strut brace and longterm issues?

nostatic · nostatic's Garage

Anyone have data or opinions on the affect of strut braces over the long term (in a coupe)? While I understand they help fight chassis flex, seems like that stress has to end up somewhere else. Is there any real downside to installing one like the Elephant Racing unit (single as I think the triangulated one would take away space I need for storage)?

Superman · 12-09-2002, 12:56 PM

I've never heard of this kind of device increasing stress somewhere else. I'd imagine it just as likely that stress would be reduced, even in other areas.

I wonder if you could USE a tower brace first, to see if it makes any difference. Folks with Targas and Cabs claim enormous improvement. FWIW, I think couples are so much more rigid that I question whether it would make much difference in your car. Solid motor mounts did nothing in my car except resonate the frame with engine noise, whereas the topless crowd seems to notice sharp improvement.

makaio · 12-09-2002, 01:02 PM

Static, for you this is the best reason for putting one in, 78silverSC gets photo kudo's:

Rouxzy · 12-09-2002, 01:56 PM

Where I come from that is what the passenger seat is for.

arcsine · 12-09-2002, 02:13 PM

As I understand the physics of it, the force that deflects the upper strut mounting is not diminished or eliminated with the addition of a strut bar. In the case of a non-triangluated bar, what you are doing is making a trapezoid out of your front suspension. The lateral forces generated during hard cornering must now move BOTH strut mountings which is twice a difficult to do as moving just one. Which is why when folks use triangulated bars, they must use an appropriate mount for it as those lateral loads will be lead to where ever the triangulating bar is attatched. Mount it to the wrong place and it will rip out.

Now Island will tell us the real story instead of my fantasy.

Superman · 12-09-2002, 02:30 PM

Yeah. Island must be sleeping.

nostatic · nostatic's Garage

I get some sunroof creak when I pull in the driveway (angled and uphill), so there's got to be some flex a higher speeds in corners. In the bicycle world I have an idea about how flex affects things. One rule is that if you beef up one area you can expose problems at another one.

I like the StPauliGirl functionality. Does anyone know if a case of Pipe will also fit?

911nut · 911nut's Garage

According to B.A. strut braces don't work, but he doesn't say why. I think it might be that the loads travel through the floorpan to the front bulkhead.
If the 911 had coilovers then you would need the strut brace for sure as the loads would be put into the strut towers and push them to the centerline of the car.

Chuck Moreland · 12-09-2002, 07:20 PM

911 tower flex is easily demonstrated when I put a preload on my strut brace. You can actually see the towers pull together with very little force.

Gary's assesment is right. Normal, non-triangulated strut braces add little rigidity since they force the towers to flex together as a parallelagram, but do not make them rigid.

Check this tech article about triangulation with a specific example relevant to the 911 discussion:

http://www.elephantracing.com/techtopic/triangulation.htm

The fact that most braces are non-triangulated is probably the reason strut braces have collected so many critics. Triangulation works.

RoninLB · 12-09-2002, 08:17 PM

Quote:

Originally posted by Superman

I wonder if you could USE a tower brace first, to see if it makes any difference. Folks with Targas and Cabs claim enormous improvement. FWIW, I think couples are so much more rigid that I question whether it would make much difference in your car. .

I kinda think a tower brace is a waste with coupes on streets.. the only reason that I use it is because there was a story of a guy doing highway speeds who slid off the road in Nebraska into a big grassy shoulder with ruts and hills.. he claimed the brace helped prevent the wheel from folding under.. it it would have folded the 911 would have rolled, go figure.............Ron

island911 · 12-10-2002, 12:36 PM

Quote:

Originally posted by Superman
Yeah. Island must be sleeping.

shhhh

Heh heh

I too like the StPauliGirl functionality. Thats about where it ends for me.

Chuck Moreland has this alarming image on his site.

Believe me, structurally, this is NOT what the 911 front clip is like.
(but that is a cool graphic for statics101 students)

The cross brace can add stffness, (esp when preloaded) but the forces at those points isn't that high.
So yeah, I'm in agreement with Bruce Anderson on this one. .. .Unless you have coil-overs, these things solve a problem you don't have.

