Power and climbing

In my never ending, and perhaps compulsive, attempt to improve my climbing, I've invested large sums in shaving grams off the weight of my bike. Also, I've struggled every season to shed a few extra pounds off of my relatively trim body. My question is this: what is it all worth -- in some real measurement such as power? There must be a relationship between the following variables - incremental weight (in lbs), velocity (mph), incline (percent), and incremental power (watts). This is simple physics:
Incremental power(watts)= constant x incremental weight(lbs) x velocity(mph) x incline (percent). In other words, for a given velocity and incline, how many extra watts are required for each lb. of extra weight? Once you know the value of the constant in the above equation, you can solve for any combination of velocity and incline. Of course extra power is required for higher velocity, but I want to isolate the weight component for climbing. Then you can make some more intelligent decisions about buying that super light wheel set, or struggling to diet off a few more lbs.

 

www.analyticcycling.com is where you need to visit!

However, if you already have a low body fat%, and the climbs are not mountains (i.e., like alpe d'huez), then dropping one or two kg either off the bike or yourself will make little difference.

For instance on a 1-mile climb loosing 2kg (either bike or body), whilst maintaining the same power will save you about 7 seconds at typical race speeds.

Also, if you are very lean, it's comparatively harder to loose weight than it is to gain power.

Lightweight wheels (for the majority of riders) makes little difference. At velocities greater than 20.0 km/hr it's far more important to use aero equipment -- this will same more time (even if it's slightly heavier).

Finally, if you concentrate on training harder with the goal set to increase your power, it's likely that you will loose body fat without actually concentrating on it.

Ric

 

Another variable u forgot to mention which may be a factor in all of this.......power to the ground is diff to that being produced at the pedals (ie. BHp Vs Hp at flywheel so to speak). A bike may sap as much as 10-15% of the power produced, through an inefficient drivetrain. Wheels should be stiff as well as light, and the frame must also have these qualities. Not all performance can be improved through lighter weight alone. Maybe this is another equation all together but i figured that u might want to consider this before u try going to derive an equation on power, weight etc.

Just an idea that u might like to think about b4 buying some superlight stuff that might be so flimsy that u would be better off with the heavier stuff. Ricstern mentions the use of aero equipment over 20km/h. U will find that this stuff is often stiffer than the lighter climbing specific equipment. Save weight where u can but keep strong and sturdy wheel and frame is my opinion. Weight can have its advantages down the other side anyway!

 

Thanks for the replies -- and they represent sound advice on where to put your focus for improved climbing. None the less, all other things being equal, there must be a weight / power trade-off in climbing. It's really simple physics- it takes a certain amount of power, to lift a given weight, at a given vertical velocity (speed X grade). My recollection of physics is simply not good enough to solve the equation -- but I believe that you will find that on a climb at a grade of 6%, at speed of less than 20 K/hr; that most of the power required to sustain this velocity will be required to overcome gravity - not to overcome frictional resistance (mostly air) and loss of energy due to wheel and frame flex. I was hoping that someone more skillful than me in basic physics, could solve the equation. Then we would have one more helpful fact to use in making decisions about equipment, training, and diet.

 

Drew JC wrote:
power to the ground is diff to that being produced at the pedals (ie. BHp Vs Hp at flywheel so to speak). A bike may sap as much as 10-15% of the power produced, through an inefficient drivetrain.

Several groups have now compared the SRM crank which measures power at the crank to the Power Tap which measures power at the hub. The difference is 2 - 2.5%, with the PT reading lower. Thus, that's how much is lost through the drivetrain.

Ric

 

Bobkny wrote:
Thanks for the replies -- and they represent sound advice on where to put your focus for improved climbing. None the less, all other things being equal, there must be a weight / power trade-off in climbing. It's really simple physics- it takes a certain amount of power, to lift a given weight, at a given vertical velocity (speed X grade). My recollection of physics is simply not good enough to solve the equation -- but I believe that you will find that on a climb at a grade of 6%, at speed of less than 20 K/hr; that most of the power required to sustain this velocity will be required to overcome gravity - not to overcome frictional resistance (mostly air) and loss of energy due to wheel and frame flex. I was hoping that someone more skillful than me in basic physics, could solve the equation. Then we would have one more helpful fact to use in making decisions about equipment, training, and diet.

>yes, at low speeds most of the resistance comes from gravity. A good ballpark figure for power can be estimated with

power = mass x gravity x vertical height gained / time

mass in kg, gravity = 9.81 m/s, height in metres and time in sec

you then need to add on the power required for rolling resistance, bearings, etc -- which will be about 30 - 50 W.

Or just go to www.analyticcycling.com !

