Coast or Pedal on a Downhill?

A triathlete asked me some time ago what he should do on downhill portions of a bike course. Should he pedal hard, pedal easy or coast? That was a great question and one that also applies to cyclists doing time trials. It does not apply to runners as their speed is not significant enough to cause substantial drag.  

On a bike as your speed increases linearly (a straight line from, let’s say, 20 to 25 mph), the power required to go faster increases exponentially. This largely is because of drag. The energy required to overcome air resistance (drag) is a function of land speed to the third power. So while it is only a 25% increase in speed to go from 20 to 25 mph, there is something like a 75% increase in the energy required to achieve that additional 5 mph.  

Why am I telling you this? Because as you go down hill and your speed increases if you want to go even faster than coasting allows the energy “expense” of the additional miles per hour is going to cost you dearly.  

The bottom line is an old adage which says that if you are riding on a fast portion of a course (down hill) ride easy; but if you are riding on a slow portion of a course (up hill) ride hard. So when riding fast on a downhill don’t expend as much energy as when riding uphill. The longer the event, the more important this is. For a sprint-distance triathlon or a short time trial you can go much harder downhill than if it was an Ironman or a long TT.

The best advice I've seen for this came from Alan Couzens, an exercise physiologist and triathlon coach. He tried to nail it down for Ironman triathletes with his “50-40-30-20-10 rule.” It goes like this (reprinted with permission of author)….   

Coast at >50km/h

Pedal easy at >40km/h

Pedal steady at >30km/h

Pedal moderately hard at >20km/h

Pedal hard at >10km/h

You can read Alan's blog on this topic for an in-depth discussion of how he came up with his rule by going here. 

Alan’s Ironman rule may not work for your race distance, but the concept remains the same: Conserve energy when the bike is going fast; expend energy when the bike is going slow. How much energy depends on how fast the bike is going, how long your race is and how fit you are. The less fit you are the more you will need to conserve energy on down hills. Based on this concept you can come up with your own rule for each race distance you do.

So the answer to the athlete’s question in the first paragraph starts with (you guessed it) “it depends.” It depends on speed, race duration and fitness.

 

Comments

[Nuno H Luz]

Hi Joe,

Interesting post. One question related to this: even you're going 50+ mph, shouldn't you pedal anyway going downhill after a hard climb (and before another one) just to keep your legs moving/help recover/avoid the dead leg feel when you start the next climb?

Thanks in advance!

Nuno

[Roger]

Thanks for the post, I think it's very clear.

For those mathematically minded I would say that power increases polynomially with speed (to the power of 3) rather than exponentially. A 25% increase in speed represents a 95% increase in required power (1.25^3).

[Joe Friel]

Nuno--Turning the pedals easy or pedaling hard? big difference.

[Nuno H Luz]

Thanks for the reply, Joe.

I meant turning the pedals easy.

Nuno

[Curt]

Joe,

In earlier writings I think you recommended increasing power when riding into the wind. Why doesn't the same advice apply in that situation (i.e. reduce power into the wind and increase power with the wind at your back)?

Thank you for your blog. It is a great source of information.

[Andy]

Great info Joe as usual.

Side question: when is the best time to grab a drink of water/open up and pop a gel/eat some food. Is it best to do that while coasting down a hill or on a flat while doing some light pedaling? Which one wastes the least? thanks!

[Joe Friel]

Andy--The worst time to come out of an aero position is when going the fastest. That's nearly always downhill. The best time is when going the slowest. But that's usually a hard uphill when it is hard to drink or eat. That leaves the flat terrain.

[Joe Friel]

Curt--I think you may be misinterpreting or misremembering something I wrote. Into the wind in a steady state event such as a TT or tri power should remain constant while speed drops. Trying to maintain speed will require a greater output of power. Of course, there are still a few 'it depends' here. If it's a short period of time into a strong wind and it's a short race then it really won't be significant to raise power into the wind. Then we're talking about how strong of a wind vs how much power to overcome it.

[Roberto]

Dear Joe,

This is a very interesting post, as usual. I'm in a "college physics" doubt: Shouldn't it be considered the kinetic energy that depends on the squared speed?
Why do you use the speed to the third power?
Regards from Chile,

Roberto

[Curt]

My apologies. It wasn't your original work. You were quoting studies in your 17 Mar 2008 Blog that suggested increasing power into headwinds:

"* Using a mathematical model Swain found that when compared with a constant effort there was a significant time savings in a cycling time trial by slightly increasing power on the uphills and into headwinds and decreasing it slightly on downhills and with tailwinds. (Swain. 1997. A model for optimizing cycling performance by varying power on hills and in wind. Med Sci Sports Exercise 29:1104-1108.)

