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Highjack! 
This thread started out as Roll Axis. I can see how front weight may relate but it's really starting to wander into an engine thread. Understandably I can do the same but.
Ralphy
Last edited by Ralphy (4/24/2012 7:47 pm)
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Yes it did. How ever its the center of mass and its location in relation to the roll axis that generates body roll or the lack there of. The single greatest contributor to the mass of a vehicle is the engine. The weight of the engine and its location in respect to the front roll center is critical on how the early Mustang platform performs in a corner. After all this thread was about how the addition of a Jag IRS changed the location of the rear roll center on an early mustang. The relocated rear roll center moved the roll axis. The center of mass acts directly on the roll axis. Reducing the engines mass plays an important roll on the vehicles over all volume of mass and through this weight reduction can move the centroid of mass. The overall weight and its distribution directly affects over all handling of any vehicle and therefore this line of thinking qualify s in the general aspect of this thread.
By the way. The 428 fe weighs 650lbs with out accessory's not 550lbs. Stock 351C 4bl weighs 550lbs with all accessory's This data is universally agreed upon by every engine weight site I have visited.
Bye Jack.
Last edited by tyrellracing (4/25/2012 2:30 am)
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We were getting a little off track, but I agree, engine weight is a critical factor, whether it's Mustang, MG, Cobra, or something else entirely. So in that regard engine choice can be pretty critical, placement certainly is, and again, as also demonstrated in the MG-Roadmaster the extra weight of the IRS can be used to our advantage while at the same time reducing unsprung weight. Certainly adding the same amount of weight at the front when converting to IRS is an option worth considering, and when it can be done in a way that doubles, triples, or even quadruples horsepower and torque then by all means that is a small detail that affects the IRS conversion.
We didn't necessarily approach it from the right direction in light of this being an IRS forum. It would have been more proper to have said something like, "The Jag IRS adds 90 lbs of weight to the rear of the car. While this dramatically reduces unsprung weight, it also opens the possibility of an engine upgrade without compromising vehicle handling." And then maybe we wouldn't have gotten bogged down in specific engine weight details. But the honest truth is that we do all like engines, we all have our favorites, and we'll talk about them almost any chance we get. Besides, engine weights, and especially block, crank, head and other component weights are a long way from being universally known and agreed on.
But in the end I guess all I was really trying to say was that with the Jag IRS and the big engine the handling of the car was remarkably neutral, something worth commenting on, and that the ride and roadholding were noticeably improved. What I do not know however, is what the effect really was on the roll center and the roll axis, going from an "ox cart" leaf spring suspension. I suppose it moved the roll center down but how much I don't know. Edit: I think this all got started by me pointing out that we aren't using any anti-roll bars, either front or rear, on this particular car as of yet. That may change of course, especially if we start putting it on the track, but for the street at this point body roll seems to be within reason.
Jim
Last edited by Jim Blackwood (4/25/2012 5:29 am)
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There is more than one way to deal with roll center height and their roll axis. Make them nonfunctional!
About 20 years ago it was common practice to set up cars to corner by putting way too stiff of springs under your car. then to make matters worse you could install heavier aftermarket sway bars. When I was young and naieve I followed suit. I got 750 lb front coil springs to replace the 450 lb units then cut two coils off to get the ride height I wanted. On the rear I added two leafs per side and de-arched each spring pack four inches. This made the car corner better by making the suspension nonfunctional and the improvement was only on very smooth roads. The grand canyon size huge down side was that the car rode so rough you could not eat before driving it or suffer side aches. It could dislodge fillings on long trips. My girl friend at the time went for one ride and would not ever get in the damn thing again! Any bumps encountered in a corner would make the car go air born and you cannot generate lateral G force when your tires are not touching the ground.. It was what every one was doing back in the eightys and is exactly what you do not want to do. It only took one trip to Seattle international race way to race in a production stock class to really see the error of my ways. In the Eightys SIR had a very poor track surface with many filled pot holes in several corners that made my creation do everything but go around a corner at speed. That's when I started buying every book I could find on suspensions and using this new found information redesigned the entire set up.
