I did a search, but didn't find too solid of an answer. Using the IRS upper arm mounts as an example, how are people adjust front roll center and why? Are you adjusting one side first, both sides equally, etc?

The front roll center determines the amount of bite the front end has. The lower it is the more bit the front will have and then just the oposite. The bit equals steering. With the lower a-arms being at the same angle or being flat all the time, the upper adjusts most of the roll center. The heighth of the front axle plays into it also. Now the part that confuses me is that IRS's view on raising and lowering is the opposite of how the drawn geometry does it. I've always viewed that to lower the RC, you close the distance between the 2 mounting points of the upper arm, using the top holes in the caster blocks. IRS says to do the opposite. As far as putting the right side in the bottom holes and the left in the top holes, this is changing the Moment Center. This moves the MC to the right making the car feel like it has more leftside weight. Below are a couple of quick links to try to help out. There are more articles about it, just have to find them.

http://www.circletrack.com/howto/1816/index.html
http://www.circletrack.com/techarticles/0304_meas/index.html

If you draw, plot, or graph it out, putting the arms into the lower holes on the mounts will raise the roll center. And vise versa.

What this does, all other things being equal, is change the distance between the CG (center of gravity) and the roll center. The way this effects your car will depend on many things. You have to draw an imaginary line between your roll center and the CG and think of it as a lever. The the longer the lever between the CG and roll center the more the car will roll.

Thats why lower roll center = more chassis roll. Not more steering, just more chassis roll.

There are many other things that effect how much steering our cars have, camber gain being one of them. If you lower the roll center on your front end your going to put the arm in an upper hole on the mount. That will make the arms flatter, and will also give you less camber-gain on the RF. You would think that having more weight transfer on the FR due to more chassis roll would give you more steering, but by having less camber-gain your almost canceling out the roll center change.

There are no magic numbers. you just have to play around with what works. I personally dont use roll center as a tuning option. Its a build thing for me. I set it and forget it. Once you find somthing that works, is best to just set it at forget it, there are so many other options to tune with these days, that adding roll center into the equation of tuning options at the track is pointless. IMO.

Dave

More chassis roll does equal more front bite. The more the front rolls over the more weight is being put on the contact patch of the tire. When the front is stiff, the weight pushs on the side of the tire causing it to slide. Same principle of a sway bar. The larger diameter bar slows the roll in the front causing a push on tighter tracks. Anyone who watches or is familiar with NASCAR has heard of the track bar also known as a panhard bar. It controls the roll center in the rear. Raising it loosens the car, lowering it tightens the car. Lower the bar the more the car rolls over.

More chassis roll does not equal more bite. More chassis roll equals more weight transfer. They do have a relationship but not a direct one. Inducing chassis roll in a car to make it turn harder is a band-aid fix to make the front bite more. And there are plenty of situations where adding chassis roll & WT (weight transfer) will make the car turn less, a good example would be; if your car is pushing because its using to much of the RF tire to make the car turn, and not effectively using the LF as well and you add more weight transfer to the RF your most likely going to exceed the circle of traction for that tire and loose grip from that tire.



Its much more possible to do that on cap tires, anyone that’s raced mod on smaller tracks with caps knows how important it is to keep the RF from working to hard to keep it from blistering and not working. But it is still just as possible to do it on foam tires to its just a different type of abuse happening to the tire, but it’s a heat/grip related problem.



Also if you increase chassis roll but decrease camber gain at the same time your not going to get more bite. Our front ends are not sophisticated enough to make the correct changes to get a real low roll center and a lot of camber gain. The more chassis roll you have due to a lower roll center the more camber gain you need, that’s where running flatter arms becomes a problem. You’ll never get the required amount of camber gain needed to accommodate more chassis roll without using a lot of static camber, which is not a good thing. Camber gain is just as important as roll center if not more. If your not effectively using the tires contact patch to make the car turn then your roll center just became completely and totally useless.



There are many other variables that have to do with how to make mechanical traction from chassis roll due to roll center height.



Last, changing a panhard bar on a 4000lb Nascar, is not the same thing as changing the roll center on our front end. That panhard bar change does not effect camber gain and instant center points like it does on our front ends. Apples to oranges.



There is no such thing as a steadfast rule for roll center – chassis roll – mechanical traction. There are to many variables to say that this does that, and that does this. If there was then there would not be all the smoke and mirrors in regards to car setup, RC or full scale.

