AZRC4Me

iTrader: (8)

Well, I'm a visual learner, so I did a couple quick mockups in Solidworks to show what's happening when you move roll centers. The dimensions are rough, but it helps show how the geometry of the suspension is affected by the changing inner camber link locations.

http://www.flickr.com/photos/73062725@N03/6592856601/

This is what we consider to be the "lower" location. The roll center is depicted by the blue circle. I'm not going to go into how to locate this, you can google it, it's a widely used practice. Notice the dimension in gray is 5.18.

http://www.flickr.com/photos/73062725@N03/6592856581/

This is the "high" location. The roll center is again shown by the blue circle. The dimension in gray is now 13.43. The roll center is lower.

So, lower camber inner camber links, higher roll center. Higher inner camber links, lower roll center. Now, what PK was explaining before is dead on, and it has to do with the moment arm created by the location of the roll center relative to the CG of the car. We can all assume safely that the CG is static while adjusting our roll centers. So, when the roll center is closer to the CG, the moment arm from the CG to the roll center is shorter. When the roll center is further away from the CG, the moment arm is longer. Now, when you have a short moment arm, it takes more force to affect something, i.e. try to break a bolt loose with a really short wrench, then try it with a long wrench. It's easier with the long wrench.

Applied to the MP9, when the inner links are in the L position, your roll center is higher, but closer to the CG so in actuality, the front end will feel slightly stiffer. When the inner links are in the H position, your roll center is lower, but further away from the CG, so the front end will feel slightly softer.

As PK said, this doesn't even take into consideration roll axis, let alone roll stiffness, and roll stiffness (read 'camber link length') has a much greater affect on vehicle handling than roll center location. Hope this clears it up a bit

shinnbad12

iTrader: (1)

^ When you refer to CG being static ,,,,, exactly what are you refering to? I was always under the impression that the CG was somewhere between the chassis and the first camber link hole on the shock towers.

I am extremely confused at this point, I have been racing with a respectable level of success and apparently have been going the wrong direction with this adjustment. The reason i'm so confused is, I feel that on the track i have achieved the result i was looking for from this adjustment. I am capable of noticing a 1mm adjustment in ride height so i am baffeled that i have been in left field all this time. ( not saying i'm right just want to understand)

Scott

AZRC4Me

iTrader: (8)

I'm saying the CG is static, i.e. not moving, when we are making this roll center change (like when you're on the bench adjusting the car). It's just for comparison purposes in the example I came up with. Obviously the CG is dynamic, i.e. moving, on the track (the CG doesn't move relative to the chassis, but relative to the track surface as the car goes through the bumps, corners, jumps, etc). The example I drew up is just to view how the roll center moves when you adjust the inner camber link holes.

I wouldn't spend too much time sweating it, if you've got your car where you want it and know what the adjustments do for the handling of you car, then you're all set. The examples I showed are just static cases, and I thought a visual of what happens when you move the inner camber links would help understanding their affect on the roll center. We race in dynamic conditions, so while the static cases are important, it's critical to feel the car on track. Sorry to confuse you, the goal was to try and make it more clear...

aaron125

iTrader: (16)

Just thought I'd add that the CG is always static - any time that one cares about where it is or wants to measure its location would be quite a headache if the CG weren't static. Of course the CG moves around when the car does, as in, under acceleration the front of the car rises and the rear of the car squats, so of course, the CG has to move to be in the correct position relative to how the car has changed. Or when braking, front dips and rear of car gets higher, so again, CG has to accomodate these changes. But the point is that whenever the actual CG needs to be located or found for some reason, it is static - until something on the car changes from the last time the CG was located.

Easiest way to understand exactly what the roll centre is and what effect there is from it's moving higher or lower is: think of the CG (just for explanatory purposes) as being located right on the middle bolt in the front shock tower, inline with and between the camber links. Now picture that the roll centre is directly underneath the CG, perhaps 15mm lower, towards the chassis. So when the car is rolling left or right in a corner, the actual forces at work are being experienced by the car at the roll centre, so the roll centre is also the axis of rotation.

The roll centre and the CG work like a pendulum such that if the roll centre is lowered, the pendulum is longer, therefore the car will generate more roll and hence more grip. If the roll centre is raised, the pendulum gets shorter, therefore, with the same force acting through the roll centre, the car will roll much less as the pendulum is so much shorter.

Incidentally, the fact that the roll centre in a car is always under the CG and in a bike (motorbike, bicycle, whatever) the roll centre is always above the CG explains exactly why a bike leans into a corner, whereas a car will always lean the opposite way in a corner, to the outside.

shinnbad12

iTrader: (1)

Aaron, thanks,,,,,,,but nobody has yet to tell me in what general area the CG is in a rc car???
The reason a bike leans to turn is not because of roll center to high,,,, its because its on two wheels{lots of experience there} if you ever been on a Quad or a four wheeler they act just like a rc car then lean the opposite direction.

