chassis re-enforcement v.101

[haygood@myway.com (Brian Haygood)]

Dave's right on track, and Allyn summed it up best (though he's off by a dangerous little bit).    Brian was very close, too.  (All opinions of mine, of course).

The brace will prevent motion of the two towers relative to each other.  Dave said that in a right-hand turn, the left (loaded) tower moves inward.  Actually it moves upward and outward, while the lower control arm mount moves inward.  K-brace and Strut brace reduce both of these horizontal motions by transferring some of the load, or stress to the other side, letting the stiffness of the unloaded side help handle the stress of the loaded side.  

The effect is more than what is suggested by saying the brace keeps the towers from moving closer to each other, as someone I've forgotten suggested.  While this is true, the noteworthy part is that one side is highly stressed, and is "anchored" to the other side.  Now even a lightly laden strut tower isn't much of an anchor, but two towers handle stress better than one.   A really nice strut tower brace is a triangulated one....

Brian suggested that the brace adds the top to the box that is the frame.  Actually it just adds a stick across this opening, which isn't nearly as effective as closing in the entire top of the box.  He prolly knew this, but I'm anal, so I'll clarify it.  To really close the top of the box, you need something more like the K-brace - that attaches to more than just two points (yeah, I know it bolts in 4 places...).  A triangulated brace would attach to something like the rain tray or firewall.  This brace would anchor each tower to the cowl of the car, providing the same type of stiffness that our mythical top of the box would.  Now before all of you break out the welder to make a triangulated brace, note that SCCA rules prohibit you from using one in the stock classes, so you'll be bumped way up in the SCCA food chain if you use a triangulated upper front brace.  

So it's very reasonable that our braces reduce flex.  Will they reduce roll?  Allyn's theoretical explanation holds true, but the dangerous part is the implication that reducing deflection of something reduces the cars motion relative to the ground in any direct way. Stiffening the front only ties it more to the rear - it doesn't tie it to the ground.  In our cars, the front is so overburdened that the rear can really help it out, so in this indirect way, roll could be reduced.  

As Allyn notes, the body flexes to appease the forces caused by roll, so it must do so in a way that encourages roll.  Adding any stiffness to this front structure will reduce the deformation of the front end, tying it in to the rest of the car, making the front end flex less relative to the car.  If you have a massive rear anti-sway bar, then your rear end is stiffer, and your now stiffer chassis lets this stiff rear keep the front flatter.  

So if more stress on the stiffened front can be transfered to the rear, then something interesting happens.  The rear can now help out more with the job of keeping the front from rolling.  In other words, the stiffer front = stiffer frame can tranfer more of the cornering force to the rear of the car.  The effect is the same as adding a stiffer rear anti-sway bar.  More weight is transferred to the loaded rear wheel, easing the burden of the loaded front wheel, which reduces oversteer and makes our cars handle better.  How much?  Allyn was right again when he said, "enough to notice? I doubt it."

So upper strut tower braces should:
1. Reduce flex of the loaded side of the front suspension - improving control of camber and other aspects of alignment at that corner
2. Increase the stiffness of the front end, making its responses feel more precise and less springy (just like poly bushings)
3. Increase the stiffness of the front end, allowing more stress to be transferred to the rear, reducing oversteer. - just probably not enough to notice.


Two final notes on improving handling in a car.  These points are often misunderstood, so that's why I'll point them out.  
1.  Traction of a tire is not linear.  For every 10% more downforce you put on a tire, it will only give you maybe 5% more cornering force (maybe 2%, maybe 8%, but never 10%).  This is contrary to what your Physics teacher taught you about friction (i.e. that downward force and friction force are proportional).  If your physics teacher had been right, then putting larger tires on your car would be useless (in fact my HS physics teacher was of this opinion!)

2. The effect is that keeping the tires evenly loaded (reducing weight tranfer) improves performance, just as wearing larger tires does (less force per unit area of tire). 

3.  Cornering forces cause weight tranfer, which causes roll - in that order.  Reducing roll of your car will not prevent weight tranfer.  Anti-roll bars can't decrease weight tranfer - they can only redistribute it. (That's the basis for the Shine setup's effectiveness, though it requires things like massively stiff springs that don't make sense for most of us).

That's all I have to say.  Hope someone got something out of it.

Brian

--- On Thu 12/26, Rabbit16v  wrote:
>From: Rabbit16v [mailto: Rabbit16v@attbi.com]
>To: scirocco-l@scirocco.org
>Date: Wed, 25 Dec 2002 23:11:36 -0800
>Subject: Re: chassis re-enforcement v.101
>
>I guess it depends on how you drive your car!
>
>
>> if the towers flex, they will do so in a way that will allow the body to
> roll more than it already has, since that very roll is whats causing the
> stress/deflection to begin with.
> enough to notice? i doubt it. just bringing up the point.
>


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