve this with the solid mounting in the front of the  original LCA point. If anything, I'd add another brace in the front to finish boxing out the frame. 

 

3 hours ago, Leon said:

 

Here are some issues I have with this:

 

- the LCA's are mounted to a frame which is hard mounted on one side and soft mounted on the other side (!)

     - this will cause a toe-out scenario under cornering which can make the car snappy and unpredictable
- they coupled diff torque reaction to the LCA pickup

- the diff is potentially mounted on isolators at the snout but hard-mounted at the case (hard to tell based on all the photos floating around)

- what's the mass save (or add)?

- other stuff, see screenshot

 

image.png.fb9c901c26a4586fe01ccf773aa770d9.png

 

Thanks Leon! I'll agree with you on the hard mount/soft mount business. I imagine the rear soft mounts (unless they are a delrin insert) are designed to accommodate some misalignment in the chassis? Unless they are a delrin or poly bushing they could be swapped out for aluminum. Looking at how the 8.8's are mounted in the S550 mustangs, this diff mount checks out, as the front two mounts on the diff have rubber isolators in them. There are also a number of kits that let you switch these to solid mounts if you want Ultimate Response (tm) haha. 

 

Toe out in a corner would be sketchy, but I'm not sure this system would allow for any more than the factory system. Something to test for sure. I still do really like being able to adjust your roll center, something nobody else is doing in the rear of these cars, at least at a retailer level. 

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1 hour ago, Ben280 said:

 

Stuck a gopro back there, saw em move. No rubber left in the bottom of my car, closest thing is the LCA pickup bushings which are delrin.

 

I do think these studs are enough to hang the rear frame on as they've done. Most "modern" cars from Nissan (s13, s14, 300zx 350z) have similar methods for mounting their subframes. The only difference would be if the upper plate is thinner than on these newer cars. They also use 4x of these, with the rear frame connecting the points. It looks to me like the Apex piece attempts to solve this with the solid mounting in the front of the  original LCA point. If anything, I'd add another brace in the front to finish boxing out the frame. 

 

 

 

I do agree that that the studs are most likely plenty to hold up to the stress of mounting the control arms in this way.  You definitely make a point when saying alot of cars use a rear subframe to hold both the differential and the control arms.  However one thing I also acknowledge is that the s30 was never designed for this.  I think this could work well, but it will need plenty of added bracing around this area.  I think some framerail reinforcement and better mustache bar bolts may be required for good longevity.

 

 I do feel though as this falls apart by the way it is bushed.  The differential in a high horsepower car is going to move around and flex quite a bit no matter what unless its mounted with about 500 lbs of concrete.  The reason why the subframe in a 240sx can mount both control arms and the diff is that it is completely isolated with correctly implemented bushings.  That alows it to flex without affecting the geometry of the suspension. 


In the senario of the apex kit there are many differences.  Since the rear of the differential is mostly hard mounted and the mustache bar is bushed, instead of the diff bushings absorbing the rotational forces, the mustache bar bushings will absorb it instead. Since it is the source of mounting for the control arms, the entire thing will twist left or right on hard acceleration.  I think this would probably cause a skew of tow in on one side and tow out on the other, but i'm not entirely sure. 

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Posted (edited)
41 minutes ago, MonocleDinosaur said:

I do agree that that the studs are most likely plenty to hold up to the stress of mounting the control arms in this way.  You definitely make a point when saying alot of cars use a rear subframe to hold both the differential and the control arms.  However one thing I also acknowledge is that the s30 was never designed for this.  I think this could work well, but it will need plenty of added bracing around this area.  I think some framerail reinforcement and better mustache bar bolts may be required for good longevity.

 

 I do feel though as this falls apart by the way it is bushed.  The differential in a high horsepower car is going to move around and flex quite a bit no matter what unless its mounted with about 500 lbs of concrete.  The reason why the subframe in a 240sx can mount both control arms and the diff is that it is completely isolated with correctly implemented bushings.  That alows it to flex without affecting the geometry of the suspension. 


In the senario of the apex kit there are many differences.  Since the rear of the differential is mostly hard mounted and the mustache bar is bushed, instead of the diff bushings absorbing the rotational forces, the mustache bar bushings will absorb it instead. Since it is the source of mounting for the control arms, the entire thing will twist left or right on hard acceleration.  I think this would probably cause a skew of tow in on one side and tow out on the other, but i'm not entirely sure. 

 

Hmmm interesting. 

 

From racing Z's for a number of years now, I think the idea that the mustache bar + diff mount will transmit a dramatic amount of lateral load under cornering is overblown. I'm sure there is some, and some FEA could probably tell us to a certain degree of certainty, but I'm also sure that there is a huge amount of flex in the rear of these cars from the factory. Once you start tossing V8's and big tires at these cars, all bets are off. 

