Sprinter 4WD Conversion Idea, GMT-800 IFS.

[luthj]

Ran into a bit of trouble. Verifying ackerman angles with current rack and outer tie rod placement. At full lock I only have about 1 degree of toe out. For 100% ackerman I need around 7 degrees of toe out. Hmm, Not sure if I messed up a measurement somewhere, or if I have a design issue. I need to play around with rack placement to see. Of course I can't really move the rack backwards due to the diff (maybe 1/2" if I shift the diff slightly). GM has about 2" of offset in their center link to clear the diff. I could fab some offset ends for the rack, but that could create some other issues.

 

[luthj]

I made a few adjustments to the model of the knuckle, I was slightly off on some dimensions. That got me near 25% ackerman. If I am okay with some significant redesign, I can shift the diff back about half an inch. That will add about 2-3 degrees to the static CV angles. I can maybe squeeze another 1/4" out of the rack to diff clearance.

Another option is to use an offset on the inner tie rod. This would allow me to use the GM tie rod. I would need to figure out something for the racks boot. My track width is about 1" wider than the GM stock width. Combined with the sprinters rack width, I have about 1" each side to use for an offset. Now an 1" rearward offset, and a move of the diff would get my rods nearly parallel with the rack, which should get me 60% ackerman? Maybe more? Now there is some extra torque on the rack, so I need to do some math to see if it would damage the racks internal bushings.

 

[luthj]

Okay, I figured out why my ackerman measurements were so off. I based my initial sketch off the 1500 front end. There are some important differences between the 2500 and 1500 with regards to steering. The 1500 has the steering arm located farther outboard than the 2500. This is because the 2500s larger brakes get in the way. To compensate, the 2500 moves the centerlink backwards. This added angle gets them the remainder of the ackerman they need. Looks like its around 80% or so.

Moving the rack (or using offsets on the rack ends) back by 1.5" from my current position gets me ~60% ackerman at full lock. With a 1" offset at the rack, and a half inch shift in the diff. Hopefully a 1" lever arm at the rack won't cause damage, and the 2 degrees on the CVs is acceptable.
Unfortunately this means I need to do major rework on my subframe. I am not sure if it would be easier to start from scratch. I would need to redraw about 100 sketches, and make around 60 bodies...

 

[luthj]

Low and behold, the race scene doth deliver an offset rod attachment. The inner joint looks very similar to the sprinter.... Of course some folks say these break steering racks... This looks like maybe a bit more offset than I would need. After some reading, this would likely result in rack failure, not to mention getting the boots to fit. Back to the drawing board I guess.

 

[luthj]

I moved the diff back 1/2", and got another bit from tightening the rack clearance. Still don't have the rack where it needs to be though. I also dropped the whole subframe another 1/2" for better oil pan clearance. That puts the total lift at 3-4" above stock. I think a 3/4" offset rack-end spacer may be the only way to get the ackerman I want. I think that is a tolerable increase in loading on the rack. I am not going to have extreme tie-rod angles either. One option for a stronger rack is to machine a bushing that presses in between the rack and housing. On a related note the GM steering arm is about 9% longer than the sprinters. The sprinter has about 6.5" of total rack travel. 6" is about the max the GM CVs can handle, So a the rack-end spacer will incorporate a ~1/4" shorter stop.


With the diff further back, the output flanges interfere with the support for the lower rear A-arm. It will be challenging to get enough support in this area, as it carries all the vehicles weight.



I am somewhat regretting the 2500 route, as the 1500 with 8.25" diff and smaller CVs would have been much easier to package! There are some minor differences in mounting, but it would be quite easy to adjust the design to accommodate a 8.25" diff. A set of spacer/adapters would be needed to adapt the 2500 CV axles to the 8.25" diff. That would be a decent compromise for those who just need the AWD/traction, and don't do much/any off-road type driving.

You can see how close the clearances are. Hence why I dropped it another 1/2". The rack mount brackets were just to close to the oil pan.

 

[Len.Barron]

It would be interesting to see how the geometry of the Express AWD Vans compare with the trucks and what you have modeled, I've never heard of anyone ever having any trouble with those 1/2ton differentials, the only problems I've ever read on them is the viscous coupler in the transfer case wears out.

 

[wjeeper]

I've never heard of anyone ever having any trouble with those 1/2ton differentials, the only problems I've ever read on them is the viscous coupler in the transfer case wears out.

The AWD express vans have a diff that's essentially S-10 sized. I think they hold up solely because of the lack of low range. Not a very robust ring and pinion combo with that kind of torque.

 

[luthj]

It would be interesting to see how the geometry of the Express AWD Vans compare with the trucks and what you have modeled, I've never heard of anyone ever having any trouble with those 1/2ton differentials, the only problems I've ever read on them is the viscous coupler in the transfer case wears out.

I can post up dimensions if anyone is interested. The 8.25" unit is essentially the same proportions, just scaled down from the 9.25" unit. Despite the "bigger is better" mantra, I have seen few failures of the 8.25" unit in my searching and personal experience. Most seem related to excessive CV angles causing case failure as a byproduct. Of course if you put big enough tires on, I am sure you could break anything. One of the issues that gives these IFS suspensions a bad rap on the web, is badly executed lifts, either without moving the Diff, or using excessive tbar preload. The resulting CV angles are ludicrous at times.

Most of my reason for choosing the 2500 gear as a donor, is due to the heavier brakes and hub assemblies. As well as having the stiffer springs. My sprinter does fine with the factory brakes (which are likely smaller than 1500 units), but I would prefer more margin for heat rejection. Plus 3600lbs is marginal for a 1500 front end.

The AWD express vans have a diff that's essentially S-10 sized. I think they hold up solely because of the lack of low range. Not a very robust ring and pinion combo with that kind of torque.

