Finding a way for my mother-in-law to use her wheelchair on rough terrain?

If you're talking about a wheelchair vs a separate device, the only one I know of is Ramp. I used to talk with the CEO, Mel Du, and his partner, Amro, last year when I was in Denver, CO.

I was impressed because Mel is paralyzed and so this isn't theoretical for him.

If you'd like to see the product, person and company, here is a link

In addition to padding and softer wheels, you should look into getting her a chair with suspension. My current chair has suspension and I'd never want to go without it now. I've heard they can be harder to push however, I don't have that problem as my chair is electric.

If all else fails, she could preemptively take Tylenol or some other mild painkiller. They work better if used preemptively.

Finding a way for my mother-in-law to use her wheelchair on rough terrain?

Have you thought about trying some sort of seating arrangement with springs build in to absorb shocks and/or additional cushions to offset her discomfort when in the forest trails?

Wheelchairs equipped with oversized balloon-type wheels help provide stability and maneuverability, all terrain wheelchairs can easily travel over a multitude of surfaces. Possibly absorbing shocks as well.

Example of an oversized balloon-type wheel.

Other options could be available if desired also.

The next best thing I could think of is to try out something like a X8-Extreme All-Terrain Power Wheelchair.

The Extreme X8 is the ultimate 4 wheel drive power chair. Primarily an outdoor chair, the X8 will take you through the softest sand beach to the rugged woods, dry creek beds and snow. The patented passive steering system and 4-1,000 watt allow you to go where other wheelchair users have never gone before.

With some luck and a some research you may be able to find a company that would let your mother-in-law road test it for a day on some possible envisioned terrain she would like to explore.

The tires look more absorbent to shock due to rough terrain. It is obvious that some land formations will not be suitable with this device. One could add additional cushions to ease the back pain of those with chronic back sufferings.

I would actually get the person interested in using such a wheelchair to get the appropriate approval of health care professionals first. They may also be in a position to let people know of where they can get grants to offset the cost. They may also be in a position to let you know where you can rent such equipment.

You've already mentioned the fifth wheel and shock absorbing wheels so I won't go there again. I'm also assuming you've done the basics, like upgrading the cushions.

Having done my fair share of pushing manual wheelchairs, even pushing them on uneven paved surfaces is hard enough, never mind going offroad. If buying an all-terrain electric wheelchair is not viable (and it's not viable for most people), then I'd suggest starting by thinking like a cyclist.

The baseline wheelchair wheels are very much like road bicycle wheels, narrow, high pressure and hard, with minimal tread, that's if they're pneumatic at all, many are just solid rubber. The chairs are effectively designed for hospital corridors, not the real world.

You can start by looking at wider softer tyres to keep the chair from sinking in and softening up the harsher bumps. Google tells me these are easily available with suitable wheels (at least locally here) for a moderate price.