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Old 05-21-2006, 05:06 PM   #1
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We have a '99 40' Alpine with 44,000 miles and have experienced two severe episodes of "porpoising" on our last trip. Porpoising is the term , I have been told, for rythmic bouncing of the front end. It happens, in our case, when traveling on old concrete highway. We usually keep our speed at 63. We have Bilstein shocks and I was told if they are not leaking they are ok. Before I start throwing $$ at trying to solve this problem, I want to ask:
Should the shocks be replaced with this mileage?
Is porpoising common?
Other than driving halfway off the road, is there another way to stop or avoid porpoising?
Your comments/suggestions would be appreciated.

Gary
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Old 05-21-2006, 05:06 PM   #2
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We have a '99 40' Alpine with 44,000 miles and have experienced two severe episodes of "porpoising" on our last trip. Porpoising is the term , I have been told, for rythmic bouncing of the front end. It happens, in our case, when traveling on old concrete highway. We usually keep our speed at 63. We have Bilstein shocks and I was told if they are not leaking they are ok. Before I start throwing $$ at trying to solve this problem, I want to ask:
Should the shocks be replaced with this mileage?
Is porpoising common?
Other than driving halfway off the road, is there another way to stop or avoid porpoising?
Your comments/suggestions would be appreciated.

Gary
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Old 05-22-2006, 03:31 PM   #3
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Yes, 44,000 is about the life span of shocks on a heavy vehicle. A shock can be worn out without leaking (an internal valve can go bad)- or maybe all the hydraulic fluid has already leaked out and it's now nice and dry.

Technically, porpoising is continued bouncing after one ridge or pothole, but I suppose the multiple bumps scenario thatyou encountered could also be called porpoising. It has happened to me with a previous fifth wheel and I know how extreme the motion can sometimes get.

But the situation you describe may be beyond the capabilities of the shocks to stop. The even spacing of the separators in old concrete highways can sometimes set up a harmonic wave in the vehicle suspension. It may happen to one vehicle at one speed, a different speed on another vehicle and not at all on a third. Wheelbase length, suspension characteristics and loading all affect this situation. Often a change in speed will stop the rhytmic pitching motion.

But if this has happened to you on two different roads, or it has happened on the same roads you used to drive with no problem, I would try changing the shocks. You are proably due anyway.
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Old 05-22-2006, 05:17 PM   #4
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Just because a shock has a lifetime warranty, and we don't know yours do, does not mean it cannot fail. The only way to really know is to dismount one of the shocks and check it for bounce and rebound. This is actually hard to do for a heavy vehicles.

There are really two ways of evaluating your description:

1) Your shocks are worn out and need replacing.

2) Your shocks are not stiff enough to handle your vehicle.

I would suspect, that if you have been happy with the ride and how the shocks handle porpoising in the past, that you have 1). The shocks are worn out and need replacing.
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Old 05-23-2006, 03:09 AM   #5
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I had that problem in my '90 Winnebago.
Mainly on the old concrete hiways.
Rear Shocks cured the problem. The fronts were fine (Monroe with coil spring assist).

Good luck and happy crusing!

"Ride it like a Ford"
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Old 05-23-2006, 08:17 AM   #6
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Gary85,
You need to include the chassis info on your motorhome, they are not all the same.
Some Ford chassis have a tendency to purpoise because of the long leaf springs in the front. Some folks have solved the problem with a front stabilizer bar, some have had luck just changing the shocks.
My '99 Ford chassis has never done it.
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Old 05-25-2006, 03:15 PM   #7
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Thank you for your replies. I made an appointment at a local repair facility, a Spartan authorized shop,btw, to check the suspension. The owner of the shop did make a comment that I will pass along. He mentioned that "if I have low profile tires (I do) they will contribute to the problem". He does not like low profile 22.5" tires. My tires are 2 years old, so changing them is out of the question.
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Old 05-26-2006, 04:37 AM   #8
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Gary, it looks like you are heading in the correct direction by taking it to a repair facility. In addition, may I suggest that you have them look into your chassis maker's computer for Service Bulletins. On ours (a Freightliner XCS), there was a Service Bulletin that would have been executed for free under warranty (that was up before we ever purchased our coach) if the owner complained. It outlined changes to be made, one of which was a replacement of shocks with a new design. I placed requests on several forums like IRV2, asking about others with XCS chassis who had porpose problems and all of the replies that I got back were about using the Freightliner recommended new shocks from the Service Bulletin versus people buying Koni or Bilsteins (which aren't even available for my chassis) as replacements. I elected to go with the Freightliner recommendation, too, and put them on myself. Actually, they were easier to install than the ones on my Ford van after I got the nuts broken loose. I've been very pleased with the results and our coach doesn't porpose at all any more.

We have similar 22.5 tires but our MH is shorter than yours and that might make a difference. Our GVW is 22K pounds (we actually run it around 19.5K pounds). More weight on those tires might be a factor, too. Our dealer initially filled them to 110lbs but we weighed all 4 corners and discovered that we could carry 95 pounds in front and 90 pounds in the rear. The air pressure may also make a difference in your ride.
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