Last edited by tyrellracing (4/25/2012 5:21 pm)
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Quite right, and there are far too many who still follow that plan of attack. But skating over bumps is no way to get around a corner. Despite the theory that a sway bar will simply level the car and somehow improve cornering in the process, the simple fact remains that at the limit you put the outside tires right on the hairy edge of adhesion. Transferring more load to them by lifting the inside tire is not going to make them grip any better under those conditions, it will either turn them to jelly or force you to run harder rubber which won't grip as well. Either way you loose traction. Then of course there are the bumps and transferring those from side to side is only going to increase the skateboarding effect.
The stiff springs do have their place, such as enabling you to lower the car and still keep it from bottoming out. But at some point, if allowable, it just becomes good engineering to trade ride height and stiffness for track width and ride quality/anti-skateboarding.
One of those books, it may have been Carrol but I can't remember, went into a very good explanation about the maximum limits of adhesion at the contact patch and how force vectors influence traction. Understanding the influence of lateral G force, braking, acceleration, and suspension movement is key to getting the fastest lap times or maximum cornering in the mountains. Once that is fixed firmly in your mind you can clearly see the results of ideas like applying the brakes to the inside rear tire in in an attempt to assist turn-in and it easily explains why cars with traction control enabled are slower. The same principles apply to sway bars. Provided body lean is not excessive, less is more.
So now, the concern becomes how to control body lean without resorting to sway bars, which in reality are nothing more than a tuning aid rather than a major suspension component. Thinking along those lines, wouldn't lowering the roll center be counter-productive? If the roll axis follows the CG then the body does not roll, right? As the roll center moves down, roll rate increases, and moving it above the CG would actually cause the car to lean into the turn. Why then, do we try to lower the roll center? And what possible reason could there be for wanting to move the roll center below the road surface? What am I missing here?
Jim
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Jim,
My thought is a higher roll center makes the chassis more ridged at that end front or rear. When the chassis goes ridged you get a higher weight transfer to the outer tires, unloading the inners/loose.
NASCAR adjusts the track bar to find the right balance from front to rear. If the rear is in a tight geometry/understeer they raise it. Raising the roll center loosens and the reverse to tighten. In a race situation for NASCAR the front is pretty well set. However because of the ease, the rear is adjusted during a race. I also think this situation shows just how valuable roll center axis is.
Don't take this as fact, it's only my analogy. Maybe irstang or phantomjock can chime in.
Oops I need to finish this! So lower is tight, higher is loose? The lower you make the whole package, front and rear. The tighter the front and rear. The better handling the chassis in total, more total grip. Lower keeps more weight on the inner tires.
I'm also thinking if you were to lower your CG only. This would in effect change your RC effects tending to increase weight on the outside by narrowing the distance between RC and CG. Somewhat minimizing the gain of the lower CG. So in order to maximize the gain in lowering it would be beneficial to lower the RC also.
Ralphy
Last edited by Ralphy (4/26/2012 8:46 am)
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If I may quote a grand-old man,(unfortunately now passed)? Allan Staniforth, in Competition Car Suspension; Design, Construction, Tuning, adds the following;
Roll Centre
...Low centres give less weight transfer to the outer wheel, smaller or nil jacking effect but high potential roll angles. Suitable and variable anti roll bars must handle this. Front and rear centres are conventionally at different heights to give a tilted Roll Axis with the lower centre at the lowest/or lightest end of the vehicle but this is by no means universal.
Nothing about perceived chassis stiffness there resulting from roll center placement, high low or otherwise. Any change in stiffness attributed to the anti roll bars, or the chassis itself.
Cheers - Jim
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Maybe I chose the wrong word. High potential roll angle, I guess would be the outcome of a lower RC. I'm looking at it as a lower chassis roll amount the higher the RC.
The part I don't understand is this, Front and rear centres are conventionally at different heights to give a tilted Roll Axis with the lower centre at the lowest/or lightest end of the vehicle but this is by no means universal. I interpret this to say on most vehicles the Roll Axis would be lower to the rear? Seems backward.
Another way I view RC. If you had a barbel set and you pushed outward/sideways at the top of the weight. Some of the force would transfer thru the bar lifting it and removing weight off the opposite sided plate and it being the inner. This would be a high RC. Then the opposite, if you pushed the weight at ground level some force would transfer thru the bar forcing down some on the opposite weight/inner. Being an illustration of a low RC. The chassis being the bar.
