I need a bit of 'splaining by some of the chassis Yodas here: Roll Center - What's it do?
This is one of those terms that I hear now and then regarding suspension design. - and no, I'm not talking about the CG - Center of Gravity - that's different, and I get the feeling that the CG rotates around the roll center.
I've done some searching on it and I think I understand what it is - it's the point (imaginary line that runs thru) where the chassis pivots as it rolls left and right in, say, cornering.
From what I've read, Road Race cars like to have the roll center down low - even to the point of actually being below ground level. But what about us with lots of suspension movement, ground clearance and the love of dirt???
I designed a Stadium Lite a few years back and purchased a roll center calculator program to help in designing the suspension. Sad thing is I didn't/still don't quite understand what it does in relationship to our rides. There were no help files or such with the program.
It was a pretty cool program; I could move the RC by modifying the suspension and such - but the calculator didn't like having 14" of travel, so I had to play with numbers that were meaningless to my project. I must have gotten the design of the car and suspension (double unequal a-arms in front, five link rear) fairly close, as the thing handled great - well, oversteered a bit - and performed much better than I would have ever dreamed - espcially for a first shot at designing/building a vehicle.
But, I have some questions:
A) How does the RC apply to us, playing in the dirt?? Is there an optimum (percentage?) height/location in relationship to the CG of the car?
II) Exactly what does the RC do? - howzit work??? - or do I have the correct idea about it as written above?
3)Will someone walk me thru the concept and how it applies to us with high(er) centers of gravity, long travel suspensions and mucho ground clearance???
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I think you understand the basics. Yes the RC is the axes that runs the length of the car that it rolls over in a corner. And Yes, it has been my experiance that in a suspension design like we use in off road racing the roll center tends to be all over the place. You must bear in mind that the roll center can change drasticly just by changing the ride hight of the car. I am no engineer but I have had a hand in building and designing a few. From my ignorant point of view I would think it would not be desireable to have the RC below ground as in some road race cars. In the last few cars I've been involved with (without looking at the design drawings) I believe the roll center on the front suspension for example was somewhere near 4-6 inches above ground and haveing between 22-24 inches of travel. You can easily check the RC by projecting a line, we'll call it line 1, starting at the top ball joint on the spindle on the left side threw the top wishbone inner pivot on the left side and out into space on the right side off the car. Then do the same, call it Line 2, with the lower ball joint on the left threw the lower wishbone pivot on the left and out into space on the right side of the car. Where line 1 crosses line 2 start line 3, extend line 3 to the center of the contact patch of the tire on the left side. Where line 3 crosses the vertical centerline of the car is your RC.
I hope you can find this as helpful, if not I have know doubt that there will be many others that can more accurately explain the concept.
I think you understand the basics. Yes the RC is the axes that runs the length of the car that it rolls over in a corner. And Yes, it has been my experiance that in a suspension design like we use in off road racing the roll center tends to be all over the place. You must bear in mind that the roll center can change drasticly just by changing the ride hight of the car. I am no engineer but I have had a hand in building and designing a few. From my ignorant point of view I would think it would not be desireable to have the RC below ground as in some road race cars. In the last few cars I've been involved with (without looking at the design drawings) I believe the roll center on the front suspension for example was somewhere near 4-6 inches above ground and haveing between 22-24 inches of travel.
Okay, you've touched on the main question I had: Where and why do we want the RC on our vehicles? - The numbers you give definitely help me relate to what I had, but why are we putting the roll center where we are?
I'm assuming it's mainly chassis dynamics - things such as weight biasing with brakes and steering to drive the car through the corners?
See if this is correct - Example is how I drive my car (live axle, no diff) - Come into corner, nail the brakes to throw the weight forward and get the rear end light, step on the go pedal and get the car into the turn - as car enters turn, it weights the outside wheels with centrepidal force of cornering and the chassis rolls about the roll center, forcing the tires into the ground by weight bias about the roll center.
So I'm assuming that the higher the roll center, the more I'm going to toss the CG around (correct?). Lower RC will enhance the cornering because of keeping the CG lower? - but then again, I use the chassis' ability to float in the suspension to help me steer and drive the car through the corners - so I need a higher CG (or RC??) to help me weight bias (load) the suspension??
See my dilema? It's got me chasing my tail - as I understand it. I may be waaaayyy off - and if I am, please tell me how and why.
Originally Posted by DUMP!
You can easily check the RC by projecting a line, we'll call it line 1, starting at the top ball joint on the spindle on the left side threw the top wishbone inner pivot on the left side and out into space on the right side off the car. Then do the same, call it Line 2, with the lower ball joint on the left threw the lower wishbone pivot on the left and out into space on the right side of the car. Where line 1 crosses line 2 start line 3, extend line 3 to the center of the contact patch of the tire on the left side. Where line 3 crosses the vertical centerline of the car is your RC.
Wow.. That one took reading three times before it made sense!! I got it now, though. Thanks for the info on how to find it. Great starting point! - it makes me understand just what the program I had was doing. Cool! I'll be able to figure out my RC on ACAD now - WOO HOO!!!!
I can see how the RC would change as the suspension is cycled. Throw in different postions for each side of the vehicle and whoa... The RC is in a constant state of motion.
Keeping the roll center constant is as important as what height it is at. Unfortunately with the large amount of travel we need for offroad it very difficult to keep the RC low or constant. Design your front suspension to get the camber, caster and travel that you want and then if possible adjust for RC. Keep in mind that RC at ride height is all you should worry about because the minute you start cycling the suspension the RC starts moving all over the place.
