watts/kg on flat vs hilly courses
I saw a post by Coach R about absolute watts having a bigger effect on flat coures than watts/kg. Which on a slow day at work got me thinking.
How do those two riders play it out on a hilly course? Would two people with different FTP/weight numbers but the same ratio ride the same time? Or does weight become the bigger issue?
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When you turn your pedals pushing watts into the gears that turn the rear wheel, there are many forces that you need to overcome.
On a flat road, the biggest force to overcome is wind resistance. This increases roughly at a function of your drag coefficient x your speed (squared). So as your speed increases, it get's exponentially harder to go even faster. So the little peeps with lower FTP but similar W/Kg will likely be smaller and have a smaller drag coefficient than a broad shouldered big dude with a huge FTP. But this is a small part of the equation as you speed starts to get above 12mph (assuming both of these people are already setup with their own close to ideal aero position)... Getting this drag coefficient as low as possible is why we spend so much money on tri bikes, deep dish wheels and aero helmets, bike fits, etc.
The other big force to overcome on any road is rolling resistance (this is much smaller than air resistance). This is why there are multiple threads on EN every yr talking about the ideal race tires, tire pressures, and latex tubes... Other things like a clean chain, cassette, and aligned deraileurs also help with minimizing friction losses in your drive-train.
But in general, a higher FTP on flat roads means More Speed! Because the things that effect the forces counteracting the Watts change very little with body mass.
Hills are a different animal. When you introduce vertical components into the equation, then you add the force of Gravity that you must overcome. And if you believe Newton's 2nd Law (which I do) then Force = Mass x Acceleration. Where the mass is essentially your body mass and Acceleration is Gravity. Therefore the force you need to overcome to go up at a given speed is directly related to your body mass. So ignoring all of the forces above for everyone on a flat road, the additional force required to take a 200 lb person straight up at a constant speed is exactly 2x the force required to take a 100lb person straight up. The math gets a little more complicated as you change speeds and grades of the hill, but the concept remains the same.
So when you climb hills, the big guy also has to fight gravity as he climbs but the little peep doesn't have to produce as much power to overcome the lower force effect. And on the way down the other side, the gravity is a multiplier for the big guy as the gravity is pulling him down the hill even faster (but once you get too fast, the wind resistance starts to overwhelm all other parts of the equation giving anyone a certain terminal velocity regardless of power). So all else being equal, 2 people with Exactly the same W/Kg (say 3.0) will roughly climb at the same speed but the heavier of the two people will descend slightly faster (thanks to gravity) and will also be even faster on the flats (thanks to a higher total FTP because the gravity effect is essentially 0 on a flat road).
So what if you compare a 100 lb chica with a 170W FTP (3.74W/Kg) to a 200lb dude with a 270W FTP (2.97W/Kg)? This gets more difficult because the big dude will likely be faster on the flats and the downhills and at very small grades of uphill. But once the hills get steep, the chica will be able to pass him uphill at the same IF. The big dude also might be able to carry his faster downhill speeds up the other side if the course is "rolling" hills. At some point, and certain courses, these two people might actually ride the same exact bike split. But if the course has enough steeper uphills, then the little chica would likely be faster. And if the course has a bunch of gradual rollers and no super steep inclines, then the big dude might actually be faster. But if these same two people rode a perfectly flat bike course then the big dude would likely be faster. All of the above assumes similar aero profiles and bike setups. Even on a flat course, the little chica in this example would crush the big dude if she has a dialed in aero position and the big dude is on a road bike with many stem spacers and no aero bars. Or even worse, if he was riding a mountain bike because now he would have a massive wind resistance and a massive rolling resistance (even if his FTP was higher and his W/Kg was higher, he would still lose).
