Rear Sway bar helps a lot!
02-27-2005, 01:33 AM
#31
Yeah, I agree with you about the pigs flying thing. 
The GRM article was what was stuck in my head actually. I was just about to go look it up to see what they had written in the past.
I was actually just reading part 20 of the first link you posted. Eyeball that and see what you come up with. I'm too tired right now to absorb it and I have to catch a flight tomorrow morning. See you all in a week!
The GRM article was what was stuck in my head actually. I was just about to go look it up to see what they had written in the past.
I was actually just reading part 20 of the first link you posted. Eyeball that and see what you come up with. I'm too tired right now to absorb it and I have to catch a flight tomorrow morning. See you all in a week!
02-27-2005, 01:39 AM
#33
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Originally Posted by Manning
02-27-2005, 01:40 AM
#34
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Originally Posted by chris luckett
Come on guys! You're engineers. Break out the math and show a proof!
I'm working on it...
Until then..
2+2=4 = I am a geenyous.
02-27-2005, 01:52 AM
#35
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Originally Posted by adrial
Unless we're talking about extremely severe angles here I dont think that the amount that the car dives/rolls has an effect on weight transfer.
02-27-2005, 06:51 AM
#37
Race Director
Adrial is correct. The weight-transfer is independent of the spring-rates or swaybars. There are two components to lateral-weight transfer under cornering. The first initial weight-transfer has on to do with cornering-G, track-width, COG height. The body-lean is only a response to the weight-transfer, not the cause of it. The 2nd component of weight-transfer has to do with lateral shift of the COG. However, the 1" lateral movement of the COG doesn't change the lateral weight-transfer that much compared to the amount caused by cornering-G forces.
Use this example, take a care with 50/50 weight distribution, say... a 944. WIth 700lbs per corner, it's laying flat on the ground. It has 1400lbs on the left side, the right side, the front wheels, the rear-wheels. Take a 300lb lead weight and place it on the center tunnel centered between all four wheels. The car remains flat with 775lbs per corner. To illustrate weight-transfer under cornering, imagine this lead weight moving to one of the rear seats, say the one behind the driver, and it moves far enough over so that it's directly between the front & rear wheels and all of its weight shifts to that side. The left-right weight distribution is now 1700lbs on the left tyres and 1400lbs on the right. Take the following example of two cars sprung differently with exactly the same weight distribution.
CAR#1: stock springs and sway bars. Moving the weight to the left rear seat causes left side to compress by 2", weight distribution is now 1700lbs on the left, 1400lbs on the right.
CAR#2: stiffer springs and sway bars. Moving the weight to the left rear seat causes left side to compress by 1", weight distribution is now 1700lbs on the left, 1400lbs on the right.
Note that the lower amount of lean on car #2 is in response to the SAME lateral weight-trasnfer. The load on the tyres are EXACTLY the same on BOTH cars. You can even weld solid the suspension with no movement at all so that the car stays flat when the weight moves over, and the weight-transfer will still be the same.
Increasing spring-rates and stiffness is only a compromize solution due to the bad geometry of the McPherson struts used in our cars. As the body leans more and more, the outside tyre leans more and more. So car#1 may end up with +1 degree camber on outside tyre (not good for cornering), while car#2 with less lean ends up with only +0.5 degree camber under the SAME amount of weight trasnfer. Car#2 will corner faster because it has more of its tyre flat on the ground, not because it has less weight-transfer than car#1. Same thing under braking.
Now cars with double-wishbone suspensions can be set up so that the tyres remain flat on the ground as the body rolls, like Ferraris. These cars don't have to use stiff springs and can out-corner our cars with more ride-comfort as well. Take the 3400lbs SupraTT which pulls 0.98g or the 3300lb Corvette C4 with 0.96g. Both having only slightly wider tyres than the 3000lb '86 951 which can only muster 0.88-0.90g (depending upon magazine doing the test).
Now swaybars serve a completely different function than springs. They are used to split the lateral weight-transfer under cornering.