Last edited by Ralphy (4/26/2012 4:01 pm)
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The truck arm suspension is the standard NASCAR rear set up these days and as crude as it is, it works. The rear roll center is easily adjusted through the pan hard bar height. But that is only one of more than a dozen methods they use to control under steer and over steer. Roundy round guys have it easy compared to road course guys as far as suspension set up and tuning is concerned The moving of the roll centers becomes far more complex to do track side when you have a GT1 car or a sports racer with a IRS. Test and tune days at PIR start this weekend so my brother and I will finally will get a chance to start dialing in his new car with out constantly looking for cops. We built a SLA set up for the front and rear and got rid of the strut suspension that came stock in the 240 Z. The front and rear suspensions are near identical with a single upper and lower ball joints on each upright using a steering link in front and a toe rod at the rear. The front is front steer and the rear has the toe rod behind the axle. The reasoning behind going front steer in front was to keep the rack from penetrating the oil pan. That is why I was asking weather or not any one knew how to set up Ackerman angle in a front steer suspension. Every thing on Ackerman angle designs I can find is for rear steer and is of no help.
Last edited by tyrellracing (4/26/2012 4:47 pm)
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I think the main diff between NASCAR and road courses. Is Roundy Rounds do changes only once, being there are only two maybe similar straights, over and over. Bristol comes to mind, 500 laps! By the way Bristol is being rebanked again trying to bring back the one line. Road courses throw so many variations at the car. NASCAR is shooting at a bulls eye, road cars are trying to hit the broad side of a barn. LOL! Harder and more varied, that's why I love road racing. For a driver the mind thought is so far more complex than circle track racing. I love oval racing also!
The one thing noticeable about the NASCAR truck arms is that they are very close to the center of the chassis. Fore and aft, left and right. Seems they want minimal effect sent thru the arms.
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I will have to add, or interpret for Allan in his statement, as rather than the full text, I chose to provide the example above.
His design and engineering efforts in the 60's and years following were in rear engined formula racing. So not "backwards." Recall these as having lighter front ends, lower slung for aerodynamic purposes (recall an earlier post on RC and aero?) and generally lower RCs forward. I commend the text to anyone interested in suspensions in general and a fine lesson in the development of the "species."
Jim
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OK, I'm puzzled. How does a higher roll center transfer weight to the outside? The way I see it, the CG is acting through the RC to influence the contact patches. Maybe it's a difference in definitions and conventions. But the CG causes the force and motion and it seems to me like simple leverage through a point which is the RC. If the CG is above the RC the weight will make the body lean outward or roll to the outside. If the CG is below the RC the weight will rotate the body in the opposite direction and make it roll towards the inside of the turn. Is this not correct? How then could this transfer weight to the outside if it is rotating towards the inside? You guys are going to have to explain this one to me, it sounds too much like black magic.
Jim
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Jim,
If you take a big old pry bar and try to un-stick a big old rock and push at the base. You won't roll the rock, you will be pushing it straight through the mud. But if you are able to push on it much higher you'll probably roll it. Imagine that rock as your outer tire and the side of the rock your pushing on toward the inner side in a turn. It raised up and rolled right? You removed weight toward the inner side.
Have you ever pushed something big and tall? Say like a tall piece of furniture, pushing at the top? Then it catches on something and tips? That's an example of a high roll center. The legs opposite the side your pushing on dig in harder the higher you push/outers. And the legs on your side have weight removed/inners. The CG never changed but the point at which you pushed did. If you were to push a bit lower you will reduce the risk, the lower you go the less.
The CG you pointed out is right. I had said the same in a earlier post. CG is a factor also.
PJ that's making more sense! I think, hmmm.......
Last edited by Ralphy (4/26/2012 7:07 pm)
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Wouldnt that depend on the location of the centroid of mass in relation to the roll axis? above the roll center creates body roll, below and it should create a jacking effect. When the center of mass falls directly on the roll axis, no body roll will be created at all. Most cars that can corner well will have the center of mass as low as practical and very close to yet still above the roll axis. The lever arm between the center of mass and the roll axis will be so short that the body roll will be minimal. That will make the negative camber gain aspect of the design less important but if the vehicle doesn't lean it will not need the increase in negative camber any how.
