Pinning rear Weisache (how do you do it)
07-29-2009, 09:50 AM
#16
Rennlist Member
Rotates fine on entry but not too much, then is slower when trying to drive at the limit of traction or just a little past through the rest of the corner, and is too tight... Am I hearing mid-corner push or understeer.
Yes, trail braking helps a little with mid-corner understeer, but is still slower than a balanced car using only throttle to hold a slight 4 wheel drift (all four tires at their maximum slip angle). That's because trail braking uses some of the available tire grip to maintain the car's balance instead of turn the car.
Just how is getting less traction in the rear going to be faster?
The real question I have is why on a car that has an almost perfect 50/50 balance is everyone running almost twice as much spring in the front as rear, then using shock valving, sway bars, tire width stagger, and alignment (huge front camber, toe out, and pinning the rear) to try to get less traction from the rear to tune out the mid-corner push?
The 944 Turbo has similar weight distribution near 50/50, front engine, rear transmission, and the front springs are the same rate as the rears. So why the front to rear spring rate stagger on the 928?
Yes, trail braking helps a little with mid-corner understeer, but is still slower than a balanced car using only throttle to hold a slight 4 wheel drift (all four tires at their maximum slip angle). That's because trail braking uses some of the available tire grip to maintain the car's balance instead of turn the car.
Just how is getting less traction in the rear going to be faster?
The real question I have is why on a car that has an almost perfect 50/50 balance is everyone running almost twice as much spring in the front as rear, then using shock valving, sway bars, tire width stagger, and alignment (huge front camber, toe out, and pinning the rear) to try to get less traction from the rear to tune out the mid-corner push?
The 944 Turbo has similar weight distribution near 50/50, front engine, rear transmission, and the front springs are the same rate as the rears. So why the front to rear spring rate stagger on the 928?
Last edited by RKD in OKC; 07-29-2009 at 10:26 AM.
07-29-2009, 10:52 AM
#17
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The spring rate difference is due to the different locations for the lower shock mounting on the front and rear. Rear is effectively at the wheel and operates at near 1:1 ratio, the front is further in-board and therefore the spring travel is smaller for the same wheel travel, altering the ratio and requiring a stiffer spring and shock to compensate.
07-29-2009, 01:03 PM
#18
Rennlist Member
MK,
Think of the rear axle on your 928 going around a hard LH corner.
You have the force PUSHING the RR tire towards the transaxle. The thin lower bar is the spring bar. This bows allowing the tire to move in towards the tranny. When this happens the front does not move in. So the rear moves in the front doesnt. This creates a toe out effect, helping to eliminate understeer in highspeed/highload corners.
I run around 1/16" toe in, or 0 toe in. When my tires were at temp I was able to eliminate understeer with simply a slight bit of heavier trailbraking. That was with 235 up front, and 295 in the rear. (not optimal I know)
Think of the rear axle on your 928 going around a hard LH corner.
You have the force PUSHING the RR tire towards the transaxle. The thin lower bar is the spring bar. This bows allowing the tire to move in towards the tranny. When this happens the front does not move in. So the rear moves in the front doesnt. This creates a toe out effect, helping to eliminate understeer in highspeed/highload corners.
I run around 1/16" toe in, or 0 toe in. When my tires were at temp I was able to eliminate understeer with simply a slight bit of heavier trailbraking. That was with 235 up front, and 295 in the rear. (not optimal I know)
I understand the Weisache to be thrust sensitive not lateral load sensitive.
My understanding of the Weisache geometry is that it is designed to adjust toe due to tire thrust not lateral loading. The flat vertical plate is designed to allow the hub (outboard end) to be able to move fore and aft, flexing the plate, while maintaining the hub in its lateral location since the plate is essentially rigid in compression. The small link at the front of the arm is used to control the toe change. Under decel the hub is pulled slightly aft by the "braking" force at the rear tire. This aft movement causes the small front link (mounted at ~45 degree angle from inboard chassis end aft and back to outboard control arm end) to rotate about it pivot end mounted to the chassis. In turn the link end attached to the control arm moves aft in inward which increases toe in. The converse of this happens under acceleration. The movement is controlled by the rubber sleeve on the control arm end of the small link, allowing for this rotation. To pin the link a second bolt is installed through the control arm into the small link forming a rigid member and eliminating then relative rotation of the small link relative to the control arm fixing the toe angle.