Rather than a single 300lbs weight moving laterally, imagine splitting that weight into two pieces and placing one on the front wheel and one on the rear. Imagine iif you split this weight into two 150lbs pieces, this would give neutral cornering. But if you break it up in to a 200lb piece on the front wheel and the remaining 100lb piece on the rear, what woudl happen? You'd get understeer because the front-wheel has to laterally-accelerate a higher load. The opposite would occur if you put 200lbs on the rear and only 100lbs on the front, you'd get oversteer.
The stiffer end gets more of the lateral weight-transfer. So a big swaybar in front will transfer the majority of the lateral weight-transfer to the front tyres. The result will be understeer. A bigger rear-bar will cause more weight to be transfered to the rear, resulting in oversteer:
To really understand vehicle dynamics, check out these publications:
How to Make Your Car Handle - Fred Puhn
Performance Handling - Don Alexander
Tune to Win - Carroll Smith
Race Car Vehicle Dynamics - Miliken
BTW - corner-weighting is balancing the weights on cross-diagonals in a static condition. It doesn't affect lateral weight-transfer, but does have an effect to the car's response to weight transfer. You can end up with a car that understeers in left-hand corners and oversteers in right-hand corners. This can be advantageous in certain conditions, like racing on ovals. Since you're intentionally throwing the corner-weights off, it's more commonly known as "weight-jacking".
Use this example, take a care with 50/50 weight distribution, say... a 944. WIth 700lbs per corner, it's laying flat on the ground. It has 1400lbs on the left side, the right side, the front wheels, the rear-wheels. Take a 300lb lead weight and place it on the center tunnel centered between all four wheels. The car remains flat with 775lbs per corner. To illustrate weight-transfer under cornering, imagine this lead weight moving to one of the rear seats, say the one behind the driver, and it moves far enough over so that it's directly between the front & rear wheels and all of its weight shifts to that side. The left-right weight distribution is now 1700lbs on the left tyres and 1400lbs on the right. Take the following example of two cars sprung differently with exactly the same weight distribution.
CAR#1: stock springs and sway bars. Moving the weight to the left rear seat causes left side to compress by 2", weight distribution is now 1700lbs on the left, 1400lbs on the right.
CAR#2: stiffer springs and sway bars. Moving the weight to the left rear seat causes left side to compress by 1", weight distribution is now 1700lbs on the left, 1400lbs on the right.
Note that the lower amount of lean on car #2 is in response to the SAME lateral weight-trasnfer. The load on the tyres are EXACTLY the same on BOTH cars. You can even weld solid the suspension with no movement at all so that the car stays flat when the weight moves over, and the weight-transfer will still be the same.
Increasing spring-rates and stiffness is only a compromize solution due to the bad geometry of the McPherson struts used in our cars. As the body leans more and more, the outside tyre leans more and more. So car#1 may end up with +1 degree camber on outside tyre (not good for cornering), while car#2 with less lean ends up with only +0.5 degree camber under the SAME amount of weight trasnfer. Car#2 will corner faster because it has more of its tyre flat on the ground, not because it has less weight-transfer than car#1. Same thing under braking.
Now cars with double-wishbone suspensions can be set up so that the tyres remain flat on the ground as the body rolls, like Ferraris. These cars don't have to use stiff springs and can out-corner our cars with more ride-comfort as well. Take the 3400lbs SupraTT which pulls 0.98g or the 3300lb Corvette C4 with 0.96g. Both having only slightly wider tyres than the 3000lb '86 951 which can only muster 0.88-0.90g (depending upon magazine doing the test).
Now swaybars serve a completely different function than springs. They are used to split the lateral weight-transfer under cornering.
Rather than a single 300lbs weight moving laterally, imagine splitting that weight into two pieces and placing one on the front wheel and one on the rear. Imagine iif you split this weight into two 150lbs pieces, this would give neutral cornering. But if you break it up in to a 200lb piece on the front wheel and the remaining 100lb piece on the rear, what woudl happen? You'd get understeer because the front-wheel has to laterally-accelerate a higher load. The opposite would occur if you put 200lbs on the rear and only 100lbs on the front, you'd get oversteer.
