peter pyce
Veteran Member
- Joined
- Nov 6, 2001
Few more and we are done with the copy-paste and then will extend with "modern" from 2006 thoughts..... Ceilidh all the way to the end:
7. Roll Centers and Weight Transfer
We're going to eventually get to the GT suspension, but there are still a few odds & ends to clean up. For this installment, you can decide whether you'll want to read it by first seeing how you do on a little quickie quiz:
Answer True or False:
1. Reducing the amount of body roll while cornering will improve performance by reducing the lateral weight transfer on the tires.
2. When a car rolls, it compresses the springs on the outside tires, which overloads the tires and reduces their effective traction. A car that rolls less will compress the outside springs less, which will load the tires less, which gives them better traction.
3. If you lower a car, it will roll less.
4. If you drop the front roll center and keep everything else the same, the car will understeer more.
5. Springs and antiroll bars are the primary factors that determine how lateral weight transfer is distributed fore & aft (which, together with camber effects, determines whether the car understeers or oversteers).
Ok, did you take the quiz? Honestly? With no peeking? Ok, then here are the answers:
(drum roll please....)......If you answered anything other than "False" for all of the questions, and if you care about suspension theory, you might want to read parts of this installment. =)
1 & 2 -- Why Reducing Roll Does Not Affect Weight Transfer
Ok, first thing is to get lateral weight transfer squared away: when a car goes around a corner, weight is transferred from the inside tires to the outside tires, and the amount of weight transferred is equal to:
(lateral g-force) * (car mass) * (CG height) / (track width)
Note that spring rate and antiroll bar rate have nothing to do with it. When a car goes around a corner, the weight transfer will occur regardless of whether its suspension is pillowy soft or absolutely locked solid. Making the springs stiffer will not reduce the weight transfer, and thus stiffer springs will not directly make the car perform better (if there's a performance benefit, it's an indirect gain from less tire camber, better control of unsprung weight, or less body movement over bumps, etc., as discussed in earlier installments).
Similarly, roll doesn't enter into the equation either (to be really accurate, it actually does to a very small extent, in that the CG shifts sideways when the car rolls -- but whilst this is a concern for big SUVs, buses, etc with a lot of roll and a very high CG relative to track width, for our cars it's small enough to basically ignore). Roll can do a lot of horrible things for handling (via tire cambers, body transients, etc.), but weight transfer is not the main issue. Hence any spring vendor that tells you that "stiff performance springs reduce weight transfer by reducing body roll" is either (A) clueless or (B) dishonest; either way you'll not want to trust him.
(Aside: So what does affect weight transfer? Track, and CG height. Here, for once, the advertising arguments do have a physical basis: if you drop the CG, you'll reduce the total lateral weight transfer, and if everything else stays the same, your handling will improve. Similarly, if you widen the track, you'll get less transfer, and if everything else stays the same, your cornering power will improve. That's why true race cars are built with the CG as low as possible and with the track width usually at the limits of regulation.
The hitch of course is the phrase "if everything else stays the same": Everything else never does stay the same. Dropping the CG by installing lowering springs causes a host of problems (discussed in this thread and more completely in the forum stickies). Widening the track by installing huge wheel spacers will destroy your bearings and screw up your steering geometry, which is also slow. All in all, "improving" your car by reducing weight transfer is not a good way to begin.)
3. Why Lowering Doesn't Necessarily Mean Less Roll
Here's another issue that has been amply discussed elsewhere, but on the off-chance there are novices who have been reading about roll centers, but don't really know what they are, here's a (very) abbreviated explanation:
a) Imagine a car with solid axles (meaning, there's literally a big beam running at hub-height from left wheel to right wheel, straight across each end of the car). Now imagine that you've drilled a horizontal hole straight through the midpoint of each axle, and that you've bolted the body of the car to the axles using those holes. If I've painted the picture correctly, you have a "suspension" where the wheels on each axle can't go up & down together, but they can pivot: if the left wheel goes up, the right wheel goes down, and vice versa. Set this strangely designed car on a level ground, steer it around a corner, and it will roll -- and the axis about which the car rolls will be the centrepoint of each axle (because that's where the body is bolted to the axles). In this situation, the axle centrepoint -- where the body is bolted to the axle -- is the "roll center".
b) In situation "a", the roll center is a physical thing -- you can see it and touch it, and it's really easy to see how the car rolls around it. There are some cars (well, there used to be...) that have such a physical roll center (chiefly some DeDion suspensions from WWII days), but unfortunately (for visualization purposes) that's not the norm. Nowadays we have suspensions where the roll center is not a physical pivot point, but a "virtual" point that moves around as the car shifts about on its suspension. But although the roll center is typically a virtual object, it controls the car's motions exactly the way that a physical pivot does. So if you have trouble imagining what the roll center does to a car, just imagine a solid, physical pivot sitting where the roll center is said to lie....