* This study involved a review of other research such as Swain's above using a mathematical model to predict how hills and wind affect performance in a cycling time trial. The authors then revised the previous models slightly but the results were largely the same as the others: Increasing cycling power on uphills and decreasing it on downhills, and increasing power into the wind and decreasing it when riding with the wind improved time trial times significantly. (Atkinson et al. 2007. Variable versus constant power strategies during cycling time trials: prediction of time savings using an up-to-date mathematical model. J Sports Sci 25(9):1001-1009.)

* Seven male cyclists did a 16.1km (about 10 miles) time trial on a CompuTrainer 3 times each. There was a simulated 8km headwind in the first half of the ride and a simulated 8km tailwind in the second half. The pacing of the 3 rides were: a) self-selected pace, b) constant power and c) variable pacing with 5% higher power into the wind and self-selected and constant with the wind. Times were significantly faster in b and c compared with a. The fastest was c. Variable pacing based on power should be used when there is a headwind. (Atkinson and Brunskill. 2000. Pacing strategies during a cycling time trial with simulated headwinds and tailwinds. Ergonomics 43(10):1449-1460.)"

Applying the laws of fluid dynamics, it seems to me you would want to apply more power with a tailwind where you would get more speed per watt due to decreased drag.

Thanks.

[Joe Friel]

Roberto--Thanks for your note. When I wrote the piece I asked my physics buddy, Bill Cofer, for his input on the issue from a physics perspective. So I've asked him to reply to your comment. Here's his reply...

"Roberto is correct… Kinetic energy does depend on the speed squared. However, as speeds increase the energy used to overcome aero drag starts to become significant since aero drag is directly proportional to the velocity cubed."

[Gary]

Joe,

Thanks for the tips. Can you relate pedal easy and pedal steady to heart rate zones? After cresting a hill on a half iron course with virtually no flats, you know that you have recovered when .... ?

[Joe Friel]

Gary--Thanks for your comment. I really can't answer that. The part you're referring to came from Alan Couzens. You'd need to ask him how he interprets those instructions (you can probably reach him through his blog address which is listed in my blog). I'd take it to mean just back off some. No measured HR implied. Especially since HR is slow to respond. On a short downhill HR may not change at all even though you are coasting. Power would be a much better indicator since it is much more sensitive. Good luck!

[Richard]

Heya this is interesting principle, I always tried to keep pedalling at about 220 W (my ftp=330) below 50kmph, this year and found I was passing loads of people who would coast and then go far too hard up the next hill, after i while i was able ot pull away from them as they have spent more energy in the long term. So I agree that just keeping the pedals turning at a reasonable rate saved a great deal of time without much sacrifice of energy. I also used this principle for a couple of 50 and 100 mile TTs and found I could gain on other competitors whilst feeling like I was recovering.
So I definite agree with the idea.

[Richard Sterry]

Good post. A few months ago I did some number crunching for mountain biking that supports your theory.

If there is a downhill section of 1 mile, the time benefit of increasing your speed from 15-20mph is just 1 minute (increasing from 20-25mph will only gain 36 seconds over the mile). However, if there is an uphill section of 1 mile, the time benefit of increasing your speed from 3-4mph or from 4-6mph is a massive 5 minutes. This supports your theory to power up the hills and rest on the descents.

Applying this strategy for the South Downs Way in Southern England, I knocked off 1:30 hours from my personal best time for the 100 mile trail.

[Supra Shoes]

Good job.No matter where we are, we must study all the way. As the proverb says that: You are never too old to learn. Thank you for your blog.

[Air Jordan]

So nice blog! But I have a suggestion that you can add some humorous language up to your article. Maybe you can write more better.

[Stuart Lynne]

For TT's you should know what your average watts will be for the duration you will be racing. Then I follow two simple rules.

Get up to speed fast when starting downhill which may take a short anaerobic burst. Then generally use up to your average power while going downhill.

Possibly a little lower if conditions immediately ahead will require over your average watts (turning into the wind or uphill.)