Lowering Roll centers
08-15-2017, 05:33 PM
#1
Lowering Roll centers
Hi guys,
I took my 92 RS with a freshly rebuilt suspension to local the autocross and I spun out twice.
Anyways, I was told to lower my rear roll center (via the Panhard bar) to help reduce this snap oversteer.
But I'm a little confused on what to do.
do I lower BOTH ends of the panhardbar?
Or just 1 side?
(My brother is a lazer operator and I'm a welder, so I can simply make my own parts from Titanium or Inconol)
I took my 92 RS with a freshly rebuilt suspension to local the autocross and I spun out twice.
Anyways, I was told to lower my rear roll center (via the Panhard bar) to help reduce this snap oversteer.
But I'm a little confused on what to do.
do I lower BOTH ends of the panhardbar?
Or just 1 side?
(My brother is a lazer operator and I'm a welder, so I can simply make my own parts from Titanium or Inconol)
08-15-2017, 09:05 PM
#3
08-15-2017, 11:34 PM
#4
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Re: Lowering Roll centers
Some upstream issues need to be addressed first. Probably good advice - a half answer, though.
Know your alignment specs?
Need more details: tires, shocks, stock springs, LCARB's, strut mounts, SFC's, bushings, stock TA, worn diff, gear ratio, etc? Spin out always on RH/LH? Do you have warning that's it's breaking loose, or does it surprise you? Any break hop into corner?
08-16-2017, 02:09 AM
#5
Re: Lowering Roll centers
Whole lot more going on than just lowering your rear 'cause someone said so.
Some upstream issues need to be addressed first. Probably good advice - a half answer, though.
Know your alignment specs?
Need more details: tires, shocks, stock springs, LCARB's, strut mounts, SFC's, bushings, stock TA, worn diff, gear ratio, etc? Spin out always on RH/LH? Do you have warning that's it's breaking loose, or does it surprise you? Any break hop into corner?
Some upstream issues need to be addressed first. Probably good advice - a half answer, though.
Know your alignment specs?
Need more details: tires, shocks, stock springs, LCARB's, strut mounts, SFC's, bushings, stock TA, worn diff, gear ratio, etc? Spin out always on RH/LH? Do you have warning that's it's breaking loose, or does it surprise you? Any break hop into corner?
245 50r16s kumho summer tire (7yrs old, but were never driven on)
KYB front struts
Moog 5662 front springs
new ball joints
J&M strut mounts
Poly a-arm bushings
Poly end links
34mm front swaybar
new steering
wonderbar
Alignment:
driver: +4.9 deg caster -0.9 camber
passenger: +5.4 caster. -1 camber
3/32 toe in
Moog cc635 rear springs
KYB gas adjust shocks
Adjustable tubular panhard bar with roto joint
Tubular lower control arms
21mm rear sway bar
Each time was in a long sweeping turn; the back just broke free without warning
tires are fairly quiet with a very slight squeel
The Track guys said these 3rd gens are prone to snap oversteoverster
Seems weird that a car with a long wheel base and a 55/45 weight balance would snap like that.
I wasn't aware our cars did that.
Most guys on this forum say to raise the front roll center, and lower the rear.
Adjustable tubular panhard bar with roto joint
Tubular lower control arms
21mm rear sway bar
Each time was in a long sweeping turn; the back just broke free without warning
tires are fairly quiet with a very slight squeel
The Track guys said these 3rd gens are prone to snap oversteoverster
Seems weird that a car with a long wheel base and a 55/45 weight balance would snap like that.
I wasn't aware our cars did that.
Most guys on this forum say to raise the front roll center, and lower the rear.
08-16-2017, 06:51 AM
#6
Member
Re: Lowering Roll centers
not for nothing never heard anyone say a third gen just spin out auto crossing.i have been auto crossing my GTA for yrS from everything bone stock to where the car is today an never once had my car just slide out an spin with out me just over driving the car...tires make a BIG difference I run nitto NTO1s on all four corners.try buying a really good set of competition tires before messing with suspension stuff..