Dave

With IRS upper arm mounts we can adjust arm position on both sides; do adjust both arms the same or usually say just the left or right side and why? Also, What do people believe is the difference between using the outer or inner holes versus raising or lowing the arm?

1st: Adjusting arms seperate from each other, ie RF bottom hole, LF top hole takes the roll center and moves it over to the left - driver side of the car, more. If you did the opposite with the arms LF - bottom, RF - top it would move the roll center to the right side of the car. Arms in the same position = roll center in the center of the front end. It really does little to the handeling other then if you have less camber gain on the RF the car will turn less.

It also changes the instant centers. The LF arm position does little of nothing to the car though. The IC -instant center of the RF is the most important part of the whole roll center debate on our front ends.

The LF arm angle, is more used to controll camber gain in the LF.

But like I said before, I dont adjust that at the track. I build it into the car and leave it. I dont see it as being a big change. For the possible roll centers that one can create with the arm positions that we have on our caster blocks, the car will never handle like it should if roll center actually ment anything. Talk to any real race car engineer, they will tell you how much roll center is just a point in space, where the car geometrically rolls around in the corner on paper. In the real world, the roll center is normally very far off where the car actually rolls around. To find the geometric center point of chassis roll, one must take into account spring rate, wheel rates, tire sidewall deflection, track banking, swaybar size and effectiveness...

oh crap I will answer the rest later, got to go to class

Dave

A Panhard Bar Is Not A Sway Bar Either, The Panhard Limits Rear End Travel, Were A Sway Bar Limits Body Roll Two Totally Different Devices

Talk to any real race car engineer, they will tell you how much roll center is just a point in space, where the car geometrically rolls around in the corner on paper. In the real world, the roll center is normally very far off where the car actually rolls around. Dave

The front and rear roll centers, define the roll axis,
which is very real in designing a race car.

-----------

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The front and rear roll centers, define the roll axis,
which is very real in designing a race car.

Yes and no. 10 years ago those were absolutes. Draw it out. Look at the ploted roll center of an modern F1 car with the geometry that they use. Look at the dynamics of one of the same cars as it corners. It does not roll around the roll center that is plotted out. That is becasue of the very large wheel rate that those cars have, as well as the tire sidewall deflection.

Roll axis is used to help understand what a car might do, but there are so many variables that will influence what a car will do in a corner.

The only thing that you could say is an absolute would be instant center. Its compleatly independent of all other influenced factors, that determine roll center.

Nash-
2nd Part: The longer any arm is, the less geometric effect it will have on the lever that is acts on. In our case, the longer the upper a-arm the less a given suspension movement will change the camber, as well as the less the roll and instant center's will move, for the same ammount of suspension travel.

If your a believer that more roll = more bite then running a longer a-arm should give you more bite, but it will also sacrafice camber gain. Our front ends are nothing but a bag of give and take.

Dave

........If your a believer that more roll = more bite then running a longer a-arm should give you more bite, but it will also sacrafice camber gain. Our front ends are nothing but a bag of give and take.

Dave

So if you have the standard associated blocks (not adjustable) would it be good to run the LF spaced out a little to reduce camber gain on that side and keep the RF shorter so you get some camber gain?

Great thread by the way. Good information.

RC

Yes. That would be correct. That also happens to be how I have been running my car, for a while now, its been the fastest front end setup that I have ever found. I use the KSG-Swift castor blocks, but run them in the bottom hole, left and right side, with the LF spaced out using the Swift castor block spacer.

Dave

JPH good info. As a sort of crew chief on a big car I know what you are saying about RC. If there were no springs or anything that is how it would work, actually roll around it. Look at Cup cars these days, there is no roll to them at all but they still got roll centers!! I will have to try the arm spacers, I got a set a while back and just haven't tried them out. I do have one question for Rickk5, how does a PHB limit rear travel? I have never seen that happen. Keep it coming...someone needs to start one on the rear of these cars so someone can finally explain the geometry behind moving the side shocks up and down and what it does from a geometry stand point.

vwal the side shocks only control role rate, or the amount of role due to a given force. The point at which the rear rolls isn't effected by the angle of the side shocks, but only the position of the pivot ball, relative to the rear axle.

vwal: I love having discussions about this stuff, I wish I had more real world experience that I could use with our cars, but right now b/c I'm still in school, I know more about the mathmatics, and the forces behind the geometry, but not the actual hands on thats what it looks like when the real car does that...

I'm glad you understand where I'm coming from on the front roll center stuff.

