Is it ok for my strut brace to push against...

[CRXer]

john,u seem to have put a lot of thought into this,but,im finding it difficult to picture where a 2point brace,whether it be front,rear,b/c pillar would be subject to a compressive(flex) load.my understanding is they are designed to tie the points together by tension.
could u describe a situation where this might be the case?

[Chriskoss]

ShiftPerformance has the ultra racing 2 point front strut bar for 200.. I think ill get this then!

[string]

The offending diaphram is a throttle damper to keep the revs up between shifts - nothing to do with cold idle (no coolant hoses) and nothing to do with fuel delivery. I've removed them on all of my B18's and didn't notice a difference.

[redefine]

john,u seem to have put a lot of thought into this,but,im finding it difficult to picture where a 2point brace,whether it be front,rear,b/c pillar would be subject to a compressive(flex) load.my understanding is they are designed to tie the points together by tension.
could u describe a situation where this might be the case?

when both the suspension pillars are compressed because of a dip or something there is a compressive force toward the middle of the car on either side at those points.

depending on the bank of a corner, it may happen there too

[JohnL]

The offending diaphram is a throttle damper to keep the revs up between shifts

Off topic but,

With some Honda engines (the Accord f22A9 being one example) the IACV is used for a similar purpose (I would suggest mis-used...). The ECU keeps the IACV open momentarily after the throttle plate has been closed (as when shifting gear), which keeps the rpm momentarily high then causes the rpm to drop more slowly than it otherwise would (should...). I think this is associated with emisions control.

This slows the speed with which smooth upshifts can be made and generally stuffs up the driveabilty. I've blanked off my IACV so that no air can flow through it into the plenum, which makes for a huge improvement, except that the AC, lights etc now causes the idle speed to drop somewhat...

- nothing to do with cold idle (no coolant hoses) and nothing to do with fuel delivery. I've removed them on all of my B18's and didn't notice a difference.

I'm not familiar with this particular engine, just that it looked like an FITV from the photo. My Accord's FITV works perfectly even though I've blocked the lines that route coolant to it (and the throttle body). Seems like ambient engine bay temp and heat soak from the plenum casting is enough heat to make it work...

[JohnL]

when both the suspension pillars are compressed because of a dip or something there is a compressive force toward the middle of the car on either side at those points.

depending on the bank of a corner, it may happen there too

Yes, but any time a significant bump loading is passed into the tower top the force will tend to push the tower top inward.

Some people seem to think that as corning forces are leveraged into the suspension linkages from the contact patch, that at the (more heavily loaded) outside suspension the lower 'wishbone' will be in compression and the upper wishbone will be in tension, and thus that the tower top on that side will be pulled outward by the upper wishbone, so a tower brace would have a tensile force applied to it.

This is actually correct, but due to the geometry of the wishbones relative to the contact patch (with the upper wishbone being so high above the lower wishbone), this outward lateral force seen at the tower top is relatively weak and of little consequence compared to the bump loadings caused by chassis roll.

By far the greatest lateral force acting upon the wishbones is acting in compressionn on the lower outside wishbone, with a relatively minor tensile force acting on the upper outside wishbone. This is why the upper ball joint can be so small compared to the much larger lower ball joint.

[redefine]

the only way i can see the top controll arm being in tension is under significant cornering, where the outside wheel will have a torque forcing the upper side of the wheel out. i cant see it happening under a general compressive load on the suspension. is there something i'm missing, like the geometry causing it? or are you talking about during cornering?

i wouldhave thought no matter what, the strut brace would always be under compression when the suspension is compressed as the suspension would put a force up at an angle through the top of the tower.

[CRXer]

With some Honda engines (the Accord f22A9 being one example) the IACV is used for a similar purpose (I would suggest mis-used...). The ECU keeps the IACV open momentarily after the throttle plate has been closed (as when shifting gear), which keeps the rpm momentarily high then causes the rpm to drop more slowly than it otherwise would (should...). I think this is associated with emisions control.

i thought its all IACV now? & dashpots got the flick when ecu's got smarter,ah...who knows...

like string said u can remove the dashpot,but u will get the emissions effect(circumstantial overrun of fuel between gears(or other throttle slam shut situations)) & also chance of engine stall at low rpms,throttle slam shut.

