Good handling at 50-60mm drop?

[diss7]

Originally Posted by CSG David

Adjust the roll center. As long as you have sufficient stroke, you just need RCA and alignment.

A rear RCA? I'm not following sorry..

How is this adjusted? Is there something I need?

Cheers

[CSG David]

Originally Posted by diss7

A rear RCA? I'm not following sorry..

How is this adjusted? Is there something I need?

Cheers

Front and rear arms need to be changed out. With the proper suspension setup, you should be okay though. RSR does design the suspension setup to be safe and comfortable.

[diss7]

Alright, just spent a couple of hours looking into rear roll centre adjustment.

From looking at some roll centre diagrams, one of the lines which seems to throw everything out is the LCA line. As the car is lowered this becomes more horizontal and moves the roll centre away.

Now on other platforms you space the lca down from the hub using a RCA or similar, which corrects, or at least better realigns this LCA angle. However that doesn't seem like an option on this platform. However, my thought a few hours ago about using the stance LCA seems to have some merit, that is if I'm understanding all this correctly.

The LCA angle that effects roll centre, I believe is actually a theoretical line from the mounting point of the shock through the mounting point of the LCA to the subframe. The stance LCA design, is based around giving more shock travel that you would otherwise have at a certain ride height. It does this but stepping down for the shock and sway mount, then stepping up to the hub. Using my setup, it would allow me to run a wheel that looked 55mm lower (and indeed the car has actually lowered this much) however because the LCA is stepped, my shock would have the same travel as if the car was only lowered 30mm. What this also means, is that the shock to lca mounting point is now at the same height that it would be if the car was only 30mm lower, which then governs the lca theoretical angle that effects roll centre. In other words, it would correct the roll centre back to where it would be with a stock setup that is lowered 30mm. Well, the LCA theoretical angle anyway, the roll centre maybe marginally different still, but in my mind it would be greatly improved.

Or have I got that completely wrong?

[diss7]

Originally Posted by u/Josh

I would be very interested in hearing your impression of the roll center adjusters if you install those without making any other changes.

No worries. Yes I'll put the roll centre adjusters in the front first, and see if I then need to add the sway bar in afterwards.

[wparsons]

To fully fix the rear roll center you also need to fix the upper arm geometry, but I don't think there's any options out there for that? Fixing the LCA geometry will definitely help, but might not be a complete solution.

[ultra]

RCA for the roll centres - you have the Moonface ones from Evasive, or Whiteline. The Robispec front arms would also help.

Going that low I think you might need adjustable lower arms to get significant camber adjustment in the rear, and possibly adjustable top hats in front depending on what you do.

[diss7]

Originally Posted by wparsons

To fully fix the rear roll center you also need to fix the upper arm geometry, but I don't think there's any options out there for that? Fixing the LCA geometry will definitely help, but might not be a complete solution.

You're right, thanks for that. I forgot about the UCA.

I'm going to go see my fabricator tomorrow. I called him earlier and he said he'd be able to sort something out for the upper and lower arms fairly easily.

[wparsons]

UCA you'd have to raise the inboard pivot, LCA you need to lower the outboard pivot. Basically you need the arms to be in the same geometry they were stock, but with the hub/knuckle higher than stock.

[raytrix]

RCA, solution solved.

[diss7]

EDIT - this diagram and indeed my understanding of the LCA in particular is not correct. The LCA needs to be treated the SAME as the suggestion I have shown for the UCA. Read the post below, but also continue to read the following posts which discuss the error.

I've getting a few PM's about this topic, mainly from people that don't want to ask the questions and be ridiculed by what I call "forum repeaters" (Someone on a forum who likes to be perseeved as being knowledgable, but is really just repeating other information they have read, and blindly accept to be fact)

Anyway, here is a rough diagram I've drawn up to demonstrate what we need to achieve to fix the rear roll centre.

Disclaimer - its certainly not drawn to scale. Rather its a demonstration of the principle. Often these types of issues/topics are overanalysed and over complicated on forums. Realistically, what I'm trying to show is how to replicated the suspension geometry of a particular height, without operating at that height.





Right. Example 1 is what we are going to look at as the ideal scenario. FWIW this has been discussed at length prior, and there appears to be common acceptance that this ideal height is stock height less 25mm. So lets work to that.
What we are looking at in particular is the two red lines. These are the angles of the UCA (Upper control arm) and LCA (Lower control arm) Its these angles that we want to replicate. You will notice the red lines are different lengths, this is because that is the particular length/angle of the arm we are worried about. The LCA line ends at the shock, because the shock bolts to the LCA. The UCA does NOT bolt to the shock (it goes around it) so we need to worrey about the entire UCA angle.

