Damping adjustment for rough, choppy pavement on AutoX course?

Hi Everyone,

I'm doing an AutoX event tomorrow at a location that has a pretty bad surface.  There are lots of seams in the asphalt, dips, and joint lines.  Last time I ran there it was really unsettling my suspension, to the point that I could feel my car bottom out probably.

I'm in an 84 VW Rabbit GTI with Koni Yellows and Ground Control Coils with 350 lb springs in the front and 450 lb springs in the rear. 

I normally just set my Konis to full hard for damping when I'm autocrossing, but in this case I'm wondering if that's the wrong idea?

Would you go full soft in this case to try and absorb the bumps?

Or half or some measure of mixed stiffness between front and back?

I'm sure I'm going to get some "just try it" responses here, but let's be real, one event isn't going to give me much "emperical evidence" here.  In 6 - 8 runs I might start to plateau my times in the 2nd session enough to see a trend line, but I won't get enough in to judge much on the effectiveness on the changes.  This is why I'm asking for advice.

Thanks.

Damping.  Dampening sounds like the process of making an object slightly wet and I don’t think that’s what you’re asking about. 

Not sure if anyone can give a good answer based on the info provided. Besides trying to improve the car’s response over the bumps, how was the balance of the car?

Haha.  Sorry you are right, I am not interesting in adding slight moisture to it! 

Especially around corners (where some of the worst bumps were last time I ran there) I felt like I was unloading the suspension and I was litterally hopping the inside rear wheel.

Here's a link to one of those runs actually if this forum will let me link to a facebook video:

rough pavement run

Is it on the bumpstops?  That looks crazy bouncy.

The places we run are rough and bumpy. The rabbits in our club are on slicks and BOUNCE. To the point that on one run, both rabbits spun out. Like mustang leaving cars a coffee. 
 

The rear wheels had at least 6" of air. Both cars. Both runs. i would run softer springs AND shock settings in the back. 

You badly need more suspension travel. There's almost no travel to be had it looks like.. Which is a major issue.. Also how big of a wheel are you running? It looks like a 15? You might consider downsizing to a 14 to get some more compliance in the sidewall as well. Yokohama is making the AD052 in a 14" size. 

If you are bouncing around, you need softer damping and maybe softer springs and more bar to compensate.

I am a believer in low and soft for the rear of a front driver, don't interrupt the front tires while they are trying to work.  My Golf had the rear end lowered severely, which looked stupid but I never had any more traction problems on corner exit.  It kept all four tires on the ground, you see, which meant it wasn't unweighting the inside front.

Too much rebound and you can jack down the suspension, not letting the shocks extend quick enough and make you run on the bump stops. That looks to be what's going on here, unless there isn't much travel from the start.

I would take out all rebound (full soft) and try it again.

T.J. said:

Damping.  Dampening sounds like the process of making an object slightly wet

Title fixed, in case that's keeping people away

_ said:

The places we run are rough and bumpy. The rabbits in our club are on slicks and BOUNCE. To the point that on one run, both rabbits spun out. Like mustang leaving cars a coffee. 
 

The rear wheels had at least 6" of air. Both cars. Both runs. i would run softer springs AND shock settings in the back. 

Wow!  Good to know it isn't just me and there are probably more experienced people coming across this problem.

spacecadet said:

You badly need more suspension travel. There's almost no travel to be had it looks like.. Which is a major issue.. Also how big of a wheel are you running? It looks like a 15? You might consider downsizing to a 14 to get some more compliance in the sidewall as well. Yokohama is making the AD052 in a 14" size. 

I am moderately lowered (close to parallel control arms) but these cars don't have a ton of suspension travel to begin with.

I'm running on 205/55/15s with 15x7 wheels.  

Out of the 8 different locations I have autocrossed at with this car it's the only one where I have ever gotten this kind of a bouncing reaction with the car.  

I think I would start by raising the car as much as the GC adjustment allows. That bit of extra bump travel might make a big difference. 

Knurled. said:

If you are bouncing around, you need softer damping and maybe softer springs and more bar to compensate.

 

I am a believer in low and soft for the rear of a front driver, don't interrupt the front tires while they are trying to work.  My Golf had the rear end lowered severely, which looked stupid but I never had any more traction problems on corner exit.  It kept all four tires on the ground, you see, which meant it wasn't unweighting the inside front.

I've had a few different people on this forum mention a slammed rear suspension height for MK1s and MK2s.  That's something I'd probably be willing to try next time considering it's solid beam and there's really nothing to disrupt for alignment in the rear of this car anyway.

I'm also not opposed to maybe trying a bigger rear bar.  Right now I'm running no front sway bar and a 24 mm Scirocco 16v rear sway bar.  Might be interesting to try and go up to an aftermarket 28.

I wussed out this morning because it honestly just wasn't a fun place to run last time and I know that the PCA is running events the next 2 weeks at a different location I've been wanting to try.

I will have to come back here next year though if I intend to run more of the SCCA calendar so any advice is appreciated!

In reply to Contradiction :

If you are lifting the inside rear, there is no point to running a stiffer rear bar.  At that point, all roll stiffness comes from the front end.  That is where lowering the rear helps on these cars - the rear CG is relatively high because the cars are so light in the floor/suspension, and because they are so light back there they prefer to lift a wheel rather than compress the suspension, so lowering the rear helps keep chassis roll minimized without having to resort to stiffening the front suspension.

And you could try the old trick of a wire tie tight on the shock absorber shaft to find out how far the shocks are actually compressing.

Unfortunately I have some experience running a lot of spring rate on koni sports on rough surfaces. As far as I could tell the shocks just aren't up to multitasking (cornering and bumps at the same time) with much more than stock springs. My advice would be raise the ride height a little and get better dampers. If that's not an option, raise the ride height a little, maybe run slightly softer springs, and get the softest most progressive bump stops you can find so that the suspension runs out of travel in as controlled a fashion as possible instead of hard bottoming out like it is in the video. Or at least make sure there are actually bump stops on the car and that they actually engage before anything solid bottoms out.

knurled nailed it.

The drawback to excessive lowering is that you get a "weight-jacking" effect from the rear suspension under braking, making weight transfer to the front happen a bit more rapidly than expected.  Its worse if you have dampers with really soft rebound and very stiff compression.

Its all about compromise, especially with FWD cars.

Koni Sports on my Si on smooth roads were very good- terrible on rough surfaces.