Sprinter 4WD Conversion Idea, GMT-800 IFS.
- Thread starter luthj
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luthj
Engineer In Residence
Working through the camber curves in bump and droop. I am considering changing my upper A arm vertical position. Right now I am around 5" bump and 3.5" droop. Any thoughts on whether I would want more travel one way or the other? Its fairly trivial to change at this point. I would just alter a couple sketches, and re-run my bumpsteer tests.
The GM bumpstop seems designed for about 3-3.4" of bump at normal ride height.
The GM bumpstop seems designed for about 3-3.4" of bump at normal ride height.
luthj
Engineer In Residence
Playing with upper arm positioning. Over a 9" travel range, My negative camber gain is going to be between 2.5 and 5 degrees. I can move it around some. I can do less than a degree in droop, and 3 degrees in bump, or set it closer to 1.5 in droop and 2.1 in bump.
I am thinking I want less camber gain in bump, as that would affect braking performance. GM appears to have the gain biased towards bump though.
I am thinking I want less camber gain in bump, as that would affect braking performance. GM appears to have the gain biased towards bump though.
luthj
Engineer In Residence
With high levels of camber gain, the front wheels will go off camber during hard braking due to dive. Depending on the tire, excessive camber can affect contact patch, and thus grip. Though with 75 aspect ratio tires, this is not nearly as much of an issue as it is with low profile street tires.
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wjeeper
Active member
How bad would the camber gain at a bump value you might actually see in a heavy braking event?
Mine was more than I wanted at extreme bump at full droop (without the CV's, and shocks installed) but it was well beyond what the CV's could handle anyway. At strapped droop/ bumped compression probably matters less than at a normal range in my book
I was way more worried about bump steer. The first iteration of steering I did had a noticeable amount and got tossed into the aborted project bin fast! I wish I had kept my calcs, they may have been helpful for you. My drawings were rudimentary 2D drawings, but hey I am a surveyor not an engineer!
Mine was more than I wanted at extreme bump at full droop (without the CV's, and shocks installed) but it was well beyond what the CV's could handle anyway. At strapped droop/ bumped compression probably matters less than at a normal range in my book
I was way more worried about bump steer. The first iteration of steering I did had a noticeable amount and got tossed into the aborted project bin fast! I wish I had kept my calcs, they may have been helpful for you. My drawings were rudimentary 2D drawings, but hey I am a surveyor not an engineer!
luthj
Engineer In Residence
Likely not much. looks like 1.5-2 degrees tops at 2.5" bump. The only exception might be hard braking on a steep downhill, where I would hit the bumpstops. I am probably worried about nothing, and I should just place the arm for optimal travel. Lots of camber change in the normal suspension travel range (for road travel) can cause odd tire wear. I think I am leaning towards a bump bias for camber gain. As traveling at speed on rough terrain (washboard etc) the droop contact patch has a lot more influence on steering feel and stability.
Since I have limited control over rack length/width There can be some advantage to camber gain, as I can use it to cancel some bumpsteer. There is a 2D kinematic method to design for zero bumpsteer. Obviously you want the tie rod to closely follow the instant center throughout the travel range. My inner tie rod joint is not in-line with the inner mount plane. But with a bit of fiddling, and with the caster/camber I was able to get less than .035" of bumpsteer (rod pull/push).
luthj
Engineer In Residence
A good consideration. I was brushing up on soft surface traction and suspension choices. My camber gain is pretty much set by GM with the upper arm length. Its currently about 0.5 deg per inch of travel average across the whole range. That is actually a bit low for such a tall vehicle (lower roll stiffness).
Given only 8" of travel, the total scrub is pretty minor. With the current design the track change is 0.34" in droop and 0.12" in bump.
At this point I think the best approach is to place the upper arm for the best BJ angles. Which is putting about 60% of the camber gain in bump. In the middle 3" of travel there is almost no camber change, so tire wear should be minimized for on road travel. I am starting with about 18 deg of low ARM angle in droop and bump. But I will push that up higher if the BJ and boots look okay. I can design some flexibility in the final placement of the bump stop and droop stop.
As a side note these 2500 brake rotors have to be nearly twice the weight of the sprinter units. The piston bores are about 25% larger area. I don't think the sprinters MC will have trouble with them. If I run out of pedal travel, I can always swap to stainless low expansion lines (not ideal). That should get me a bit less expansion loss.
Given only 8" of travel, the total scrub is pretty minor. With the current design the track change is 0.34" in droop and 0.12" in bump.
At this point I think the best approach is to place the upper arm for the best BJ angles. Which is putting about 60% of the camber gain in bump. In the middle 3" of travel there is almost no camber change, so tire wear should be minimized for on road travel. I am starting with about 18 deg of low ARM angle in droop and bump. But I will push that up higher if the BJ and boots look okay. I can design some flexibility in the final placement of the bump stop and droop stop.
As a side note these 2500 brake rotors have to be nearly twice the weight of the sprinter units. The piston bores are about 25% larger area. I don't think the sprinters MC will have trouble with them. If I run out of pedal travel, I can always swap to stainless low expansion lines (not ideal). That should get me a bit less expansion loss.