If you look at the swift web site for the Maveick cars, we have a graph up there for what our side shock locations will do to the rear end of the car. They are wheel rates too, which is the best info to look at since it takes into account all other suspension variables. The graphs are not labled with any shock travel over weight figures (have to keep somthing to ourselfs) but you can get an idea of what is hapening.

Other cars without the inline rear shock's will only have a different curve at the end of the shock travel. swiftracingproducts.com

Dave

I wish I had more real world experience that I could use with our cars, but right now b/c I'm still in school, I know more about the mathmatics, and the forces behind the geometry, but not the actual hands on thats what it looks like when the real car does that...



You are right in that one. Hands on combined with book learning is the best way to fully understand how race cars work. I fully understand that what works on full size cars doesn't always work on small cars. But I do know this much. I'm not trying to telling you or anyone else that they don't know what they are talking about because I'm still learning about Roll Center myself but I have been around Stock cars all my life and for the past 14 years have had hands on experience on working on and seeing what different changes do to a car. For the last 8 years I've been apart of the desicion making on setups too. I have also driven these cars and know what each change feels like. I'm not trying to say I know more than anyone because I don't, but with front roll center, I do know this, the lower it is the looser the car will become because the front is getting more bit. Then you have to play caught up with the back. Higher RC tightens the car up. It has more to do with it than just camber gain. The top arm length changes how the feel handles too. This is do to camber gain. As I have said before, I don't fully understand roll center but I have seen what it does. With both size cars. As far as a panhard bar, it does both in a sense, change the rear roll center and limits the travel. When the bar is high, it puts the roll force in a straighter line that is perpendicular to the ground. The lower it is the more force is going towards the ground. So, a high bar will loosen and help the car turn more and also reduces roll. Low bar tighten and help gain bite and allows more roll. My question is this though, with a pancar not having a PHB, can the height of the T-Plate to the same thing? With the angle of the side shocks determining roll resistance, will this affect how loose or tight the car will be, ex. more angle looser car? Just a little info on how a big car feel, even though it seems there isn't much roll in them, there is. When you hit the corner, especially with a soft suspension, low PHB, good bite tires, and flat tight corners, the car feels like its going to flip over. Look at a dirt late model, they show it more because they run a higher center of gravity. An asphalt late model feels the same but doesn't show it.

JPHRacer & Mike : Now I am confused. Based on what was said I assumed we were saying the all things equal raising the shock angle would only affect the rate at which the roll would occur.

According to the graph, the rates are not only progressive at different rates, but the roll does not "match" for all 3 settings. Why is that? Static starting point (level) is equal. End roll is the same (or is the actual roll affected as well?)

M.P.

Here is a great book to buy and read to under stand more on Roll Centers among other things. Paved track Stock Car Technoloy by Steve Smith.

http://www.ssapubl.com/index.cfm?CategoryID=1&do=list&start=11

I helped me with Both types of Cars big and little.

Jason

...someone needs to start one on the rear of these cars so someone can finally explain the geometry behind moving the side shocks up and down and what it does from a geometry stand point.

The math part of it is really pretty simple..
if you look at the back of your pod, and move it, you'll see the
top plate, where your sideshocks mount, moves in an arc.
Because it does not move too much, it moves nearly horizontally.
For the sake of explanation, move it 1/4" to the left.
What you want to do, is mount the shock, so that it also moves 1/4".
Let's say that 8 degrees makes it near perfect.
If it is at say 30 degrees, it will only move a percentage of that 1/4".
If you have a 10# spring there, the car 'thinks' that the spring is,
your percentage x 10#.... which equals the 'wheel rate'.
It may react, like a 6# spring..
So when you look at a guy's car that has a 'gold' spring, before you
put that same spring on your car, you need to also look to see
if your shock angle is similar.. If your shock angle is more severe,
you'll need a slightly stiffer spring.

Dan, I do see how on the Swift site it shows it being softer at the start of travel with more angle. It would be nice to know the amount of travel they used there, we don't need the weights, but the distances would be nice to see what kind of movement we are talking about here. So on your math example I get confused. I understand it starts softer but do I need to go through and measure my lengths at a bunch of different travels to see at what point they cross linear and such? I wish my car was here so I could do ti right now!! I think I am getting what you guys are saying I just need my car and some calipers!!!!

