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Old 04-15-2009, 11:15 PM   #1
penderperson
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Default How do you guys determine the best rideheight?

hey all,

Just fine tuning my new Vorshlag-AST suspension and i was wondering what factors you use in determining how low to go and preferablly no TLAR(that looks about right).

-I'm currenlty running at 14.25"f/13.75"r fender to centercap and roughly 6" f/6.25" r rocker panel to ground.
-I have about 2.35" remaining travel at rideheight front and 2.75" remaing travel rear(tips on better measurements would be great, i used a zip as a tape measure b/c it was flexible enough).
-450lbs f / 500lbs r with 27mm sway front/ 22mm sway rear
-bumpstops can still be cut(almost 2" long now)


Not sure how the control arms line up or how to measure the angle.


Thanks for the help,
Rob
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Old 04-16-2009, 06:49 AM   #2
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The dicipline might be of help here, but it sounds like you are right there for autocross or aggressive street. IIRC, the generally accepted ideal heights for a GD are 14.25F and 13.5 or 13.75R when measured from the axle centerline to the top-center of the arches.

Keep your control arms/lateral links from being past flat at rest, they should angle down slightly (inner mounts higher than outer mounts) or be level.

Jay
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Old 04-16-2009, 07:48 AM   #3
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Quote:
Originally Posted by Storm View Post
The dicipline might be of help here, but it sounds like you are right there for autocross or aggressive street. IIRC, the generally accepted ideal heights for a GD are 14.25F and 13.5 or 13.75R when measured from the axle centerline to the top-center of the arches.

Keep your control arms/lateral links from being past flat at rest, they should angle down slightly (inner mounts higher than outer mounts) or be level.

Jay


What shock/coilovers are those numbers with? What camber?

What i'm thinking is that these shocks have extra travel(any my GC camber plates add .75" of travel) available and my camber is about 3*, both of which should allow me to lower my rideheight without getting into a bad camber curve or hitting bumpstops(IMO, even below the accepted 14.25/13.75 heights).

Last edited by penderperson; 04-16-2009 at 07:58 AM.
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Old 04-16-2009, 08:07 AM   #4
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What do you like to see for a track car - not a fully built up one?

Thanks

Ray
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Old 04-16-2009, 08:08 AM   #5
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I'm not sure shocks or camber play to much into it. You can only go so low, until you're hurting the car's performance.

My car is currently at 13.75fr and 14rr. Front control arms are not quite parallel with the groud yet. I could probably drop .5. But everything is going to be dictated by the front control arms.

For weight transfer I think you want some forward rake. Especially for auto-x if you're into that. If it's for looks do whatever.

The only way you'd gain extra travel is to extend the shock into the engine compartment (gc plates) above the original mounting location. And even then the control arm is only going to come up so far, by it's design. I think the point is don't take the front arms past parallel.
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Old 04-16-2009, 08:27 AM   #6
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All of this is definately geared towards autox, i understand a lot of concepts, but not quite how to apply them.

Quote:
Originally Posted by mccanixx View Post
I'm not sure shocks or camber play to much into it. You can only go so low, until you're hurting the car's performance.
Shocks with more travel can go lower before they bottom out(the shorter the shock body, the more travel at a given rideheight). If a shock did bottom out the spring rate caused by the bumpstop would kill your handling/grip.

Quote:
Originally Posted by mccanixx View Post
The only way you'd gain extra travel is to extend the shock into the engine compartment (gc plates) above the original mounting location. And even then the control arm is only going to come up so far, by it's design. I think the point is don't take the front arms past parallel.

I do have the GC plates and shorter shock bodies, so i'm thinking i have an extra 1-1.5" over a standard setup, add to that the possibility of cutting the bumpstops say just 1/2" and the numbers get large. This probably means the control arms are the determining factor.

From my research, its not the control arm's relation to the ground(i.e. parallel), but rather their relationship via the angle from cotnrol arm mounting point to ball joint to upper mounting point(camber plate). Once the angle from control arm-ball joint-camber plate passes 90* you're loosing camber, at some point you're not gaining camber as fast(85*? 88*?), but you're still gaining it. So how do i measure this angle and analyze the camber curve?