This is only my opinion. Feel free to design your front suspension for the ultimate RC, just be carefull what you have to give up. Quality travel with good caster and camber is more important then optimizing the RC
Sickrick, and the Displaced Desert Racers.
Not sick in that "extreme" kind of way, its more of a weak stomach thing.
I can see how the RC would change as the suspension is cycled. Throw in different postions for each side of the vehicle and whoa... The RC is in a constant state of motion.
I'm also a little fuzzy on this subject, but I think this would be called IC, or Instant Center.
Sick - great info. That would explain why my first build worked - I concentrated more on the suspension dynamics as it cycled - camber, caster, etc. - Heck, I don't even know where the roll center ended up!
Thanks!
I see you're in Minnesota - Great tracks up there! I LOVE Crandon!!! - too bad they dropped the Stadium Lites... - I managed to win the "World Champinship" round back in '99. Woo Hoo!!! lol...
I may be off base on this one, but here's what I've picked up so far (most of it from rock crawling, less experience with desert applications)
A higher roll center works well for rock crawling because it minimizes body roll off camber because you decrease the moment arm, or distance between the COG and the RC. For rocks, basically the higher the RC, the better.
One problem with a high RC for desert is that, as you said, the COG rotates about the RC. You can also see that as the wheels deflect about the RC when you hit bumps at speed, since the COG tries not to shift around too much. A higher RC means the COG has to actually deflect sideways when the suspension compresses unevenely, ie one side compresses while the other droops.
Imagine, for fun, your roll center was somehow about 10 feet off the ground. when you went around a corner, the body would actually lean INTO the corner, like on a motorbike. Now imagine the left side of the car hits a bump. The roll center deflects about 3 feet to one side. This will have 2 effects. One, it will jar the car and its occupants pretty hard as they get shoved sideways, and TWO, it will work against the suspension. The mass of the car at cog will resist sideways deflection, and will prevent the suspension from deflecting freely.
You need a reasonably low RC to allow the body to roll to the outside in a corner to weight those tires and get them to dig in, like you need weight transfer when braking and accelerating to add traction.
None of that helps to answer your question about how high your RC should be however....
I'm also a little fuzzy on this subject, but I think this would be called IC, or Instant Center.
Roll center has to do with how the suspension acts when a force tries to roll the body, ie cornering or side-hilling.
Instant center has to do with how the suspension acts with forces in the longitudinal axis, ie braking and accelerating.
Instant center for a linked rear can be found be extending (looking from the side) a line through the upper and lower links. The IC is their intersection point. You need to know it if you want to calculate your squat/antisquat percentage for your suspension.
Okay, you've touched on the main question I had: Where and why do we want the RC on our vehicles? - The numbers you give definitely help me relate to what I had, but why are we putting the roll center where we are?
I don't think anyone will tell you "X" is the perfect RC it must go here. As Rick has said there are many other things to consider first.
Below is a jepg of the last front suspension I help design. In discussions about the design we talked about Camber at Ride Hight as well as the Camber Curve, Contact Patch, Caster, Scrub, Bump Steer, Wheel Travel, etc. The only thing that was mentioned about RC was. "It can't be below ground". The criteria was as follows. First we desided what the camber should be at ride hight. Then we said that we wanted as little scrub as possable. Scrub is the amount the tire moves in and out from the longitudinal centerline of the car as the tire moves threw the travel. We didn't want alot of scrub because this tends to keep the car going much straighter in the straight ahead rough sections then a car with alot of scrub, as well as giving less bump indused feedback into the steering wheel. Have you ever looked at video footage of a car comming straight at you down a wooped out road and noticed that the car is all over the road, left to right and the poor driver is chaseing it like mad just to stay on the road? That is because the design either has too much scrub or too much bump steer or both. Both will negitively effect the handleing of the car. You will need to make that judgement as to how much you want it to effect your design. How perfect do you want it to be? Like anything with these cars it is a series of compromises. The biggest question is where do you want to focus your design? Next we played with the king pin inclination & contact patch. You can see by the picture that the king pin inclination is not perfect to the center of the contact patch. By moving the contact patch out board of the king pin intersection point you can make the car handle as if it had more caster than you may have built into the design. For example if you think you want, for the sake of argument, 8 degrees of caster you could design the set up to have 5 degrees and offset the king pin intersection point a little to make up for the remaining 3 degrees. However this is not an exact science. There is no exact amount to offset the intersection point to come up with exactly 1 degree of caster that I'm aware of. It is one of those thing that you can play with in your design and that you will need to make the determination of what is acceptable to your design criteria. At some point you will need to look at the design as a whole and make adjustment according to what you think will be acceptable for your application as the above mentioned articals relate to each other. Example, We had obtained the amount of scrub that we wanted as well as the amount of camber we wanted at ride hight and the king pin offset we felt was acceptable along with very little bump steer. Got the camber curve we felt was good and then actually limited the wheel travel a bit because at full droop we really started to lose the camber curve and the bump steer went absolutly crazy. After it was all finished we checked the RC to see where it was and as you can see from the picture it is close to what I origonally thought. After mearsureing the RC it is 3 inches above the ground at ride hight.
This is my therory about the hight of the RC. The higher the RC (within reason) the more stable the vehicle will be in the corners. The lower the RC, the more the car will roll about the centerline. I think that if you actually measured some cars out there you would find most have a high RC and that is why you don't very often see a anti sway bar on the front. You will usually only see them on the rear and then most often they are on trucks because of the solid axel.
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Originally Posted by DUMP!
