Air Bags, Bump Stops & Getting High

[Marley]

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.

[markopolo]

Has it had a wheel alignment? That's what I'd do first.

[Marley]

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".

[booster]

Originally Posted by Marley

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?

[Marley]

Originally Posted by booster

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.

[booster]

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.

[Marley]

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...

[markopolo]

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

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/

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.

[BBQ]

.

I would not remove the bump stop.

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

Have you checked the shocks?


:-O

[booster]

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.

[Marley]

What made me wonder about the bump stops was 3 things:
1. Mercedes parts diagrams show just a small bump stop at the frame
2. The Sumo Spring replacement for the bumper stop sits right on the axle and,
3. My bump stops have a 2'' gap to the axle and are firm rubber

So, in essence, I'm only getting a small amount of movement in the rear suspension before contacting the bump stops.

The 'ride' isn't that bad at all really it is the handling at highway speeds where it feels like the back end has too much input into the steering. Sitting all the way in the back and looking out the front window (and watching steering) you can really see and feel what is happening. Steering correction and the front moves a little, the body leans then another correction is required to counter the sudden direction change. This is mainly a problem precipitated by wind and/or 'ruts' in the asphalt.

There is a Sprinter upgrade kit that includes larger sway bar, Koni shocks and sumo springs that maybe the answer. Plus an alignment after that.

[booster]

Originally Posted by Marley

What made me wonder about the bump stops was 3 things:
1. Mercedes parts diagrams show just a small bump stop at the frame
2. The Sumo Spring replacement for the bumper stop sits right on the axle and,
3. My bump stops have a 2'' gap to the axle and are firm rubber

So, in essence, I'm only getting a small amount of movement in the rear suspension before contacting the bump stops.

The 'ride' isn't that bad at all really it is the handling at highway speeds where it feels like the back end has too much input into the steering. Sitting all the way in the back and looking out the front window (and watching steering) you can really see and feel what is happening. Steering correction and the front moves a little, the body leans then another correction is required to counter the sudden direction change. This is mainly a problem precipitated by wind and/or 'ruts' in the asphalt.

There is a Sprinter upgrade kit that includes larger sway bar, Koni shocks and sumo springs that maybe the answer. Plus an alignment after that.

I think your assessment sounds reasonable, and goes back to what causes these things. When the van rocks, it may feel like the rear is what is moving, but really the entire van is moving, including the front. As weight goes on and off the front wheels, their traction will change, and the changes in the suspension height will cause some self steering. Add to that the fact that the wind is trying to push you offline, and the fact that all the weight on the back axle gives it more traction than the front (which lets the front move over more than the rear), and you can have a handful.

Since the handling works backwards to most people's thought process, it can get confusing, but you are likely correct that the solution is in the rear, even though I think the problem is front traction.

Generally, in the vans we have found that the addressing of the issue as nearly pure understeer will help a lot, although most of the experience is with Fords, Chevies, and Dodges. They are similar designs, though.

As mentioned earlier, increasing rear spring rate, increasing rear sway bar rate, and increasing shock damping rate are the first places to start, after tire pressures and alignment.

If you look at the Sprinter improvement kit, these are the things it addresses.

As to the parts in the kit. If the swaybar is as big a bar as they make, no problem. If it is smaller, I would go with the biggest. Koni makes great shocks, with very good control and damping, but they can be pretty harsh for some people's taste. Generally, you can get the control of the Koni shocks with better ride characteristics with Bilsteins, if they make the correct ones for your weights. They have variable damping rate valving so they can be smoother on small bumps and firmer on big ones. As I mentioned, I am not a fan of using rubber beehives as active suspension parts, as they just don't come close to real spring characteristics. I would much prefer higher rate springs if that is what is needed to limit travel on bumps. The fact that you have the airbags should really make it so you don't need either the Sumo or normal spring upgrade, as they allow you to carry the load OK, it appears. Assuming that, the very big sway bar would even out the side to side issues of rocking and combined with better shocks might be fine.

