RSR Type Engine mount cross bar

[911pcars]

The add'l metal along the top and bottom edge will add the needed rigidity reduced by the add'l holes. Net effects are offsetting, weight-wise. Has anyone compared?

Sherwood

[Nate2046]

I would NOT drill the standard bar. That thing is flexy enough without removing a bunch of material. I bought the Rennline version sold here to replace the bent OEM unit in my car. It's definitely a way sturdier piece and fairly cheap as far as upgrades go.

[911pcars]

Originally Posted by Nate2046

I would NOT drill the standard bar. That thing is flexy enough without removing a bunch of material. I bought the Rennline version sold here to replace the bent OEM unit in my car. It's definitely a way sturdier piece and fairly cheap as far as upgrades go.

I'm not totally sold installing holes is a bonafide upgrade unless the mount becomes both sturdier and lighter, but it does look racy.

Sherwood

[Flieger]

Originally Posted by 911pcars

The add'l metal along the top and bottom edge will add the needed rigidity reduced by the add'l holes. Net effects are offsetting, weight-wise. Has anyone compared?

Sherwood

Ah, that is where you are wrong. You are moving the material around to change the distribution of the rigidity basically. By punching holes and adding I beam flanges of less mass than was removed, the whole thing will become more rigid in the fore-aft direction and in the vertical direction. Lateral will be less rigid by an almost immeasurable amount.

This is a case of making the best use of the least amount of material so that the material is not understressed from a % of yield strength point (extra weight) while being as rigid as possible.

[911pcars]

Originally Posted by Flieger

Ah, that is where you are wrong. You are moving the material around to change the distribution of the rigidity basically. By punching holes and adding I beam flanges of less mass than was removed, the whole thing will become more rigid in the fore-aft direction and in the vertical direction. Lateral will be less rigid by an almost immeasurable amount.

This is a case of making the best use of the least amount of material so that the material is not understressed from a % of yield strength point (extra weight) while being as rigid as possible.

I think I'm saying the same thing in the context of comparing the weights of two examples.

Sherwood

[Flieger]

OK. I guess I did not understand what you were saying fully.

The rigidity/weight ratio goes up, but the strength goes down a bit if you reduce the weight.

[Walt Fricke]

This one has held up fine for over 10 years of track only use. The machinist (an engineer) and I winged it on hole size and placement vs. how much web to leave in between. And no reinforcement either. I welded nuts on for the chassis mount bolts, and luckily have not bent those ends off despite being lazy and not counterholding.

I welcome fore and aft flex, as it makes getting the long bolts started easier - race motors go in and out a lot more than street motors.

Now if I could figure out the optimum pattern of holes in that exhaust support square tubing, I'd happily get to work saving an ounce or two.

Two of the holes in the motor mount proper are really useful. The small one upper right just under the crosspiece lets you get a socket wrench on the mount nut behind it. And the big hole opposite the pulley means you can get a wrench in on the pulley bolt easily.

The outermost two holes on the cross piece are a bit much, in retrospect.

Perhaps my good luck with this not failing might be due to using solid motor/transmission mounts? Less flexing leading to fatigue failures?

[Mr9146]

Originally Posted by Walt Fricke

Perhaps my good luck with this not failing might be due to using solid motor/transmission mounts? Less flexing leading to fatigue failures?

Naaaah. It's because you're not making any real horsepower!

[Flieger]

Well, I guess the holes were small enough and you cleaned up the edges enough that the stress risers are small. It will still fail sooner theoretically, but it should be a LONG time.

[jeff eelkema]

[QUOTE=Walt Fricke;6318497]


I welcome fore and aft flex, as it makes getting the long bolts started easier - race motors go in and out a lot more than street motors.


Just curious, is some fore and aft flex going to cause gearshift issues or gear retention issues on acceleration or deceleration? Or does the gearbox mount do enough to stop any movement when the car is driven hard?

[Walt Fricke]

With solid mounts there is zero movement of the gearbox. The flex which sometimes helps me with installation is only in the rear mount crosspiece.

I usually leave the transmission in when pulling my motor. Saves lots of time. I used to chain the transmission up to the shock tower crossmember to support it.

Then I noticed it didn't need support! Not only did it not sag as you would expect when the motor was removed, but it even seems to rise up a little bit at the back.

I'm not recommending ignoring supporting it (lots of stress on the nose piece and so on), but it isn't budging if you have solid mounts.

