Sprinter 4WD Conversion Idea, GMT-800 IFS.

[luthj]

I just agreed to buy the whole front end off a 02 2500 suburban with 3.73 gears. Going to put a deposit on it tomorrow, and pick up when I'm back in town on the 16th. $750 out the door, and I don't have to lay in the mud!


Here is a section view. The steering rack is a close fit, but will clear the upper diff mount. I need to measure my drive-line angle at the trans, as the front pinion needs to come up a bit.

 

[whitenoise]

After pursuing several other avenues, I have narrowed in on a 4WD conversion approach for my 2004 T1N sprinter.

First of all, thank you for sharing your adventures with the forum. As a fellow engineer, I would like to hear your rationale for converting rather than buying a different vehicle (apart from cost of buying and time invested, or is that it). Something drastic during your travels must have prompted taking on a surgery of this magnitude. The cost delta between a 2wd and a 4x4 van is massive for sure, and I am sitting here thinking whether 4x4 is really worth pursuing so this discussion would be really helpful.

 

[luthj]

First of all, thank you for sharing your adventures with the forum. As a fellow engineer, I would like to hear your rationale for converting rather than buying a different vehicle (apart from cost of buying and time invested, or is that it). Something drastic during your travels must have prompted taking on a surgery of this magnitude. The cost delta between a 2wd and a 4x4 van is massive for sure, and I am sitting here thinking whether 4x4 is really worth pursuing so this discussion would be really helpful.

It was a tough decision. We don't need 4x4 very often, but when we need it, we are usually very far from help. Our needs are really just better traction on slippery surfaces; snow and sand. We don't rock crawl, or do technical trails, and our van has good ground clearance as it sits. We don't plan on using the extra capability, but our experience has shown that we end up needing every few months, even if we don't plan on it! I have had the van slide into ditches due to "quick mud" under grass, gotten trapped being unable to turn around on crap roads, etc. Not very often, but often enough. If we had any larger of a vehicle ( 21ft long and 8,800lbs), I don't think we would bother with the 4x4. The size of the vehicle itself is just to limiting.

We considered getting a different vehicle. But the amount of time (and decent amount of money) we have in our current vehicle is hard to sell. We would never get the value back in a sale. Our van has known history with us, its unmolested, lower miles (165k now), and has proven itself. The interior fit-out is good for our needs, and I have templates/designs so that I can make changes easily. In addition I am intimately familiar with maintenance, repair, and diagnosis on the T1N sprinter platform. The T1N platform combines some modern safety features with low complexity, and a good diesel powerplant.

The advantages of the approach I have chosen, are that I don't need to slice/dice the vans chassis. Custom parts are few, and the driveline bits I am adding are reliable. I believe it will be much less work (not counting design time) than a solid axle swap on just about any other van platform. Our next major travel plans (2021?) may be a circuit of the PanAmerican highway. Which will include winter driving, often in mountainous terrain. So having the 4x4 on standby gives us more options for planning.

Finally, I really enjoy this type of project (at least for now!). So the work and design is interesting.

Here is an example where you suddenly find yourself on a track/road which no way to turn around, and rough conditions. We were driving a forest service road, and in about 1/4 mile it went from single track to no-track. Then it started raining... With a fair bit of winching and pucker moments, we got through after 4 hours. With 4x4 I would have been able to back up the steep incline to a turn around spot.

 

[Buddha.]

I don't know if you care anymore now that you've found a donor but from what i can tell the HD chevy axle changed in 2011. In 2007 the 1500 was all new but the HD's retained the chassis until 2011 when "something" about the HD axle was improved.

 

[luthj]

I don't know if you care anymore now that you've found a donor but from what i can tell the HD chevy axle changed in 2011. In 2007 the 1500 was all new but the HD's retained the chassis until 2011 when "something" about the HD axle was improved.

Good info thanks. I have a couple other folks who are interested in a DIY using my subframe design. So its good to know what vehicles are compatible.