So back to the short answer I started with, "It depends!" But in general, on flat courses, bigger FTP people are faster. And on hilly courses, W/Kg is more important. Doesn't really matter though, the important things are the same for everyone:
1) Increase your FTP
2) Decrease your weight
3) Have a clean bike and dial in your aero position
4) Execute like a Ninja
5) lather, rinse, repeat
Sincerely,
Big Dood (aka, slow UP the hills, fast DOWN the hills and fastER on the flats)
If you are going straight up, then the only significant resistance is gravity, and W/kg therefore becomes the only significant measure. Absolute watts don't matter. EXCEPT that people measure W/kg for just their bodies and not their bodies plus their bike...and bike weights don't vary that much, so the effective W/kg is actually a bit higher for the big guy than for the little gal with the same (body-only) W/kg. This "limit" is effectively the case at grades of about 6% or more for normal human beings (speeds of ~10 mph or less)
On a flat, smooth road, John is absolutely right again... the main resistance is now wind. That resistance goes as a LINEAR function of your absolute "aero-ness" (the drag coefficient) and as the square of speed. The driving force is the watts you put out. If your drag coefficient went up linearly with your mass, then W/kg would also work for on the flats. However, even though ~80-85% of the drag coefficient is due to the rider's body, it goes up much slower than on a 1:1 basis with rider mass. This is because the drag coefficient is mainly related to frontal area, and a lot of your mass is "shielded" by the front of you. A longer torso would present no more surface area to the wind at all than a shorter one at all in the ideal case where you were absolutely flat. So bigger guys have more drag, but not enough more to take away their advantage in the flats.
On the downhills, bigger guys usually go faster, but here I dispute John's explanation. Actually gravity affects everything proportionately with mass. Two spheres of the same density and shape (but different size) will fall at the same rate from any height. In a vacuum, a feather and a bowling ball fall at the same rate.
I think the reason that bigger guys tend to go faster downhill is a little counterintuitive. Since wind resistance is obviously pretty significant at 30-40 mph (!), you might think the smaller peep would go faster. I think the reason that bigger guys tend to go faster downhill is that the case isn't ideal (starting from a dead stop with no watts applied by the rider). A downhill necessarily comes right after a crest or a flat. Those are places where a big guy has an advantage and the big guy STARTS the downhill going faster and/or can get going to a higher speed in the gentle part of the hill at the top before gravity really takes over.
This said, it points out how the EN way of riding hills give the chica a huge advantage over other chicas - that is, conserve energy going up the hill and ride fast across the crest. As you begin the downhill, you'll be at an advantage and able to get to your terminal velocity a lot faster than someone who crosses the crest at a crawl.
And I would be at a severe disadvantage if I tried to get into a physics debate with the Professor since it's been 15 yrs since I even pretended to be an engineer. So yes, I will concede that 2 balls of different masses rolling down a slope of known grade in a vacuum will indeed do so at the same rate if the only force acting on them is gravity. But, if the effect of gravity is not helping the bigger person, it is also not hindering the bigger person either since he doesn't have to provide any additional force to lift his weight. Therefore, assuming he was pedaling at his goal watts, it would be no worse than on a flat road and we already agreed that this would be faster than the light Chica pedaling at her goal watts on the same road. And yes, when they both spin out and start coasting, it is likely that the bigger dude is starting his coasting at a faster speed to continue accelerating forward with the aid of gravity until he reaches his terminal velocity.
And his final point for anyone we haven't put to sleep yet and is still actually reading this is that the EN way of riding, DEFINITELY maximizes this downhill speed advantage regardless of your weight class. Because your (power + gravity) trumps (gravity alone) 100% of the time.
One of the valuable aspects of riding with power is observing the interplay of rider weight, % grade, FTP, w/kg, wind resistance, etc as you ride with cyclists of widely varying body types and fitness levels. This becomes a powerful pacing tool on race day, as you observe the people around you making mistakes and gain confidence from the fact that you are not.
