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 Post subject: Re: Suspension Dynamics.
 Post Posted: Tue Apr 14, 2009 6:37 pm 
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Beretta; I don't have any videos files of the car on this computer and the ones I have are way too big to post and there in video format. I know its difficult and I have the advantage of knowing what it does but if you look carefully at the youtube video you will see the rear of the car rise as it first moves. Its better if you use one of those keep video programmes and download the video. But you can detect that the car launches up and forward sort of like a cat leaping from a crouch to get ya.

Bubstr; The IC for our car at rest is 7.75" above the ground. I run the lower bar flat and its a 19.6" long bar. At 650 lbs of gearbox input shaft torque in first gear it varies the bar angle 2.8 degrees and this places the IC to just under the height of the CG of the sprung mass on the start line. This is important to get right because the videos show the first thing that happens is the tyre wrinkles and squashes and diff separation occurs before the car does any moving worth talking about. What I have the car setup to do is point the lower bar which has a lot of forward force on it, so that it doesnt go higher than the CG. If it goes higher than the CG the suspension oscillates. The reaction from the CG drives the bar downward etc if you like to think of it that way and the tyre shakes. Then you have to set the shocks hard and then it looses traction on the bumps down track.
When you point the lower bar close to but not over the CG in the dynamic situation you get a stabilising effect because the reaction load into that bar is straight down it and movement variations dont cause the loading to vary much so the tyre receives nice smooth weight. I know you cant actually load a bar with a pivot at both ends with anything other than straight down the shaft its not that Im talking about. It's the force that reacts from the bar into the diff bracket that is what you have to consider. Doing what I do keeps that force in a stable direction and amount. It definitely helps.


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 Post subject: Re: Suspension Dynamics.
 Post Posted: Sat Apr 18, 2009 7:38 pm 
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Scott Smith wrote:
I can't help but think you are using roundy round thinking here.

Round track racing doesn't have a 10th of the torque or tire adhesion a drag car does (forward bite).

I have seen long bar combos and short bar combos on a scale car with a torque wrench applying torque to the axle and there is a major difference.
As your IC gets longer it also gets further away from the neutral line which diminishes anti squat progressively.

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Scott, do you see that neutral line that runs from the rear tire contact patch to above the front axle at COG height. On the first picture the IC is right on that line. This produces no squat or anti-squat. The second picture, the IC was moved forward but not adjusted upward. This put it below the neutral line, thus making the rear compress. If it was adjusted up along with out and stayed on the neutral line the results would be the same as picture one. In picture three the setting is shorter but not adjusted down to keep IC on the neutral line. If it was on the neutral line, it too would have the same results as the first picture. This only shows why we have compression or separation in the rear. We don't necessarily need to have compression in the rear to have weight transfer. You can watch the front suspension travel and see exactly how much weight transfer you have. If it's not on the front, there is only one other place it can be. This compression or separation induced by IC only determines what we do with the weight we transfer. Do we direct it on a compressed rear which is softer than a separated rear and let it mush, or a well separated rear that is hard and distort tires or find the happy place in between?

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 Post subject: Re: Suspension Dynamics.
 Post Posted: Sun Apr 19, 2009 10:47 am 
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Beretta, could you finish the measurements that shrinker posted? I would like to see the results and would like to see everyone's opinion on the results. If you would not mind.


Coy

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 Post subject: Re: Suspension Dynamics.
 Post Posted: Sun Apr 19, 2009 11:23 am 
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shrinker wrote:
For those interested here are some specs ---Our drag car has 5388 Lbs Ft of torque at the rear axle that's without factoring in torque converter multiplication. The wheel base is 107" that's a lift at the front of 604 lbs in just torque reaction. The front weight is 1211LBS; rear is 1174; CGH is 13" the IC is 35.55" the car runs average times of 8.25 1/4 mile with 60' times of 1.153 sec.


Is this a misprint?? CGH of 13" above the ground? Have you added a large amount of weight VERY low in the car? I'd be curious of the way this was calculated since I have done some work in this area recently and that seems very low.

Rick


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 Post subject: Re: Suspension Dynamics.
 Post Posted: Sun Apr 19, 2009 1:53 pm 
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rick360 wrote:

Is this a misprint?? CGH of 13" above the ground? Have you added a large amount of weight VERY low in the car? I'd be curious of the way this was calculated since I have done some work in this area recently and that seems very low.

Rick

It's gotta be a typo. He'd need slicks that could pull more than 4 g's to see any air under the front tires.
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 Post subject: Re: Suspension Dynamics.
 Post Posted: Sun Apr 19, 2009 6:15 pm 
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82Blackbird wrote:
Beretta, could you finish the measurements that shrinker posted? I would like to see the results and would like to see everyone's opinion on the results. If you would not mind.


Coy




I have them and will post later,,,,,,,,,,,

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 Post subject: Re: Suspension Dynamics.
 Post Posted: Sun Apr 19, 2009 7:51 pm 
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Beretta wrote:
82Blackbird wrote:
Beretta, could you finish the measurements that shrinker posted? I would like to see the results and would like to see everyone's opinion on the results. If you would not mind.