IMHO, it just so happens that for most cars/suspension parallel to the ground keeps the control arms happy and out of 90*.
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Old 04-16-2009, 09:51 AM   #7
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the angle of the control arm (inner pivot to ball joint) directly impacts your roll center- the closer to parallel (or past parallel) the lower the roll center goes and the further away it gets from the cg (the rc migrates at a much greater rate than the cg does)

travel is certainly one factor in determining ride height, roll center another and as mentioned above- rake as well, aero another, but not for autox

for travel measurements- you need the stroke length of both the front and rear dampers, the MR, top deflection (you can figure 0 here w/ a metal camber plates), static ride height and droop travel- with that you can calculate bump travel
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Old 04-16-2009, 10:04 AM   #8
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Quote:
Originally Posted by BIGSKYWRX View Post
the angle of the control arm (inner pivot to ball joint) directly impacts your roll center- the closer to parallel (or past parallel) the lower the roll center goes and the further away it gets from the cg (the rc migrates at a much greater rate than the cg does)
Do you think roll center is more important than the rate you go towards positive camber, aka camber curve?

I seem to remember reading(and i strongly agree) that for autox, you get into such stiff spring rates and roll bar combos that the car only rolls what 2*(vs. 4* stock) and you have a minimal effect of handling (unless overdone) Vs. the camber curve which dictates when you are loosing your optimum contact patch.


Quote:
Originally Posted by BIGSKYWRX View Post
for travel measurements- you need the stroke length of both the front and rear dampers, the MR, top deflection (you can figure 0 here w/ a metal camber plates), static ride height and droop travel- with that you can calculate bump travel
Got any examples/spreadsheets or references for the equations?


All in all, i'm only talking about going from 14.25" to 13.75", or some other small change, i'm mainly looking for something mathmatical to back it up with.
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Old 04-16-2009, 10:45 AM   #9
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Do you have any data logging equipment in the car?

-Duncan
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Old 04-16-2009, 11:22 AM   #10
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the roll center directly impacts the camber curve- there is also some really good info out there now on the role of SAI and caster on dynamic camber curves- that may trump some of thought on raising roll centers via camber plates (pushing the strut tops inboard raises the roll center- good, but increases in SAI-bad)

BumpTravel = ShockStroke-(FullDroop-TopMountDeflection (practically 0 for metal camber plates)-Ride Height)xMR (MR for the WRX is .96)

I think dunk is probably getting to the gist of the matter- you have what most consider good ride heights to start w/- some fine tuning might be in order from there and data logging (and/or times or even seat of the pants feel) would help you quantify some of that

also look through John's (ButtDyno) thread on his STX setup- he has some first hand experience experimenting w/ rake
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Old 04-16-2009, 11:37 AM   #11
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First off your ride heights look good from a generic stand point. The point is to prevent your Control Arm Pivot Points from going past parallel during an aggressive turn. Stiff Springs and you can go lower (not much). Soft springs and higher.

Quote:
Originally Posted by penderperson View Post
Do you think roll center is more important than the rate you go towards positive camber, aka camber curve?
Roll center is more important. Why? Because the further that Roll Center (RC) gets away from the Center of Gravity (CG) the more your car will tilt/sway/lean in a turn. Think of it like a wrench. With the RC near the CG you're wrenching with a 6 inch long wrench. Can't get a lot of leverage there. Now move that RC away from the CG and you've got a 3 foot breaker bar capable of loosening any bolt on your car. The more your car rolls the longer the camber curve graph is. That is the suspension moves through a longer arc with every thing else being the same (springs,bars,damping). That means you have to run more static negative camber to in order to get that contact patch flat when you turn. The more static camber you run the worse the braking and acceleration is on those tires because the contact patches are smaller.

Last edited by 68Cadillac; 04-16-2009 at 11:45 AM. Reason: English is hard.
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Old 04-16-2009, 11:49 AM   #12
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Quote:
Originally Posted by penderperson View Post
hey all,
-450lbs f / 500lbs r with 27mm sway front/ 22mm sway rear
Personally, I think with those spring rates your front sway bar is too stiff. Your springs are already resisting roll quite well. My advice would be to reinstall the OEM bar and try that. You'll be surprised the level of grip you'll get.
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Old 04-16-2009, 03:45 PM   #13
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^^^ Maybe on street tires...... Not with race tires though.

Jay
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Old 04-16-2009, 05:23 PM   #14
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I've been working a bit on a program that you can put suspension geometry into, and it'll graph the roll center. It increments roll by .25 deg at a time, and rolls the car around the roll center at each step, not just around the CG. Disclaimer: I've haven't put any knowledge of wheel rates / roll rates into it, nor made any attempt to do anything with force-based roll-centers. It also isn't using real impreza suspension geometry, although I've attempted to get control arm length, SAI, and caster in the ballpark (if anyone's got real numbers, I'd really appreciate it).