The stiffer end gets more of the lateral weight-transfer. So a big swaybar in front will transfer the majority of the lateral weight-transfer to the front tyres. The result will be understeer. A bigger rear-bar will cause more weight to be transfered to the rear, resulting in oversteer:
To really understand vehicle dynamics, check out these publications:
How to Make Your Car Handle - Fred Puhn
Performance Handling - Don Alexander
Tune to Win - Carroll Smith
Race Car Vehicle Dynamics - Miliken
BTW - corner-weighting is balancing the weights on cross-diagonals in a static condition. It doesn't affect lateral weight-transfer, but does have an effect to the car's response to weight transfer. You can end up with a car that understeers in left-hand corners and oversteers in right-hand corners. This can be advantageous in certain conditions, like racing on ovals. Since you're intentionally throwing the corner-weights off, it's more commonly known as "weight-jacking".
Last edited by Danno; 02-27-2005 at 07:08 AM.
02-27-2005, 10:52 AM
#38
Originally Posted by Danno
BTW - corner-weighting is balancing the weights on cross-diagonals in a static condition. It doesn't affect lateral weight-transfer, but does have an effect to the car's response to weight transfer. You can end up with a car that understeers in left-hand corners and oversteers in right-hand corners. This can be advantageous in certain conditions, like racing on ovals. Since you're intentionally throwing the corner-weights off, it's more commonly known as "weight-jacking".
I need to re-read you post when I get back. Even more Cliff's notes version please: How does the relation of roll center to CG effected by dynamic ride height changes not impact how weight tranfers while driving the car?
Now off to the airport.
02-27-2005, 12:22 PM
#39
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Originally Posted by Danno
How to Make Your Car Handle - Fred Puhn
Performance Handling - Don Alexander
Tune to Win - Carroll Smith
Race Car Vehicle Dynamics - Miliken
BTW - corner-weighting is balancing the weights on cross-diagonals in a static condition. It doesn't affect lateral weight-transfer, but does have an effect to the car's response to weight transfer. You can end up with a car that understeers in left-hand corners and oversteers in right-hand corners. This can be advantageous in certain conditions, like racing on ovals. Since you're intentionally throwing the corner-weights off, it's more commonly known as "weight-jacking".
Performance Handling - Don Alexander
Tune to Win - Carroll Smith
Race Car Vehicle Dynamics - Miliken
BTW - corner-weighting is balancing the weights on cross-diagonals in a static condition. It doesn't affect lateral weight-transfer, but does have an effect to the car's response to weight transfer. You can end up with a car that understeers in left-hand corners and oversteers in right-hand corners. This can be advantageous in certain conditions, like racing on ovals. Since you're intentionally throwing the corner-weights off, it's more commonly known as "weight-jacking".
I was hoping i wouldn't have to break out the old library of car books....
If you have an equally balanced vehicle with 100 lbs. at each corner with the same spring rate and a spring height of 10 units, and then you then shorten the RF spring height 1 unit, does that not transfer weight off of the LR?
02-27-2005, 12:53 PM
#40
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As Danno said, Adrial is correct. Weight transfer is a function of wheelbase, track width, location of CG, and acceleration (acceleration meaning change of velocity in any direction - braking, turning or throttle application). Spring and shock rates affect how the chassis responds to the weight transfer, but they don't change the amount of weight transfer.
Lowering the ride height by installing performance springs changes weight transfer by lowering the CG, but the increased stiffness of the springs doesn't doesn't affect it. A car with stiffer springs leans less not because there is less weight transfer, but because the springs more strongly resist the same amount of weight transfer.
Check out Danno's book recommendations.
Matt
Lowering the ride height by installing performance springs changes weight transfer by lowering the CG, but the increased stiffness of the springs doesn't doesn't affect it. A car with stiffer springs leans less not because there is less weight transfer, but because the springs more strongly resist the same amount of weight transfer.
Check out Danno's book recommendations.
Matt
02-27-2005, 10:21 PM
#41
Race Director
"Even more Cliff's notes version please: How does the relation of roll center to CG effected by dynamic ride height changes not impact how weight tranfers while driving the car?"
