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Discussion Starter · #1 ·
2021 3500 Sierra Denali SRW is used to tow a 30ft tag along enclosed trailer with race car, tools, spares, etc. Trailer weight loaded (estimate) is 11-12k lbs.

I noticed on our last long tow that the rig is very "bouncy" at highway speeds when encountering things like road surface level changes at construction zones or on/off certain overpasses that may have a different level than the rest of the highway. Annoyingly bouncy, bordering on "holy crap, I sure hope that hitch is hangin' on!"

I'm not talking about excessive speeds here 65-75mph max.

I plan to measure tongue weight - don't have that yet - but I've towed this with other trucks and not encountered similar problems.

Also, just for completeness of info, I use a Reese load leveling hitch.

My question to the Duramax brain trust is whether you all think think this is best addressed with upgraded shocks (I see a lot of folks on other posts have converted to Bilsteins), or an airbag?

I do like the ride quality of the truck unloaded and don't want to alter it much with aggressively valved dampers.

What do you think?
 

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In a new truck that new I’m not sure I’d replace the shocks as a first step… they’re obviously not worn out. Getting the tongue weight compared to your truck’s available payload will tell us how close to max you may be, which can cause the symptoms you described. Where did you have your tire pressure set? Properly figuring that number is also key to supporting the load, but still maintaining some dampening if possible by not overinflating.
 

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Airbags. Have them on my 2020 and it helps take out the harshness of those bumps when towing our 37’ TT.

I have the WirelessOne controller so I can air up/down as I cruise down the road to find that sweet spot.
 
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Discussion Starter · #4 ·
In a new truck that new I’m not sure I’d replace the shocks as a first step… they’re obviously not worn out. Getting the tongue weight compared to your truck’s available payload will tell us how close to max you may be, which can cause the symptoms you described. Where did you have your tire pressure set? Properly figuring that number is also key to supporting the load, but still maintaining some dampening if possible by not overinflating.
IIRC rear tire pressures are at ~85, front ~65-70 psi.

It does not feel at all like tire pressure related spring rate: this is a low frequency oscillation not high.
 

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2021 3500 Sierra Denali SRW is used to tow a 30ft tag along enclosed trailer with race car, tools, spares, etc. Trailer weight loaded (estimate) is 11-12k lbs.

I noticed on our last long tow that the rig is very "bouncy" at highway speeds when encountering things like road surface level changes at construction zones or on/off certain overpasses that may have a different level than the rest of the highway. Annoyingly bouncy, bordering on "holy crap, I sure hope that hitch is hangin' on!"

I'm not talking about excessive speeds here 65-75mph max.

I plan to measure tongue weight - don't have that yet - but I've towed this with other trucks and not encountered similar problems.

Also, just for completeness of info, I use a Reese load leveling hitch.

My question to the Duramax brain trust is whether you all think think this is best addressed with upgraded shocks (I see a lot of folks on other posts have converted to Bilsteins), or an airbag?

I do like the ride quality of the truck unloaded and don't want to alter it much with aggressively valved dampers.

What do you think?
Which other vehicles have you used to pull this trailer and had a better experience?

I ask because if you're used to an older 3/4-ton truck the springs are likely less stiff, and will thus feel more forgiving on bumps. If that's so, then increasing the spring rate with airbags is going to produce the exact opposite of the effect you're looking for.

As a concrete example, my 3500 SRW became a lot bouncier when I went to the 19.5-inch tires (and ran them at 120 psi...), and became bouncier still when I added the Timbren bumper springs. But its ability to handle big weight has been enhanced by both of those modifications.

As another example, try driving a big rig with air-sprung rear end without a trailer/load on. The rear axles may actually leave the ground intermittently.

Edit: I see that you're referring to a low frequency "bounce" which certainly doesn't sound like the spring rate is too high -- I'll give it some more thought.
 

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Discussion Starter · #6 ·
Which other vehicles have you used to pull this trailer and had a better experience?

I ask because if you're used to an older 3/4-ton truck the springs are likely less stiff, and will thus feel more forgiving on bumps. If that's so, then increasing the spring rate with airbags is going to produce the exact opposite of the effect you're looking for.

As a concrete example, my 3500 SRW became a lot bouncier when I went to the 19.5-inch tires (and ran them at 120 psi...), and became bouncier still when I added the Timbren bumper springs. But its ability to handle big weight has been enhanced by both of those modifications.

As another example, try driving a big rig with air-sprung rear end without a trailer/load on. The rear axles may actually leave the ground intermittently.

Edit: I see that you're referring to a low frequency "bounce" which certainly doesn't sound like the spring rate is too high -- I'll give it some more thought.
Right.

Previously I towed it with an '08 F350 DRW with airbags. That rear suspension was stiff as a board and great to tow with, just sucked around town without the load. Briefly, I towed it a few hundred miles with a late model RAM 2500 (borrowed when my F350 engine caught fire and left me stranded...). The RAM 2500 didn't bounce like this, but it did certainly drop low in the rear and lifted the front - more so than the 3500 does.
 

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Right.

Previously I towed it with an '08 F350 DRW with airbags. That rear suspension was stiff as a board and great to tow with, just sucked around town without the load. Briefly, I towed it a few hundred miles with a late model RAM 2500 (borrowed when my F350 engine caught fire and left me stranded...). The RAM 2500 didn't bounce like this, but it did certainly drop low in the rear and lifted the front - more so than the 3500 does.
Are your overload springs engaged with this trailer at rest? If not, I wonder if the initial suspension travel's low spring rate (due to only the main packs being engaged) is causing it. I know that's a stretch, but...I'm stretching. Can't think of any reason why a truck like yours should be squirrelly in front of a 10-12k trailer.