Example 2 shows what happens when the car is lowered. The arms are no longer at the ideal angles. What are the ideal angles and how are they calculated, go do some homework on roll centres; for this example, all you need to know is that they are not as per example 1, which we are looking at as our ideal setup. Note the arch gap (tyre to fender is closer) Not only is the roll centre off, you also have shortened the shock, which may or may not have limited your suspension travel.

Example 3. This is the lowered height of Example 2, but with the control arm angles of Example 1. How has this been achieved?
The LCA angle has been corrected by using a stepped designed LCA. Remember we only need to worry about the angle from the subframe to the shock mount. But if we used a straight LCA, then it would hold the hub down and not achieve the height we want (refer example 1.) The LCA in Example 3 steps, so that it achieves the desired angle from the subframe to the shock, and then steps up from this point, which raises the hub; which achieves the ride height. There are very few arms that do this currently; I've only found two. Stance (boo) and a Japanese arm I found last night; but she's exxy.
The UCA solution is different. The entire arm needs to be at the same angle as example 1. We can do this two ways, both are drawn. (You do NOT need both UCA's that are drawn in example 3, only one or the other) UCA A is mounted on the original mount at the subframe, but needs to be mounted LOWER on the hub.
UCA B is mounted on the original mount on the hub, but is mounted HIGHER on the subframe.
Either solution chosen for the UCA is going to require fabrication work. I will be going to see my fabricator and discussing with him which is the better option; it may be a case that A is not possible, so have to go B, or vice versa. It could also be an option that you do a bit of both, raise the mount to the subframe slighty, and lower it on the hub slightly.

One further bonus of the above example 3, is that you have retained the suspension travel OF THE SHOCK as example 1. Depending on the whee'tyre you run, and offset, and the camber, you may hit the inner guard before your suspension runs out of travel. Don't ask me how much room you have, if you want to know jack up the oppostie corner of the car to compress the corner you are interested in. TBH I dont think its too much concern if you're running a sensible wheel/tyre/offset at say 55mm drop. But I will check.

Despite what people who are trying to portray themselves as purists will tell you, you can be lower that what is accepted as ideal, and have good handling. Its identifying and corrected the geometry that has been thrown off. If you have the same ideal geometry at a lower ride height, your car will have a lower COG (centre of gravity), and ideal roll centres, best of both worlds.

Hopefully this answers the questions I'm being PM'd about.

[wparsons]

^^ Unfortunately, a bent/curved LCA isn't going to fix the geometry, you need to change the relative position of the inner and outer pivots to be back to stock. The geometry doesn't care if the LCA is straight or curved, just where the pivots are.

To correct the rear RC you need to lower the outer pivot point and raise the inner.

[Racecomp Engineering]

Originally Posted by wparsons

The geometry doesn't care if the LCA is straight or curved, just where the pivots are.

Yup...

It's the pivot points that are important.

EDIT: not saying that you're not on the right track, but your diagrams are incorrect.

- andy

[diss7]

Originally Posted by Racecomp Engineering

Yup...

It's the pivot points that are important.

EDIT: not saying that you're not on the right track, but your diagrams are incorrect.

- andy

Could you elaborate/assist on what is the solution?

In my eyes, the LCA pivot points have been addressed. It's the pivot points between along the lca between the shock mount and the subframe mount. The LCA angle after this is governing hub height, not the geometry that affects roll centre.

I'm quite happy to be proven wrong and shown the right answer. But, I don't see why the LCA needs to be treated the same as the UCA when it's pivot points are different..?

[Racecomp Engineering]

Originally Posted by diss7

Could you elaborate/assist on what is the solution?

In my eyes, the LCA pivot points have been addressed. It's the pivot points between along the lca between the shock mount and the subframe mount. The LCA angle after this is governing hub height, not the geometry that affects roll centre.

I'm quite happy to be proven wrong and shown the right answer. But, I don't see why the LCA needs to be treated the same as the UCA when it's pivot points are different..?

The shock mount is not a pivot point....the outer pivot point is at the hub. The shock mounting point doesn't have anything to do with roll center location.

You might be confusing the geometry of the rear with how it is up front with a strut. The front strut is technically the upper suspension arm for that end of the car, but for the rear the shock is not a suspension linkage/arm. You could take the shock out and support the car with a jack and the roll center height wouldn't change (if you jacked the car to the same height).

- Andy