Rear Shocks vs. Rear Sway Bar

[Flojd]

What would be better upgrade for the Si sedan that does both DD and auto-x:
1. Rear Koni shocks, or
2. Progress rear sway bar

Rear sway bar is a bit cheaper but I am worried about my rear end dancing around while driving kids to school. Noticed how the guys with rear sway bar are 3-wheeling around tight corners in auto-x which looked odd. Also, I would like rear shocks to decrease back wallowing in slalom.
However, I might be totally wrong, so asking for opinions?

I am not worried abot SSCA classing and rules as we don't follow them by letter here up north - both upgrades are fine by local rules.

[hukdizzle]

The shocks without a doubt. They will help with handling more than the RSB once tuned properly to your driving style.

[CivicSiRacer]

Shocks 1st.

[littlewing1208]

I 3 wheel around with stock rear sway bar in autocross but yes get the dampers.

[testify]

Do you already run in STX? Because the rear bar is not stock legal. Also, like littlewing demonstrated, the OEM rear bar is stiff enough to cause the car to do the three wheel motion, which is going to reduce traction as much as is necessary. More rotation can be achieved by simply adjusting the PSI in your rear tires.

Personally, I think that you should do the shocks, but I think you should do them on the front as well. The fronts are probably going to give you more overall performance than anything you can do to the rear. Also, I've found that with the right technique, I can get the car to rotate just fine in 100% stock trim.

just my .02

[littlewing1208]

Trail braking FTW.

[Flojd]

So, looks like everyone prefers shocks. My only worry is that having rear shocks firmer than the front ones may cause more understeer.

[SHG_Mike]

Dampers should not really be used to control the balance of a car. Springs control weight transfer and balance, the dampers should then control the spring. IMO a rear bar would in no way make the car unsafe on the street.

[littlewing1208]

Originally Posted by SHG_Mike

Dampers should not really be used to control the balance of a car. Springs control weight transfer and balance, the dampers should then control the spring. IMO a rear bar would in no way make the car unsafe on the street.

I assume you don't autocross then?

Springs only control rate of weight transfer, not amount of weight transfer.....just like dampers . Only wheelbase, track, CoG vertical height and lateral/longitudinal Gs control amount of weigh transfer.

A rear bar would make it hopelessly uncompetitive and is unnecessary given how well the car handles when properly driven in G Stock form.

[littlewing1208]

Originally Posted by Flojd

So, looks like everyone prefers shocks. My only worry is that having rear shocks firmer than the front ones may cause more understeer.

But you said you only want to do rear dampers? Anyway, firmer rear dampers (front a rebound perspective) will make the car less likely to understeer in transition situations.

[SHG_Mike]

Originally Posted by littlewing1208

I assume you don't autocross then?

Springs only control rate of weight transfer, not amount of weight transfer.....just like dampers . Only wheelbase, track, CoG vertical height and lateral/longitudinal Gs control amount of weigh transfer.

A rear bar would make it hopelessly uncompetitive and is unnecessary given how well the car handles when properly driven in G Stock form.

I have... and I dont like it. I run DE days, worked and driving for skip barber racing and currently work for a motorsports team that competes in both SCCA club and Grand Am racing.

You are correct about springs in that they control the rate at which weight transfers, but they can also control amount. When a car rolls or dives you are effectively changing ride heights in that given instance. That will add or subtract weight on each given wheel for that height that it is at while rolling diving squating etc. Hence the purpose of corner balancing. So with stiffer springs a car will not change from its static corner weight as much as a car with stock or soft springs. IMO in general you should not adjust shock valving to change the balance of the car.

I guess i missed where he only wanted to run gstock and have very small amounts of modifications allowed. It can work in that case but should not be used ad a general tuning method.