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Len.Barron
Observer
Yeah, if you look at the MC bore on a 2500hd pickup you can get an idea of just how much volume they use. IIRC they are 37mm, a lot bigger than most you'll see.
luthj
Engineer In Residence
Looks like the size used on the burban is 37 mm. Sprinter uses a 23.8mm. The 3500 sprinter uses a 25.4 mm unit.
Of course that's only half the story. Without stroke length the bore is not super useful as a comparison. As the hydroboost Isn't directly comparable with a vacuum boosted unit. At least I have an option of upgrading to the 3500 sprinter unit if necessary.
The more directly comparable 1500 suburban vacuum master cylinder is 34 mm.
I'm hoping I don't need to make any modifications to the master cylinder brake system but who knows? I have seen several solid axle conversions that use the factory master cylinder and big Ford brakes up front so that gives me some hope.
Of course that's only half the story. Without stroke length the bore is not super useful as a comparison. As the hydroboost Isn't directly comparable with a vacuum boosted unit. At least I have an option of upgrading to the 3500 sprinter unit if necessary.
The more directly comparable 1500 suburban vacuum master cylinder is 34 mm.
I'm hoping I don't need to make any modifications to the master cylinder brake system but who knows? I have seen several solid axle conversions that use the factory master cylinder and big Ford brakes up front so that gives me some hope.
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luthj
Engineer In Residence
Measured the GM calipers. Pistons are 2.15". I don't want to pull the sprinter apart, but a quick google shows rebuild kits for 1.685" front calipers. So that is 21.6% larger. Pretty well within the realm of sanity. Without assist and pedal ratios is hard to proceed further. The reason for GMs MC bores being so much larger is likely driven by a lower pedal ratio.
With the GM setup, the 37mm MC needs about 1.3cm of displacement to have the pads contact the rotor. Plus extra to account for line expansion. With the sprinters factory brakes this is 1.8cm. With the GM calipers and the MB MC, the displacement is 2.9cm. I need to look the sprinter MC to see what kind of travel it has. I would guess at least 1.2"/ 3cm.
I can't find the 1500 series front piston diameters, but the rears are 45mm or 1.77".
For additional comparison the GM rotors are 325mm and the sprinter rotors are 285mm.
With the GM setup, the 37mm MC needs about 1.3cm of displacement to have the pads contact the rotor. Plus extra to account for line expansion. With the sprinters factory brakes this is 1.8cm. With the GM calipers and the MB MC, the displacement is 2.9cm. I need to look the sprinter MC to see what kind of travel it has. I would guess at least 1.2"/ 3cm.
I can't find the 1500 series front piston diameters, but the rears are 45mm or 1.77".
For additional comparison the GM rotors are 325mm and the sprinter rotors are 285mm.
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luthj
Engineer In Residence
The farthest I can compress the sprinters brake pedal is about 1.1-1.2" which is 2.8-3cm. Assuming another 20% of travel that nets 3.4-3.6cm of MC travel. So its looks feasible. It will be close though. The larger sprinter MC would get the apply distance down to 2.76cm.
Brake balance is going to be affected significantly. The rotors are ~12% larger diameter, and the pistons are 21.6% larger. So net brake bias will be shifted 36% more towards the front. Currently the brake bias is somewhere around 60/40. So with the GM rotors that will become around 75/25? The sprinters ESP/ABS system is brake bias aware, and during heavy braking events will reduce rear brake force to prevent oversteer/spinout. Its a dynamic calculation though, so it just calculates the apparent weight transfer/brake force using the pitch rate and possibly yaw rate when turning.
For long descents, The GM front brakes have about 3 times the heat rejection ability as the MB rears, so that level of balance should be acceptable for preventing one axle from overheating/fading before the other.
Brake balance is going to be affected significantly. The rotors are ~12% larger diameter, and the pistons are 21.6% larger. So net brake bias will be shifted 36% more towards the front. Currently the brake bias is somewhere around 60/40. So with the GM rotors that will become around 75/25? The sprinters ESP/ABS system is brake bias aware, and during heavy braking events will reduce rear brake force to prevent oversteer/spinout. Its a dynamic calculation though, so it just calculates the apparent weight transfer/brake force using the pitch rate and possibly yaw rate when turning.
For long descents, The GM front brakes have about 3 times the heat rejection ability as the MB rears, so that level of balance should be acceptable for preventing one axle from overheating/fading before the other.
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luthj
Engineer In Residence
Still plugging along on small stuff. It will be warm tomorrow, so if I can find the time I will check a local junkyard for a 242D Tcase, and a few random bits I need from GM vehicles. Its going to be some time before I am ready to tear into my van. So I am undecided if I want to do the work of pulling a jeep NAG1 trans. They are heavy, and not a fun job without a good trans jack. At some point I am going to put the Tbar and most of the left side suspension together. Then make a mockup on the floor. That way I can get some measurements of the Tbars positioning. I don't want to put much bending on the Tbar.
Len.Barron
Observer
While it's tedious work, mocking everything up, connnecting all the parts always shows something that you would have otherwise missed, it's also a great opportunity to clean and paint older parts...none of this is a valuable as standing in front of it with your hands on you hips and staring at it...I'm convinced that is the magic..
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