 

[mgmetalworks]

Sometimes projects of this magnitude are just fun, challenging and great learning experiences. I get asked all the time how I justified the Cummins swap in my van. Financially, it doesn't make much sense. It will never pencil out with fuel mileage savings or "I use the van for XXXX so I needed it".... It was just a project I wanted to do. luthj has a good list of reasons for doing what he's doing but I love to see people do stuff like this just to do it. This forum has been stale with the same ol' same ol' stuff for so long that there's an actual thread about how dead it's been... I'm stoked to see someone going after a cool idea and documenting the process. No rationale needed!

 

[whitenoise]

Projects are cool. For someone who's actively traveling to cut up their primary vehicle and take on a project of this magnitude is even cooler, and most certainly wasn't a decision taken lightly.

@luthj what is your plan to preserve the original front-end geometry or at least evaluate the new suspension geometry. As I'm sure you know, the changes you're considering will impact ride and handling, potentially degrading some aspects of it. If you're going to put in all the work to develop a kit (or more precisely the knowledge base for DIYers to replicate your setup) it would be nice to properly engineer it from the get go.

 

[luthj]

Projects are cool. For someone who's actively traveling to cut up their primary vehicle and take on a project of this magnitude is even cooler, and most certainly wasn't a decision taken lightly.

@luthj what is your plan to preserve the original front-end geometry or at least evaluate the new suspension geometry. As I'm sure you know, the changes you're considering will impact ride and handling, potentially degrading some aspects of it. If you're going to put in all the work to develop a kit (or more precisely the knowledge base for DIYers to replicate your setup) it would be nice to properly engineer it from the get go.

We are about ready to stop traveling and enter a work phase for a couple years. So the van will have some downtime while it gets maintenance and upgrades.

The geometry of these suspensions is not very tough to replicate, or improve for that matter. Compared to a race car which is very sensitive to roll center, squat/dive, and lateral weight transfer, these larger vehicles are more forgiving.

With the strut setup on the sprinter, I am mainly concerned with the following specs.

Caster
camber gain
anti-dive
trailing distance
steering pivot offset
ackerman angle

The sprinter uses parallel lower A-arm pivots, so no appreciable anti-dive, Caster is around 3 degrees, I am going for 4-5, but the strut upper mount and wheel wells are the limiting factor. The steering pivot offset is basically set by the GM knuckle and BJ placement. I am not going to be changing the track width much. The ackerman angle on the steering is going to be a bit higher than previous, this is due to the steering rack getting pushed forward a couple inches by the diff housing. Its not ideal, but should not result in oversteer given the camber/bump steer gains in body roll.

Really most of the geomtry was set well by GM and mercedes. I am making pretty minimal modifications, with the exception of the GM upper A-arm being discarded. This means less caster gain, and the camber gain rate is reduced. The roll center is going to be lower and farther out than I want, but its still better than the lifted-to-the sky or slammed-to-the-ground mcpherson strut vehicles.

Anyways, I will know more once I plug all the GM dimensions into my model. I can do a motion study to see how everything reacts in roll, bump, and with steering inputs.

 

[whitenoise]

I have been reading your build thread due to a personal interest in the T1N platform, and I really like your can-do attitude with all aspects of your build, including your response above. I have professional experience with suspensions in most kinds of land vehicles, all the way from single-seater formula cars to mining trucks, and most things in between. I agree with a lot of what you've said with a few important exceptions, so I hope you will take the below in the right spirit.

1) Achieving the right balance between ride, handling, NVH, durability and cost in a vehicle program is not trivial, and unfortunately a lot of it hinges on getting the geometry right. A lot more than most people, even most automotive industry people think. Even pro-level Race cars actually have a tiny fraction of design considerations in comparison to a passenger car or van though they may be more sensitive. Luckily you don't have to consider all the things that an OEM might have to, but that doesn't mean caution shouldn't be exercised when changing anything from its original state. I don't mean actual components, I mean the underlying geometry. I think a lot of the durability concerns become non-existent because you're grafting on something from a similar GAWR vehicle (I hope).