[littlewing1208]

Originally Posted by SHG_Mike

You are correct about springs in that they control the rate at which weight transfers, but they can also control amount. When a car rolls or dives you are effectively changing ride heights in that given instance. That will add or subtract weight on each given wheel for that height that it is at while rolling diving squating etc. Hence the purpose of corner balancing. So with stiffer springs a car will not change from its static corner weight as much as a car with stock or soft springs. IMO in general you should not adjust shock valving to change the balance of the car.

Again, springs absolutely will not change the *amount* of weight transfer for a given instance of lateral or longitudinal acceleration. It just means that said amount of weight that is going to transfer will occur at a faster rate. The only way to reduce the amount of weight transfer for a given negative longitudinal acceleration rate (let's talk braking in a straight line, ie dive) is to reduce the mass of the car, lower the center of gravity height or increase the wheel base. Obviously with stiffer springs, the car will dive to a lesser degree, but the amount of additional weight on the front tires is the same as an identical car decelerating at the same rate with soft springs.

For a given acceleration (ie lateral G, or longitudinal braking G or acceleration G) the force due to said acceleration is F=m_car*accel_CoG and said vector acts horizontally at the veritical location of the center of gravity. Let's again consider braking dive.

This force vector points forward. It creates a moment M = F*height_at_CoG due to height of the vector above the ground where the tires make contact. This moment causes a change in force on the front and rear tires relative to static (as in no acceleration). In doing so it adds weight to the front and subtracts it from the rear.

The amounts are as follows:

F_added_front = F*height_at_CoG/w
F_removed_rear= F*height_at_CoG/w

where w is the wheelbase of the car.


So as you can see spring rates have nothing to do with weight transfer. The same applies in roll (ie lateral acceleration).

[hukdizzle]

Originally Posted by littlewing1208

Again, springs absolutely will not change the *amount* of weight transfer for a given instance of lateral or longitudinal acceleration. It just means that said amount of weight that is going to transfer will occur at a faster rate. The only way to reduce the amount of weight transfer for a given negative longitudinal acceleration rate (let's talk braking in a straight line, ie dive) is to reduce the mass of the car, lower the center of gravity height or increase the wheel base. Obviously with stiffer springs, the car will dive to a lesser degree, but the amount of additional weight on the front tires is the same as an identical car decelerating at the same rate with soft springs.

littlewing1208 is correct, it is simple physics. I do not have the book with the equations to give but there is a great post here on a genesis coupe forum that should help you understand and also provides the equations as well.

Roll Stiffness and Weight Transfer – What you should know - Genesis Forum: GenCoupe Hyundai Genesis Forums

He is quoting almost exclusively from Bill Milliken, and that guys research and publication is one of the major benchmarks for the race car design/testing/research.

[littlewing1208]

Originally Posted by hukdizzle

littlewing1208 is correct, it is simple physics. I do not have the book with the equations to give but there is a great post here on a genesis coupe forum that should help you understand and also provides the equations as well.

Roll Stiffness and Weight Transfer – What you should know - Genesis Forum: GenCoupe Hyundai Genesis Forums

He is quoting almost exclusively from Bill Milliken, and that guys research and publication is one of the major benchmarks for the race car design/testing/research.

Haha, I don't even have to go to M&M to get this info. This is 9th grade physics .

I did my example in longitudinal terms to avoid roll center etc.

That was a good example in that link.

A lot of people get caught up in body roll without actually understanding why it is bad. The main point is roll = change in suspension geometry. As we know in a macstrut car, roll can lead the suspension into lesser negative camber and possibly positive camber which is not good. Plus there is dynamic toe to account for. On SLA suspensions like double wishbone/multilink etc, you gain negative camber which is good, but there is still dynamic toe to account for. In a race car situation, you want A) grip and B) predictability so very stiff springs and/or sway bars are nice in these regards since your suspension will move so little that for a given turn, you know exactly how the car is going to react time and time again. Further, body roll represents a movement of the driver. When going 100 mph on the uphill esses at VIR, the driver does NOT want to be whipped around because tenths of a degree more or less steering input due to your body moving can be the difference of perfect negotiation and hitting a tire wall.

