Front rack for front suspension bike? What are some good options for a sturdy front bike rack (for carrying 30 to 40 pounds) on a front suspension bike?
Nothing is suitable for your requirements as-stated.

30-40 pounds is 15-20 kilograms.  That's like carrying two or three complete bicycles on your handlebars.  The weight will decrease the quality of your steering and will pre-load the suspension at all times.

That much added weight will increase inertia, make the bars slow to turn, and once turning will resist stopping.  So it will feel laggy and lethargic.

Additional weight also increases the chance of a front-wheel step-out on turning.  Normally a bike has a balance of about 40% of your whole weight in the front to 60% of your weight on the rear, or 40/60 to 30/70.  Adding 15 kilos on the front will make it 50/50 or worse, front-wheel heavy.



Experience it

For a simple demonstration to help make the point clear, go find something weight about as much as your heaviest intended load.  A half-bag of Cement, 40 blocks of butter, or 20 litres of water would be ideal (don't use small children as test weights, they're squirmy and loud when dropped.)

Then put the weight on top of your bars or on your forearms.  Stand over your bike, and try to shuffle or scoot your bike back and forth, and round a turn or in a circle.  Feels super unstable, right?  Especially if you bump through something that's not flat, or some gravel.  Now imagine that on the road ?



Alternative Solutions:

The bike can carry substantial loads.  Your problem is limiting it to "over a suspension fork"

1) Lock out the suspension fork, if you can.  Downside of this is that you're now riding with all the weight of a useless suspension fork.  To use your suspension, you'll have to unbolt the front rack completely.   Additionally, there's a chance the lockout will self-unlock, if you hit a large-enough impact.   This will upset any rack, because its mounting points have just moved in relation to each other.

2) Change your suspension fork for a rigid fork, and fit a normal front rack permanently.   Here's a heavy-weight front rack rated for "up to 45 lbs/20.5 kilograms"

  Notice how it bolts to the front wheel axles, and has mounts higher up that attach to your upper forks (ie where your bike has sliding stanchions)

3) Cargo bike!  Probably outside budget, but you could have a massive load carrying ability on your bike.  Some designs have the wheel out the front, some have two wheels, some have the load at the rear.





4) Bike Trailer - again not exactly what you're asking for, but at least this option is removable easily.  Most good designs use some sort of permanently mounted hitch on the rear axle which stays on your bike, and the trailer disconnects with a quick-release coupling like a car trailer ball joint, or some sort of kingpin like a truck trailer.




I personally use an old kid's trailer as a cargo trailer, and it works well enough.  Mine's rated to 20 kilos of liveweight, and I've found 50 kilos to be a comfortable maximum.  I have towed 100 kilos in it but that was ridiculous.



Options like Panniers and back packs aren't really suitable for that level of mass.  Backpacks really should be under 5 kilos, and ideally not even used at all.  Panniers (carrier bags) are generally cloth and are generally limited to 10-20 kilos depending on design and construction.

Final thought - depends a lot on what you're moving, but sometimes paying money for delivery works out better financially.  If you only get big things like this once a year, its hardly worth spending a lot of money on it.  Delivery fees can be surprisingly affordable or ridiculous, so plan ahead.