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Re: chassis rigidity??
[Re: jcc]
#1156065
01/14/12 06:38 PM
01/14/12 06:38 PM
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Joined: Feb 2011
Posts: 4,302
Nebraska
72Swinger
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Nebraska
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Ok, would you say that a stiffened chassis will make a car react quicker? I would, also if you say hit a pothole? With the normal unstiffened chassis do you think the suspension is gonna travel the same amount as hitting that same pothole with no chassis flex? See you are thinking about ride, im talking about what my car does before and after. So in the context that you are using yes my car is probably rougher from a "ride" stand point because now I am getting deeper into the rates of my tbars,shocks and swaybar that used to get help from the chassis soaking some of the impact up. But I am certainly not gonna go to a smaller tbar to improve my "ride".
Mopar to the bone!!!
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Re: chassis rigidity??
[Re: AndyF]
#1156067
01/18/12 03:57 PM
01/18/12 03:57 PM
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Joined: Jan 2010
Posts: 157
hawaii
pauly v.100
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hawaii
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Quote:
Yep, that is one of my pictures. I have a whole folder of shots taken of Tim's Valiant at Portland International. Hopefully he gets the car back to the track this year. He has it apart right now working on the front suspension.
...My inspiration!!
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Re: chassis rigidity??
[Re: A57_RT ]
#1156070
01/20/12 09:56 AM
01/20/12 09:56 AM
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Joined: Feb 2010
Posts: 1,645
Phila. Pa.
Mattax
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Phila. Pa.
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Quote:
How important and what is too much?
It's relatively important.
Generally, there can never be too much.
From a design and tuning point of view (in other words both in theory and practice) one wants a chassis that has effectively inconsequental compliance relative to the suspension system.
As usual what one does on specific vehicle will come down to projected tires and application. The mopar chassis was fairly decent. But even there, notice the factory decided to make various improvements or additions. If you are going to autocross or do track days on R compound tires, then definately add at least all the factory extra bracing. Also, check all the welds whenever possible. For example if the K frame is out, check it over -reinforce and repair seams or spots (within the liitations of rules you'll be running under of course).
How much flex depends on the chassis/body style and the loads input. The cross braces in the back of my coupe eventually tore from the floor pan. Was that due to poor assmebly alone? I suspect that was only a contributor, the other part of the equation being increased spring rates - bad or unpaved roads and lots of autocrosses on R comps. OTH Mitch autocrossed his Challenger very competitively with more tire with no problems that I'm aware of. At the time, SCCA Street Prepared rules didn't even allow welded frame connectors (now that's allowed with restriction on attachment points). On the other hand, anyone running a nationally competitve C Prepared car has pretty much braced the heck out of it for very good reason.
Herb Adams's Chassis book has a good pictorial overview of effective and ineffective bracing.
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Re: chassis rigidity??
[Re: jcc]
#1156071
01/20/12 10:11 AM
01/20/12 10:11 AM
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Joined: Feb 2010
Posts: 1,645
Phila. Pa.
Mattax
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If much stiffer springs are installed, and the chassis is not stiffened it is possible that chassis will become a noticible portion of the action. The reverse is not true. That does assume the chassis was an insignificant portion of the motion to begin with. If it was a portion of the equation, then springs, tires, and shocks will be doing a little more work. This is good, however.. A stiffer frame will generally better transmit noise and vibration. Fast response is largely dependent on what is called the high frequency damping. The main damper is of course the shock, but the bushings and tires are significant as well. Quote:
Quote:
Makes perfect sense to me....
Then consider this, say we have a car that you are going to stiffen up the chassis by any means you desire. Say you want/expect the car to have the relative same handling characteristics after mods are completed ( and as mentioned a post or two before chassis stiffening will make car more consistent/predictable).
So if for simplicity we imagine the car has say 4? "springs" of different rates/resonances IE tires at a certain pressure, the TB's, the chassis flex (which really is a spring as long as it returns to original shape after removal of road loads) and the drivers seat suspension. To keep matters very simple. If all these "springs" say add up to some magical value "X", removing or reducing any of them will reduce the "springiness" of the system, and in order to keep the same ride stated above, something or combination needs to be soften to maintain same ride quality. However without any readjustment, the suspension being more rigid may exhibit less shake feel more predictable, and allow an increase in other "spring" rates until chassis flex(spring) reappears due to greater loads or another spring becomes too soft for the new application.
Bottom line, increasing Chassis stiffness does not automatically require increase spring rates for same ride, and that makes sense to me, and I guess we still disagree  
Last edited by Mattax; 01/20/12 10:20 AM.
