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Old 05-27-2012, 05:08 AM   #1
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First time towing - Porpoising help

Towed the AS for the first time! Trip was from Virginia to New York about 9 hours... All I can say is concrete road ways suck! Most of Pa the AS and truck bounced up and down like a porpoising dolphin. I'm towing a 31 foot Limited with a 2003 GMC Sierra Hd 1500... Towed the AS just fine... Was wondering what adjustments can I make to my current set up to reduce this effect.
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Old 05-27-2012, 05:33 AM   #2
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Few questions....

1) What hitch are you using?

2) Tire size and load rating on your truck

3) When you say 1500 HD for your tow vehicle, is that the rebadged 3/4 ton that GM offered for a few years with the 8 bolt wheels, 16" load range E tires, etc?
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Old 05-27-2012, 07:31 AM   #3
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Some concrete roadways are worse than others, especially as they age. We did the hitch adjustment routine and it may have helped, but the solution is to get off those roadways whenever possible. When not possible slow down as needed to minimize the effect, and this could be a lot. I think its very bad for the truck and trailer (and us) so I do everything I can to avoid those roads.

Some swear by the "Airsafe" hitch and claim this will relieve the problem. The Airsafe can be used with a standard weight distribution/sway control hitch. I'm tempted.

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Old 05-27-2012, 10:17 AM   #4
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Good replies so far, type of hitch is really important is it weight distributing?

Another question have you replaced or when was the last time replaced you the shocks on your nearly 10 year old truck?

If the shocks are even slightly worn out (meaning not enough to be noticeable when not towing) they can still be a major problem while towing.
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Old 05-27-2012, 11:26 AM   #5
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The 2003 1500 (1/2 ton) was really virtually the same as the former 2500 (3/4 ton) GMT 800 series trucks. They did have eight lug wheels and I suspect that the axle ratings/spring rates/etc. were the same as the standard duty 2500 series. GM also had a heavy duty 3/4 ton 2500 series that had the frame that was approximately 2" thicker at the same time. The HD 2500/3500 with the thicker frame could accommodate the height profile of the Allison 1000 series automatic transmission. All of these vehicles had the same wheels (standard and optional) but the HD 1500 and standard duty 2500 series had semi-floating rear axles where the HD 2500 and 3500 series had full-floating rear axles. The HD 1500 also came with the 6.0 L engine and the 4L80E transmission. In essence, 3campers' tow vehicle is the has the same drive train as my 3/4 ton Yukon XL/Suburban.

I agree that having the weight-distributing hitch properly set up and decent shocks on the tow vehicle is essential. However, the expansion joints on a lot of concrete roadways can be really uncomfortable even with an optimum setup of tow vehicle and trailer.
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Old 05-27-2012, 12:30 PM   #6
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I have an Airsafe and it reduces this effect tremendously... makes for a much nicer day.

Bouncing can be related to shocks being worn out... when it is unhooked, jump up and down on the read bumper of your truck, then jump off. If it bounces at all, the shocks are worn out. It should just pop back up to level.
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Old 05-27-2012, 02:10 PM   #7
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Yes, I've heard Airsafe helps. This supposedly happens more with heavier tow vehicles. Though I have a buddy who tows with a half-ton and it happens just as much.

It's always the fault of seams on aging concrete highways. Nothing to do except go slowly until you get to a better stretch of highway. I've got my list of regional bad stretches of road and will avoid them until they receive an asphalt topcoat. Getting down as low as 50mph can get the jerking into an acceptable range. I know a stretch where 45mph is too fast...
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Old 05-27-2012, 02:19 PM   #8
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Before getting the AirSafe hitch, porpoising was a fact of life, We had porpoising with two SOB's and our AS using two different hitches. Since the AirSafe, two years and 34 states later, we've had not one occurrence of porpoising, even on the roughest highway in the USA, US-90 in LA.

Slowing down is still a good idea, even with the AirSafe.

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Old 05-27-2012, 02:24 PM   #9
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My 2012 2500HD did the same thing when we left the dealer's lot. Not a real big problem since that one instance. We have a 30' as well and are using an Equalizer setup.
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Old 05-28-2012, 06:29 AM   #10
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Quote:
Originally Posted by Midnightmoon View Post
Few questions....

1) What hitch are you using?

2) Tire size and load rating on your truck

3) When you say 1500 HD for your tow vehicle, is that the rebadged 3/4 ton that GM offered for a few years with the 8 bolt wheels, 16" load range E tires, etc?
All that info above about the truck is correct. The tires are 17in BFG's with an "E" load rating. The hitch is a "Pro Series" V-5 rating. Weight distribution bars are 1,200 lbs. Shocks? Hitch came with the AS!

