Suspension question
Thread Starter
Melting Slicks
Joined: Mar 2009
Posts: 2,300
Likes: 115
From: Odessa FL
St. Jude Donor '10-'12-'13-'14-'15-'16-'17
Suspension question
Ok so the plan was to install C5 Z51 springs (front and rear), C6 Z06 shocks and C6 Z51 sway bars (front and rear) with new metal end links on my 2000 coupe with base suspension. Install went fine and no real issues. Have not really pushed the car yet in twisties so I can’t comment on handling improvements, but it does feel tighter and flatter. Now the problem or maybe the opportunity. I got the sway bars as take offs from what was supposed to be a 2007 C6 Z51. The bars looked almost new and the bushings looked as if they were installed yesterday, so I was very pleased and there is no concern that they were not in good shape. I was under the car yesterday and was measuring the bars in anticipation of potentially replacing the bushings with poly ones. The front bar measured at 31mm but the rear bar measured at 28mm. Quick research tells me the bars are then actually from a C6 Z06 not Z51. More research indicates that unless I am going to run rear tires in excess of the stock 275 width, that the Z rear bar may be more detrimental to than improving handling, but there seems to be different takes at what width the rear tires would either require or benefit from the rear Z bar. I know that the stock C6 Z06 rear tire is a 325/19 but is it necessary to go to that size to justify the rear Z bar. So, first question for all that have experience is at what width would the Z rear bar become beneficial and not detrimental, 285, 295, 305, 315, etc. Currently running 17/18 stock rims so if I want to change rims to facilitate wider rears to make the rear bar compatible trying to gage size required to make that happen. Second question is if I keep the Z rear bar and go to wider rear tires and rims, should I now replace the C5 Z51 springs (C5 Z06 or aftermarket as I know the C6 springs are not interchangeable also know that he front Z51 spring is the same as the front Z06 so really only looking at rear) or would the C5 Z51 springs still be compatible with the C6 Z06 shocks and bars combination. The car will rarely be tracked (occasional auto cross and drag run but primarily a street, back road twisty romp) so I want to retain some level of comfort while street driving. The conventional wisdom on the forum as of late is that the C6 Z06 shocks and C6 Z51 bars are a compatible and improving combination, so last question is should I just replace the rear bar with a C6 Z51. Also have not had an alignment yet, but wanted to wait to solve this issue first. Thanks for any input.
Forgot to mention, that also installed was the Elite 1/4 tunnel plate.
Forgot to mention, that also installed was the Elite 1/4 tunnel plate.
Last edited by alxltd1; Apr 8, 2011 at 10:57 AM.
Melting Slicks
Joined: Aug 2004
Posts: 3,003
Likes: 89
From: Indy IN
You probably aren't as far off as you think
Before you do anything, you should drive the car at an autocross test day and see what it is doing with the present setup. You probably aren't as far off as you think, and here's why:
You need to think about total roll stiffness, which is the combined roll stiffness of springs and bars.
What you have, compared to a C5Z is a bit more bar in the back coupled to a softer spring. Your rear spring rate is about 12% lower in the back than a C5Z, and you have the same front spring as the Z.
Your rear bar is a lot stiffer (because stiffness is the cube of the diameter increase), but you would have to go through linkage ratios and see what that means relative to spring stiffness.
But your front bar is stiffer than the Z front bar by 1mm, which erases some of the rear bar effect.
In relative terms your rear bar is probably 2 or 3mm too big, but that's not a huge amount.
You can do a couple of things.
After you drive the car decide what it's doing and see if it's too loose. If it is, the easiest thing to do is stiffen the front sway bar bushing. Go get a set of poly bushings for the front only, and if it is loose, put those in first. That should get you in the ball park because these cars have stiff bars, changing the bushing stiffness is the easiest way to tune the suspension. At the Proving Ground they routinely fine tune sway bar stiffness by changing the stiffness of the bar bushings and you can do it too. Sway bar bushing stiffness is an easy way to fine tune your balance, and it works really well on cars with big bars because a little bit of flex in the bushings makes a big difference in the relative bar stiffness.