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Too many words! -- Just kidding.
Pictures - I'll put some together this weekend - unless someone else beats me to it!
All good stuff though - and helps everyone get a better understanding by framing it in their own "words"and context!
Cheers - Jim
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Ralphy, are you saying the rock is the car body (assume CG or mass centroid is halfway up I guess)? So what is the lever? Is the base of the rock the contact patch? If the lever establishes the RC then something is backwards here, since the lever is acting on the rock rather than the other way around. If the rock acts on the lever then the rock will tip over when the lever is low and it will slide when the lever is high. It's the car body, Center of Gravity, CG or Mass Center that's doing the pushing, not the roll center. Unless the definitions of these terms are somehow contrary to reason.
Jim
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Great exchange of thoughts here.
I made a front roll center diagram of my '65 Mustang convertible with RCMotorSportsInc RC107 and RC is 2.25 inches above ground. I'm happy with this for now and I will set up the rear with the RC a bit higher.
When one thinks about the lateral and gravitational forces of your car in motion I think moment center would be a better term for what we are calling the roll center. The attached Circle Track articles on suspension dynamics explain it better than I would be able to do:
Part One
Part Two
Some of the suspension set up stuff does not apply since the articles emphasize circle track racing, but the importance of achieving a balance between front and rear suspension systems comes across pretty clear.
Oh. Engine weights. Take a look at Ralphy's "Engine Thread" in the lounge.
Last edited by irstang (4/27/2012 10:04 pm)
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There are a number of terms that have been used to describe the geometry.
Roll Center (Centre) seems to have been around for the longest and universally accepted - though maybe not the most accurate.
Dynamic Roll Center is a good one as it helps describe the actions and movements as the suspension moves through its range.
Moment Center is certainly a good choice and helps explain the vector sums and relationships as well.
Every science, art, and endeavor has its term of reference. Its learning the "Code" that is often more challenging than the practice itself!
Cheers - Jim
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Jim,
Think of it as a three piece hinged assembly. The base holding this assembly in place is at the road surface/tire contact. The chassis is the center section of the assembly. The lower you can make this hinge opperate the more even it will operate. With two links UCA/LCA you can move this point up or down by changing the position of both.
Take a pile of Janga blocks and push them. If you push at the top they will tip and transfer weight toward the side/direction your pushing. Push the pile at the base and the pile will stay level and weighted more evenly.
Ralphy
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So if the roll couple is like a shovel stuck in the ground with centrifugal force and gravity acting on the handle at the CG, moving the moment center (roll center) above the handle, (shovel upside down sticking into the ceiling) would cause the car body, (ceiling) to plant the inside tires. Which is what I've been trying to say all along. So the same question: Why do we try to get a low roll center? Isn't it better to shoot for a short roll couple? No roll couple at all? Even possible negative roll couple?
I do get the point about matching the roll angle front to rear. Wouldn't the best way to do that be to use similar geometry front and rear? (Of course this obviously means IRS, how apropos).
Jim
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As a motorcycle rider, I once wondered about the possibility of designing a car with the CG below the roll center. The problem is that a car does not behave like a motorcycle leaning in a turn. A motorcycle rider is able to position his mass at will, positioning CG to maximize downward force on the tires. Something we can't do.
When a car goes into a turn, the mass of the car creates a lateral force no matter which way the body rolls. With a low roll center, body roll from a higher CG tends to load the outside tire more vertically than horizontally. This increase in downward force helps plant the tire, creating more force to counter the effects of a turn. A high RC, with a short couple, loads the tire more horizontally than vertically making the tire easier to slide rather than bite into the road surface. No roll couple would not create any downward force, creating only a shear force, at the outside tire. I believe, 'pushing' is the term race drivers use to describe the tendency of the tire to slide.
Sketch up some A-arms for placing the RC above the CG and things get really scary. All kinds of jacking forces come in to play. Attempting to make the inside tire bite better is of dubious value when all the weight is transferring to the outside tires in a turn. The terms squirrelly and crashing come to mind.
