Roll Center, Roll Couple, Roll Axis, How to lower Efficiently?

[Felipe340]

Alright, I read an article a while back that discussed lowering your car and the pros and cons. If anyone is interested it is in the August 2005 Sport Compact Car issue the article is "Making it Stick: Tweaking roll center, roll axis and steering geometry". Anyway I learned a lot and figured there may be some of us that would enjoy discussing this topic, especially the roll center and roll axis topics.

Basically it states that geometry of our suspensions when lowered obviously lowers the center of gravity (CG) which is a benefit, but it lowers the roll center (RC) (point at which the car pivots when rolling) much more then the CG, therefore lengthening the roll couple (RC) (difference between the two) and therefore basically enduces more lean towards the outside in a turn, decreasing inner tire grip as well as other things.

I just got my first Protege not too long ago after numerous ford probes (basically mazda mx6s) and would like to discuss this to figure out how to lower my car while maintaining the most available grip though turns by all my tires or even increasing it. The article states that adjustment of links on the control arm could fix or even improve this action.

Anyway, something interesting to discuss I thought.

 

[low_psi]

Wow, I'm glad things like this are being brought up for discussion.

Did they use a double a-arm suspension as an example or a strut-type?

I'll post more in a bit.

 

[aMaff]

What kind of info are you looking for? Do you race? or is this strictly a street car? Road racing or Autocross? The setup essentially boils down to "What do you want to do with it?"

 

[apexlater]

You need to find someone in your area that is good at setting up cars to help you out....seriously.

Otherwise, you go with someone else's starting point and tweak it from there to your liking.

Orrrrrr....you can just start throwing stuff onto the car and figure it out as you go (not the best idea).

If you are going to try and setup the car yourself, do it in stages, with one component at a time so that you understand what that piece does.

Tip: As long as the lower control arms are not beyond parallel to the ground you will be fine. It's when the car gets lowered so much that the suspension geometry gets really ugly when you go beyond horizontal.

 

[Felipe340]

The article discusses both setups, but I think the only important one is ours, basically the MacPherson strut setup. To basically explain how our setup works together:

If you were to extend an imaginary line inwards towards the car from the ball joint on the control arm through the control arms inner pivots, then one inwards from a 90 degree angle from the strut axis on it upper mounting point, their intersection would be the instant center. If another imaginary line were extended from this instant center to the center of the tire contact patch, where this line crosses the center of the car is the roll center. The roll center is "virtual pivot point in space that a car rotates around when subjected to cornering forces"

It may all seem complicated, but if you were to see a picture it would really help (I wish a had a scanner). If you try to invision it you can see how this point would lower quicker than the center of gravity, therefore damaging a cars basic cornering geometry by lowering it and negating most actual gains made by lowering your car.

As to the second question, when I got this car I really wasn't dreaming of a 12 second drag car, but I was dreaming of a car that can tear up a curvy country road. Maybe a little autocrossing too, but mostly a car that leaves others in the dust around curves. So yes, this question most definitely is important to me, because thats all I desire in this car.

 

[nocar]

This is an issue with the nissan spec v's. The way the suspension is setup if you go lower than 1.3 or 1.5 inch you decrease cornering ability. Im not sure what it is with our cars but yeah id like to know the facts as well because i do not trust anyone telling me how to run my suspension because in the end whos the one going off the road? So if you can get a copy of that on here it would definetely help.

 

[xelderx]

Here is what I've run into in all my trials and errors.

It is virtually impossible to go too far with regards to the values you are talking about with the Protege and here is why. You can't lower the Protege enough to where it starts to have negative returns without first lowering it too far to avoid hitting the bumpstops under load and normal suspension travel. Even cutting the bumpstops in half doesn't allow enough room to alter the suspension in a negative way. I'm running 450lbs springs on my car and I set the suspension up to where it just barely hit my half bumpstops under racing conditions. I run 16 inch rims with a 225/45 tire and I can get at least one finger between each fender and tire. Even with my high springrates I can't lower the car far enough to do damage and still keep full suspension travel. You will create more obvious problems with extreme lowering like hitting the bumpstops so there is really no reason to go far enough to cause negative handling returns.

 

[low_psi]

Ok so lets talk about roll couples. On any MacPherson strut suspension, the RCenter is going to be very low, and the CG is going to be fairly high(street car) so the roll couple is always going to be larger than you'd like. But having a low RC, even an underground one, is better than having a high RC, which will induce jacking forces and give horrible camber curves. The Mac strut design has positive camber gain under load already, so we don't need any more camber challenges.

I think the benefit of a lower CG from lowering our cars offsets the potential negative effects due to reduction in RC height. I feel this way because one way to combat a larger Rcouple is to increase the roll resistance of the springs. A lot of lowering springs are stiffer, which helps to increase roll stiffness to counteract the roll couple. You run into other problems though if you increase spring resistance soley to combat Rcouple changes. Upper camber plates are a good solution to remove compliance from the system and raise the RCenter by adjusting the inclination of the strut tube. This creates the same effect as pushing the lower balljoint outward, but is easier and cheaper.