I am pretty sure the CG is below the camber link holes on the tower.

This may sound simple or dumb but this is the easiest way i have found to describe the effect on roll that the camber link has. :::

If you have a free standing wall and you want to support that wall so it dont blow over(aka body roll) where would you put the support board or "kicker board". The higher the placement the more support, the lower the placement the less support. Same as a camber link is to a shock tower (highest point of the car), move the link up you get more support (less roll), move it down and less support(more roll)

Scott,

aaron125

iTrader: (16)

It is very simple to find out the exact position of the (instant) roll centre Scott, takes about 2 mins, if that - I'll email you and show you how to find it.

Re: bike leans in, just coz on 2 wheels, why couldn't the bike also lean out when cornering - or conversly, a car leaning in while cornering? It is specifically due to the position of the roll centre relative to the CG. Have a look at this illustration, might explain a bit better.

c2k

Hello shinnbad12,

in this link you might find the info you are looking for . Specifically the section numered 2.3 explains very the cg point and how it is calculated.

http: //users.telenet.be/elvo/
Cheers,

C2K

Matthijs

iTrader: (1)

When I first started buggyracing I made a small document with some usefull info from Mark Pavidis postings on The Grid, here is his info (mind you, it was the mp7.5/mp777 setuptalk ;-))

John, I believe I have read that, and it contradicts the book Tune to
Win, which is what I read. I can assure you that if you use the H
block, your car will change direction quicker, and respond better in
high bite conditions. If you use the L block, it changes directions
slower, and there is more uptake and body roll.

Roll center is way too complex to even clarify here. It also changes
dramatically with suspension movement, which we have allot of in off
road cars. Let me try to explain:

When the car is at rest with normal droop, the roll center is found by
drawing a line from the HUB towards the CHASSIS on both the upper and
lower arms, and extending it past the opposite side of the car to an
intersection point. Then you take that intersecting point, and draw a
line back to the centerline of the contact patch for the respective
tire. THEN, after doing that for both sides, where the last two lines I
just described intersect, THAT is your roll center.

The location of that imaginary point in relation to the CG determines
how much body roll will occur.

Now, to complicate things even further, when the suspension is at full
droop (like when it's in the air), the lines are drawn in the opposite
direction (out away from the body).

Now if that didn't confuse you enough, if you change the front roll
center (moving the camber link) and you don't do anything to the rear,
you will most certainly screw up the ROLL AXIS, which is death to a
good handling race car. That's what makes cars try to lift one wheel or
the other entering or exiting a corner. Roll axis issues.

AZRC4Me

iTrader: (8)

In your example, are you assuming the "kicker board" is a constant length or does it get longer the higher you move it and shorter the lower you move it? In my mind, you'd need to lengthen the kicker board as you raise it to get more support. This is getting into roll stiffness, which is related to how long your camber links are.

Here's a link on how to calculate your vehicle's CG:
http://www.longacreracing.com/articles/art.asp?ARTID=22
I've never heard of anyone doing it in RC, but it's possible. No two cars would have the same CG unless they were identical in every way possible. Change a screw or nut and this affects your CG (albeit a small affect). And your CG is dynamic during a race...your burning off fuel...probably a small change, but a change nonetheless.

toyota 2jz

iTrader: (21)

i currently run a tki2 and some people tell me that i cant run associated wheels and some say i can on the tki2.they are saying that the wheel offsets are different.i have 4 sets unmounted and really dont want to waste them

can anyone shed some light on this? i am a little confused here lol
sorry for such a noob question

thanks

RAlmeida

iTrader: (2)

Just run them

yamahamxracer

iTrader: (62)

Anyone have a good base setup for brushless converted tki2 for this indoor track?:
http://www.youtube.com/watch?feature...&v=2PZ5F4WY8MQ

aaron125

iTrader: (16)

Why can't you just put them on your car and find out right there and then? No need to put tires on the wheels, just stick em on your MP9 and see how they fit.

pitpop

not to ... BUT...

there are plenty of high profile drivers who have the roll center adjustments backward. I can assure you that lower inner camber link locations provide less body roll, and H settings provide more. Try this and it will help you understand:

Some day when you have time on your hands, or even breaking in an engine, drive your car in a parking lot. Put your front camber link high, and your rear camber link low. Drive figure eights and you will see that the rear wheel will lift off the ground when you turn. This is because the front is rolling more than the rear, screwing up the roll axis. If you reverse it (rear high/ front low) the front inner wheel will come off the ground giving you the stiff roll bar effect I referred to earlier.

Back to your regularly scheduled programming..

hookem34

iTrader: (17)

Lol! Love the animations! So front high and rear low will give more front roll and essentially more turn in while front low and rear high will creat more push at corner entry. Does that sound about right?