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Old 05-09-2017, 05:24 PM   #1
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Default Air Bags, Bump Stops & Getting High

After putting on some miles now with a trip to Vancouver Island I have pretty good idea how the 06 GWV Sprinter 2500 handles the hills and twisty roads.

Slow speed handling is great but the high speeds have the weird wandering thing going on so I'm looking into upgrades or whatever to correct it.

The rig has sway bars, air bags (with in dash control) etc and was ordered from the factory with heavy duty everything including shock mount reinforcing.
Checking things out I have found it has what looks like aftermarket rear bump stops that extend down about 4" but do not touch the axle. I tried adjusting the air bags to drop the rig height down so the bump stops would sit on the axle but nothing seems to change regardless of pressure.

So my thinking is that this is whats causing the sudden corrections at higher speeds i.e. when you steer to correct a 'wander' the rear leans so the bump stops come in contact which makes the suspension firm up (like braking to set the suspension before a high speed corner when racing).

Here is a bit blurry picture with (left to right) tire, leaf spring, bump stop, sway bar.
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Old 05-09-2017, 06:52 PM   #2
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Has it had a wheel alignment? That's what I'd do first.
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Old 05-09-2017, 07:22 PM   #3
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New tires and wheel alignment. Also out-of-province safety inspection.

When I say wandering it would probably be more accurate to say "steering corrections required because of wind or road surface effects on vehicle".
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Old 05-09-2017, 07:38 PM   #4
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New tires and wheel alignment. Also out-of-province safety inspection.

When I say wandering it would probably be more accurate to say "steering corrections required because of wind or road surface effects on vehicle".
The size of the correction is usually pretty important. If you are moving the steering wheel an inch or so, it can get to be uncomfortable for a lot of people. Needing to correct a bunch is usually a variation of understeer, or push in Nascar, meaning the front tires are not holding as well as the rear, so the front moves more in the wind or in reaction to a tilting of the van.

Understeer in wind can normally be improved by increasing front tire pressure, adding bigger rear sway bar and/or increasing rear spring rate, getting better shocks that are higher damping rate. Increasing positive caster when aligned can also help, as can making sure the front is toed in the right amount that is usually about 1/16-1/8".

The long bump stops you see are probably Timbrens, which are sold to be like a helper spring and increase load capacity. I am not a big fan of them because they tend to be very progressive in spring rate, stiffening up too much in short compression. The suspension will often feel like a semi bottoming out and then a kind of "pitching" back off the bump stop. If you need more rear spring rate, which is likely, a new set of rear springs is not all that expensive and much better than Timbrens, IMO. Reducing your airbag pressure might make it worse because the Timbrens will hit more often. Running the bags way up high might be something to try.

Tire brand can also make a big difference. What are you running?
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Old 05-09-2017, 08:45 PM   #5
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The size of the correction is usually pretty important. If you are moving the steering wheel an inch or so, it can get to be uncomfortable for a lot of people. Needing to correct a bunch is usually a variation of understeer, or push in Nascar, meaning the front tires are not holding as well as the rear, so the front moves more in the wind or in reaction to a tilting of the van.

Understeer in wind can normally be improved by increasing front tire pressure, adding bigger rear sway bar and/or increasing rear spring rate, getting better shocks that are higher damping rate. Increasing positive caster when aligned can also help, as can making sure the front is toed in the right amount that is usually about 1/16-1/8".

The long bump stops you see are probably Timbrens, which are sold to be like a helper spring and increase load capacity. I am not a big fan of them because they tend to be very progressive in spring rate, stiffening up too much in short compression. The suspension will often feel like a semi bottoming out and then a kind of "pitching" back off the bump stop. If you need more rear spring rate, which is likely, and new set of rear springs is not all that expensive and much better than Timbrens, IMO. Reducing your airbag pressure might make it worse because the Timbrens will hit more often. Running the bags way up high might be something to try.

Tire brand can also make a big difference. What are you running?
Thanks for the informative reply. To start I am running Nokian Rotiiva AT. https://www.nokiantires.com/winter-t...tiiva-at-plus/

Factory rating for vehicle was 3880kg but GWVans increased that to 4182kg (I'm assuming with air bags). New springs may be an option.

Also factory rear sway bar is 7/8'' not the beefier 1-1/8'' aftermarket one.
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Old 05-09-2017, 08:52 PM   #6
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That is a really small bar for that much weight. Of course the geometry of how it is designed makes a difference in rate, but for comparison our Chevy with 5400# on the rear has a 1 3/8" bar that we installed because it had no rear bar in it. The change was very noticeable and positive.
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Old 05-10-2017, 12:49 AM   #7
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The other part of the title (getting high) was to do with the air bags raising the height of rear of the RV. As I said it doesn't seem to make much difference what the pressure is, although I haven't actually measured it or changed the load much either.

I did pull the bump stops out and they are a Mercedes part - must have been part of the order that Great West Vans put in for this one. Went for a drive and it did seem better, as in that it didn't seem to dart when steering correction applied. Wasn't really windy though.

Will need upgrades when I take this along...
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Old 05-10-2017, 12:00 PM   #8
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If it were me, I'd contact Sprinter Engineering to confirm the GVWR increase by the upfitter is in fact accurate. RV Upfitter label errors are not that uncommon.

The GVWR is derived from more than just suspension parts.

Links:

Can You Boost Your Payload Ratings? - PickupTrucks.com News

Quote:
GVWR and GAWR validation covers brakes (we don’t recommend any full-size truck running anywhere near GCWR without trailer brakes), frame, wheel bearings, springs, suspension arms/bushings, steering pumps and gear, tires, and box integrity.
https://www.airliftcompany.com/works...capacity-gvwr/

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In spite of air spring’s leveling capacity, they do not in any way increase the GVWR or GCWR of the vehicle. It’s up to the operator to ensure the vehicle complies with the GVWR/GCWR at all times.
My understanding of bump stops is that they are primarily there to protect chassis/suspension/steering components from damage. I most definitely would not remove them.

Measurements are a key part of the process to gather info and form the basis for the adjustments that come later. You need a flat and level surface to work from. When you add a lot of weight that far behind the rear axle the front of the vehicle will become lighter. The rear will become heavier by more than just the weight of the motorcycle and carrier. You should be able to see the change in wheel well height measurements.

Restoring the front and rear ride heights to (or near to) the measurements you had before adding the load should help the handling. There's a limit to what can be achieved by air bags alone as you still have to remain within GAWR (both axles) and GVWR.
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Old 05-10-2017, 12:13 PM   #9
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I would not remove the bump stop.

You already have the airbags and the ride is still unsatisfactory?

Have you checked the shocks?


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Old 05-10-2017, 12:36 PM   #10
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I would agree that totally removing the bump stops is a good idea, although that is what Air Ride has you do when you put the bags in a Chevy. They have a stop in the bag itself to prevent overtravel. The bags Marko has even have a real rubber bumpstop in them, which I wish ours did.

I think the question would be if the tall stops are also being used as overloads like a Timbren. If so, replacing them with a stop that just limits damaging travel would be OK, as long as the airbags are there.
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