Watts vs Speed

['Dale]

In the gym I get a speed of around 32km/hour and watts of around 160 when riding a moderate pace.

 

What will this mean on the open road if I ride alone (time trial) in terms of kilometres per hour?

Difficult to say... so many variables:

Wind resistance.

Drafting.

Inclince.

Road surface quality.

Edited December 10, 2010 by AirBender

[NickD]

Cycling Power

Most of us don't want to spend large amounts of money on a Cycling Power Meter for our bikes, and unless we are training very hard and with particular goals it is more of a luxury than a necessity.

But it is still useful and interesting to know how much power we can generate when we cycle and an understanding of cycling power also makes it clear why we can all increase our speed quite quickly when we start, but it becomes progressively harder to make small improvements as we get faster!

The basic idea is that we need power to overcome air resistance (more important on flat / downhill sections) and gravity (important when going up hills!). There is also a small amount of friction to overcome, both in the bike and between the bike and the road (hence the use of smoother, thinner tyres).

As a guide, below are some approximate cycling power figures for various speeds of cycling:

Speed - kmh (mph) Power (watts) Increase in power needed

to increase speed by 2.5kmh

20 (12.5) 75

22.5 (14) 95 20

25 (15.6) 120 25

27.5 (17.2) 148 28

30 (18.7) 180 32

32.5 (20.3) 218 38

35 (21.9) 262 46

37.5 (23.4) 311 49

40 (25) 366 55

Figures assume a flat road on a windless day. The exact figures will change according to rider and bike weight, air temperature and position on the bike but the principle is the same and the figures are quite similar.

The numbers explain immediately why cyclists find it hard to keep getting faster - the faster you are going, the larger the increase in power needed to keep improving - for example, increasing your power by 50% (no mean feat) will only increase your speed from 15.6 mph to 18.7 mph.

Similarly, increasing your average speed from 20 kmh to 25 kmh requires that you produce an extra 45 watts of energy, while the same increase in speed, from 35 to 40 kmh, needs an increase of over 100 watts - and you need to already be producing over 250 watts, which is already quite an achievement for most of us apart from shorter stretches of road!

This is because, in maths speak, 'wind resistance increases with the square of the velocity' - in non-maths terms, going 25% faster needs much more than a 25% increase in the power you produce!

At lower speeds air resistance is much less important (ie much easier to overcome) but the faster you travel the more of your energy goes in just pushing the air out of the way. Hence of course the reason to try and be quite aerodynamic on your bike - the faster you are riding, the more important this becomes.

It is useful to find a flat stretch of road a few miles long if possible, and every month or two see what average speed you can maintain on the same stretch or road - this will give you a guide to your power output, so you can compare your own performance over time. It is also worth noting your weight at the same time, so you can see a 'cycling power to weight' figure.

You can also use a 'cycling power calculator' (eg this one here) to come up with an estimate of your power.

It is this number that gets you up the hills faster and that you want to improve! For example,if you are gaining speed on hills but losing lots of weight over the same period it is possible that your actual cycling power is unchanged, but your 'power per kilogramme' is increasing.

Maximum power output is of course much higher than the average you can sustain over a long distance, at least 50% extra would be typical - so if you can cycle at 15.6 mph for a long distance it is probable that you can manage at least 19 mph for short distances.

Note: It is said that top riders produce 450 - 500 watts over reasonably extended stretches (e.g. a hill climb) and that sprinters at maximum output during the last 100 metres of a race produce 1000 - 1200 watts. Most of us will have to settle for something rather less!

[pastapouch]

Aha! I like it. So I'm actually a 30% better cyclist than you woesies who get into pelotons, because I ride my races solo. PPA, I want a re-seeding!

I'm also racing solo.... right at the end of all bunches.

As previously said... ride alone at that same effort and then compare. and remember, get real, and don't sit in bunches. I'm watching you.

[Woofie]

Cycling Power

Most of us don't want to spend large amounts of money on a Cycling Power Meter for our bikes, and unless we are training very hard and with particular goals it is more of a luxury than a necessity.

But it is still useful and interesting to know how much power we can generate when we cycle and an understanding of cycling power also makes it clear why we can all increase our speed quite quickly when we start, but it becomes progressively harder to make small improvements as we get faster!

The basic idea is that we need power to overcome air resistance (more important on flat / downhill sections) and gravity (important when going up hills!). There is also a small amount of friction to overcome, both in the bike and between the bike and the road (hence the use of smoother, thinner tyres).

As a guide, below are some approximate cycling power figures for various speeds of cycling:

Speed - kmh (mph) Power (watts) Increase in power needed

to increase speed by 2.5kmh

20 (12.5) 75

22.5 (14) 95 20

25 (15.6) 120 25

27.5 (17.2) 148 28

30 (18.7) 180 32

32.5 (20.3) 218 38

35 (21.9) 262 46

37.5 (23.4) 311 49

40 (25) 366 55

Figures assume a flat road on a windless day. The exact figures will change according to rider and bike weight, air temperature and position on the bike but the principle is the same and the figures are quite similar.

The numbers explain immediately why cyclists find it hard to keep getting faster - the faster you are going, the larger the increase in power needed to keep improving - for example, increasing your power by 50% (no mean feat) will only increase your speed from 15.6 mph to 18.7 mph.

Similarly, increasing your average speed from 20 kmh to 25 kmh requires that you produce an extra 45 watts of energy, while the same increase in speed, from 35 to 40 kmh, needs an increase of over 100 watts - and you need to already be producing over 250 watts, which is already quite an achievement for most of us apart from shorter stretches of road!

This is because, in maths speak, 'wind resistance increases with the square of the velocity' - in non-maths terms, going 25% faster needs much more than a 25% increase in the power you produce!