As you noted bigger tires on the back will help get rid of any oversteer that you might find, but that's a lot more expensive way to do it. I doubt that you would need to go anywhere beyond a 315 to take care of it so you aren't going to need to go to huge tires. Even going to a 305 or 295 might get it done, depending on what you do with the fronts.
What I would do is look for a set of OEM C5Z wheels. They are as light as you can get for a good street wheel, and you can put whatever tires you want on them, including the fronts that you have on the car now. That and the fact that they aren't expensive is what makes them great.
If you want to upgrade tires, go with the Z wheels, put a set of 275's on the fronts and a set of 315's on the back and if the car is loose or tight you can tweak the bar bushings accordingly.
With that setup the car should hook like glue with a good set of sticky tires on it. The C5 really needs something like a 275 to get the front end to hook up. Just rememeber that what you want to get in terms of balance is the front end to bite and the back end to follow you around. If you feel the back end doing a lot of work you have too much rear roll stiffness.
You need to think about total roll stiffness, which is the combined roll stiffness of springs and bars.
What you have, compared to a C5Z is a bit more bar in the back coupled to a softer spring. Your rear spring rate is about 12% lower in the back than a C5Z, and you have the same front spring as the Z.
Your rear bar is a lot stiffer (because stiffness is the cube of the diameter increase), but you would have to go through linkage ratios and see what that means relative to spring stiffness.
But your front bar is stiffer than the Z front bar by 1mm, which erases some of the rear bar effect.
In relative terms your rear bar is probably 2 or 3mm too big, but that's not a huge amount.
You can do a couple of things.
After you drive the car decide what it's doing and see if it's too loose. If it is, the easiest thing to do is stiffen the front sway bar bushing. Go get a set of poly bushings for the front only, and if it is loose, put those in first. That should get you in the ball park because these cars have stiff bars, changing the bushing stiffness is the easiest way to tune the suspension. At the Proving Ground they routinely fine tune sway bar stiffness by changing the stiffness of the bar bushings and you can do it too. Sway bar bushing stiffness is an easy way to fine tune your balance, and it works really well on cars with big bars because a little bit of flex in the bushings makes a big difference in the relative bar stiffness.
As you noted bigger tires on the back will help get rid of any oversteer that you might find, but that's a lot more expensive way to do it. I doubt that you would need to go anywhere beyond a 315 to take care of it so you aren't going to need to go to huge tires. Even going to a 305 or 295 might get it done, depending on what you do with the fronts.
What I would do is look for a set of OEM C5Z wheels. They are as light as you can get for a good street wheel, and you can put whatever tires you want on them, including the fronts that you have on the car now. That and the fact that they aren't expensive is what makes them great.
If you want to upgrade tires, go with the Z wheels, put a set of 275's on the fronts and a set of 315's on the back and if the car is loose or tight you can tweak the bar bushings accordingly.
With that setup the car should hook like glue with a good set of sticky tires on it. The C5 really needs something like a 275 to get the front end to hook up. Just rememeber that what you want to get in terms of balance is the front end to bite and the back end to follow you around. If you feel the back end doing a lot of work you have too much rear roll stiffness.
Last edited by Solofast; Apr 8, 2011 at 12:34 PM.
Thread Starter
Melting Slicks
Joined: Mar 2009
Posts: 2,300
Likes: 115
From: Odessa FL
St. Jude Donor '10-'12-'13-'14-'15-'16-'17
Before you do anything, you should drive the car at an autocross test day and see what it is doing with the present setup. You probably aren't as far off as you think, and here's why:
You need to think about total roll stiffness, which is the combined roll stiffness of springs and bars.
What you have, compared to a C5Z is a bit more bar in the back coupled to a softer spring. Your rear spring rate is about 12% lower in the back than a C5Z, and you have the same front spring as the Z.
Your rear bar is a lot stiffer (because stiffness is the cube of the diameter increase), but you would have to go through linkage ratios and see what that means relative to spring stiffness.
But your front bar is stiffer than the Z front bar by 1mm, which erases some of the rear bar effect.
In relative terms your rear bar is probably 2 or 3mm too big, but that's not a huge amount.
You can do a couple of things.