Had it on the scales almost empty:

Steer axle 3380
Drive axle 3420
Trailer axle 6140
Gross weight 12940

Hope that helps...

Mark
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Old 05-28-2012, 07:17 AM   #11
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Porpoising happens when less-then-perfect roads cycle (on and off) more weight to a given location then the suspension is adequately capable to support. So you have two real general choices of resolution: distribute the weight so it is not concentrated in one location (which is what we are trying to do with weight distribution bars), or modify the suspension such that it can handle the weight. I guess you could say there is a third choice: don't drive where the road are less than perfect.

The problem is that there are so many different opinions out here on how to go about making the related changes, that any advice given starts a big debate ... If you want to read about the multiple arguments on how to fix this problem, there are multiple threads that already exist and provide hours of reading. Use the search feature using buzz words like porpoising, suspension, weight or distribution.
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Old 05-28-2012, 08:01 AM   #12
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Quote:
Originally Posted by 3campers View Post
Had it on the scales almost empty:

Steer axle 3380
Drive axle 3420
Trailer axle 6140
Gross weight 12940

Hope that helps...

Mark
Mark, It would be interesting to see what your truck axles weigh without the trailer attached, so there would be some comparison of before/after weights. Those weights look good at first glance, but they would be more meaningful compared to unhitched weights.
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Old 05-28-2012, 11:10 AM   #13
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Joint Spacing

I know of several concrete roads in the State where the joint spacing of the concrete slabs or bridge decks harmonizes with the wheel spacing of the truck, or the spacing between the rear wheels of the truck and the trailer wheels, just about perfectly. IMHO the only solution in that situation is a different route or a different tow vehicle!!! This is with respect to a tow vehicle/trailer combination that is otherwise very smooth on even the bumpiest of roads.
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Old 06-01-2012, 08:02 AM   #14
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Here are the weights for an unloaded 2001 Chev regular cab long bed 2500HD with a 8.1 L and Allison trans:

steer axle: 3280
drive axle: 2460
gross: 5740

Weighing the truck separately will let you figure out the tongue weight. At a CAT scales, you can re-weigh within 24 hrs for $1. I weigh the whole set-up, pull off the scales and pay, ask for a re-weigh, then unhook and weigh just the tow vehicle.
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Old 06-01-2012, 10:09 AM   #15
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Quote:
Originally Posted by GetOutDoors View Post
Porpoising happens when less-then-perfect roads cycle (on and off) more weight to a given location then the suspension is adequately capable to support. So you have two real general choices of resolution: distribute the weight so it is not concentrated in one location (which is what we are trying to do with weight distribution bars), or modify the suspension such that it can handle the weight. I guess you could say there is a third choice: don't drive where the road are less than perfect.
BINGO!! I agree.
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Old 06-01-2012, 11:31 PM   #16
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[QUOTE=GetOutDoors;1152980]Porpoising happens when less-then-perfect roads cycle (on and off) more weight to a given location then the suspension is adequately capable to support. So you have two real general choices of resolution: distribute the weight so it is not concentrated in one location (which is what we are trying to do with weight distribution bars), or modify the suspension such that it can handle the weight. I guess you could say there is a third choice: don't drive where the road are less than perfect.


I agree with the last sentence, only.

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Old 06-01-2012, 11:46 PM   #17
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Doug, tough to do in WA State.
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Old 06-02-2012, 12:00 AM   #18
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Hi, all the adjustments, or modifications, to your vehicles can't change the shape of the roads you drive on.
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Old 06-02-2012, 05:29 AM   #19
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Cut and pasted from another thread on a similar subject:

"We have towed our 19-foot Bambi from Phoenix to the SF Bay Area numerous times with both a 3/4 ton Chevy crewcab with a long wheelbase and a 1/2 ton Tundra CrewMax with a short bed. The stretch of I-10 between Indio and LA is really bad with harmonic dips that get both rigs bucking like a bronco. I don't think switching tow vehicles will help much; the real problem is the roadway. We now take some parallel side highways that are much smoother to avoid this stretch."

There is a lot of discussion on truck forums, too. It's not the tow vehicle suspension or shocks, or the hitch, or the weight distribution in the trailer; it's the roadway. The old concrete slabs sag in the middle and pitch up at the joints; and when the frequency (speed) of the tow vehicle and trailer match the length between the joints, random bouncing becomes harmonic porpoising. The only way to reduce this effect is to speed up or slow down to reduce the harmonic oscillation. This will not stop the bouncing, only reduce the magnifying effect. Or, use another route until road crews grind down the high spots at the joints or repave with asphalt.
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Old 06-05-2012, 08:35 AM   #20
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Doug, tough to do in WA State.
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LOL ... but, wait until you hit MT ... even worse ... NOT LOL ...
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