Personally, if I had a van that had already been "rerated" to effectively tell you to run it overweight, I would never, ever, hang that much weight out the rear on a hitch carrier. It may put you over the rerated spec, even, and it will be even further away from the original weight allowances. Weight on the back like that can really make a van porpoise on dips and such, because of the reduced and constantly varying front wheel weight and suspension height. I think I would use a small trailer for the bike, which would also allow you to offload some of the other van weight into the trailer. Having the bike on the back will also essentially make it impossible to get your front and rear ride heights back to where they are without the bike, with the front going high if you bring up the rear. This will mess up your alignment settings and handling, even beyond the weight loss. You will likely have to choose to have it aligned right with the bike on or off, with the other alignment being off some.

I think it was Mike Wendlund that got rid of his earlier Sprinter, which was also a single rear wheel one, because his wife wouldn't drive it due to being to hard to control in the wind. The newer ones have wind assistance built into them IIRC, probably for that reason.

[jrobe]

There are tons of great discussions on the Sprinter forum about improving the ride quality and handling for Sprinters.

https://sprinter-source.com/forum/index.php?

I think you could summarize most of this by the following order of Sprinter suspension upgrades (in order of preference and magnitude of effect)

1. Better tires and correct tire pressure.
2. Better rear shocks (this is a biggie, the stock shocks are lousy) - the custom tuned Fox shocks from Agile Off Road and Van Compass would be the best followed by Koni's
3. Better front struts - Koni's probably best currently available
4. Rear leaf spring upgrades - like the leaf spring assist products like the ones from Upscale Auto and Sumo Springs
5. Bigger rear sway bar - improved handling but worse ride quality
6. Sumo Springs

[Marley]

Thanks for being bouncing board for ideas.

I do have off-road motorcycles and 4 wheel SxS with high and low speed adjustments on the shocks and know how that can help.
I also have a Haulmark enclosed trailer but Yamaha TW200 adds about 280lbs plus the carrier so we are under 350lbs which is within the 500lb hitch rating.
I have driven the rig with the bike on back and the handling wasn't any worse. If it is an issue then I'll get an E-bike instead...or use the scooter.

[booster]

Have you been to the scales yet? That would probably be a good thing to do before you decide on anything else.

[Marley]

Yes. Full of fuel but no fresh water:

Front Rated 1751kg. Actual 1540kg
Rear Rated 2431kg. Actual 2160kg

Total Rated. 4182kg. Actual 3700kg
9220lbs. 8160lbs

MB Rated at 3880kgs
8550lbs

[booster]

Originally Posted by Marley

Yes. Full of fuel but no fresh water:

Front Rated 1751kg. Actual 1540kg
Rear Rated 2431kg. Actual 2160kg

Total Rated. 4182kg. Actual 3700kg
9220lbs. 8160lbs

MB Rated at 3880kgs
8550lbs

Did that include people and typical camping supplies?

[markopolo]

Originally Posted by Marley

Yes. Full of fuel but no fresh water:

Front Rated 1751kg. Actual 1540kg
Rear Rated 2431kg. Actual 2160kg

Total Rated. 4182kg. Actual 3700kg
9220lbs. 8160lbs

MB Rated at 3880kgs
8550lbs

I see where you got the 4182 KG previously posted now. You're adding the front and rear axle ratings. There's not much point in doing that because the limiting factor is the GVWR at 3880 KG.

GAWR Front Axle 1751 KG
GAWR Rear Axle 2431 KG
GVWR 3880 KG

Your at 3700 KG on the scales so you only have 180 KG remaining before you exceed the GVWR.

It's good to get the actual front and rear weights at the weigh station like you did but that info is to make sure neither exceeds the corresponding axle rating.

That trailer might be useful when you need extra capacity. The GCWR looks to be 13,550lbs. The GVWR vs actual weight is still a big factor though as there is not much capacity left for tongue weight if staying within 10% to 15% tongue weight of a loaded trailer when towing.

[booster]

There is also an MB spec on hitch weight that says at 400# you need a weight distributing hitch, so at 350# you would be very close on that with no wheels of a trailer for stability. I think you should really add the weight of the spare tire on the rear door and all that hardware, as it is essentially the same place as the hitch, and will be nearly 100#.

[Marley]

Weights today with hitch carrier, bike, fresh water, most supplies, myself and 2 border collies:

Kilograms
800 front drivers --- 740 front passenger

1190 rear drivers --- 1190 rear passenger

All 4 wheels on 3860
Rear 2 wheels only 2350

70*f ambient - mildly windy - steering/driving reasonable (no white knuckles)
60lbs in Firestone air bags
No rear bump stops
60psi front tires
70psi rear tires