Z-main, We have raced in South Bend a few times, used to be one crappy track, not sure about today though!! I guess my driver has never said he feels as though the car is going to flip. He usually complains when we don't have a big enough bar or a big enough RR spring that the car is rolling over, but we try to keep that from happening. On big cars, the lower the rear RC the bigger the RR spring you need and the higher the less spring you need. But anyway, enough with big car rear suspensions, we are talking about little cars!!

My question is this though, with a pancar not having a PHB, can the height of the T-Plate to the same thing? With the angle of the side shocks determining roll resistance, will this affect how loose or tight the car will be, ex. more angle looser car? Just a little info on how a big car feel, even though it seems there isn't much roll in them, there is. When you hit the corner, especially with a soft suspension, low PHB, good bite tires, and flat tight corners, the car feels like its going to flip over. Look at a dirt late model, they show it more because they run a higher center of gravity. An asphalt late model feels the same but doesn't show it.
Z-main:
I’m not going to argue that having a lower roll center does induce more roll, but on cars with such a high left side weight bias, its not always going to add more bite. And roll does not equal bite. Weight transfer is not directly related to roll. Look at go karts. No chassis roll, yet tons of bite and lots of weight transfer.

You don’t need chassis roll get front bite, or rear bite. All your doing with chassis roll its trying to control it in a manner to harness it. If your using chassis roll to induce more RF dynamic weight transfer then your going to loose weight from the LF. The key to making a oval car work, is to use both LF and RF tires equally in the corner. Its finding the balanced setup.

Its simple physics, a more equal tire loading will give you more grip. To really understand what I am talking about, you need to know the characteristics of our, or any race tire. We can all agree that as a tire is loaded more, the more grip it will have. But being that we are working with tires that are either made up of foam or rubber or a combination of both, the characteristics are very similar. The more you load a tire the more grip it will create, but not equally with the amount that its loaded. If a tire is loaded 10 oz. it will make 5 oz. of sheer grip (just play numbers), if you increase that load to 20 oz. due to the laws of adhesion and the characteristics of rubber the grip of the tire will only go up to say 8oz.



Its a non linear curve that your dealing with. That makes it easy to see how you can add all the chassis roll and dynamic weight transfer onto the RF that you want, but you will only make so much grip due to the effects of transferring the weight. This gets exacerbated when you add heat into the situation. By unloading a tire to make another tire grip more your unbalancing the car. An unbalanced car = an inconsistent car that will wear our tires harder and be less free in the corners. It will also use less of the available grip in both of the front tires traction circles, then if you were to setup the car to better utilize both front tires to make the car turn. This would be accomplished with less chassis roll and a higher roll center.


onto the next...


The height of the t-plate pivot balls is the rear roll center.


Mproy- What becomes effected is the time it takes for the shock to build up enough resistance to counter the chassis roll. The more angle, the more the shock needs to travel in order to counter the forces that the chassis is placing on it. It will have more roll resistance as the travel increases. It’s a different adjustment from going to softer or stiffer springs.



A spring change will yield more or less chassis roll depending on if you go softer or stiffer. The shock angel change will effect how the chassis roll happens. The number one position (bottom hole) will yield more initial chassis roll (given the same shock and spring package) but will build more roll resistance as it travels more, and will return the car to center-flat, much quicker. The number 3 position (top hole) will yield more chassis roll resistance, but, being that the roll resistance does not ever go up or down it’s a much slower reaction. Both into the corner and out of the corner.



To tell you what to use, or how to tune with it, will depend on what the car is doing, track conditions ect…

Dave

DW, your makin ny head hurt with all that there thinkin ;)

good stuff though !

Z-main:
Weight transfer is not directly related to roll. Look at go karts. No chassis roll, yet tons of bite and lots of weight transfer.
Dave
[/font]

Because for the most part, a car will always transfer the same amount
of weight. It's how the car 'reacts' to the transfer, that all of this tuning
theory stuff is about...

Chassis roll is a product of weight transfer, more accurately than weight transfer
is a product of chassis roll....

Yes. You are correct, its a bad example, on my part, for the situation that I was trying to explain.

Your an engineer, arent you? What is your take? Its always nice to hear other explinations, or therories.

Rich: come on dude. Whats an M.E. student suposed to do? They cram all this crap up in our heads, its fun to try to use it every so often, even if we are wrong. :roll:

Dave

Here is a great book to buy and read to under stand more on Roll Centers among other things. Paved track Stock Car Technoloy by Steve Smith.

http://www.ssapubl.com/index.cfm?CategoryID=1&do=list&start=11

I helped me with Both types of Cars big and little.