08-16-2017, 10:25 AM
#7
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Re: Lowering Roll centers
Tires (rubber) do have a shelf life = 5 years.
You have a lot of "mild" parts. That's OK. You're out there having fun, that's what counts.
You should get SFC's. Stiffening the chassis is #1. It will also help to preserve the car's body = No Brainer.
A car can only handle as good as the front suspension is able. You merely "tune" the rear to match the capabilities of the front. The KYB's and Moog springs are mild (being kind, here). You really need better performance parts before going after roll center changes. Without hesitation, I recommend #750-#850 front and #175-#200 rear. There is no concern for harshness at all, and my personal recommendation is slightly higher than this. For now, 'you gotta run what you brung.'
Yes, raising front RC and lowering rear RC is a good idea and not incredibly expensive. Extended ball joints will raise front RC, and UMI has axle side only brackets or both side brackets for lowering the PHB. Lowering your entire ride height should be considered, as well. You should come up with a long range plan and budget according to how you plan on using your car (application, application, application).
You could get the axle side only and experiment with small incremental changes to understand what is happening. It is an excellent mod. But it is just a misguided band-aid for proper rubbers, dampers, and springs.
You really need to step back, formulate a proper plan and budget, and gain more understanding in order to achieve your goals. Regardless, the axle side lowering bracket will be a part of that plan. So if you are dead set on trying just this mod only, it is not wrong per se.
Search under "roll center", "RC", "center of gravity", "CG", "grip", "ext BJ", etc. You are going to have to take the time to learn, and you're going to have to spend some money. Is this really where you are at in life right now? Shortcuts will end up costing you more.
Keep having fun!
You have a lot of "mild" parts. That's OK. You're out there having fun, that's what counts.
You should get SFC's. Stiffening the chassis is #1. It will also help to preserve the car's body = No Brainer.
A car can only handle as good as the front suspension is able. You merely "tune" the rear to match the capabilities of the front. The KYB's and Moog springs are mild (being kind, here). You really need better performance parts before going after roll center changes. Without hesitation, I recommend #750-#850 front and #175-#200 rear. There is no concern for harshness at all, and my personal recommendation is slightly higher than this. For now, 'you gotta run what you brung.'
Yes, raising front RC and lowering rear RC is a good idea and not incredibly expensive. Extended ball joints will raise front RC, and UMI has axle side only brackets or both side brackets for lowering the PHB. Lowering your entire ride height should be considered, as well. You should come up with a long range plan and budget according to how you plan on using your car (application, application, application).
You could get the axle side only and experiment with small incremental changes to understand what is happening. It is an excellent mod. But it is just a misguided band-aid for proper rubbers, dampers, and springs.
You really need to step back, formulate a proper plan and budget, and gain more understanding in order to achieve your goals. Regardless, the axle side lowering bracket will be a part of that plan. So if you are dead set on trying just this mod only, it is not wrong per se.
Search under "roll center", "RC", "center of gravity", "CG", "grip", "ext BJ", etc. You are going to have to take the time to learn, and you're going to have to spend some money. Is this really where you are at in life right now? Shortcuts will end up costing you more.
Keep having fun!
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08-16-2017, 12:43 PM
#8
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Re: Lowering Roll centers
not for nothing never heard anyone say a third gen just spin out auto crossing.i have been auto crossing my GTA for yrS from everything bone stock to where the car is today an never once had my car just slide out an spin with out me just over driving the car...tires make a BIG difference I run nitto NTO1s on all four corners.try buying a really good set of competition tires before messing with suspension stuff..
in stock form thirdgen handles well but the snap oversteer comes when push to limit and certain turns you have very sticky tires so it wont be noticeable as much due to extra grip but with stock to mid level tires you will notice it the power doesn't get transferred to the ground as it should , tire shocks and springs does help and since you been driving for awhile your probably just accustomed and adapted to the characteristics of a thirdgen in stock form also the lower to mid range tq delivery and make it more prone as well so there's a couple factors in play
tires shocks struts and springs is good start as long all bushings are good to go
Tires (rubber) do have a shelf life = 5 years.
You have a lot of "mild" parts. That's OK. You're out there having fun, that's what counts.
