# Why is it more difficult to start a bike race than to be in the middle of a bike race?

When the bikers are starting at the blocks and then the starting gun goes off, the speed at which they are going at at that time is slower than at any part, like during the middle of the race.

Why is this? The question says that you must relate air resistance to this. What I know about air resistance is that it builds up as something increases speed.

So how come it is more difficult to start a biking race (at the starting block) than it is to just ride during the middle of the race? Wouldn't there be more air resistance as you start moving?

At the beginning of a race your bike and the air around you are at rest.

To accelerate the bike (and some air) a force needs to be applied (on the pedals) which depends on the mass of the bike (and rider) and the air.

If you reach a time when you are moving at constant speed then the mass you are accelerating is much reduced - it is just the air - and so the force you need to exert on the pedals is also much reduced.

Speeding the bike up requires more force because now you have to accelerate more mass - the air and the bike.
Once the bike has stopped accelerating the force applied on the pedals will be reduced but will be larger than before the acceleration phase because the air is being accelerated by a greater amount as the bike is now moving faster.

A short simple answer is that to start a bike race you need to accelerate the bike up to the desired cruising speed, and that will need a lot of force.

One you are a the speed you wish to cruise, you only need to maintain that speed (by fighting air resistance) and that requires less fore than the acceleration process.

To illustrate my point, consider the ideal scenario of no, or very little air resistance (which is what bikers try to achieve). In that case, you will only need to use force/energy to accelerate at the start of the race. Basically to built up kinetic energy. Once you are at your desired speed, no extra force is needed and you can cruise without spending any energy.

At the end of that day it biold down to a comparison between how fast your wish to cycle and how fast you wish to accelerate. The faster you cycle the more air resistance you will get, but also the faster you accelerate the more force you will need.

• Thanks, but why does it require more force to get to the desired cruising speed? Isn't there only more air resistance at a larger acceleration? – Christopher Uren Aug 4 '17 at 8:22
• In the end it's a comparison between desired acceleration and desired cursing speed. See the extended answer. – Mikael Fremling Aug 4 '17 at 8:30
• So basically, it takes more force to accelerate to a desired speed? – Christopher Uren Aug 4 '17 at 8:39
• @ChristopherU'Ren Basically, yes. – Mikael Fremling Aug 4 '17 at 8:50

It takes a while to accelerate up to cruising speed, Yes. There is a greater air resistance at greater accelerations. there is a greater air resistance also while accelerating in general than while at cruising speed (constant velocity).

There is also the biological factor to consider - it does take a while to warm up properly, even if you have done a warm up.

And then the tactical element to a bike race - at the start people are shuffling for position, trying to spread out and stay behind others so as not to take the bulk of the air resistance. They don't want to be going full pelt right out of the gate.

A partial answer is that one reaches a 'second wind' when the aerobic oxygen uptake and blood circulation are established at a higher-than-resting rate. Part of the answer, too, is that the major blood vessels in the legs have their own valves, and accomplish a lot of blood pumping, after reaching the steady fast pace of cruising speed. At the start, it's only your heart doing that work.

As a cyclist, I always felt better at speed, too, because a lot of my riding was in summer weather, and airspeed aids evaporative cooling.