Don't get too complicated yet. The instaneous roll-center is the pivot that the CG rotates around:
As the CG moves laterally under cornering, yes, there is a shift in weight. Note that there are TWO types of weight-transfers. The most significant part is the external G-forces from cornering and is additive on top of the outside tyres in addition to the weight of the car. There's no way around this weight-transfer, the faster you corner, the more this weight-transfer will be. The 2nd weight transfer is in due to lateral shift of the CG in response to the cornering-Gs. You can calculate it using a SINE function of the angle of body-lean. This amount is minimal; it's very, very small compared to the shift in weight from cornering forces. Let's compare the numbers:
Weight-transfer due to cornering G-forces = 600-800 lbs
Weight-transfer due to CG shift = 30-50lbs
TOTAL weight-transfer = 630-850lbs
So the most you can affect weight transfer by removing all body-lean and suspension movement is 30-50lbs. Such as converting your car into a go-cart with zero suspension-travel using a solid bar instead of struts and springs. In that extreme case, you can only save 30-50lbs of weight-transfer anyway, not a significant amount compared to how much is beyond your control due to cornering-Gs.
Now, it's possible to have the CG shift over significantly enough to cause detrimental weight-transfer, but you'd have to really tilt the car over such that the CG is directly over the outside tyres. ln which case, the 1400lbs of lateral-weight trasnfer due to CG-shift would be significant; but you'd probably need 5000lbs of lateral-G cornering-forces to do that. I don't think you can get sticky enough tyres to do that:
Now the primary issue here is camber control and keeping the outside tyre flat on the ground, NOT limiting body-roll or weight-transfer. More sophisticated suspensions can minimize camber-change (relative to the ground) as the body rolls. Such as the E46 M3:
This was the '02 OTC where Navid's E46 dusted everyone. We all had similar treadwear-60 R-compound DOT tires. I was pulling around 1.0-1.05Gs in the corners and you can see that I've got much less body-lean them either of the M3s. The lighter E30 M3 was probably around 1.1-1.2Gs. The E46 M3 was pulling around 1.25Gs despite the much heavier car and much larger amounts of body-roll. Modern suspensions can keep the tyres flat regardless of body-lean. This last years' OTC has an EVO that was pulling around 1.3-1.4Gs in the corners on treadwear-220 street tyres!!
Do the math behind the physics and it all makes sense. Miliken's book is the ultimate standard. The "Physics of Racing" series that someone mentioned distills it down to the basic concepts and basic equations. I've got that on my 951 RacerX website.
Don't get too complicated yet. The instaneous roll-center is the pivot that the CG rotates around:
As the CG moves laterally under cornering, yes, there is a shift in weight. Note that there are TWO types of weight-transfers. The most significant part is the external G-forces from cornering and is additive on top of the outside tyres in addition to the weight of the car. There's no way around this weight-transfer, the faster you corner, the more this weight-transfer will be. The 2nd weight transfer is in due to lateral shift of the CG in response to the cornering-Gs. You can calculate it using a SINE function of the angle of body-lean. This amount is minimal; it's very, very small compared to the shift in weight from cornering forces. Let's compare the numbers:
Weight-transfer due to cornering G-forces = 600-800 lbs
Weight-transfer due to CG shift = 30-50lbs
TOTAL weight-transfer = 630-850lbs
So the most you can affect weight transfer by removing all body-lean and suspension movement is 30-50lbs. Such as converting your car into a go-cart with zero suspension-travel using a solid bar instead of struts and springs. In that extreme case, you can only save 30-50lbs of weight-transfer anyway, not a significant amount compared to how much is beyond your control due to cornering-Gs.
Now, it's possible to have the CG shift over significantly enough to cause detrimental weight-transfer, but you'd have to really tilt the car over such that the CG is directly over the outside tyres. ln which case, the 1400lbs of lateral-weight trasnfer due to CG-shift would be significant; but you'd probably need 5000lbs of lateral-G cornering-forces to do that. I don't think you can get sticky enough tyres to do that:
Now the primary issue here is camber control and keeping the outside tyre flat on the ground, NOT limiting body-roll or weight-transfer. More sophisticated suspensions can minimize camber-change (relative to the ground) as the body rolls. Such as the E46 M3:
This was the '02 OTC where Navid's E46 dusted everyone. We all had similar treadwear-60 R-compound DOT tires. I was pulling around 1.0-1.05Gs in the corners and you can see that I've got much less body-lean them either of the M3s. The lighter E30 M3 was probably around 1.1-1.2Gs. The E46 M3 was pulling around 1.25Gs despite the much heavier car and much larger amounts of body-roll. Modern suspensions can keep the tyres flat regardless of body-lean. This last years' OTC has an EVO that was pulling around 1.3-1.4Gs in the corners on treadwear-220 street tyres!!