[JohnL]

the only way i can see the top controll arm being in tension is under significant cornering, where the outside wheel will have a torque forcing the upper side of the wheel out. i cant see it happening under a general compressive load on the suspension. is there something i'm missing, like the geometry causing it? or are you talking about during cornering?

Yes I agree. All I'm saying is that there must be multiple forces that work in different ways to place a tower brace in both tension and in compression. Any momentary tensile force will deduct from any momentary compression the brace may be in, and vice versa.

There is some opinion floating around on the net (can't recall where I saw this) that the predominant load on a tower brace is tensile due to the outward force acting upon the upper wishbone (or the top of a Mac Strut) when cornering, but IMO this ignores the affects of vertical loadings deflecting the tower top laterally inward.

i wouldhave thought no matter what, the strut brace would always be under compression when the suspension is compressed as the suspension would put a force up at an angle through the top of the tower.

I would expect (though have no data to prove) that compressive loads in tower braces are predominant, with the greatest loads occuring when a wheel hits a bump etc., largely because in this circumstance it's probably a compressive load only.

Loads on a tower brace in roll are likely to be + compressive - tensile = a generally lessened compressive load. However I suspect the load peaks might be 'spiky', i.e. in roll (ignoring loads caused by bumps) there may well be brief moments where the compressive load is fairly high, and moments when the load becomes slightly tensile... maybe.

It occurs to me that it would be relatively easy to check the direction of these tower forces by disconnecting one end of a tower brace and setting up some sort of marking system, e.g. a pencil taped to the bar and rubbing on a peice of cardboard taped to the other tower top, or something similar.

[string]

My quick take on the subject:

On either wheel, the UCA and LCA must be forces of opposite sign. For them to be both under compression, or both under tension, the contact patch force must be applied between the two arms, which is impossible since this would mean the LCA mounts the upright below the surface of the road.

During roll, at the front axle, the inside wheel is pulling the nose of the car and the outside wheel is pushing. The outside LCA is therefore under compression and the UCA under tension of a lower magnitude. The inside LCA is under tension and the UCA is under compression in similar ratios to the outside, but at much lower overall magnitude due to smaller contact patch forces thanks to lateral load transfer.

Static negative camber adds an inwards lateral force at both wheels resulting in a static tension in the UCA and thus the strut brace. During roll the effects of lateral load transfer and tyre load sensitivity means you lose more "grip" unloading the inside tyre than you gain from loading the outside tyre. Therefore you are losing more total inwards camber thrust than you are gaining during roll reducing the strut brace tension the more lateral load is transfered. As as aside, from this result it can be seen that [as long as your particular tyre agrees] static camber is always beneficial for lateral force in the direction you want, the more lateral load transfer the better your resultant camber thrust (i.e. FWD cars, which are trying to minimise front load transfer experience these benefits to a much lesser degree than a front stiff RWD).

The only totally compressive force on the strut brace is from the compressed springs levering the upper control arms inwards off the contact patch. Whether or not this counteracts all the other tensile loads would be a complete guess on my behalf and a quick thought experiment can't resolve them like a full analysis could.

In summary, I'd be inclined to agree with someone claiming that a functional strut brace is important. In addition, it should be fairly clear that having strong lower control arm bushings is very important.

[redefine]

There is some opinion floating around on the net (can't recall where I saw this) that the predominant load on a tower brace is tensile due to the outward force acting upon the upper wishbone (or the top of a Mac Strut) when cornering, but IMO this ignores the affects of vertical loadings deflecting the tower top laterally inward.

this is what i was saying, and i agree with you

i understand that. i think i just misread your post. haha my bad

[Chriskoss]

Physicians please move this discussion to another thread , and thankyou all kindly for the useful input