Strut Bar weld in or??

[NoLift911]

I need to install a strut bar as part of my suspension refurb and was looking around at different ones. The weld in seems the most straightforward but most don't offer any kind of preload adjustment. I noticed some (not all) of the factory RSR bars did not either. How important is the preload? For the weld in bars I doubt you will be able to pull any additional camber out of them if attached to the strut towers? Correct me if I am wrong.

Anyway any thoughts on the weld in vs the bolt on strut top appreciated...and yes I searched and read but didn't really seem to be a consensus on the best way to go about.

For a street/DE car - mostly DE.

Thanks -

[HarryD]

While no expert, part of your Strut Bar selection decision needs to be based on your intended use.

For some sanctioning groups, a welded in brace system moves you to a higher class which may or may not be desirable. If you are just out for fun, I suspect the difference is academic.

My Weltmeister is totally removable and I can add some preload to the towers by merely turning the bar.

[Brando]

If you're not doing a full on track car, bolt in is fine. I've got a Rennline bar from a courteous PP member and it works great for intended use.

[Flieger]

I got an adjustable one specifically because my non-adjustable bolt-in one could only be inserted in such a way that the chassis was all pretzel shaped and would not align the front wheels properly. I am not a fan of "pre-load" and twisting the chassis for more camber.

[5:04]

Not to Hijack, but I had been wondering about the size of the Porsche ones vs that of what I've seen in other manufacturers. The ones that I see for 911's look like ones we used to joke about using as towel racks since they were flimsy enough to bend by putting your weight on it. I planned on welding a bar in since I wasn't too keen on the way they looked since it seemed like they weren't as "substantial" on the strut bars I'm used to.

As an example here is one for a BMW made by Mason Engineering.

[Canada Kev]

Weight up front from the engine may have something to do with it. As well, from what I've read, the bar just has to hold the shock towers together, as cornering forces want to pull them apart rather than force them together.

[euro911sc]

I think the thickness difference is simply driven by the fact that the other makes have the engine up there in the front trunk... WTF we're they thinking? The engine clearly goes in the back!

I am not sure that it makes much difference, physics wise, between bolt on vs. weld in. Many of the weld in type are actually tied into the cages and function as part of the cage as well. You can have a weld in and bolt in bar... the mounts are welded into the strut tower with a bar bolted on to the mounts. Having a bar that screws around to change length and pre-load may be a nice configurable feature, IMHO.

Your biggest choice, I believe, is should you go with 2 point or 3 point (also called triangulated). There are also some 'X' bar systems that tie the strut towers together with the front sheet metal. Some of these are even removable.

Another main choice may be weather to attach to the strut towers or to the strut mounts. Not sure there is a difference in physics here either, but it is a choice none the less.

-Michael

[Flieger]

If they are loaded in tension, then they are strong enough, particularly if steel. If compression, they can bend/buckle in the extreme case. They are not designed to, nor should they be seeing, bending moments.

If you figure the outside tire does most of the lateral force, then the strut towers should be pushed together.

How much they move has not really been measured. It cannot be much unless one has RSR/935 type tires and speed.

[Canada Kev]

While not the end all and be all, I suspect that Weltmeister might be considered somewhat of an authority:

"The design people at Weltmeister built a testing device to record shock tower movement under load. Their tests revealed a surprise, shock towers do not collapse inward! Rather, they spread apart under hard cornering. The cornering force from the tire’s contact with the road, up through the strut, acts like a lever pulling the top of the struts apart. Only 15% of the camber change could be attributed to shock tower movement. The other 85% took place in the rubber mounts on the top of the shock absorber!"

I can see it, though... Like any lever, the strut, when a force is exerted (the outside tire being pressured inward), would move in the opposite direction at the other end, which is outward.

[911st]

Using and adjustable bar to pull the struts together can gain camber but closes the fender to hood gaps. Thus, only worth considering if some class rules do not allow proper methods to gain proper camber.

Removable may be handy if one may have to fill there trunk with something that would not fit other wise or maybe service the AC for example.

The BMW strut brace looks like in might not only reduce in and out movement but might also be designed to reduce shuffling movement of the front end. Think of a box with no top or bottom. It is easy to push it out of square. Just a guess.

[911pcars]

Some of the factory race cars use weld-on brackets on the inner strut tower area, but use a fixed-length bar to connect the two sides.

If you want adjustability (good), replicate the above weld-on brackets but with a threaded bar and Heim-joints.

Sherwood

[NoLift911]

Ok so next question - who sells the brackets (I know RUF does with the bar, non-adjustable)? And who sells the bar with the adjustability for use with the weld on brackets?

[Flieger]

I have the weld-on mounts. It was a long time ago so I do not remember where I got them from. I got my adjustable strut bar from Clint at Rebel Racing. Aluminum tube, tapered ends, Heim joints.

I still think the strut mounts must press inward. The strut is like a lever, located on the bottom by the ball joint and A-arm and located on the top by the monoball (in my case). The wheel's cornering force is transmitted through the spindle, into the strut. The strut then wants to pivot about the lower ball joint, because the force transmitted through the ball joint and A-arm has the suspension pan to hold the bottom of the strut towers together. On the top, the compression has only the sides of the box to resist. It just makes senses that they would get closer together, assuming that the outside wheel does most of the work.

Although, now that I think more about it, there is the lever of the tire radius, since the tire force is applied in the ground plane. That would reverse the direction of the moment on the strut.

That would mean that a strut suspension would put the top strut mount in tension, favoring the thin lightweight bars on 911s.

I suppose all one needs to look at is the instant center for each wheel, or the roll center for each axle. That location is where the lateral force vectors point to, so that would give the direction and magnitude of the moments on the struts, and one could then find whether the top is in compression or tension. A high roll center would be compression and a lower roll center would be in tension. It would be extremely hard to make a strut suspension that had a roll center high enough to be in compression, from that perspective.

[911pcars]

Originally Posted by P-Dawger

Ok so next question - who sells the brackets (I know RUF does with the bar, non-adjustable)? And who sells the bar with the adjustability for use with the weld on brackets?

For weld-on brackets, check Rebel Racing as suggested; also Smart Racing Products and others. As for the threaded tube and rod ends, aluminum or steel? I suggest Speedway Motors. Select the correct length after determining span.



Sherwood

[Trog]

ER solution:

[NoLift911]

Looks good Trog - what does all of that weigh? Do you notice a difference in the structure during conering etc?

I know Chuck makes good stuff and was thinking about that very setup - then I thought a little bit simpler.

[Trog]

Maybe 10-15lbs. It's all light-weight aluminum. Really stiffens up the front-end.

[proffighter]

This solution I do not like, locks a bit weak:

[Flieger]

I am glad someone else thinks that, too. Any vertical load on that joint or any internal moments on that spot and the truss collapses, if it were not pre-tensioned.

I like the BMW design.

[KTL]

Don't give Weltmeister too much credit. Their design is extremely flawed in that it restrains the strut shock shaft. The shaft MUST be allowed to pivot. Case in point is Weltmeister's bracket breaks if you put enough load cycles on it. It breaks because the shock shaft pivot action is so great that the bracket can't handle the load.

In terms of weld in vs. bolt in, i'd say weld in if you're really serious. The weld in is much stiffer and if you plan to attach anything to it, like say a helmet side net forward attachment, this is key. I have my side net attached to my bolt in bar and I can see the load placed on the bar by the ratchet can easily make the bar flex. I plan to weld in my next bar after I get my forward oil tank completed.

Of course this is very extreme for a street car. Just pointing out that my install of the side net connection revealed to me how easy it is to flex the adjustable cross bar.