Tuning shock absorbers

[April Dillon]

I've been frustrated for ages by these setup guides which simplify damper tuning to "increase front bump for more oversteer" etc. only to find that this isn't really the case, or the truth of the matter is more complicated than that. So it's been a breath of fresh air to find this article, which despite being more vague, probably shines more light on this black art than anything else I've ever found.

http://f1elites.com/4d/?p=46

Could someone who's more familiar with chassis tuning post their thoughts? Would you back up what this guy's saying, or has he got the wrong end of the stick?

 

[Jim Cole]

Interesting way of describing transitional information for spring dampers. In some aspects the OP is correct but you are right he is very vague and a couple of the things he mentions to do will get you into trouble and will make setting up the car a lot more difficult.

First remember that before anyone even attempts to make changes to the setup of the car, they should be very proficient and comfortable with car and track. I would suggest that being able to do 5+ laps with laptimes around .5 to .8 apart is where you want to be before you attempt a change. The reason for this is because you need to be that good to see the differences from what you are changing. I don't mean to imply that some of you can't see the differences, but some of the changes might not be seen if your laps times are what you are going by. Also remember that the spring dampers are usually the last thing you adjust to try and get the car balanced as the rest of the suspension is more suited for the rougher tuning required first.

Spring dampers are designed to control the weight transfer to and from any given wheel at any given time. They make it to where the weight either hits hard onto the tires or soft depending on settings. Controlling the weight transfer allows for better grip where we want it and will help reduce lap times a little because of it. Car weight control is usually described as balanced when you can enter, negotiate and exit a turn without a ton of thought and control input to keep the car from losing control. If you need to make constant changes in your steering input, throttle input and brake input to be able to make a corner, then your car is not in balance.

Keep in mind that any change that is made to spring dampers will most definitely have effects on areas besides what we are trying to get straightened out. As most of the suspension tuning we do is for road courses and not ovals, the beginning tuner should always keep the settings to where they adjust symmetrically, or where you adjust one side and the other side automatically adjusts for you.

A couple of small notes to remember. For the most part anytime you adjust bump on one end of the car, you should adjust rebound in the opposite direction on the other end of the car, there are exceptions, but the beginner need not worry about that just yet. Also note that testing your adjustments on a straight line are not going to give you much information on the changes. For RWD cars, pretty much the quicker you get the weight on the back tires the faster you can accelerate, and the faster you get it on the front tires, the harder you can brake, but only in a straight line. For negotiating turns, sudden weight changes put the car out of balance and can cause you to lose control very quickly.

There are charts all over the internet on how to adjust your spring dampers for different conditions that you are experiencing. Keep in mind that most of that information out there refers to stiffening or softening the damper. To stiffen, you would raise the numeric value for the bump or rebound which ever you are adjusting. To soften it would be the opposite. For instance if the car won't turn and wants to keep going straight with no brake input then it is likely that the weight isn't on the front wheels enough to allow them to work. To fix this you would adjust the front bump to make it stiffer and make the rear rebound lighter in order to let the weight move onto the front tires. For acceleration, RWD cars want the weight on the back, but FWD cars want to keep the weight on the front as long as possible. To keep the weight on the front, you would stiffen the front rebound and soften the rear bump.

I would suggest that anyone attempting to make changes to a setup that is just learning, talk to someone who is experienced in it so that they can help explain what each setting does and when you would change it. I would also suggest that unless you are an alien and can feel the differences immediately, run several laps to find out if your change made a difference. A general rule of thumb though is change gears first, change springs next, then anti-sway bars and then dampers. Once you have done this, go back and start at the beginning and do it all again until your car is not improving with your changes.

 

[April Dillon]

In preparation for the next S2000 race, I wanted to do exactly that. There's a few places where it'll bounce and slip a little bit, the rear wheels will spin suddenly, etc. And this is the case on all tracks. It could be I'm pushing too hard, but I figure if I can establish a base setup I can drive more comfortably, then learning to drive better should come easier, right?

