I've recently rethought dampening a bit and I feel stupid for not thinking this way in the first place. I am now running more pack in front than rear but like balancing spring rates it's all about leverage. Many people already run more in front but now I have a logical excuse to do it. I prefer to think through why things work rather than to just blindly accept that they do. A shock that is mounted further inward on the arm like front shocks are have a shorter stroke which is why they are shorter. This means total piston speed per the same amount of suspension (wheel) travel over the same amount of time must be lower which means that the velocity of the oil through the holes in the piston will be correspondingly slower. This is less pack even though the total hole area may be the same. By running a "higher pack" piston with smaller holes we can increase the fluid speed through the holes to more equalize the velocity of the oil through the pistons in the same amount of time with the rear.

I tried simplifying things a bit rather than calculating piston speed. I'll play with piston speed calculations later but for now what I did on my old RC10 experimental beater car was to see what the ratio was between the front and rear springs when balanced and then used the generalization that the ratio between total hole area in the front to rear pistons should also remain about the same as this number. This left me with a #1 piston in the rear and a #3 up front in the Associated shocks. I have also tried using the bounce test to determine oil weight too. Not for total dampening but rather to compare wheel rates with the goal of keeping them the same with oil. It isn't terribly easy to see though. When I was running the same pistons all around, I ran a higher weight oil up front. Now with more pack up front, I run a lower oil weight there. I'm still playing around but so far it works quite well. I personally love having everything balanced.

Fred is using the shock itself to set droop good or bad ? My shocks fall right at 125mm droop on my car in the rear so i took the droop screws out and the arms hang even and don't always have to check the droop measuements on the rear. What i want to know is if the shock piston slapping the bottom of the shock body all the time is bad or good and can it actually be an efective way to set droop more permantly without the droop screws ? Also would like to know about up travel washers , if the arm when compressed goes passed the chassis in up position can i put a spacer in to keep from going so far up in the shock body and will this affect the shocks performance while on the track ? Just some minor questions i was hoping to get cleared up. Thanks .

I don't have a vehicle with droop screws. If I did, I'd use them. Since I don't, I use internal shock limiters. I'd prefer droop screws only because if you land really wrong off of a jump you don't run the risk of stripping the shock bottom out. Another reason for using droop screws if you have them is that there is a lot of stress being put on a shock shaft if the piston bottoming is what stops movement. Especially in a wreck. What is truly holding the shock shaft in preventing it from coming out? Is it just a little e-clip? I also don't like a suspension to hit full compression when the shock piston hits full compression. This can damage the shock too. At the very least run a small spacer/stop on the shaft. What I have done on some of my 1/10 buggies is to run very small springs on the shock shafts themselves such as old Associated RC12L/10L front springs. You don't even see them but they absorb some of the impact of a bottom out situation.

Thanks.

Although I've sort of thought calculating the effective spring rates in the past was a waste of time and prone to error compared to the oil-less shock and drop method for balancing, I've come to realize that I may need to bring out the calculator for my SC10. I had it set up with a two sets of balanced springs, the softer of which was silver springs front and rear. I ended up wanting an even softer setup, and went down to green springs front and rear with the same shock mounting positions. This resulted in the true f/r balance shifting to being slightly softer in the rear. I've loved how the thing has handled since then (although some other things were changed at the same time......yeah, yeah, I know). Now I am planning to calculate the effective spring rate ratio front to back. The number itself doesn't really mean much unless you want to maintain the that ratio when you change the overall effective spring rate to a stiffer or softer set. They're gonna add some more jumps on my home track, and I'm thinking I'll need a little bit of a stiffer setup, but want to try to keep the same f/r ratio. So, I'm going to attempt to calculate how to best do that based on the various options for shock mounting positions, springs, etc. The spring balancing method Fred has explained here is still a very important part of the whole thing, and serves as the starting point (ratio value of 1).

Anyone else doing this?

I did that when I switched from AE springs to Durango or Kyosho springs. I switched brands because the AE springs don't collapse uniformly, bending off to the side under load. I also did it to do exactly what you are doing, find a stiffer matching set. Never verified it with slow motion video footage.

I had a question asked to me in a pm and I thought it would be good to address it here. I'm going to simplify the conversation a bit but the end question was "could your tuning method possibly be flawed?". Of course it can! What I mean by that is that this method is a guide and nothing more. It is also a bit of my thoughts and logic on how to proceed and why. I have always maintained that this is how I set things up as a starting point. If I can get everything about the car as neutral as Switzerland through some simple logic, fine tuning it from there shouldn't be terribly difficult.