Chuck Moreland · 12-10-2002, 01:27 PM

The lateral force excerted on the strut tower is indeed significant. The strut/hub/wheel/tire forms a lever arm with the ball joint as a fulcrum. The ratio between sides is roughly 4:1, hence the lateral force at the top of the strut is roughly 1/4 the lateral force generated by the tire.

Let's look at a real example. SC or Carrera carries about 1080 lbs on the front tires. A sticky race tire might have a coefficient of friction of 1.6:1, hence potentially 1728 lbs of lateral force (neglecting front/rear weight transfer affects of sway bars and springs).

That means 1728/4 = 432 lbs of lateral force at the strut top!

I can assure you 432 lbs of lateral force will cause elastic deformation of the strut tower.

If this was a good street tire with a coefficient of say .9 the strut tower would still see about 243 lbs of lateral force. That it is still plenty to move the tower and change the camber setting.

Yes I made some simplifying assumptions in the example. But my point stands.

Coil over vs tbar has no impact on the merits of strut braces. The coil over force vector is nearly perpendicular to the strut brace (read as irrelevant).

island911 · 02:38 PM

Yes, and I can assure you 432 lbs of lateral force will cause elastic deformation of the strut tower. . . even with your strut brace! SO-what!? Everything Flexes!
It all comes down to how anal you want to be about deflection.
As I said, preloading *is* an advatage of these to add stiffness. But I don't see the necessity here.

As it is, the shock is a long stick, pushing on that point. . . it's whimpy by comparison to the structure of the body; so it's going to flex too.

. .so why not have a brace for the shock length?

Anyway, nostatic's SC, in my opinion, isn't going to reallize the tiny gain in camber stiffness.

don911 · don911's Garage

I can only comment on the weltmeister brace because this is the one in my car. The weltmeister brace improved my car's handling quite a bit. On the autox track, I noticed less push and far better turn in. The turn in is what I noticed the most.
I agree if you just link the tops of the struts together, the effect may be very small. The welmeister brace also links the top of the shock to the strut. The top of the shock goes thru a big rubber donut in the upper mount. All the flex in the big rubber donut is also taken out.(cheap monoball effect)
Without the brace, my camber was changing enough that the top of my tire was rubbing on the fender during hard, sweeping corners. That doesn't happen with the brace installed.
IMHO, if you're autoxing or doing track events, the strut brace can improve your car's handling.

Chuck Moreland · 12-10-2002, 05:26 PM

A triangulated brace cannot flex in the plane of triangulation. It can fail, but it can not flex. That is the nature of triangulation and the reason a triangulated brace is effective where a parralellagram brace is not. My "statics 101" write up covers that.

The strut is actually a very stout stick that is up to the loads presented.

The shock tower is merely two layers of sheet metal. It's plenty rigid to forces vectors directed fore/aft and up/down, but its shape provides limited cross car rigidity. It flexes relatively easily.

Has anyone ever seen a competitive 911 race car that does not have a strut brace?

TimT · 12-10-2002, 06:42 PM

I just love it when engineering types get into techo arguments, we get wound up at what makes most people doze off to dreamland.

Every purpose built 911 racecar I have seen (not counting very early cars) early has some sort of heavy strut tower brace. If the guys with the slide rules didnt decide to start installing strut tower braces on the cars, Im sure it was the mechanics that made the decision ( based upon fatigue cracking and other "hints" they may have seen) that did.

The cornering forces a street driven car sees are many times less than a race car. Does a strut tower brace work? yes absolutely. Does a person who uses the car for sunday brunches need a strut bar, no.

Chuck good point about the vectors being perpendicular. Island good point about the benefits on a street car.

Oldporsche · 07:08 PM

Well I think you guys are working pretty hard at this.

The strut brace will help probably in each instance however it will help some body types more than others.

Chuck has some very good points. The cross section of the car in the towers/suspension area is actually a trapazoid. (top of triangle cut off) In other words it will give a little.

Cars built >'73 had a channel formed in the floor pan around the peripheral that was deeper than the <'74 cars. You can check this out. This was an important factor in making of the turbo targa. I think I can find the article.

