Posted By: A57_RT
chassis rigidity?? - 01/13/12 05:12 PM
How important and what is too much?
chassis rigidity??
Posted By: amxautox
Re: chassis rigidity?? - 01/13/12 05:57 PM
Depends what the car will be used for. All out racing, street/road course/autocross, or street only with some spirited driving? Too stiff and the car will 'hop' and 'chatter' across the road or track.
Posted By: A57_RT
Re: chassis rigidity?? - 01/13/12 06:09 PM
All I can do without the sunday dive part..
So far ive an 8 point that the rears hit right above the rear sliders, and front spring boxes (reinforced), front frame rails/ trans cross tunnel (reinforced), frame connectors of course and ive a rear cross bar at the very rear of the frame rails which X-s to the front.
Its rigid enough a floor jack will lift three tires with no flex.
Just hope its not overkill.
So far ive an 8 point that the rears hit right above the rear sliders, and front spring boxes (reinforced), front frame rails/ trans cross tunnel (reinforced), frame connectors of course and ive a rear cross bar at the very rear of the frame rails which X-s to the front.
Its rigid enough a floor jack will lift three tires with no flex.
Just hope its not overkill.
Posted By: autoxcuda
Re: chassis rigidity?? - 01/13/12 06:35 PM
Quote:
Depends what the car will be used for. All out racing, street/road course/autocross, or street only with some spirited driving? Too stiff and the car will 'hop' and 'chatter' across the road or track.
That is the spring and shock combination creating the "skipping" Best case is the chassis is as stiff as it needs to be (pretty ombiguous answer). But meaning just as much stiffinng (weight) to make it stiff and not just have a heavy overdone tank out there.
Bob Reed has SCCA GTII championship under his belt and thinks his you reach a limit with these old cars because it lack a full cage. He can cut as fast laps with his 300 SRT-8 as his 68 Barracuda .
But Bob has driven Tiim Werner's 68 Valiant for comparison with full cage and thinks that the cat's meow. Especially with big 1.10 or larger T-bars
That's one person's take on it.
This is Tim's Valiant in action...
Posted By: 72Swinger
Re: chassis rigidity?? - 01/13/12 07:46 PM
Taking chassis flex out of the picture will rear the ugly head of inadequate suspension components.
Posted By: A57_RT
Re: chassis rigidity?? - 01/13/12 10:59 PM
I will post the cars specs in the proper section but to be brief,t I think im covered semi well, the RMS front coil over setup #550 springs,RMS sway bar, WW front disc, Dr diff rears, zero arch road race springs on the rear, afco spring sliders, QA1 double adjust on all 4 corners and way more motor then I think I need.
Quote:
Taking chassis flex out of the picture will rear the ugly head of inadequate suspension components.
Posted By: Grizzly
Re: chassis rigidity?? - 01/14/12 03:50 AM
Autoxcuda:
That photo that you attached is stinkin' awesome.
That photo that you attached is stinkin' awesome.
Posted By: 72Swinger
Re: chassis rigidity?? - 01/14/12 04:31 AM
My point was that a stiff chassis will need absorbtion from its suspension. I stiffened my front unibody up a bunch and it is now glaring that I seriously need bigger tbars and better shocks bigtime.
Posted By: jcc
Re: chassis rigidity?? - 01/14/12 04:56 AM
Quote:
My point was that a stiff chassis will need absorbtion from its suspension. I stiffened my front unibody up a bunch and it is now glaring that I seriously need bigger tbars and better shocks bigtime.
Are you sure about that, because as I understand it, if we are talking the about driving the same circuit, and the only change was to stiffen the chassis, it would seem some of the compliance was taken out of the suspension equation, meaning a softer set-up would be needed to have exact same performance, am I missing something?
Do you mean stiffen to maintain correct geometry?
Suspension tuning is based on compliance and resonance of all springy parts, tires, tire pressure, TB, shocks, bushing compliance, control arm compliance, chassis compliance, etc. If you make one stiffer, some other combination needs to be softer to maintain same ride quality.
Posted By: autoxcuda
Re: chassis rigidity?? - 01/14/12 05:46 AM
Quote:
Autoxcuda:
That photo that you attached is stinkin' awesome.
That is Tim Werner's Valiant.
