Coilover & Bypass Shock Tuning Article

[Sandy Rut]

found this on a www written by a bloke called Justin from a mob called 'Revenge Racing'.

Its hard to read but have found it a huge help:

Shock Tuning, from scratch.
I often get asked "what’s wrong with my shocks" by people who are frustrated with the ride of their car and just need some help. I have also seen many people ask questions on this board about this subject and so I thought it may be a good idea to list the things I would do to get a good riding car from start to finish. Remember this is a generality as well as just my .02 so use it if you like or don't if you disagree.

Ok, here we go. This could be long but it will help. The order in which you should proceed is spring first, valveing second and rate nuts last. (Rate nuts being the aluminum collars inside the spring threaded on the shock body)

Springs: The preferred spring rates for most cars will require 1" to 2" preload to get the desired ride height for your car. With the suspension extended (car jacked up off the ground) loosen the upper spring collar that controls pre-load. You want it to loosen up to the point that the coil springs become loose enough to rattle on the shock. Now tighten the preload collar a turn or so until the springs are just held in place and can't rattle around much. This is ZERO preload for your application. Depending on the spring heights being used there can be any where from 0 threads showing on the shock body above the preload collar to 4 inches showing. What ever it is, measure from the preload collar to the top of the shock, shock bolt, top of the threads, what ever is a constant so that you know what 0 preload is for later. If you have 4" of threads showing or 0 threads you need to change the height or length of the springs to correct this but we won't go into that right now. Now that you have a starting point, thread the preload collar down the shock until you get the ride height that you want in your car. This must be done with nitrogen in the shocks and preferably an average load in the car. This may take a few attempts with the jack and don't forget to settle the suspension a bit by jumping up and down on it or rolling the car back and forth. If you have the ride height you want but have less than 1" of pre-load then chances are that you have too much spring rate. If you have more than 2" of preload then you have too little spring rate. It is better to go to the light side (3" preload) than it is to go to the stiff side when it comes to spring. If you are running two springs (upper and lower per shock) then it is a good idea to change spring rates between them. Always go lighter on the top and heavier on the bottom. This way the car can enjoy the plush ride that a light spring can help with and also get a stiffer rate when the secondary rate nuts come into play for those big G outs. I don't think it is a good idea to have MORE than 200lbs difference between the upper and lower spring rates(400 upper and 600lb lower for instance) because more than that tends to be something you can feel considerably as the suspension cycles. As you transition from the upper spring to the lower it may be more of a jarring feeling than a smooth one. Remember that if you need more or less spring on your car you may be able to just change the upper one way or the other to get the rate you need. Also, never raise your car or lower the pre load collar in an effort to stiffen up compression. That is the shocks job.

I know this is out of order but since you are playing with springs and may have them off the car now is the best time to measure them for secondary rate nut placement later. Every shock and spring set up is different. The correct way to set the rate nut is to figure out how tall the lower spring is at full compression. Most cars will bottom on the bypass (or should) not the coil over so the right way to check this is to take the springs off the car and let the suspension compress to full bottom. Here you can measure the coil over length from the lower spring plate upwards. This will be important soon. Now, figure out the total length of the lower spring if it were compressed to bind. You can do this by adding up the thickness of the individual coils multiplied by the total number of them to get total spring height compressed. Now add the height of the spring divider. Add that to the lower spring perch measurement you got by compressing the suspension and you now have the lowest possible setting for a secondary rate nut with out binding the spring and shattering the spring divider. As long as you don't go below this number you can play with the position of the rate nuts to see what works for you. Remember compressed lower spring height plus spring divider added to the position of the lower spring perch at suspension compression gives you the lowest point at which you can run a rate nut (from spring perch upward) with out problems. This should be the last thing you adjust when doing suspension. Every thing else is first. In order from start to finish, spring rates correct (1-2 inches of pre-load to get the ride height you want. Less than an inch you have too much rate, more than 2 inches you have too little rate) next valveing, compression and rebound to get the ride the best you can, last, secondary rate nuts to help with the G outs. If you have bypasses then a rate nut is less important to you if the valveing is correct. If you have coil over only then the rate nuts will be a considerable improvement for you.