I believe there was a smaller ~7.25" diff unit that was used in the light trucks. Given its small size, that may have been the route that GM took with the vans.

Edit: Here is a photo of the S10 7.25" unit. Looks just like the 9 and 8" units, just smaller, and no disconnect.

Here is the express unit. Looks pretty similar.

 

[luthj]

With the Diff flanges and rear bushing so close, getting enough structure is challenging, as I can't place any vertical members. I settled on boxing as much in as I possibly can.



I am leaving 1/32" gaps on one or two sides of most of the parts. Laser cutting is really accurate, but leaving a bit of room makes assembly a lot easier. The same goes for all the tab/slot groups.

I was pretty good about keeping my base geometry in sketches near the beginning of my part tree. But I still did a few dumb things with my order of operations, so I went back and created some master sketches, and re-ordered the tree. Now I only need to change the master sketches to adjust component placement. Its been something like 8 years since I did multibody modeling like this.

 

[luthj]

Anyone have experience with the helical gear type LSD on the front? I have used them before on a small track car (rear wheel drive) with good results. For ~600$ it seems like a reasonable upgrade. I will be resetting the backlash and diff preload anyways when I replaced the bearings. So its not really any added work.

https://speedmaster79.com/chrysler-...ine-torqueworm-lsd-limited-slip-differential/

 

[luthj]

Made a lot of progress today. I finally feel that I have something that will meet all the requirements.

Here we are at full droop. I need to check the upper BJ articulation limit. If I am over in droop, I will drop the upper A-arm pivot 3/4".



Here is full bump. Bump steer is nearly zero, Ackerman is 65%. Track width is 65-68" depending on wheel selection. Shown is the sprinter wheel, but a adapter would be needed. GM Dually wheels (16x6.5) with 120mm offset could be used with a spacer to set track width anywhere between 65-70" as desired. Total travel is 7", and is approximately symmetric. More might be possible, but I haven't put the CVs through their range to see what happens.



Plenty of minor design work left, but the geometry and clearances are all there (at least I think so).

The torsion carrier may require modifications. On the Passenger Side frame rail is positioned well, so a drop bracket/link should be easy. On the DS the fuel tank covers the frame rail. There is about 10" free to the outboard of the fuel tank (my grey water tank lives there currently). Any bracket that connections to the frame rail would need a ~8" offset to clear the factory fuel tank. The carrier will just clear the fuel tank. I may cut an inch of height off the carrier, and reinforce with a piece of tubing for the sake of ground clearance.

 

[Len.Barron]

Anyone have experience with the helical gear type LSD on the front? I have used them before on a small track car (rear wheel drive) with good results. For ~600$ it seems like a reasonable upgrade. I will be resetting the backlash and diff preload anyways when I replaced the bearings. So its not really any added work.

https://speedmaster79.com/chrysler-...ine-torqueworm-lsd-limited-slip-differential/

I'm running the eaton truetrac in my front 9.25, I've used them many times in the past in all kinds of vehicles and got nothing but good service/performance. I went with the truetrac rather than the factory unit simply because of my history with them, I'm sure the factory unit performs about the same, given the packaging and overall design being the same I can't imagine there being any real differences.

 

[Len.Barron]

I can post up dimensions if anyone is interested. The 8.25" unit is essentially the same proportions, just scaled down from the 9.25" unit. Despite the "bigger is better" mantra, I have seen few failures of the 8.25" unit in my searching and personal experience. Most seem related to excessive CV angles causing case failure as a byproduct. Of course if you put big enough tires on, I am sure you could break anything. One of the issues that gives these IFS suspensions a bad rap on the web, is badly executed lifts, either without moving the Diff, or using excessive tbar preload. The resulting CV angles are ludicrous at times.

Most of my reason for choosing the 2500 gear as a donor, is due to the heavier brakes and hub assemblies. As well as having the stiffer springs. My sprinter does fine with the factory brakes (which are likely smaller than 1500 units), but I would prefer more margin for heat rejection. Plus 3600lbs is marginal for a 1500 front end.

I was suggesting that because of the frame/controlling arm mounting differences that they may have had to make compromises on steering/CV angles and suspension travel that would differ from that of the suburban and truck...nothing that really matters unless you wanted to know if you were shooting for some optimum goals when something less provides good service.
Your decision to stick with the 9.25" diff is sound, they are really tough units and the big brakes are excellent on the 2500hd trucks I've owned.

 

[luthj]

An often neglected but critical part of the GMT-800 suspension is the bumpstop. This serves two purposes. The first is to limit the bump travel. This is served by the metal nub on the back of the A-arm. The second is to create progressive spring rates. This is done via the rounded rubber bumper. The torsion bars have a linear loading profile (not progressive). For the ideal ride, a progressive rate is desired. This bumper performs that function, adding progressive resistance for the last 2-3" of bump travel. The sprinter has a similar design, and gap ratio. At neutral ride height (about 55% droop, and 45% bump), the bumper should have a 1/4" gap or so. I want to be able to adjust my ride height some, so I need to work out a spacer system to move the bumpstop plus/minus 1/4" in order to maintain factory ride quality.

 

[luthj]

I was scoping for wheel speed sensor and tone ring placement. There is enough room to wrap the outer CV with a tone band.

The Diff output flanges have a bit of clearance. I may be able to weld a tone wheel on the back side, and attach the sprinters WSS to the diff case. The sprinters sensor is about 3" long, so clearance will be tight. I do believe the sensor can tolerate a 2-3 degree non parallel. I am not sure how much play the flanges have. The Sprinters WSS can tolerate ~.03 gap, maybe .060". I guess I need to put the diff back together and check!