c) Now let's do some physics (shudder): suppose the car body in "a" has a center of gravity that lies 18" above the ground. How do we get it so that this car does not roll at all when it goes around corners? That's easy: we give it 36" wheels (overall diameter, wheel & tire together). In that situation, the physical roll center is sitting at 18" (axle height), which means the center of gravity (CG) is sitting directly on the roll axis (sorry -- the "roll axis" is the line connecting the two roll centers -- here it's a horizontal line running fore & aft 18" above the ground). Since the CG is sitting on the roll axis, there's no tendency to roll. So the car corners flat -- even though we don't have any springs or antiroll bars.
d) What happens if we make the wheels, say, 48" in diameter, so that the CG of the car body (at 18") is lower than the roll axis (at 24")? That's interesting. Now the body of the car is basically hanging down from the roll centers (from the physical pivots, in this example), and it'll actually bank into turns, the same way a bucket will "bank" into turns if you carry it by the handle.
e) Conversely, what happens if we drop the roll center way down, all the way to the ground? The car rolls, a lot, as now the CG is above the roll center.
f) In practice, automotive roll centers always like below the CG (situation "e"). Given that, a car's tendency to roll depends upon how far the CG is above the roll center. The greater that distance, the more the car wants to roll.
g) Now, if you go to the "Lowering" sticky at the top of the forum, you'll see that if you lower a McPherson strut suspension, the roll center drops faster than the CG -- which means that the distance between CG and roll center grows, which means that you have more tendency to roll This phenomenon is really well-discussed elsewhere, so we'll say no more about it.
h) What we will mention in passing, however, is what happens at the back of the Golf/Jetta IV: for everything other than the R32, we have a twist beam rear axle, and the roll center in such a setup lies right in the middle of the twist beam. If you go look at this axle on your car, you'll see that it doesn't behave like the McPherson front: if you lower the rear of the car, the CG drops (because you're lowering the car), but the twist beam doesn't drop quite as much (because of the way the trailing arm is pivoted: the wheel end doesn't drop at all, while the pivot end drops with the body....and the twist beam is in between). Hence when you drop the rear end of these cars, you simultaneously lower the CG while slightly reducing the tendency to roll. Thus you're free to lower the rear end of the car pretty much all you want -- theoretical roadholding will improve because of the (modest) reduction in roll and (modestly) lowered CG.
And so, if you've ever wondered why the Shine setup looks so, um, interesting, that's why: Shine keeps the front high, for all the roll center and camber gain issues discussed in the previous installments, while dropping the rear for less weight transfer and less roll.
(continued next post)
7. Roll Centers and Weight Transfer
We're going to eventually get to the GT suspension, but there are still a few odds & ends to clean up. For this installment, you can decide whether you'll want to read it by first seeing how you do on a little quickie quiz:
Answer True or False:
1. Reducing the amount of body roll while cornering will improve performance by reducing the lateral weight transfer on the tires.
2. When a car rolls, it compresses the springs on the outside tires, which overloads the tires and reduces their effective traction. A car that rolls less will compress the outside springs less, which will load the tires less, which gives them better traction.
3. If you lower a car, it will roll less.
4. If you drop the front roll center and keep everything else the same, the car will understeer more.
5. Springs and antiroll bars are the primary factors that determine how lateral weight transfer is distributed fore & aft (which, together with camber effects, determines whether the car understeers or oversteers).
Ok, did you take the quiz? Honestly? With no peeking? Ok, then here are the answers:
(drum roll please....)......If you answered anything other than "False" for all of the questions, and if you care about suspension theory, you might want to read parts of this installment. =)
1 & 2 -- Why Reducing Roll Does Not Affect Weight Transfer
Ok, first thing is to get lateral weight transfer squared away: when a car goes around a corner, weight is transferred from the inside tires to the outside tires, and the amount of weight transferred is equal to:
(lateral g-force) * (car mass) * (CG height) / (track width)
Note that spring rate and antiroll bar rate have nothing to do with it. When a car goes around a corner, the weight transfer will occur regardless of whether its suspension is pillowy soft or absolutely locked solid. Making the springs stiffer will not reduce the weight transfer, and thus stiffer springs will not directly make the car perform better (if there's a performance benefit, it's an indirect gain from less tire camber, better control of unsprung weight, or less body movement over bumps, etc., as discussed in earlier installments).
Similarly, roll doesn't enter into the equation either (to be really accurate, it actually does to a very small extent, in that the CG shifts sideways when the car rolls -- but whilst this is a concern for big SUVs, buses, etc with a lot of roll and a very high CG relative to track width, for our cars it's small enough to basically ignore). Roll can do a lot of horrible things for handling (via tire cambers, body transients, etc.), but weight transfer is not the main issue. Hence any spring vendor that tells you that "stiff performance springs reduce weight transfer by reducing body roll" is either (A) clueless or (B) dishonest; either way you'll not want to trust him.