So to Marks question about pinning the Weisache and whether pinning will help: If under steer is occurring under decel then pinning may be helpful, as it will eliminate the toe in generated by the Weisache suspension. If it is occurring during accel pinning will work against the desired result, as the Weisache geometry adds toe out and aids in rotation.
07-29-2009, 02:38 PM
#21
Rennlist Member
Thread Starter
are you sure? I think you might have it backwards. as the rear wheel is compressed it TOES IN, not toes out, to counteract oversteer. going around this left hand turn you mention, if the rear toes in under compression, it fights the outward slip because it is pointing the rear of the car to the apex. I am really talking about 80mph skid pad type turns , like turn 2 at laguna (see video ) the car cant be throttle oversteered at all. its tight. more throttle, more push, even with rocking the car front and back with the throttle. it doesnt want to rotate.
I have a lot ot time on the big slicks and a car that is REAL tight with a tremendous amount of trail braking turns. the back is way to tight. I cant get it to rotate, unless the tires are worn, and then its not the right kind of rotation as it is a slip and let go type of slide. It acts best when the rear is a controlled looseness that doesnt break away. Ive had it before a few years back, I just have to find the aligment that does it.
I have a lot ot time on the big slicks and a car that is REAL tight with a tremendous amount of trail braking turns. the back is way to tight. I cant get it to rotate, unless the tires are worn, and then its not the right kind of rotation as it is a slip and let go type of slide. It acts best when the rear is a controlled looseness that doesnt break away. Ive had it before a few years back, I just have to find the aligment that does it.
MK,
Think of the rear axle on your 928 going around a hard LH corner.
You have the force PUSHING the RR tire towards the transaxle. The thin lower bar is the spring bar. This bows allowing the tire to move in towards the tranny. When this happens the front does not move in. So the rear moves in the front doesnt. This creates a toe out effect, helping to eliminate understeer in highspeed/highload corners.
I run around 1/16" toe in, or 0 toe in. When my tires were at temp I was able to eliminate understeer with simply a slight bit of heavier trailbraking. That was with 235 up front, and 295 in the rear. (not optimal I know)
Think of the rear axle on your 928 going around a hard LH corner.
You have the force PUSHING the RR tire towards the transaxle. The thin lower bar is the spring bar. This bows allowing the tire to move in towards the tranny. When this happens the front does not move in. So the rear moves in the front doesnt. This creates a toe out effect, helping to eliminate understeer in highspeed/highload corners.
I run around 1/16" toe in, or 0 toe in. When my tires were at temp I was able to eliminate understeer with simply a slight bit of heavier trailbraking. That was with 235 up front, and 295 in the rear. (not optimal I know)
07-29-2009, 02:46 PM
#22
Burning Brakes
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The entire final chapter of this book is dedicated to the Weissach rear axle. Unfortunately not included in the preview, but worth getting the book for a good read on the 928 rear suspension.
http://books.google.co.za/books?id=l...sec=frontcover
http://books.google.co.za/books?id=l...sec=frontcover
07-29-2009, 02:51 PM
#23
Rennlist Member
Thread Starter
yes, mid corner push is what I see. trail brake can get over the entry push, but after you are constant speed, the rest is up to the traction balance. If you have mid corner push, no trail braking in the world is going to make a bit of difference. its way after the fact. trail braking uses the some of the kinetic energy of the car and transfers it to the front wheels so that you can have more grip to set the car and rotate the lighter rear for an optimal angle entering the turn.
less traction in the rear will be faster as im trying to accelerate out of the turns and the fronts are pushing and the rears are not slipping and cannot even be made to slip. the more power applied, the more the fronts plow. Not good and not fast, becase if you could get the rears to the limit of slip, then you would be much faster, as you say, with all 4 wheel with equal slip as you exit a turn.
I think, in regards to your last comment, the other types of cars with different mixes of spring rates cant be compared as was stated by another poster. motion ratios and geometry determine this. our fronts have a motion ratio of over 5:1. 1" of spring compression for 5" of tire movement. the rears are not as much. other cars may have entirely different leverage points.
less traction in the rear will be faster as im trying to accelerate out of the turns and the fronts are pushing and the rears are not slipping and cannot even be made to slip. the more power applied, the more the fronts plow. Not good and not fast, becase if you could get the rears to the limit of slip, then you would be much faster, as you say, with all 4 wheel with equal slip as you exit a turn.