Nerd Alert. Jess told me I made her head hurt and forbid me from talking more physics on an EN forum. BUT... I couldn't stop thinking about this on my ride this morning, so I'm prolly gonna be in trouble for posting more of my thoughts on this so stop reading now if you are a normal person. My old hobby long before triathlon was skydiving. I have nearly 100 jumps (formation stuff, high altitude, night, etc). I was ~225lbs back then and I fell faster than just about everyone. For me to fall at the same speed as my little 140lb friends I had to pretend to hug a giant beach ball while falling (to catch as much wind as possible) and they would arch as hard as possible to be more aerodynamic. Body weight has a huge effect on terminal velocity. If you care to learn more, click here: http://en.wikipedia.org/wiki/Terminal_velocity But basically, if we're NOT in a vaccuum, then then terminal velocity (downward) is where the force created by the falling object is perfectly counteracted by the force of the drag of the wind (or whatever medium you are in). And since your Force down = Mass x Acceleration where the Acceleration is gravity, then the force you apply in the downward direction is greater if you are heavier and therefore the higher your terminal velocity. So to translate this into two cyclists of different weights going downhill (on a real course with real air slowing you down), even if they are not pedaling and started from a stop or from the same speed, the bigger dude will go down a steep hill faster than the little chica in most circumstances.
Even with what I said in the paragraph above, everything said in prior posts is still relevant in that no matter your weight class, EN style of riding will simply make your starting speed faster that you carry downhills.
Theoretically, two people of different weights but the EXACT same W/kg (including the bike and full kit), same training, execution, aero-ness, etc... The one with the higher total FTP will be slightly faster on a hilly course. But that is theoretical and almost irrelevant because as stated above, we all need to try to raise our FTP, stretch how long we can hold certain percentages of this FTP, get our weight down, and execute like Ninjas.
Sorry guys.... "Forbid" is a strong word, but I tried!
I'm gonna go take a couple Excedrin now...
I see you go for inexpensive hobbies, my friend....
My favorite execution story is during IMLoo 2011 ---- there was one hill that was moderately steep and a couple hundred yards long. Well, sitting on the side of the road were a couple of High School aged girls cheering everybody on. LOTS of my fellow racers (but certainly not the EN contingent) were HAMMERING this hill. I was 'toodling' along, holding my watts, and just sitting up and thinking about "gumdrops and fairies and unicorns".. Since I was going so slow compared to the others, both times I went by these girls they shouted out to me (with concern in their voice):
"You can do it!!!! Just keep going!!!!"
Like Rich says, if you are doing the opposite of most everyone else, you are probably executing correctly.
<< is a nerd..... and is enjoying this geekery. </p>
Sorry Jess, for the headache and the notion that I am enjoying this thread.
To take math out of the equation, but give an example. I ride frequently with a female friend who has a powermeter. As it turns out we compare current riding power frequently during our rides. She is aways amazed at the difference between hers and mine as we ride evenly together.
I'm 6'2" and 200 lbs and she is a small frame 130 lbs (its amazing that female training friends have no problem telling you their weight
As we are riding side by side we compare power sometimes. Note that we are both in aero but her bike fit has her lower than my fit. The gist is:
* on flats, I am pushing about 40w-50w more than her to keep even
* on hills (7-8% grade) I am pushing 100w-120w more than her to keep even
So yes, I have a higher FTP than she does, but I've found weight makes a big difference even on the flats.
I imaging this is due to:
* bigger folks are wider so the wind resistance is more to overcome
* weight makes a difference in rolling resistance even on flats with exactly the same frame
Just my non-scientific observational experience.
Mike.
My only take away from this whole thread is... Get the FTP and w/kg has high as you can and present as little of yourself as possible to the relative wind!
Just good stuff.....
@Mike M.... nice comparison in a real world scenario
Big boys rule downhill as gravity helps
Great Thread!
@JW you wrote.... But that is theoretical and almost irrelevant because as stated above, we all need to try to raise our FTP, stretch how long we can hold certain percentages of this FTP, get our weight down, and execute like Ninjas.
I feel as if I'm splitting these 4 areas up.
1. OS - Raise FTP 2. Race specific Prep - Stretch percentages of holding FTP & get weight down 3. RR & Race day - Be a Ninja
For me personally I stress the importance of point 2 in Long Distance Triathlon. Last year I got my FTP up, but couldn't hold the percentages. This year I have sets holding those %'s for 40-50 minutes a few times in every long ride instead of FTP interval work. I'm ok with losing a few FTP watts for holding that 85% for HIM and 75% for IM.
Any insights or experience with this?