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I've been reading a lot on anti-sway bars here over the last few weeks.
It seems many STI owners are scared to run large rear bars because they're afraid of lifting the inside rear wheel, as if this magically and suddenly reduces the car's grip. Obviously a tire not touching the ground can not grip, right? In every thread on rear sway bars, someone always brings up rear wheel lift and why big rear sways are bad.
The popular solution here is to run a smaller rear sway or softer rear springs. Either lowers the effective wheel rate in the rear, which (if the strut allows) gives a longer effective suspension travel. And what does this accomplish? It produces understeer, pure and simple. You're changing the handling bias. It's no wonder so many STI owners complain of understeer. I've never met a crowd so willing to stick with just a huge front sway bar, and the STI is certainly not the only camber-starved car out there.
The only reason this is reducing rear wheel lift is because the softer rear suspension is forcing the front tires to instead control body roll. Since under braking and turning your front tires are already saturated, grip up front is reduced while grip in the rear increases. This would be no different than if you ran softer rear springs or firmer front springs. Running a smaller rear sway does not normally increase the total grip of the vehicle, and the trade is almost never worth it if the handling bias becomes off-neutral.
Increasing droop travel beyond the spring travel and adding a helper spring in the rear does not improve the situation. A helper spring simply makes your linear springs progressive; they work the same as a very progressive coil spring. It makes the end of your suspension travel very soft, and while this suddenly soft rear suspension may keep a tire planted, it'll only do so by changing the car's handling bias towards understeer. Progressive springs are not good for performance as they create jacking forces and can lead to unpredictability. The weight shift of the car has not changed because the center of gravity has not changed (it will have risen, if anything, due to the progressive spring rate).
I've even read someone suggest running using firmer rear springs just so that he/she could run a smaller rear sway bar, thus preventing rear wheel lift. It won't! Sways and springs together make up your wheel spring rate; the wheels don't know the difference. The total weight shift of the car does not change. I've read elsewhere online that sway bars decrease a car's grip by transfering weight to the outside tire, and this is complete BS. The weight transfer is no different than it is with firmer springs, so long as the total wheel rate is the same.
Now, I'll concede that too much inside rear wheel lift can be a bad thing, but only if it's prominent in steady-state turning. In that case, it's a sign of having too much weight transfer, NOT that your handling bias is off. Reducing weight transfer as much as possible is always beneficial and really helps make the most of your tires: that's why we see cars with aluminum roofs and such. But if you're getting lift under heavy braking and turning, then what's the big deal? The rear tires aren't used much for braking anyway, especially on a front-heavy car. On a street car like the Impreza the Cg may never get low enough to prevent wheel lift under all circumstances. Besides, you will NEVER lift a rear wheel under acceleration when you need the rear wheels planted most.
As I said earlier, a wheel lifts for one reason only: there is no weight on it. So, how do we fix this? You can either widen the track or lower the center of gravity. Only the latter is really practical. If you are getting rear wheel lift and, for whatever reason, are troubled by it, lower your car. It's simple. Lowering too much creates its own set of problems, but that's a different topic. The point is this: lifting a rear wheel is a weight shift problem, not a handling bias problem. It should be solved by reducing the weight shift in your car, not by changing the handling bias towards understeer.
Now, lots of people here run big front sways with the stock rear sway, and they're happy with that. Great- I'm sure it is a drastic improvement over stock due to the reduction of body roll. However, running a large rear sway also reduces body roll and will thus keep the front tires on-camber too. However, the rear sway reduces body roll without overwhelming the front tires at the same time, and thus the improvement in front grip will be far more pronounced. Having more front grip than rear creates oversteer, and thus it's important to keep a balance.
From all that I've read, it seems most guys who really love their vehicle's handling run nearly equal front and rear sway bars. This sure makes more sense to me than running a much larger front sway (you stock class autocrosser's aside). Lastly, my own experience says that you want high-quality shocks if you run large sway bars, because your spring rate because much higher in turns than it is on straights. The shocks need to be good enough to dampen this much higher rate or you will feel instability.
Sorry for the rant here, but I got tired of reading about inside rear lift and seeing people run small (or stock) rear sways because of it. There are plenty of good reasons to run a smaller rear sway, but that's not one of them.