I think, in regards to your last comment, the other types of cars with different mixes of spring rates cant be compared as was stated by another poster. motion ratios and geometry determine this. our fronts have a motion ratio of over 5:1. 1" of spring compression for 5" of tire movement. the rears are not as much. other cars may have entirely different leverage points.
Rotates fine on entry but not too much, then is slower when trying to drive at the limit of traction or just a little past through the rest of the corner, and is too tight... Am I hearing mid-corner push or understeer.
Yes, trail braking helps a little with mid-corner understeer, but is still slower than a balanced car using only throttle to hold a slight 4 wheel drift (all four tires at their maximum slip angle). That's because trail braking uses some of the available tire grip to maintain the car's balance instead of turn the car.
Just how is getting less traction in the rear going to be faster?
The real question I have is why on a car that has an almost perfect 50/50 balance is everyone running almost twice as much spring in the front as rear, then using shock valving, sway bars, tire width stagger, and alignment (huge front camber, toe out, and pinning the rear) to try to get less traction from the rear to tune out the mid-corner push?
The 944 Turbo has similar weight distribution near 50/50, front engine, rear transmission, and the front springs are the same rate as the rears. So why the front to rear spring rate stagger on the 928?
Yes, trail braking helps a little with mid-corner understeer, but is still slower than a balanced car using only throttle to hold a slight 4 wheel drift (all four tires at their maximum slip angle). That's because trail braking uses some of the available tire grip to maintain the car's balance instead of turn the car.
Just how is getting less traction in the rear going to be faster?
The real question I have is why on a car that has an almost perfect 50/50 balance is everyone running almost twice as much spring in the front as rear, then using shock valving, sway bars, tire width stagger, and alignment (huge front camber, toe out, and pinning the rear) to try to get less traction from the rear to tune out the mid-corner push?
The 944 Turbo has similar weight distribution near 50/50, front engine, rear transmission, and the front springs are the same rate as the rears. So why the front to rear spring rate stagger on the 928?
07-29-2009, 02:57 PM
#24
Rennlist Member
Thread Starter
ah, so its under thrust loads only, and really under braking only, which is a slight force due to limited rear brakes and compression braking. It would explain why I cant get off the throttle mid corner and have the rear end rotate, like most other cars, expecially 911s, right?
so, no issues under accel, so thats not an issue at all, in fact i could use more toe out under accel to loosen the rear up, even on major high speed drifting turns, im seeing a push and its scary because you loose exit control. the trajectory is set and there is nothing you can do excep slow down and thats not an option in a race. (or it is, but it slows you way down).
So, if you are sure about this, then my only real option is to get the rear toe set to 0 as it is at 1/8+" right now . that might help!
I totally had a wrong understanding of the Weisache . I thought it acted under compression toeing in only under compresssion , accel or decel. But, if thats wrong, Ill deal with the reality. sounds like you got it down!
Thanks,
mk
so, no issues under accel, so thats not an issue at all, in fact i could use more toe out under accel to loosen the rear up, even on major high speed drifting turns, im seeing a push and its scary because you loose exit control. the trajectory is set and there is nothing you can do excep slow down and thats not an option in a race. (or it is, but it slows you way down).
So, if you are sure about this, then my only real option is to get the rear toe set to 0 as it is at 1/8+" right now . that might help!
I totally had a wrong understanding of the Weisache . I thought it acted under compression toeing in only under compresssion , accel or decel. But, if thats wrong, Ill deal with the reality. sounds like you got it down!
Thanks,
mk
I understand the Weisache to be thrust sensitive not lateral load sensitive.
My understanding of the Weisache geometry is that it is designed to adjust toe due to tire thrust not lateral loading. The flat vertical plate is designed to allow the hub (outboard end) to be able to move fore and aft, flexing the plate, while maintaining the hub in its lateral location since the plate is essentially rigid in compression. The small link at the front of the arm is used to control the toe change. Under decel the hub is pulled slightly aft by the "braking" force at the rear tire. This aft movement causes the small front link (mounted at ~45 degree angle from inboard chassis end aft and back to outboard control arm end) to rotate about it pivot end mounted to the chassis. In turn the link end attached to the control arm moves aft in inward which increases toe in. The converse of this happens under acceleration. The movement is controlled by the rubber sleeve on the control arm end of the small link, allowing for this rotation. To pin the link a second bolt is installed through the control arm into the small link forming a rigid member and eliminating then relative rotation of the small link relative to the control arm fixing the toe angle.
