OK, while I think I know quite a bit about suspension design and geometry, the more I learn the more I wonder...

For example, in many cases the instant centers and the roll center are very easy to figure out.

But here's my curiosity...

On long travel buggies that have truely parallel A-arms, how can you find the instant center? Going off of all of the formulas I have seen, it's impossible to calculate the instant center if the A-arms are parallel. Or what about some Buckshot cars, that have a further distance between the A-arm pivots on the chassis than on the spindle? That would seem to make finding the instant center even more impossible... if that is possible.

Either this is a situation where instant centers cannot be measured, OR, I am just missing something in the equation.

Suspension experts, please chime in.

You would be correct in thinking that you can't get an instant center measurment on a parrallel arm system. In theory, instant center would be placed infinitely in the direction of the arm plain at any partictular point in travel. You may think that since that point should be infinitely wide that the instant center should be low and in theory this is true but in reality it doesn't act that way. What it does is create a system where the instant center changes in height faster throughout travel than a raised plane upper arm system does. This makes the side forces on the front suspension during a turn uncontrolable and irratic. In english, when you turn, the front of the car will want to raise up in height as soon as the suspension droops in any way basicly pushing the spindles together as the car raises in a turn.

You mentioned the Buckshots having taller frame mounts than spindle pivot points and that will make the instant centers even more irratic or quick to change in height which makes the car want to "pinch" the suspension together in a turn. Add a taller ride height to the equation and the problem is worse.

In my opinion, instant centers and "Roll Centers" can only be considered an advantage in an off road car or long travel suspension from the parrallel arm position (parrallel to the ground) upward through compression. Anything below this point will always "pinch" suspension together. Now if a car was designed to have a parrallel arm system at ride height and people actualy ran it that way then you could take advantage of these principals to an extent since doing this also gives you a very inconsistent camber curve which in turn gives a lot of track width scrub which can be worse in a long travel car than no roll center.

To finish, most of these principals are needed for road racing style suspension to keep the tire in constant contact throughout all movements of the suspension. This works well when the suspension has 3" of travel but near impossible to do "correctly" throughout 18" or more travel. I feel that even though most builders don't use these theories in there cars (whatever the reason may be) if used in moderation and correctly the best performing system can be achieved for your intended use. But to say that a car "must" have this or that is just not true or necessary in the world of "long travel" My .02 Justin

were can I learn s*** like that?????????

My understanding is that "Roll Centers " and "Reaction Points," (Instant Centers) are only calc'ed at ride height. Since CG would change with the vehicle moving up and down in the range of travel, It's pretty much a moving target that couldn't be hit. Throw in polar moments and braking dynamics and it's gets out of hand.

I was advised to only use these calc's at one static ride height, (steady state), to determine the effectiveness of a particular mount system points. The points locations will help determine general effectiveness of the config's contribution to the handling and the transfer of weight/leverage into the chassis thought the mounts.

Like the added pic:



Carl

Oh man, now my head hurts. There's a reason I'm not an engineer, and that's pretty much it.

It's really pretty easy to understand stuff!

Look at the diagram. That's the most basic diagram I've got to explain the idea. The roll center is like where the centerline of the chassis is anchored in a turn.

Imagine that pivot below ground like the bottom pic. The chassis would be more influenced to roll in a turn since the RC has more leverage on the CG. And might be more predictable while turning in rough terrain since it's more responsive to steering input. It would plant weight faster than the other pics.

The middle pic would be moderately easy to lean. And kind of nuetral to directional changes. It might not gain from suspension pivot changes.

The top one would be less responsive to steering input. It might be harder to upset it in rough terrain. This one might be a slow turner since it's tranferring weight slower.

Really neat stuff!