Do the math behind the physics and it all makes sense. Miliken's book is the ultimate standard. The "Physics of Racing" series that someone mentioned distills it down to the basic concepts and basic equations. I've got that on my 951 RacerX website.
Last edited by Danno; 02-27-2005 at 10:56 PM.
02-27-2005, 11:16 PM
#42
Race Director
On the other issue of corner-weighting:
"If you have an equally balanced vehicle with 100 lbs. at each corner with the same spring rate and a spring height of 10 units, and then you then shorten the RF spring height 1 unit, does that not transfer weight off of the LR?"
Yes, in our universe, matter cannot be created or destroyed out of thin air. If you take it away from one spot, it has to go somewhere else. Ok, let's take the instance of a car that's perfectly balanced:
CAR#1
_L__|__R_
100__100
100__100
The cross-diagonal sums are identical LF+RR=200lbs=RF+LR and this car would handle as best as possible due to even weight-distribution. Let's add 100lbs of weight-transfer due to cornering-Gs in a right-hand turn. Assuming even-springs rates and sway-bars such that the lateral weight-transfer under cornering is split evenly between front & rear tyres:
CAR#1 - right-hand turn w/100lbs cornering force
_L__|__R_
150__100
150__100
The weight-transfer is evenly split and the car corners with neutral balance. Let's try that weight-jacking trick (or a bad corner-balancing job):
"... and then you then shorten the RF spring height 1 unit, does that not transfer weight off of the LR?"
Yes, it does:
CAR#2
_L__|__R_
105__95
95__105
Note that due to the rigid body of the car, it remains flat. But the suspension is like the legs of a table. When you shorten one, it reduces the load on that leg AND the corner on a diagonal opposite. The reduced weight on that diagonal is ADDED to the opposite diagonal pair (the total mass of the table or car has to remain the same). The car basicaly teeter-totters on the heavier diagonal, like a table with a short-leg. You end up with a car that's not corner-balanced on cross-diagonals:
LF+RR=210lbs
RF+LR=190lbs
Note that left/right AND front/rear weight-distribution is exactly the same as before at 50/50; you have not transfered ANY weight to the left side or the right side overall. Not front to rear or rear to front either, you've just changed the diagonal weights. So now under cornering, we have opposite effects depending upon whether it's a right-hand turn:
CAR#2 - right-hand turn w/100lbs cornering force
_L__|__R_
155__95
145__105
This car will understeer in right-hand turns and on left-hand turns:
CAR#2 - left-hand turn w/100lbs cornering force
_L__|__R_
105__145
95__155
... it will oversteer. Note that the 100lbs of weight-transfer is still split evenly between the front & rear tyres because you haven't changed spring-rates or sway-bars. The difference between left & right-turn behaviors is one reason to corner-balance a car (unless you're running on ovals.). Also the outside-tyre that's loaded heavier will reach its traction limit first and slide first, thus reducing the potential maximum cornering speed, because the other tyre hasn't reached its limit yet.
"If you have an equally balanced vehicle with 100 lbs. at each corner with the same spring rate and a spring height of 10 units, and then you then shorten the RF spring height 1 unit, does that not transfer weight off of the LR?"
Yes, in our universe, matter cannot be created or destroyed out of thin air. If you take it away from one spot, it has to go somewhere else. Ok, let's take the instance of a car that's perfectly balanced:
CAR#1
_L__|__R_
100__100
100__100
The cross-diagonal sums are identical LF+RR=200lbs=RF+LR and this car would handle as best as possible due to even weight-distribution. Let's add 100lbs of weight-transfer due to cornering-Gs in a right-hand turn. Assuming even-springs rates and sway-bars such that the lateral weight-transfer under cornering is split evenly between front & rear tyres:
CAR#1 - right-hand turn w/100lbs cornering force
_L__|__R_
150__100
150__100
The weight-transfer is evenly split and the car corners with neutral balance. Let's try that weight-jacking trick (or a bad corner-balancing job):
"... and then you then shorten the RF spring height 1 unit, does that not transfer weight off of the LR?"