Another thing to think of is that believe it or not, not every vehicle out there has the best suspension or chassis layout design. Yes the big boys do mess things up sometimes! Don't think that just because they've all got time and money in something that it's going to be perfect. Clearly there are differences among vehicles out there and they all required time and money to develop. Little things from potential quality control issues, to build mistakes, or even geometry can mess some things up. If you have a suspension arm that moves more freely on one side than the other, don't expect things to all be fine later on. Perhaps the suspension binds as it compresses for some reason. This may have the same feel as increasing the roll stiffness with suspension compression.

This tuning method does 3 things. 1 is that it gives you order to tuning so that you know how to address certain handling issues rather than just going in circles all day. 2 is that it helps you get a decently handling base setup. By all means make little changes one at a time to see if you can improve. 3 is that it can potential show you if a vehicle has certain design faults or not.

I just recently found this thread and I'm still back at the beginning of the roll center discussion. I happened to find this website that has some visual components that might help some of us slower people understand.

http://www.racecartuner.com/03/304.html

Thought I would interject some more vehicle dynamics into the discussion. Since many of you may be unfamiliar with the terms I'm about to describe, a google search on "suspension natural frequency" may prove useful.

What you are doing with the drop test is setting the front and rear sprung mass natural frequencies (NF) the same. You could also do this more accurately by measuring spring rates, motion ratios and static weight distribution. The advantage of this is that it would allow you to figure out every possible combo without ever having to do the test. But, do we actually want the same natural frequency front and rear?

Well, standard theory says no, we do not. It turns out that exactly even natural frequencies is the worst combination for good ride over bumps. Because the front will hit the bumps first, you want the rear to have a slightly higher NF (stiffer) so that it can "catch up" to the front. This is a common set-up for production street cars. Oddly enough, the second best combo is to have the front NF significantly higher than the rear. This is a common set-up for road racing cars.

Now I have yet to do much experimentation in this area with offroad rc cars but I suspect a slightly higher rear NF (and I do mean slightly, not some massive difference) would prove to be better, especially in washboards, woops, ruts and blown out conditions. The car should go through such bumps more level and composed. However, it is possible that even NF could have some advantages in other areas, such as take off and landing of jumps. I do not know that it does, just saying it's possible.

I suspect that when setting up via the drop test, it is very difficult to get the NF exactly even. Friction and other factors are going to throw it off. Plus we don't exactly have an infinitely fine series of adjustments available to us. In a way, this could be a good thing since having them be a bit different front to rear is usually better but if you do get it spot on, that could be the cause of the bounce some have reported.

That is not going to make the front softer, it will make it stiffer. Moving the bottom shock mount out increases the motion ratio, making it stiffer. Yes, moving the top in will make it a bit softer but not as much as moving the bottom out makes it stiffer.

As for damping, these are coil overs. So you are moving both the spring AND the shock the same amount. If you make a change that stiffens the wheel rate, you have also stiffened the damping and vice versa. The damping ratio between the shock/spring doesn't change.

Street cars typically have a stiffer rear rate for the simple reason that they try to account for various situations where more weight may be added such as additional passengers or cargo. It's tough to try to get an optimal setup for all situations when variables can keep changing. In other cases a manufacturer may just use what they already have for price reasons. Weight in fuel is also constantly changing. I wouldn't use a street passenger car as a basis for comparison. Real race cars may be limited in geometry due to a design envelop specified by the rules which means unconventional solutions to deal with them. Current F1 cars are a prime example.

Okay call me a noob but I have looked and can't find exactly what I am looking for.
To decrease roll without sway bars or shock changes I do.......
Thanks!

I was not comparing, merely mentioning it. There is plenty of data out there supporting the statement that rear NF slightly higher than front NF yields a smoother ride (except when heavy damping, say 70% of critical or more is used, then higher front NF is usually better). Are there other reasons to do it or other reasons to not do it? Sure but that doesn't change the theory or the data.

The only ways to reduce roll are:

1. Increase roll resistance (stiffer springs or bars)

2. Widen the track width.

3. Lower the CG.

4. More anti-roll in the suspension geometry (Mark Ortiz's term, or more generically, raise the roll center)

5. Go slower.

Thanks!
1. Don't want to otherwise it can't handle the bumps
2. Can't
3. I'll try it!
4. So basically just raise the roll center of front and rear?
5. If I go any slower I'll be going backwards