Of course I am not making enough horsepower. Who ever is? For years it was 222 or so RWHP with the 2.7. The 2.8s are putting out 250 RWHP. I want at least 300 flywheel, but I'm not there yet, blast it. If that will break this piece, fine with me - at least I would have the power. I've got a stock piece in the trailer.

The one failure I observed (real RSR piece, too, not home brew) led to two laps of sparks before the driver realized it had happened. No serious damage.

I'd not consider holes in the crossmember for my street/track SC. Why? It has to make weight when raced, and the small reduction the holes make, the even smaller change in CG location, and the vagaries of highway driving (not a track with a tow truck handy and a trailer with spares and tools at hand) suggest not doing low value/some risk things.

[strokher racing]

How much lighter than stock is the rennline brace?

[RWebb]

Originally Posted by Walt Fricke

Now if I could figure out the optimum pattern of holes in that exhaust support square tubing, I'd happily get to work saving an ounce or two.

make the center of it out of CF with the ends of inconel on screw threads to adjust for length

oh yeh - I drilled my cross bar too and powder coated it silver - not very functional, but...

[tazzieman]

Originally Posted by strokher racing

How much lighter than stock is the rennline brace?

Very little i.e. not significant enough for me to go and compare weights. It's a solidly made piece , even though it's full of holes. I can bend the stock one with my hands.

[Toddlaunchbaugh]

Great thread. But does any one know what the weigh savings actually is,

[RWebb]

engine support crossbar (stock 2.7L vs. 1986 with holes drilled) 3.0 -0.52 2.5 drilled 3.2L bar - RSR type

[Toddlaunchbaugh]

Thanks , Half a lb for $225 if you purchase one. I still struggle with justifying $100/lb

[panzerfaust]

Originally Posted by Flieger

Ah, that is where you are wrong. You are moving the material around to change the distribution of the rigidity basically. By punching holes and adding I beam flanges of less mass than was removed, the whole thing will become more rigid in the fore-aft direction and in the vertical direction. Lateral will be less rigid by an almost immeasurable amount.

This is a case of making the best use of the least amount of material so that the material is not understressed from a % of yield strength point (extra weight) while being as rigid as possible.

All things being equal, yes adding the large hole will make it weaker

But if you swage the hole by altering the cross section it can become lighter and stronger than the original part with holes. I’m surprised it hasn’t been done yet

[Walt Fricke]

If this is what you see done in sheet metal reinforcements, isn't this material too stiff to be done easily? Wouldn't you need a hefty die and very strong press?

I'm nut sure why making it more rigid in the fore and aft direction is all that important. Porsche just made holes for the RSRs, didn't it. Solid transmission mounts will hold resist those forces.

Does swaging holes in sheet metal make if more resistant to bending when under compression? But here compression on one side is resisted by tension on the other, isn't it?

I'm not following the "lighter and stronger" part, though, assuming the holes are the same size. Isn't tensile strength more or less governed by the smallest cross sectional area? Which is at the 0 and 180 degree point on each hole? I've tried to keep those consistent hole to hole, though I have no idea what a reasonable minimum would be.

Since my home brew ones haven't failed, the old 356 guys would say I probably made them too small. Keep going until things break, then go back t the next smaller size seemed to be the mantra from the Swiss cheese era of lightening.

[911pcars]

My engine support bar weights:
- Factory support bar (below): 3 lb 1.4 oz (1400.47 gm)
- "Drilled", then reinforced along top and bottom edges, 3 lbs 13.4 oz (1740.66 gm)


For comparison, an "RSR" spec support bar:


BTW, early support bars use a nut/bolt (M12x92mm) attachment to each chassis mount. Later support bars delete the nut - the end boss is threaded to accept a hex bolt (M12x95mm). Unsure of why the difference in bolt lengths.

The solo bolt threaded into the bar end does not provide a counterhold during loosening and tightening procedures. This results in stress applied to the joint between the round end boss and the support bar. Numerous R&R cycles often/sometimes results in fatigue failure at that junction.

Short of using an RSR-spec support bar, I suggest reaming out (removing) the internal threads on later bars, then add a nut and washer attachment to mimic the early support bar mount. The hex bolt head thus provides a proper counterhold when tightening or loosening the clamping nut.

Early support bar:


Late support bar:


Sherwood