In order to load the front wheels when braking and the rear wheels when accelerating very quickly, would I want to increase the bump resistance and reduce the rebound resistance in all four corners? Or is that a recipe for disaster?

 

[Jim Cole]

Marcus, what you are experiencing may not even be an issue that needs the dampers adjusted. Snap oversteer and sudden pushing can be caused by other things. For instance if your car only some times suddenly swings the back end around on a turn, and does it at the same spot on the track each time then your suspension could be bottoming out. If this is the case then changing dampers won't fix anything, but changing ride height will, or spring pressure. If the car pushes straight in all corners, it could be brake bias is set too far forward, if the back comes around on all corners, the bias could be too far to the rear.

All this in mind, why don't we start with the basics. Have you looked into raising the ride height any to see what happens? As I mentioned in the other post, start by adjusting ride height and spring pressure. You want to try and balance the height and the spring pressure to where you run fairly light springs and low height, but you don't want to bottom out. You can watch your replays to see if there is any sparks. If there are, your suspension has just bottomed out. Watching the replay will also show you when your tires are locking and which ones are actually locking up. This information will help to determine where to move the brake biased.

What car are you driving? Perhaps I can give one a spin and make a fairly stable setup for a starter for you. The setups would need gearing adjust for each track, but that is a no brainer

 

[Ryan Callan]

The simplest way to play with oversteer and understeer is through the anti-roll bars, rear wing and diff settings - especially on an S2000 FWD car.

Stiffer rear bars than fronts induce understeer, the idea is have as soft a front bar as you can (in theory), softer settings equal more mechanical grip. But that brings higher tyre heat, less responsiveness and balance issues relative to the rear settings. One way to get around any understeer there is to lessen the rear wing or vice versa.

And transition with diff settings can make a big difference - a higher coast equals a more stable car with better braking, but less mid corner speed due to higher understeer - a higher power gives better acceleration but again higher understeer - and if the difference between the two coast settings is too high you may find unwelcome body movement upon on/off throttle transition.

This is pretty much just FWD S2000 relevance.

 

[April Dillon]

The car is the 6 speed BMW 3 series from the STCC class.

I've already set the brake bias to 58/42 to prevent the front from locking up, and that feels good. I've got minimum downforce at the back to avoid drag, as I don't seem to miss it in high speed corners. My tire pressures seem to be a good compromise for the whole circuit, judging by watching the temperatures on SimAdapter. I've got the springs as soft as they'll go, and the car doesn't appear to bottom out, even when the back end snaps away.

I only have trouble with the back end in slow corners (1st or 2nd gear). Everywhere else, it handles great. That's why I was wondering whether it was related to me doing something stupid with my right foot.

It could be a sign that I need to work on the differential some more. My understanding is that too far from the 'optimum lock' can cause the back end to become unstable at low speed under power, amongst other things.

I did an experiment with the GTs in Endurance Series earlier tonight, tweaking the shocks, and I can deffinately feel the difference, for better or worse. Although I didn't take the time to see what effect it had on my lap times.

 

[Ryan Callan]

Well if it's any comfort the BMW 320 is one of the hardest cars to drive and fathom in the entire RACE series

 

[April Dillon]

Figures. Oh well, that kind of technical challenge is one of the things which got me interested in racing sims in the first place.

I'll try tweaking the antiroll bars. My front wheels are well below the optimum 90 degrees celsius, so some more heat in there certainly couldn't hurt.

 

[Jim Cole]

Don't mess with the anti roll bars. I have run a few laps on that track with a rear wheel drive car and the track is bumpy as all get out where you are talking about. It also exhibits a little bit of off camber in the turn as well. Take it easy through the turn and don't start to accelerate hard until you are out of the hairpin and halfway through the left hander.

As with all RWD cars, you have to gently apply the throttle coming out of a turn or the car will gently turn you around.

 

[April Dillon]

Ah ok, it is bumpy as hell, and I don't just suck at coping with it. That's good to know. I've got to put in a lot more practice before tomorrow's race anyway, I'm still not confident I'll survive right through to the end. Thanks Jim.

