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.
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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).
Step 2: Bottom Junction
To make the bottom junction:
First, screw the brake post into the suspension element on the left and right sides of the bike.
Next, attach the angle bracket to the top of the brake post using a screw (M6).
It might be possible to attach the angle bracket to the frame directly by screwing some kind of large screw into the brake post hole, thereby obviating the need for buying brake posts and probably increasing stability. It may eventually wear down the finish where the angle bracket is attached, however (maybe add a layer of rubber from an inner tube between finish and angle bracket?).
Step 3: Rear Support to the Axel Area
This step is pretty straightfoward, possibly with a twist.
This step will depend on what kind of frame you have.
Attach the bottom part of the bike rack to your rear suspension element according to the instructions for that particular frame.
My frame has a long threaded rod with an eyelet at the end of it. The eyelet had to be attached somehow to the frame, it was intended for extra brazed-on nuts near the axel present on some bikes. I had wanted to attach directly to the axel itself for maximal strength (like some of the high end frames for suspension bikes), but decided against this approach, because it would complicate my removing the tire to fix a flat...
I drilled holes into my bikes rear suspension element bilaterally. It impressed the guys at the bike store that I was willing to void my warranty after one month for a DIY experiment.
Attaching the right side was no problem -- jus tscrewed it into the hole I drilled and threaded.
On the left side, however, the rod with the eyelet had to somehow clear the disc brake apparatus to attach to the frame. I didn't want to bend the rod, and so, following a suggestion by the local bike store, I built a "tunnel" of 2 M7 nuts, and passed my M6 screw through this and into the rear suspension element of the bike. Attach a nut and make really sure all this is securely tightened.
Step 4: Assembly -- Final Touches
First, attach the short segment of the L-shaped vertical support to the bottom of the bike rack with a few screws and nuts.
Then, once the bike rack is already attached to the bike (by the supports leading to the axel area), affix the long part of the L-shaped vertical support to the small angle bracket, and you have yourself a solid bike rack in place.
Remember, this rack rides on the rear suspension element, not the bike frame. This means it does not itself enjoy the benefits of suspension -- don't put your laptop, fine china, or kitten on it. It is ESSENTIAL to test make sure the front of the rack won't collide with the back of the bike frame at any point. You can test this by adjusting your suspension choke to be way soft and then lean down on the bike while eyeballing the rack. If it comes close to colliding, you'll have to loosen the screws on the angle bracket and angle the rack posteriorly away from the frame and re-tighten it in that position. Mine doesn't come close to colliding.
Do a pre-flight check:
- Recheck that all the screws are tight.
- Trim nonessential metal off the angle brackets with the saw to protect your fingers, tires, and pant legs. Trim enough from the angle brackets, if necessary in your case, to let you remove the tire without fiddling with the angle brackets.
- Then put your pannier(s) on the bike (40kg of self-roasted coffee beans, or a boombox for Friday's critical mass, perhaps), and go for a test ride!