For my R129 SL track day car, I'm resisting adding fender flares to the rear, front might need altered a bit.

Take off Pirelli World Challenge slicks seem to be available but won't work so well on 9" wheels. ( see sizes below )

So, when checking rear track widths I've found the 95 to 00 C class W202 is 57.6 In 1464 MM / 90 - 01 R129 SL 60.0 In 1528 mm The 2.6" difference would keep the tires under the wheel wells. This is comparing cars with the same wheel offset.

I know the hub carriers are the same between W202 and R129.
I'm pretty sure the lower control arm and other bits are the same as well.

So the question is, has anyone gone down the path of using a W202 sub frame in a R129 or narrowing a R129 sub? ( or a similar swap ) This isn't a question of can I do it and more of a can I save some steps. Car is getting gutted , caged so doing more welding isn't an issue.

In any event I'm leaning to using a aluminum case 8.8 Ford center section from a Explorer so axle lengths will need to be altered no mater what I do. The 8.8 offers many ratios and limited slip as well. Also, doing work for a scrap metal yard allows me to get parts for scrap price.

Thanks

Fronts are 275/645-18's(275/30-18), 10 - 11.5 wide wheel
Product Code: 2128700
Compound: DH
Size: 275/645R18
Measuring Rim: 11
Rim Width Range: 10.0-11.5
OD (inches): 25
SW (inches): 11.7
TW (inches): 10.0
Rev (Rev/Mile): 796


rears are 305/645-18's( 305/30-18)
Product Code: 2128300
Compound: DH
Size: 305/645-18
Measuring Rim: 11.5
Rim Width Range: 10.0-12.0
OD (inches): 25.5
SW (inches): 12.3
TW (inches): 11.8
Rev (Rev/Mile): 800

I for one would like to know as well. I have seen people go the other way with other subframes and cars. But they basically just made new mounts for the subframe to chassis locations. This way all the control arm points are just factory.
If you are going to this much trouble for different offset why not just get different wheels with the correct offset. Or if you are going to go through the whole rear suspension just redo the mount locations for the control arms to the track that you want.

I have 9 x 18 35 ET AMG monoblocks on the rear of another SL. There is maybe 1/2" left to move the wheel to the inside before hitting one of the suspension arms. ( Pretty sure it is the froward toe link ) Also the inner fender / frame section limits movement.
I think some 98 and up cars have 9 1/2" on the rear so 10" is likely the upper limit without cutting metal.

There isn't much room to move towards the fender.

The least intrusive swap would be a C class rear sub with a 198 MM diff as the SL used this past 95 ish.

As for wheels the plan is to widen steel R class 18" spares, this will allow me to run any offset.

I guess I do not understand then. Narrowing the rear end would have the same effect on the wheel and tire position as using a different offset in the wheel would it not.
As far as interference with the lower control arm that can be solved with an offset in the arm. That arm has very little to no effect on the geometry when you move it inboard at the spindle to make room for the wheels. I did this on my w124. I have approx. a 48 et with 9.5" rims. If you really did not want to move this arm the way I did it would be very easy to make a bent arm that gave the exact same geometry as stock with no clearance issues. My long term plan is to do this.

My goal is to use 10 to 11" wide wheels on the rear. Since I want the wheels under the fenders, a 11 wheel will hit the suspension towards the inside. Since there is a limit to how far I can go towards the center of the car before hitting something, I'm going to cut narrow the rear sub frame or use a C class sub frame.

This would be an alternative to getting a slight amount of distance but I don't think it will be enough before the arm gets springy.

I do not think that you would have to have that much bend in it. The arm is a brake strut for all practical purposes. It also provides the caster role for the rear suspension. A dom tube bent for clearance would have more than enough strength for this.

Did some real life measurements today.

97 C280 rear suspension with 18" x 9" 35 ET AMG Monoblock wheels

Measurements done with suspension in droop: Between the wheels inside 48" , outside 68" , track 58" , hub to hub 58 13/16" .

97 SL320 rear suspension with 18" x 9" 35 ET AMG Monoblock wheels

Measurements done with wheels on the ground: track 59 1/2" ( no other measurements yet. )

So, it looks like 1 1/2" reduction in width is possible using a C class rear sub not the calculated 2.6".

More data

Since the lower control arms between the C and SL are the same I measured between the sub frame bolts. Looking from the rear on the 99 C230 I just picked up, it is 18 3/4" , the 92 500 SL sub I have is 20" No numbers for a 97 E320 yet. Not accounting for wheel width, the C is 1 1/4" narrower. ( These numbers should be the same body for body on the other cars I checked previously. )

The 99 C230 rear diff is a 185 MM 1.1 L unit , boot to boot axle length is 10 3/4" ( this can vary some due to boot location. )

99 C 230 wheels are 15 x 7 37 ET

Another addition.

Measured lower control arm bolt spacing looking from the rear on a 97 E320, it is 21 1/4"

I wasn't expecting the W210 E class to be wider than the R129 SL