(Aside: So what does affect weight transfer? Track, and CG height. Here, for once, the advertising arguments do have a physical basis: if you drop the CG, you'll reduce the total lateral weight transfer, and if everything else stays the same, your handling will improve. Similarly, if you widen the track, you'll get less transfer, and if everything else stays the same, your cornering power will improve. That's why true race cars are built with the CG as low as possible and with the track width usually at the limits of regulation.
The hitch of course is the phrase "if everything else stays the same": Everything else never does stay the same. Dropping the CG by installing lowering springs causes a host of problems (discussed in this thread and more completely in the forum stickies). Widening the track by installing huge wheel spacers will destroy your bearings and screw up your steering geometry, which is also slow. All in all, "improving" your car by reducing weight transfer is not a good way to begin.)
3. Why Lowering Doesn't Necessarily Mean Less Roll
Here's another issue that has been amply discussed elsewhere, but on the off-chance there are novices who have been reading about roll centers, but don't really know what they are, here's a (very) abbreviated explanation:
a) Imagine a car with solid axles (meaning, there's literally a big beam running at hub-height from left wheel to right wheel, straight across each end of the car). Now imagine that you've drilled a horizontal hole straight through the midpoint of each axle, and that you've bolted the body of the car to the axles using those holes. If I've painted the picture correctly, you have a "suspension" where the wheels on each axle can't go up & down together, but they can pivot: if the left wheel goes up, the right wheel goes down, and vice versa. Set this strangely designed car on a level ground, steer it around a corner, and it will roll -- and the axis about which the car rolls will be the centrepoint of each axle (because that's where the body is bolted to the axles). In this situation, the axle centrepoint -- where the body is bolted to the axle -- is the "roll center".
b) In situation "a", the roll center is a physical thing -- you can see it and touch it, and it's really easy to see how the car rolls around it. There are some cars (well, there used to be...) that have such a physical roll center (chiefly some DeDion suspensions from WWII days), but unfortunately (for visualization purposes) that's not the norm. Nowadays we have suspensions where the roll center is not a physical pivot point, but a "virtual" point that moves around as the car shifts about on its suspension. But although the roll center is typically a virtual object, it controls the car's motions exactly the way that a physical pivot does. So if you have trouble imagining what the roll center does to a car, just imagine a solid, physical pivot sitting where the roll center is said to lie....
c) Now let's do some physics (shudder): suppose the car body in "a" has a center of gravity that lies 18" above the ground. How do we get it so that this car does not roll at all when it goes around corners? That's easy: we give it 36" wheels (overall diameter, wheel & tire together). In that situation, the physical roll center is sitting at 18" (axle height), which means the center of gravity (CG) is sitting directly on the roll axis (sorry -- the "roll axis" is the line connecting the two roll centers -- here it's a horizontal line running fore & aft 18" above the ground). Since the CG is sitting on the roll axis, there's no tendency to roll. So the car corners flat -- even though we don't have any springs or antiroll bars.
d) What happens if we make the wheels, say, 48" in diameter, so that the CG of the car body (at 18") is lower than the roll axis (at 24")? That's interesting. Now the body of the car is basically hanging down from the roll centers (from the physical pivots, in this example), and it'll actually bank into turns, the same way a bucket will "bank" into turns if you carry it by the handle.
e) Conversely, what happens if we drop the roll center way down, all the way to the ground? The car rolls, a lot, as now the CG is above the roll center.
f) In practice, automotive roll centers always like below the CG (situation "e"). Given that, a car's tendency to roll depends upon how far the CG is above the roll center. The greater that distance, the more the car wants to roll.
g) Now, if you go to the "Lowering" sticky at the top of the forum, you'll see that if you lower a McPherson strut suspension, the roll center drops faster than the CG -- which means that the distance between CG and roll center grows, which means that you have more tendency to roll This phenomenon is really well-discussed elsewhere, so we'll say no more about it.
h) What we will mention in passing, however, is what happens at the back of the Golf/Jetta IV: for everything other than the R32, we have a twist beam rear axle, and the roll center in such a setup lies right in the middle of the twist beam. If you go look at this axle on your car, you'll see that it doesn't behave like the McPherson front: if you lower the rear of the car, the CG drops (because you're lowering the car), but the twist beam doesn't drop quite as much (because of the way the trailing arm is pivoted: the wheel end doesn't drop at all, while the pivot end drops with the body....and the twist beam is in between). Hence when you drop the rear end of these cars, you simultaneously lower the CG while slightly reducing the tendency to roll. Thus you're free to lower the rear end of the car pretty much all you want -- theoretical roadholding will improve because of the (modest) reduction in roll and (modestly) lowered CG.
And so, if you've ever wondered why the Shine setup looks so, um, interesting, that's why: Shine keeps the front high, for all the roll center and camber gain issues discussed in the previous installments, while dropping the rear for less weight transfer and less roll.
(continued next post)