ITAC News.

[Gregg]

So, in effect, you're saying that the transmission mount (#14) material on an 88-91 CRX and Civic can be changed since it mounts to the front crossmember (#2)...a piece which also serves to locate a part of the front suspension (the radius rods which go back to the lower control arms)?

 

[tnord]

The BMW rear subframe is attached to the chassis at three points. One on each side and one on the diff. Since the diff is attached to the subframe, the diff bushing (which is what BMW calls it) locates the subframe...ergo, bushings are free. Chuck

if BMW calls it a diff bushing in the FSM, i'm not sure how that is "free."

when you drop the diff to change FD, does the bushing come out with it or stay on the subframe?

nothing personal of course, you were a good guy when i met you at the arrc last year, and i don't blame you at all for trying to take advantage of a failure in the rules. i'm just not sure i buy the arguement.

 

[JoshS]

if BMW calls it a diff bushing in the FSM, i'm not sure how that is "free."

when you drop the diff to change FD, does the bushing come out with it or stay on the subframe?

nothing personal of course, you were a good guy when i met you at the arrc last year, and i don't blame you at all for trying to take advantage of a failure in the rules. i'm just not sure i buy the arguement.

I don't have my FSM handy, but the parts catalog just calls it a "rubber mounting."

But to answer your question up there ... there is no bushing between the diff and the subframe. The subframe is a stressed member of the subframe (called a "rear axle carrier"), they are rigidly bolted together. The assembly of the two parts mounts to the unibody at 3 locations (2 on the axle carrier, one on the differential cover).

I use a stock bushing there because I think it keeps the rear trunk floor from turning into a twisted hunk of sheet metal, but I think it would be absolutely legal to put something else in, because "Bushing material, including that used to mount a suspension subframe to the chassis, is unrestricted." I think the whole thing (combo of the axle carrier + diff) is the subframe, and I don't think I'm stretching to say that.

Here's a pic, attached. The red circles are the three suspension mounting points. Just to be really clear, the three circled bushings are the only bushings in the picture. The differential case and the axle carrier are rigidly bolted together (with the four big bolts you can see.)

But the E30/Z3 rear suspension argument is totally different than Jeff's engine mount argument, if I understand it right. I think that argument claims that any bushing that is attached to the suspension subframe is somehow a suspension bushing ... I don't buy that at all. The subframe bushings that are explicitly allowed are those "used to mount a suspension subframe to the chassis." Just because an engine that is also bolted to a suspension subframe via a bushing doesn't make that bushing a suspension bushing.

 

[EV]

i mostly agree jeff, but to me the engine mount rule gave another handout to FWD cars. not to mention that many would disagree with the bushing rule as it is anyway, so using that to justify the engine mount rule change sounds a lot like creep/falling further down the rabbit hole.

just so nobody gets all huffy about this whole deal, i would support engine mounts if the same thing applied to trans and diff mounts in RWD cars.

Travis, I am sure you are using oem engine mounts on your miata, and not the Mazda mounts that the spec miatas are running right? Of course, rumor has it, there is no way to tell the difference between them dispite the fact that the SM mounts are reportedly stiffer.

 

[spawpoet]

I think that argument claims that any bushing that is attached to the suspension subframe is somehow a suspension bushing ... I don't buy that at all. The subframe bushings that are explicitly allowed are those "used to mount a suspension subframe to the chassis." Just because an engine that is also bolted to a suspension subframe via a bushing doesn't make that bushing a suspension bushing.

But the GCR doesn't say only suspension bushings are free. It says "Bushing material, INCLUDING that used to mount a suspension subframe to the chassis, is unrestricted." While itincludes suspension subframe bushings in saying they are unrestricted it doesn't limit those bushing that can be modified to only suspension bushings in any way. It simply says bushing material is unrestricted. The intention may have been to limit changes to only suspension bushings, but the wording doesn't reflect that. Moreover the front subframe on many cars IS the front suspension subframe. If that's the case, any bushings on it are free by that wording.

 

[chuck baader]

Chris, I agree. Unfortunately, my BMW motor mounts and trans mounts do not meet the definition of "bushing". I feel, in the Honda example above, the "bushing" test is passed and that mount can be upgraded/changed. Chuck

 

[Knestis]

We've had the "material" conversation here in the past, Chris, but short version is, the point at which the "material" of the "bushing" is "unrestricted," the design of the "bushing" is equally unrestricted.

If I can substitute hard rubber for soft rubber parts, I can substitute poly for rubber parts, poly and steel for the poly parts, Delrin and steel for the poly parts and air and some differently-shaped steel for the old steel parts, (etc.) and end up with a completely different animal - as long as it doesn't serve an otherwise disallowed function.