Yes, it does:
CAR#2
_L__|__R_
105__95
95__105
Note that due to the rigid body of the car, it remains flat. But the suspension is like the legs of a table. When you shorten one, it reduces the load on that leg AND the corner on a diagonal opposite. The reduced weight on that diagonal is ADDED to the opposite diagonal pair (the total mass of the table or car has to remain the same). The car basicaly teeter-totters on the heavier diagonal, like a table with a short-leg. You end up with a car that's not corner-balanced on cross-diagonals:
LF+RR=210lbs
RF+LR=190lbs
Note that left/right AND front/rear weight-distribution is exactly the same as before at 50/50; you have not transfered ANY weight to the left side or the right side overall. Not front to rear or rear to front either, you've just changed the diagonal weights. So now under cornering, we have opposite effects depending upon whether it's a right-hand turn:
CAR#2 - right-hand turn w/100lbs cornering force
_L__|__R_
155__95
145__105
This car will understeer in right-hand turns and on left-hand turns:
CAR#2 - left-hand turn w/100lbs cornering force
_L__|__R_
105__145
95__155
... it will oversteer. Note that the 100lbs of weight-transfer is still split evenly between the front & rear tyres because you haven't changed spring-rates or sway-bars. The difference between left & right-turn behaviors is one reason to corner-balance a car (unless you're running on ovals.). Also the outside-tyre that's loaded heavier will reach its traction limit first and slide first, thus reducing the potential maximum cornering speed, because the other tyre hasn't reached its limit yet.
02-27-2005, 11:35 PM
#43
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Me: A loose sway bar might produce unpredictable handling characteristics as you experienced if it was totally unbolted and then binded up against something as the weight transfer was occuring. This would transfer the weight rather abruptly and could push the tires beyond their adhesion threshold.
Danno: Now swaybars serve a completely different function than springs. They are used to split the lateral weight-transfer under cornering....The stiffer end gets more of the lateral weight-transfer. So a big swaybar in front will transfer the majority of the lateral weight-transfer to the front tyres. The result will be understeer. A bigger rear-bar will cause more weight to be transfered to the rear, resulting in oversteer
I think we're saying the same thing here.
Danno: Now swaybars serve a completely different function than springs. They are used to split the lateral weight-transfer under cornering....The stiffer end gets more of the lateral weight-transfer. So a big swaybar in front will transfer the majority of the lateral weight-transfer to the front tyres. The result will be understeer. A bigger rear-bar will cause more weight to be transfered to the rear, resulting in oversteer
I think we're saying the same thing here.
02-27-2005, 11:43 PM
#44
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Originally Posted by Matt
As Danno said, Adrial is correct. Weight transfer is a function of wheelbase, track width, location of CG, and acceleration (acceleration meaning change of velocity in any direction - braking, turning or throttle application). Spring and shock rates affect how the chassis responds to the weight transfer, but they don't change the amount of weight transfer.
Lowering the ride height by installing performance springs changes weight transfer by lowering the CG, but the increased stiffness of the springs doesn't doesn't affect it. A car with stiffer springs leans less not because there is less weight transfer, but because the springs more strongly resist the same amount of weight transfer.
Check out Danno's book recommendations.
Matt
Lowering the ride height by installing performance springs changes weight transfer by lowering the CG, but the increased stiffness of the springs doesn't doesn't affect it. A car with stiffer springs leans less not because there is less weight transfer, but because the springs more strongly resist the same amount of weight transfer.
Check out Danno's book recommendations.
Matt
02-27-2005, 11:46 PM
#45
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Originally Posted by Danno
... it will oversteer. Note that the 100lbs of weight-transfer is still split evenly between the front & rear tyres because you haven't changed spring-rates or sway-bars. The difference between left & right-turn behaviors is one reason to corner-balance a car (unless you're running on ovals.). Also the outside-tyre that's loaded heavier will reach its traction limit first and slide first, thus reducing the potential maximum cornering speed, because the other tyre hasn't reached its limit yet.
In your experience, how close can you get the corner weights on a 944?