It seems many STI owners are scared to run large rear bars because they're afraid of lifting the inside rear wheel, as if this magically and suddenly reduces the car's grip. Obviously a tire not touching the ground can not grip, right? In every thread on rear sway bars, someone always brings up rear wheel lift and why big rear sways are bad.
The popular solution here is to run a smaller rear sway or softer rear springs. Either lowers the effective wheel rate in the rear, which (if the strut allows) gives a longer effective suspension travel. And what does this accomplish? It produces understeer, pure and simple. You're changing the handling bias. It's no wonder so many STI owners complain of understeer. I've never met a crowd so willing to stick with just a huge front sway bar, and the STI is certainly not the only camber-starved car out there.
The only reason this is reducing rear wheel lift is because the softer rear suspension is forcing the front tires to instead control body roll. Since under braking and turning your front tires are already saturated, grip up front is reduced while grip in the rear increases. This would be no different than if you ran softer rear springs or firmer front springs. Running a smaller rear sway does not normally increase the total grip of the vehicle, and the trade is almost never worth it if the handling bias becomes off-neutral.
Increasing droop travel beyond the spring travel and adding a helper spring in the rear does not improve the situation. A helper spring simply makes your linear springs progressive; they work the same as a very progressive coil spring. It makes the end of your suspension travel very soft, and while this suddenly soft rear suspension may keep a tire planted, it'll only do so by changing the car's handling bias towards understeer. Progressive springs are not good for performance as they create jacking forces and can lead to unpredictability. The weight shift of the car has not changed because the center of gravity has not changed (it will have risen, if anything, due to the progressive spring rate).
I've even read someone suggest running using firmer rear springs just so that he/she could run a smaller rear sway bar, thus preventing rear wheel lift. It won't! Sways and springs together make up your wheel spring rate; the wheels don't know the difference. The total weight shift of the car does not change. I've read elsewhere online that sway bars decrease a car's grip by transfering weight to the outside tire, and this is complete BS. The weight transfer is no different than it is with firmer springs, so long as the total wheel rate is the same.
Now, I'll concede that too much inside rear wheel lift can be a bad thing, but only if it's prominent in steady-state turning. In that case, it's a sign of having too much weight transfer, NOT that your handling bias is off. Reducing weight transfer as much as possible is always beneficial and really helps make the most of your tires: that's why we see cars with aluminum roofs and such. But if you're getting lift under heavy braking and turning, then what's the big deal? The rear tires aren't used much for braking anyway, especially on a front-heavy car. On a street car like the Impreza the Cg may never get low enough to prevent wheel lift under all circumstances. Besides, you will NEVER lift a rear wheel under acceleration when you need the rear wheels planted most.
As I said earlier, a wheel lifts for one reason only: there is no weight on it. So, how do we fix this? You can either widen the track or lower the center of gravity. Only the latter is really practical. If you are getting rear wheel lift and, for whatever reason, are troubled by it, lower your car. It's simple. Lowering too much creates its own set of problems, but that's a different topic. The point is this: lifting a rear wheel is a weight shift problem, not a handling bias problem. It should be solved by reducing the weight shift in your car, not by changing the handling bias towards understeer.
Now, lots of people here run big front sways with the stock rear sway, and they're happy with that. Great- I'm sure it is a drastic improvement over stock due to the reduction of body roll. However, running a large rear sway also reduces body roll and will thus keep the front tires on-camber too. However, the rear sway reduces body roll without overwhelming the front tires at the same time, and thus the improvement in front grip will be far more pronounced. Having more front grip than rear creates oversteer, and thus it's important to keep a balance.
From all that I've read, it seems most guys who really love their vehicle's handling run nearly equal front and rear sway bars. This sure makes more sense to me than running a much larger front sway (you stock class autocrosser's aside). Lastly, my own experience says that you want high-quality shocks if you run large sway bars, because your spring rate because much higher in turns than it is on straights. The shocks need to be good enough to dampen this much higher rate or you will feel instability.
Sorry for the rant here, but I got tired of reading about inside rear lift and seeing people run small (or stock) rear sways because of it. There are plenty of good reasons to run a smaller rear sway, but that's not one of them.
