Do I need pannier, or spacer rails to install saddlebags?

Quick  answer: probably. I will explain why in the second section of this guide. 
But let's be fair and explore all the options, because in some cases, you can live happily without.

To know if you can mount saddlebags without rails, you need to answer the following question:

Can I install a saddlebag securely on my motorcycle? So let's define, securely.
The saddlebag must never touch the rear wheel, and always stay about 3 inches away from the exhausts. It has to be attached from the top, and also anchored somewhere on the lower end to prevent the saddlebag from flapping up and down.
As you can imagine, pannier rails are helpful when it comes to ticking all the boxes.

With some motorcycles, it's just not possible.

For example on the Ducati Scrambler below, you can't mount saddlebags without some kind of luggage rack.

This Scrambler does not provide any support for a saddlebag. There is no place to anchor the lower end of your bags either. So if it looks empty below the seat, like it does on this Ducati Scrambler, you need spacer rails.
Here's an example of good saddlebag holders on a Ducati 800 full throttle (official spacers from Ducati). These would offer ample support for either the Loxx, or Saddleback system. If you don't have them, go for our Ducati Scambler Spacers, which will allow you to choose any of our mounting systems.

Another problem would be high exhausts too close to where the saddlebags would be. If you have less than 3 inches between your saddlebag and the exhausts, it could burn your saddlebag, or more likely damage its content. Once again, saddlebag holders can help prevent that issue, although you might have to go for smaller saddlebags even with saddlebag holders.

Sometimes it works out.

We offer two universal mounting systems that can work without saddlebag holders.
The LOXX system, and the SADDLEBACK system.

However, the motorcycle must be suitable. If the frame offers anchor points, prevents any contact with the rear wheel, and the motorcycle doesn't have high sporty exhaust, then mounting a saddlebag without rails is an option.
Sometimes, all it takes is to choose a smaller saddlebag for it to work.

Here's an example with the Triumph Tiger 1200.

This Tiger 1200 features handles towards the back seat, the frame offers several tubes to latch on, one side has no exhaust, and the position is way up, away from the wheel. So a Mini Patriot SB - as featured on the photo above - can be safely attached on the left side of the Triumph Tiger without a dedicated saddlebag frame - which is made possible by our Saddleback system.

We've also seen it working with Royal Enfield, vintage BMW's, Honda's classic Triumph's etc.
Sometimes it works out great, and sometimes it's far from ideal. It could mean having the saddlebags positioned in a way that doesn't leave much room for a passenger, just because you have to align your bag with the suspension for example. Sometimes, having the bag rubbing against the suspension is an issue, as it can be very abrasive.
So be thorough and test your setup.

Here's a customer sharing his perspective on the matter. He owns a Bonneville T100, a set of the Mini Patriots SB, and he does not use spacers.

When mounting a single saddlebag without spacers, often the only option is to support the back of the bag with the suspension.
With the LOXX system, the top end will be locked to the separate leather support, which can be secured under your seat for example. 
With the SADDLEBACK system, you have two straps that you can loop around the seat's frame, or a grab rail for example.
The lower end of you bag can be tied to the suspension or anywhere on the frame.

If you are mounting a set of two saddlebags with the Saddleback, it gets easier as you can tie them together and balance them out on the seat.
The following video illustrates a method that you can try out.

Here's another example, where the rider adds a tail bag on top.

To conclude on this topic, here's one example where mounting a set of saddlebags without spacers works perfectly well.
This customer followed the method illustrated in the videos above. The saddle belts are tucked below his seat.

Now let's move on to the second part of the answer.

Why you should favour spacers to mount your saddlebags.

In many cases, having saddlebags directly mounted to the motorcycle frame isn't the best option. It might be doable, but it doesn't mean it's the most secure, or even better aesthetically.
On spacer rails, you will find the setup more secure, more practical, and often better-looking.

On the custom T120 below, thanks to the rails, the saddlebags don't rub against the suspension. Access to the backside (so you can reach the straps and unmount the bag) is easier, and the spacers give adequate room for the blinkers.

It's more secure that way because the rails offer a dedicated support for the bags. There is no room for the unexpected. They allow you to anchor the saddlebag easily from top to bottom. Often they also enable more distance between bags and exhausts.

It's also faster because this extra piece of frame adds a further dimension to your motorcycle that's easy to interact with. You don't need to rely on a tube tucked away below your seat, or deal with knots on your motorcycle's frame.

Finally, it looks better in some cases because the position of the saddlebag is optimal. Let's not forget that it leaves more room for a passenger too.
Without proper support, you will be tempted to bring the saddlebag forward so it rests fully against the suspension. It works fine with some motorcycles, but on others, it might look odd.

So how can I find spacers for my motorcycle?

1. Check out our saddlebag holders section.
Maybe we have exactly what you need. If you own, or plan on ordering Longride saddlebags, it's a good idea to choose Longride supports as well. This means that your rails will likely be compatible with all our mounting systems. This is particularly important for the SIDEKICK system that requires specific frames.

2. Do an internet research.
Our LOXX and SADDLEBACK mounting systems are universal, so they'll work with any spacers. 

3. Ask a garage to build you custom luggage holders. This could be your only option if your motorcycle is customised or a classic vintage, and no stock parts can be found anymore. Building a spacer frame isn't rocket science, we even have handy customers who built them themselves. Of course you need the right tools and experience.

4. Ask us to build them.
When enough of you guys come to us with the same request, we get to work and design what you need. That's how we decided to develop the Moto Guzzi saddlebag holders for example. We developed them and had them tested by real customers.

Above, a custom Yamaha XJR with custom-made side racks. 
It looks great with the saddlebags on, it's secure and practical.