 

I take a lot of reference from how the cars that these parts come out of are used. The S550 Mustang for example from the factory has bushings inset into the front of the diff, and the rear rides in another set of bushings. There are a number of kits for people who track the cars to solid mount the diff (aluminum bushings for the front and rear). A local autox friend of mine has this upgrade in his 2017 Mustang and is making about 850hp. No cracked diffs yet! 

 

Good points tho!! This is one of the more radical mods for the rear of the Z car, I'm excited to discuss and see if the community can improve it for V2!

Edited by Ben280

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Posted (edited)
3 hours ago, Ben280 said:

Thanks Leon! I'll agree with you on the hard mount/soft mount business. I imagine the rear soft mounts (unless they are a delrin insert) are designed to accommodate some misalignment in the chassis? Unless they are a delrin or poly bushing they could be swapped out for aluminum. Looking at how the 8.8's are mounted in the S550 mustangs, this diff mount checks out, as the front two mounts on the diff have rubber isolators in them. There are also a number of kits that let you switch these to solid mounts if you want Ultimate Response (tm) haha. 

 

Toe out in a corner would be sketchy, but I'm not sure this system would allow for any more than the factory system. Something to test for sure. I still do really like being able to adjust your roll center, something nobody else is doing in the rear of these cars, at least at a retailer level. 

 

If the mustache bar bushings are made solid then some of the concerns are alleviated. I don't disagree regarding compliant toe, factory rear suspension probably also goes toe-out in cornering, but that's something I would aim to fix if I were redesigning the suspension.

 

I do disagree about the diff mounting. From a quick google, you can see that the aft bushings are pressed into the Mustang's subframe. The inner metal of the bushing directly connects to the diff and allows the diff to move within the confines of that bushing. That's the proper use of a diff bushing.

 

What I'm seeing on the Apex setup is that the bushings are sandwiched between the diff and the subframe. Unless the bushings are actually metal spacers (don't see a reason why that would be though), this is a huge no-no for that fastened joint. I smell either a fatigue failure coming on for the diff mount bolts or the bolts backing out completely. As was mentioned previously, as it sits, that bushing would only have a chance at isolating longitudinal forces. In short, that's absolutely not the correct application of a diff bushing, if that is the case.

 

image.png.6e0497fd9fcbf4da887f0735b6064411.png

 

image.png.ef218a83a4950989b204400c871513bf.png

Edited by Leon

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Posted (edited)
45 minutes ago, Leon said:

If the mustache bar bushings are made solid then some of the concerns are alleviated. I don't disagree regarding compliant toe, factory rear suspension probably also goes toe-out in cornering, but that's something I would aim to fix if I were redesigning the suspension.

This is clearly one of the biggest issues with the kit.  It is an easy design change and fix.

 

45 minutes ago, Leon said:

do disagree about the diff mounting. From a quick google, you can see that the aft bushings are pressed into the Mustang's subframe. The inner metal of the bushing directly connects to the diff and allows the diff to move within the confines of that bushing. That's the proper use of a diff bushing.

This is what I was talking about when referencing other vehicles with subframes like this.  It is a great idea to move towards this kind of setup in my opinion.  However the rear diff mount definitely needs to be rethought out.  It is also a relatively easy design change and fix. I see how the solid mounts can work in a s550, but the subframe and entire frame of the car is much stronger looking to handle it.  the way the apex kit is it would be relatively easy to twist the rear side of the mustache bar.  

 

51 minutes ago, Ben280 said:

Good points tho!! This is one of the more radical mods for the rear of the Z car, I'm excited to discuss and see if the community can improve it for V2!

This is what this kind of discussion is about.  TTT has had their problems as well, its all about improving on it.  I hope Ohm hops on to talk about some of these problems.  If so this can turn into an amazing kit.

Edited by MonocleDinosaur

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10 hours ago, Ben280 said:

Instagram is rife with photos of their knuckles, control arms, front subframes. A little less so with these big rear suspension conversions, but they're out there. Makes sense for a part that costs a couple thousand dollars. 

 

https://www.instagram.com/p/B7-IwogJtBi/

https://www.instagram.com/p/B8Eizm5pSz7/

https://www.instagram.com/p/B9j1UvHnyoq/

https://www.instagram.com/p/B-N_tOFHCBG/

https://www.instagram.com/p/B-0kBOXntko/

 

I am personally using their tie rods and knuckles, after snapping a Arizona Z Car knuckle in half.

 

I wouldn't consider five "rife".  I do like to see an uncommon word used though.  Thank you.

 

If ohmster just put the factory dropdowns back in and connected them to the subframe, the usage of the mustache bar pins would be a strengthening move, not a replacement of the factory setup.  Then put a bushing on the back diff mount to isolate it from the suspension sub-frame, creating the typical rubber mounted diff situation.  Just bench-thinking...

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Looks like they're using the old style, cantilevered front diff mount, using the four small bolt holes from the original strap.  Puts a levered load on those bolts. Not sure what's going on with their other parts.  Kind of hard to see in the car.  They need a professional photographer.

 

image.png.f56dd5438e7a1b98722893f3c1d9b862.png

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