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Re: chassis rigidity??
[Re: Mattax]
#1156072
01/20/12 11:04 AM
01/20/12 11:04 AM
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Joined: Dec 2003
Posts: 22,696
Bitopia
jcc
If you can't dazzle em with diamonds..
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If you can't dazzle em with diamonds..
Joined: Dec 2003
Posts: 22,696
Bitopia
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Quote:
If much stiffer springs are installed, and the chassis is not stiffened it is possible that chassis will become a noticible portion of the action. The reverse is not true. That does assume the chassis was an insignificant portion of the motion to begin with. If it was a portion of the equation, then springs, tires, and shocks will be doing a little more work.
Wanting to be clear here on the points you are making:
"the reverse is not true" could you state the reverse, and I am not sure I agree until clarified
If chassis was "insignificant portion", why is it being stiffen?
"If it was a portion of the equation, then springs, tires, and shocks will be doing a little more work." Doesn't the analogy of removing say a coil from a coli spring or shortening a TB which then actually increases its rating apply here? 
Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.
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Re: chassis rigidity??
[Re: jcc]
#1156073
01/20/12 02:36 PM
01/20/12 02:36 PM
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Joined: Feb 2010
Posts: 1,645
Phila. Pa.
Mattax
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Sure. I'll try  If the chassis has a much higher spring rate than the springs (and tires) then increasing its rigidity is going to have a minimal effect on the overall response to any input. I think this will make more sense when we get to your last question below. Input could be a distance traveled, like a 2" deep step in the road surface. But it also could be weight transfer in a corner, or braking or accelerating. It's just more equations to figure out. hahahaha Why stiffen the chassis then? Well most folks here are increasing the tire traction, and often spring rates. Some chassis (eg convertables) already start with less rigidity and others were intended for uses where it could be an issue. So the factory dealt with the potential issue. Transfering the weight to the rear the way drag racers did even in the 60s was more than a production chassis was deisgned for. My co-worker used to put his GTX on jackstands after a night at the dragstrip so it would settle back to normal. The other reason is that its a variable you just want to make sure is eliminatated from the picture if at all possible. It's a big undamped spring and another variable. Is it essential? Well no. There have been successful vehicles with lots of chassis movement, Morgans come most immediately to my mind. Basically in the real world its a pretty good bang for the buck IF for almost any use involving slicks or r-comps. The downsides are weight, cost, obstructing other uses/access to interior and time to design and test. In the end its about what the rules allow and what makes sense for you. Is it analogous to increasing the spring rate? Well yes and no. Yes the equivalent spring rate has increased, assuming it acts like a linear spring. No, that's not the equations for dynamic response. Basically a differential equation is solved for using Mass that is sprung, the mass(s) that are unsprung, the spring rates and damping rates that are connecting the sprung to unsprung masses. Dynamic analysis is about system response to input (distance, force) over time. Notice how mass shows up again? Everyone wants to put on big this and bigger that or for you to buy their gizmogadjet, but the real game is reducing mass.
Last edited by Mattax; 01/20/12 02:46 PM.
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Re: chassis rigidity??
[Re: ahy]
#1156076
07/12/12 01:28 AM
07/12/12 01:28 AM
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Joined: Nov 2005
Posts: 3,456
Fly Over States
PHJ426
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Fly Over States
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Thinking of this from a basic statics class standpoint they always make a general assumption in that first statics class that a car or any object is treated as a "Rigid Body" Meaning there is no flex in that structure. That way inputs or forces to the rigid body can be calculated with exact results to the "Rigid Body." There are many books available out there for vehicle dynamics and chassis tuning,,,,anyone check into this one. http://www.amazon.com/Vehicle-dynamics-damping-Jan-Zuijdijk/dp/1449059163Here is a ppt from a Ford presentation at a conference regarding chassis rigidity and vehicle dynamics: http://www.altairhtc.com/europe/EHTC2011...2%20eteipen.pdfThere is a ton of information out there on chassis stiffening and rigidity. Like was said before the best way to do this is from an engineering stand point with a 4 post shaker and figuring out exactly where to add the extra weight when you need to do more than just seam weld the rockers, floors etc etc. A few points on the Mopar unibody are a given: Subframe connectors and torque boxes are the starting point to connect everything together and go from there. Where to stop just before you make your A body the same weight as a 57 Imperial. Speaking of that remember the one article was done on a C body or was it an Imperial where they basically took all the doors etc sawzall'ed off the roof etc etc of this large Mopar to see how fast it would go 1/4 mile with half the car laying on the ground in the pits?
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