K

EDIT - I'm not saying that's what the rule originally intended (I'm pretty sure it was not envisioned to be the case) but that's how it's currently operationalized.

 

[spawpoet]

We've had the "material" conversation here in the past, Chris, but short version is, the point at which the "material" of the "bushing" is "unrestricted," the design of the "bushing" is equally unrestricted.

If I can substitute hard rubber for soft rubber parts, I can substitute poly for rubber parts, poly and steel for the poly parts, Delrin and steel for the poly parts and air and some differently-shaped steel for the old steel parts, (etc.) and end up with a completely different animal - as long as it doesn't serve an otherwise disallowed function.

K

EDIT - I'm not saying that's what the rule originally intended (I'm pretty sure it was not envisioned to be the case) but that's how it's currently operationalized.

So that I am clear then, if it fits the definition of a bushing as it sits on the car in it's stock form, it's free as in the FWD engine mount pointed out above? That is how I am seeing it. If this is the case we already have a discrepancy on motor mounts where some cars should be free to change their mounts (if the stock part is a bushing), and others cannot change their mounts as in my case where the Z mounts are not "bushings" per the GCR definition in their stock form (they are just rubber blocks). I'm sorry if I'm digging up topics I missed, and I just want to make sure I apply the rules appropriately to my vehicle. To me the discrepancies allowing some mounts to be open while limiting other to stock as demonstrated in the BMW's is all the more reason to open the mount rules up as discussed a few months ago. Fix the locations to stock locations and allow all transaxle mounts to be free to use whatever material desired.

 

[JeffYoung]

I agree there is enough lack of consistency and clarity that this should be cleaned up.

I thought on this some last night, and I agree with Josh on the INTENT of the subframe rule, it probably was not intended to cover engine mounts that are "bushings." But that "including" language is not limitng and suggests that anything that is a bushing is unrestricted.

And, of course, there is no reason to say that some cars that have "bushings" for engine mounts can change them while others that don't cannot.

Werd!

Material should be unrestricted on all mounts/bushings, so long as the suspension pickup point and engine/diff/tranny location are unchanged -- in my view.

 

[JoshS]

But the GCR doesn't say only suspension bushings are free. It says "Bushing material, INCLUDING that used to mount a suspension subframe to the chassis, is unrestricted." While itincludes suspension subframe bushings in saying they are unrestricted it doesn't limit those bushing that can be modified to only suspension bushings in any way. It simply says bushing material is unrestricted. The intention may have been to limit changes to only suspension bushings, but the wording doesn't reflect that. Moreover the front subframe on many cars IS the front suspension subframe. If that's the case, any bushings on it are free by that wording.

That section you are quoting is in the "Suspension Mounting Points" section of the ITCS. Context matters. Not all bushings are unrestricted, only Suspension Mounting Point Bushings. Hard to believe that anyone sees fit to ignore the context.

And yes, the front subframe is often both an engine cradle and a suspension subframe. That doesn't mean that any bushing on it is a suspension bushing, to me that's a huge stretch.

 

[Knestis]

>> So that I am clear then, if it fits the definition of a bushing as it sits on the car in it's stock form, it's free as in the FWD engine mount pointed out above? ...

If I'm following, then NO. That "bushing material" clause is ...5.d.6 in the GCR. "5" is "Chassis" and "d" is "Suspension Mounting Points."

It's too much to extend that to engine mounts, which are NOT addressed (or allowed) in the ITCS and are explained under different subsections in the specs for categories in which they may be modified - even if they do meet the definition of a bushing.

Allowances sort of cascade logically through rules that take what in my business we call a "nested" structure. That's reflected in the way the rules are structured. You can't take an allowance and generalize it UP in that organization.

K

EDIT - What Josh said. I should know better than to go for a coffee refill in mid-post.

 

[spawpoet]

That section you are quoting is in the "Suspension Mounting Points" section of the ITCS. Context matters. Not all bushings are unrestricted, only Suspension Mounting Point Bushings. Hard to believe that anyone sees fit to ignore the context.

And yes, the front subframe is often both an engine cradle and a suspension subframe. That doesn't mean that any bushing on it is a suspension bushing, to me that's a huge stretch.

I'm not obtuse Josh. I just missed the context. It happens, and I appreciate you pointing it out. I was actually quoting it from Jeff's earlier post, and wasn't really looking in the GCR myself. Seeing that the bushing allowance is only listed in the suspension sub-section, I see what you (and then Kurt) are saying. In this case I'll backtrack again then, an I don't feel that the bushings on the Datsun (the whole reason I'm asking all this) mustache bar are free to change. That is a clearly a drivetrain subframe, and not a suspension subframe. No different than the transmission cross-member. I apologize if muddied the waters, and again appreciate the clarification. I'm not here to torture the rules, but as it applies to my vehicle I'd like to "get it right".