At lower speeds air resistance is much less important (ie much easier to overcome) but the faster you travel the more of your energy goes in just pushing the air out of the way. Hence of course the reason to try and be quite aerodynamic on your bike - the faster you are riding, the more important this becomes.

It is useful to find a flat stretch of road a few miles long if possible, and every month or two see what average speed you can maintain on the same stretch or road - this will give you a guide to your power output, so you can compare your own performance over time. It is also worth noting your weight at the same time, so you can see a 'cycling power to weight' figure.

You can also use a 'cycling power calculator' (eg this one here) to come up with an estimate of your power.

It is this number that gets you up the hills faster and that you want to improve! For example,if you are gaining speed on hills but losing lots of weight over the same period it is possible that your actual cycling power is unchanged, but your 'power per kilogramme' is increasing.

Maximum power output is of course much higher than the average you can sustain over a long distance, at least 50% extra would be typical - so if you can cycle at 15.6 mph for a long distance it is probable that you can manage at least 19 mph for short distances.

Note: It is said that top riders produce 450 - 500 watts over reasonably extended stretches (e.g. a hill climb) and that sprinters at maximum output during the last 100 metres of a race produce 1000 - 1200 watts. Most of us will have to settle for something rather less!

 

I wonder what equipment and riding styles this is based on.

I know for instance that old school 32 spoke wheels will take about 30 watts more power to drive at 40km/h than zipp 808's.

Tri bars, riding on hoods or in drops are fair differences as well.

 

Looking at the power figures, it reminds me how feabile we are against international pros.

Most of the local half decent guys will rake up 400-500 watts on a PPO test.

Go and look at power figures from some big races and some of these guys are spending minutes riding above 500 when they weigh less than my garden rake.

 

Another interesting fact.

I know that Stefan Ihlenfeldt (CT market rider) his power in a sprint was measured at 1300. Now he is not a big guy at all! I believe around 60kg or so.

[Garfield2010]

Thanks. Basically what I want to know is at what wattage should I train on a trainer in the gym to simulate going 30km/hour on a time trial for 90km's. I though the gym bike might take into account wind etc since it is suppose to simulate a real bike..... What I mean is why would the gym bike say 30km/hour if it actually means 27km/hour on the road. I want to gym like I ride ouside.

 

Wind etc is not an issue for me as with circle routes it cancels out itsself mostly?

[Woofie]

Thanks. Basically what I want to know is at what wattage should I train on a trainer in the gym to simulate going 30km/hour on a time trial for 90km's. I though the gym bike might take into account wind etc since it is suppose to simulate a real bike..... What I mean is why would the gym bike say 30km/hour if it actually means 27km/hour on the road. I want to gym like I ride ouside.

 

Wind etc is not an issue for me as with circle routes it cancels out itsself mostly?

 

Are you cycling for 3 hours in a gym?

 

Flip. 20 minutes on those bikes is long enough.

 

Why dont you just aim a higher? like 38.

If you bomb out then at least you dont have to phone your wife to fetch you then.

[Garfield2010]

No! I normally cycle for one hour but I ride a steady yet comfortable pace. I ride in the gym cause I work in the CBD and there are no races currently on weekends. I have done the 94.7, but want to do the 90km time trial in the Ultra Triathlon in 3 hours on the 16th of Des.

[Woofie]

No! I normally cycle for one hour but I ride a steady yet comfortable pace. I ride in the gym cause I work in the CBD and there are no races currently on weekends. I have done the 94.7, but want to do the 90km time trial in the Ultra Triathlon in 3 hours on the 16th of Des.

 

Well then that is simple.

Either go do some hard spinning classes or ride it as hard as you can for an hour.

 

It you can ride at 40 on the indoor bike for an hour, there is no way that you would manage 30 for 3 outside.

 

Intervals on the indoor bike will be benificial as well.

 

Just remember to have rest days if you are doing hard days.

[Garfield2010]

I do spin classes and I have done numerous races..... well see what happens.

[riaanb]

Wind etc is not an issue for me as with circle routes it cancels out itsself mostly?

 

Isn't that like saying that on circular routes the downhills cancel out the uphills?

[JA-Q001]

One problem with gym stationary bikes are that you are not training exactly the same mussels. You do not sit in the same position, your bike does not move with you from side to side and there are no side forces like wind to lean against. Another reason why you can hold a certain amount of watts on that and not outside. But you can still build basic cycling fitness. Further, doing a circular route does not cancel out wind factor. With the wind from the front you loose more time compared to a tail wind. This is due to the wind resistance increasing exponesially. Also, you can only compare your watt output with someone that has the same combined weight as you and your bike, that is if you go slower than 25km/h, anything faster and your personel aerodynamics will start to have an effect.

[JA-Q001]

And downhills dont cancel out uphills on a circular route aswell. If you have a 1km climb with a 1km downhill, and you whant a ave speed of 30km/h. Now say you ave 20km/h uphill the uphill takes 3min, now you have 1 min left to cover the 1km downhill, thats a 60km/h ave downhill. If your climbing speed decreases to 15km/h, that takes 4min to cover the 1km climb and you have no chance to cover the 1km downhill for a 30km/h overall ave.

[riaanb]

And downhills dont cancel out uphills on a circular route aswell. If you have a 1km climb with a 1km downhill, and you whant a ave speed of 30km/h. Now say you ave 20km/h uphill the uphill takes 3min, now you have 1 min left to cover the 1km downhill, thats a 60km/h ave downhill. If your climbing speed decreases to 15km/h, that takes 4min to cover the 1km climb and you have no chance to cover the 1km downhill for a 30km/h overall ave.

 

Exactly, hence my point about the wind!