What is the mm size of the OEM rear sway?

 

I don't buy this. They would have to account for so many different factors this would be fruitless. Do they account for bushing deflection as well?

I have always believe that it is best to put the car on the frame supports to install bracing. I believe the cars are accurately built on the assembly line unloaded and ready to do. The degree of chassis flex that occurs when the car is loaded is indicative of how good that chassis is at maintaining its proper shape. Do you really think the designers do the backwards math of the effect of the car just sitting under its own weight to come up with the actual design schematics? I don't

But I also think it is really not all that critical, otherwise the stock brace would not have oval holes allowing installation of the bar with the car on the ground or in the air.

Marcus

 

I see you are new here...the stock bar size varies depending on the year of the car and the transmission option.

I suggest you poke around a bit more. Also this post is off topic. We are not talking about sway bars; we are talking about shock tower bars. There is a difference. Again some poking around should come up with the answer on that too.






Welcome

Marcus

 

The amount of deflection can be pretty accurately calculated and it can be very accurately measured. I know for a fact the engineers factor in the amount of flex. Think about the fact that the car has positive camber. This in itself is compensating for tire deflection and flex in the suspension. Bushing deflection is most definitely factored in as well, the engineers know exactly how much the bushings are going to give under load.

Every chassis has deflection and flex no matter how stiff it is. Every car is going to be different sitting on it's tires vs sitting on the frame.

You drive the car on it's tires, not on the frame, why would you put it on the frame to install braces?

It would be like doing a suspension alignment with the car off the tires. How the car sits unloaded is completely irrelevant.

 

Ok, we've had discussions before, and I've been impressed with your posts. But I believe you are actually wrong here.


So, you are saying they design the car, then run a simulation to find the flex, then change the chassis design to compensate for the flex. I don't know what self-respecting engineer would do that, but I don't want that car. I can certainly agree they'd factor in the flex when they are coming up with their suspension geometry and their alignment settings and such...but when they are designing the chassis? No sir, the chassis is designed to resist flex. Not allow it. Adding load to the car before bracing it will tie things together just a bit out of whack.


This is not true. The camber goes positive when the car is unloaded for the same reason it goes negative when you load or lower it. It is the natural cycle of the suspension geometry to move this way. The loaded outside wheel gets a flatter contact patch in the corner if it's got negative camber. The unloaded inside wheel gets a flatter contact patch when going through the corner if it's camber goes positive. This is not compensation for anything. It is normal suspension geometry. Geometry that is better maintained when braced against movement when the load is introduced. Hence, bracing the car while unloaded will lock in the baseline geometry as intended when the geometry was designed.


Yes I am sure they do want to know how much the bushings deflect under load, but not to the end to compensate for it in chassis design. The compromise here is how hard of a bushing should they use. They design the chassis, then the suspension geometry, then they figure out what bushing gives the best compliance versus dampening balance for the tune of the car. They don't pick bushings first, then design how much chassis distortion will compensate for the deflection of the bushing they picked.


Obviously you want to align the car on the ground because that is where the chassis and tires are settled in. The argument here is really whether or not you could get a better alignment with braces installed before you put the cars down versus if you put the braces on while the car was on the ground.


I can see the direction you are thinking in...but there is a flaw in your logic. How the car sits unloaded versus loaded is irrelevant to the ultimate alignment setting you achieve as once the car is loaded you adjust the settings to compensate for deflection in the chassis, tires and bushings. With respect to the chassis and bushings we play the hand we are dealt.

I think we need to do an experiment

We will check a car's alignment both ways. With the brace put on before before the car is loaded, and one put on with it loaded. Would that be a good test?

Marcus

 

You're right that the chassis is supposed to resist flex but even the best most rigid chassis flex. The TL's chassis is among the strongest torsionally. I remember an article comparing it to the C5 Vette in terms of torsional rigidity and the C5 was praised for it's frame rigidity when it was released.

However, all it takes is a drive the hear the interior noises as it slowly rolls over a driveway ramp unevenly to know the chassis flexes. Find a body gap like the door to door sill. Wedge your finger in there and then roll over some bumps unevenly, even slowly and you will get your finger pinched. I've done it with my TL. The factory's use of the front strut tower brace and the built in rear braces (the reason we don't have fold down rear seats) are proof the engineers were trying to stiffen it.


My suspension example was a bit extreme but think about why negative camber on the outside during hard cornering is necessary to maintain a flat contact patch. If the tire was perfectly rigid, the bushings had no give, and nothing in the suspension and frame flexed, 0 would be optimal but we know this isn't the case.

My whole point being, the engineers do know that no matter how rigid the chassis is, it's going to flex when you take the weight off the suspension and it's going to flex again when you put the weight back on. This is a known data point.

Cars have gotten much better. I remember as a 17 yr old doing transmission R&R I could put a cheap FWD car on the rack and press the bumper a few times and watch the car bow near the center. It would send a shudder down the chassis, pretty scary. My GN, it's a H-frame car and in stock form had visible chassis flex. In fact, I kept cracking the roof at the pillar from the flex of launching and that's just the engine trying to twist it up. Case in point, watch drag cars like mine that only lift the left front tire during the launch. If the chassis were perfectly rigid, it would lift both or none, but not one. The engine rotates clockwise so it pulls up on the driver's side framerail.

I've got the flu right no and a major headache so if this makes no sense I'll try and clear it up when I'm well.

Edit: I meant to say negative in place of positive camber in the quote you quoted me on.

 

No worries man, I have the same flu and headache at the moment.

I just got off the phone with an associate of mine at Honda. He is a designer, not an engineer, but he does know much about the vehicle design and races endurance events with factory support and such. Most important he has an inside word on what is involved with the design of the car. Google : lawrence huang honda design

Here is the deal.The frame points used to support the car on stands are known to be good reference points. Indeed the, while the design of the chassis is all based on the center-point of the front wheel, it is an impractical reference to measure from. So manuals on checking frame straightness reference the jack points. Therefore supporting the car on 4 level jack stands at the jack points is a great way to true the frame for installing braces.

Now here is the interesting part: If the same car is put on level ground without any extra weight in it, tires inflated properly and evenly, brand new tires with the same tread life, etc, ALL that precaution taken into account, the frame is in the same shape sitting on the tires as when it is supported by stands. This of course is within the scope of practicality and reasonable limit. While things might move under the vehicle's own weight it is such a minimal amount it is not of consequence for us to debate really. On level ground with good suspension and equal tire inflation, installing the braces will have the same effect on the chassis as if you did it while the car was supported.

It seems we are going to have to meet in the middle on this one. Technically you could put bracing on the car and lock in its shape with the car on the ground or in the air, either way is fine. However it is critical that the car be perfectly level, and if you are doing it on the ground all tires should be in the same condition (ideally new) with the same pressures, all weight removed from the car. Indeed a good reason for getting the car supported for bracing it is because it is easier to level the car on stands than on the tires given the different variables that will be eliminated from the equation.

MIND YOU, this is all true for when the car is STATIC. In dynamic situations of course the chassis becomes a mess of distortion. Which is why we definitely would not recommend installing braces when the car is moving (please appreciate the technical irony inherent in this comment). Also we are talking about later model cars as well which are more rigid that older ones. But the same is still true. Just sitting there the cars are designed sitting on the ground, and when you pick them up they really don't change much at all.