Their bikes are either too large, or too small, or just setup wrong for their bodies.
We all have different length arms, legs, and torsos, and the manufacturers know this, and accommodate bike frame sizes from small to extra large.
So, how do you size a bike to your body, like a well tailored suit? You need to understand a few basics of bicycle fitting before buying a bike.
As I mentioned, get a good bicycle manual like Zinn and the Art of Road Bike Maintenance or Zinn and the Art of Mountain Bike Maintenance, and read the chapter on bike fit first.
When choosing a bike, one of the important lengths is your reach to the handlebars.
This is determined by the top tube length of the bike frame.
The top tube is the tube that goes from the seat to the steerer, or head tube.
The steerer tube houses the shaft that is attached to the front fork and is fastened to the handlebars.
Each manufacturer has slightly different top tube lengths, so try sitting on the bike you're interested in and see how you feel when your hands are on the handlebar grips.
If you feel cramped, and your elbows are sticking out, the top tube is too short.
Another test is to have a friend hold the bike steady while you're sitting on it.
Place your pedals at the 9 and 3 o'clock position, and turn the handlebar toward the knee that is facing forward.
If you hit your knee with the handlebar well before the handlebar is close to 90 degrees to your body position, the top tube is too short.
In either case, you need to look at the next larger size frame.
If you find it difficult to reach he handlebar grips with your arms locked out and fully extended, the frame is too big.
Go to a frame one size smaller.
Another important length is the seat tube.
The seat tube is the tube that holds your seatpost and saddle.
It is connected to the rear of the top tube, and extends down to the lowest point of the frame called the bottom bracket housing.
The bottom bracket housing holds your bottom bracket which is like the crankshaft on an engine.
The crank set, pedal arms, and pedals are attached to either end of the bottom bracket.
And of course, you are the engine that turns the pedals.
The last tube is the down tube.
This tube connects the steerer tube to the bottom bracket, to form a triangle, which gives strength to the frame.
A triangle in engineering is the strongest form of a structure.
Unless you're building a custom bike, the top tube, seat tube, and down tube lengths are preset by the manufacturer in sizes ranging from small to extra large.
Bike manufacturers do not standardize sizes; some run slightly smaller, and others slightly larger.
There are also geometry differences in the overall height of the frame, and length of the seat and chain stays that affect how the bike handles and rides.
The seat stays run from the top of the seat tube, to the rear wheel axle.
The chain stays run from the bottom bracket and meet the seat stays at the rear axle.
Another factor to consider is the weight of the frame.
The lighter the frame, the quicker the bike will feel, because there is less weight to move as you pedal.
However, a slightly heavier frame with lighter components can zero this out, and actually be lighter when fully built.
The lightest frames I've owned have been Cannondale and Ellsworth frames for mountain bikes.
For road bikes, there are many to choose from, using high tech materials like carbon fiber and titanium.
If you are still deciding on a bike brand, ask your friends about the bikes they ride, and find out the positives and negatives of each brand.
Take their bikes for a test ride and see how it feels.
I've had some bikes that just felt totally wrong for me, and they didn't last long before I was looking for something else.
Once you've selected a bike, it's time to fine tune your body fit to the bike.
Remember, the bike is going to be tailored to fit you, and you only.
Fitting the bike to your body is critical for efficiency, center of gravity, comfort, performance, and safety.
Once you have the right size frame, the top tube length should be correct as described.
If, unfortunately, you have one of those bodies that falls between a small and medium, or medium and large frame, a choice has to be made.
The top tube length can be adjusted to your reach by making the tube tube appear longer to your arm length.
There are different length stems that are shorter or longer to accommodate for a shorter or longer reach.
The stem is the component that attaches to the fork's steerer tube shaft that goes through the head tube.
The other end of the stem bolts to your handlebars.
I don't recommend a stem length longer than 90-100mm.
Anything over that length will affect your center of gravity and change the steering geometry.
This can create a dangerous situation where the front wheel turns unpredictably because of the center of gravity being shifted too far forward on the bike.
It's hard to describe, but an extra long stem can cause the steering to feel slow initially, and too sharply responsive as you keep turning the handlebars into a turn, leading to a crash.
In this case, go to the next larger frame, and if the reach is too far, try a stem that's less than 90mm and see if you can get the reach adjustment just right.
Between these two frame sizes and the right stem, you will find one will feel more natural and comfortable than the other.
Once the frame, top tube, and stem length are tailored to you, it's time to adjust the saddle height.
My adjustment is slightly different than Zinn's, but no saddle height adjustment is written in stone.
However, there is a very small window that you want to stay in when making saddle height adjustments.
My method is to sit on the bike, leaning against a wall, or having a friend hold your bike steady.
Press the front or rear brake, or both.
In case of a sudden loss of balance, this will keep the bike from moving backwards or forwards, and you ending up on the ground.
With your bare feet, place one pedal at the 6 o'clock position.
The heel of your foot should just barely graze the top of the pedal with your leg fully extended and locked at the knee.
When you get the position right, lock down your saddle.
When you put on your biking shoes, you're now adding around a 1/2" or more to the bottom of your feet.
That's why I use the barefoot method.
If the saddle is too low, your legs will not reach optimal extension, and you'll lose a lot of power and efficiency.
You'll probably get tired a lot faster, too.
If the saddle is too high, you can over extend your knees and cause injury to them.
You may also experience a lot of saddle discomfort and pain, as you're rocking side to side trying to reach the pedals.
When you have the saddle height right, try increasing the seat post height in 1/16" increments until you start to lose power, then slowly back it down in 1/16" increments until you feel the most power in every pedal stroke.
I don't recommend doing this initially, but only once you have everything dialed in, and you are comfortable on your bike.
You'll find there is a very narrow range of adjustment to the seat post height if you follow the method I mentioned above.
If you're adjusting your saddle much more up or down, start over, using the method I described.
Making too many adjustments at one time can confuse yourself into thinking an adjustment made an improvement or made something worse, when it didn't.
Make one adjustment at a time, until you hit the sweet spot, before moving on to other areas of your bike.
Saddle tilt is another essential adjustment for both comfort and efficiency.
If you feel numbness and pain in your prostate area, tilt the saddle down toward the front of the bike in small increments.
If you feel pain behind your sitbones, tilt the front of the saddle upwards.
You should find a balance between both that should allow you to ride comfortably.
If you can't find a sweet spot, slide the saddle forwards or backwards a little at a time on the seat rails.
If all adjustments fail, look at different saddles.
But let me warn you, I've been through countless saddles, and have found very few that work for me.
Everyone has a different rear end, and finding the right saddle can be frustrating.
Try all the methods I've described first before giving up on your saddle.
My riding is pretty demanding.
I ride a high end mountain bike over 30 miles, the last 2.
5 miles is a grueling uphill grind.
The total ride takes me 3.
If your ride is not as demanding and much shorter, you may find your saddle fits you perfectly once adjusted.
Now that I'm getting older, my rear end seems to be getting more demanding, and I'm about to try a $200 saddle that I hope works.
Once your saddle is adjusted properly, your seat post height is right, and your top tube length is optimized, you'll find yourself covering miles on your bike efficiently and effortlessly, not even feeling any discomfort.
You'll be tired at the end of your ride, but you'll be looking forward to your next epic bike adventure with excitement and enthusiasm knowing your bike is now a finely tuned machine.