Valveing: Once the springs are close the next thing to do is adjust the shocks. For the sake of typing, I am going to assume you have bypass shocks so the adjusting of them is easy to do. All of these things apply to a coil over (accept bump stage) but you have to take a coil over apart to change valveing where a bypass can be done externally as long as the internal valveing is close. For the sake of definitions, more compression means more compression resistance, or slower compression of the suspension or a stiffer ride. Less compression means less resistance, or faster compression of the suspension or softer ride. More rebound means more rebound resistance, or slower extension of the suspension. Less rebound means less resistance to the extension of you suspension, faster tire extension. There are many ways to adjust suspension and all work but what "I" like to do is to start with the shocks as loose as possible. Set your compression and rebound adjusters all the way out or as loose as they can be. Next, run the whoops. Try them at 20 mph and slowly run up to 60mph. If you have some experience with this then you can feel what the suspension is doing but if not then the best thing is to watch the car from another car or video a section of whoops at different speeds and play it back in slow motion. If your compression is too loose it still rides great in the whoops. If compression is still too stiff the chassis will rise up with each jarring hit of a whoop. The goal is to have compression loose enough to allow the suspension to compress over the whoops without causing the chassis to rise up with each hit. Use some common sense here since a huge whoop can hit pretty good no matter what you do. I then stiffen compression until the chassis starts to "shock" upward and then back off a bit. If the rebound is too loose then it will still ride good in the whoops unless it is WAY too loose in which case the car will rise upward shortly after you hit a whoop. I know this sounds the same as stiff compression but they are different. If compression is stiff and moves the car you will feel jarred by the front side of the whoop as you go over it. If it is a rebound issue then the front of the whoop feels fine but after that the chassis raises up smoothly without any jarring affect. Another way to tell is if the chassis bobbing around stops when you get into the throttle, probably loose rebound. If there is too much rebound then the engine will rap up in rpm between each whoop since the tire isn't allowed to extend fast enough to keep contact with the ground. Again, video is easiest when doing this so try it and see how you do.

Next, G outs and final rebound adjustments. Once you are happy with the whoops then move on to some G outs. If you have a 3 tube or more bypass then the shortest compression tube is the closest thing you will have to a G out adjustment. Hit a few G outs and if the car tends to bottom out during what you would consider to be normal dunning for YOU then stiffen the short tube adjustment a turn or two at a time until your G outs become livable. Remember that every car can be bottomed out if you like so keep this at the level you intend to run the car. If you still can bottom the car with ease even with the short tube all the way tight then try the secondary rate nuts. If you can get the ride you like without using up all the short tube adjustments then great! Try the rate nuts also and just loosen your short tube adjustment to get a smoother ride in other places. If you have a two tube bypass then you may want to try the rate nuts first and if they are not enough then you have to stiffen you main compression tube to help out in the G out. This also sacrifices your whoop ride to stiffen only as much as you can stand it. If you don't have a bypass shock then you have no bump stage to speak of so your only adjustment in a G out are the rate nuts. Now go find a jump somewhere you are happy with and jump it. If your car hits the landing and sticks, then great. But if your car hits the landing and then bounces up a time or two you need more rebound. Stiffen it up and try it again. Personally, I like to have as little rebound as possible just short of having a car that bounces up a second time on jump landings. This will keep the tire on the ground the most and save trannies and CV's. Also, run the air pressure you intend to use mostly when doing these adjustments. I like to have a lot in my tires, 16-18lbs and let the suspension do its job not the tire sidewall. Variable tire pressures will affect both compression and rebound so keep it consistent. If you can't get your adjustments to go far enough one way or the other then your next step is the valve stack in the shock. Stiffen or loosen the stack just as a coil over guy would do and then start your testing over again. Bypass adjusters allow you to adjust a certain range of what the valve stack is set up to do. If you are out of this range then changing the stack will move your adjustable range one way or the other.

This description of shock tuning is a simple one. There are many other variables to consider that, simply put, would take too long to type. Free bleed, bypass valve porting, flow restrictions, shaft speeds relative to hydraulic flow potential, reversion, nitrogen temperatures, unsprung weight, and CHASSIS DESIGN to name a few. Assuming that all of these things aren't affecting you in a negative way then following these steps should get you really close to a nice riding car. Good luck guys.
Justin at Revenge Racing

[Sandy Rut]

here's the link
http://www.revenge-racing.com/rr/