And that car has the motor AndyF built in Mopar Muscle magazine. I believe AndyF took the picture too.
Posted By: AndyF
Re: chassis rigidity?? - 01/14/12 06:22 AM
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.
Posted By: Kern Dog
Re: chassis rigidity?? - 01/14/12 06:24 AM
That Valiant looks great. This new forum is really getting me interested in cornering performance again!
Posted By: AndyF
Re: chassis rigidity?? - 01/14/12 06:33 AM
I have lots of pictures of Tim's car since the last couple of builds were done in my shop. Here is a shot of the XV brace up front. You can also see the down bars in the engine compartment.
Posted By: AndyF
Re: chassis rigidity?? - 01/14/12 06:38 AM
Here is a shot of the front down tube. I think that the down tube needs to be tied into the shock mount so that might be one of the mods that we add this winter. Just a couple of pieces of steel tying the shock mount to the sheet metal and the roll bar tube should do the job.
Posted By: Grizzly
Re: chassis rigidity?? - 01/14/12 06:47 AM
Quote:
This new forum is really getting me interested in cornering performance again!
Yes, when a Valiant and a Lotus are in the same picture in the same corner, you have my attention.
Posted By: Grizzly
Re: chassis rigidity?? - 01/14/12 07:08 AM
Quote:
Here is a shot of the front down tube. I think that the down tube needs to be tied into the shock mount so that might be one of the mods that we add this winter. Just a couple of pieces of steel tying the shock mount to the sheet metal and the roll bar tube should do the job.
Would a light gusset work in this area? There appears to be a nice metal pad to weld to there. There would still be an opening left to get access to the shock nut.
Where the circle is, would it be a noticible gain to have tubing tying the shock towers together (like some of the Mustangs come with)?
Posted By: 72Swinger
Re: chassis rigidity?? - 01/14/12 08:34 AM
Quote:Yeah im positive about that. Think about that flex as an extra shock that gets taken away from the equation. Now your existing shocks and tbars are doing more of the absorbing and the same bump you hit 100 times before now makes you bottom out. Makes perfect sense to me....Quote:
My point was that a stiff chassis will need absorbtion from its suspension. I stiffened my front unibody up a bunch and it is now glaring that I seriously need bigger tbars and better shocks bigtime.
Are you sure about that, because as I understand it, if we are talking the about driving the same circuit, and the only change was to stiffen the chassis, it would seem some of the compliance was taken out of the suspension equation, meaning a softer set-up would be needed to have exact same performance, am I missing something?
Do you mean stiffen to maintain correct geometry?
Suspension tuning is based on compliance and resonance of all springy parts, tires, tire pressure, TB, shocks, bushing compliance, control arm compliance, chassis compliance, etc. If you make one stiffer, some other combination needs to be softer to maintain same ride quality.
Posted By: 72Swinger
Re: chassis rigidity?? - 01/14/12 08:43 AM
Quote:Absolutely love the Valiant Andy! web page mine is round tube but seems to work nicely.
I have lots of pictures of Tim's car since the last couple of builds were done in my shop. Here is a shot of the XV brace up front. You can also see the down bars in the engine compartment.
Posted By: TC@HP2
Re: chassis rigidity?? - 01/14/12 06:17 PM
Quote:
How important and what is too much?
How important is a good foundation when building a house? Same concept. If your chassis is not adequatly braced to support the suspension, you allow the uni-body to become an active, yet inconsistent participant in the suspension system. The key is how much bracing and support is adequate without tremendous increases in sprung weight. Combine that with the need to mount the engine, trans, other components and the possibility of any rules restrctions, and that is the challenge facing ultimate rigidity.
In a street car, simply mods like torque boxes or subframe connectors are adequate. The more spirited you use the vehicle, the greater the benefit of additional bracing, up to competition levels where cage structure then has to be taken in to consideration.
Posted By: jcc
Re: chassis rigidity?? - 01/14/12 08:28 PM
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
Posted By: 72Swinger
Re: chassis rigidity?? - 01/14/12 10:38 PM
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".
Posted By: 68440fish
Re: chassis rigidity?? - 01/16/12 04:04 AM
Quote:Quote:
This new forum is really getting me interested in cornering performance again!