So to Marks question about pinning the Weisache and whether pinning will help: If under steer is occurring under decel then pinning may be helpful, as it will eliminate the toe in generated by the Weisache suspension. If it is occurring during accel pinning will work against the desired result, as the Weisache geometry adds toe out and aids in rotation.
My understanding of the Weisache geometry is that it is designed to adjust toe due to tire thrust not lateral loading. The flat vertical plate is designed to allow the hub (outboard end) to be able to move fore and aft, flexing the plate, while maintaining the hub in its lateral location since the plate is essentially rigid in compression. The small link at the front of the arm is used to control the toe change. Under decel the hub is pulled slightly aft by the "braking" force at the rear tire. This aft movement causes the small front link (mounted at ~45 degree angle from inboard chassis end aft and back to outboard control arm end) to rotate about it pivot end mounted to the chassis. In turn the link end attached to the control arm moves aft in inward which increases toe in. The converse of this happens under acceleration. The movement is controlled by the rubber sleeve on the control arm end of the small link, allowing for this rotation. To pin the link a second bolt is installed through the control arm into the small link forming a rigid member and eliminating then relative rotation of the small link relative to the control arm fixing the toe angle.
So to Marks question about pinning the Weisache and whether pinning will help: If under steer is occurring under decel then pinning may be helpful, as it will eliminate the toe in generated by the Weisache suspension. If it is occurring during accel pinning will work against the desired result, as the Weisache geometry adds toe out and aids in rotation.
07-29-2009, 05:17 PM
#25
Mark all "normal" cars toe out under braking and in on acceleration. Think about a typical upper and lower a-arm car how the pieces would flex (bushings, etc.). Under braking the car is still pulling forward so the parts flex back at the outsides and toe out. Obviously reverse under acceleration. Porsche was ahead of the curve with what are now the 5 link systems. They use a toe control link to flex the spindle to compensate for the normal bushing movement under force. The nice thing about a 5 link is many are less prone to wheel hop (ever seen a supra launch). I realise my last comment leaves many variables on the table.
As for the higher spring rate in the front, it's not just the weight in the car, but where. The engine makes the front of the car have a much higher center of gravity so it needs more roll stiffness to counteract it effect. Think of a weight on top of a 20 foot pole trying to lean.
As for the higher spring rate in the front, it's not just the weight in the car, but where. The engine makes the front of the car have a much higher center of gravity so it needs more roll stiffness to counteract it effect. Think of a weight on top of a 20 foot pole trying to lean.
07-29-2009, 06:11 PM
#26
Rennlist Member
As for the higher spring rate in the front, it's not just the weight in the car, but where. The engine makes the front of the car have a much higher center of gravity so it needs more roll stiffness to counteract it effect. Think of a weight on top of a 20 foot pole trying to lean.
07-29-2009, 07:04 PM
#27
Rennlist Member
I was just curious because of the huge difference. If the lever is that much shorter maybe there is not enough front springs. However, on my GTS I added front tire pressure and firmed up front rebound, both made the mid-corner and exit push worse.
07-29-2009, 07:44 PM
#28
I realize this is only one aspect, but does anyone think about the track width. Porsche had the track width wider in the front stock. Perhaps that's an easy way to help. Wider in back means back has more control and vice versa. If it's higher speed corners have you wondered about too much rear downforce/not enough front?
07-29-2009, 07:51 PM
#29
This is kinda' redneck (for lack of better description) but you can monitor the effects of downforce by installing tps type sensors on the suspension members with rods and data logging their position. Do averaging to see how it effects the ride height front and rear at speed.
07-29-2009, 09:10 PM
#30
Rennlist Member
Thread Starter
what lever are you referring to? the front motion ratio is not .5 its more like .2. I have pictures of 5-7" of movement for only 1" of shock travel. in the rear its about double that. this is the main reason why the fronts will be at 900lbs and the rears 500lbs on a race car. If you notice the rear shock bottom almost rubs on the bottom of the rear tire, while the front shock is much more inboard. thats the difference in a nutshell.
mk
mk