You should get SFC's. Stiffening the chassis is #1. It will also help to preserve the car's body = No Brainer.
A car can only handle as good as the front suspension is able. You merely "tune" the rear to match the capabilities of the front. The KYB's and Moog springs are mild (being kind, here). You really need better performance parts before going after roll center changes. Without hesitation, I recommend #750-#850 front and #175-#200 rear. There is no concern for harshness at all, and my personal recommendation is slightly higher than this. For now, 'you gotta run what you brung.'
Yes, raising front RC and lowering rear RC is a good idea and not incredibly expensive. Extended ball joints will raise front RC, and UMI has axle side only brackets or both side brackets for lowering the PHB. Lowering your entire ride height should be considered, as well. You should come up with a long range plan and budget according to how you plan on using your car (application, application, application).
You could get the axle side only and experiment with small incremental changes to understand what is happening. It is an excellent mod. But it is just a misguided band-aid for proper rubbers, dampers, and springs.
You really need to step back, formulate a proper plan and budget, and gain more understanding in order to achieve your goals. Regardless, the axle side lowering bracket will be a part of that plan. So if you are dead set on trying just this mod only, it is not wrong per se.
Search under "roll center", "RC", "center of gravity", "CG", "grip", "ext BJ", etc. You are going to have to take the time to learn, and you're going to have to spend some money. Is this really where you are at in life right now? Shortcuts will end up costing you more.
Keep having fun!
You have a lot of "mild" parts. That's OK. You're out there having fun, that's what counts.
You should get SFC's. Stiffening the chassis is #1. It will also help to preserve the car's body = No Brainer.
A car can only handle as good as the front suspension is able. You merely "tune" the rear to match the capabilities of the front. The KYB's and Moog springs are mild (being kind, here). You really need better performance parts before going after roll center changes. Without hesitation, I recommend #750-#850 front and #175-#200 rear. There is no concern for harshness at all, and my personal recommendation is slightly higher than this. For now, 'you gotta run what you brung.'
Yes, raising front RC and lowering rear RC is a good idea and not incredibly expensive. Extended ball joints will raise front RC, and UMI has axle side only brackets or both side brackets for lowering the PHB. Lowering your entire ride height should be considered, as well. You should come up with a long range plan and budget according to how you plan on using your car (application, application, application).
You could get the axle side only and experiment with small incremental changes to understand what is happening. It is an excellent mod. But it is just a misguided band-aid for proper rubbers, dampers, and springs.
You really need to step back, formulate a proper plan and budget, and gain more understanding in order to achieve your goals. Regardless, the axle side lowering bracket will be a part of that plan. So if you are dead set on trying just this mod only, it is not wrong per se.
Search under "roll center", "RC", "center of gravity", "CG", "grip", "ext BJ", etc. You are going to have to take the time to learn, and you're going to have to spend some money. Is this really where you are at in life right now? Shortcuts will end up costing you more.
Keep having fun!
to OP
Good start point for spring rates def not harsh at all
and I also went higher up front 1000lb rear kept the same had adjustable rear sway
daily driver as well
and there is a lot of good reading on this forum about GC Roll Center etc it is dense but worth it
but it does add up $$ and there is a lot of time developing and tinkering on your setup and research and research
08-16-2017, 02:54 PM
#9
Re: Lowering Roll centers
Tires (rubber) do have a shelf life = 5 years.
You have a lot of "mild" parts. That's OK. You're out there having fun, that's what counts.
You should get SFC's. Stiffening the chassis is #1. It will also help to preserve the car's body = No Brainer.
A car can only handle as good as the front suspension is able. You merely "tune" the rear to match the capabilities of the front. The KYB's and Moog springs are mild (being kind, here). You really need better performance parts before going after roll center changes. Without hesitation, I recommend #750-#850 front and #175-#200 rear. There is no concern for harshness at all, and my personal recommendation is slightly higher than this. For now, 'you gotta run what you brung.'