Carl

Carl, checking these points at ride height is great for a baseline of reactions but to get a "handleing" suspension set up close to "perfect" as you can then you need to check these points through travel. By changing upper arm angles and lengths you can pretty easily manipulate your roll center. The goal would be to have as close to 0 roll center change as posible. The more it moves through the cycle or dive or body roll the more unstable the front will feel during a turn. Remember that these things can be achieved with small travel numbers (3" or so on a road car) but are worthless once a long travel suspension goes into droop. Now try to accomplish these things by matching car center of gravity and roll center through chassis movements if you really want to smoke your noodle. Justin

We were dealing with this on our race car last night. Measure and inputting them into the computer to get the roll center and instant centers. With the small changes we were making it was moving the roll center 7-10 inches each time. I can see how this would be useless or very difficult to achieve on a car with a lot of travel.

Guys, you're getting a bit too technical about this stuff.



Let's just stick to ferrofluid shocks controlled by GPS triggered mapping and call it good.

Justin, dynamic roll center isn't physically possible nor would it be advantageous to have one. That would , in effect be a changing pivot point system and/or a variable Center Of gravity. What a nightmare!

You can however, change the effect of a certain roll center with everything else on the car. Springs rates and shock mounting comes to mind.

Body roll is good. But when, where and how much is the key.


Carl

Guys, it's not meant to be a dynamic positioning system. It's a static tool to predict the weight transfer before, during and after a turn or directional change.

Wow.

Carl

Carl, I am not sure we understand each other. I am not talking about moving arm pivot points at will but only stating that the position of a given roll center moves up and down in relation to the chassis as the suspension is cycled. Keeping this movement to a minimum in road racing is key. Justin

When building a car that is why you buy a chassis with arms from a company that has it already figured out.
DON~~~

Justin,
It's impossible to keep that point static, or not moving. Since it is a function of a projected point from a moving component (control arms). The arm moves, the point moves.

I'm not sure it would be beneficial to keep it's movement to a minimum. It's just a quantitative measurement of static planes that intersect at fixed points and is only used for reference.

Now if you wanted to take dynamic data, you could use it for reference. But that is a huge undertaking. And the data would have to be taken on a controlled surface, (asphalt).

Carl

Don,
Just because they sell cars, might not mean it's been properly done. Who's around to check up on it?

Quote
Justin, dynamic roll center isn't physically possible nor would it be advantageous to have one. That would , in effect be a changing pivot point system and/or a variable Center Of gravity. What a nightmare

Here you say that a "dynamic" or moving roll center isn't possible. That is why I said that roll centers always move with suspension movement.

Quote
It's impossible to keep that point static, or not moving. Since it is a function of a projected point from a moving component (control arms). The arm moves, the point moves.

Here you agree with what I said that it moves.

Quote
I'm not sure it would be beneficial to keep it's movement to a minimum. It's just a quantitative measurement of static planes that intersect at fixed points and is only used for reference.


Since the higher or lower a roll center is moved the more unstable a car can "feel" while turning, keeping it's movement to a minimum can be a good thing. Remember this is all good on a road course but just about toss is out the window on a long travel design. Every one has an opinion, this is just one of mine and I have plenty more where that came from. Is is beer 30 yet? Justin

and just how do you know they have it figured out?

Well i think if you hook up the thingamajig to the whatchamacallit and hit it with a really big hammer it would work

Ok, I understand now.

I agree that it moves. I don't agree with trying to minimize it's movement(?)

This is what you said."

"Keeping this movement to a minimum in road racing is key. Justin"

Ok, the theory of a "roll center measurement" moving is what I believe is the rub here.

I've never heard of, or read of, a "dynamic roll center measurement." It's a reference point that is based on one suspension setting: Ride height. To make a "roll center" measurement at full compression, and full droop, is just not done. It doesn't relate to anything.

That's not the definition of "Roll Center."

" A theoretical point about which the chassis rolls , and is determined by the inherent design of the front or rear suspension. Each end of a particular vehicle has a different roll center. A "Roll Axis" is an imaginary line drawn through both front and rear roll centers. How a car rolls in a corner depends on the relationship between the roll axis and the car's CG.
The closer the "Roll Axis" is to the CG, the less it will roll from mechanical efforts.