Ric

 

Ric - thanks for the equation. Since I'm used to looking at my speedo, and since I'm familiar with the grade of the hills I climb, I converted this equation into terms I'm more comfortable with. Furthermore since I'm used to the U.S. measurment system in miles and lbs., I converted from metric to U.S. equivalents as follows:
Metric: Power(watts)=weight(kgm) x 2.96 x speed (k/hr) x grade
US : Power(watts)= weight(lbs) x 1.84 x speed (mile/hr) x grade
Last year, I won a 5 mile local hill climb TT in my age group (age 55+). The climb had average grade of 6%, and my average speed was about 13 mph. At the time, the combined weight of myself, and all of my equipment and clothing was about 180 lbs. If these equations are correct, I had to expend about 260 watts, simply to overcome gravity!
Here are some observations:
1. Once you start climbing, and speed drops under 20kph, weight is dominating factor ( assuming a fixed power output).
2. Even small changes in weight -- can have a measureable effect on results. For example, I won this event by a mere 8 sec. margin.
3. In the larger scheme of things, since weight has a linear effect, while friction grows exponentially, on an average rolling course, aerodynamics are a much more important consideration than weight.
Since I'm somewhat arithmetically challenged I would greatly appreciate someone checking my arithmetical conversions.

 

Hi Bob,

very quickly working it out with your data, i get 278 W plus you need to add on the other power to overcome various resistive forces, about 40 W = 318 W.

I also, quickly plugged the figures in at analyticcycling.com and got 344 W (total). That's certainly an excellent figure, looking at a power to mass ratio of about 4.8 W/kg.

As a rough guide on a flat 10m TT circuit with that type of power you should be around 21-mins

Cheers
Ric

 

Ric - thanks for the time and thought. Based on your analysis, I've simplified my climbing power equation:
Power(watts)=2 X mass(lbs) X Speed(mph) x grade(ratio)+ additonal power required to overcome friction.
The additional power term of course, increases exponentially with speed; but at climbing speeds is around 40-50 watts.
Your assessment of my 10K TT is not correct - but that's because I gave you incomplete data on my hill climb TT. During the hill climb, we had a stiff gusty head wind. So, the actual additional power term was much higher than expected. My best 10K time last year was under 15 min; and my 40K TT was 59:50. This year, I have been doing 20K TT tests on my Computrainer at around 29 min. On the Computrainer, I do my 2 min. repeat intervals at around 370 watts; and I can sustain efforts at around 350 for periods up to 1 hr.
Although I've used power as an intensity measurement on my Computrainer for many years, since I had no way to measure power outdoors, I found power to be less useful. Now that an inexpensive and flexible power measurment system is available from Polar, I plan to use power as my main measurement of training intensity. In another thread, I asked about the Polar system, and the replies were very positive. I'd be interested in your thoughts.
Bob

 

Hi Bob,

No poblems. I haven't checked your simplied equation, yet...

the '10' i was refering to was miles, not km. sorry, if i confused the issue by swapping back to old fashioned numbers!!

If you can sustain 350 W for an hour, then your higher intensity efforts should be much higher than 370 W, i.e., there'd be no real benefit to riding at 370 W for 2-mins.

Caveat 1: i haven't used a Polar S710, so information i give is on third hand knowledge -- although i feel it is good

Caveat 2: i sell the Power Tap, so, if people think i'm suggesting the PT is good because i sell, then i should point out i don't sell to the US market -- so i've nothing to gain from that.

Okay, from what i understand there are some problems with the S710, in that power won't always be spot on, is an average of 5-secs (so peak power would be low), they don't work well on a trainer (when you would want to use it for specific work) and there are problems with gearing and cadence affecting the readings, which doesn't happen with the SRM or PT.

The S710, is very dependent upon the way that it's installed.

I recommend the PT, and that's what i use (all the time).

here's some external links,

http://external.nj.nec.com/homepages/sandiway/bike/feather/tt/s710/index.html

http://www.schererfamily.com/Joisey_Scherers/Chris/cycling/polar/polars710.htm


cheers
ric

 

Ric - this thread is morphing into a new topic. Is that OK - or should I start a new thread? In reply to some of your comments:
Yes - the bane of my athletic career is lack of speed. As a runner or as a cyclist, I've always been able to sustain close to my maximum speed for extended periods -- but my maximum speed is too slow- especially for the type of short high speed races that are common in my age group over here. That is one of the reasons I've been doing more ITT's of late.
The PT seems like a less troublesome installation than the Polar. The reason I avoided the PT system, was that you needed a dedicated wheel. If I built an every day wheel on the PT hub, I wouldn't want to use it for my TT's. On the other hand, if I built a TT wheel on the PT hub, I probably wouldn't want to use it every day. Of course you could build 2 wheels; but that's starting to get very expensive. Here are some questions I have about the PT system:
1. Is there a rim that you would recommend with the PT hub, which would be sturdy enough for every day; but aero enough for TT's?
2. Is the PT system output stable enough that you could use it real time to monitor your effort during a TT?
Best regard and thanks for the help- Bob

 

Seems fine to leave it here, i think that's the nature of the beast on the internet, in that threads just drift!

It's most likely that both your power at lactate threshold, and at VO2 max need raising. I see no reason why they can't be raised.

It's true you do need a dedicated wheel with the Power Tap. Here's what i do/did:

Option 1) I got a 24-hole PT and laced it to a Mavic CXP14 rim (well, i didn't -- my local bike shop did!!). The CXP14 is pretty much the same as a CXP30, i.e., it's 30-mm deep and therefore vaguely aero. I use this for training and racing, i weigh about 68kg. I had the wheel laced with normal DB spokes.

Being that you're in the US, i'd most likely order Velocity deep V rims (30-mm) -- i'd have used them because you can get them in different colours, and that's cool! Unfortunately, they're almost impossible to get in the UK!

I'd also use Sapim CX Ray spoke, which are bladed spokes that don't require the PT hub to be slotted. They're lighter than most spokes and tougher too, but a little pricey.

Option 2) try to find a 40-mm deep aluminum rim (i think FIR make some????) or a 40-mm carbon or a deep 58-mm carbon rim (e.g., Zipp 303 and 404 rims i think)

Option 3) build it on to something like an Open Pro, box section rim. Then for TT's fit some disc covers to it (available from bike shops, quite cheap). From memory, I think disc covers are reasonable in comparison to a proper disc. I have a feeling that Kraig Willet did some tunnel testing with them not to long ago and the results might be up at his site. i can't recall the URL of the site, please someone help me out!

One of the first things you'll notice when training outdoors with power is how variable or stochastic the power is. It varies not only with the intensity, but with e.g., road grade, surface, environmental conditions, etc.

This can be somewhat unnerving at first, but as you start to ride at a higher intensity (e.g., TT effort) the power tends to be more modulated, because you are always trying ride hard, but never too hard. I certainly use my Power Tap (SRM would work too) to pace/modulate all my training and to a lesser degree my road racing.

cheers
Ric

 

Ric - I just went to the Grabber web page. Evidently they have a new model named the PT Pro. One of the new features is the ability to modulate the display rate specifically for TT's. They also use a standard wired cadence pick-up on this model. The Pro model also seems to have expanded memory functions, as well as a more flexible way of sampling intervals. The MSRP is about $200 more than the old model -- but the improved functions -especially the display control for TT's seem very attractive. Do you have any thoughts or experience with the new "Pro" model?
By the way, I like your suggestion of building a semi-aero wheel using a 30-40mm aluminum rim. There are many currently available in the US market.
Best regards - Bob

 

Damn!! That's what happens when you answer queries late at night!! Yes, i have both systems the normal and the Pro, the only difference is the computer and the (proper) cadence meter. The hubs are the same except for the funky yellow cover!

Yes, you alter how quickly the display is updated, i vary it depending on what i'm doing along with the recording rate (e.g., sprint sessions are set to 1sec update and record, endurance are set slower).

There's various functions you can do with the Pro, too, such as just have it as a HR monitor, and it also has a clock on it (rather than just a stopwatch on the normal one).

Wish we had a great range of rims here!

Cheers
Ric

 

Ric - After some deliberation, I decided to order the Polar 720 power system. I already own a Zipp 900 disk for TT's; and I was reluctant, at this time, to give it up for a PT wheel. I expect delivery on the Polar system this week. Should you be interested, I'll give you my assessment in a few weeks.
Regards - Bob

 

Steve - thanks for the encouragement. Any suggestions or tips on installation and use? I plan to install the power system on my TREK team TT bike, and I plan to use it during TT's, as a real time monitor of my actual effort. Most of our TT courses are relatively flat; and my typical gearing is in the 53/15-11 range, depending on wind conditions. Also, I regularly use a Computrainer indoors. How comparable are the power output measurements between the Polar and Computrainer systems? I saw a link to a detailed analysis be someone I believe named Wong, which addressed this question-- but I would be curious about your opinion.
Thanks - Bob

 

From the information i've heard (i.e., third hand!) i believe that there will be no correlation between the S710 power and the Computrainer power. As i understand it, the S710 doesn't work (well) on a(ny) trainer.

However, i'd be pleased to hear how you get on with it, so please keep us updated.

Ric

 

Sorry for the delay in this report -- but I wanted to give the Polar power monitor a fair shot. Well it failed in all regards:
Very tricky installation with poor instructions.
After initial installation- very erratic results - no correlation to Computrainer or to perceived effort.
Tried reducing air gap -- but additonal rubber pads supplied by Polar US support insufficient.
My overall assessment is that this device is not ready for prime-time. In fairness to Polar however, I should point out that I tried to install the monitor on my TREK Team TT bike, with a very short chain stay, and unusal BB and seat stay geometry, requiring a tricky routing of the monitor wires. Also, I should mention that I'm delighted with the 720 monitor computer- that is once I got the USB IR port to work. But that's another story.

 

Ha! - www.biketechreview.com

I did test a CH aero wheel cover in the wind tunnel at Texas A&M. I'd love to be able to give the info away for free, but I am still paying the entry off! For a small fee, the wheel cover and several other wheel sample data points are available for download.

As far as power meters go, I had the opportunity to outfit a single bike with all three systems simultaneously. I wrote a review, that if it doesn't put you to sleep, has some relevant information in it. It is the article titled "3/5/2003 - Comprehensive Power Meter Review"

at:

http://tinyurl.com/3x36

-kraig