Yes, raising front RC and lowering rear RC is a good idea and not incredibly expensive. Extended ball joints will raise front RC, and UMI has axle side only brackets or both side brackets for lowering the PHB. Lowering your entire ride height should be considered, as well. You should come up with a long range plan and budget according to how you plan on using your car (application, application, application).
You could get the axle side only and experiment with small incremental changes to understand what is happening. It is an excellent mod. But it is just a misguided band-aid for proper rubbers, dampers, and springs.
You really need to step back, formulate a proper plan and budget, and gain more understanding in order to achieve your goals. Regardless, the axle side lowering bracket will be a part of that plan. So if you are dead set on trying just this mod only, it is not wrong per se.
Search under "roll center", "RC", "center of gravity", "CG", "grip", "ext BJ", etc. You are going to have to take the time to learn, and you're going to have to spend some money. Is this really where you are at in life right now? Shortcuts will end up costing you more.
Keep having fun!
You have a lot of "mild" parts. That's OK. You're out there having fun, that's what counts.
You should get SFC's. Stiffening the chassis is #1. It will also help to preserve the car's body = No Brainer.
A car can only handle as good as the front suspension is able. You merely "tune" the rear to match the capabilities of the front. The KYB's and Moog springs are mild (being kind, here). You really need better performance parts before going after roll center changes. Without hesitation, I recommend #750-#850 front and #175-#200 rear. There is no concern for harshness at all, and my personal recommendation is slightly higher than this. For now, 'you gotta run what you brung.'
Yes, raising front RC and lowering rear RC is a good idea and not incredibly expensive. Extended ball joints will raise front RC, and UMI has axle side only brackets or both side brackets for lowering the PHB. Lowering your entire ride height should be considered, as well. You should come up with a long range plan and budget according to how you plan on using your car (application, application, application).
You could get the axle side only and experiment with small incremental changes to understand what is happening. It is an excellent mod. But it is just a misguided band-aid for proper rubbers, dampers, and springs.
You really need to step back, formulate a proper plan and budget, and gain more understanding in order to achieve your goals. Regardless, the axle side lowering bracket will be a part of that plan. So if you are dead set on trying just this mod only, it is not wrong per se.
Search under "roll center", "RC", "center of gravity", "CG", "grip", "ext BJ", etc. You are going to have to take the time to learn, and you're going to have to spend some money. Is this really where you are at in life right now? Shortcuts will end up costing you more.
Keep having fun!
I don't want to mess with springs until my 6.0 LS is ready. I still don't know if the 305 is heavier or not
the tires are for sure an issue.
I was looking at getting 275 30r18s on all 4 corners and put the C7 z51 brakes on it.
I guess I need to just practice more untill the cash is available
08-16-2017, 07:27 PM
#10
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Re: Lowering Roll centers
LSX will be lighter by about 80 to 160lbs depending on different things iron heads, t5 or auto etc.
I ran 800, 950, and 1000lb Front 150, 175 and 200lb on both SBC and LSX Thirdgen with LSX it is a bit lighter up front and more balance front to rear and front capabilities where increase so all options are viable despite the weight but 800 and 200 rear are good start for both LSX and SBC
I wouldn't go a 30 profile tire to thin of a side wall and tire height would be effected as well a 35 is best you can fit up to 285 and depending on wheel specs up to 295 with not much effort corvette 18x 10.5 wheels can be found for cheap
I ran 800, 950, and 1000lb Front 150, 175 and 200lb on both SBC and LSX Thirdgen with LSX it is a bit lighter up front and more balance front to rear and front capabilities where increase so all options are viable despite the weight but 800 and 200 rear are good start for both LSX and SBC
I wouldn't go a 30 profile tire to thin of a side wall and tire height would be effected as well a 35 is best you can fit up to 285 and depending on wheel specs up to 295 with not much effort corvette 18x 10.5 wheels can be found for cheap
08-17-2017, 10:58 AM
#11
Re: Lowering Roll centers
new suspension has 800miles on it
245 50r16s kumho summer tire (7yrs old, but were never driven on)
KYB front struts
Moog 5662 front springs
new ball joints
J&M strut mounts
Poly a-arm bushings
Poly end links
34mm front swaybar
new steering
wonderbar
Alignment:
driver: +4.9 deg caster -0.9 camber
passenger: +5.4 caster. -1 camber
3/32 toe in
Moog cc635 rear springs
245 50r16s kumho summer tire (7yrs old, but were never driven on)
KYB front struts
Moog 5662 front springs
new ball joints
J&M strut mounts
Poly a-arm bushings
Poly end links
34mm front swaybar
new steering
wonderbar
Alignment:
driver: +4.9 deg caster -0.9 camber
passenger: +5.4 caster. -1 camber
3/32 toe in
Moog cc635 rear springs
KYB gas adjust shocks
Adjustable tubular panhard bar with roto joint
Tubular lower control arms
21mm rear sway bar
Each time was in a long sweeping turn; the back just broke free without warning
tires are fairly quiet with a very slight squeel
The Track guys said these 3rd gens are prone to snap oversteoverster
Seems weird that a car with a long wheel base and a 55/45 weight balance would snap like that.