2) Before you cut anything off, I suggest measuring the existing front suspension geometry as well as possible. This has to be done with the vehicle sitting at its correct ride height (i.e. loaded with your gear), full fuel, correct tire pressures and wheels straight ahead. The idea is to capture all hard-point locations for a motion study, so you know what the current status is, and what your new setup should replicate as closely as possible.

3) I would be careful about changing spring rates and especially front stabilizer bar sizing from original. Directional response (over/understeer) is controlled more by this than any other single thing. It is difficult to evaluate this quickly without the right software but hand calcs can get you quite close despite being tedious. Roll stiffness goes up as a square of the track width, so keep that in mind. For example, a tiny (50mm or 2%) increase in track width might stiffen roll by 5-7% and change the front to rear roll couple balance by a similar amount.

4) Exercise caution when changing anything related to the static steering geometry. Commercial vehicles like the sprinter usually have barely adequate power steering systems, and anything you do to increase the steering system loads (such as the caster change you mentioned, which is a HUGE change) will come back to bite you later. I'm not saying the pinion will shear teeth off but a power steering pump failure is not out of the question.

5) Evaluate ackerman at a low and high steer angle. Say 20 and 40 degrees at the inside wheel. You should be able to achieve at least 60% of full ackerman at both positions.

6) Your comment about minimal modifications may be true. But its better to know rather than assume. Sometimes the smallest changes produce unexpected results, so it will be wise to evaluate in advance before cutting any metal.

Good luck with the project and keep posting!

 

[luthj]

The feedback is appreciated.

1. The GM GAWR for the front is equal to or greater than the sprinter, so no worries there. My sprinter is lifted 2" from stock with a subframe spacer kit. I immediately noticed the change in roll center, which increased the tendency to oversteer. Not dangerous, but definitely noticeable. With the GM lower control arms being longer, I think its possible to remedy this without changing the upper strut mount (not an option).

2. I will have an accurate model of the sprinters front geometry. Since there will be no cutting, I can always swap the subframe over if I have concerns.

3. One of the nice things about the GM front end is that is has similar travel to the sprinter (8 vs 7"), and has similar weight (2500s with the diesel). There are several torsion bar options, which if needed I can swap to change spring rate. My van is tall and heavy. I have changed the rear sway bar to reduce body roll. In my case I also have adjustable fox shocks. I have used these to dial in the low speed rebound damping, which has really made a big difference. The sprinters front spring rate is too soft for a fully loaded vehicle, even with the front sway bar (which is marginal with a high roof van). I am not planning on changing track width (hopefully). Really I think my van drives great for its height and weight class. My only real complaint is that it has too much roll steer, and the lower caster makes it a bit sensitive to cross winds.

4. I hear you with regards to the steering. Due to the lift, my van has reduced caster from factory specs (for north america). In europe they were another 1" lower (with same strut mount), and had even higher caster angles. I need to keep steering ratio roughly the same anyways, as the stability control relies on the steering wheel angle sensor.

5. I will see how the motion study pans out. If necessary I may need to shift the diff back half an inch. This will reduce total wheel angle and increase turning radius some.

6. I will be making a full scale mockup, (in addition to the CAD model) to verify. I am confident that conservative choices for the basic geometry, and using the factory (GM and MB) choices as baseline will provide acceptable performance. I am sure there will be some tweaking with spring rates, sway bars, etc.

 

[luthj]

Picked up the parts today. A little bit crusty, but its all there. Seems pretty tight for 180k miles or so.

IMG_20190116_140557 by J Luth, on Flickr

IMG_20190116_143836 by J Luth, on Flickr

IMG_20190116_143849 by J Luth, on Flickr

IMG_20190116_144539 by J Luth, on Flickr

Getting the torsion keys off the bars was a pain, took about 20 minutes with a 4 pound sledge and air hammer.