That said, here are some observations I've made:
1) The lower the roll center is, the faster it moves to the side as the car rolls. With reasonable ride height, it seems to move toward the inside of the turn, which seems like it should cause the car to push down toward the ground (this is where I need to better understand force-based roll centers).
2) Making a small change in ride height (like .5 inches) has a big impact on how fast the roll center moves from right to left, as well as up and down.
3) When you go too low on your ride height, the roll center will move down and towards the inside for the first bit of roll, but then suddenly teleport upwards and way to the outside, and start jumping around all over the place. This can't possibly be good for getting the chassis to take a set.
4) You can have the a-arms at or a bit below level without the roll center or camber curves getting too bad as long as you keep the body roll to a minimum. Unfortunately, I can't actually calculate body roll without correct geometry and wheel rates / roll stiffness. :/

Weight transfer is defined by cornering force, CG height, and track width. Body roll is defined by cornering force, CG height, track width, roll couple (CG minus RC), and roll stiffness. Roll stiffness is defined by springs and swaybars. What this means is that when you change rear ride height, you're changing the way the roll stiffness works on the car. A higher rear suspension setting would need a softer swaybar to have the same body roll for a given cornering force, and a lower rear end would need a stiffer swaybar to have the same body roll.

Some books talk about how having the roll-center lower in back than in front will make a car oversteer, while higher in back and lower in front will make the car understeer. Whether this applies to double a-arm only or also to strut, I'm not sure about. Looking at the plots my program does, and trying to think about how a Subaru feels, I think you'd want the rear roll center *slighly* lower than the front. I think this would make the following things happen:
1) Picture the car rolling around the roll center. If the roll center is lower, the car is going to move sideways more, and if it's higher, it's going to move sideways less. Having the rear try to push sideways more seems like it would help to counter the inherent understeer in the chassis.
2) With the higher rear roll couple, the rear is going to do more of its weight transfer through body roll, letting you use rear roll stiffness to move weight transfer backwards more. But you can't go too crazy with this at high cornering loads, or you lift inside rear wheel and understeer ensues. I think this is why stiff rear swaybars are fun on the street, but not as effective for autox/track.
3) With the rear roll center a bit lower, it will move to the inside a bit faster than the front, and I theorize this could help keep the rear (especially inside rear) pinned down a bit more, but I haven't figured out the math to prove/disprove that theory yet. Rotating the car around a point to the inside of the centerline should make the car move downward, not just roll / move sideways.
4) Having the rear roll center more towards the inside of the turn than the front will make the roll axis point a bit toward the outside of the turn. Like this: -/ as opposed to this -| or this -\. With the first one (rear RC more towards the inside), I have a theory (again, no math) that this will make the weight more naturally want to transfer to the outside rear wheel, which I think will keep the front from rolling too much and picking up the inside rear wheel, overloading the outside front tire, lowering the front center, blowing out your front camber curve, etc. It makes sense in my head, but like I said, I have no math, and haven't tested it.

Throwing all my wishy-washy over-analysis to the side, I think the bottom line is you want to keep the front a-arms level (note the balljoint pivot is above the outside end of the arm), to make sure your front end is going to behave well. Then you can play with rear ride height and rear swaybar to tweak the car balance, and that you might have to increase rear roll stiffness when you lower rear height to really see the expected changes.

-Mike
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Old 04-16-2009, 06:01 PM   #15
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Quote:
Originally Posted by penderperson View Post
All in all, i'm only talking about going from 14.25" to 13.75", or some other small change, i'm mainly looking for something mathmatical to back it up with.
I can't help you with math, but I can say that in early 2007 I went from 13.75 f, 14.25 r to 14 f, 13.5 rear and the car went from feeling really good to sucky understeer. So, relatively small-seeming ride height changes can have really drastic impacts on how the car feels.

After a) wanting to calm the car down a little and b) reading a handful of posts and PM's from people whose experience I trust, I lowered the car a little this year to 13.6/13.6 from last year's 13.8/14.0. I did cut 15mm or so off the bumpstops to help facilitate this. Ideally I would have gotten a shorter bumpstop that was harder rather than just cut the soft part off the stop I had but it's good enough for government work. Sadly I still haven't gotten a ****ing dry event in to get a feel for how things have changed.

I'm not especially smart, just observational data
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