Weight affect on downhills

(Jeff Doughty PVC WBR (B)) #1

I am 68kg. I have entered 68kg into the rider stats. I get dropped on the downhills all the time. I will be riding consistently with a group and be in their draft at the top of the hill. They will be around 2w/kg throughout the downhill and they speed away, even with me pushing 2.7-3.1w/kg… I don’t think this makes any sense. At first I thought it was my imagination but it is very consistent.

I thought maybe everything would even out on the uphill. It doesn’t seem to. At the same w/kg I am not passing anyone on the uphill.

To me this basically means being lightweight is a disadvantage, and in my opinion a fairly significant one.

(Chuck Kozlowski ZSUNR) #2

Jeff, I weigh 82 kg just for reference. Downhills are my friend.

(Jeff Doughty PVC WBR (B)) #3

I watched you guys fly away on me every time today… There was basically nothing I could do. Even at an extra watt/kg I couldn’t keep up. Not sure I’ve ever experienced that outside. Maybe I’m just being a crybaby

(Eric C. (Zwift HQ)) #4

Added weight will always have an advantage on the downhill, just like being light will have an advantage on the uphill.

During testing, I am one of the heavier people in the office and can take off after the KOM gates but the lighter riders (admittedly also with a bit more physical ability) leave me in the dust when climbing.

I don’t necessarily think our model is off but, of course, I won’t claim it’s bulletproof. There will be continual refinements as Zwift develops and we’ll definitely be taking a closer look at things like this.

(Jeff Doughty PVC WBR (B)) #5

Thank you for the reply Eric. I am not saying anything is wrong necessarily. Just giving my experience :slight_smile: out of curiosity is the Cda for a bigger rider different than a smaller one?

(Eric C. (Zwift HQ)) #6

Funny enough, I don’t think so because all of our riders are the same “physical” size despite varying weights. I’m hoping Jon can come in here and school me on it when he gets a chance!

(Chuck Kozlowski ZSUNR) #7

@Jeff, the same observation that you have made had been made by a few others as well so I think there is something to this line of thought. It is good to see Eric (Zwift software wizard) weighing in (pun intended) on this discussion. One of the guys I ride with regularly (Jeff Knisely) is much lighter than me and roars up the hills and I can pass him on the downhill segments.

(Eric C. (Zwift HQ)) #8

I’m eating all them donuts for a reason!

(Chuck Kozlowski ZSUNR) #9

If the aerodynamic drag (CdA) is the same for everyone regardless of their mass then that might indeed explain why the bigger riders are bombing down the hills (which I personally think is awesome).

(Jeff Doughty PVC WBR (B)) #10

That’s it. I ordering a large pizza and eating a sleeve of Oreos every night! :slight_smile:

(Eric C. (Zwift HQ)) #11

That’s what I call a recipe for fitness going downhill fast. /puns

(Jeff Doughty PVC WBR (B)) #12

I’d really like to hear Ryan’s from (Best bike split)s impression on this. He has this stuff dialed in like no other (that I know of anyways)

(Percy Zahl) #13

Good point, find a estimate for a weight dependent Cda – and may be even adjust the rider models appearance accordingly a bit – you got height and weight, so it’s a good basics parameter set :wink:

While (slow) climbing the Cda is pretty irrelevant, but descending at 60mph… sure it is!

(Andrew Williams) #14

The downhill thing is tough for me at 61kg. In real life, nobody can really blow me away on a downhill unless I am not aware of what’s going on, but that’s because I can make myself very slippery and never lose the draft. On Zwift (and especially with AIs), once people slip away, it’s lights-out. They slip away at a rate which is far greater than I have ever seen in actuality.

I would also like to see proportional CdA. My speeds on the flat sections are a bit slower than they probably would be given the same conditions in RL.

Regarding uphill, w/kg seems to fall in line with reality.

(Andrew Williams) #15

Cycling physics are somewhat complicated too. While we might compare eachother via w/kg (and that is certainly the industry-standard for comparing relative fitness level between people of different sizes) those comparisons only yield predictable results on uphill grades, where power-to-weight takes over as the primary forces.

For downhill it’s not w/kg that matters. It’s mass/CdA. The Galileo test doesn’t count here, since we aren’t measuring gravitational acceleration, but terminal velocity. Weight matters.

For flats, it’s Watts/CdA. Weight matters almost nil in real life unless you are accelerating. People have arguments about this on the internet ad nauseum, but the physics are the physics. Since Zwift currently gives everyone the same CdA (whether you are 5’1 and 110lb or 6’9 and 240), the advantage goes to the heavy watt-mashers.

So, on balance, Zwift currently doesn’t really favor the lightweights. At 50 feet per mile, it’s marginally better to be heavy as long as you are fit enough to make big power.

(Josh Currier) #16

6’6" 120kg here, and while I love destroying everyone on the downhills in the game I do think the rapid acceleration is pretty unrealistic.

(Jeff Doughty PVC WBR (B)) #17

It definitely makes group riding a challenge. Last night during a group ride with 3 other riders (all heavier than me) I had to either push 3+w/kg to keep up with them on the descent while they rested and did 1.5 w/kg or so… or I had to sacrifice 7 or more seconds and then push 5-6w/kg to catch them on the flats. The guys I was riding with last night, Michael, Chuck and Greg were cool and didn’t just leave me behind, but it was definitely unrealistic and frustrating. It seemed to be a bigger issue now that the island is reversed and the steep hill comes into play. It makes the reverse Green Jersey nearly impossible to compete at. I did a 950 watt (avg.) sprint and still only got 7 seconds, my belief being that I didn’t have much speed coming into it due to my weight.

I think entering a more realistic CdA for bigger riders would balance the benefit of extra mass. It seems to work in real life. If modeling CdA into the equation and having different equations for flats, uphill and downhill is too much, something should probably be done to the modeling of the downhill algorithm.

I will now go eat a wahhhh burger and some french cries, then wash it down with a few whinekins … it can’t hurt :slight_smile:

(Chuck Kozlowski ZSUNR) #18

It is fun for me to bomb down the Zwift hills (@ 82kg). I would like an option for helium/hydrogen balloons to deploy on my bike (perhaps the ‘b’ key) on the uphill segments as the lighweight riders zip past me. @ Jeff, that was a great ride yesterday.

(Jeff Doughty PVC WBR (B)) #19

It really was Chuck. I had a blast. You missed Michael going nuts and hitting every sprint and flying up the hills for his last 2 laps. I had planned a 30 mile endurance ride but that did not exactly happen!

(Percy Zahl) #20

Yeah, put this in a relation… 60kg vs. 120kg – no pedaling same Cda and wooooosh the big rider is gone at nearly 2x speed – a little hold off via Cda would do good :wink: I as a light rider normally have no problem just sitting on a big rider’s wheel on any decent. I guess every one knows how much of an difference that makes, on a say 6% decent in a pack having need to control speed gently braking not to run in to the rider ahead – or even pedaling bad up front!