A first look shows the lower A-arm pivots are actually offset slightly. The axis are parallel front/back, but they don't share the same width from center.

First impressions are that everything is meaty. Gm is pretty conservative with these 2500/3500 trucks. The brakes and bearings look substantially heavier than the sprinters factory bits. I am going to try and pull one side apart for measurements. From there I will do motion studies in solidworks, and do a rough mockup on the shop floor to simulate everything. I have a used sprinter steering rack, so I can use that without pulling my van apart.

The torsion bars are as follows, this is a 2500 suburban with 6.0 engine.
GK 8615
15528963 (LH)
15528964 (RH)

According to this site they are 8600lb/ft rate, which is a bit less than the max available of 9000. Should be good for a fully loaded van. I do have options as low as 4500lb/ft if needed.
https://www.gmfullsize.com/tech/torsion401.html

 

[luthj]

The GM forums were not lying when they said the torsion keys and bars were a pain. Got them out though. I am surprised how good the condition of this front end is. All parts except brakes appear original. All boots are tight, axles have minimal play, though the drivers side inner has a bit of play. One rotor has pad that has no lining, likely due to a stuck caliper slide pin.

Brakes are very heavy, large venting space, big piston bores. The hub/brake assy+axle+control arms has to be at least 95lbs. I need to add more dead lifts to my workout routine... Diff is pretty light, as its a mostly aluminum case. Diff lube may be original, but had good viscosity, and was pretty clean. I will drain and flush to check for any debris. It will need a reseal, and probably output shaft bearings. Pinion bearings feel good, but I will check the backlash to be certain.

After looking at the wheel options for the GM 8 bolt hubs, I think I will go with that pattern all around. Wheel adapters will be needed at the rear. 90% certain I will need to get custom ones made, but thats the nature of the beast.

 

[Len.Barron]

couple cans of "Easy Off" and a pressure washer...you'll be good to go. Oxy/Act torch does wonders to ease getting those stubborn bits to separate.

Are you going to try and stay with the sprinter master cylinder/brake assist or switch over to hydroboost/gm master cylinder?

 

[luthj]

At this point I have no plans to change the sprinter's MC/booster. It works well, and I am trying to keep this projects scope under control! I am running smaller tires which should be fine with the GM brakes. If it comes down to it, I think there are 17" wheel versions (GMT900?) with larger rotors and calipers that will bolt on.

I need to get some measurements of the GM calipers pad and piston area to compare to the sprinter. The ratios look similar, so I don't see any major brake issues cropping up. The Sprinters ESP/ABS does dynamic brake bias control electronically, so in theory it will be okay with modest changes.

I am hoping to find some time today to get the parts cleaned up, and maybe do a bit of teardown. I don't have my ball joints tools with me currently, so I need to make a trip to the auto parts store for a loaner.

 

[Buddha.]

The GM forums were not lying when they said the torsion keys and bars were a pain. Got them out though. I am surprised how good the condition of this front end is. All parts except brakes appear original. All boots are tight, axles have minimal play, though the drivers side inner has a bit of play. One rotor has pad that has no lining, likely due to a stuck caliper slide pin.

Brakes are very heavy, large venting space, big piston bores. The hub/brake assy+axle+control arms has to be at least 95lbs. I need to add more dead lifts to my workout routine... Diff is pretty light, as its a mostly aluminum case. Diff lube may be original, but had good viscosity, and was pretty clean. I will drain and flush to check for any debris. It will need a reseal, and probably output shaft bearings. Pinion bearings feel good, but I will check the backlash to be certain.

Heresy! Everyone knows GM's IFS is garbage and a solid axle Dodge or Ford is the way to go , lol.

I should have mentioned in my earlier post that when GM changed the chassis on the HD's in 2011 they also went away from the long used wheel bolt pattern of 8x6.5 which is relevant if youre switching the sprinter to a chevy bolt pattern. You don't want to buy the wrong wheels.