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I'm working on a couple of cars that use the MRRC/Monogram Sebring Chassis. With the short wheelbase on these cars (Porsche 911S's) and the inline motor configuration, I need to add some weight to the rear of the car; about 10 gms. But, I'm having a hard time finding a good location for adding weight. There's not a lot of clearance on the underside of the car, so it's challenging to place weight there. The inside is mostly irregular surfaces, not much in the way of flat area to speak of.

Does anyone have experience adding weight to the rear of this type of chassis? If you do, then where and how did you add it? Thanks.

Audi1

Lead sheet along the bottom of the chassis. You can get it from various slot suppliers or golf club stores.
I have also seen folks glue brass sheet to the bottom of the chassis (I would try contact adhesive or good ol' super glue.

I have the Sebring chassis under a Chaparral.

There are Slot.It wheels at all four corners. I fitted Paul Gage 21105 tires on the rear.

The car on MDF is very neutral. No weight added.

Here's a few places.

On my Sebrings I use peel and stick golf weights on the bottom along the chassis adjustment channels like in SRQ's second picture seems to lower the center of gravity quite well.

I ended up using less weight than I had planned, based on some track testing (Carrera plastic track). Both cars now weigh about 76 gms; so about 5gms of weight added to the chassis in each car; slightly more in the FC-130 car.

I usually shoot for somewhere between 60:40 and 65:35 (rear:front), but these cars are heavy in the front and I'm running more like 41% in the front now. Adding more weight to the rear did not make the cars faster and didn't make them easier to drive. I would call them "predictable" now; in terms of handling and that seems like a reasonable place to start.

It's hard for me to get used to 14k rpm motors when I spend most of my time running 20+k rpm motors; whether they're the old Mabuchi black stripe motors from Scalextric and Fly or the newer Slot.It and NSR motors. Smooth, but definitely not rockets in my hands at 10V!

Thanks for all the suggestions on how to locate weight in a Sebring chassis.

Audi1

The only other thing I do when there is not a lot of space, is mix up, some jb weld, and pour it into, the small areas. Admittedly not a lot of weight, but it gets to areas otherwise inaccessible.
One positive is that it flows to the lowest point so keeps your cog as low as possible.
Our rules state no weight under the chassis , so everything must go inside which can make it even more of a challenge. Anyway just another option for you.
Cheers
Bob