I think think the biggest problem with lowering our cars is trying to maintain the struts' range of motion to keep the damper working optimally. Right now I have a very small range of motion in the OEM Tokicos, which I am trying to address with the Koni inserts I'm working on. Second, I want to raise the RCs by running more static negative camber (something most tires can deal with). That should decrease the RCouple back towards OEM lengths. Third, lower the CG and reduce weight!!! This will obviously reduce lateral weight transfer, which was previously increased by lengthening the RCouples and help in just about every other area of the car's dynamics.

 

[Felipe340]

Yeah, beyond horizontal is bad, but even if it is not beyond horizontal when sitting, if it goes beyond when hitting a hard corner it can also be a huge drawback. Ever feel that nasty understeer, of course it is usually present in front wheel drives anyway, but can be temporarily greatly pronounced in the conditions I just stated.

I believe, and the article confirms this, that there are two ways to combat these problems. The first, stiffer springs, helps but the problem is still there, just slightly compensated for. The second is an actual solution, which is reworking or at least preserving the geometry of the suspension. Notice that there is room between the bottom of the control arm and the rim. If you can extend the ball joint downwards toward the bottom of the rim, it will change the angle though the inner pivot, raise the instant center, and effectively raise the roll center to be closer to the center of gravity. Stiffer springs on top of this will improve handling even further. Its a mouthful and a lot to take in, but it makes sense.

Now, most nissans have kits readily available to accomplish this. Where do we find someone who can create parts for us that will allow us to accomplish this, or how do we do it ourselves?

 

[nocar]

holy crap can you like draw that out on paint or something lol

 

[Felipe340]

Here is what I've run into in all my trials and errors.

It is virtually impossible to go too far with regards to the values you are talking about with the Protege and here is why. You can't lower the Protege enough to where it starts to have negative returns without first lowering it too far to avoid hitting the bumpstops under load and normal suspension travel. Even cutting the bumpstops in half doesn't allow enough room to alter the suspension in a negative way. I'm running 450lbs springs on my car and I set the suspension up to where it just barely hit my half bumpstops under racing conditions. I run 16 inch rims with a 225/45 tire and I can get at least one finger between each fender and tire. Even with my high springrates I can't lower the car far enough to do damage and still keep full suspension travel. You will create more obvious problems with extreme lowering like hitting the bumpstops so there is really no reason to go far enough to cause negative handling returns.

Not to be argumentative, but yes hitting bump stops can cause problems and maybe do damage, but thats not really what I'm addressing, I'm searching for the way to maximize cornering grip through perfecting geometry and minimizing extreme weight shift to outer tires and away from inner tires through corners.

 

[nocar]

im a suspension noob in terms of modifying suspension but what about wider tires? would that maximize cornering grip, Some stx girl on the forum said less rotational mass is what makes handling better

 

[nocar]

can u use this pic to explain what ur talking about plz at all

 

[low_psi]

Forgive me for the large image.

AWR makes camber plates that work with a variety of struts.

The debate on keeping the lower control arm horizontal is tricky. If you decline it so that under load it begins to level out horizontally, you start with a VERY low Rcenter and long Rcouple. If its horizontal statically, then under load it will incline, creating jacking forces due to the raising Rcenter. You have to find a middle ground, because a lot more is going on that effects the geometry.

 

[Felipe340]

Heres another, must have been working at the same time:

 

[low_psi]

im a suspension noob in terms of modifying suspension but what about wider tires? would that maximize cornering grip, Some stx girl on the forum said less rotational mass is what makes handling better

Reducing mass is good, rotational mass even more so. If you can widen the track and tire section, its usually a good thing... but to an extent. Wider tire=more grip, but harder to heat up. Like everything else, you have to pick your tires to match your setup. There's going to be a compromise between a 185mm and 385mm width tire. For our cars, a 215 or 225 provides plenty of contact patch while still fitting inside the fenders.

 

[low_psi]

Can you guys PM those attachments? IE won't let me view them.

 

[xelderx]

Not to be argumentative, but yes hitting bump stops can cause problems and maybe do damage, but thats not really what I'm addressing, I'm searching for the way to maximize cornering grip through perfecting geometry and minimizing extreme weight shift to outer tires and away from inner tires through corners.

Ok...I thought the original question was about going past the point of no return and creating negative handling characterists. Now we are talking about changing geometry to improve handling. That's two different things.

 

[xelderx]

Also I hope everyone understands that this will be the most confusing thread on this entire forum. You have now entered into rocket science territory.

 

[Felipe340]

Well, altering, controlling, avoiding, really any of these. Basically I am wondering what needs to be done to improve handling on levels beyond the blind buying of better springs and just assuming handling gains are made. I know that a stiffer, lower ride is not all that is needed, and I'm trying to understand how real handling gains can be seen with the protege, even if it means some alteration. Also where would me and other protege owners go to get parts to do these sorts of things if we were interested.