Early small bumper cars (<'74) with the smaller rim channel were prone to flex more than the later ones. The targas were probably the worst. My first car was a '72 Targa. I could go around a real tight corner and watch the dashboard move in relation to where I was sitting. It was even worse in a friend's car which was in addition to being a '72 targa was a clipped car too. When I drove it, I actually was beginning to feel impaired. The input sensations to me from the car did not match the reactions I was looking at from the vehicle. Just plain weird.

In my '70 911E Coupe, I worked a lot on the allignment with the above mentioned friend. He had a Bee-Line frame machine in his body shop on which we did the allignments. This way we could monitor all four wheels at a time. When we went to work on the camber of the front end we found that we had run out of movement with the top of the struts and still had too much negative camber. No, the car was lowered only 1" below stock height. We did this to make up for the lack of front spoiler when I added the ductail at the rear. I didn't want to lower the car because I was trying to get the suspension and allignment to work for me as much as possible at the autocross.

To gain enough room for the most amount of negative camber but to keep it within factory specs, we had to put a porta power between the front towers. This causes a gap in the allignment of the sides of the front hood. I look for this gap on all early cars. From what I can tell, if the gaps on the front hood are parallel, there's a better than even chance that the front end has enough negative camber to be out of factory specs. (my observation)
It has also been my observation that the small bumper cars that were "adjusted" like I described above, soon lost their gaps at the hood with just normal street driving. So the towers will move about in normal operation of the vehicle. I confirmed this observation with Chuck Stoddard in a discussion with him about the use of Koni shocks on the front end of early (<'74) cars. His recommendation were to actually make sure the red konis are set on full soft. Don't even think about using the yellow shocks. He said also that my observations about the front hood gap were correct. He's seen a few Porsches over the years.

Thus the camber bar, trussed (triangulated) or not, will help the early (<'74) Targas more than the later (>'73) Coupes. The trianqulated models will help keep the allignment more stable than the untriangulated ones.

My $.02 worth.

Good luck,
David Duffield

john_colasante · 08:10 PM

Consider this. Quite a few factory racing 911's used simple strut braces to tie the front shock towers together including many early factory race cars, all type 964/993 Cup cars, all type 964/993 RSR, type 993 GT2, etc. I like to think that the Porsche race engineers at Weissach know a little about these cars to think these devices might be of some benefit.

So, who do you believe Roland Kussmaul or Bruce Anderson? My money's on Roland...

EDIT: I didn't see TimT's response when I wrote this which basically says the same thing.

Randy W · Randy W's Garage

My experience agrees with what Don and David posted. IMO a Weltmeister strut tower brace will help a pre-'74 911 (even a coupe) survive longer if it is driven aggresively. Long hood gaps are very revealing...

A Quiet Boom · A Quiet Boom's Garage

I'd like to relate a few things I've witnessed. First a simple home built strut brace in my '66 made a vast improvement in several areas including turn-in, reduced chassis rattling/creaking and improved steering feel. Examining the area in question I believe the brace can only help chassis longevity much like subframe connectors on front engine rear drive cars prevent permanently twisted chassis. My experience with drag racing front engine cars has shown me that despite ADDING 150lbs of roll cage the car will launch straighter and go faster despite the increased weight IF the rollcage ties the suspension pickup points together. Suspension design for street vehicles assumes a rigid chassis from the factory with small allowances made for some chassis flex but inorder for a suspension to properly work it's attachment points MUST NOT flex in relation to one another, engineers make concessions to chassis rigidity to allow more usable room inside the vehicle, engine compartment and trunk and since a strut brace takes up that valuable room it would not be an option for most cars. If anything I believe that the brace will extend the useful life of the unibody by preventing excess flex that can be encountered even during hard street driving. As Chuck stated triangulation is the key here for best results but even a simple brace connecting the struts will transfer some of the force on the inside strut to the outside strut and provide some benefit. If you look at the back of a 911 you'll notice the factory unibody effectively has a "built in" brace between the rear shocks and I'd bet they'd have done the same in front if it wouldn't have made an already small trunk even smaller. My experience with bracing is that the cars without it are the ones that suffer the damage. At least for me the benefits far outweigh the negatives (cost and loss of trunk functionality).