I haven't touched the anti-roll bars yet. All I've done is add a bit more preload. That's helped a lot, especially in the tight left hand corner before the hairpin right. It's not much faster, but it is more managable. I tried setting preload to 5 (1 higher) afterwards, and that wasn't any more stable, but was much slower.

 

[Jim Cole]

For RWD cars I normally set the diff to 30/60/1 sometimes 30/60/2 depending on how things feel. This is mostly preference as you have to get used to driving with the diff like that. You have to be steadier and smoother with the throttle, but the result is a bit faster corner exits if done right.

As a general rule of thumb though, if you are breaking the back end loose coming out of the corners you can try and lower the value for the diff in the power setting area. This will allow power to be transferred to the inside wheel more and may cause that wheel to break loose, but the outside wheel should still keep traction. The downside to this is slower corner exits, but more stable exits until you get proper throttle control.

For the Coast setting on the diff, pay attention to your car when you are braking. If you have the bias correct and the car still wants to come around as you enter the corner, you may need to lower the value of the coast setting. The higher the value the more effective your brakes are though, so balance it out.

 

[Koray Can]

...I would also suggest that unless you are an alien and can feel the differences immediately, run several laps to find out if your change made a difference. A general rule of thumb though is change gears first, change springs next, then anti-sway bars and then dampers. Once you have done this, go back and start at the beginning and do it all again until your car is not improving with your changes.

I've read somewhere (sorry can't find the URL at the moment) that in DTM they determine damper settings by telemetry data alone because drivers just cannot tell the difference. If you're serious about damper settings you may have to use i2 pro to check damper velocities and see where the fast dampers are switched on, etc.

 

[Jim Cole]

If drivers can't tell the difference, then there is a bug in the mod. The difference may be slight, but a driver that knows his car can tell when it doesn't take a corner the way it needs to. Also, if the drivers can't tell the difference, why even bother changing it?

 

[April Dillon]

I think by DTM, he doesn't mean the mod.

I've found that's sometimes the case with some tuning options on a car, where I won't be able to tell the difference it's making until I check the clock after a flying lap. Especially with gear ratios when I don't have a speedometer, radiator/brake duct settings, that sort of thing. As a driver, I don't notice these things. It's only when checking my lap times and data aquisition that I'll see what kind of a difference those things are making.

 

[Jim Cole]

There are some things that won't be noticeable for handling, but will make a difference on top speed. Dampers are one of those items you should notice in handling every time you make a change. If you can't tell the difference, then perhaps something else is wrong in the setup that is preventing you from feeling it or seeing it. If that is the case, you would pretty much need to fix what ever else is wrong before you go back to adjusting dampers. Trust me when I tell you that if you adjust the dampers a little bit, you are going to be able to tell if your car can no longer make a corner or has the backend come around on you when it didn't before the adjustment.

 

[Brendan Crosby]