Coy




I have them and will post later,,,,,,,,,,,



Thanks alot.

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 Post subject: Re: Suspension Dynamics.
 Post Posted: Mon Apr 20, 2009 6:08 am 
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shrinker wrote:
The wheel base is 107" that's a lift at the front of 604 lbs in just torque reaction. The front weight is 1211LBS; rear is 1174; CGH is 13" the IC is 35.55" the car runs average times of 8.25 1/4 mile with 60' times of 1.153 sec. The front wheels come in the air at 1.76G precisely and it maxes the G at 2.4 to 2.7 depending on track grip at launch. It spins the tyres at 2.8 g, I have never had more g than that.

Can't make any sense of this at all. If the CGH is a typo and we ignore it, the acceleration at which the fronts lift ("1.76G precisely") would indicate a CGH of 30.9 inches. This would be as high as the 13 is low.
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 Post subject: Re: Suspension Dynamics.
 Post Posted: Mon Apr 20, 2009 8:26 am 
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It doesn't make sense if you dont factor that the effect of a fourlink is to shorten the wheel base weight distribution. When the rear axle torque reaction is sufficient to relax the springs the rear axle weight increases to the same as if the axle is located at the IC. The car has a 35" IC at 7.75" above the ground and the rear springs have extension. The CG of the sprung mass of the car is indeed that low. The CG of the whole car is not. The car is very low construction with dry sump and the lowest possible crank height. The car has aluminium panels and a light weight driver. The gearbox and everything else is below 13" off the ground. The rear axle is not part of the reaction that causes a body to lift or to go into the air. The rear axle is the thing that pushes the body. The rear axle weight is at 17" above the ground and that raises the whole CGH but its not the rear axle that goes into the air is it. I know the car lifts the front at 1.76 G because when we run on low traction tracks or with different rpm launches etc it still just lifts the front wheels at 1.76G on the data logger.

What I am saying is incorrect about the physics presented on sites is this; there is a ultimate reaction pathway from the CG down to the tire, That is most certainly the reaction, but that doesnt mean its the action pathway. The action pathway is from the axle centerline into the body. The reaction of the axle is a rotation about the tire patch but its not a fixed pivot its a torque about the patch. The reaction of the axle torque is horizontal to the ground, it cant be anything else. The forward acceleration of the body has inertia and that inertia pathway ends at the tire patch and get to the tire patch via the axle bearing it travels vertically downward through the rim etc to the ground. Thats weight transfer.
The torque of the axle has to be reacted somewhere and because the ladderbar etc is somewhere in front of the axle it's a vertical lift of the body so therefor there is a downward reaction through the axle to the ground.
Here is an example, ride an older style BMW motorbike and feel the lift of the shaft drive then ride a more modern one with a Paralever shaft drive, the bike doesnt lift anywhere near as much. The paralever system has a IC further forward than the short drive shaft of the older bikes. Attach a sidecar to the bike and sit in the side car and you can see it. It obvious. So why do engineers keep telling me that weight transfer only comes from forward acceleration? I see it on bikes , on dynos, everywhere?

I cant fathom the page 12 on billyshopes site; how can you have a line under the ground when you add the rear axle weight? I cant agree that there is a forward force at the tire contact patch. Take off on gravel and which way to do the stones go??? Perhaps Im doing it backwards to the norm I don't know. But the force is not forward at the tire contact patch, its backward. The way tires wear indicates rearward force. Just feel it on your feet when you take off, you push backward not pull your feet forward to move away. On a car its backward at the ground to go forward at the axle.
I think I will just be content to work this out my way as it does agree with what I can measure and I can use it to effect what I need and it always works for me. I guess that's what most people want to get with physics anyway.


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 Post subject: Re: Suspension Dynamics.
 Post Posted: Mon Apr 20, 2009 4:17 pm 
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shrinker wrote:
...the rear axle weight increases to the same as if the axle is located at the IC.

And what if the IC is located at infinity (parallel links)?

The problem lies in an understanding of that which is called a "free body diagram." I googled "free body diagram," hoping to find a site which would lead those interested to an understanding of an engineering analysis of the forces and moments acting on the car during launch. Unfortunately, the examples were either too "easy" or were presented in an engineering jargon that would not be easily understood. So, I was forced to resort to a lengthy post I made on the subject in another forum:
http://www.hotrodders.com/forum/free-bo ... 14650.html

If you don't want to accept my presentation, I would urge you to look at those other sites.

(I will head off an argument which might arise regarding the application of a dynamic free body analysis to a non-rigid body: Yes, the entire car has components which experience deflections and motion during launch. These deflections result in transient events, such as the separation at the rear of the car and the twisting of the front. Once these events have stabilized...which requires only a few milliseconds..., the car acts like a rigid body and the application of the free body diagram is fully justified.)
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 Post subject: Re: Suspension Dynamics.
 Post Posted: Mon Apr 20, 2009 6:07 pm 
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nlmsc33 wrote:
It would take 3 or 4 seconds before this car would untwist.

