Suspension...understanding how it all works together

Struts, spring, anti-rollbars, braces and the like.

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Suspension...understanding how it all works together

Post by WRXish »

Ok, I'm going out on a limb here and offering my observations about suspension as a total system and how each piece of that total system contributes to the overall picture. These observations are offered for consideration from my fairly broad experience of screwing around with suspension hardware over the last 40 years or so, both for street use, autocross and track use. Its not my first rodeo and the overall understanding related to subarus in particular has only within the last year come together for me. I own 3 subies and each of them is set up completely differently from the other.

I will start with the mostly stock 2008 legacy. I upgraded brakes to slotted and drilled rotors and ceramic pads and added a camber kit for the rear suspension since this particular car had way too much negative camber in the rear. It was a serious tire wear issue and that solved the problem. I also changed wheels and tires and gave the car a custom alignment. It came from the factory with 205/50-17 bridgerocks that sucked...this gave the car reasonable turn in, reasonable braking, and so so cornering grip, with a comfortable cushy ride. I wanted more of all those things except was willing to sacrifice ride comfort to some extent. Enter 17x8" wheels with 235/45-17 high performance all season tires. So the wheel and tire change gave me a seriously firmer ride than I expected. Its still quite streetable and comfortable on a road trip. There is considerably more impact harshness now than there was before, not ridiculous of course because the rest of the suspension is stock as delivered from subie. But that is my point here, I changed one thing and it transformed the handling of the car. Much better response, better turn in, better grip at the limit and much better braking. The point here is that if you start with the wheel and tire package that fits your car and gives you the look that you want, you may also get the better performance that you are seeking too without ever turning anything but a lug wrench. For street use only, this is a wonderful setup on this particular car. It cost me ride comfort and a couple of MPG because the tire is taller and wider than stock so it impacts gearing and available torque. All reasonable tradeoffs for what was gained.

My 1991 legacy wagon on the other hand has been a test car for some things, and I wanted the car lowered so I ended up doing coilovers with the spring rates dialed in for moderately aggressive street use but suitable for autocross too. So on the front the springs are 420 inch pounds and the rears are 280 inch pounds, a very satisfactory combination when combined with the factory wheel and tire combo, because remember now, the tire sidewall height has a direct impact on the total suspension spring rate, because the tire is a spring too. Combined together with the coilover, it gave a firm but pleasant ride on the street. Problem is, I had already put 205/50-16 wheels and tires on the car with the GRX struts and factory springs. Now with both of those changes I now had a seriously firm riding car that was satisfactory on good roads but not so much on the back roads of VA where I drive all the time. Still I liked it overall and drove it that way for a good while, As luck would have it, I had the wheels that came off the 2008 legacy laying around and a set of of 215/45-17 tires also laying around. I married em up and put em on the car. The good part of that combo was sharper turn in, better braking, better cornering grip, and what I thought was a better looking setup. The bad part was it killed ride comfort by making impact harshness really brutal, introduced tramlining when going down the highway, killed some torque and cost me a couple of MPG. This car doesnt have any torque (yet!) that it can give up to an unwise gearing change, nor does the 25 year old body need the trauma of brutal impact harshness either. So, after a reasonable period to evaluate this change, I put the 16" wheels back on it. And I like it much better. One other change I may try is to put 205/50-17 tires on to replace the 215s and see if that is a better overall combo.

To further illustrate how impact harshness can be tuned out by doing different things, consider my 2009 WRX. factory suspension and stock wheels and tires 225/45-17 yielded a comfortable ride that left something to be desired in terms of performance. Again I upgraded suspension first and got a result that was too harsh. I decided the Bilstein shock and spring package that was offered at the time was the way to go. Installed em, and performance was much improved, but it was way too harsh to do a road trip in. So to fix that and get more grip, I put 235/45-17 tires on 8" rims and that improved the ride comfort and gave me more grip and better braking, better turn in, it transformed the car into a very quick capable high speed transport device. But it is very stiff still and would be tiring on a long trip, but fun to drive for sure.

So when you are considering upgrading suspension, if you can try a simple wheel and tire change first and see how close that gets you to where u want to be. Chances are for many of us, it will be close enough.

We havent spoken about sway bars at all because in reality they dont affect ride comfort except under one very specific situation. When both wheels are moving up and down together, the sway bar is doing nothing, its just along for the ride, but if you hit a bump that affects just one wheel, the spring rate of the sway bar gets added to the spring rate of the wheel absorbing the bump and so single wheel bump rates will be higher than if both wheels move together. Sway bars if selected correctly will reduce body roll, and give you the steering characteristics you desire, that is more or less understeer or oversteer. For those who dont understand those two terms, understeer is when you go into a turn too fast and the front tires lose grip before the back tires, the opposite is true for oversteer.

Finally lets look at alignment settings...this will fly in the face of some you guys who run much more aggressive settings than this, but this is based on my experience playing with alignment settings over the years. For street use and very occasional autocross use, you should be able to get away with something like 1/2 to 3/4 degrees negative camber in the front and a 1/4 to a 1/2 degree negative camber in the rear on a stiffened up suspension and retain good even tire wear. Probably could push those numbers negative by a factor of 2 for a totally stock suspension and still get decent tire wear. This assumes you run zero toe front and rear. All out race cars will have those numbers determined by taking tire temps at the track and adjusting accordingly.

Have I said all there is to say on these subjects? Hell no, but this is a starting point. Think about how you will use the car, and what you want the end result to be and realize that performance characteristics fly in the face of creature comforts more often then not. Choose your parts carefully grasshopper! :)
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Re: Suspension...understanding how it all works together

Post by Legacy777 »

Well said and thanks for sharing.

Sometimes....less is more ;)
1990 Legacy (AWD, 6MT, & EJ22T Swap)
2020 Outback Limted XT

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