Sharp Bouncing

jeffreyduncan wrote:
Morning.

I have a 2015 Chevy 3500 HD short bed, Crew with the duramax. It's setup with RS9000's in the rear (set at 9) and stable loads. Front shocks are Bilstein 5100. I have a Hellwig stabilizer in the rear. Truck is stock otherwise. No airbags.

I have a fairly heavy camper and took this setup out over the weekend for my first road trip. Everything went very well and I am generally pleased with the way it handles. However - I notice that I get an annoying fast bounce (I think from the rear). It's not a porpoise. It's more of a chucking. It happens a couple times per second it seems, when I am rolling over small bumps at highway speeds or over 40mph. It's not constant, and any larger bumps or dips are handled fine. It's more of an annoyance than a drivability issue.

I am thinking it's because my auxiliary springs are so pre-loaded with the stable loads. They are almost flat, but they aren't so flat that they touch the lower springs. i have maybe and inch and a half left. The truck sits nice and level though.

Would anyone have a suggestion to calm the sharp bounce and improve the ride?

I was thinking my options might be:

Add air bags.
Change the Bilsteins in the front for RS9000.
Install stiffer rear aux springs (SuperSprings).

Any insight would be appreciated.

I have the same truck and camper as you, except that I have the 6.0 instead of the DM. I also have a Hellwig big wig adjusted to max. I disconnect the sway bar when the TC is not loaded to return some articulation to the suspension.

FWIW, you want the Bilstein 4600 series shocks, not the 5100s.

I have Bilstein 4600s on all four corners, and they're a definite improvement over the stock "fake" Ranchos.

It sounds like you have lower Stableloads? I went with the uppers to avoid what it sounds like you're talking about - the sudden engagement of the upper overload leafs when you hit a bump. With the upper stableloads, all leafs are engaged at all times with the TC loaded. I find the loaded ride to be surprisingly good. My only ride issues are hitting large potholes offroad while off camber. Even with upgraded shocks and a sway bar, it sets up a momentary oscillation that can be disconcerting.

Being that we have essentially the same setup except for (as I suspect) the type of Stableloads installed, I would put money on it being the aggressive net spring rate change from the engagement and disengagement of the upper overload leafs.

My main problem right now is the one you mentioned with the rubber bumpers not engaging the front of the bulkhead. I'm thinking about having some 1" UHMW PE spacers cut to install behind the rubber bumpers.

whizbang wrote:
Wrong Bilstein shocks in front. 5100's are Bilstein's off road shock. You want the payload shocks is #4600.

Aha! I will try the 4600 then. The two Ranchos I have for the back arrived with broken adjusters. I sent them back and got replacements and they are installed, but I hesitate to get them again. Thanks for pointing out I had the wrong ones up front. Hopefully this will at least help. No wonder I noticed the front end coming up when I hit the brakes in reverse! I didn't think they were stiff enough. Duh.

If the front shocks are for "off road" I would replace them with Ranchos so all four corners have the same shock. JMHO

Wrong Bilstein shocks in front. 5100's are Bilstein's off road shock. You want the payload shocks #4600.

The fact that you get the problem at higher frequency bumps rather than lower frequency bumps makes it sound like you have hit some sort of resonant frequency that your dampers aren't handling. Maybe try to find a piece of local road that induces it, then try different adjustments on the dampers either way to see if one of them improves it. I.e. don't just try making the dampers firmer, try making them softer too.

It could be that the resonant frequency of bounce of the tyres (which probably relates to their pressures) may be hitting a frequency the dampers are failing to damp.

Steve.

I am sure it is road surface induced. It only lasts a second or two and it doesn't happen all the time. The strange thing is that I don't FEEL any bumps in the road when it happens, and it recovers quickly. The larger dips and bumps are handled fine.

I was going to try to lighten up on the RS9000's which are set at 9, but I thought that might be worse. I'll move them down a notch and see what happens.

I don't know the rear axle weight yet, but I am sure I am at or slightly over the limit of 7050. I plan to weigh it Columbus Day weekend at a CAT scale.

The camper is an Arctic Fox 811S. Center of gravity is over the rear wheels. Yes, I know it's a 4300 pound camper loaded. I'm very happy with how the truck handled except for the annoying choppy bounce every now and then. It didn't seem to matter whether I had a full 51 gallons of water or not - I tried full/empty and not any significant difference.

I am really thinking it's induced by the helper springs being loaded like they are.

OR, it could be the way the camper sits in the bed. This truck has a large lip at the forward end of the box, on the top of the bed. The rubber bumpers on the camper barely touch the front of the bed, and the exterior wall of the camper hits the lip. Bad enough that it squeaks and on this trip I had to grease it or I would have gone crazy. I am going to have to modify these rubber stops on the camper to get them out another inch or so to keep the camper off the bed rail.

Finally, I have a rubber truck mat in the bed - maybe there is some give to that?

What about adding Cabover struts? I have heard pros and cons, but generally people say they help.

Try adjusting the rear shocks first.

Camper?
Axle weights?

Thx.