As a fan of NBA basketball, I'm convinced 3 or 4 seconds is essentially equal to eternity. But, if it did actually take that long, that is certainly an exceptional case. Usually, a car will either quickly squat or rise and that's it. High horsepower pro mod cars exhibit an interesting transient: The left front will pop up in the air and result in a high transient loading of the right rear, causing the car to veer to the left out of the hole. But, even this violent transient is short lived. Unfortunately, many tuners act on this leftward turn and statically preload the left rear, making the rear loading more unequal after the transient has subsided.
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 Post subject: Re: Suspension Dynamics.
 Post Posted: Mon Apr 20, 2009 11:05 pm 
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Billy; I can tell you what happens when we use a IC thats further forward, the car spins the wheels down track and spins them more off the line. Run a short IC and it doesnt. I have never run it with parallel bars.
I am trying to get time to read and understand your stuff on hot rod site. I'll have to draw it all out as per the description.


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 Post subject: Re: Suspension Dynamics.
 Post Posted: Tue Apr 21, 2009 8:42 am 
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shrinker wrote:
Billy; I can tell you what happens when we use a IC thats further forward, the car spins the wheels down track and spins them more off the line. Run a short IC and it doesnt. I have never run it with parallel bars.
I am trying to get time to read and understand your stuff on hot rod site. I'll have to draw it all out as per the description.


This is where we disagree. It is not the forward but The forward with out moving the IC up to keep it's relationship to the neutral line that creates a different anti-squat value. Think of the compression or separation as a reaction to which side, (up or down) of the neutral line, that the IC is. To see this grab a folding carpenter rule and unfold two lengths and grab the bulk of it in your hand. With the other hand put the end to your palm and compress your hands. You grab hand is forward force. Your palm hand is the far end of the neutral line. The middle joint represents your IC. By putting a bend at the middle joint you can get three results, two of varying degrees.
One, the rule will remain straight, if it started straight, (center joint directly on neutral line).
Two, with rule bent slightly upward at the center joint, (center joint showing a high IC), This will want to increase the angle of the center joint. If you think of the length close to the pushing hand as the rear links you can see force raising car at joint and opposite force pushing down at tire.
Three, with rule bent slightly downward at the center joint, (center joint showing a low IC). This will want to increase the angle of the center joint down ward. If you look at the length of rule representing the rear links, you will see the direction of force has changed.

Now if we had no other lines of force going on, we may see the chassis dragging the ground or rear wheels traveling under even forward as far as linkage would allow.

Can you see even if we increase the length of rule close to the pushing hand, (making IC longer), The results are very similar. Try it. Instead of one length by pushing hand use two lengths, but only make one IC change at the same place we did before. It still depends on vertical relationship of the IC to the neutral line. Also notice what this does to your angle of the sum of rear links. Yes the longer IC did make a less drastic change for distance IC is above or below neutral line. In real life this will be seen as abrupt or velocity of reaction, but not really an issue as everything happens so fast and the difference is slight..

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 Post subject: Re: Suspension Dynamics.
 Post Posted: Tue Apr 21, 2009 9:12 am 
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Mel, The twisting chassis thingy. Yes seen it and even had one to not that extent. The explanation I have for it is my chassis wasn't rigid enough. Only a 6 point cage that was acceptable them days and even cheated legally with wall thickness in main rails.

Now think about this. With a flexible chassis, some forces are absorbed and stored in the chassis, making adjustments very hard, but also makes things like toque roll not as effective also. Solid chassis cars still use this flex. Go karts use it. It makes car into a spring.

Now when the wheels leave the ground, it takes any resistance to force out. The little up or down arrows disappear. Although the engine is still making torque to spin that rear axle housing in a clockwise direction, It no longer has ground to push off of. This leaves only stored energy in chassis that cancels it's self out in a short period of time. You can see this in any of the Pro Classes. front left wheel comes up, as soon as right wheel unloads the torque is canceled. This may not be right but it is the way I see it. Watch, Up Up and plop down with both.

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 Post subject: Re: Suspension Dynamics.
 Post Posted: Tue Apr 21, 2009 1:34 pm 
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Bubstr wrote:
Now when the wheels leave the ground, it takes any resistance to force out.

I'm going to depart on a tangent here, but I think many will find it interesting. As Bubstr points out, as soon as that left front lifts, there's no resistance to a moment at the front. Well, suppose we start with only 3 wheels? No, I'm not suggesting that anyone rush out and start building a tricycle, but let's think about it. If the 2 front wheels were combined into a single, centrally located wheel, these suspension forums would pretty much dry up and disappear. All of the twisting and veering right or left problems would disappear and all that would be left would be the amount of hit an individual preferred.

But, the NHRA would simply throw you into a motorcycle class, so all the effort would be wasted. Of course, if you mounted a suspension beam on a central bearing and kept the 4 wheels....

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