I have read many posts about lowering the rear with longer bolts, wedge kits and reversing the rubber on the mounts but have not seen anyone talk about the rear spring. What if you bought a rear spring like the the ones from VBP which is a composite and dearched from the original several inches. Wouldn't the rear be lowered by installing this spring and would it not be lowered as much as the difference in the twp arcs? Comments


[Modified by vstol, 12:19 AM 4/10/2003]


[Modified by vstol, 3:03 AM 4/10/2003]

 

All other things being the same, it should be lowered by the differernce in height between the old tip and the new tip.

Think of it this way, when you install the longer bolt you effectively lower the tip(s) of the spring from the wheel mounting point(s). This lowers the center of the spring, and so the body, by the same amount.

 

makes sense to me. So if the VBP springs arch is 2-3" less than stock then you would have the option to lower the car that amount. Has anyone used their rear composite springs?


[Modified by vstol, 3:04 AM 4/10/2003]

 

You might contact the mfg; 2-3" seems like too much drop.

 

What you are missing here is that the VBP spring has a higher spring rate than your factory spring. That's why it's not arched as much. Let's say that the rear of your car weighs 750 pounds at one corner, and you have a spring rated at 250 lbs per in. The spring is going to move 3 inches from rest when the weight of the car is put on it. If you have a spring rated at 400 pounds per inch, the car will settle down less than 2 inches (instead of 3. The weight of the car doesn't change), so, in order to wind up with the same ride height, the spring has to start out with less arch in it, because it's not going to move as far given the same load.

Now, it's not quite that simple since the end of the spring is not at the same point at the contact patch of the tire (it's a shorter "lever"), but it's the same principal. The reason VBPs springs have less arch is because they have a higher spring rate.

Ask them what the rate is on the spring they want to sell you and compare that with your factory spring. I have a chart if you don't know what your factory spring is (I need the year and suspension option).

Dan

 

Dan I spoke with the tech rep at VBP and he said he recomends a 400lb spring. He also said that the spring has 2-3" less arch then the original, which would lower the car about that amount. I have this 96 suspension under a 64 Corvette Grand Sport that I am building and want to lower the rear. Having said that the car has not even settled after connecting the suspension to the frame and adding new dog bones. The axles are still 2" or so from being level. I understand what you are saying. The suspension was from my 96 LT4 Collectors, big brake car. I take your offer to fiqure out what I have and would get. I will tell you the 96 spring is arched pretty good and the car is light, about 2800#. Thanks Kevin

 

Kevin, I'm not sure I can tell you what you need, but here's some more info. There were 3 different rear springs in 96, and the stiffest is rated at 39.9 N/mm which converts to 228 lbs/inch. So the factory spring will move more than VBPs 400 lb spring when you put the weight of the car on it, so it has to have more arch if the ride height is going to come out the same. Now, it is possible that the car will be lower with VBPs spring IF it is dearched more than necessary to compensate for the difference in spring rate, but, the change in ride height is not going to be equivalent to simply the difference in the amount of arch.

The lightest rear spring for 96 was 26 N/mm or about 150 lbs/in.

I have two different rate factory springs for my 85 (Z51 at 57.2 N/mm and FE1 at 39.9 N/mm) and they have a different arch, and I've run them both and the ride height is the same for both on my car.

I was having trouble getting shocks that worked well with my Z51 spring and when I called VBP their recommended solution was one of their sport springs which is rated at 400 to 440 lbs., and I couldn't see how a stiffer spring was going to help a situation where the shocks didn't have enough dampening for even the Z51 spring at 320 lbs/in. (so, I didn't go for it).

VBP will let you return their spring for a refund though within 30 days if you don't like it, so I guess you can try it.

OK, having said all that I will make a recommendation. First, decide what spring rate you need or want for your car depending on what your goals are. If this is an autocrosser, then get the VBP spring. If you're building a driver or touring car, stay with the spring you have. Use longer bolts to lower the car if you need to adjust the ride height (in either case no matter which spring you choose). And don't worry about the half shafts running parallel. I don't think they're supposed to, even on the factory setups.

Hope that helps,
Dan

 

I can't help you with picking your spring rate, but I'd look for another tech over at VBP, or maybe another vendor. For them to say a stiffer spring with less arch would lower the car by the difference of the arch makes no sense. As danno said, that would only be true if the springs were the same stiffness.

Think of lowering the rear with longer bolts like this:

Assume the sprung and unsprung weight does not change. Unsprung weight will make the spring to flatten. The spring is mounted below the rear suspension. The sprung weight is supported on the middle of the spring, on the bottom of the differential carrier. The spring is arched down towards each side of the car. The axels are above the spring. Since the bolt is between the bottom of the hub and the top of the spring, if you use a longer bolt, the wheels are "pushed" further up into the wheel wells.

They wheels are not really pushed, because the sprung weight is suspended from the spring, it's just suspended ontop of the spring.

Hope that made some sence... Good luck with your project :cheers: