BC Racing Coilover Question
01-24-10, 09:36 PM
#17
Tech Info Resource
iTrader: (2)
if you compress the spring completely using more than recommended preload, you are definelty going to change the static height, but you are also reducing the amount of travel the spring is going to have. how can you say it wont affect ride quality?
With linear springs like in the BC coilovers, when you put the weight of a corner of a car (probably around 700lbs) over your spring it compresses to absorb that weight. With 400lb spring, it will compress about 1 3/4". So, if you start with the lower collar so that it is just tight enough to hold the top of the spring against the upper perch this would be 0 preload. Raise the collar 1 3/4", and you now have 700lb's of preload. (arbitrary #'s for the sake of illustration.) Since you have preloaded the spring, the same weight as the car's corner, when you put the car on that spring, it will not compress at all, and will simply sit on top of the spring. If you were to hit a bump, the force would still compress the spring the same amount it normally would if you had set the spring to 0 preload. Now if you set the spring to say 900lbs of preload, when you hit the bump (say its a bump that generates around 250lbs of upward force), the spring will compress much less b/c the spring is already beyond the load specified to absorb the bump. Since there is only 50lbs beyond the preload, the spring will only compress about 1/8th inch, and the car will likely be launched over the bump. Preloading can provide you with more suspension travel in certain situations. In the ideal world, having the spring at 0 preload would place the piston inside the shock exactly halfway through it's stroke (middle of shock) once the car's weight is resting on the spring. This is almost never the case. by preloading the spring, you can adjust the amount of rebound, or compression travel you have in the shock. If you preload like 200lbs, this will give you more compression travel, and will help keep your car from bottoming out when cornering.
With linear springs like in the BC coilovers, when you put the weight of a corner of a car (probably around 700lbs) over your spring it compresses to absorb that weight. With 400lb spring, it will compress about 1 3/4". So, if you start with the lower collar so that it is just tight enough to hold the top of the spring against the upper perch this would be 0 preload. Raise the collar 1 3/4", and you now have 700lb's of preload. (arbitrary #'s for the sake of illustration.) Since you have preloaded the spring, the same weight as the car's corner, when you put the car on that spring, it will not compress at all, and will simply sit on top of the spring. If you were to hit a bump, the force would still compress the spring the same amount it normally would if you had set the spring to 0 preload. Now if you set the spring to say 900lbs of preload, when you hit the bump (say its a bump that generates around 250lbs of upward force), the spring will compress much less b/c the spring is already beyond the load specified to absorb the bump. Since there is only 50lbs beyond the preload, the spring will only compress about 1/8th inch, and the car will likely be launched over the bump. Preloading can provide you with more suspension travel in certain situations. In the ideal world, having the spring at 0 preload would place the piston inside the shock exactly halfway through it's stroke (middle of shock) once the car's weight is resting on the spring. This is almost never the case. by preloading the spring, you can adjust the amount of rebound, or compression travel you have in the shock. If you preload like 200lbs, this will give you more compression travel, and will help keep your car from bottoming out when cornering.
NO you DON'T want the shock at mid-stroke when you're at normal ride height, ideally you want it at 1/3 travel rebound and 2/3 compression.
The shock or the spring will limit full travel. Either the shock bottoms out when the piston hits the end of the cylinder or the spring goes into coil bind. In either case, spring rate goes infinite and the only thing left to absorb bumps is the tire's sidewalls.
You sound like you've really never tuned a chassis for a performance application. Many years ago I very unsuccessfully argued your position with a real chassis tuner. What you say is intuitive, but it's wrong. Just like cutting springs raises the spring rate (not at all intuitive, but absolutely true every time.) So, preload can't affect the spring rate - that's a given. I agree it can change full compression load if the spring doesn't go into bind, but for any given bump, the spring is going to compress to the same length unless the shock bottoms. As you said, the one thing it can change is the amount of force on the system at full extension, but even that has no impact on ride quality except in the case of a severe washboard where the tires are no longer in contact with the pavement.
So ask yourself a couple of questions -
1. What is the length of the spring at normal ride height?
2. Does changing the preload change the length of the spring at normal ride height?
(NO, it can't if the suspension compresses at all when the corner is loaded, and it the suspension doesn't compress with the corner loaded something is VERY wrong.)
3. Does the spring's length change at full compression with more preload?
(NO it can't. Either the coil binds, or the shock's piston stops the suspension from compressing any further, and in either case, spring rate goes infinite. Adding preload can change which of these occurs first).
So the real problem - spring RATE determines ride quality (stiffer or softer) and preload can do nothing to affect spring rate. Damping rate also combines with spring rate to alter ride quality - damping is to prevent the spring system from oscillation, so damping rate and spring rate are inexorably linked. Changing preload will only change ride height and which of the two possibilities on compression happen first - coil bind or shock piston hitting the end of the cylinder and in either case, when this happens spring rate goes infinite and ride quality goes to total crap.
See attached diagram for a graphic on what happens with preload. Notice with a Force X the spring's length is identical regardless of preload. This is the situation on the car, the only difference you'll see is the ride height of the car with more preload will be higher because you've increased the distance from the bottom of the spring to the bottom shock mount by moving the collar higher on the shock body. In either case the spring is the exact same length. Nothing else changes.
So, I realize you think the ride height changes because you're shortening the allowed space for the spring between the shock body and the upper mount, but that's not what is really happening. You're increasing the distance between the lower collar and the bottom of the shock. The spring's height isn't going to change one bit because you're not changing the load on the corner. If it is possible to compress the spring more by increasing preload (shock travel is gone before coil reaches coil bind), then you'll move higher on the sloped line at maximum shock travel (minimum shock length) and there will be more maximum force which is the effect you're referring to, but again, this is a ride height adjustment, not a ride quality adjustment since the spring's rate is unchanged, AND you can only add preload to increase this until you hit coil bind.
It didn't make sense to me when I first heard it, but it's reality and it's the way you need to consider this if you expect to tune suspension for performance.
01-25-10, 09:17 AM
#19
The BCR coilovers come with the factory preload set. So you don't have to mess with it.
If you want to check just mnake sure you have at least 3mm of preload, I wouldnt go over 4 or 5mm. But you do need to make sure all 4 corners are the same if you change it.
Feel free to PM us with any questions on the BC stuff
If you want to check just mnake sure you have at least 3mm of preload, I wouldnt go over 4 or 5mm. But you do need to make sure all 4 corners are the same if you change it.
Feel free to PM us with any questions on the BC stuff
01-25-10, 09:03 PM
#21
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iTrader: (2)
Once you have this set, then you loosen all the bolts holding the bushings, jack the corner with a floor jack until the chassis rises off the jackstand, bounce the corner a few times to settle it, then tighten the bushing bolts so the suspension is at neutral height when you tighten the bushings. This will minimize bushing wear and influence on ride height.
01-27-10, 04:41 PM
#23
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01-27-10, 11:35 PM
#25
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iTrader: (2)
Exactly. Ideally the car would be on the ground, but jacking the corner is the next best thing. That's how I've been doing it for years.
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