Carl

when you come out of a bowl and you need to whip a u turn to go into the next bowl the BIG V8 car efforlessly slides around immediatly and does not push.

I enjoy this type of banter between these two. So, CarlP. In your opinion can you provide a general observation/critique on front suspension set-ups of various manufacturers of sand cars that you have seen in say the past year? Limit it to 5-10 that seem to stand out one way or the other. It certainly would be interesting reading. Since you are in AZ, why not start with say....................Revenge, Tatum, Sandwinder and Sand Limo.

Thanks for the insight, always interesting reading.

Just add a bigger motor to negate any front suspension problems.


Hey Bartender,
whip me up a roll center with a touch of instant center on the rocks.. Oh and make is a double..

^^^^^ ^^^ At this point I am with Chummin.

"Remember this is all good on a road course but just about toss is out the window on a long travel design."

Finally someone said it! Rehashing data from books written about cars with 3" of travel is silly.

One thing that is never mentioned is the lack of good off-road rims with a deep back set to correct the spindle conflicts and compromises.

That's not entirely true. They both have wheels and have suspension and brakes and an engine. So the things we're talking about are "Physically Universal." Whether a car has 2" of travel or 20", physics is everything.

Anything That rolls or has mass is affected by universal truths. Like it or not.

Spindle talk:
And instead of looking for a custom wheel, just design the spindle for the wheels that most people use. That's what I did. Now, any guy can put a generic wheel on and be completely accurate.

I really like these conversations as well.

Because just the fact that there are some that are striving to improve their product and will talk about these things is great.

Justin,
About the RC. There are other chassis dynamics you CAN control so the roll center becomes less of a "moving target." You can incorporate them without major difficulty and be THE ONLY guy that has a grasp on vehicle dynamics on the planet.

Carl

Carl, wait till you see the new Silencer. I think you will like the "chassis dynamics" we have encorporated into this car to control the issues in this thread as well as many others rarely touched upon when talking about "off road" suspension. Thanks for the terminology chess game, I enjoy it every time with you. Justin

I can't wait to see this new rail!

I noticed on your site that your listing "0 track change" on the suspension specs on one of your models.

I personally applaud anyone that can recognize that the wheels flailing from side to side as they go up and down is not a good thing.

Finally , someone that is paying atention to the details of chassis dynamics!!

Carl

Aside from here on the board, where can you get GOOD reading on the finer points of chassis dynamics?

Heres something for you guys to talk about.....

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What a fantastic thread! We're talking about motorhome tire pressure right? Man you guys can get technical!

but what is the fluid angle in a can of soda during a 3g turn on a 15* incline? say doing about 40?

May I suggest something.......

http://www.amazon.com/gp/product/156091199...glance&n=283155





wow...thats a tight turning radius to achieve a 3g turn goin 40....
anyways....the answer all depends if you are moving at a constant velocity or not....if you are...then its 15 degrees....geez...
physics 101-centriptal force....newton's laws....

Carl & Justin:

Thanks for taking the time to post. It is refresing to see opinions going back and forth without any drama involved.

I prefer this chassis design series..

Engineer To Win

and yes, it's about road race cars, but still the best reference around..

Good stuff. If you guys keep it up, I'll have to start carrying a backpack with text books and a calculator to work just to keep up with the posts.

so we heard on how different a-arm setups on the front end affect roll centers...
but what about the roll center in the rear?
5-point vs trailing arm whether its attached to the frame by one point or two)...all that jazz...

Time to crack open the textbooks.. ^^

If they are winning races with the same suspension.
DON~~~

Oh...

we have another thread for that!

In the sand that doesn't mean the suspension is right.. On an asphalt circle track maybe, but not in the dunes.

I might be waaaaaay out in left field on this one, so please be gentle. I didn't think you can do anything with the basic design of a "Semi Trailiing Arm Rear Suspension" to control the influence of roll.

I looked at that for a while and the best that I could muster up was to reduce the negative affect of that design. And that method is very limited in it's current applications. Don't ask me who.

But the multi-link is the way to go for that particular aspect.

But, the current VW design rear suspension is pretty well thought out and implemented. In other words, you can't argue with it's success. But, nobody has jumped out of the box very often.

The Chenowth Magnum multi-link was "THE" car to have for best suspenion control in Stadiium stuff. Although Jerry Whelchel took some championships with a Nye Frank design that had HUGE spread mounts on a trailing arm setup. It capitalized on camber change .

Carl

Carl, I have to agree with you here. I don't feel there is much a trailing arm system can to in the way of manipulating roll center. In theory, a five link or multi link system can control all aspects as well or at least the same as a-arms can in the front with the added control over "squat", hence the success 5 links have enjoyed in stadium racing. I am far from fluent in multi link rear suspension so I will stop right here on this subject but I know that a trailing arm rear system is easy to sell to a potential customer where as a link system tends to drive people away when it comes to marketing and car sales in general (just my experience). This is why we never went that way in the past. Even though it may be better,that doesn't mean it will be accepted as such. Justin

Is having "0 track change" all that critical on a sand car? I defenately agree on a road race car, or even a full off road race car.

Just conjecture here but yes, I believe that it is. Here's why, and there's more than one school of thought here.

1) There is such little traction in the dirt/sand that It's of paramount importance that you maintain all the valuable traction that you can for traction and directional control, and minimize all deviations in the form of erratic movements (track change).

2) There is such little traction available in the dirt/sand that the small adverse movements in any direction have little to no affect on traction or directional control.

I go with the first because:

The minute deviations in footprint control of the tires almost always will have a negative affect in the forward motion of the vehicle from the friction of moving sideways. Sure you could always just get more cubic inches, but that cost money. And accurate wheel control shouldn't.



Got time for a short story?
I had a fabber friend that was toying with these "a-arm " things in the early 80's. He conjured up an equal length, parallel arm car that had 24" of travel. But it also had 5" of track change per wheel. A noted Off-Road racer of the time., was interested in his car and asked to test it. He did and said that he was unsure of the chassis. His words were that it never did the same thing twice when it landed on the front end. It came back and we disected the results. Track change was the culprit. It either broke tractionon the landing wheel, or it pushed the chassis sideways away from the wheel with the most traction.


Carl

Good read! Wish you guys would stick to one damn thing though I was tracking with RC until you brought instant in. The weazl's brain only has enough bytes to process one task at a time

In that case Aaron, you should concentrate on keeping that Ford engine from blowing up.

Carl,
While I agree with you, I think we both know that not much has changed over the years with respect to track change. Sure, some desert race cars/trucks have been built with geometry that lessens the track change (scrub), but most have not. Even some of the top names that win lots of races still have a lot of track change (I'm sure you watch the desert racing videos like I do, easy to see on those).

Which begs the question... what are the adverse effects of trying to reduce/eliminate track change? My first thought? Such a massive and odd-looking camber curve throughout the suspension cycle that it's a hard sell to people that don't fully understand "why" it is done.

Remember that there IS a big difference between sand and dirt.

Sand gives you the 'feel' of traction, but really you have none (although this depends a lot on the condition and composition of the sand). What gives you thrust (either forward [propulsion] or sideways [turning]) in the sand is the actual displacement of the sand.

The obvious is roost on acceperation, but also sand moves UNDER the tire. This movement causes force on the tire, moving the car either forward or to the side.

This will greatly affect the assumptions on suspension and steering.
Sand really IS a different animal than other off-road surfaces.
It is a little like water in that it moves (obviously not as much) and displaces so easily.

Everyone says that they use all of these formulas to make their suspension perfect, but i think they just eyeball it. A trial and error thing, lol. Im not smart enough to do all this math. If its straight and it goes up and down its perfect for me, lol.

Too much info!!!