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Wow, that helped a lot. Also sort of explains why you might want to push the roll center below the road surface in some cases, especially with a low CG to get more down force on the tire. BUT, here is where it does not make sense. Total maximum down force is limited to the weight of the car plus aerodynamics. If we disregard the aero for a bit, car weight is all we have. Now it is true, we can concentrate that all on the outside tires. We can also shift as much of it as we want to the inside, theoretically. IF all tires were planted squarely, the ONLY advantage to concentrating the down force on the outside tires is that they are on a radius to the corner center point that is shorter by the width of the car than the inside tires. However, this is enough of a margin to win races in some cases. What advantages would there be to even downforce? Well, if all four tires are equally loaded then all four maintain the same tread temperature so on a road course all are equally sticky and the tread compound can be optimized in a narrower band. With outside loading the outside tires get hot while the inside tires cool down so temp variations are wider and the tires must work in a wider temp range. Is this enough to use as a competitive advantage? Perhaps, but apparently at this point the suspension geometries are still in the way of doing this and a new design would be needed to get rid of the jacking forces mentioned above. What geometry would do this? I don't have a clue. But if you don't have to fight roll all the time than camber changes present much less of a problem and it seems that most of the development on the double A suspension has been to try and control camber.
Jim
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Roll Center and Wedge.
So what we've gathered here. I should assume in my case lowering my rear RC would be beneficial to me. Since my vehicle is near 50/50 weighted. This may explain to me why I see so many Cobras lose it in the rear. If I lower my LCA's inner mount, I will increase the distance out with regard of my IC? Which will also give a lower RC.This is also good info pertaining to why I should keep my half shafts level and not higher at the differential.
Improper track bar adjustment
Ralphy
Last edited by Ralphy (4/30/2012 7:50 pm)
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Here's another question. If my LCA and UCA/half shaft are parallel where is my RC? Since my IC is infinite, I have no point to draw from?
Ralphy
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Take the midpoint at infinite distance.
That beer drinker has got WAY too much coordination.
Jim
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Which would put the RC at app. ground level and make the roll axis lower in the rear. WOW! If I do move my LCA's inner point, I need to be very exact. Since the closer you get to parallel the more rapid the RC would lower.
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Ralphy -
You can do the analysis very simply: (BonesApart)
You'll note the camber gain is linear. The Camber Curve is displaced -1.5 degrees, as that is the preset camber on this example.
Also note, the roll is 2 degrees - and the body roll gives exactly 1 degree of camber change - for each degree of body roll.
Also note the EXTREME migration of the RC - in this example that works out to over 200 feet! But yes, it is a Ground Level.
Cheers - Jim
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I've asked the MG guys where the RC is on the front of the MGB. One guy came back and said it was at ground level. I don't know if that is true or not, but it is common to see the LCA's inclined to the inside.
Jim
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Have you guys made a diagram of your front suspension to determine the roll center?
Mine looks like this:
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Jim,
From what I have seen, most front LCA's are parallel to the road surface. So if this holds true for you, your RC can not be at the road surface or very unlikely. Since the control arm that is parallel will be the height of your IC. My C3 rear has the halfshaft at level per factory, therefore my IC would be at the HS height. Now the distance out of the IC is the question. In irstangs pic his IC is at 6 1/2" height. You would almost have to have both upper and lower pointing downward toward the center of the chassis or almost parallel to parallel control arms to get a RC at road height. I'm just running this by you so when you look at your front end you can make a fairly good guess.
Also I would guess parallel control arms would be rare to not. Since if they were you would fight to get any positive camber. And now I just read your post again. You said the lowers are inclined to the center, this would raise your IC even higher than the LCA.
phantomjock and I got in a discussion about lowering. PJ, why would I not want to just lower my car changing my position of control arms and lower my RC? First thought is bump steer then maybe a change in the amount of camber change? There are bump steer ends made for the MII. There are ball joint manufactures that make extended BJ's for lowering as PJ once showed me. There are 2" lowering spindles also. But I want to go only 1".
Thinking more about it, extended ball joints will not move my rack back in the home position. It would increase the distance between the upper and lower control arm. Plus keep the LCA in the home position. This would seem to also reduce the degree of camber increase. I wish they made a 1" drop spindle for the MII.
If you have a Jag IRS and set it up as Day says with the bone level. Your IC will be at the LCA height. Length/distance out unknown. Giving it a fairly low RC.
Last edited by Ralphy (5/01/2012 11:37 pm)