 

[JeffYoung]

LOL..this one is muddy!

Chris, here's a though -- I think Josh would agree with you that the mustache bar bushings are free as suspension subframe bushings. Look at what he is calling subframe bushings in his picture on the Z3 (and I agree with him).

How is that different than the mustache bar, which holds the diff carrier in place, which holds the halfshats which keeps the rear suspension attached to the car.

On the Z cars, I think the mustache bar bushings and the tranny mount bushing are "free."

 

[JoshS]

LOL..this one is muddy!

Chris, here's a though -- I think Josh would agree with you that the mustache bar bushings are free as suspension subframe bushings. Look at what he is calling subframe bushings in his picture on the Z3 (and I agree with him).

How is that different than the mustache bar, which holds the diff carrier in place, which holds the halfshats which keeps the rear suspension attached to the car.

On the Z cars, I think the mustache bar bushings and the tranny mount bushing are "free."

A quick google search turned up this diagram of a 240Z rear suspension:

It doesn't look like you need the diff to locate the rear suspension. The suspension control arms are mounted to the subframe, which is attached to the chassis at four points, independent of the diff. You can remove the diff and the subframe would still be rigidly bolted to the chassis at four points, right?

Seems different than the E30/Z3.

 

[Z3_GoCar]

With #2 being the mustache bar and is mounted with #1 the rear a-arm mount would be mounted on the rear bracket, I can't tell if the braket is attached to the mustache bar or not. If the bracket's are conntected to the mustache bar then yes #1 should be changed with any material.

 

[tnord]

Travis, I am sure you are using oem engine mounts on your miata, and not the Mazda mounts that the spec miatas are running right? Of course, rumor has it, there is no way to tell the difference between them dispite the fact that the SM mounts are reportedly stiffer.

1) my miata was converted to ITA before the SM motor mount rule change.
2) i think there is a way to tell the difference
3) not sure how your post is relevant anyway, the mazdacomp mounts would be illegal.

 

[spawpoet]

With #2 being the mustache bar and is mounted with #1 the rear a-arm mount would be mounted on the rear bracket, I can't tell if the braket is attached to the mustache bar or not. If the bracket's are conntected to the mustache bar then yes #1 should be changed with any material.

The mustache bar is stand-alone. It mounts to the diff and the chassis, and not to the suspension subframes. You can take the diff out by disconnecting it from the driveshaft/halfshafts, disconnecting the front diff mount, and unbolting the sides of the mustache bar, and the unit will slide right out with the mustache bar attached. IMHO (now ) this means the bushings in the mustache bar should remain stock. Ditto for the tranny crossmember, which is not a suspension subframe on a Datsun z-car. The BMW is different in that the diff mounts to the subframe which mounts to the chassis.

 

[JeffYoung]

Ok, I generally agree with the logic, other than this.

Look at the Datsun Z suspension.

Look at the Z3 suspension.

They are essentially identical -- a trailing arm IRS set up. One gets "free" bushings and the other arguably does not.

Does that make any sense?

 

[Z3_GoCar]

Ok, I generally agree with the logic, other than this.

Look at the Datsun Z suspension.

Look at the Z3 suspension.

They are essentially identical -- a trailing arm IRS set up. One gets "free" bushings and the other arguably does not.

Does that make any sense?

If the picture that Josh posted is correct there are a couple of differences:

240Z: uses more of a chapman strut set up, and you can solidly replace all the suspension bushings without worrying about tearing out chassis mounting points. Also, you could use a coilover on the rear to lower/corner balance all four corners, also you can tie the rear upper mounts into your cage.

The Z3 uses a semi-trailing arm, more like the older 510. Sure we could replace the differential bushing but then what would we do when the sheet metal crossmember that's spot welded on tears off the sheetmetal trunk floor. Our rear springs are also burried so adjusting and corner balancing with them are out. Also, because the upper spring pad is burried we can't tie this into our cage and are limited to ~700lbs (~300lbs at the wheel) rear spring rates. Only way around this is to move the springs to a true coilover set up and fab new upper shock mounts tied into the cage.

Best bet is to use the STU/STO allowance and fab a double-A arm rear suspension.

 

[JeffYoung]

They are essentially teh same save for spring/shock location. On the Z, the spring/shock is part of the hub assembly; on the Z3 it is on the trailing arm.

Both use large a-frame based trailing arms, and flexible halfshafts. It's essentially the same suspension, the only difference is that on the Z3 the diffcarrier is integral to the "subframe" while on the Z it bolts in/on.

Why should bushings be free on one and not the other? Makes no sense.