Riding with a pillion passenger on a motorcycle
Motorcycles are simple devices - couple of wheels, a frame, and a whopping big engine. Seems a shame to only transport a single person around on one of these things, doesn't it? Well, that's probably why many motorcycles have a pillion seat - a seat directly behind the driver's seat.
Riding with a pillion passenger is quite an interesting experience, primarily because another human being isn't just dead weight - add 12 stone (~170 lbs / 75 kg) dead weight to the saddle bags and back box on the back of your bike, and the experience of riding doesn't change all that much. Add the same weight's worth of human being, however, and you're up for an entirely different surprise.
As it turns out, riding as a pillion passenger is the most dangerous form of transportation - statistically, mile for mile - you could do on the road. As such, in order to pass the UK motorbike test, you have to be able to answer the following questions... And even if you aren't taking your motorbike test, if you're planning to ride with a passenger, knowing the answers might be a good idea:
Things you may need to adjust on your motorbike
Most of the things that need to be adjusted are related to the extra weight on your bike.
Step 1 is to check (and increase, if necessary) the tyre pressure, primarily in you rear tyre, so the bike's handling isn't unduly affected by the extra weight.
Step 2 is to check (and increase, if necessary) the rear suspension - if it needs to be pre-tensed, then most motorbikes have a way of adjusting it to make it harder. You do not want your suspension to bottom out with a passenger.
Step 3 is to check (and adjust, if necessary) your mirrors - the extra weight might mean that your bike is tilting upwards
Step 4 is to check (and adjust, if necessary) your headlight - blinding oncoming traffic isn't good for your health
Things that change on your bike
When riding with a pillion passenger, there are a few things that change a little bit, which means that you have to adjust your riding style accordingly.
First off, your bike is heavier, so your braking distances will increase
Secondly, your bike is still heavier, so your acceleration will decrease. This might mean that you need bigger gaps in traffic than when you're on your own.
Thirdly, you have someone behind you, who will want to look forward. They do this by sitting off to one side, but that means that if you need to do a lifesaver (the quick look over your shoulder to be 100% sure there's nobody in your blind spot), you may have to look around further
Fourth, you'll have relatively less pressure on your front tyre, so your steering will become lighter. This will affect your stability, both at speed and when going slowly
Things you need to tell your passenger before you set off
Riding on the back of a motorcycle is actually surprisingly difficult, and there are a few things the passenger needs to know and do before getting on the bike.
Before getting on the bike, make sure they're wearing a helmet (in the UK, it has to be DOT approved), that it is fastened correctly, and that they're wearing appropriate gear for a motorcycle. Loose things (like shoe-laces etc) should be tucked in, and ideally, the passenger should be wearing the same level of protection as the rider - full leathers are better protection than, say, jeans and a T-shirt.
When sitting on the motorcycle, sit behind the rider, facing forward, with one foot on each passenger foot-peg. Riding side-saddle is strongly discouraged, and would be considered to be unsafe by many police forces around the world.
Hold on to the grab rails or the driver
Don't make signals for the driver - waving cars past or flipping the bird at another driver could cause dangerous situations
Lean with the driver - not more, and not less - if you remain upright, the bike won't turn as predicted, which can cause accidents.