Edit: Another thought. Did you adjust the WDH for the new truck? If your coupler is higher than it was on the other trucks then the tension in your chains or whatever is used to apply the weight distribution may be less in this configuration, which could contribute to the effect. Maybe try tightening them up if possible.
 

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Discussion Starter · #8 ·
Are your overload springs engaged with this trailer at rest? If not, I wonder if the initial suspension travel's low spring rate (due to only the main packs being engaged) is causing it. I know that's a stretch, but...I'm stretching. Can't think of any reason why a truck like yours should be squirrelly in front of a 10-12k trailer.

Edit: Another thought. Did you adjust the WDH for the new truck? If your coupler is higher than it was on the other trucks then the tension in your chains or whatever is used to apply the weight distribution may be less in this configuration, which could contribute to the effect. Maybe try tightening them up if possible.
Great suggestions! Thanks I will check that out.
 

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Airbags would definitely help but it’s important to know your numbers and have the tires properly inflated for the load + properly adjusted WDH as someone else mentioned. That way the fundamentals are solid and you can go from there in adding equipment.
 

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Don't come to Alberta then. Our roads here are so bad. You could turn cream into butter flopping all over the piss poor highways.
 

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Discussion Starter · #11 ·
Update - numbers show that while my total weight is within range, the tongue weight is way out of range. Truck + loaded trailer = ~22000 lbs. So far so good. Tongue weight = 3140l bs. Not so good as the rating is 2000 lbs. Waiting now for some longer tie-down straps so I can shift the car back a bit and re-arrange some spare parts to alleviate tongue weight. Will update again with handling characteristics after tongue weight is properly adjusted.

Also, by the way, my Reese WDH seems to be adjusted fine, as the ride height change over the front axle with and without the WDH are right in spec for the hitch.
 

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Update - numbers show that while my total weight is within range, the tongue weight is way out of range. Truck + loaded trailer = ~22000 lbs. So far so good. Tongue weight = 3140l bs. Not so good as the rating is 2000 lbs. Waiting now for some longer tie-down straps so I can shift the car back a bit and re-arrange some spare parts to alleviate tongue weight. Will update again with handling characteristics after tongue weight is properly adjusted.

Also, by the way, my Reese WDH seems to be adjusted fine, as the ride height change over the front axle with and without the WDH are right in spec for the hitch.
Holy...!

If your truck is a CCSB, that tongue load is applying about 4400 additional lbs to your rear axle (multiplier is roughly 1.4 for that geometry) and taking about 1250 lbs off the front axle. No wonder it handles like junk. With that distribution of weight your rear tires are very likely over their rated capacity as well, and structural failure of the crappy round-tube GM receiver is not out of the question.
 

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Discussion Starter · #13 ·
Holy...!

If your truck is a CCSB, that tongue load is applying about 4400 additional lbs to your rear axle (multiplier is roughly 1.4 for that geometry) and taking about 1250 lbs off the front axle. No wonder it handles like junk. With that distribution of weight your rear tires are very likely over their rated capacity as well, and structural failure of the crappy round-tube GM receiver is not out of the question.
Hmmm... your comment makes me think I made an error in calculation. Here are the actual numbers form the scale.

No trailer: Truck weight 8440. Steer axle: 4880. Drive axle: 3560
Trailer: Steer axle 4060. Drive axle: 6700. trailer axle: 11360. Total 22120

I equated the increased drive axle weight of 3140 as the tongue weight. But on second thought, it should be more along the lines of 2,320, which is the difference between the total truck weight before and after connecting to the trailer. Which comports with your estimate of 1.4x multiplier tongue to axle weight.

Still over the 2000 lb rating, but not grossly so.

Thanks for pointing out my error.
 

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Hmmm... your comment makes me think I made an error in calculation. Here are the actual numbers form the scale.

No trailer: Truck weight 8440. Steer axle: 4880. Drive axle: 3560
Trailer: Steer axle 4060. Drive axle: 6700. trailer axle: 11360. Total 22120

I equated the increased drive axle weight of 3140 as the tongue weight. But on second thought, it should be more along the lines of 2,320, which is the difference between the total truck weight before and after connecting to the trailer. Which comports with your estimate of 1.4x multiplier tongue to axle weight.

Still over the 2000 lb rating, but not grossly so.

Thanks for pointing out my error.
Glad that's sorted out. So your trailer weight is 13680 and your tongue weight is 2320/13680=17% of that. That's high but not stratospheric like I thought it was based on the initial numbers. I bet if you shift the weight back to get the tongue load under 2k and then use a good WDH it will handle just fine. My old travel trailer had numerous "modifications" that caused its tongue load to be 1700 lbs (on a 10k trailer, so about the same distribution as your trailer) and with a cheap Pro-Series Etrailer WDH (and no sway control) it handled absolutely fine. I routinely ran it at 75 MPH and the only time it ever got squirrelly was in the presence of extreme crosswinds going through the midwest.
 

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Discussion Starter · #15 ·
Thanks for your help, JD. I'll be shifting weight around a bit to get the numbers better and will update when I see the results.
 
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