Back rack good for commute?

You are 100% right, a backpack on the back while riding is a very bad idea. However, I cannot wholeheartedly encourage you to use a back rack:

• If you plan to put your backpack directly on the back rack, forget it. It won't stay where you want it, and either start interfering with your rear wheel, your drive train, or simply fall onto the pavement. Back racks are simply not designed to secure a bag.

• If you plan to put your backpack into a basket that rides on the back rack, this will work for a time. However, typical bike baskets are simply not built to endure being used on a bike for long. They will start to deform quickly, and pretty soon the two wire teeth that secure the basket to the back rack will break off.

As such, the smart idea (imho) is to forget about your backpack, and switch to a bike bag that clips to the side of your bike. Afaik, these are designed for normal wear and tear on a bike, they are typically waterproof, and I assume that they will last some years. (I do not have personal experience with these as I've been opting for a DIY, extra heavy duty solution myself over the past decade.)

If you want to try a carrier/parcel rack, then there are plenty to choose from. Yes they are definitely useful, if they suit your riding style and luggage needs.

Seatpost cantilevered vs struts Some carriers are simply beams that clamp around your seatpost and go backward. Their capacity is somewhere around 5 kg/10 pounds.

Beam rack and special rack bag. These have cloth panniers that unzip and provide extra storage.

There are also carriers that have some form of attachment down on the rear dropouts - these are often rated to 20/25 kg or 40-50 pounds, and can also carry panniers/saddlebags as well as a top-bag.

This one has 4 attachment points, which are all bolts rather than clips or straps. It is set up for a dedicated rack bag, and also clip-on panniers.

If you have rear suspension then this affects how the carrier can be mounted - often rear suspension means you can only have a beam rack or have to mount the rack to the seat stays only, like this:

Front racks are a thing too - they go over your front wheel and provide a platform or a basket for carrying things.

that one is also set up for side panniers over the front wheel.

Top-of-rack bags in lieu of the backpack, you can get dedicated bags that clip onto your carrier top and unclip to go inside. Some clip onto the handlebars via a permanently mounted attachment point.

Personally I wear the backpack because its more aerodynamic :-) There are showers at work and I have spare clothes, so sweating is just part of the ride.

You can also explore frame bags, which essentially fill the triangle in your frame.

Lastly, if you need to carry your bag away from the bike and like the idea of backpack straps, there are panniers that have backpack straps which can be stowed under a cover while on the bike, and the hooks can be stowed when wearing the bag. For example:

When I commute a short distance, I currently use a backpack, but agree it's not ideal.

I occasionally do a long commute though, and in that case I use a large saddlebag (mine's 15 l but I roll it to about half size. With what I keep in work that's enough. The downside is it isn't easy to carry off the bike. It's not designed to take a shoulder strap, though one day I'll find a way to fit one; it's also a bit fiddly to get on and off the bike.

Another option for light stuff is a handlebar bag (this is the new version of the one I have). I have a couple with shoulder straps, and another that's a real pain to take off/put on. The former are good for clothes/lunch, but get in the way of handlebar-mounted lights, and you won't get a laptop bigger than about 10" in there (my chromebook just fits diagonally). Over about 3kg and they can affect the bike handling

Counter to the answer that 'cmaster - reinstate monica' gave, you CAN put your backpack directly in a basket on a rack. It's easy!

• Install a rear rack as many other answers suggest. If you're not comfortable doing this yourself, have a shop do it for you. Get a full rack, not one that relies on just clamping to the seatpost.
• Get a milk crate as a basket. Don't laugh! Milk crates are often easy to find and can be free if you look around hard enough. They're tough and are not affected by water or temperature extremes. They have wonderful handles that assist in moving your bike around.
• Attach the milk crate to the rack using metal hose clamps. You might be tempted to use zip-ties. Zip-ties do work (for a while), but become brittle with age and will break at the worst possible time (when your crate is heavily loaded).

Putting your back-pack in your milk create is easy! Just throw it in! The high walls will make sure your bag stays in. If you're concerned about it falling out, use an old innertube tied between the handles on the side. (An old innertube makes a great tie-down for other things in your crate as well!)

Your bike is now much more useful. You can stop for a bag of groceries which fit nicely in the rack (and your back-pack can go on your back for that leg home from the store).

Do a google image search for "milk create bike" and you'll see some examples.

Yes, very. I have commuted with the backpack on my back rack for years, partly also because after a while my back started to hurt from somewhat heavier loads on daily commutes. By the way: If the issue is only sweat I can highly recommend backpacks which rest only on your shoulders and your lower back, like this one: The back facing side has two crossing fiber glass poles pushing it outward so that the entire back is exposed to the air.

Back to the back rack: My typical setup consists of a rather arbitrary back rack (the price is only important for reduced weight) combined with one or two inner tubes as bungee cords, run lengthwise. Inner tubes have excellent elasticity and longevity even when exposed to weather.

The exact setup depends on the shape of the bike, the back rack and the tail-light (!). The latter can serve as an excellent "hook" which holds one end of the inner tube if it looks like this: These lights are surprisingly stable. The other end would be looped around the frame or the frame-side end of the back rack. If there is no light or other "hook" you can make a knot (which doesn't adapt easily to varying loads).

The optional second inner tube can be longer and hold large loads, or can secure a load from sliding sideways by leading it like "wings" over the outer edges of a load.