But again, weight transfer is the same given an amount of lat/long acceleration, it just occurs at a faster rate.

Weight transfer is more interesting as it relates to tire grip and tire saturation and the non linear (more logarithmic) relationship between normal force and tire tractive force.

[SHG_Mike]

Well you are obviously more versed in the specific math then I, and I must admit I have had a slight bit of misconception with weight transfer. I have been more of a 'learning by doing' guy in my career, and suspension setup has not been my specialty. I have been around motorsports and building and tuning cars for many years now. That article did bring up the fact that balance is achieved with relative roll resistance, with springs or bars. Changing spring rate or bars front only or rear only WILL change the amount of weight transfer as per that article. So as far as making your overall stiffer your are correct in that more or less weight transfer will not occur. However changing rear springs to stiffer ones, will transfer more weight in the rear. I suppose I understand methods and changes that should be made when a balance issue is there, but not the exactly the math to back it up. IMO unless you have no other option springs should be used to change balance not damping rate

Can you answer this for me? or point me in a direction that can answer this? Why do static ride height adjustments, which changes individual contact patch weight, change balance, while in motion car's the amount of pitch or dive(roll stiffness or the car's relative height to the tarmac), does not affect individual tire contact patch weight. Is it purely a change in CoG and geometry change? I find it hard to believe when an 1/8" of height change makes a difference.

Thanks for the articles they were a good read.

[littlewing1208]

Originally Posted by SHG_Mike

Well you are obviously more versed in the specific math then I, and I must admit I have had a slight bit of misconception with weight transfer. I have been more of a 'learning by doing' guy in my career, and suspension setup has not been my specialty. I have been around motorsports and building and tuning cars for many years now. That article did bring up the fact that balance is achieved with relative roll resistance, with springs or bars. Changing spring rate or bars front only or rear only WILL change the amount of weight transfer as per that article. So as far as making your overall stiffer your are correct in that more or less weight transfer will not occur. However changing rear springs to stiffer ones, will transfer more weight in the rear. I suppose I understand methods and changes that should be made when a balance issue is there, but not the exactly the math to back it up. IMO unless you have no other option springs should be used to change balance not damping rate

Can you answer this for me? or point me in a direction that can answer this? Why do static ride height adjustments, which changes individual contact patch weight, change balance, while in motion car's the amount of pitch or dive(roll stiffness or the car's relative height to the tarmac), does not affect individual tire contact patch weight. Is it purely a change in CoG and geometry change? I find it hard to believe when an 1/8" of height change makes a difference.

Thanks for the articles they were a good read.

I was only talking about total weight transfer in the general sense. That's why I used a longitudinal example to keep roll centers and TLLTD out of the equation (Lean Less: Grassroots Motorsports Magazine Articles good article). TLLTD coupled with a conversation about tire saturation makes for an interesting topic that doesn't come up much.

I'm not 100% sure what you are asking in the second paragraph, but let me attempt to tackle it. Corner weighting doesn't affect right/left weight distributions or front/rear weight distribution. The only way to change those weight distributions is to move physical mass front/rear, left/right within the car. However, you can tune corner weights purely with ride height if you have adjustable height spring perches. The basically analogy is as follows: take a chair with 4 legs. Lengthen 1 (or 2 corners diagonal) of them and the two adjacent legs have less load on them....lengthen it a bit more and one or both of them will lose contact with the ground and you'll need a package of sugar to balance the table .

Let's apply this to a car:

So when you raise the ride height of a coilover you are simply raising the height of the lower perch relative to the lower attachment point. If there was just a 500lb bag on top of this corner, the spring would remain equally compressed but it would be a bit higher from the ground. However on a 4 contact patch car, each corner being suspended, when you raise that ride height the adjacent corners need to extend a bit to 'keep up' and in doing so, the spring is no longer the same compressed height anymore so the F=-kx is such that there is less load on those to corners. Then the corner you raised and the opposite corner have to take up the load lost on the other 2 corners.