As I noted in Mr. Blaine's thread in the How To section on selecting springs when going to coilovers, I have some questions (bolded below for clarity), as I'm considering adding coilovers to my build. I hope this thread, intended to discuss the technicalities of the dual-spring coil-over shock, isn't seen as jackassery because I'm simply trying understand the dual-spring coil-over setup on a high level. Since we have some very smart engineers and non-engineers here, I'd like to embark on a path of enlightenment vis-a-vis the coil-over shock.
First Topic - Preload
Someone asked about preload, and I, too have that question. How does the tuner decide on the amount of preload? Or better yet, what factors are considered when determining preload? Blaine mentioned that his tuner suggested 1" in the front and 2" in the rear, and later said that he wasn't sure why they preferred that. On other vehicles I've been involved with (both 2 and 4 wheels and one with skis and a track), the main reasons for preload were 1) to keep the spring seated (not bouncing off the seats during dynamic loading) and 2) to finely adjust ride height.
Second Topic - Dual-springs and Cross-over
The dual-spring shock can be used to produce a progressive wheel rate. Progressive means that it takes more force to compress the first inch than to compress the last. The wheel rate is the spring rate measured at the axle because the axle is what controls the tire position. Progressive wheel rates have been around for a long time, and they are used on most vehicles with wide variations in degree. Progressive rates can be achieved in numerous ways - with a single progressive-rate spring, with dual-springs, with linkages, with jounce bumpers on the shock, or with bump stops, as on a stock TJ. Generally, the progressive rate is used to prevent a harsh bottoming of the suspension, which is hard on vehicle and driver alike.
I presume that the traditional reasons for the progressive rate apply with TJs, too. Off-road, solid axle shocks end up being comparatively long, and that seems to make dual-springs a very good way to get a progressive wheel rate. Also, solid axle suspensions have relatively small linkage effect, so the wheel rate and the spring rate are not much different. Therefore, using spring rate instead of wheel rate removes some calculations.
Using Mr. Blaine's method to determine the combined spring rate based upon corner weights and desired ride height makes good sense to me. I've been doing some analysis of the dual-spring set-up following his method, and two big questions come to mind. Knowing that the way dual-spring setups work, you have an initial combined spring rate as Mr. Blaine explained. As the shock continues to compress, at some point the rate changes because one of the springs can no longer compress further (either when it reaches solid height or when it's movement is restricted by the cross-over rings on the shock body). At this point, the spring rate becomes that of the remaining spring (the one that can still compress). The spring rate increases (usually, but i may always increase - I haven't thought about that). This gives the progressive rate. My first question is what parameters are considered in order to determine where to put this rate change in the shock's travel range? Should it occur below the desired ride height, at the desired ride height, or above the desired ride height? It can be adjusted by moving the cross-over rings on the shock body, and it seems that is an important parameter to get right.
The second question is related to the first. Mr. Blaine gave a great way to determine the combined spring rate, but what determines an ideal second spring rate (the one of the spring that's still compressing as the shock bottoms)? This is limited by keeping the differences in your spring rates no larger than 100 lb/in. Is this the primary reason for this guidance? This question came about as I started analyzing a spring setup similar to that mentioned in Mr. Blaine's How To post. The springs picked were from Eibach. Both were 2.5" I.D. springs. The top spring was Eibach number 1400.250.0150 (14" long, 150 lb/in, 4.72" solid height). The bottom spring was Eibach number 1600.250.0200 (16" long, 200 lb/in, 6.30" solid height). Using those springs, with 1" of preload, here's how the spring curve changes when you adjust the cross-over ring's position (the vertical axis is spring force in pounds and the horizontal axis is shock compression in inches):
Since articulation is an important parameter for a rock-crawling TJ, I would suspect that the right end of that curve is important. Too much spring force, and articulation is hindered because vehicle weight is not enough to compress the high-side spring, causing the body to tilt and causing a rollover. So, my thoughts are that both the transition point and the final spring force should be considered in a setup (along with that final spring rate), but having no practical experience with these vehicles, I'm still low on the learning curve. My goal is to become my own suspension tuner, rather than use someone else, as I've done that with other types of vehicles in my former career.