Yes, when a Valiant and a Lotus are in the same picture in the same corner, you have my attention.
The Lotus was probably in the way just slowing him down. Is the youtube video of Tim's car passing the Focus still out there?
Posted By: pauly v.100
Re: chassis rigidity?? - 01/18/12 07:57 PM
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!!
Posted By: 72Swinger
Re: chassis rigidity?? - 01/18/12 08:20 PM
Pauly whassssup!
Posted By: pauly v.100
Re: chassis rigidity?? - 01/19/12 07:37 AM
Haa!
alive and well brother!
I'm very close to goin in for the inspection so that I can drive, but still a ways to go before the interior is done.
alive and well brother!
I'm very close to goin in for the inspection so that I can drive, but still a ways to go before the interior is done.
Posted By: Mattax
Re: chassis rigidity?? - 01/20/12 01:56 PM
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.
Posted By: Mattax
Re: chassis rigidity?? - 01/20/12 02:11 PM
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.
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
Posted By: jcc
Re: chassis rigidity?? - 01/20/12 03:04 PM
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?
Posted By: Mattax
Re: chassis rigidity?? - 01/20/12 06:36 PM
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.
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.
Posted By: BigSugar
Re: chassis rigidity?? - 07/12/12 02:01 AM
Wow. Great thread !! Hopefully we can pick this up where it left off,
Ok so we focus on chassis stiffening with some of the available aftermarket braces or home made bracing most of the stiffening is forward in the chassis well ahead of the rear axle, seems to me this area must also require additional bracing seeing as this is where the torque hits the pavement . So other than rear torque boxes and subframe connectors are there any other areas in the back half that could benifit from gusseting or welding body seams.
While im in the thread 66-70 B bodys had some inhearant weak or flex areas due to the large side opening of the design, cracks would show up at the bottom of the A pillar at the cowl as well as the top of the A pillar in the door opening, as well as the front of the quarter panel at the shoulder of the door opening. Is stiffining the chassis going to accentuate the flex in these areas or reduce it.
Stiffening the the chassis is inevitably going to lead to the purchace of better quality shocks and bushings and higher rate springs as well as modern tires.
Is all this additional rigidity going to rear it head again in the inhearant flex zones in the chassis / body ?
Great thread
Ron
Ok so we focus on chassis stiffening with some of the available aftermarket braces or home made bracing most of the stiffening is forward in the chassis well ahead of the rear axle, seems to me this area must also require additional bracing seeing as this is where the torque hits the pavement . So other than rear torque boxes and subframe connectors are there any other areas in the back half that could benifit from gusseting or welding body seams.
While im in the thread 66-70 B bodys had some inhearant weak or flex areas due to the large side opening of the design, cracks would show up at the bottom of the A pillar at the cowl as well as the top of the A pillar in the door opening, as well as the front of the quarter panel at the shoulder of the door opening. Is stiffining the chassis going to accentuate the flex in these areas or reduce it.
Stiffening the the chassis is inevitably going to lead to the purchace of better quality shocks and bushings and higher rate springs as well as modern tires.
Is all this additional rigidity going to rear it head again in the inhearant flex zones in the chassis / body ?
Great thread
Ron
Posted By: ahy
Re: chassis rigidity?? - 07/12/12 04:07 AM
Stiffening the chassis (frame connectors, torque boxes ect.) should reduce flex that tears unibody parts on high HP/hard driven cars. Less movement=less damage.
Posted By: PHJ426
Re: chassis rigidity?? - 07/12/12 05:28 AM
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/1449059163
Here is a ppt from a Ford presentation at a conference regarding chassis rigidity and vehicle dynamics: http://www.altairhtc.com/europe/EHTC2011...2%20eteipen.pdf
There 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?
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/1449059163
Here is a ppt from a Ford presentation at a conference regarding chassis rigidity and vehicle dynamics: http://www.altairhtc.com/europe/EHTC2011...2%20eteipen.pdf
There 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?
Posted By: ChrisUSCT
Re: chassis rigidity?? - 10/20/12 04:38 AM
Here's a video of some in house testing that we did using a 1970 Challenger. We installed our new inner fender braces and the results were better than expected. The factory front end has a good amount of flex.
Uni-Body Rigidity Test
Uni-Body Rigidity Test