Control arms to increase shock travel, autocrossers?
 

I plan on a mild drop with Bilstein B14 PSS coilovers.

I thought that using LCA's that increase shock travel might be a good idea (VooDoo13 or Stance).

Are there negative consequences handling-wise to using this method versus using taller than factory top hats for more shock travel?

I asked about this a while ago and no one with any real knowledge said anything about it. Maybe they missed my thread....

I think a down side to using that style of control arm is that the lower arm will be at a different angle than the upper arm, causing some geometry issues and likely messing with your camber curve. At least that is was what I gathered when trying to do the same research you are doing now.

A benefit to the LCA is that you gain bump travel between the shock body, bump stop and top hat, so if you're bottoming out the bump stop but not the shock internally you need this. If you're bottoming out the shock internally only then the top hat will help you.

The shape of the arm doesn't change the geometry like that. What matters is where the pivots are, not if it's curved/bent. There is a slight change in the geometry to the shock with it being curved, but that is a tiny difference and doesn't change the roll center or camber curve.

This.

Interesting.

So if there is no down side, why doesn't everyone run their suspension this way? Other than cost...

So in the BRZ/FRS with a sub 2" drop, which do you generally bottom out on? Or is it shock brand dependent?

mostly because it's non-standard

it turns 1" drop springs into a 2" drop and so forth. introduces potential issues where your max bump travel on the shock may be beyond reasonable in terms of other components. your bump stops may no longer save your axles from exploding or the roll center becoming subterranean.

it's just another adjustable component you can tune. IMO, I'd *only* buy LCA's with a dropped shock mounting point, and even if I was happy with my previous ride height / bump travel you can always just run longer rear shocks and add helper springs to your setup to get some extra droop easily.

If you had a coilover that could be set at stock height, couldn't you use the control arms to drop the rear of your car and keep all your rear travel? Or am I missing something?

Droop travel is still important too...bump is nice but droop is still worth having.

- Andy

Andy,

any chance you guys would come out with a speedway / afco eyelet shock mount onto a steel plate that fits the rear shock towers? it'd be nice to just run double eyelet shocks instead of the stock stuff.

*sarcasm* While we're at it... let's throw in some drop spindles to save your CV joints and steering. Scrub Scrub Scrub.
http://image.truckinweb.com/f/tech/1...p_spindles.jpg

\sarcasm
As the guys have mentioned, offset lower control arms will give you more shock travel for the same body height... similar to raised top hats. The net effect on your geometry is pretty minimal.

You only need this if your desired ride height is pushes the shock into a significantly non-linear region (in the system dynamics sense). If you're at this point, I'd really look at how gnarly the geometry has gotten. Underground roll centers and steep camber/toe curves. Moving (and reinforcing) some of your points might be able to get you back in the sweet spot (area A):
http://www.auto-ware.com/setup/camb_cur.gif

Yes and no... I wonder how much additional travel is really usable inside the fender above stock max bump travel...

Using a drop arm would require that coilover to be extended like you mentioned, but the coilover would need to be designed around a longer bump stroke and longer spring to make it usable.

Part of the reason for shorter stroke is due to the nature of stiffer (read heavier sprung) coilovers. They're typically use springs that are much stiffer than stock. As a loose example: if an OEM spring is say 200lb/in, it may require 4" of compression to settle under the vehicle's weight (droop). If an aftermarket coilover has a 400lb/in spring, then it would only need 2" of compression/droop to settle, but since you're 1" lower, that leaves only 2" of bump room (3 -1 = 2). Another reason is the risk of spring binding if they are compressed too much. This breaks things. The only way around this and regain more droop/bump is to use highly progressive springs (where the spring rate becomes stiffer as it's compressed) or helper springs. Progressive springs are harder to valve for, and helper springs aren't really usable since they're really soft and designed to be compressed flat.

more like this

http://sp-tec.com/shop/goods_image/A29_I1.jpg

/completelyserious

also... how does the drop upright save your CV's? distance from the differential to the wheel centerline is the same either way, the axles know about as much about suspension geometry as most of ft86club.

Totally dependent on ride height and shock.

Only good if you have coilovers, and most people looking at slamming their car aren't worried about bump travel. From what I've seen, most people serious enough about tracking their cars to consider this also keep the ride height high enough that it isn't a big concern.

That's a mighty cool upright! Nissans get all the toys.

I see where you're coming from, and most likely I'm just a little slow tonight. Please bear with me while I walk through it from the other direction.

The suspension controls the distance between a point on the sprung mass and a point on the unsprung mass. Sprung mass point = body. Unspring mass point = LCA-to-knuckle connection

Drop spindles simply extend the offset between the wheel center and the LCA-to-knuckle connection. Since the wheel center is connected to the ground, the net effect is a "body drop" equal to the offset.

If you use adjustable spring perches to raise the ride height back to its original value, it will extend (rebound) the neutral position by the offset * motion ratio. As many people above have pointed out... that's not a bad thing.

Now that we're back at the original ride height, we look at the CV angle. Both the differential and the wheel are at the same height as before... so the CV angle doesn't change. High Five, SWIM!

Offsetting points isn't an area that I know much about. I've heard about this on hotrods... but it seems just as welcome on track cars, if the control arms are sufficiently overbuilt (stresses will be different, especially in cornering).

Here's another way to look at it...

A car being lowered in a typical manner puts the suspension control arms in a somewhat compromised position, ultimately affecting roll center negatively. The "drop upright" simply repositions the control arms of the suspension back to their ideal positions so the suspension's geometry isn't changed, but the car is still lowered. The end result is that you can relocate the wheel's position (to get lower) without ruining the handling.

This is effectively the same thing as the Whiteline roll center correction kit for the front end. Lower the car, but relocate the ball joints back to their original positions.

Also, the axle is still at a slightly different angle using these drop uprights, this won't change unless the diff is relocated. The purpose of these is not to correct the drive axles, but the suspension geometry.

if you lower the car 2", all the axle sees is a 2" drop. it doesn't know or care if it got there by coilovers, lowering springs, drop knuckles, or anything else, all it knows is the wheel centerline is 2" above where it used to be in relation to the differential.

Correct, we're saying the same thing...? I was responding to your post where you were asking how it saves the cv joint and I am agreeing it only corrects suspension geometry, it does nothing for the drive axle angles... I misunderstood your post because it appeared you were asking a question.

@wheelhaus okay, I misread "the axle is still at a slightly different angle using these drop uprights". when you said different angle, it threw me off, my bad.

I can look into it if you're seriously interested. No in-house CNC but we've got peoples we can talk to and/or work with. Not a bad idea.

- Andy

I'm not sure you'd need NC machines to create such a piece, you could probably farm the plate out to a waterjet / laser cutter and have them cut it out of mild steel, then with a simple fixture you could hold the steel clevis shock mount to it and construct the piece as a simple weldment. powdercoat it and pop two studs into it and you've got a cheap, durable mount which is easy to manufacture.

it's sprung weight anyway, and if you're milling it out of aluminum billet there's a lot of material waste and extra setups to chamfer both sides and such, even with a 5-axis machine.

you might even consider running some simple FEA on the plate... if you could decide that strength wouldn't be negatively impacted, it would be nice to add a hole into the plate and include a rubber grommet large enough for a braided hose to pass through so people have the option of running remote compression canisters

and yeah, I'm serious about planning on running such a setup on my own car, if you think it's something worth bringing to market I'd rather buy the part off the shelf than go through the trouble of making them myself.