I've been reading a lot of racecar-engineering magazines and it seems to me that seperating bump and rebound damping is not a mainstream type of setup. Often engineers will describe a shock absorber setup in terms of the damping ratio. The damping ratio is the actual damping coefficient in [N/m/s]/damping coefficient that will yield critical damping also in [N/m/s]. For example 2500[N/m/s]/3000[N/m/s] = 0.8333 which would be the damping ratio. Critical damping in the context of a suspension system means that if you were to sit down on the car making the suspension lower and then stand up off of it the body will return to it's original position as fast as possible without oscillating. A critically damped system will have a damping ratio of 1 and an undamped system will have a ratio of 0 theoretically oscillating forever if disturbed. The way you get the damping ratio is an equation called the quarter car model and seems to be pretty much the core of suspension setup at least when you're using math that is. you have to adjust the spring rates and damping rates for wheel rates before plugging them into the calculation and I've applied all of this to sim racing with a few cars such as the Saleen S7 in GTR2. What I found was quite surprising. According to my reading the slow setting which we are told influence body control for the most part should yield a 0.7- 1.2 damping ratio, quite a large range with 0.7 making the suspension the fastest reacting (studies have shown that 0.4-0.8 return to their original position faster than critical damping even though they oscillate a little bit) while still being controllable and 1.2 being the best for working heat into the tires. Damping ratios for the fast settings should be 0.3 to 0.4 allowing quite a lot of oscillation so that when going over a series of bumps the suspension is almost already reacting to the next bump. The pattern that I keep seeing over and over in the .hdc files of simbin cars and all mods with no exceptions so far that I've seen is that the slow rebound setting is always the highest initial value by far next is the slow bump which will be half of that at most. The fast rebound setting will start out at the same value as slow bump or sometimes even higher and the fast bump will be almost too low to do anything. Most of the time you aren't able to set all of these damper settings so that they fall into these damping ratio ranges, a lot of the time you can get most of them though and the results are quite remarkable, but it's clear that this very basic but powerful calculation, which is really just a ballpark description of what the car is actually doing since the tires and weight of the suspension components complicates things, (it's best when unsprung weight if very low compared to sprung weight and the tires have a very high spring rate compared to the wheel rate of the suspension) is not widely used when developing physics models. In the case of the saleen the highest damping ratio it achieved was 0.7 with slow rebound, 0.5 with slow bump and I don't remember the fast settings other then that you couldn't get them into acceptable ranges. The result of this is that cars in GTR2 are a bit sluggish when you come from something that has been adjusted into these ranges, especially in high speed sections most evidently at spa the cars sort of want to weave all over the track as the body's movement isn't controlled properly, granted I'm not trying to say that this isn't something that one can get used to and for all I know that's how the real life dampers really behave and it was/is a problem that often needs to be faced by race teams but I have yet to see any evidence for this. In cars that I've been able to put tune these damping ratios on such as the MMG F1 mod I try to set both bump and rebound the same rate or at least near it which looks weird in the setup menu but means that the damper has a uniform damping ratio. Sometimes I find that a small difference is desirable and I think this is where the "bump setting should be 1 or 2 clicks lower than rebound" advice came from, if both the bump setting "7" and rebound setting "7" were the same 6000[N/m/s] hypothetically then having a slightly lower bump setting such as "5" may get you 5500[N/m/s] and this may be desirable for certain cars in certain situations, but I don't think half the damping force is reasonable. Another thing is that it seems to me from MoTeC histogram analysis and experiment that the switchover from slow damping to fast damping may be too high. In the case of simbin touring cars I'm pretty sure it's 90mm/s but it looks like the suspension doesn't move this fast in response to weight transfer. And so this has been a very long winded explanation of why I think the damper setting ranges we have to work with are oftentimes insufficient for getting to ideal damping ratio ranges (at the right suspension speeds) where I've found the most constructive handling differences can be explored.

 

[Brendan Crosby]

http://www.vehicledynamics-expo.com/07vdx_conf/day_3/nowlan.pdf here's exactly what I was trying to explain. I really think it makes a difference especially in race consistency because the car becomes so precise and predictable.

 

[Jim Cole]

Brendan, that gets way into what the OP was trying to avoid He wanted something simple to understand and implement rather than something that would take a math major to understand.

 

[April Dillon]

Actually, quite the opposite. I wasn't learning anything from the simple implementations, and I'm very interested in the science and engineering behind racing cars. So at a glance, what Brendan posted looks perfect to me. Thanks.

 

[Brendan Crosby]

Ya, it would be great if I was able to give simple and generally effective tips on the subject, but I have none and I don't think anyone else does either. Unfortuneatly it takes a math major to comprehend what it takes to get you into the ballpark of effective damper tuning and even then it's a ballpark estimate. If you ever hear someone or something claiming to contain the holy grail of damping their full of it, unless of course it's active suspension but that's a whole 'nother subject. This is only to be used as a launchpad for experimentation otherwise you're looking for a needle in a haystack. Also it's not like I'm saying this will get you seconds of improvement every lap, and it doesn't necessarily lead to the best setups ever, as always it depends heavily on the specific track. For the most part I've only found it gives you a few tenths at best, it really shines in a race, the longer the better.

Those are my thoughts, I sincerely hope they're helpful