Is it more efficient to...

barrybridges
barrybridges Posts: 420
edited April 2013 in Road general
Odd question, but been thinking about this and not sure what the answer is.

Let's say that there's a hill near to where you live. If you maintain a constant effort up the hill and then down the hill, your average speed is 10mph uphill - and then 20mph downhill.

Is it more efficient to put more effort in on the uphill section, or the downhill section. Or - does it make no difference whatsoever?

In other words, which of these would be faster or more efficient for the same hill:

15mph uphill, then 20mph downhill
10mph uphill, then 25mph downhill

I'm assuming that in answering this question there's a difference between the 'faster' solution and the 'least energy' solution?

Comments

  • iF the hill is symmetrical, then increasing the lower speed is faster than increasing the higher speed.
    This is because you spend more time going slowly to cover the same distance.

    Personally, i'm a bit cack at descending, so i work on the ups and recover on the downs. Sadly, i'm still not fast!
  • colsoop
    colsoop Posts: 217
    Work hard on the uphill section. You can recover on the downhill.
  • dhobiwallah
    dhobiwallah Posts: 272
    In your simple example above negativelycra is correct in time taken. But it takes more effort to increase your 10mph by 50% going uphill than your 20mph by 25% going down.

    Not sure how you would even go about calculating it, but common sense would suggest that 'resting' on the uphills won't get you very far very fast!

    Try is next time - On your way out power up the hills and freewheel down to recover. On your way back power down the hills and freewheel up and see what time you get home :P
  • rolf_f
    rolf_f Posts: 16,015
    Not sure how you would even go about calculating it, but common sense would suggest that 'resting' on the uphills won't get you very far very fast!

    I think 'resting' is probably the wrong word. I'd assume more like just spinning up at a comfortable pace and then hell for leather going down and vice versa.

    Wind resistance I think is the key here. At speed you are putting all your energy into overcoming wind resistance - on the climbs gravity. But you have to climb the climb however long you take over it and the quicker you are up it the less time you spend struggling. Bottom line is you can gain minutes on a climb and seconds on a descent so the conclusions are clear.

    We had a hill climb up Norwood Edge a while back - the fastest were around 7-8 minutes and the slowest about 15. On Sunday we descended the same hill - that took two minutes. Of course, I was a wheezing wreck after the climb but even taken at a more sustainable pace it wouldn't have taken me more than a couple more minutes. No matter how fast you try to descend, it probably isn't going to make more than about 10 seconds of difference. The descent is therefore almost irrelevant.
    Faster than a tent.......
  • jibberjim
    jibberjim Posts: 2,810
    Air resistance is the dominant force, so you work harder wherever your speed is below your average speed and easier when above it.
    Jibbering Sports Stuff: http://jibbering.com/sports/
  • cougie
    cougie Posts: 22,512
    If you're racing - its best to ease it back on the climb so that you're not too knackered at the top to take advantage of the descent.
  • diy
    diy Posts: 6,473
    There is a hill near me that according to strava I can do at 12mph at 170bpm and 8mph at 145bpm. That is an order of magnitude less effort for a reduction of 4mph.

    Having said that, there is another hill, where I can pull 34mph at 150bpm but 38mph is the full flat out 170bpm.

    The point here is that we are looking at a bell curve. getting 1 or 2mph more is quite easy in the lower effort zones, getting it at the other end when you are flat out is almost impossible.
  • sungod
    sungod Posts: 16,535
    as others said, if the gradient is steep, even if you really kick it going downhill, it'll make little difference to speed - i've been on long straight descents where speed just maxed out at 80-90kph, even pedaling at 6-700w made only a few kph difference, in that situation you may as well coast and enjoy it, whereas using more power going uphill will give an almost linear increase in speed

    scenario:

    - rider+bike weights equal for both riders
    - equal gradients, say 15%, which should be steep enough to keep the speed of the faster rider below that where aerodynamic drag has a significant impact (this is the critical bit)
    - length of race, 1km up, 1km down...

    rider 1 250w uphill, 250w downhill
    rider 2 500w uphill, coast downhill

    on the ascent, rider 2 will crest while rider 1 is only a bit more than halfway up, then rider 2 will coast down at much higher speed

    rider 2 probably reaches the bottom before rider 1 reaches the top, and as a plus will have used less total energy to hand out the drubbing - on the ascent, both will use about the same total energy (raising the same mass through the same height uses the same energy), but rider 1 must still expend extra energy on the descent, most of which will be squandered due to drag
    my bike - faster than god's and twice as shiny
  • rolf_f
    rolf_f Posts: 16,015
    diy wrote:
    There is a hill near me that according to strava I can do at 12mph at 170bpm and 8mph at 145bpm. That is an order of magnitude less effort for a reduction of 4mph.

    But you are at 145 bpm for 30% longer than you were at 170.
    Faster than a tent.......
  • Monty Dog
    Monty Dog Posts: 20,614
    cougie wrote:
    If you're racing - its best to ease it back on the climb so that you're not too knackered at the top to take advantage of the descent.

    What kind of soft-tapper races do you do? Go hard up the hills and harder over the top to make sure the wheezers don't get back on.
    Make mine an Italian, with Campagnolo on the side..
  • centimani
    centimani Posts: 467
    colsoop wrote:
    Work hard on the uphill section. You can recover on the downhill.
    Depending on the kind of hill, but this is the inly thing that allowed me to increase my average speed appreciably.

    Having done the same kind of loops/distances over the years, I know exactly what my times will be over say 40 miles, give or take 5 minutes. Some time ago I wanted to up the pace and break through the plateau I seemed to find myself at.
    Rolling hills would describe my terrain locally, nothing too long, 1000ft ascent over 40 miles.
    Attacking the hills and recovering on the descents made quite a difference to me. When I say recover on the descents (or just at the top of a hill)..I don't mean freewheel, just take it easier and get my breath back.
    Over that year, I upped my average speed from 15.5 mph to just under 18 mph....a frikkin lot of hard work it was as well. :?

    Edited to say...just read the post headline..is it more efficient ?
    I doubt its more efficient, harder and faster, but not more efficient.
  • bernithebiker
    bernithebiker Posts: 4,148
    jibberjim wrote:
    Air resistance is the dominant force, so you work harder wherever your speed is below your average speed and easier when above it.

    This.

    In a nutshell.
  • good question, there is a hill next to my house and i have to go up it all of the time, i tend to sprint up it and then continue at a ride pace.

    I only sprint it to warm me up!
  • diy
    diy Posts: 6,473
    Rolf F wrote:
    diy wrote:
    There is a hill near me that according to strava I can do at 12mph at 170bpm and 8mph at 145bpm. That is an order of magnitude less effort for a reduction of 4mph.

    But you are at 145 bpm for 30% longer than you were at 170.

    I can go all day at 145, well at least 5-6 hours. I can't go for more than 10 mins at 170.
  • declan1
    declan1 Posts: 2,470
    I find it's easier to climb faster then recover on the descent. If you get into an aero position when going down, it's hard to go much faster if you pedal.

    Road - Dolan Preffisio
    MTB - On-One Inbred

    I have no idea what's going on here.
  • seanorawe
    seanorawe Posts: 950
    Work hard going up, blow out of your @rse going down
    Cube Attain SL Disc
    Giant CRS 2.0