When I bought my full suspension mountain bike a few years back, I had a seriously hard time finding any bike racks that met the following realistic criteria:
1. Were suitable for full suspension bikes.
2. Cost less than about $40.
3. Were actually worth their weight in terms of loadbearing. Most full suspension bike racks -- even the super pricey ones -- are rated only for 10 kg or so.
4a. Most of the weight should NOT be loaded onto the saddle post, but rather downward to the tire.
4b. Vertical supports along the sides to make them compatible with clip-on Ortlieb pannier-bags like the Office Bag. Racks without side supports (there are several out there) would either let the bag flail into the tire, or otherwise -- since usually only one bag was applied -- could lead to torsion forces.
I used to carry textbooks and groceries in a backpack or courrier bag. I often had back pain. I've never again had back pain since using this bike rack, since I carry no extra weight whatsoever on my body while biking.
While I do not intend to do failure testing by loading my bike rack until it gives, I can say that I use it with my Ortlieb Office Bag filled alternately with vegetables and juice from the farmer's market or massive textbooks around town, as well as repeatedly traversing rough terrain with ALL my heavy rock-climbing stuff (60 meter alpine rope, 2 harnesses, carabiners, webbing, 1kg water, food, etc.), and there has been no failure, and no sign of instability or bending, in two years now.
I totally love instructables!! Since reading an article here about coffee roasting long ago, I've been roasting my own fair-trade organic beans for years now! Instructables is my favorite website ever (sharing first with indymedia) -- so I've decided it was time to participate!
Step 1: Materials
You'll need some stuff...
A plain old bike rack with conventional mounting hardware. I had one laying around in my attic that is rated to 40kg. It has holes along the flat horizontal panel that makes up most of the rack, if your rack doesn't have holes you'll need to drill some or decide on another way to affix the rack to the top junction of the vertical support. [price: already had mine, costs about $15 bucks new]
Some M6 screws and nuts (about 10 of each), and about 3 extra nuts one size larger (M7). Larger screws might add stability, but mine works fine with M6. [price: 2 clams]
Some type of steel to make the vertical supports. I used flat L-shaped steel intended for holding up curtains (I think). It is about 2 to 3 mm think and roughly 2 cm wide. I decided not to use aluminum to save weight --- I wanted supports that could stand up to real loads and offroad abuse. [price: 4 clams]
2 small angle brackets, about 2cm x 2cm. These will form the lower junction between brake post and vertical support. Mine were also intended for curtains and made of the same steel. [price: about 1.50]
If you have disc brakes like me, you'll need TWO brake posts that screw into the empty holes on the rear suspension element. These MUST have an M5 or M6 threading at the top, which usually allows you to secure your caliper brakes in place (though not in my case). Somebody please add a comment about what these are formally called. [price: about 4 clams for both]
(I didn't buy these, but rather used them at our local bike cooperative, which has a space for about 6 people to repair their bikes themselves on premises while paying about $1 per hour to use all the tools and get advice for complicated repairs)
-- Powerdrill, with about a 5mm bit.
-- A cute little hand tool to apply a threading to the holes you drill, with a 6mm bit.
-- A little hand saw (optional, for trimming the angle brackets if needed).