I wasn't aware our cars did that.
Most guys on this forum say to raise the front roll center, and lower the rear.
Adjustable tubular panhard bar with roto joint
Tubular lower control arms
21mm rear sway bar
Each time was in a long sweeping turn; the back just broke free without warning
tires are fairly quiet with a very slight squeel
The Track guys said these 3rd gens are prone to snap oversteoverster
Seems weird that a car with a long wheel base and a 55/45 weight balance would snap like that.
I wasn't aware our cars did that.
Most guys on this forum say to raise the front roll center, and lower the rear.
I would say with the Moog 5662's up front you are not getting too much dive on the nose under braking (which would drop the front RC fast if it was in fact diving- but I think not too much as stated). So why when set in a long sweeper you are spinning? here is my speculation. I do not speculate you are getting onto the brakes while already set into that long sweeper, if you are then brakes will make this issue even worse by tossing the car over the outside front tire. I suspect you are hitting a bump or dip in that sweeper and your choice of shocks are not helping control the situation. They are not very good dampers to control chassis movement so you chassis is abruptly reacting to road changes. Yes lower the Rear rc will be helpful, but you will still have issues with chassis control and the weight of the car moving about before the car will react to a corner.
My suggestion is to try another course layout and see if the car still does this with the same type of corner but hopefully one that is more smooth pavement. HOWEVER< yes do still make a rear panhard relocation bracket and play with it. You should easily be able to at least move down the chassis side 1-1.5". I just did this on a friends car who also was at aprox stock ride height. Left corners were fine but rights were tossing him in a high siding effect due to panhard jacking geometry.
08-17-2017, 11:03 AM
#12
Re: Lowering Roll centers
ps- I was half way around the world in Istanbul Turkey working on that car. That spacer is merely a 5/8" deep socket I drilled out- was in a pinch...lol. Not a lot of resources when I am that far from home. Point is though, make sure you brace a spacer tube in place of the old panhard location before you weld or tighten things down.
You want to make the bar level. If you can get the center of the bar down about 1/2" to 1" from its current position but maintain a level bar from ground height to each bolt center on both sides of the panhard then you are good. Within 1" is acceptable- perferably on the axle side being lower then the chassis side without anyone in the car if it is not going to be level.
You want to make the bar level. If you can get the center of the bar down about 1/2" to 1" from its current position but maintain a level bar from ground height to each bolt center on both sides of the panhard then you are good. Within 1" is acceptable- perferably on the axle side being lower then the chassis side without anyone in the car if it is not going to be level.
Last edited by SlickTrackGod; 08-17-2017 at 11:06 AM.
08-17-2017, 11:49 AM
#13
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Re: Lowering Roll centers
In this pic, I am going through the Carousel at Road America. My instructor kept telling me I was doing fine - keep pushing, but I felt that the rear was wanting to come around. I'm doing between 75-85 mph, here. 90 mph for a street tire car is pretty good. Had QA1's in this pic, but ditched for RideTechs. The lean is actually pretty good, but suspect as Dean mentions - a panhard geometry issue (need chassis side bracket). Track is clock-wise, so more RH's.
Usually, street tires give some warning.