Jason

You are so Right!!
I personally think knowledge is speed.

Paved track Stock Car Technoloy by Steve Smith book is awesome.
I would recommend it to anyone who wants to have a better understanding
about chassis technology.

Our team broke 3 world records using the setup theories on full sized racing for Filmore and Dunn Motorsports sponsored by Western Auto Race Team back in 1994-96.

It works...

Let me set the record straight. Many people are very confused of the concept of roll centers. The "roll center" itself is an old fashion way to describe the lateral jacking forces of a front suspension.

What you need to understand is simple. When a lateral (side) force acts into the suspension from the tire, it is reacted by the suspension. When the car corners the tires develop lateral forces that are opposing the centripetal acceleration of the car. The interaction of the forces between the tires and the chassis creates weight transfer from the inside to out side tires. The amount of this weight transfer is very simple to calculate.

WT=Lateral Acceleration*CGheight/Track width

The complicated part occurs in how the weight transfer is distributed across the front and rear of the car. This weight transfer distribution is a function of four major factors:

1. The springs
2. The anti-roll bars
3. The suspension geometry (instantaneous centers or "roll centers")
4. The weight transferred within the unsprung mass (very high in the solid axle rear ends of our RC oval cars)


The amount of weight transfer that acts through each of the
Mechanisms is a function of:

1. What the springs rates are
2. What the anti-roll bar are
3. Where the instantaneous centers are located at
4. How much unsprung mass there is and how high the unsprung mass CG is.

The more roll stiffness that is carried through one end of the or higher the roll centers are on one end of the car, the more the distribution of the lateral load transfer will be that is carried by that end of the car.

As a quick example, if you increase the front anti-roll stiffness, you will transfer more of the lateral load across the front and an equal amount less in the rear. This will add dynamic wedge (more load on the RF and LR and less on the LF and RR.) This will in most case tighten the car up (Not in all cases!)

Now onto the "roll centers". A roll center is a mathematical construction of both the LF and RF instantaneous centers. The instantaneous center (IC) is the mathematical point in space where the lateral force of the tire acts into the chassis. If the instantaneous center is higher, the more the lateral force will lift up on that corner of the chassis. In other words if the RF IC is raised, the lateral force created by the RF tire will lift up on the RF corner of the car, creating a moment that will resist the roll of the car. If the LF IC is raised, the lateral force created by the LF tire will pull down more on the LF corner of the car, once again resisting the roll of the car. Just the opposite occurs if the IC's are lowered.

The instantaneous centers are determined by the angles of the upper and lower A-frames on a duel wishbone independent suspension (Where the line of the upper A-frame and lower A-frame intersect). On a Std. Associated front suspension there is only one A-frame. The other line is created by taking a perpendicular line of the Kingpin where it intersects through the lower pivot ball.

If you want to be able to understand this better, try using my suspension geometry solver. It will give you a general idea of how moving different linkages change the IC position

S & B

Well I tried to upload the file and it wouldn't let me. Maybe Hank can repost the link to the file.

Never mind.:)

The instantaneous centers are determined by the angles of the upper and lower A-frames on a duel wishbone independent suspension (Where the line of the upper A-frame and lower A-frame intersect). On a Std. Associated front suspension there is only one A-frame. The other line is created by taking a perpendicular line of the Kingpin where it intersects through the lower pivot ball.

If you want to be able to understand this better, try using my suspension geometry solver. It will give you a general idea of how moving different linkages change the IC position

S & B


Very good information S & B!!

What about chassis flex in R/C cars effecting the spring rates.

Most oval pan cars made today use thin graphit chassis.
I personally would like to find a thicker chassis with no flex.

If you have that program please pass it on would be interested in it.
If you can send it to rbartuccio@comcast.net

Thanks again.

Thank you for the post.

M.P.

The more roll stiffness that is carried through one end of the or higher the roll centers are on one end of the car, the more the distribution of the lateral load transfer will be that is carried by that end of the car.

As a quick example, if you increase the front anti-roll stiffness, you will transfer more of the lateral load across the front and an equal amount less in the rear. This will add dynamic wedge (more load on the RF and LR and less on the LF and RR.) This will in most case tighten the car up (Not in all cases!)


The second part is in refrence to having an increased spring rate and in no relation to roll center, correct? Or are you talking about anti-roll stiffness as a by-product of moment center height?

Dave