After you drive the car decide what it's doing and see if it's too loose. If it is, the easiest thing to do is stiffen the front sway bar bushing. Go get a set of poly bushings for the front only, and if it is loose, put those in first. That should get you in the ball park because these cars have stiff bars, changing the bushing stiffness is the easiest way to tune the suspension. At the Proving Ground they routinely fine tune sway bar stiffness by changing the stiffness of the bar bushings and you can do it too. Sway bar bushing stiffness is an easy way to fine tune your balance, and it works really well on cars with big bars because a little bit of flex in the bushings makes a big difference in the relative bar stiffness.
As you noted bigger tires on the back will help get rid of any oversteer that you might find, but that's a lot more expensive way to do it. I doubt that you would need to go anywhere beyond a 315 to take care of it so you aren't going to need to go to huge tires. Even going to a 305 or 295 might get it done, depending on what you do with the fronts.
What I would do is look for a set of OEM C5Z wheels. They are as light as you can get for a good street wheel, and you can put whatever tires you want on them, including the fronts that you have on the car now. That and the fact that they aren't expensive is what makes them great.
If you want to upgrade tires, go with the Z wheels, put a set of 275's on the fronts and a set of 315's on the back and if the car is loose or tight you can tweak the bar bushings accordingly.
With that setup the car should hook like glue with a good set of sticky tires on it. The C5 really needs something like a 275 to get the front end to hook up. Just rememeber that what you want to get in terms of balance is the front end to bite and the back end to follow you around. If you feel the back end doing a lot of work you have too much rear roll stiffness.
You need to think about total roll stiffness, which is the combined roll stiffness of springs and bars.
What you have, compared to a C5Z is a bit more bar in the back coupled to a softer spring. Your rear spring rate is about 12% lower in the back than a C5Z, and you have the same front spring as the Z.
Your rear bar is a lot stiffer (because stiffness is the cube of the diameter increase), but you would have to go through linkage ratios and see what that means relative to spring stiffness.
But your front bar is stiffer than the Z front bar by 1mm, which erases some of the rear bar effect.
In relative terms your rear bar is probably 2 or 3mm too big, but that's not a huge amount.
You can do a couple of things.
After you drive the car decide what it's doing and see if it's too loose. If it is, the easiest thing to do is stiffen the front sway bar bushing. Go get a set of poly bushings for the front only, and if it is loose, put those in first. That should get you in the ball park because these cars have stiff bars, changing the bushing stiffness is the easiest way to tune the suspension. At the Proving Ground they routinely fine tune sway bar stiffness by changing the stiffness of the bar bushings and you can do it too. Sway bar bushing stiffness is an easy way to fine tune your balance, and it works really well on cars with big bars because a little bit of flex in the bushings makes a big difference in the relative bar stiffness.
As you noted bigger tires on the back will help get rid of any oversteer that you might find, but that's a lot more expensive way to do it. I doubt that you would need to go anywhere beyond a 315 to take care of it so you aren't going to need to go to huge tires. Even going to a 305 or 295 might get it done, depending on what you do with the fronts.
What I would do is look for a set of OEM C5Z wheels. They are as light as you can get for a good street wheel, and you can put whatever tires you want on them, including the fronts that you have on the car now. That and the fact that they aren't expensive is what makes them great.
If you want to upgrade tires, go with the Z wheels, put a set of 275's on the fronts and a set of 315's on the back and if the car is loose or tight you can tweak the bar bushings accordingly.
With that setup the car should hook like glue with a good set of sticky tires on it. The C5 really needs something like a 275 to get the front end to hook up. Just rememeber that what you want to get in terms of balance is the front end to bite and the back end to follow you around. If you feel the back end doing a lot of work you have too much rear roll stiffness.
Melting Slicks
Joined: Aug 2004
Posts: 3,003
Likes: 89
From: Indy IN
None of those have any measurable effect on handling. More power or torque would tend to make the rear end loose, but you are going to have to feather the throttle in any case and not just dump on the power on corner exit, so it doesn't matter. If you hammer the power on corner exit the base car will oversteer. The Z might be worse, but in a lower gear in an autocross course you always have more power than you can use with a base engine car so it doesn't matter.