Has anyone got any insight on these ponderings? Is this the wrong venue for this type of discussion?
First Topic - Preload
Someone asked about preload, and I, too have that question. How does the tuner decide on the amount of preload? Or better yet, what factors are considered when determining preload? Blaine mentioned that his tuner suggested 1" in the front and 2" in the rear, and later said that he wasn't sure why they preferred that. On other vehicles I've been involved with (both 2 and 4 wheels and one with skis and a track), the main reasons for preload were 1) to keep the spring seated (not bouncing off the seats during dynamic loading) and 2) to finely adjust ride height.
Second Topic - Dual-springs and Cross-over
The dual-spring shock can be used to produce a progressive wheel rate. Progressive means that it takes more force to compress the first inch than to compress the last. The wheel rate is the spring rate measured at the axle because the axle is what controls the tire position. Progressive wheel rates have been around for a long time, and they are used on most vehicles with wide variations in degree. Progressive rates can be achieved in numerous ways - with a single progressive-rate spring, with dual-springs, with linkages, with jounce bumpers on the shock, or with bump stops, as on a stock TJ. Generally, the progressive rate is used to prevent a harsh bottoming of the suspension, which is hard on vehicle and driver alike.
I presume that the traditional reasons for the progressive rate apply with TJs, too. Off-road, solid axle shocks end up being comparatively long, and that seems to make dual-springs a very good way to get a progressive wheel rate. Also, solid axle suspensions have relatively small linkage effect, so the wheel rate and the spring rate are not much different. Therefore, using spring rate instead of wheel rate removes some calculations.
Using Mr. Blaine's method to determine the combined spring rate based upon corner weights and desired ride height makes good sense to me. I've been doing some analysis of the dual-spring set-up following his method, and two big questions come to mind. Knowing that the way dual-spring setups work, you have an initial combined spring rate as Mr. Blaine explained. As the shock continues to compress, at some point the rate changes because one of the springs can no longer compress further (either when it reaches solid height or when it's movement is restricted by the cross-over rings on the shock body). At this point, the spring rate becomes that of the remaining spring (the one that can still compress). The spring rate increases (usually, but i may always increase - I haven't thought about that). This gives the progressive rate. My first question is what parameters are considered in order to determine where to put this rate change in the shock's travel range? Should it occur below the desired ride height, at the desired ride height, or above the desired ride height? It can be adjusted by moving the cross-over rings on the shock body, and it seems that is an important parameter to get right.
The second question is related to the first. Mr. Blaine gave a great way to determine the combined spring rate, but what determines an ideal second spring rate (the one of the spring that's still compressing as the shock bottoms)? This is limited by keeping the differences in your spring rates no larger than 100 lb/in. Is this the primary reason for this guidance? This question came about as I started analyzing a spring setup similar to that mentioned in Mr. Blaine's How To post. The springs picked were from Eibach. Both were 2.5" I.D. springs. The top spring was Eibach number 1400.250.0150 (14" long, 150 lb/in, 4.72" solid height). The bottom spring was Eibach number 1600.250.0200 (16" long, 200 lb/in, 6.30" solid height). Using those springs, with 1" of preload, here's how the spring curve changes when you adjust the cross-over ring's position (the vertical axis is spring force in pounds and the horizontal axis is shock compression in inches):
Since articulation is an important parameter for a rock-crawling TJ, I would suspect that the right end of that curve is important. Too much spring force, and articulation is hindered because vehicle weight is not enough to compress the high-side spring, causing the body to tilt and causing a rollover. So, my thoughts are that both the transition point and the final spring force should be considered in a setup (along with that final spring rate), but having no practical experience with these vehicles, I'm still low on the learning curve. My goal is to become my own suspension tuner, rather than use someone else, as I've done that with other types of vehicles in my former career.
Has anyone got any insight on these ponderings? Is this the wrong venue for this type of discussion?
